-11th volcanic rocks in nearby areas eugene i....
TRANSCRIPT
GEOLOGY AND GEOCHEMISTRY OF THE VOLCANIC ROCKSIN THE RIVER MOUNTAINS, CLARK COUNTY, NEVADA
AND COMPARISONS \-11TH VOLCANIC ROCKS IN NEARBY AREAS
Eugene I. SmithDepartment of Geoscience
University of Nevada, Las VegasLas Vegas, Nevada 89154
ABSTRACT
Two chemically distinct suites of igneous rockwere formed in late-Miocene time in the RiverMountains. Initial volcanic activity was alkalic innature and built a stratovolcano of andesite anddacite in the southeastern part of the range. Thisepisode of activity was followed by the eruption offlows of subalkalic dacite. These lavas are commonlywell flow-banded and are interbedded with mud-flowbreccias, thin pyroclastic units, and flows of alkaliolivine-pyroxene basalt and andesite. Rare earthelement data suggest that fractional crystallizationwas not important in producing either the alkalic orsubalkalic suites, rather they may have been formedby the partial melting of a heterogeneous crust(probably Precambrian in age). The basalts wereprobably derived from the mantle by partial melting.
The River Mountains are chemically distinct whencompared to nearby volcanic ranges. The volcanicsection in the River Mountains is more alkalic, anddisplays a narrower range of Si02 values thanvolcanic rocks in either the McCullough or EldoradoMountains. In addition, alkali basalts in the RiverMountains show stronger iron enrichment than those inthe other ranges. Also, the basalts of the RiverMountains are nepheline normative, whereas those inthe McCullough and Eldorado Mountains are hypersthenenormative. Both the McCullough and Eldorado rangescontain two major groups of lava flows that areseparated by an ash-flow tuff. However, there aresmall but important differences in chemistry thatthrow doubt on the direct correlation of unitsbetween ranges. The distinctive chemistry of thevolcanic sections in the River, Eldorado andMcCullough ranges suggests that each area records thedevelopment of a separate cogenetic assemblage ofvolcanic rocks.
The structural geometry of the River Mountainsis characterized by fault-bounded blocks each havingdifferent structural orientation. Thispattern was probably imposed on the range by threemajor structural events. The first and mostspeculative of these events was the detachment of thevolcanic section and possibly part of the Precambriancomplex from the Precambrian basement along a lowangle fault. Evidence for this structural episodeincludes: 1) a low-angle fault on Saddle Island(just to the east of the River Mountains) that isdefined by well developed foliation (myloniticfabric(?)) in Precambrian chlorite schist andamphibolite. The fault separates an upper plateschist and gneiss terrain intruded by pegmatite,granite, rhyolite and basalt from a lower plateintruded only by pegmatite. Since two lithologicallyand structurally different terrains are juxtaposedalong this fault, displacement was probablyconsiderable. 2) Intensely brecciated volcanic andplutonic rocks to the west of Saddle Island mayreflect shattering during transport. These rocksmay lie just above the detachment fault. 3) Farther
41
to the west, low-angle faults are common in thevolcanic section. These faults are probablyreadjustment structures in the upper plate.
High-angle normal faults truncate the low-anglefaults and are the dominant structural feature of therange interior. These faults record a northeastsouthwest directed extension that was probablyimposed on the range by the strike-slip faults of theLake Mead Fault system. An earlier extensional eventof unknown origin is indicated by numerous east-westtrending dikes.
The strike-slip faults of the Lake Mead Faultsystem bound the River Mountains to the north andsouth and may be responsible for transporting therange as far as 40 km to the southwest from anoriginal site in the northern part of the BlackMountains of Nevada and Arizona. Stresses imposed onthe range during strike-slip faulting may haveproduced the discordant orientation of structuralblocks within the range.
INTRODUCTION
Areas adjacent to the Colorado River in Nevada,Arizona and California display numerous structuresthat were formed during a major Tertiary-agedextensional event. These structures include:1) low angle detachment faults well documented in the\-Ihipple and Rawhide Mountains of California andArizona (Davis and others, 1979; Shackelford, 1976);2) listric normal and associated low angle faults inthe Eldorado Mountains of Nevada (Anderson, 1971);3) strike-slip faulting in the Lake Mead Region(Longwell, 1960; Anderson, 1973; and Bohannon, 1979);and 4) numerous normal faults bounding and withinmost of the ranges adjacent to the Colorado River(Longwell and others, 1965; Anderson, 1978; Steward,1980).
The River Mountains lie within this extensionalterrain, and are composed of a thick pile ofTertiary-aged dacites, andesites and basalts. Theyrepresent one of four major volcanic ranges in thewestern part of the Lake Mead area (Fig. 1); besidesthe River Mountains, these include the McCulloughMountains (Longwell and others, 1965; Hewitt, 1956;Bingler and Bonham, 1973; Anderson, 1977), theEldorado Mountains (Anderson, 1971) and the Cleopatravolcano in the northern part of the Black Range(Anderson, 1973). Chemical data are presentlyavailable for the McCullough Range (Kohl, 1978;Johnson and Glynn, 1981), the Eldorado Range(Anderson, 1978; Johnson and Glynn, 1981), and theRiver Mountains (this report).
Previous investigations in the River Mountainswere mainly reconnaissance in nature. Longwell(1936) mapped a small part of the eastern RiverMountains including the Precambrian rocks of SaddleIsland as part of his study of the Boulder reservoirfloor. Subsequently, he mapped the entire range, and
described the River Mountains stock as well as thelimestone blocks that are wedged into the volcanicsection just to the north of Boulder City (Longwell,1963). McKelvey and others (1949) studied a smallpart of the northern River Mountains near the ThreeKids manganese mine and recognized the anticlinalnature of the northern half of the range. As part ofthe site survey for the River Mountains tunnel,geolog~sts from the U.S. Bureau of Reclamationcompleted a detailed geological strip map along theroute of the tunnel (Westcamp and Pavlovich, 1968).Anderson and others (1972) dated the rocks of theRiver Mountains as late-Miocene by the K-Artechnique, and later Anderson (1977) mapped a smallpart of the southern River Mountains near BoulderCity as part of his work in the Boulder City IS'quadrangle. Recently Brenner and Glanzman (1979)suggested that the River Mountains are the site ofa large caldera. This hypothesis was refuted byBell and Smith (1980) in their geological study ofthe Henderson Quadrangle. The general geology of theRiver Mountains and the relationship of strike-slipfaulting to volcanism were discussed by Smith (1979,1981).
This paper is based on the current geologicaland geochemical research now being conducted at theUniversity of Nevada, Las Vegas. The paper presentsthe first chemical data for the volcanic rocks of theRiver Mountains and speculates on the petrogeneticand structural evolution of the mountain range.Chemical comparisons of the volcanic section in theRiver Mountains with volcanic rocks in nearby rangesshow that each mountain range is composed of achemically distinct volcanic pile and that eachvolcanic section probably originated from a differentsource area. No key unit extending from range torange was identified. This report is based on workin progress, therefore interpretations may change asmore data becomes available.
BRIEF DESCRIPTIONS OF MAJOR ROCK TYPES
Volcanics of Powerline Road and Bootleg Wash
The Powerline Road and Bootleg Wash volcanicsequences (Bell and Smith, 1980) represent thethickest accumulation of volcanic and volcaniclasticrock in the River Mountains (Fig. 2). The PowerlineRoad section is composed of numerous flows of bandeddacite containing plagioclase, biotite and hornblendephenocrysts. Flows are interbedded with pyroxene andolivine basalts, thick sections of mud-flow breccia,thin pyroclastic units, and autobrecciated daciteflows. Sources for the dacite flows are smallstructurally elongated dacite domes. The source forthe basalt section is probably in the northern partof the range where a thick section of basalt,basaltic agglomerate and breccia crops out.
The volcanics of Bootleg Wash underlie thePowerline Road units, and are composed of an upperdacite flow containing plagioclase, biotite,hornblende and quartz phenocrysts; a centralepiclastic unit locally interbedded with a daciteflow; and a lower unit of andesite and andesitebreccia. The volcanics of Bootleg Wash arerestricted in occurence to the sourthern margin ofthe interior valley (Fig. 2).
Volcanics of River Mountain and adjacentvolcanic sequences
Porphyritic andesite flows of the volcanics of
42
Figure 1. Index map of the Lake Mead region. BCBoulder City. BCP-Boulder City pluton, CVCleopatra Volcano, DP-Devil Peak, EDEldorado Mountains, FM-Frenchman Mountain,LM-Lake Mead, M-McCullough range, RPPRailroad Pass Pluton, TM-Table Mountain,VM-Virgin Mountains, \{RP-Wilson RidgePluton. Lined areas represent theapproximate distribution of volcanic rocksof Tertiary age. The shaded patternrepresents the approximate distribution ofintrusive rocks of Tertiary age.
River Mountain (Bell and Smith, 1980) form the RiverMountains stratovolcano in the southeastern part ofthe mountain range. These andesite flows are cut bydikes and sills of dacite. Coring the stratovolcanois a quartz monzonite stock, that was dated byAnderson and others (1972) at about 13 m.y. old.The stock is surrounded by an aureole of intenselyintruded and altered volcanic rock of thestratovolcano. This transition zone from stock tovolcanic rock (the aureole) was first described byLongwell and others (1965). Wedged into the southernpart of the stratovolcano are blocks of well-beddedlimestone, identified in part as the Banded MountainMember of the Bonanza King Formation (Wernicke,personal communication, 1981). The limestone blockswere partially consumed by intrusive rock related tothe stock (Smith, 1981). These exposures probablyrepresent exotic blocks that were emplaced as aresult of strike-slip motion along the Lake MeadFault System. The nearest source area for thelimestone lies 60 km to the northeast in the SouthVirgin Mountains (Fig. 1). The coeval relationshipbetween the emplacement of the limestone blocks,volcanic activity, and strike-slip faulting suggeststhat igneous activity and strike-slip faulting werecontemporaneous in this area (Smith, 1981).
To the east of the River Mountains stratovolcanois a thick section of steeply dipping dacite (wellflow banded with phenocrysts of plagioclase,hornblende and biotite) and basalt (augite andplagioclase phenocrysts) that may be equivalent tothe Powerline Road volcanic section. These flowsare separated from a thick section of breccia (mudflow and autobreccia) by a low-angle fault. Theupper plate breccias, named the volcanics of TeddyBear Wash by Bell and Smith (1980), are composed ofnumerous angular fragments (up to 1 m in size) ofplagioclase-biotite dacite; little or no structure(bedding or banding) is observed within this massiveunit. The impressive cliffs along the eastern flank
,;q"'/'
.;:,'iJ,/;.'
//
//
//'{
"
'j
I, ii /, .--' Boulder
,/'-'-'-'-' City,//
.-,..--- ...---
Figure 2. General Geologic map of the River Mountains, Clark County, Nevada.
43
: : Saddle• Isla
\\\
\\\\
"L.-·.-.-'-'--'.--'
miles
EXPLANATION
of 0]\': .,\' Intrus~veRoad -."
VolcanicsPowerline
Volcanics ofBootleg Wash
Alkali Basaltsand Andesites
rock
B Exotic Blocks oft;;;;j Lime stone
Limestone andSandstone
numerous plagioclase-biotite bearing dacite dikes.Several low angle faults, originally described byAnderson (1973), separate highly altered and intrudedlavas below from less highly altered and rarelyintruded lavas and volcaniclastic rocks above.Because of similarities in lithology, I infer thatthe volcanics of Red Mountain are the highly alteredequivalents of the volcanics of the River Mountainstratovolcano.
~Volcanic Source Illl11 Poss ible Volcanic
~ Areas WU]Source Area
Lying to the west of the River Mountainstratovolcano are the volcanics of Red Mountain (Belland Smith, 1980). This unit is formed by highlyaltered and oxidized gray to light red porphyriticlavas of intermediate composition, and is cut by
Precambrian Rocks on Saddle Island and adjacentbrecciated exposures
Techniques
GEOCHEMISTRY
Major elements and Ba were analyzed by theinductively coupled argon plasma (ICAP) technique,and x-ray fluorescence spectroscopy (XRF) was usedfor Rb and Sr (for samples done at Technical ServiceLaboratories in Toronto). In five additional samples,the major elements and Sr and Zr were analyzed byAtomic Absorption Spectrophotometry and XRF in theRock Chemistry Laboratory at the University of Nevada,Las Vegas. The rare earth elements (REE) wereobtained by instrumental neutron activation analysis
Precambrian rocks crop out on Saddle Island(Fig. 2) and form two lithologically and structurallydistinct terrains. The northern part of the islandis characterized by a basement of amphibolite andchlorite schist intruded by dikes and small stocksof: 1) red coarse-grained porphyritic granite(quartz, orthoclase, biotite), 2) pegmatite (quartz,orthoclase, plagioclase, muscovite), 3) porphyriticrhyolite (quartz, plagioclase, and biotite), and 4)basalt (plagioclase, pyroxene). Although the age ofthese intrusions is unknown, a Precambrian age forthe basalt and rhyolite dikes can be ruled out sinceseveral of them intrude a carbonate breccia ofprobable Paleozoic age (described below). At MoonCove near the northern tip of Saddle Island areseveral exposures of red quartzite, and carbonatebearing breccia (Fig. 2). The quartzite is similarin lithology to the Cambrian Tapeats Sandstone;alternatively it may correlate with quartzite in thelate-Precambrian section. The breccia containsclasts of limestone, granite and quartzite; thedolomite clasts are almost certainly Paleozoic inage. These units lie unconformably on aPrecambrian erosion surface of moderate relief, andrepresent the ~nly "in situ" Paleozoic deposits inthis area. The southern half of the island is formedby chlorite schist, well foliat~d quartzo-feldspathicgneiss and amphibolite and is intruded by pegmatitedikes. Separating these terrains is a major lowangle fault (strike N 80 E, dip 30 to the north) thatpasses through the prominent saddle that gives theisland its name. This low-angle fault will bediscussed in detail in the structure section of thispaper .
Severe brecciation is common in volcanic andplutonic rocks (probably Tertiary in age) just to thewest and south of Saddle Island. Near the BoulderBeach Marina (Fig. 2), low hills composed ofintensely autobrecciated granite are intruded bydikes of brecciated biotite-bearing lamprophyre.Contacts between granite and dike rock can be tracedfrom clast to clast in the breccia in a continuousmanner. To the west of these outcrops are exposuresof brecciated dacite flows of Tertiary age, that arealso intruded by lamprophyre dikes.
"0
"0/ .... ..
D ..
.. ....
DDikes
~Autochthon on~ Saddle Island
~ Allochthon on~ Saddle Island
Y Dip-slip Fault
./Low-angle Fault
Volcanics ofRiver Mountain
River MountainStock
Volcanics ofRed Mountain
;/ GeneralStructuralOrientation
/I Strike-slipFault
12A
10
8
:\\V ..6 I>-\~~ :\\V
~~~
.. '<>.;;'O~
4 ....
R..·.·.. ·.·.··.U
D
oN
::.::6
N
'"Z
.. Volcanics of Powerline Roado Volcanics of Bootleg Wash" Volcanics of the River MountainsD River Mountains Stockv Volcanics of Teddy Bear Wash
O+--...:-::..;..:::.:-~~...:....::..:~..::..;..:=---,..:..::..--.-,-----.---r-.---i
46 50 60 70
Weight Percent Si02Figure 3a. Harker variation plot that differentiates
between the alkalic and subalkalic suites.The field boundary is from Irvine andBaragar (1971).
of the mountain range just to the west of Lake Meadare formed by these breccias.
44
at the Phoenix Memorial Laboratory of the Universityof Michigan. Accuracy for REE with concentrationsless than 1000 ppm is 10 percent except for Nd, Tband Yb which is 20 percent. The multielementstandards G2, BCV-l, and HBVO-l were used as internalstandards for all of the above determinations. Atotal of 20 new chemical analyses were obtained forthe River Mountain rocks (Table 1). Rock names arefrom the chemical classification of Irvine andBaragar (1971), and were determined by a Fortran IVprogram by Smith and Stupak (1978).
Results for the dacites
Two chemically distinct suites of dacites wereidentified in the River Mountains. Rocks of theRiver Mountain stratovolcano, the Teddy Bear Washbreccias and several dikes that extend from the
River Mountain stock, but not the stock itself, showalkaline affinities. On the other hand, dacites ofthe Powerline Road, Bootleg Wash and the intrusiverocks of the River Mountains Stock are distinctlysubalkaline (Fig. 3a). The two suites show severalimportant differences in chemistry. For example, therocks of the alkalic suite are lower in CaO and Fe203than comparable rocks of the subalkalic suite(Figs. 3b and 3c). Within the subalkalic suite,rocks of the Bootleg Wash sequence can easily bedistinguished from the dacites of the Powerline Roadsection by lower values of A1203, Sr and Ba (Figs. 4aand 4b). The rocks of the alkalic suite representthe initial volcanic pulse in the River Mountains.This episode was followed closely by the eruption ofthe rocks of the subalkalic suite.
The dacites show light rare earth element
Table 1. New Chemical Analyses for Volcanic Rocks in the River Mountains.l
Unit
N2,3
o.
SiO~
A1203TiO?Fe203CaOMgOMnONa20K20P205LOI
TOTAL
BaCrZrRbSrLaCeNdSmEuTbYbLuThHfSc
Na20/K20FeO/ (FeO+MgO)Ba/SrRb/SrSum REELa/LuEU/Sm
Tpb
3
47.1114.521. 94
12.0710.85
7.080.152.951.320.881. 99
100.86
2191. 63186.52312.00
38.831020.33125.24274.19113.9015.10
3.932.721.890.33
18.667.74
29.56
2.230.632.150.04
537.30372.22
0.27
Lam
47.1114.321. 73
10.785.706.880.133.535.431.681.69
98.98
3049.3062.78
126.05482.00153.64360.14168.15
20.735.112.792.270.35
17.958.91
23.79
0.650.616.330.26
713.18438.97
0.25
Tpd
5
67.0714.36
0.484.082.560.910.063.904.350.161.41
99.34
1472.6249.20
172.00175.39272.0058.80
109.2439.115.241.180.731. 520.26
19.044.704.85
0.950.825.410.82
216.08222.97
0.23
Tpa
2
56.5215.81
0.909.216.053.890.113.742.410.591.03
100.24
2355.9588.06
374.0099.63
874.00120.24258.52105.55
13.583.331.632.370.37
16.027.95
14.64
1.550.702.700.11
505.58324.97
0.25
Tbd
67.6612.18
0.575.632.060.420.012.407.650.220.75
99.55
774.35175.83
199.2630.0047.66
102.7438.475.811.110.311.420.22
14.824.057.48
0.310.93
25.816.64
197.64216.64
0.19
Tba
59.5113.141.146.914.591.410.093.035.320.503.26
9,'0.90
1120.7098.33
160.6633.0053.55
120.2556.53
7.492.090.542.000.29
10.755.05
14.93
0.570.83
33.964.87
242.74184.66
0.28
Trs
66.0414.83
0.535.922.611.430.063.953.680.180.10
99.33
1512.00177.36
135.80121.00
52.27101. 0051.46
5.201.130.941.770.27
14.015.025.93
1.070.81
12.501.12
214.04193.59
0.22
Tra
53.4915.011.087.805.153.750.094.123.200.654.71
99.05
1375.50161.85
99.15351. 00
74.51163.57
75.4910.60
2.510.732.440.36
20.736.63
14.57
1. 290.683.920.28
330.21206.97
0.24
Trd
65.5814.69
0.484.851. 391.120.044.735.480.121.12
99.60
1576.1092.35
127.8466.0057.69
105.49"35.14
5.231.090.221.630.29
17.905.584.51
0.860.81
23.881. 94
206.78198.93
0.21
Ttbd
56.6116.96
0.795.794.393.030.073.985.200.501.44
98.77
16211.5035.62
139.04405.00
75.94157.15
77 .899.122.221.111.690.30
15.745.81
11.04
0.770.664.010.34
325.42253.13
0.24
lAnalyses performed by Technical Services Laboratories, Toronto, Canada; UNLV Rock ChemistryLaboratory; and the Phoenix Memorial Laboratory of the University of Michigan.
2Number of analyses averaged
3Major elements as weight percent; trace elements as ppm.
Unit symbols: Tpb = Powerline Road basalts, Lam = lamprophyre dike, Tpd = Powerline Road dacites,Tpa Powerline Road andesites, Tbd = Bootleg Wash dacites, Tba = Bootleg Wash andesites,Trs = River Mountains stock (quartz monzonite), Tra = andesites of the River Mountain stratovolcano,Trd = dacites of the River Mountain stratovolcano, Ttbd = dacite (clast from breccia in the TeddyBear Wash sequence).
45
4
10
12~-------------------....,
Basalts in the River Mountains are olivinenormative and strongly alkalic in character. Ironenrichment is greater than for the typical basin-andrange alkalic basalts described by Leeman and Rogers(1970) (Fig. 6). REE patterns for the basalts showextremely high light REE enrichment (Fig. 7). Forexample, La is enriched from 340 to 480 times that ofchondrites, and the lamprophyre dike shows 600 timesenrichment. All rocks display steep slopes for theheavy REE, and either have a shallow Eu anomaly orlack it entirely. These trends are similar to thosefor alkalic basalts in other areas (for example,Campbell and Gordon, 1980). The andesites are alsostrongly alkalic, but do not show the same extremesof light REE enrichment displayed by the basalts(Fig. 7). The fact that the basalts and andesitesshow greater light REE enrichment than the associateddacites suggests that the mafic rocks do notrepresent the parent material for the dacites (byeither a process of partial melting or fractionalcrystallization). The basalts and andesites probablyformed independently of the dacites by partialmelting of mantle material.
equivalence of REE patterns for the alkalic andsubalkalic suites, and the lack of a prominent Euanomaly suggests that feldspar and/or hornblendedominated differentiation was not important for thegeneration of either suite or in the formation of onesuite from the other. Rather, it is probable thatboth suites were derived from source rocks alreadyenriched in the light REE, such as the granites andgneisses of the Precambrian basement. This situationis currently being modeled and the results will bereported in another paper.
Results for the basalts and andesites
70
6 06 060 0
50 60
Weight Percent Si02
o River Mountains (Subalkalic)6 River Mountains (Alkalic)
2 +----,....:---,-----,,---,--;--,----'---,------,----.----.---.--.----146
14
12
fi6
10(')
0N
QlU- 8...t::Ql
0 0UQ;
0... 6....t::Cl'w$:
8
0C1l 0
0
U... 6t::QlUQ; 0
0...... 4.t::Cl
Ow$:
2
o 0oo
oo
Conclusions
Based on the chemical data presented above thefollowing tentative scenario for the formation of theRiver Mountain volcanic suites can be presented.
1) Alkalic andesites and dacites formed theRiver Mountain stratovolcano in the southeastern partof the range. Activity centered on the RiverMountain stock resulted in several east-west trendingdikes that cut the western flank of the volcano.
Modern chemical data is available for volcanicrocks in several nearby ranges. Anderson (1971),and Johnson and Glynn (1981) reported analyses forthe Eldorado Mountains, and Kohn (1978), and Johnsonand Glynn (1981) provided chemical data for theMcCullough range. Unfortunately there is an inherentdanger when comparing chemical data from differentrock laboratories, because of differences (sometimesmajor) in analytical precision and accuracy. As aconsequence, the reader should take care whenevaluating conclusions presented in this paper thatwere made by comparing different data sets. We are
2) Subalkalic dacites with associated brecciasand thin pyroclastic units produced the voluminousvolcanics of Powerline Road. These units eruptedfrom numerous domes adjacent to the stratovolcano.During this event the River Mountain stock wasintruded into the core of the stratovolcano. Daciteeruptions were accompanied by extrusion of alkalicbasalts and andesites with sources mainly in thenorthern part of the range.
CHEMICAL CO~ITARISONS WITH NEARBY RANGES
7060
Weight Percent Si02
50
o River Mountains (Subalkalic)" River Mountains (Alkalic)
O+----,----.-----,,---,--i--,----'---,------.,.---.----.--,-----,-------i46
Figure 3b. The volcanic rocks of the alkalic suiteare generally lower in total iron (asFe203) than the rocks of the subalkalicsuite.
3c. The volcanic rocks of the alkalic suitein the River Mountains have lower valuesof CaO than similar rocks of the subalkalic suite.
enrichment, lack a prominent Eu anomaly, and have arelatively flat heavy rare earth pattern (Fig. 5).Modelling studies for the formation and subsequentdifferentiation of these suites are currently underway, so little can be said about their petrogenesisat the present time. However, visual inspection ofthe REE and major element plots allows severalspeculative conclusions to be made. For example, the
46
70
....
60Weight Percent Si02
50
2o River Mountains.. Bootleg Wash Volcanics• Trachytes and latites of the McCullough Range
O+--.-,--'-r-,------,-,------.r--.--,....:-.--r--.-----l46
12
10
8
o·
66 •
o " 00 M
"40
B
1000
BA
G
A
G
E G
Q,Q, GI:"
C/) 100
GG
....c::Q)(J
Q;0.......c::O'l
'cu3:
('I)
oN«
Z) Based on the alkali-lime index (Peacockindex), the River Mountains samples are more alkalicthan rocks of the other ranges (for the RiverMountains the alkali-lime index is 55; the EldoradoMountains 57 and the McCullough range 57).
attempting to rectify this problem by analyzing alarge number of rocks from each volcanic range in theRock Chemistry Laboratory at UNLV. Hopefully, in thenear future a common set of chemical data forvolcanic rocks in the Lake Mead area will beavailable for comparative studies.
1) The volcanic rocks of the River Mountains arelimited in compositional range when compared to theEldorado and McCullough samples (Fig. 8). Rocks witha SiOZ content of greater than 69% are not presentin the River Mountains, but are common in the Mt.Davis, Patsy Mine and Tuff of Bridge Spring sectionin the Eldorado range and in the Erie Tuff andMt. Davis volcanics in the McCullough range. There
is a compositional gap between 49 and 53% SiOZ forthe River Mountains section. In comparison, it iscommon for rocks of the Mt. Davis and Patsy Minesequences in the other ranges to have SiOZ valuesthat fall within this gap. Most of the intermediateand felsic rocks of the River Mountains have SiOZvalues between 57 and 69%, but only a few samples ofthe Patsy Mine and Mt. Davis volcanic sequences havevalues of SiOZ that fall within this range. Animportant exception is the latite and trachyte domeflow complex in the McCullough range. These rocks,however, differ from the River Mountains samples bybeing higher in AlZ03 and lower in normative albite(Figs. 4a and 9).
The following chemical characteristicsdistinguish the volcanic rocks of the River Mountainsfrom those in the Eldorado and McCullough ranges:
3) Basalts in the River Mountains show strongeriron enrichment than either the basalts of theMcCullough or Eldorado Mountains (Fig. 6). Also thebasalts of the River Mountains are nephelinenormative, whereas basalts in the Eldorado andMcCullough Mountains are hypersthene normative. Interms of total volume, basalts and andesites arecomparatively rare when compared to the other ranges.
Comparison of the River Mountains with theEldorado and MCCullough Mountains
Overall chemical variation in the RiverMountains is similar to that of the Eldorado range inthat early flows (Patsy Mine volcanics) are alkalicand later activity (Mt. Davis volcanics) issubalkalic (see Anderson, 1978). A similar historyis weakly displayed in the McCullough rocks (Kohl,1978). It should be noted that the River Mountainslack a major ash-flow sheet that separates thealkalic and subalkalic suites of rocks such as thatfound in the other volcanic sections.
'" Volcanics of Bootleg WashG Subalkalic DacitesB Alkalic BasaltsA Alkalic Andesites
10 -1-----,---,---,---r-----j500 1000 1500 2000 2500 3000
Ba(ppm)
Comparisons between the volcanic rocks in theMCCullough Range and Eldorado Mountains
In the Eldorado Mountains, Anderson (1971)divided the volcanic section into three mappableunits (from youngest to oldest): the Mt. Davisvolcanics, the Tuff of Bridge Spring and the PatsyMine volcanics. Kohl (1978) in his study of thewestern McCullough range recognized the same threeunits, however he named the ash-flow unit the ErieTuff, a name used earlier for the same unit by Barton
Figure 4a. The volcanic rocks of Bootleg Wash can bedistinguished from the other volcanicrocks of the River Mountains by beinglower in AlZ03. Also the trachytes andlatites in the McCullough range (Kohl,1978) are higher in A1 Z0 3 than comparablerocks in the River Mountains.
4b. The volcanic rocks of Bootleg Wash can bedistinguished from the other volcanicrocks of the River Mountains by havinglower values of Sr and Ba.
47
and Behre (1954) and by Hewitt (1956). A comparisonof chemical data of Anderson (1978) and Kohn (1978),and the recent data of Johnson and Glynn (1981)throws doubt on the direct correlation of unitsbetween ranges. Chemical evidence includes:
1) Mt. Davis volcanics in both ranges aredistinctly bimodal in character, however moresilicic lavas are found in the McCullough range (upto 77% Si02) (Fig. 8).
2) The McCullough lavas have higher CaO contents(9-12%) than their Eldorado Mountain counterparts(usually less than 8%).
3) The Tuff of Bridge Spring in the EldoradoRange is distinctly higher in Si02 (Fig. 8), andlower in Zr, Ba and Sr when compared to the ErieTuff in the McCullough range (Table 2 and Fig. 10).These differences suggest that either the Erie andBridge Spring tuffs are different units or that theErie Tuff represents a phase of the Tuff of BridgeSpring not recognized in the Eldorado Mountains.
400 -,-----------------------,
100
'E't:lc:o
..c:U'c:.g~..c:Q)
u 10c:oU
Alkalic Dacites
LuTbSm EuNd1+--.-----,---,----,---,----------1
La Ce
Subalkalic Dacites
..
F
Figure 5. Rare earth element plot comparing thealkalic and subalkalic dacites of the RiverMountains.
• River Mountains
(-1 Eldorado Mountains'-'o McCullough Mountains
a Basin and Range Basaltsfrom Leeman andRogers(1970)
4) The Patsy Mine volcanics in the EldoradoMountains have a greater compositional range (51-76%Si0 2 ) than the Patsy Mine lavas in the McCulloughrange (centered at 52%). Silicic lavas common in thePatsy Mine sequence in the Eldorado Mountains areapparently lacking in the McCullough range (Fig. 8).
5) Andesites and basalts in the McCullough rangedisplay greater enrichment in iron than similar rocktypes in the Eldorado Mountains (Fig. 6).
Discussion
In conclusion, important chemical differencesbetween the volcanic rocks in the River, McCulloughand Eldorado Mountains make direct correlations ofrock units between ranges doubtful. It follows thatthe volcanic sequences in each range erupted fromseparate source areas.
Differences in chemical characteristics ofvolcanic rocks between ranges suggests that each arearecords the development of separate volcanic pileseach containing a cogenetic sequence of lavas.Development of these volcanoes was probably coevaland spans an age from 12-15 m. y. ago (Anderson andothers, 1972).
The Tuff of Bridge Spring in the EldoradoMountains probably did not originate in the LakeMead area. Flow direction studies using thetechniques of Elston and Smith (1970), and Rhodesand Smith (1972) suggest a source to the southwestfor this unit, possibly in the Mojave Desert ofCalifornia (Brandon, 1979). The Erie Tuff in theMcCullough Mountains may be a local unit originatingin a latite and trachyte dome complex in the westernpart of the McCullough range as originally suggestedby Hewitt (1956), or it may have as its source thevolcanic center at Devil Peak (Fig. 1) described byWalker and others (1981).
;;A. ·CfZj.,' ,,---
lit • ", ",.. ". '----------,'.
The presence of an ash-flow unit between twopredominately mafic sequences does not necessarilyinfer that the unit above the tuff is the Mt. Davissequence and the unit below is the Patsy Minesequence. Supporting geochemical andgeochronological data must be presented tosubstantiate these correlations.
Figure 6. A(Na2
0+K20), M(MgO), F(FeO+O'9Fe
20
3) plot
comparing the volcanic rocks of the River,McCullough and Eldorado Mountains.
48
Chemical differences in felsic rocks betweenranges may be due to the partial melting of aheterogeneous crust of Precambrian igneous andmetasedimentary rocks. The exposed Precambrian rocksof southern Nevada vary considerably in compositionfrom rapakivi granite (Volborth, 1962) to chloriteschist. Partial melting of such a crust may resultin the observed compositional differences in felsicrocks between ranges. Basalts and andesites ingeneral show smaller compositional differences thando the more silicic lavas, and may originate by thepartial melting of a relatively homogeneous mantle.
degrees south to 40 degrees north. Preliminaryanalysis of foliation measurements suggests that thelower plate is folded with east-,.,rest axespredominating. Just to the south of the fault plane(in the lower plate) a poorly developed foliation inchlorite schist trends northwest (N 20 W) and dips tothe west (25 to 55 degrees). Adjacent to thedetachment fault chlorite schist and amphibolitedisplay a well developed foliation (myloniticfabric?) that is coplanar with the fault plane(N 80 E strike, 30 degrees north dip). Within this
10...,-----------------,
STRUCTURAL GEOLOGY8
McCullough Mountains
River Mountains
52 56 60 64 68 72 76 80
- Eldorado Mountains---- -- D
Q r.;-::;:0
~~I-- - r- c--
0 0 0 ~Ip 0 pO 0 10 0~~ 'f~ P
rPl01 Pip P P P P Iplp 0 p p PI I I
4
6
2
2
8
pO+..L.:....j-:-I-+..:...L:.~yp_+..:..L:.._r_'_r-_r----l
44 48
6
4
8
2
o44 48 52 56 60 64 68 72 76 80
6
4
10
10-.----------------,
...(!).cE:JZ
o+=-J.::..~=+-..I.-"""-+;....I.-+--bf
44 48 52 56 60 64 68 72 76 80
700~----------------------'
Q)
·E"C<:o.l:U"<:o
'..~...<:Q)
g 10oU
Low-angle faults
100
The River Mountains have been affected by atleast three major structural events. The earliestand most speculative of them is the westward motionof the entire range on a west-dipping detachmentfault. The most impressive evidence for thedetachment surface is a major low-angle fault withinthe Precambrian complex exposed on Saddle Island onthe west shore of Lake Mead. The fault separateslithologically different terrains (see section onrock types above) and may either represent thedetachment plane itself or it may reflect adetachment fault higher or lower in the stratigraphicsection. Foliation developed in chlorite schist inthe allochthonous terrain constantly strikes N50 E±lOdegrees, and dips steeply (60 to 70 degrees to theeast). On the other hand, foliation in the lowerplate is mainly east-west with dips varying from 50
Basalts
AndesitesWeight Percent Si02
Histograms showing the distribution of SiOJ
values for volcanic rocks of the McCullougnEldorado and River Mountains. For theMcCullough and Eldorado Mountains D=Mt.Davis volcanics, P=Patsy Mine volcanics,stippled pattern=Tuff of Bridge SpringErie Tuff. For the McCullough range L=latite, T=trachyte. For the River MountainsP=volcanics of Powerline Road, R=volcanicsof River Mountain, TB=volcanics of TeddyBear Wash, B=volcanics of Bootleg Wash.
Figure 8.
LuTbSm EuNd1+---,-----.-----,--,---,.---------...,
La Ce
Figure 7. Rare earth element plot comparing thealkalic basalts and andesites of the RiverMountains.
49
Table 2. Chemical Comparisons of theTuff of Bridge Spring and the Erie Tuff
Erie Tuff
Ba 128 139 153 106Sr 74 97 106 74Zr 231 239 210 276Ti 1790 2080 2080 2010
Tuff of Bridge Spring
Ba 677 369 263 460Sr 339 247 201 349Zr 462 389 224 307Ti 3440 2800 2180 3520
Data from Johnson and Glynn (1981)
Orthoclasee River Mountains
o Trachytes and latites ofthe McCullough Range
oo
oc
o 0 0
oCOD
00
Ba(75%)
Figure 9. Normative orthoclase-albite-anorthite plotcomparing the trachytes and latites of theHcCullough range to the volcanic rocks ofthe River Hountains.
Zr
Anorthite
oo
Albite
/_,..-Tuff of/~. ":, Bridge SpringI" ,
'-'~,,;)
cigure 10. Ba, Sr, Zr plot (using the dat a ofJohnson and Glynn, 1981) comparing theTuff of Bridge Spring in the EldoradoHountains to the Erie Tuff in theHcCullough Range.
Sr(75%)
The Tertiary section to the west of SaddleIsland was affected by the detachment faulting intwo significant ways. First, volcanic and intrusiverocks just to the west of Saddle Island are highlyshattered. These units are intruded by lamprophyredikes that are also intensely brecciated. The entiremass may be autobrecciated and may reflect an underlying detachment fault. Second, within the volcanicsection in the River Hountains there are numerouslow-angle faults. All of these faults strike to thenortheast and dip to the west. They roughly parallelthe detachment surface on Saddle Island, and mayprovide evidence for readjustment of the upper plateduring detachment sliding (Fig. 12).
mylontic(?) zone, pegmatitic dikes are rotated intothe plane of foliation and in places well-developedboudinage is formed (Fig. 11). The low-angle faultis only exposed at one locality, on the east side ofSaddle Island. Here, well-foliated chlorite schistgrades upward into a two meter thick zone ofbrecciated and altered schist. Above this zone is aband of finely brecciated rock about 20 em thick andabove this band is the fault surface (commonlymineralized by chalcedony). At this locality thefault strikes N 80 E and dips 30 to the north.
In summary, the Tertiary volcanic section in theRiver Hountains and part of the Precambrian basementmay have slid to the northwest (probably by gravity)off a Precambrian metamorphic complex. The age ofdetachment faulting is probably late-Tertiary sincevolcanic rocks (Hiocene in age) were faulted andbrecciated during the sliding event. The fault onSaddle Island probably represents substantialdisplacement, since it juxtaposes lithologically andstructurally distinct terrains, however, it mayormay not be the detachment surface itself. Hotionalong the Saddle Island fault was distributed withinthe rocks of the two plates adjacent to the fault.Rocks of the upper plate behaved in a brittle mannerand formed local zones of breccia. In the lowerplate, chlorite schist and amphibolite developed apervasive foliation. Farther from the fault in theupper plate, volcanic rocks riding above the detachment surface were autobrecciated, and slip adjustmentalong low-angle faults occurred.
High-angle faults
In the River Hountains the low-angle faults arenot continuous, but seem to be cut by a set of highangle dip-slip faults (Fig. 2). This relationshipsuggests that detachment and extension representseparate, but possibly closely related events. In
other detachment terrains, detachment and high-anglefaulting are more closely related. For example, inthe Eldorado Hountains, high-angle faults apparentlytranslate into low-angle structures (Anderson, 1971),and in the Whipple Hountains, high-angle faults inthe allochthon terminate against the detachmentfault. The high-angle faults in the Whipple
50
Mountains may reflect the extension of the upperplate during gravity sliding (Davis and others,1979).
In the River Mountains, most of the high-anglefaults strike to the northwest; faults in the westernpart of the range dip steeply to the west (60 to 80degrees), and faults in the eastern part of the rangedip steeply to the east. This fault geometrysuggests that the range was extended in a northeastsouthwest direction, since normal faults usually formperpendicular to the direction of maximum extensionalstress. This stress direction may be related toforces imposed by the strike-slip faults of the LakeMead Fault System (discussed below).
An earlier extensional event is revealed by aset of east-striking dikes in the central andsouthern parts of the range (see Fig. 2). The originof the north-south stress orientation required toproduce these structures is at present unknown.These dikes are displaced by the northwest trendingnormal faults.
Strike-slip faults
High-angle faults merge with arms of the LakeMead fault system (Bohannon, 1979). Thisrelationship is clearly seen along the southernmargin of the range where dip-slip faults drag to theeast as they approach a left-lateral strike-slipfault (Fig. 2). The relationship between the twofault sets is unclear along the northern margin ofthe range because of widespread cover by Tertiary andQuaternary-aged sediments. However, it is extremelyprobable that the range is separated from theFrenchman Mountain-Rainbow Gardens area to the northby a major fault (probably another arm of the LakeMead Fault System) that trends parallel to Las VegasWash (Bell and Smith, 1980).
The River Mountains may have moved as far as40 km to the southwest as a result of motion alongthe Lake Mead Fault System. Fault displacement wascalculated by locating similar rock types on bothsides of the fault and measuring the amount ofapparent offset. Specifically, intrusive rockssimilar to those in the River Mountains stock cropout in the Black Range of Nevada and Arizona (theWilson Ridge Pluton of Anderson, 1978) (Fig. 1).If the River Mountain stock is indeed the northwardextension of the Wilson Ridge Pluton then at least40 km of strike-slip displacement is required toproduce the present outcrop configuration.
Fault-bounded blocks with different internalstructural orientations are an importantcharacteristic of the River Mountains. Apparentlythe structural geometry of the upper plate in theRiver Mountains differs from other well kno\vndetachment terrains. For example, the Eldorado Rangeis characterized by volcanic ridges that consistantlystrike north-south (Anderson, 1971). In the WhippleMountains volcanic units are faulted but constantlydip to the southwest (Davis and others, 1979), and inthe Eagle Pass detachment in southeastern Arizona(Davis and Hardy, 1981), Tertiary units have arelatively constant orientation. The discordantorientations of structural blocks in the RiverMountains may be the result of the late-stageinternal disruption (as revealed by the rotation ofstructural elements) of the mountain range by strikeslip faulting (a structural ingredient that is notimportant in the other detachment terrains). The
51
structural complexity of the River Mountains,therefore represents the complex interaction ofdetachment, high-angle and strike-slip faults.
As suggested by Smith (1981) strike-slipfaulting and igneous activity may be coeval alongthe southern margin of the River Mountains. Here,blocks of limestone, probably Cambrian in age, havebeen wedged into the volcanic section along splaysof the left-lateral strike-slip fault that boundsthe range to the south. These carbonate blocks weresubsequently cut by intrusions related to the RiverMountains stock. Rocks of the stock were dated byAnderson and others (1972) at 12.6 m.y. old. Theserelationships strongly suggest that faulting andintrusive activity occurred contemporaneously aboutthis time.
SOURCE AREAS FOR VOLCANIC ROCKS
Vent areas for the volcanic rocks are scatteredthroughout the range. The River Mountainsstratovolcano is the source for most of the alkalicandesite and dacite; the stratovolcano may also bethe vent area for some of the more recent subalkalicdacite flows. Numerous domes and dome complexesrepresent the source areas for the dacite flows ofthe Powerline Road sequence. The domes are normallyobsidian rich and are characterized by steeplydipping flow foliation. Several of the domes areassociated with thin pyroclastic units. Commonly theflow banding within a dome is aligned subparallelto the structural grain. This suggests thatextrusion and extension may be contemporaneous insome areas of the mountain range.
In the central part of the range there is akinney-shaped area (2 km by 1 km in size) where rocksare highly altered and locally mineralized (magnetiteand quartz), and flow banding in dacite is usuallysteeply dipping (Fig. 2). This area may representthe source for many of the dacite flows in thecentral and northern part of the River Mountains.The source for the alkali basalts in the PowerlineRoad section probably lies in a thick section ofpyroxene basalt, breccia, and agglomerate preservedin a north-trending graben in the north-central partof the range (Fig. 2).
Bell and Smith (1980) demonstrated that there isno structural or petrological evidence for a calderain the River Mountains as originally suggested byBrenner and Glanzman (1979). No major ash-flowsheets are evident in the mountain range and nonearby ash-flow units can be conclusively related toa source area in the River Mountains.
SU11MARY
Initial alkalic volcanism formed a largestratovolcano in the southeast part of the RiverMountains. The volcano was mainly composed ofhornblende andesite and numerous sills and dikes ofbiotite-hornblende dacite. Volcanism became moresubalkalic with the eruption of the lava flows of theBootleg Wash and Powerline Road sequences. Thesubalkalic flows originated from local domal sources,and possibly from a major source area in the northcentral part of the range. Also, the stratovolcanomay have been the source for some of the subalkaliclavas as evidenced by a plug of subalkalic quartzmonzonite and monzonite in its core (the RiverMountains stock). The subalkalic dacite flows areinterbedded with alkali pyroxene-olivine basalts and
Figure 11. Photograph of well-foliated chloriteschist just below the low-angle fault onSaddle Island. Pegmatite dikes and quartzveins (just above box) form well-developedboudin. For scale, the box isapproximately 8 em wide.
thick sections of mud-flow breccia.
Chemical data suggest that fractionalcrystallization was probably not important inproducing either the alkalic or subalkalic suites,rather they were generated by partial melting of aheterogeneous crust of Precambrian rocks.
Chemical data indicate that the River Mountainsare chemically distinct when compared to nearbyvolcanic ranges. Volcanism in these nearby areas,however, was coeval with the activity in the RiverMountains. During the period 12 to 17 m.y. ago therewere at least four additional centers of activity in
Figure 12. Photograph of a low-angle fault within theTertiary volcanic section in the RiverMountains. The fault strikes to the northwest and dips to the west and may reflectthe readjustment of the allochthon duringdetachment faulting. For scale, the lightpatch in the upper right of the photographis approximately 10 em high.
52
the Lake Mead area besides the River Mountains(the McCullough, Eldorado, Cleopatra, and Devil Peakvolcanoes).
Three structural episodes affected the RiverMountains. The first and most speculative event isthe detachment of the range from the Precambrianbasement along a major low-angle fault. Thevolcanics of the River Mountains according to suchan interpretation would represent the upper plate ofthis detachment complex. Extension occuring mainlyafter detachment faulting produced north,,,est-trending,high-angle, dip-slip faults. The event waspreceeded by an enigmatic north-south extensionalepisode revealed by numerous east-west oriented dikes.Finally the range was cut by arms of the Lake Meadfault system. During this deformational episode theRiver Mountains may have been transported 40 km fromthe northern part of the Black Range to their presentlocation. The shear stresses imposed on the rangeduring this structural event rotated blocks withinthe range thus forming the discordant structuralpattern that is characteristic of the River Mountains.Along the strike-slip fault on the south margin ofthe range many of the normal faults show evidence ofleft-lateral drag, and blocks of Cambrian limestone"ere wedged into the flanks of the River Mountainsstratovolcano. The River Mountains thereforerepresent an alkalic ann subalkalic volcanic complexthat was disrupted by low-angle, high-angle andstrike-slip faults.
ACKNOWLEDGEMENTS
I especially thank John D. Jones and Ward Rigotof the Phoenix Memorial Laboratory, University ofMichigan for completing the REE analyses. Financialsupport by the Department of Energy (D.O.E.) to thePhoenix Memorial Laboratory through the UniversityResearch Reactor Assistant Program and ReactorFacility Cost Sharing Program made much of this workpossible. Funding for many of the major elementanalyses was provided by a grant from the UniversityResearch Council of the University of Nevada, LasVegas. This support is gratefully acknowledged. Iwould also like to thank several fellow geologistsfor helping me (either knowingly or unknowingly)formulate my ideas on the development of the RiverMountains. Among them are R.E. Anderson, H.F. Bonham,J.W. Bell, Brian Wernicke, J.C. Kepper, andS.M. Rowland. I, however, take full responsibilityfor the conclusions presented in this paper.John Young, Charles Meyers, Joe Parolini and MarkReese helped me occasionally in the field. I alsothank Walter Raywood for keeping our analyticalequipment in working order, and for helping mecomplete the chemical analyses done in our RockChemistry Laboratory.
REFERENCES CITED
Anderson, R.E., 1971, Thin skin distension inTertiary rocks of southeastern Nevada:Geological Society of America Bulletin, v. 82,p. 42-58.
------~7' 1973, Large magnitude late Tertiary strikeslip faulting north of Lake Mead, Nevada:U.S. Geological Survey Professional Paper 794,18 p.
______~-' 1977, Geologic map of the Boulder CityIS-quadrangle, Clark County, Nevada: U.S.Geological Survey Quadrangle Map GQ-139s.
, 1978, Chemistry of Tertiary volcanic rocks------~i-n-the Eldorado Mountains, Clark County, Nevada,
and comparisons with rocks from nearby areas:U.S. Geological Survey Journal of Research,p. 409-424.
Anderson, R.E., Longwell, C.R., Armstrong, R.L., andMarvin, R.F., 1972, Significance of K-Ar agesof Tertiary rocks from the Lake Mead region,Nevada-Arizona: Geological Society of AmericaBulletin, v. 83, p. 273-288.
Barton, P.B. Jr., and Behre, C.H., 1954,Interpretation and evaluation of the uraniumoccurrences near Goodsprings, Nevada: U.S.Atomic Energy Commission., RME-3l19.
Bell, J.W., and Smith, E.I., 1980, Geological mapof the Henderson Quadrangle, Clark County,Nevada: Nevada Bureau of Mines and Geology Map67.
Bohannon, R.G., 1979, Strike-slip faults of theLake Mead region of southern Nevada: inArmentrout, J.J., Cole, M.R., and Terbest,Harry: Cenozoic Paleogeography of the WesternUnited States, Pacific Coast PaleogeographySymposium 3, The Pacific Section, Society ofEconomic Paleontologists and Mineralogists,p. 129-139.
Bingler, E.C., and Bonham, H.F., Jr., 1973,Reconnaissance geologic map of the McCulloughRange and adjacent areas, Clark County, Nevada:Nevada Bureau of Mines and Geology Map 45.
Brandon, C.F., 1979, Flow direction studies of theTuff of Bridge Spring, Nevada: Senior Project,University of Wisconsin-Parks ide (unpublished).
Brenner, E.F., and Glanzman, R.K., 1979, Tertiarysediments in the Lake Mead area, Nevada: inNe~vman, G.W., and Goode, H.D., eds., 1979Basin and Range Symposium: Rocky MountainAssociation of Geologists and Utah GeologicalAssociation, p. 313-323.
Campbell, I.H., and Gorton, M.P., 1980, Accessoryphases and the generation of light rare earthelement enriched basalts - a test fordisequilibrium melting: Contributions toMineralogy and Petrology, v. 72, p. 157-163.
Davis, G.A., Anderson, J.L., Frost, E.G., andShakelford, T.J., 1979, Regional Miocenedetachment faulting and early Tertiarymylonitization, Whipple-Buckskin-RawhideMountains, Southeastern California and WesternArizona: in Abbot, P.L., ed., Geologicalexcursions in the southern California area,San Diego, San Diego State University, p. 75108.
Davis, G.H., and Hardy, J.J., 1981, The Eagle Passdetachment, southeastern Arizona: product ofmid-Miocene listric (?) normal faulting in thesouthern Basin and Range: Geological Societyof America Bulletin, v. 92, p. 749-762.
Elston, W.E., and Smith, E.I., 1970, Determinationof flow direction of rhyolite ash-flow tuffsfrom fluidal textures: Geological Society ofAmerica Bulletin, v. 81, p. 3393-3406.
53
Hewitt, D.F., 1956, Geology and mineral resourcesof the Ivanpah quadrangle, California andNevada: U.S. Geological Survey ProfessionalPaper 275, 172 p.
Irvine, T.N., and Baragar, W.R.A., 1971, A guide tothe chemical classification of volcanic rocks:Canadian Journal of Earth Science, v. 8, p. 523548.
Johnson, Cady, and Glynn, Jack, 1981, Uraniumresource evaluation, Las Vegas quadrangle,Nevada, Arizona and California: Department ofEnergy, Final Report for Contract DE-AC-1376GJ01664, 64p.
Kohl, M.S., 1978, Tertiary volcanic rocks of theJean-Sloan area, Clark County, Nevada and theirpossible relationship to Carnotite occurrencesin caliches: M.S. Thesis, University ofCalifornia, Los Angeles, unpublished, 116p.
Leeman, W.P., and Rogers, J.J.W., 1970, Late Cenozoicalkali-olivine basalts of the Basin-Rangeprovince, USA: Contributions to Mineralogy andPetrology, v. 25, p. 1-24.
Longwell, C.R., 1936, Geology of the Boulderreservoir floor, Arizona and Nevada: GeologicalSociety of America Bulletin, v. 47, p. 13931476.
, 1960, Possible explanation of diverse-------s~t-ructural patterns in southern Nevada: American
Journal of Science, v. 258-A (Bradley Volume),p. 192-203.
, 1963, Reconnaissance geology between Lake----~M~e-a-d and Davis Dam, Arizona and Nevada: U.S.
Geological Survey Professional Paper 374-E, SIp.
Longwell, C.R., Pampeyan, E.H., Bower, Ben, andRoberts, R.J., 1965, Geology and mineraldeposits of Clark County, Nevada: Nevada Bureauof Mines and Geology Bulletin 62, 2l8p.
McKelvey, V.E., Wiese, J.H., and Johnson, V.H.,1949, Preliminary report on the bedded manganeseof the Lake Mead region, Nevada and Arizona:U.S. Geological Survey Bulletin 948-D, 101p.
Rhodes, R.C., and Smith, E.I., 1972, Directionalfabric of ash-flow sheets in the northwesternpart of the Mogollon Plateau, New Mexico:Geological Society of America Bulletin, v. 83,p. 1863-1868.
Shackelford, T.J., 1976, Structural geology of theRawhide Mountains, Mojave County, Arizona:Unpublished Ph.D. dissertation, University ofSouthern California, Los Angeles, l75p.
Smith, E.I., 1979, Late Miocene volcanism in theRiver Mountains, Clark County, Nevada (abs.):Transactions of the American Geophysical Union,v. 61, no. 6, p. 69.
, 1981, Contemporaneous volcanism, strike-slip-----faulting and exotic block emplacement in the
River Mountains, Clark County, southern Nevada:Geological Society of America Abstracts withPrograms, v. 13, no. 2, p. 107.
Smith, E.I., and Stupak, W.A., 1978, A Fortran IVprogram for the classification of volcanicrocks using the Irvine and Baragarclassification scheme: Computers and Geoscience,v. 4, p. 89-99.
Steward, J.H., 1980, The Geology of Nevada: NevadaBureau of Mines and Geology Special Paper 4,136 p.
Wall~r,J.D., Beaufait, M.S., and ZeIt, F.B., 1981,Geology of the Devil Peak area, SpringMountains, Nevada: Geological Society ofAmerica, Abstracts with Programs, v. 13, no. 2,p. 112.
Volborth, A., 1962, Rapakivi-type granites in thePrecambrian complex of Gold Butte, Clark County,Nevada: Geological Society of America Bulletin,v. 73, p. 813-831.
\vestcamp, D.P., and Pavlovich, R.L., 1968, Finalconstruction geology report, River MountainsTunnel and portal structure, volume 1: Bureauof Reclamation, Region 3, Southern Nevada WaterProject, unpublished report, 88p.
54
THE MULE M0 UNTAIN S THRUS T IN THE t~ ULE M0 UNTAINS CALIF 0 R NIAAND ITS PROBABLE EXTENSION IN THE SOUTHERN DOME ROCK MOUNTAINS, ARIZONA;
A PRELU~INARY REPORT
Richard M. TosdalU.S. Geological Survey345 Middlefield Road
Menlo Park, California 94025and
Departm ent of Geological SciencesUniversity of California
Santa Barbara California 93106
ABSTRACT
The ~1ule Mountains thrust in the Mule Mountains,Calif., and its probable extension in the southern Dam eRock Mountains, Ariz., is a southward dipping thrust faultthat superposes Preca mbria n(?) or Mesozoic(?) graniticrocks and a Precam brian gneiss com plex over Early and (or)Middle Jurassic(?) metavolcanic and volcaniclastic rocks.Mylonitic rocks and fabrics are present along the thrustfault, and synkinematic crystalloblastic fabrics in themetamorphic rocks of the lower plate. In the southernDome Rock Mountains, a gradational stratigraphic contactbetween Early and (or) Middle Jurassic(?) metavolcanic andvolcaniclastic rocks and structurally lower but youngerupper Mesozoic metasedim entary rocks indicate these rocks,part of the Mc Coy Mountains Form ation and equivalentLivingston Hills Form ation, may be as old as MiddleJurassic(?) and not of Late Cretaceous(?) or younger age.The age of movement along the thrust fault is constrainerlto be post-M iddle Jurassic(?) and pre- middle Tertiary, andprobably of late Cretaceous to early Tertiary age.
INTRODUCTION
Thrust faulting, regional metamorphism, nappeform ation, and intrusion of synkinem atic granite areimportant elements of the late Mesozoic to early Tertiarytectonics of the eastern Mojave f)esert, Calif., and northernSonoran Desert, Ariz. (Haml1ton, 1971; Pelka, 1973; Haxeland Dillon, 1978; Howard and others; 1980; Reynolds andothers, 1980; Anderson and Ro wley, 1981; Cro well, 1981;Miller and others, 1981; r~ay and others, 1981). Throughoutthis region, crustal shortening of uncertain magnitude isrecorded by the thrust fa ults; Reynolds and others (1980,p. 50) suggest sam e tens of kilom eters of tectonic transporton thrust faults in the area of Salome, Ariz. Varyingstructural thicknesses of mylonitic rocks are present alongthe thrusts. Prograde metamorphism of lower plate rocksaccom panied translation along the thrust faults; whereasretrograde metamorphism occurred synkinematically at thebase of the upper plate. These thrust faults comprise twosystems (Fig. 1): the outboard Vincent thrust, and arecently recognized but incom pletely studied inboard syste In(e.g. Crowell, 1981; Tosdal and Haxel1982).
The inboard system of thrust faults occurs within anorth west-trending belt that extends at least fro mnorthernmost Sonora and south-central Arizona (May andothers, 1981) to the Salome-quartzsite-Blythe area ofArizona and California (Reynolds and others, 1980). TheMule Mountains thrust that superposes Precam brian(?) orr~ esozoic(?) pl uta nic roc ks ato p varyingly meta morphosedJurassic(?) volcanic and volcaniclastic rocks in the MuleMountains, southeastern California (Pelka, 1973; Crowell,1981) and in the southern Dome Rock Mountains, southwestern Arizona (this paper), is part of the inboard syste m
55
of thrust faults (Fig. 1). The brief description ofthese rocksand structures presented are preli min ary observations, and1'1111 be greatly expanded by further sturlies of the kinematics and regional relationships of the Mule r~ountain
thrust.
MULE MOUNTAINS
Along the ,"1 ule Mountains thrust (Figs. 1, 2A), a zoneof mylonitic rocks separates Precambrian(?) or M~sozoic(?)
porphyritic granodiorite and Preca mbrian gneiss of theupper plate, from the weakly to moderately metamorphoserlEarly and (or) Middle Jurassic(?) rhyolitic to dacitic volcanicand volcaniclastic rocks of the lower plate. The thrust faultstrikes north westerly and dips about 300 to 50 0 S[1.
Penetrative foliation that rJeveloped in both thelower-plate metavolcanic rocks and upper-plate blastomylonitic granodioritic gneiss parallels that the in themylonitic rocks along the fault. The width of the zone ofblasto mylonitic granodioritic gneiss at the base of theupper-plate varies dram atic ally in thickness along the thrustfault, with foliation rapirlly diminishing in intensitystructurally upward. All macroscopic trace of the foliationhas disappeared 300 m above the thrust, although localizedzones of blastomylonitic gneiss occur sporadically throughout the porphyritic granodiorite. Quartz rods and tral1s ofbroken micas and feldspars present throughout the metamorphic rocks define a penetrative lineation within theplane of the foliation that plunges to the south-southwest.A late stage north west-trending crenul atio n lineationslightly folds the prim ary lineation.
The porphyritic granodiorite, a tabular body extendingacross the entire width of the mountain range (Fig. 2A), isbounded on the northeast by the Mule Mountains thrust, andon the south by the gneiss com plex. With the exception ofthe narrow zone adjacent to the Mule Mountain thrust,scattered zones of blastom ylonitic rock that parallel thethrust, and a structurally disturbed portion of the othervliseintrusive contact with the gneiss com plex, the porphyriticgranodiorite has igneous textures and mineralogy. Creamcolored, com manly I'lith a pinkish tint, potassium feldsparphenocrysts are the distinctive feature of the porphyriticgranodiorite. These phenocrysts range fro m 1 c m to 6 em insize, and are variable both in their habit and distribution,occurring as subparallel tabular crystals, blocky crystalsthat com pose much of the granodiorite or as scattered, randomly oriented crystals, usually of sm aller size. The age ofthe porphyritic granorliorite is unknown, but petrologicresem blances to other dated plutonic rocks of southeasternCalifornia suggest one of several Precam brian ages (Silver,1968, 1971; L. To Silver, in Powell, 1981). However,else where in the south I'lest-;-porphyritic granodiorite andmonzogranite plutons are also of Mesozoic age (Silver, 1971;L. T. Silver in Powell, 1981); hence, a Mesozoic age for the
Area ofLC Figure 2A
40I
KILOMETERS
()
(
\\\
~\,;",~~I
")/.
(7
~~Blythe (~
1-------\lO)----------.::=.------~----,LQ:
Figure 1. General geologic map of the lower part of Colorado River, California and Arizona,showing distribution of inhoard thrust system and associated rocks, and outhoard Vincentthrust system. Modified from Strand, (1962); ,Jenning (1967); Wilson and others, (1969);Miller (1970); Pelka, (1973); Dillon (1975); Haxel, (1977); Crowl, (1979); W. Hamilton(oral commun., 1981); CD, Castle Dome Mountains; CH, Chocolate Mountains; CM, CargoMuchacho Mountains; CW, Chuckwalla Mountains; DR, Dome Rock Mountains; LC, LittleChuckwalla Mountains; LM, Laguna Mountains; KM, Kofa Mountains; MC, McCoy Mountains; MD,Middle Mountains MG, Muggins Mountains; ML, Mule Mountains; MMT, Mule Mountains thrust;MR, Maria Mounta ns (includes both Big and Little Maria Mountains); PL, Palomas Mountains;PM, Palen Mounta ns; PV, Palo Verde Mountains; TM, Trigo Mountains; TP, Trigo Peaks.
56
EXPLANATION
D Volcanic and sedimentary rocks (Tertiary)
Granitic and sedimentary rocks(late Mesozoic and (or) early Tertiary)
INBOARD THRUST SYSTEM
UPPER PLATE
~~~If{i1 Sedimentary rocks (Paleozoic)
E"1~~ Gneiss and granite (Precambrian;may include rocks of Mesozoic age)
.,...,......" ...""""" THRUST FAULTS
LOWER PLATE
I; :;;,""1 Sedimentary and metasedimentary:"'.' rocks (late Mesozoic)
Granitic rocks (Mesozoic)
VINCENT THRUST SYSTEM
UPPER PLATE
• Gneiss and gran,ite (Precambrian,,, ", "", and MeSOZOIc)
"V v v v Q CHOCOLATE MOUNTAINS THRUST
LOWER PLATE
Orocopia Schist (Late Cretaceousand (or) early Tertiary)
CONTACT
FAULT
57
porphyritic granodiorite cannot be discounted.Polymetaillorphic melanocratic to leucocratic gneiss
derived from granitic to granodioritic, gabbroic, andultra mafic pl utonic roc ks, ann suborrlinate a fll ounts ofsedimentary(?) and (or) volcanic(?) protoliths form themajority of the rocks corn prising the upper plate. Crystalloblastic fabrics fornl ed under am phibolite facies conditionsare characteristic of the gneiss com plex; although, extensive areas of mylonitic fabrics, unrelaten to the ~~ule
Mountains thrust, are present. A Precambrian(?) age isinferred for the gneiss co III plex.
Certain rock types, ann their relative age relationswithin the gneiss cOin plex of the Mule Mountains are si milarto those characteristic of the Preca mbrian San Gabrielterrane (Silver, 1971; L. T. Silver in Po well, 1981). The SanGabriel terrane is widely distributed in the eastern Transverse Ranges, southeast Califol'l1ia, as far east as the LittleChuckwalla Mountains (Powell, 1981), located some 10 kmwest of the ~~ule Mountains (Fig. 1). Correlation of thegneiss complex of the Mule Mountains to the gneiss composing the San Gabriel terrane has previously been suggestedby Cro~lell (1981), but confirmation of this correlation musta \'iait further work. It is interesting to note that in theOrocopia ~1ountains, 70 km west of the Mule r~ountains, theSan Gabriel terrane forms the upper plate of the Vincentthrust (Cro~lell, 1981; PO~fell, 1981).
A cofliugate syste m of gabbroic to granodioritic dikesare the youngest rocks of the upper plate. These rocks areconverted to greenschists and blastolnylonite near andwithin the deform ation zone at the base of the upper plate.The age of these dikes are unkno ~1Il, but are provisionallyassigned a Mesozoic age.
The rhyolitic to dacitic metavolcanic andvolcaniclastic rocks of the 10~ler plate (Fig. 2A) consist ofvarying amounts of relict em bayed quartz, broken plagioclase phenocrysts, and recrystallized pumice lapillisurrounded by a ground mass com posed of quartz, albite,white mica, epidote, chlorite, and tourm aline. Severalwhite quartzite and quartzite-cl ast metaconglo merate bedsare interstratified with the volcaniclastic strata. An Earlyand (or) Middle Jurassic(?) age is assigned to themetavolcanic rock sequence on the basis of their simllarityin appearance and petrology to volcanic rocks of knownEarly to Middle Jurassic age elsewhere in southeasternCalifornia (Daniel Krum menacher, in Pelka, 1973), in southcentral Arizona (Wright and others, 1981), and northernSonora, Mexico (Anderson and Silver, 1978). Crowell (1981)tentatively has correlated these metavolcanic rocks toweakly meta morphosed dacitic and andesitic volcanic rocksin the southernmost McCoy Mountains, located some 12 kmnorth of the Mule Mountains; there, the meta morphoseddacitic and andesitic volcanic rocks structurally overlie theMcCoy Mountains Formation of Mlller (1944; Pelka, 1973).Alter-natively, in the northern McCoy Mountains wea klycleaved, rhyodacitic hypabyssal rocks of Early Jurassic age(Daniel Kru mmenacher, in Pelka, 1973), deposition allyoverlain by the McCoy Mountains Formation of Mlller(1944)of late Cretaceous(?) or younger age, may prove to be abetter stratigraphic correlation for the metavolcanic rocksin the Mule Mountains.
SOUTHERN DOME ROCK MOUNTAINS
The Dome Rock Mountains are com posed of upperMesozoic metasedimentary rocks, considered to be equivalent to the Mc Coy Mountains-Livingston Hills Form ationsas defined by Harding (1980), and Early to Middle Jurassicmetavolcanic and granitic rocks (Wllson and others, 1969,L. T. Silver, in Cro~ll, 1979). Scattered investigations ofthese rocks rcro~ll, 1979; Harding, 1980; Marshak, 1980;
114'49
A
TIMING OF MOVEMENT
The majority of the rocks corn prising the 00 In e Rock110untains are wea kly to I~ oderately rn eta morphosed sandstone, siltstone, and conglorn erate (Crov/l, 1979; Marshak,1980) of the 7,000-m-thick McCoy Mountains-LivingstonHills Form ations (Harding, 1980; Harding and others, 1982).In the McCoy i~ountain, Calif., the central and southernDome Rock Mountains, Plornosa r~ountains, and theLivingston Hllls, Ariz. (Fig. 1), the sedim entary or metasedimentary rocks of the McCoy Mountains-Livingston HillsForm ations rest de position ally on or are interstratified withsiliceous volcanic and metavolcanic rocks of known orinferred Early and (or) Middle Jurassic age (figs. 1, 2R,i1iller, 1970; Pelka, 19 73; Crov/l, 1979; Harding, 1980). Inthe southern Dome Rock Mountains (Fig. 28), the sequenceis overturned, and the Early and (or) Middle Jurassic(?)metavolcanic rocks grade structurally dOI·lnward into themetasedimentary rocks. This gradational contact betweenthese metavolcanic rocks and upper Mesozoic metilsedimentary rocks strongly implies the depositional age of theMcCoy Mountains-Livingston Hllls Formations may be as oldas Middle and (or) Late Jurassic (Harding, 1980) and not lateCretaceous(?) or· younger (Miller, 1970; Pelka, 1973).Recognition of a probable Jurassic depositional age for all,or at least the basal part, of the McCoy r~ ountainsLivingston Hill Form ations has several im portantim plications for the stratigraphic relation of thesesedim entary rocks to the Jurassic volcanic are, and to theJurassic tectonics of the region.
Harding and others, 1982) have revealed highly corn plex and,as yet, incom pletely understood geologic relations. Athrustfault in the southern Do me Rock Mountains (Figs., 1, 28) isgenerally on strike Itlith the Mule r'10untains thrust (Fig. 1),superposes si mII ar rocks (Fig. 28), and is inferred to be anextension of the Mule r~ ountains thrust. The thrust strikeseast-northeast ilnd dips moderately south-southeast. Themylonitic foliation along the thrust parallels the crystalloblastic foliation in the lovler-plate rocks, and contains apenetrative quartz-rod ding and mic a-strea king lineationthat plunges soutlHlest. The bearing of this lineation issimllar, but not identical, to the quartz-rodding and brokenrnica and feldspar lineation noted in the r1ule Mountains(Fig. 2A).
3 KILOMETERSI
+- + +
oI
+ + + ++ + +
+ + + + +
+~(+r++ + + +
+ +++++ ++ +
+ ++ +
+ +
:'::;:'-7\\7//==:~~fl \\=I/~ ~;/~-~ --//) \\ ~ fI-1'
33' 32
33'29
EXPLANATION
UPPER PLATE
The youngest rocks involved in thrusting andmeta morphis m are the weakly to rn oderately meta mor-'phosed Me Coy i10untains-Livingston Hllls Form ations oflate Cretaceous(?) or younger age (Miller, 1970; Pelka,1973), although, a Middle and (or) Late Jurassic(?) age nowseems more likely for these rocks. Middle Tertiary volcanicand sedim entary rocks, prior to limited and local movem entalong detachment faults (Frost and others, 1981; D. Sherrodand R. i1 Tosdal, unpub. datil, 1982) deposition ally overlaythe Precambrian and Mesozoic crystalline rocks. Tilerefore,existing data broadly constrains the movem ent of the Muler~ountains thrust to late Mesozoic (late Jurassic(?)) and (or)early Tertiary times.
Volcanic and sedimentary rocks(middle Tertiary)
MULE MOUNTAINS THRUST
Gneiss (Precambrian)
Porphyritic granodiorite(Precambrian or Mesozoic)
~~
LOWER PLATE
L»"~I Metasedimentary rocks (Upper Mesozoic)
Metavolcanic rocks (Jurassic?)
Contact
Throughout western Arizona, crystalline thrusts of theinboard thrust system have been demonstrated to be lateCretaceous and (or) early Tertiary in age (R eynolds andothers, 1980; Haxel and others, 1981). The simllaritybetween the r~ule Mountilins thrust and thrusts of theinboard thrust system suggests movem ent along the Mulei~ountains thrust may also be late Cretaceous and (or) earlyTertiary in age.
?",,,""" Gradational contact--Queried where uncertain
.~ Attitude of foliation and lineation
-- -? Fault--Dashed where approximately located;queried where uncertain
Figure 2a. General ized geologic maps of the i~ule
Mountains thrust; A, northern Mule r10untains,Cal if., B, southern Dome Rock Mountains,Ariz. See figure 1 for locations of theserna ps •
58
SOME REGIONAL QUEST!()NS
SOlne 40 km south of the last outcrops of thesouthward-dipping Mule Mountains thrust is a simllar, butdisti net, myl onitic thrust fa ult, the Choc 01 ate Mountainsthrust of the Vincent thrust system. The ChocolateI~ountains thrust places Precam brian granite and gneiss andMesozoic granitic rocks over the ensimatic Orocopia Schist,which consists of dOlninantly quartzofeldspathic schist andminor ferrom anganiferous III etachert, III etabasite, siliceousmarble, and meta-ultralnafic rock (Dillon, 1975; Haxel,1977). In its northernmost outcrops in the Chocolate andTrigo Mountains (Fig. 1), the Chocolate Mountain thrust dipsvaryingly north. Mov e ment alo ng the Choc 01 ate Mountainsthrust is also inferred to be late Cretaceous and (or) earlyTertiary in age (Haxel and Dillon, 1978).
Despite the effects of middle Tertiary large-scalefolding of the di Fferent Preca mbrian and Meso zoiccrystalline rocks and the two thrusts (Cameron and Frost,1981), some im portant questions are raised by their closegeographic proximity (Fig. 1), the northward dip of theChocolate Mountains thrust and the southward dip of theMule Mountains thrust and the inferred late Cretaceous and(or) early Tertiary age for thrust movements. vJhat is therelation between these two thrust faults? Does the riuleMountains thrust represent a sym pathetic, intracontinentalthrust related to moveln ent along the Chocolate tiountainsthrust (Crowell, 1981); or, does the Mule Mountains thrustrepresent a shallower exposure of the Chocolate Mountainsthrust (Dillon, 1975); or do these two thrust faults share notectonic relationship except formation by simllar geologicprocesses? If the Mule ~10untains thrust is continuous withthe Chocolate Mountains thrust, vlhat is the relationbetween the lower plate rocks, the Orocopia Schist and theEarly and (or) Middle Jurassic(?) volcanic rocks? If therocks of the two lower plates are related, could theprotolith of the Orocopia Schist be sedim entary rocksoriginally deposited in the accretionary wedge of an EarlyJurassic(?) subduction zone, and subsequently, during thelate Cretaceous and (or) early Tertiary, overridden by anallochthon that may have been exotic to North America(Vedder and others, 1982), or part of the continental margin(Crowell, 1981)? The answers to these questions I'lhichremain unresolved, may yet be found in the discontinuouserosional windol'/s of Precam brian and Mesozoicmetamorphic and granitic rocks exposed between the MuleDome Rock Mountains and the Chocolate-Trigo Mountains(Fig. 1).
ACKN0 l~ LEO GEM EN TS
In particular I would like to thank Nobie Amamoto,Karen Johnson, and Diane Stevens of the U.S. GeologicalSurvey for their generous help in preparation of themanuscript. Version of the manuscript were greatlyimproved through the freely given advice of J. C. Crowell,Gordon Haxel, D. J. May, and R. E. Powell.
REFERENCES
Anderson, J. L., and Rowley, M. C., 1981, Synkinematicintrusion of peraluminous and associated metaluminousgranitic mag mas, vlhipple Mountains, California:Canadian Mineralogist, v. 19, pt. 1, p. 83-101.
Anderson, T. H., and Silver, L. T., 1978, Jurassic magmatismin Sonora, Mexico [abs.} Geological Society ofAm eric a Abstracts with Programs, v. 10, no. 7, p. 359.
Cameron, T. E., and Frost, E. G., 1981, Regionaldevelopm ent of mcdor antiforms and synform scoincident I'lith detach ment fa ulting in California,Arizona, Nevada, and Sonora, [M exico] [abs.]:Geologic al Society of America Abstracts withPrograms, v. 13, no. 7, p. 421-422.
59
Crol'lell, ,J. C., 1981, An outline of the tectonic history ofsoutheastern California, in Ernst, vi. G., ed., Thegeotectonic developm ent of California (\1. l~. RubeyVolume No.1): Engle\'lOod Cliffs, N. J., Prentice Hall,p. 583-600.
Crowl, lL ,J., 1979, Geology of the central Dome Rockriountains: Tucson, University of Arizona, M.S. thesis,76 p.
Dillon, ,J. T., 1975, Geology of the Chocolate and Cargo~luchacho Hountains, southeasternmost California:Santa 8arbara, University of California, Ph. D. thesis,405 p.
Frost, 1::. G., Caneron, T. E., Krum menacher, naniel, andMartin, I). L., 1981, Possible re<Jional interaction ofmid-Tertiary detachment faulting I'lith the SanAndreas fault and the Vincent-Orocopia thrust system,Arizona and California [abs.} Geological Society ofAmerica Abstracts with Programs, v. 13, no. 7, p. 455.
Hamllton, ('Jarren, 1971, Tectonic fralnel'lOrk ofsoutheastern California [abs.} Geological Society ofAIn eric a Abstracts I'lith Progra ms, v. 3, no. 2, p. 130131.
Harding, L. 1::., 1980, Petrology and tectonic setting of theLivingston Hills Form ation, in Jenney, ,1. p., andStone, Claudia, eds., <;tudiesin western Arizona:Arizona Geological Society Digest, v. 12, p. 135-145.
Harding, L. 1::., Coney, P. J., and 8utler, R. F., 1982,Stratigraphy, sedilll entary petrology, structure, andpaleornagnetislll of the r1cCoy Mountains--LivingstonHills Forn ations, southeastern California andsouthl'festern Arizona [abs.]: Geological Society ofAmerica Abstracts with Progra ms, v. 14, [in press].
Haxel, G. 8., 1977, The Orocopia Schist and the ChocolateHountain thrust, Picacho-Peter Kane Hountain areil,southeasternmost California: Santa 8arbara,University of California, Ph. D. thesis, 277 fl.
Haxel, Gordon, ilnd Dillon, John, 1978, The Pelona-OrocofliaSchist and Vincent-Chocolate r10untain thrust system,sOlIthern California, in Howell, D. G., iind McDougall,K. A., 1978, eds., Mesozoic paleogeography of the1'1 estern United States: Los Angeles, Society ofEconomic Paleontologists and Hineralogists, PacificSection, Pacific Coast Paleogeography Symposiu:n 2,p. 453-469.
Haxel, Gordon, Tosdal, R. M., Hay, n. J., and \'lright,.J. 1::.,1981, Latest Cretaceous and early Tertiary thrustfaulting, regional metamorphism, and graniticplutonism, south-central Arizona [abs.], ~ Howard,K. A., Carr, H. n., and Hiller, D. r1., eds., Tectonicframework of the Mojave and Sonoran neserts, California and Arizona: U.S. Geological Survey Open-FileReport 81-503, p. 42-44.
Howard, K. A., Miller, C. F., and Stone, Paul, 1980,Mesozoic thrusting in the eastern Mojave Desert,California [abs.]: Geological Society of AmericaAbstracts with Progra ms, v. 12, p. 112.
Jennings, C. I~., Compiler, 1967, Salton Sea sheet ofGeologic map of California: Division of Mines andGeology, State of California, scale 1:250,000.
Harshak, Stephen, 1980, A preliminary study of Hesozoicgeology in the southern Dome Rock Mountains,southwestern Arizona, in Jenney, J. P., and Stone,Claudia, eds., Studies m western Arizona: ArizonaGeological Society Digest, v. 12, p. 123-133.
May, D. J., Tosdal, R. M., and Haxel, Gordon, 1981,Imbricate thrust faulting of Late Cretaceous or earlyTertiary age, Organ Pipe Cactus National Monument,southwestern Arizona [abs.} Geological Society ofAmerica Abstracts with Progra ms, v. 13, no. 2, p. 95.
Miller, D. M., Howard, K. A., and Anderson, J. L., 1981,Hylonitic gneiss related to emplacement of aCretaceous batholith, Iron Mountains, southernCalifornia, in Howard, K. A., Carr, M. D., and Miller,eds., Tectonic framework of the Mojave and Sonoran
Deserts, California and Arizona: U.S. GeologicalSurvey Open-Flle Report 81-503, p. 73-75.
1·1111er, F. K., 1970, Geologic map of the Quartzsitequadrangle, Yuma County, Arizona: U.S. GeologicalSurvey Geologic Quadrangle Map GQ-841, 3 p., scale1:62,500.
Ml1l er, W. J., 1944, Geology of the Pal en Springs-Blythestrip, Riverside County, California: California Journalof Mines and Geology, v. 40, p. 11-72.
Pelka, G. ,J., 1973, Geology of the Mc Coy and PalenMountains, southeastern California: Santa Barbara,University of California, Ph. D. thesis, 162 p.
Powell, R. E., 1981, Geology of the crystalline basementcom plex, eastern Transverse Ranges, southernCalifornia: constraints on regional tectonicsinterpreta tions: Pasadena, California Instit ute ofTechnology, Ph. D. thesis, 441 p.
Reynolds, S. ,J., Keith, S. B., and Coney, P. ,J., 1980, Stackedoverthrusts of Precambrian crystalline baselTJent andinverted Paleozoic sections em placed over Mesozoicstrata, west-central Arizona: Arizona GeologicalSociety Digest, v. 12, p. 45-50.
Silver, L. T., 1968, Precambrian batholiths of ArizonaCabs.} Geological Society of ,1\ merica, CordllleranSection, Programs, 64th Annual Meeting, Tucson,Ariz., 1968, Progra ms, p. 109-11 O.
1971, Problems of crystalline rocks of the Transverse- Ranges Cabs.} Geological Society of Am eric a
i\ bstracts with Progra ms, v. 3, no. 2, fl. 193-194.Strand, R. G., Com piler, 1962, San Diego-El Centro sheet of
Geologic In ap of California: California Division ofMines and Geology, scale 1:250,000.
Tosdal, R. M., and Haxel, Gordon, 1982, Two belts of lateCretaceous to early Tertiary crystalline thrust fa ultsin southwest Arizona and southeast California Cabs.]:Geologic al Society of America Abstracts withProgra rn s, v. 14 Cin press].
Vedder, J. G. Ho well, D. G., and Mclean, Hugh, 1982,Stratigraphy, sedim entation, and tectonic accretion ofexotic terranes, southern Coast ranges, California:(Paper presented at Hedberg Conference onContinental r1 argins, 1980).
Wilson, E. 0., r100re, R. T., and Cooper, J. R., 1969,Geologic map of Arizona: Arizona Bureau of Minesand U.S. Geological Survey, scale 1:500,000.
Wright, J. E., Haxel, Gordon, and May, D. J., 1981, EarlyJurassic uranium-lead isotopic ages for Mesozoicsupracrustal sequences, Papago Indian Reservation,southern Arizona Cabs.]: Geological Society ofAmerica Abstracts with Progra ms, v. 13, no. 2, p. 115.
60
REGIONAL GRAVITY AND ~~GNETIC EVIDENCE FOR A TECTONIC WEAKNESS ACROSS SOUTHWESTERN ARIZONA
Douglas p. KleinU.S. Geological Survey
Denver, Colorado
ABSTRACT
The Basin and Range ~rovince of southwest Arizonais traversed by a northwest-trending belt of magneticanomalies that separates a region of relatively highresidual gravity to the northeast from a region ofgravity minimums to the southwest along the Mexicanborder. The magnetic anomalies form a fairly uniformzone of 20-30 km width whose magnetic intensityexceeds the otherwise generally quiet magneticbackground pattern by 200-400 nT. The exposed rockassociated with the magnetic anomalies consistspredominantly of Mesozoic to Tertiary metamorphic,intrusive, and volcanic material that appears to capthe more fundamental magnetic sources estimated byprofile analysis to lie 6 km or more below thesurface. The anomalies may reflect a locus ofintrusion, metamorphism, or basement structuraldefects that accompany a primary weakness between twodifferent crustal regimes.
INTRODUCTION
Colorado Plateau (~) is distinguished from the Basinand Range Province by generally larger amplitude andshorter dimension magnetic anomalies, and by a mergingof anomalies into northeast trends that presumablyreflect Precambrian basement contrasts (Sauck,1972). To the southwest, these trends terminate at orare attenuated on crossing T, and sou.thern Arizona ischaracterized by a northwest grain in the magneticpattern which follows the tectonic trends of Mesozoicand Tertiary age features (King, 1969). The residualgravity (Fig. 3) is less definitive across T, butthere are patterns of northwest-trending anomalies inthe Basin and Range Province that do not have anextension north of the transition T. These trends aremos t conspicuous sou th and sou theas t of Phoenix andMorenci. The grouping of magnetic anomalies along MBand the broad, elongated gravity minimums southwest ofME are noteworthy parts of the general northwestgeophysical grain of the Sonoran Basin and RangeProvince that will be discussed subsequently.
Figure 1. Regional map showing the magnetic belt(MB), the Sonoran Basin and Range Province (SB),the Great Basin (GB) llasin and Range Province,the Colorado Plateau (~), and major volcanicfields: High Plateau (HP), San Juan (SJ), SanFrancisco (SF), Datil-White Mountain (D), andPinacate (P). Hachured line indicates easternedge of the Mesozoic-Tertiary Cordilleranorogenic belt (Drewes, 1978). To the west arethe Sierra Nevada (S) and Baja (B) intrusivecomplexes.
Regional aeromagnetic and gravity patterns insou thern Arizona are examined for their relationshipto regional geology and tectonics with particularfocus on a belt of prominent magnetic anomalies (MB inFig. 1) that trends southeast across the lower part ofArizona from about lat. 350 N. on the Arizona-NevadaState line. This area is part of the llasin and Range~rovince generally east of California between theColorado Plateau (CP) and Sonora, Mexico (see Fig.1). For the purposes of identification, in contrastwith the Great llasin region of Nevada (Gll) , this areawill be referred to as the Sonoran Basin and RangeProvince (Sll).
The data for this study are the ResidualAeromagnetic Map of Arizona (Sauck and Sumner, 1970;Sauck, 1972) and the Residual Gravity Map of Arizona(Aiken, 1975, 1976). The aeromagnetic data wasacquired on north-south lines at 5-km spacing; theflight altitude was 9000 feet (2740 m) above sealevelexcept for a narrow strip bordering New Mexico thatwas flown at 11 ,000 feet (3350 m). These data areappropriate for identifying geophysical featureshaving dimensions greater than about 3 km, and aremore than adequate to identify the geophysicalsignature of tectonic trends that measure in the rangeof tens of kilometers or more.
REGIONAL GEOPHYSICAL SETTING
A simplified version of Sauck and Sumner's (1970)residual magnetic intensity map is shown in Figure 2,and Aiken's (1975) regional llouguer gravity map ofArizona in Figure 1. The magnetic data show a cleargeophysical separation across the transition T betweentwo magnetic provinces that coincide roughly with theBasin and Range, and Colorado Plateau Provinces. The
61
3"
Now Moxlco
- 41'"
"".
"".
34°
33°
32· N
35·
36°
Contour intorval 20001Contours label@d in oT-:-1oo
110·"'"RESIDUAL MAGNETIC INTENSITY-Scale In mi"s
100 10203040
r'om Sauck &rid Sumner. 1970
RESIDUALAEROMAGNETIC
MAPof ARIZONA
[I]] Loss than 400 oT
Figure 2. Simplified residual aeromagnetic map of Arizona (modified from Sauck andSumner, 1970), showing the inferred magnetic boundaries of the regions discussed inthe text: the Colorado Plateau (CP); the transition zone (T); the Sonoran Basin andRange Province subdivided into regions, SBT (Sonoran Basin transition), SB (SonoranBasin) and SBP (Sonoran Basin Papago area); the magnetic helt (MB), the extrapolatedline of the Jemez Lineament (JL); the Ajo trend (AJ); and the Cananea Line (CL).
62
Scale in miles
10 0 10 20 30 40
RESIDUALGRAVITY MAP
of ARIZONA
35'
37'
110·RESIDUAL GR W
Conto Jf . AVITY" Interval 10 mglill
ca Less than -20mgal
1110
1975from Aiken,
Figure 3197' Simplified5). Zon residuales T and ME_ _are
Bouguerfrom fi gravi tygure 2.
map of Arizona (modified f rom Aiken,
63
Other geophysical data compliment the gravity andmagnetic contrasts between the Colorado Plateau andthe Sonoran Basin and Range provinces. Crustalthickness south of the transition T is 20-30 km,compared with 30 to more than 40 km (increasingnortheast) for the Colorado Plateau (Smith, 1978).The greatest change in crustal thickness is along thegeneral region of the magnetic transition T. Thistransition also correlates with the average elevationscomputed over 10 x 10 squares (Strange and Woollard,1964; and see Blackwell, 1978) which varies from lessthan 1 km to greater than 1.5 km northeast across azone roughly marked by T. Heat flow reflects themajor provinces by shOlving generally higher values inthe Basin and Range Province (Blackwell, 1978).Electrical conductivity averaged vertically tosubcrustal depths similarly increases in the Basin andRange Province compared with the Colorado Plateau asinterpreted from geomagnetic variometer arraymeasurements (Gough, 1974).
GEOPHYSICAL SUBPROVINCES OF SOUTHHEST ARI7.0NA
Parts of the magnetic belt in southwest Arizonahave been attributed to batholithic intrusion (Sauck,1972) but there is generally no obvious correlation ofgravity and magnetic anomalies in the belt. The mostimportant correlation in gravity data appears to be apronounced gravity low southwest of the magnetic belt(Fig. 3) • The belt may be considered a separatesubprovince, but based on its relationship to residualgravity it is probably more correct to consider it asa boundary between subprovinces. The transitionalsubprovince (SBT) to the north of the magnetic beltgenerally shows more intense magnetic anomalies andhigher residual gravity than does the region to thesouth (SB and SBP). The gravity low to the southwestis over a region that is characterized by mixedJurassic to mid-Cretaceous granitic rocks andmetamorphosed Jurassic-Triassic sedimentary rocks(King, 1969). These rocks may be expected to havelower intrinsic density, as well as to have been moresusceptible to tectonic strains that could decreaseformation densities than the more prevalentPrecambrian basement (King, 1969) north of themagnetic belt. Haxel and others (1980) documentgeological differences between the southern andnorthern parts of the Papago area (SBP) that are inaccord with the hypothesis of a major variation ofterrane type across the magnetic belt.
An apparent discontinuity in the magnetic belt isroughly centered on long. 112 0 W. (SBP) and is boundedby two north-northwest magnetic trends. Thewesternmost trend (AJ) extends toward Ajo andessentially terminates the magnetic highs of thesouthwestern segment of the magnetic belt. The othertrend (CL) correlates with the Cananea Line(Hollister, 1978, p. 124). The magnetic belt maycontinue southeast of CL but it is noticeablyfragmented in the southeast corner of Arizona (Fig.3) • The possibility that the apparent break in thebelt may be related to thermal demagnetization ratherthan being a structural defect in the belt can beargued from higher than normal drill-hole geothermalgradients (Hahman and others, 1978) in the region oflong. 112° W. However, this geothermal evidence isweak because temperature sampling northwest of 1120 30'is sparse (Kenneth Hollett, U.S. Geological Survey,pers. commun.) and deep crustal electromagneticinvestigations that could provide complimentaryevidence are lacking. The AJ trend, that marks thewestern boundary of the discontinuity, coincides witha zone of recent volcanism (17 m. y. or less, Stewartand Carlson, 1978) and is in a region that is
64
remarkably aligned with a southwest continuation ofthe Jemez volcanic lineament (JL). The area ofhypothetical intersection of the magnetic belt withthe trends AJ and JL shows a pronounced minimum inresidual gravity (Fig. 3) such as might be expectedover a major volcanic collapse structure. Thesouthwest and southeast segments of the magnetic beltcould be interpreted to show a northeast transverseoffset of about 50 km in this region. Horizontaldisplacements along this direction and of this orderof magnitude have been suggested in southeast Arizona(Drewes, 1978), but supportive geological evidence islacking in southwest Arizona.
The continuous nature of the magnetic belt acrosssouthwest Arizona, and possibly into southeastArizona, suggests that the locus of anomaly sourcesmay reflect a "root zone" of a suite of rocks that mayhave identifiable consistency in composition or age atthe surface. A preliminary test of this possibilitywas performed by attempting to relate the magneticanomalies to mapped geologic units. Surfaceprojections of probable source bodies were estimatedfor the larger magnetic anomalies in the Sonoran Basinand Range Province whose residual contour closuresexceeded 500 nT. Table I shows the number ofanomalies associated with each of the magneticsubprovinces and with each of several generalized rocktypes based on the geologic map of Arizona (Hilson andothers, 1969). The "ambiguous" category refers toanomalies over alluvium or over complex suites ofdifferent rock types. Most of these ambiguouscorrelations were in southeast Arizona, east of long.112° W. Table I does not differentiate between the"Laramide" (Cretaceous-Tertiary) rocks and the earlierMesozoic rocks because of possible inconsistenciesbetween the State geological map and more recentgeological mapping (Gordon Haxel, U.S. GeologicalSurvey, pers. commun.). The major inconsistenciesoccur wes t of Tucson where new geological mappingshows discrepancies with the older work, mainlybetween "Laramide" and earlier Mesozoic intrusions,and between Precambrian and Mesozoic metamorphicrocks.
Tertiary-Mesozoic rocks make up the largestnumber of source body-surface rock correlations forthe Sonoran Basin and Range Province as a whole andalso for each of the geophysical subprovinces and themagnetic belt. More than 35 percent of the largeranomalies in each subprovince are related to TertiaryMesozoic rocks, whereas less than 20 percent arerelated to Quaternary and Tertiary basalts. Most ofthe basalt associations occur along the magnetic beltin or just northwest of the Ajo area of southwestArizona. Precambrian rock contributes a relativelysmall percentage to the association of surface rocksto larger magnetic anomalies in general, and none forthe region southwest of the magnetic belt. Thepredominance of Mesozoic-Tertiary age rocks inassociation with the larger magnetic anomalies withinand south of the magnetic belt is noteworthy; butexcept for the basaltic rock percentages, the resultsin Table I do not identify any geological uniquenessto the magnetic anomalies within the magnetic belt ascompared with the bordering subprovinces.
Although many surface outcrops seem to reasonablycorrelate with magnetic anomalies within the magneticbelt, the half-width (10-15 km) of the belt indicatesthat the main locus of sources may be at intermediatecrustal depths and that the surface rocks may becapping the more fundamental sources. Nine profilesacross the magnetic belt were analyzed for depth-tosource based on dike and fault models using Herner's
method (1953; see also Hartman and others, 1971; Jain,1976). Figure 4 shows the profile lines, along withabbreviated aeromagnetic contours, and the location ofmajor known mineral deposits. The magnetic data weresampled at 2-km intervals and are plotted on Figure5. Several depth solutions result from passingdifferent filters over the profiles; generally,confidence that reliable source parameters are foundis based on a clustering of solutions. '!'he methodprovides a 2-dimensional zone in which the top of thesource is inferred. The horizontal dimension of this"source zone" is indicated by a heavy line below theanomaly in Figure 5; the vertical depth range of thezone is shown next to the heavy line. The "sourcezone" is taken as an approximation of the maximumdepth to the top of the source because a widermagnetic body at shallower depth might also satisfythe anomaly.
The magnetic source depths (Fig. 5) show a largeoverall range along the belt, 6-31 km, but the medians(Fig. 4) average between 10 and 20 km depth. Theresults are consistent with indications from thedimensions of the magnetic belt that near-surfacerocks are not the fundamental sources of anomalies.The surface rocks, however, may provide a clue to theage of the crustal feature that produces the anomaliesof the magnetic belt. The surface rocks may also insome cases be related to the deeper crustal feature asigneous derivatives or direct continuations with thedeeper material.
DISCUSSION
The northwest-trending locus of large, possiblymid-crustal depth, magnetic contrasts that separatestwo crustal regimes of apparently different averagedensity will eventually have to be put in the contextof regional tectonics. Such a synthesis will requirefurther data and reasoning to infer the direct cause
of the anomalies that may be crustal intrusions,metamorphism or structural relief of buried crustalunits. The present aim is basically to point out theprominence of this belt and to document some possiblecorrelative features.
The magnetic belt is generally parallel to thenorthwest-trending tectonic grain of southernArizona. This regional grain, or specific parts ofit, has been identified as the "Texas Lineament"(Hertz, 1970). The significance of lineamenttectonics is a source of debate (Gilluly, 1976;Marshall, 1979), but one implication is a deep-seatedcrustal weakness that could give rise to magneticcontrasts either by displacement or by providing aconduit for intrusion. The magnetic belt is notstrictly a linear feature, but tectonic events thathave occurred at numerous times at least since earlyHesozoic time in southern Arizona (Coney, 1978) mayhave distorted its original form.
An inferred Jurassic magma tic arc (Coney, 1978)crosses southern Arizona with a trend similar to thatof the magnetic belt. A magmatic arc would beexpected to create a zone of intermediate-compositionigneous rocks against or within a sedimentary wedgethat would produce a magnetic signature. A magmaticarc would also produce a deep-seated crustal flawwhose geophysical signature may remain relativelystable over subsequent deformation events, and itl<1Ould concentrate a source of intrusive material forsubsequent differentiation into various crustal rocksand, possibly, economic mineral concentrations.
The fact that the Ajo, Lakeshore, and Silver Bellporphyry copper deposits, as well as several lesserdeposits, occur on the periphery of the magnetic beltindicates the possibility that the magnetic belt isrelated to .a crustal flaw that guided magma, andlocally, economic concentrations of minerals to the
Table I. Correlations between magnetic anomalies and generalized surface geologyin southern Arizona
Surface-rockcorrelation
Number of anomalies in each subprovince1
HB SBT' SB + SBP Total
Quaternary and Tertiary basalts ----- 8
Lower Tertiary-Hesozoicmetamorphic, intrusive& volcanic rocks ----------------- 16
Precambrian granites andgranitic metamorphic rocks ------- 6
Subtotal ------------------------- 30
Ambiguous ------------------------ 12
Total ---------------------------- 42
3
10
5
18
q
27
13
o
14
22
12
39
11
62
29
91
I See Figure 2. -- For identification of subprovinces.
65
1100
.....
.Aercra.gnetlcs frOQ Sauck. and Sumner (1910)Klnernls frOllll Mardirod.llD (l973)
®250
- CUJ!IIIllati01l production to 1973(rrllUoon </If dol1Artl)
112
- ll!.lJlJ than $1- $1-100
- $100-2000
- $1000-4000
no circle
ooo
- mlg'Detic sllOlDlllle.o
- III'1nenll depoo1ts
- leflll thAn 300 nT
- gold-9ilver- ~per
- copper--.:)l,bdeIlll.tm.- lead'-':lnc
- greater tb.mn 500 nT
352 ~:o33'
A
A
AC
CHl'1>
®A337 A
Pigure 4. fup showing the location of magnetic profiles (a-h, see figure 5), mineraldeposits Hardirosian (1973), and abbreviated magnetic contours (Sauck and Sumner,1970). The profiles shoH interpreted source-locations (heavy-line segment) andsource-depth in kilometers.
- 8 -18 km depth
IIII
-1- _
~- 10 -20km depth
_ 6 -24 km depth
i.
"I AI"",,1(17ki
- 10-18 kmdepth
-- 6-20kmdepth
20 40 km
Horizontal Scale
VN=--. ,
e.~ :_ _ 290,T
I •I~
II
Ia. I
III
I~- 14 - 22 km depth
c.
i/\J"""~
-- 9·31kmdepth
upper crust. Major base-metal mineral deposits(Hardirosian, 1973) are shoHn on Figure 4. About 30percent of these deposits are on or close to the edgeof the magnetic belt, Hhereas the area of this beltcomprises about 15 percent of the region from TsouthHard. From the standpoint of random chance, thisis not a strong argument that the ore occurrences arerelated to the belt but the possibility cannot bedismissed. The mobilization of mineralized fluidslYOuld not have to have occurred simultaneously withthe intrusions. The near-surface migration anddeposition of minerals may be controlled by secondarycrustal features that could distribute deposits somedistance laterally from the root zone.
Figure 5. l1agnetic profile data (see figure 4) acrossthe magnetic belt shoHing the source-depth inkilometers, range of solutions and the anomalypeak-trough \qidth and amplitude. The verticalbars range from 300 to 600 nT. The left of theprofile is alHays oriented to the south or to thesouthHest.
DreHes (1978) postulated the southeastcontinuation of the Cordilleran orogenic thrust beltfrom the region of Nevada through the 10Her part ofArizona based on horizontal displacements of at leas t20 km, possibly more than 100 lan, 'in southeastArizona. The timing of this thrusting Hould be partlysimultaneous Hith or l'lOuld postdate the Cretaceous Tertiary (Laramide) orogenic event and itsmineralization. If the source roots for the magneticbelt are at depths approaching 10 km or more, thenthey are probably beneath the postulated thrustregime, and any surface correlations of rock suites tothis root Hould be fortuitous unless the horizontaldisplacements involved Here close to the resolvabledimensions considered in the present data. Themagnetic belt itself is not a critical argument for or
66
of directGeophysics,
against major thrusting hecause a reLHively thinupper crust regime may operate tYithout significantlyaffecting the deeper magnetic anomaly sources, and athicker thrust regime could create a quasi-linearmagnetic source as the result of thermal metamorphismarising from crustal stress or as the result ofdLsplacement of magnetic crustal units.
CONCLUS IONS
The major magnetic feature of the U!lsin and RangeProvince in south\qest Arizona is a northwest-trendinghelt of magnetic anomalies that contr!lsts with agenerally quiet magnetic pattern in this region. Thishelt has a general consistency in its average magneticmorphology along most of its length with anomalyamplitudes exceeding 300 nT across a zone 20-30 kmwide. Surface correlations with the anomalies of themagnetic belt are mostly Hesozoic and Tertiary agerocks that may only be cappings to source roots at amaximum depth of about lO-;W km. There appears to hea transition in residual gravity across the helt thatsuggests a regime of less dense crustal material tothe soutlwest. The evidence is consistent \qith thespeculation that this belt represents a tectonicHeakness that separates two different &lsin and Rangeterranes. The magnetic belt may continue in southeastArizona, east of a discontinuity in the region oflong. 1120 Iv. tYhere the helt intersects a hypotheticalsouthHest continuation of the Jemez lineament. Thediscontinuity is bounded to the west by a magneticzone extending northwest toward Ajo, and on the eastby the Cananea Line.
The trend of the magnetic belt in southVlestArizona corresponds with the regional tectonic grainand correlates in a general sense with a magmatic arcproposed by Coney (1978). If the belt reflects !l zoneof unique crustal intrusion, it could possibly be asource for the creation of differentiated fluids thatmineralized the region. However, in considering thebelt as a continuous unit across all southern Arizona,only about 30 percent of the known mineral depositsare within or along its borders.
ACKNOtVLEOGEHENTS
Professor John Sumner of the University ofArizona provided the regional magnetic and gravitymaps used for this report. Data digitization andcompilation were done \qith the assistance of Hike Baerand Hike Broker of the n.s. Geological Survey inDenver. This study \qas performed in part as apreliminary geophysical analysis for the Ajo 10 x ,:>0
sheet; U.S. Geological Survey's "Conterminous U.S.Hineral Resource Assessment Program", (CUSHAP).
REFf<:R"NCES CITfW
Aiken, C. L. V., 1975, Residual Bouguer gravityanomaly map of Arizona: Laboratory ofGeophysics, University of Arizona, Tucson.1976, The analysis of the gravity anomalies of
------Arizona, Ph.D. dissertation, University ofArizona, Tucson.
Blackwell, D. D., 1978, Heat flow and energy loss inthe western United States, in Smith, R. ll., andEaton, G. P. , eds. : Geological Society ofAmerica llemoir 152, p. 175-208.
Coney, P. J., 1978, The plate tectonic setting ofsoutheas tern Arizona: New Hexico GeologicalSociety Guidebook, 29th Fieln Conference, p. 285290.
67
Dre"es, H. , 1978, The cordilleran orogenic heltbet'ween Nevada and Chihuahua: Geological Societyof America llulletin, v. S9, p. 641-657.
Gtlluly, J., 1976, Lineaments--Ineffective guides toore deposit s: Economic Geology, v. 71, p. 1 ';071514.
Gough, D. 1., 1974, Electrical conductivity under\qestern North America in relation to heat flow,seismology and structure: Journal of Geomagetismand Geoelectricity, v. 26, p. 105-1~1.
Hahman, H. R., Sr., Stone, C., and 1'1itcher, J. C"1978, Geothermal energy resources of 4.rizon!l,preliminary map, geothermal map no. 1: Bureau ofGeology and Hineral Technology, Geological SurveyBranch, University of Arizona, Tucson.
Hartman, R. R., Teskey, D. J., and Friedberg, J. L.,1971, A system for rapid digi tal aeromagneticinterpretation: Geophysics, v. 36, p. 891-918.
Haxel, G., Hright, J. E., Hay, D. J., and Tosdal, R.H., 1980, Reconnaissance geology of the Hesozoicand Lower Cenozoic rocks of the southern PapagoInnian Reservation, 4.rizona; A preliminaryreport: Arizona Geological Society Digest, v.12, p. 17-29.
Hollister, V. F., 1978, Geology of the porphyry copperdeposits of the western hemisphere: Society of}lining Engineers, American Institute of llining,Hetallurgical, and Petroleum Engineers, Inc., NewYork, 219 p.
Jain, S., 1976, An automatic methodinterpretation of magnetic profiles:v. 41, p. 531-541.
King, P. B., 1969, Tectonic map of North America,sheet 2: U.S. Geological Survey.
Harnirosian, C. A., 1973, Hining districts and mineraldeposits of Arizon!l (exclusive of oil and gas),map: l1ining club of the southVlest, llox 17226,Tucson, Ariz.
Harshall, B., 1979, The lineament-ore association:Economic Geologv, v. 74, p. 942-940.
Sauck, H. A. , 1972, Compilation and preliminaryinterpretation of the Arizona aeromagnetic map:Ph.D. dissertation, University of Arizona,Tucson.
Sauck, H. A., and Sumner, J. S., 1970, Residualaeromagnetic map of Arizona: Dept. ofGeosciences, University of Arizona, Tucson.
Smith, R. B., 1978, Seismicity, crustal structure, annintraplate tectonics of the interior of thewestern Cordillera, in Smith, R. B., and Eaton,G. P., eds., Cenozoic tectonics and regionalgeophysics of the western Cordillera: GeologicalSociety of America Hemoir 152, p. 111-141.
StetYart, J. H., and Carlson, J. E., 1978, Generalizedmaps showing distribution, lithology and age ofCenozoic igneous rocks in the western UnitedStates, in Smith, R. B., and Eaton, G. P., eds.,Cenozoic~ectonics ann regional geophysics of thewestern Cornillera: Geological Society ofAmerica Hemoir 152, p. 263-264.
Strange, H. E., and Hoollard, G. P., 1964, The use ofgeologic and geophysical parameters in theevaluatton, interpolation, and prediction ofgravity: Aeronautical Chart ann InformationCenter, USAF, St. wuis, llissouri Contract AF23(601)-3879 (Phase 1), 225 p.
Herner, S., 1953, Interpretation of magnetic anomaliesat sheet-like bodies: Sveriges GeologiskaUndersok., Ser. C. C. Arsbok 43, N:06.
Hertz, J. B., 1970, The Texas lineament and itseconomic significance in southwest Arizona:Economic Geology, v. 65, p. 166-181.
Hilson, E. D., Hoore, R. T., and Cooper, J. R., 1969,Geologic map of Arizona, scale 1 :500,000: U.S.Geological Survey State Geologic }mp Series.
HINERAL OCCURRENCES IN THE I!HIPPLE HOUNTAINSWILDERNESS STUDY AREA, SAN BERNARDINO COUNTY, CALIFORNIA
James Ridenour, Phillip R. Hoyle, and Spencee L. Hillettu. S. Department of the Interior, Bureau of Mines
Western Field Operations CenterE. 360 Third Avenue
Spokane, Washington 99202
ABSTRACT
The Hhipple Mountains Wilderness Study Area (HSA)encompasses approximately 85,100 acres in SanBernardino County, California. An examination ofmines, prospects, and mineralized areas within andnear the study area was conducted during the springof 1980, by U. S. Bureau of Hines personnel under theprovisions of the Federal Land Policy and HanagementAct.
U. S. Bureau of Hines statistical files indicatethat about $84,000 worth of copper-gold-silver orewas produced from mines in the Whipple miningdistrict, exclusive of Copper Basin, from 1906 to1969. Prospecting targets were the ubiquitous occurrences of chrysocolla, malachite, and hematite infracture zones both above and below the Whippledetachment fault, the dominant structural feature inthe HSA.
The predominant element assemblage consists ofbarium-manganese-iron-copper-gold-silver and is foundin a wide variety of host rocks ranging in age fromPrecambrian (?) to Hiocene. Petrographic analyses ofselected copper-gold-silver occurrences indicatethese elements are constituents of mineral assemblages largely composed of quartz, sericite,chlorite, and sulfides which have been altered tosecondary copper and iron oxide minerals. Theseassemblages are indicative of an epithermal toperhaps lower mesothermal temperature range.Structural analysis of fractures associated withthese occurrences indicates distinct populationsabove and below the detachment fault that are theresult of crustal lengthening in the lower plate anddislocation of the upper plate. The younger agelimit for these occurrences is believed to be theOsborne Hash Formation (mid-Hiocene); the older agelimit for these occurrences is thought to be early inthe dislocation process, or late Oligocene, as issuggested by a 24.5~0.7-m.y. K-Ar date of sericitefrom one of the localities. Collectively, theseindicate a temporal relationship between mineralization and dislocation in the Whipple Mountains.
The conceptual mineralization model suggested bythese relationships is a low to moderate temperaturehydrothermal system spatially related to the detachment fault and chlorite breccia zone immediatelybelow it. Communication of fluids into the upperplate was provided by listric fault and antitheticfault conduits formed during dislocation of theallochthon. The sources of metallic elements forthis model are thought to include pre-dislocationmineral deposits and syntectonic volcanicmetallogenic pulses.
INTRODUCTION
General Statement
Under the provisions of the Federal Land Policyand Management Act (Public Law 94-579, October 21,
69
1976), U. S. Bureau of Hines personnel conducted anexamination of mines, prospects, and mineralizedareas within and near the \fuipple Hountains Hilderness Study Area (WSA). Field investigations wereconducted from mid-Harch to mid-Hay 1980, t-lith subsequent short visits to the area in September 1980and October 1981. During our investigations, 643hard rock samples and 39 petrographic samples werecollected from the 49 mines and prospects shown onfigure 1. Analytical results of these samples,detailed descriptions of sample local ties, maps ofworkings (where applicable), and background dataregarding mineral potential of the study area, arecontained in a report at the Bureau of Mines WesternField Operations Center, Spokane, \Iashington.
A synopsis of our work plus geological, geochemical, and geophysical investigations of the studyarea conducted by the U. S. Geological Survey will bepublished as a Summary Bulletin and ~W-series maps inthe near future.
The purpose of this paper is to report ourobservations of the study area's mineral occurrenceswith emphasis on their geologic setting, and tocomment on their time-space relationships to detachment faulting, probable origin, and mode of distribution.
Previous Investigations
Library and file research revealed manyreferences concerning geology and mineral resourceswithin and near the WSA. Davis and others (1980)present their latest findings regarding structure,stratigraphy, petrology, geochronology, kinematics ofdetachment and listric faulting, and geologic historyin the Ilhipple Hountains. The reader is referred tothis publication for a more detailed list of previousinvestigations dealing with these subjects. Recentpublished mapping includes Dickey and others (1980)in the southern part of the study area and a compilation of the Needles 1° X 2° sheet by Stone andHoward (1979). Site specific geological studies inthe vicinity of the study area have been conducted byFrost (1980) and Anderson and Frost (1981). Earlymining activity in the area has been recorded by Root(1909), Bancroft (1911), and Jones (1920). Specificdescriptions or notations of various mines,prospects, and mineralized areas in the region aregiven by Bailey (1902), Turner (1907), Aubury (1908),Graeff (1910), Stevens (1911), Hilson (1918),Cloudman and others (1919), Tucker (1921), Tucker andSampson (1930, 1931), Noble (1931), Trask and others(1943), Tucker and Sampson (1943), Eric (1948), Trask(1950), Wright and others (1953), Trengove (1960),and Fleury (1961). Several previous property examinations (unpublished) have been made by U. S. Bureau ofHines and California Division of Hines and Geologypersonnel. The tectonic history of the Vidal-Parkerregion has been presented by Carr (1981). A historical overview of mining in the California desert hasbeen written by Vredenburgh and others (1981).Studies on Cordilleran metamorphic core complexes, of
0."(/",
0-;>
1--~
X Blue HeavenGroup
X Gold Crown Group
X Peacock CopperGroup
Copper Basin
~
Co
--r-34°15'
+
o 1 2 3 4 5 MILESI i \ I' I 'I I; i !o 1 ~ 3 4 5 6 7 8 KI LOMETERS
~
Red Eagle GroupX
~() o
~<) r-- ~P~r---- "-<I 99\C ..
~~'
)r ~ Prospect
C j~ Crescent""'\~ Group
1lT\\ ) 't Mangane"
Vrr- ~X~s~ecr J/~"'- r X MO;;;;;;';;King
\<- ~ v FMonarch fHornMine~~ .-......:.;w
Van" ......--..9J ~&. Prospect 'X. X Homt'r Group
v.. Prospect F . kef ,o/ate MinerX.... XNlc
X "'ftPrQspectProspect ~
...x~/aCkjaCk Group
+
+
-l-I
Blue CloudMine
)~"\..~
X Lizard Group
WlO\.lN11'-\NS
X Riverview Mine
d",-,...y--r2lJ..-.7~!,n ","oX 'y~. ~ '<""___
~-4.-____ ~ 'r
+
+
X Dickie
IN\-\\??\..t:
X War Eagle Mine
Whipple Detachment
Fault (double hatchure' X Black Mesas
on upper Plat/e) X Thunderbird
f' X Blue Bird
\-~ X. Lucky Green Group
~ ---.. --"" ---Double MNo. 1X~
X p';;;;e>-' '\rX Prospect P
X Golden Arrow
\+
o(Y)
~
+X Sleepy Burro Group
New American Eagle-X Mine
Pros'Jecr X ProspectProspect X X X Prospect
o & W Mine X X A tkinson Group
.....o
Figure 1. Mines and prospects in the Whipple Mountains WSA.
Ivhich the lihipple Hountains is but one, have beenrecently compiled by Crittenden and others (1980) andConey and Reynolds (1980). Schuiling's work (1978)on the mineral assemblages of the SacramentoHountains was used as a comparison to the assemblageswe observed in the lIhipple t1ountains.
Nining in the Study Area
Bureau of Hines statistical files show that 5,123tons of ore have been reported from 56 mines in theHhipple mining district beginning in 1906 and endingin 1969. The historic value of the ore is estimatedat about $84,000; approximately 48 percent of thevalue came from gold and about 45 percent fromcopper, with silver, lead, and zinc constituting theremaining seven percent of the ore's value. In addition to gold-copper ores, about 2,500 tons ofmanganese IVere produced from five mines between liorldliar I and the 1950's. Huch of the mining activityhas been limited to intermittent extraction of relatively small, high-grade pockets of mineralizedrock. Prospecting targets were the ubiquitous occurrences of chrysocolla, malachite, and hematite infracture zones. Underground mining IVas accomplishedprimarily by sinking shafts and subsequent driftingalong the mineralized zones. Production recordsindicate mining activity was most prevalent prior toliorld liar I and from the depression years throughliorld liar II.
HINERAL OCCURRENCES
General Statement
The investigation of mineral potential of theHhipple Hountains liSA has given us a unique opportunity to merge our observations regarding itsmineral deposits with recent information on metamorphic core complexes in general and the lihippleHountains in particular. Because detailed studies ofthe origin, temperatures of formation, geochemistry,and age of the deposits are beyond the scope of ourinvestigation, we hope that our observations willstimulate more definitive studies of the economicmineral assemblages of this and other metamorphiccore complexes.
Geologic Setting
The dominant structural feature of the study areais a low-angle detachment fault IVhich juxtaposescontrasting lithologic terranes across its surface.As described by Davis and others (1980, p. 79), thefault
•.• separates a lower-plate assemblage ofPrecambrian to t1esozoic or lower Cenozoicigneous and metamorphic rocks and theirdeeper, mylonitic equivalents from anallochthonous, lithologically varied upperplate.
The allochthonous units (Davis and others, 1980, p.80)
••• include Precambrian to Nesozoic crystalline rocks, Paleozoic and tlesozoic metasedimentary rocks, Hesozoic metavolcanicrocks, and Tertiary volcanic and sedimentaryrocks.
Immediately belolV the lihipple detachment fault liesan altered and highly disturbed zone termed thechlorite breccia zone (Frost, 1980). This zone is
71
capped by a 2-to 25-cm-thick, resistant, flintymicrobreccia (Davis and others, 1980, p. 112);consistent striae directions of N 50±10° E havebeen measured on its surface, indicating transportdirection of the allochthon.
Salient geologic history of the Whipple~lountains, as documented by Davis and others (1980),includes Late Cretaceous and/or early Tertiarymylonitization and metamorphism accompanied bysynkinematic intrusion of adamellite to tonalitesheets or sills, follolVed by Oligocene (?) to middleNiocene dislocation of the allochthonous terrane.The lihipple llountains were asymmetrically domed afterdislocation, and erosion has removed most of theupper plate terrane from the core area, leaving onlya few isolated klippen of allochthonous rocks exceptin the east-central part of the study area.
Description of Assemblages
The commodity assemblages in the WhippleHountains consist of barite-manganese-iron andcopper-gold-silver. The barite-manganese-iron assemblage is found, in significant amounts, almostexclusively in upper plate rocks. Copper-goldsilver-bearing occurrences, on the other hand, arefound in a IVide variety of host rocks in both 10lverand upper plate positions and are megascopicallyindicated by the presence of chrysocolla, malachite,and iron oxides. Associated with these commoditiesis a pervasive chlorite-dominant propylitic alteration. The barite-manganese-iron-copper-gold-silverassemblage and its alteration products suggest epithermal temperatures associated IVith Tertiary volcanism (Park and l~cDiarmid, 1970, p. 344-345). Eightsamples IVere selected for detailed petrographic description from seven localities within and near thestudy area. Descriptions of these samples aresummarized in Table 1. We believe these samples tobe representative of the ubiquitous copper-goldsilver-bearing occurrences we observed during ourfield investigations.
Petrographically the mineral assemblages arecomposed largely of quartz, sericite, chlorite, andsulfides (now mostly altered to secondary copper andiron oxide minerals). Although not all constituentsare always present in a given sample, where they dooccur the sequence is fracturing and concomitantquartz-sericite-chlorite alteration followed bydeposition of sulfides.
Although there are some important differencesbetIVeen the assemblages described here and thoseinvestigated by Schuiling (1978), IVe believe hisassemblage B (quartz+minor pyrite+hematite+chrysocolla+barite) most closely approximates theWhipple Nountain assemblages. Temperatures offormation of the Sacramento Hountains assemblagesranged from 120 0 C to more than 310 0 C (epithermal tolower mesothermal); paragenetic sequences indicatethat certain constituents IVere deposited in stagesIVithin distinct ranges of temperature and salinities(Schuiling, 1978, p. 53), perhaps indicating multiplestages of deposition. Overall alteration andgeologic setting of these assemblages appearscorrelative with the lfuipple Nountain assemblages.
Structural Patterns
Structures that control the occurrences of theassemblages described here include tension gashes,shears, and faults. These occur throughout the studyarea in nearly all types of rock in both upper and
"'"I\)
Name
Whipple Wash prospect
Crescent Group
Lortie Group
Setting
Lower plate
Upper plate
Upper plate
Table 1. Petrographic Summary
Rock type Assemblages (minerals in decreasing order of abundance)
Chlor i te brecc ia I quart z-ch lor i te-s eri ci te-sulf ides-limoni te-c lay
Quartz vein I quartz-chrysocolla-limoni te-malachi te-serici te
Brecciated porphyritic grani te I quartz-c lay-limoni te-ch rysocolla
Paragenetic Summary
Fracturing followed by simultaneous introduction ofsilica, serici te. chlori te-c lay, and sulf ides.
Fracturing followed by introduction of vein quartzand chalcopyri te accompanied by silification andsericitic alteration; further fracturing followedby oxidation of chalcopyri·te to limonite, chrysocolla, and malachite, accompanied by deposition ofchalcedony.
Brecciation followed by introduction of chalcopyrite andits alteration to limonite and chrysocolla; alterationof plagioclase to clay.
Nickel Plate Mine Upper plate I Adamellite microbreccia
Lucky Green Group Lower plate I Granitic cataclasite
Blue Bird Mine Lower plate I Graphic granite breccia
New American Eagle Mine I Lower plate I Sadie dacite porphyry (dike?)
New American Eagle Mine I Lower plate 1 Siliceous volcanic breccia
quartz-clay-limonite-chrysocolla-sulfides I Microbrecciation followed by introductfon of quartz,pyrite, chalcopyrite, and sericite. Later argillicalteration of plagioclase.
qua rtz-seri ci te-chalcopyri te-limoni te-malachi te I Fracturing followed by quartz-serici te-sulffde minerali-zation; supergene alteration of chalcopyrite tolhnoni te and malachi te.
quartz-seric i te-nontroni te (? )-bari te-chrysocolla-chlori te-limoni te I Brecciation followed by introduction of quartz. nontronite (?). sericite, barite, and sulfides. Supergene alteration of sulfides to limonite and chrysocolla.
quartz-chlori te-sulf ides-serici te-limoni te Def ormation followed by introduction of quartz. sulfides.chlori te, and sericite. Supergene limonitic alteration.
quartz-limonite-malachite-clay Brecciation followed by introduction of quartz and sulfides.supergene alter<:1-tion of sulfides to limonite, malachite.and clay.
Nlower plate position; they have been identifiedwithin a zone approximately 1200 feet both above andbelow the detachment surface. Faults, particularlylistric normal ones, can be traced for several miles;shear zones and tension gashes can usually be tracedalong strike for only a few hundred feet.
Two hundred and eighty-six attitude observationsof mineralized structures were separated into lowerand upper plate populations and analyzed by computer. The resulting Pi diagrams (figure 2) show aconcentration of poles in the lower plate orientedN 26° H, 48° NE; upper plate structures show adominant cluster at N 45° II, 28° NE, and a secondarycluster centered about N 46° II, 66° SH. Upper platedata confirm the suspected structural control ofthese occurrences by NE-dipping listric andsympathetic normal faults and Slrdipping antitheticand bedding plane faults. The dominant lower platestructural grain is within limits predictable fortension fractures resulting from the N 50~0° Edirection of crustal extension and direction ofmovement of the upper plate.
DISCUSSION
Our conclusions regarding the mineral potentialof the Hhi pple l'lountains liSA were based on theobservation that the majority of the mineral occurrences observed at the mines and prospects we mappedand sampled were spatially related to the detachmentfault and the premise that they are temporally related to the dislocation process. Because groundpreparation is the dominant physical control on themineral occurrences, a temporal link between theprocesses of ~islocatlon and mineralization seemsintuitive. As younger rocks, e.g., Osborne lvashFormation, have not been deformed by the dislocationprocess and are not known to contain occurrences ofthe type described here, a younger age limit ofl3.5~.0-m.y. (Kuniyoshi and Freeman, 1974) isplaced on the mineralization and suggests that, ifthe processes were coeval, mineralization may haveceased with cessation of dislocation or shortlythereafter. He must also assume that if themineralization process continued into Osborne Hashtime, these rocks were unaffected.
a. n 176
o
N
'I
=
Pole density
00.1%
01.2%
~2.3%
_3.4%
_4-5%
~5.6%
_6·7%
The older age limit of mineralization is harderto define. These occurrences are found in rocks thatmay be as old as Precambrian (Davis and others, 1980,p. 86) or as young as Niocene (Davis and others,1980, p. 95). Paragenetic relationships at the LuckyGreen Group indicate that mineralization resulteuduring extensional fracturing discordant to earliermylonitic fabric; those at the pr03pect in lowerHhipple Hash indicate that sulfides formed during orafter formation of the chlorite breccia. Finally, aK-Ar date on sericite from the Nickel Plate Nine isplaced at 24.5~0.7-m.y., indicating probableformation of mineralization at this locality duringearly stages of dislocation.
In summary, we interpret the mineral assemblagesas having been transported in a low to moderatetemperature hydrothermal system, coeval with dislocation of the allochthon, and deposited infavorable physical and geochemical environments. Asthe dislocation process spanned some ~10 millionyears and apparently was not a continuous process, asindicated by local erosion into the chlorite brecciazone followed by renewed dislocation (Davis andothers, 1980, p. 95), so must mineralization havealso been somewhat discontinuous, if the two processes were coeval. He believe the main conduit for
73
b. n 110
Figure 2. Pi diagrams of mineralized fractures ina. upper plate and b. lower plate rocks in thelIhipple Nountains HSA.
these fluids was the detachment fault and chloritebreccia zone, and that the feeder conduits into theupper plate were listric and antithetic faults. Thestructurally lmver limit of mineralization, as indicated by the study area's barren core, may simplyreveal the boundary beyond which confining pressuresexceeded hydrostatic pressures of the hydrothermalsystem.
Because this model requires either predislocationor syntectonic sources (or both) for the metallicelements such as copper, gold, and silver, we offerthe following possibilities:
1. The work of Anderson and Frost (1981)demonstrates that a Cretaceous adamellite, asignificant piece of which is in Copper Basin, wascut by the detachment fault and transported by thedislocation process into the l!hipple Nountains area.
Hhile this suggests its mineralization may significantly predate dislocation and, therefore, may havebeen transported, at least in part, to its presentsite, the hypogene minerals associated with thisdeposit, as described by Heidrick (1980, p. 20),indicate an epithermal, volcanic-derived assemblage,the constituents of which are also present throughoutthe Ivhipple llountains and apparently temporallyassociated with dislocation. Therefore, the CopperBasin mineralization may be a consequence ofdislocation rather than a contributor to the process.
2. The extreme western part of the study area isdominated by nonmylonitic rocks intruded by anorthHest-trending dike s'varm whose lithologiesappear to range from rhyolite to andesite. A dacite(?) from this swarm has been dated at 4l.9~6.7-m.y.
(Eric Frost, personal communication). In apparentassociation Hith this dike swarm are podiform tolensoid and disseminated occurrences of sulfides atthe D&\J Hine (Aubury, 1908, p. 337) and NeH AmericanEagle Hine (Hilson, 1918, p. 4-6; Hright and others,1953, p. 65). Because the sulfides formed later thanthe host rock, as indicated by paragenetic relationships at the New American Eagle l.fine, and the dikeSHarm is in the lmver plate and thus truncated by thedetachment fault, We infer that these occurrences mayhave been one possible source for remobilizedmetallic elements.
3. Volcanic activity in the area both prior toand during dislocation has been documented by averagedates of 27.8~.8 and l7.7~0.6-m.y. for GeneCanyon and Copper Basin volcanic rocks, respectively(Anderson and Frost, 1981, p. 36). In addition, Carr(1981, p. 18) cites evidence of intermediate tocaIn-alkaline volcanism ranging in age from about 22to 18 m.y., and basaltic volcanism beginning about 15m.y. ago in the Hhipple !!ountains and vicinity. Ofparticular importance to this system is any syntectonic volcanism that could have supplied metals tothe transporting conduit.
CONCLUDING COHHENTS
Hhile we are relatively comfortable with themineralization model presented here, we realize thatmuch further Hork is necessary to define its parameters and that its application to detachmentterranes elseh'here may not be valid. Nevertheless,using this model, we have defined areas in theHhipple Hountains Hhere we believe there is a greaterlikelihood of discovery of potentially significantmineral resources. Because of dominant structuralcontrol of the occurrences, we believe larger scalestructures should be examined in detail and withmodern exploration methods and tools. In addition,thicker sections of upper plate crystalline rocks maybe likely hosts for entrapment of ascending hydrothermal fluids generated during dislocation as wellas favorable terrane in which to explore for significant remnants of predislocation mineral deposits.
ACKNOHLEDGEHENTS
He wish to acknowledge the assistance of the manyindividuals who contributed to our knowledge of thestudy area. Special thanks are due Eric Frost, SanDiego State University, who not only provided most ofthe geologic and structural information necessary toour understanding the occurrence of the study area'smineral deposits, but also tirelessly answered ourquestions and offered much encouragement. LawfordAnderson and Greg Davis, University of SouthernCalifornia, contributed clarifying points, through
74
personal communication, regarding petrologic andstructural aspects of the study area. Conversationswith Sherman Harsh, U.S. Geological Survey, havehelped us understand the probable geochemical conditions under which the minerals were formed. He alsowish to thank Bill Rehrig, Phillips PetroleumCompany, for sharing his observations of similarmineral occurrences and for critical review of themanuscript. Thanks are extended to our colleagueSteve Hunts for his technical review and many helpfulsuggestions for improvement of the manuscript. Hethank Keith Howard and the U. S. Geological Surveylaboratory for geochronological determination of aspecimen collected during our investigation. Petrographic thin sections were prepared by PacificPetrographics, Spokane, Hashington; descriptions ofthese were done by Koehler Geo-Research Laboratory,Grangeville, Idaho. He are indebted to ClaytonHorlock and Julia Bosma for their assistance duringfield investigations. Finally, appreciation is extended to the U. S. Bureau of Hines for allowing usto prepare and publish this paper.
REFERENCES CITED
Anderson, J. L., and Frost, E. G., 1981, Petrologicgeochronologic, and structural evaluation of theallochthonous crystalline terrane in the CopperBasin area, Eastern Hhipple Hountains, southeastern California: unpublished report to theHinerals Division, The Louisiana Land andExploration Company, LakeHood, Colorado, 63 p.
Aubury, L. E., 1908, The copper resources of California: California State Hining Bureau Bulletin50, 366 p.
Bailey, G. E., 1902, The saline deposits of California: California State Hining Bureau Bulletin 24,.216 p.
Bancroft, Howland, 1911, Ore deposits in northernYuma County, Arizona: U. S. Geological SurveyBulletin 451, 130 p.
Carr, II. J., 1981, Tectonic history of the VidalParker region, California and Arizona, in Tectonic framework of the Hojave and Sonoran Deserts,California and Arizona: U. S. Geological SurveyOpen-file Report 81-503, p. 18-20.
Cloudman, H. C" Huguenin, E., and Herrill, F. J. H.,1919, San Bernardino County, in Hines and mineralresources of California: Report XV of the StateHineralogist, Part VI, California State HiningBureau, p. 775-889.
Coney, P. J., and Reynolds, S. J., 1980, Cordilleranmetamorphic core complexes and their uraniumfavorability: U. S. Department of Energy ReportGJBX-258(80), 627 p.
Crittenden, H. D., Jr., Coney, P. J., and Davis,G.H., eds., 1980, Cordilleran metamorphic corecomplexes: Geological Society of America Memoir153, 490 p.
Davis, G. A., Anderson, J. L., Frost, E. G., andShackelford, T. J., 1980, Hylonitization and detachment faulting in the Hhipple-Buckskin-RawhideMountains terrane, southeastern California andwestern Arizona, in Cordilleran metamorphic corecomplexes: Geological Society of America Memoir153, p. 79-129.
YDickey, D. D., Carr, H. J., and Bull, H. B., 1980,Geologic map of the Parker ffiv, Parker, and partsof the Hhipple Hountains SH and Hhipple Hashquadrangles, California and Arizona: U. S.Geological Survey Hiscellaneous InvestigationsSeries Map 1-1124.
Eric, J. H., 1948, Tabulation of copper deposits inCalifornia, in Copper in California: CaliforniaDivision of Mines Bulletin 144, Part 3,
p. 199-429.Fleury, Bruce, 1961, The geology, origin, and eco
nomics of manganese at Roads End, California:unpublished M. A. Thesis, University of SouthernCalifornia, 96 p.
Frost, E. G., 1980. Appraisal design data, Structuralgeology and water-holding capability of rocks inthe l/hipple Hash area, San Bernardino County,California: Hhipple Hash pump storage project:United States Hater and POlver Resources Service,P. O. 0-0l-30-D71l0, 88 p.
Graeff, F. H., 1910, Nitrate deposits of southernCalifornia: Engineering and llining Journal,v. 90, p. 173.
Heidrick, Tom L., 1980, Examination of Copper Basinmineralization and alteration, in Mylonitization,detachment faulting, and associated mineralization, Hhipple Mountains, California, and BuckskinNountains, Arizona: Arizona Geological Society1980 Spring Field Trip Guide, p. 19-21.
Jones, E. L., Jr., 1920, Deposits of manganese orein southeastern California: U. S. GeologicalSurvey Bulletin 710-E, p. 185-208.
Kuniyoshi, S" and Freeman, T., 1974, Potassiumargon ages of Tertiary rocks from the easternHo jave Desert: Geological Society of AmericaAbstracts with Programs, v. 6, no. 3, p. 204.
Noble, L. F., 1931, Nitrate deposits in southeasternCalifornia: U. S. Geological Survey Bulletin820, 108 p.
Park, C. F., Jr., and llacDiarmid, R. A., 1970, Oredeposits: 2d ed., II. H. Freeman and Company, SanFrancisco, 522 p.
Root, H. A., 1909, llining in western Arizona andeastern California: The llining Ilorld, v. 30, no.20, p. 929-932.
Schuiling, II. T., 1978, The environment of formationof Cu+Ag-bearing calcite veins, SacramentoMountains, California: unpublished 11. S. Thesis,University of Arizona, 60 p.
Stevens, H. J., 1911, The Copper Handbook, v. 10:H. J. Stevens, Houghton, Michigan, 1902 p.
Stone, Paul, and Howard, K. A., 1979, Compilationof geologic mapping in the Needles 1° X 2°sheet, California and Arizona: U. S. GeologicalSurvey Open-file Report 79-388.
Trask, P. D., 1950, Geologic description of the manganese deposits of California: California Division of Nines Bulletin 152, 378 p.
'----~I~la--n-g~n:~~s~:~o~~t:·~fa~~l~~:~~~~~·aS~~m~:;~'report, in Nanganese in California: CaliforniaDivisionof llines Bulletin 125, p. 51-215.
Trengove, R. R., 1960, Reconnaissance of Californiamanganese deposits: U. S. Bureau of Mines Reportof Investigations 5579, 46 p.
Tucker, H. B., 1921, Hines and mineral resources, LosAngeles Field Division, Part I, in Report XVII ofthe State Mineralogist - llining in Californiaduring 1920: California State Mining Bureau,p. 263-390.
~__~--_' and Sampson, R. J., 1930, San BernardinoCounty, Los Angeles Field Division, in Mining inCalifornia: Report XXVI of the Stat;/lineralogist, v. 26, no. 3, California Division of Nines,p. 243-325.
____~--_' and Sampson, R. J., 1931, San BernardinoCounty, Los Angeles Field Division, in llining inCalifornia: Report XXVII of the State llineralogist v. 27, no. 3, California Division of Mines,p. 243-406.
------;c-::-' and Sampson, R. J., 1943, Mineral resourcesof San Bernardino County, in Quarterly chapter ofState Hineralogis t' s ReportXXXIX: CaliforniaJournal of Nines and Geology, v. 39, no. 4,
75
p. 421-609.Turner, H. Y., 1907, The sodium nitrate deposits of
the Colorado: Mining and Science Press, v. 94,p. 634-635.
Vredenburgh, L. M., Shumlmy, G. L., and Hartill,R. D., 1981, Desert fever: An overviel, ofmining in the California Desert ConservationArea: Living Press Hest, Canoga Park,California, 323 p.
Hilson, P. D., 1918, American Eagle Mine: unpublished private report for the Eagle SulphideCopper Company, 11 p.
Hright, L. A., Stewart, R. M., Gay, T. E., Jr., andHazenbush, G. D., 1953, Mines and mineraldeposits of San Bernardino County, California,in California Division of Mines, v. 49, nos. 1and 2, p. 49-192.
TECTONIC IMPLICATIONS OF CENOZOIC VOLCANISMIN SOUTHEASTERN CALIFORNIA
Kent MurrayCalifornia Energy Co~~ission
1111 Howe AvenueSacramento, California
ABSTRACT
Volcanic rocks of early-to-middle Cenozoic agecrop out within a number of block-faulted mountainranges in southeasternmost California. Geologicmapping, and K-Ar age determinations of the volcanicrock within the region, have disclosed a major transition from calc-alkaline intermediate and silicicvolcanism to fundamentally basaltic volcanism. Thespace-time distribution of the various volcanic associations provide constraints on the plate interaction history along the west coast of North Americain early to middle Cenozoic time.
Volcanism of calc-alkaline intermediate andsilicic composition was initiated approximately33 Ma and probably reflects an early Cenozoic subduction system off the coast of western North America.The termination of subduction, inferred from theyoungest calc-alkaline silicic volcanic activity, occurred from 25 to 22 Ma. A southeastward timetransgressive transition to fundamentally basalticvolcanism is then recorded from 19 to 13 Ma acrosssoutheastern California. The age of the volcanictransition, in accord with previously published volcanic-tectonic models, overlaps with the probableage of inception of Basin and Range faulting, theprobable age of inception of the San Andreas faultsystem in southern California, and is consistentwith a southward migration of the Pacific-CocosRidge.
I NTRODUCTI ON
The rapid development of plate tectonics theorywithin the last decade has prompted renewed researchof the volcanic history and geophysical characteristics of the Basin and Range province (Atwater, 1970;Scholtz and others, 1971; Thompson and Burke, 1974).As a result of this research effort, the voluminousCenozoic volcanic rocks of the southern Basin andRange have been interpreted in terms of plate interaction between the North America, Pacific, and Farallon plates (for example, McKee, 1971; Lipman andothers, 1971, 1972; Christiansen and Lipman, 1972;Noble, 1972; Cross, 1973; Cross and Pilger, 1974,1979). In these studies, calc-alkaline volcanicrocks of broadly andesite composition are presumedto form from magmas generated along subduction zonesby partial melting of oceanic crust, and/or by melting of the adjacent upper mantle (Green and Ringwood,1968; Hatherton and Dickinson, 1969; Ringwood, 1977).By contrast, abundant basaltic and associated minorrhyolitic volcanism presumably reflects crustal extension which permits the rise of denser magmas generated by partial melting in the asthenosphere(Scholtz and others, 1971; Christiansen and Lipman,1972; Pilger and Henyey, 1979).
From studies of space-time distributions ofCenozoic magmatic rocks in the western United States,several investigators have noted patterns of change
77
in the record of igneous activity (for example, Armstrong and others, 1969; Armstrong and Higgins, 1973;Cross, 1973; Armstrong, 1974; Cross and Pilger, 1974;Silberman and others, 1975; Snyder and others, 1976).
If generation of calc-alkaline volcanism isgenetically related to subduction and basaltic magmatism to crustal extension, then the changes in igneous activity with time must reflect changes in plateinteractions. Based on recent detailed mapping andnewly obtained K-Ar age-determinations in the Chocolate, Palo Verde and Picacho Mountains of southeastern California (Crowe, 1973; Crowe and others, 1979;Murray and Crowe, 1976; Murray, 1979, 1980, 1981,1982, in press), this paper evaluates the timing andnature of the regional calc-alkaline to basalticvolcanic transition in southeastern California(Fig. 1) in terms of recent plate tectonic theory.This information is then used to test and refine detailed volcanic-tectonic models (Lipman and others,1971; Christiansen and Lipman, 1972) which have beendeveloped to interpret the complex space-timepatterns of Cenozoic igneous and tectonic activityin terms of plate configuration changes.
A fundamental part of the Lipman modelrelates extension within the continental plate andsubsequent basaltic volcanism to collision of theEast Pacific Rise with a continental margin trench.The resulting direct contact of the American andPacific plates progressively terminated subductionand replaced it with a north-south propagating rightlateral fault system.
A corollary of the model is that a time lagshould exist between the cessation of subduction andthe cessation of arc magmatism. Active arc magmatismpresumably reflects the presence of a descendingslab of subducted lithosphere directly beneath thesite of the arc (Lipman and others, 1971, 1972;McKee, 1971; Christiansen and Lipman, 1972). Wheresubduction is arrested at the surface, the cessationof arc magmatism will be deferred until the trailing edge of the detached descending slab has passedbeneath the arc. Similarly, if subduction is renewed, the inception of arc magmatism will reflect asimilar lag ti~e required for the leading edge ofthe descending slab to reach an appropriate depth.This lag time is a function of spreading rates, inclination of the subduction zone, and promimity ofthe descending slab to the spreading center (see forexample, Cross and Pilger, 1979; Pilger and Henyey,1977). Consequently, the lag time could range fromone to several million years.
IMPLICATIONS OF VOLCANISM
In the context of plate tectonics, there is afundamental difference between basaltic magmas, andandesitic to dacitic magmas because of their presumed
«zoN
"0
"."" O',y
" " l'co.
I
I. /'••"!1' :
USA· '. _.r--,f" ---~'., Y
~------MfXICO ~
itN
IMPERIAL
~) 10 t<ln11;;,=oml===1='
-------- ..--
Figure 1. Location map of the area of study showing the general distribution of volcanic rocks in southeasternCalifornia (Strippled area), modified from Jennings (1977). Af: Algodones fault; B: Blythe; C: ChuckwallaMountains; Cf: Chiriaco fault; If: Imperial fault; LC: Little Chuckwalla Mountains; MW: Milpital Wash.;M: Little Mule Mountains; NC: northern Chocolate Mountains; 0: Orocopia Mountains; PV: Palo Verde Mountains;SCf: Salton Creek fault; SAf: San Andreas fault; SHf: Sand Hills fault; SC: southern Chocolate Mountains;Y: Yuma, Arizona.
origins; however, the orIgIn of high-silica rhyolite,primarily ignimbrites, is less certain (Cross andPilger, 1979). As noted previously, calc-alkalinemagmas of intermediate composition generally are considered to be generated by partial melting of oceaniclithosphere or asthenosphere along subduction zones.Scholtz and others (1977) suggested that, owing tolow fluid pressures and high density, basaltic magmas are less capable of penetrating the lithosphereunder compressional conditions. However, under extensional regimes, basaltic magmas rise more easilythrough the normally faulted and extended lithosphere.Reduction of pressure by extension also may cause increased partial melting and increased volume of magma (Cross and Pilger, 1979). Christiansen and Lipman (1972) argued that abundant late Cenozoic basaltic volcanism in the Basin and Range province wasclosely associated with rhyolitic but not andesiticor dacitic volcanism producing a "bimodal suite".Recent map plots of isotopic ages in the western
United States however, reveal that the distributionof rhyolite more closely resembles that of the intermediate-composition volcanic rocks than thatof basalt (Cross and Pilger, 1979). Further, fieldmapping and K-Ar ages of high-silica rhyolites andintermediate-composition volcanic rocks in southeastern California confirm the overlap in age andstratigraphy of the intermediate and silicic composition rock units and clearly distinguish themfrom the patterns defined by the hasaltic rocks.
Volcanic fields in southeastern California(Fig. 1) show fundamental changes in configurationand position through time (Fig. 2 A-C). Cenozoiccalc-alkaline volcanism in southeastern Californiabegan in mid-Oligocene time (Fig. 2A) following apronounced period of early Cenozoic magmaticquiescence (Cross, 1973; Cross and Pilger, 1979).Space-time distribution of the youngest calc-alkalineactivity, obtained from the stratigraphically
78
N
1
Figure 2. Index map of southeastern Californiaadapte~ from Crowe and others, 1979; Outline of theMounta1n ranges follows Figure 1. A. Distributionof K-Ar ages documenting the age of inception ofcalc-alkaline volcanism in southectstern California.Ages shown are the oldest K-Ar ages for individualvolcanic centers; the true age of inception of volcanism is older by a small but unknown amount. 8.Distribution of the youngest K-Ar ages for the silicic volcanic sequence of Crowe and others (1979) andMurray (1980; 1981). These ages represent the cessation of intermediate and silicic volcanic activity1n southeastern California. They correspond to butare,slightly younger than the probable age of terminat10n of subduction in coastal southern California.C. Di~tribution of K-Ar ages for the oldest cappingbasalt1c sequences following the transition.
highest ignimbrite activity in southeasternC~lifor~ia, reflects the termination of arc magmat1sm (F1g. 28). Due to the inferred lag time, however, between cessation of subduction and cessationof arc magmatism, the isotopic ages indicated on
Fig. 28 would be slightly younger than the cessationof subduction. Subduction off the west coast ofNorth America at the latitude of southeastern California, therefore was probably terminated 22 to 25Ma. Figure 2C indicates the late Cenozoic historyof basaltic volcanism in southeastern California.These dates relfect the oldest basaltic volcanismin southeastern California following the transitionin volcanic associations.
Plate Interactions Along the Western United StatesInferred from the History of Igneous Activity
It is apparent that the patterns of volcanic andtectonic activity in southeastern California in theearly to middle Cenozoic should not merely reflectthe interaction between the Pacific North America andparallel plates but can be used to constrain theirmovements, and refine tectonic models based on seafloor spreading data alone.
Early to middle Cenozoic intermediate and silicic volcanism commenced in southeastern Californiaabout 33 Ma following pronounced magmatic hiatusthat may mark the presence of a proto from Andreasfault system in coastal California (Crowe and others1979; Cross and Pi 1ger, 1979). The occurrence of 'intermediate and silicic volcanism in the southernBasin and Range of California suggests that a shallowing dipping subduction zone was in place off thewest coast of California at this time. This data forf~r example, supports Coney's (1972, 1916) assertlon that the Farallon, North America relativeplate convergence was perpendicular to the trendof the continental margin opposite southern California,and Arizona, between 40 and approximately 30Ma. P11ger and Henyey (1977, 1979) incorporatingAtwater and Molnar (1973) work, presented modifiedplate reconstruction from 38 Ma to the present andcompared these with an early Cenozoic palinspasticrestoration of the western United States, includingeast-west Basin and Range extension (their Fig. 4,pg. 198). They concluded that the initial contactof the Pacific plate with North America occurredopposite the southern Coast Ranges, the TransverseRanges and the northern Continental Borderland between 29 and 21 Ma., or slightly later and farthernorth than the first contact predicted by Atwater andr~o 1nar (1973).
, ,The distribution of isotopic ages as depicted1n F1gure 2B and 2C therefore clearly indicates amiddle to late Cenozoic age for the transition ofarc magmatism typified by calc-alkaline volcanism toan extensional environment typified by fundamentalbasaltic volcanism. Moreover, although Fig. 2C isbased on limited K-Ar age determinations, it suggeststhat the transition was time-transgressive and migrated progressively southeastward, into southeasternCalifornia.
Plate interaction history subsequent to Atwater(1970) and Atwater and Molnar (1973) have adequatelydescri~e~ plate interaction history subsequent tothe ln1t1al contact of the two plates. Two triplejunctions, separated by the San Andreas fault transform was created, and the transform lengthened ast~e triple junction continued to separate. Subduct10n of remnants of the Farallon plate terminatedpro~r~ssively northward and southward of the pointof 1n1tlal contact, corresponding to the northwardmigration of the Mendocino fracture zone and thesouthward migration of the Pacific-Cocos Ridge. Figure 3 is a schematic presentation of the history of
A
B
c
79
this plate interaction history (following Atwater,1970) based on the work of Pilger and Henyey (1977,1979). The north to south trend of cessation ofintermediate and silicic volcanism of southeasternCalifornia (Fig. 2B) correspond to the historypredicted by these modern plate reconstructions(assuming a small time lag) which describe a progressive southward termination of subduction, corresponding to the southward migration of the PacificCocos Ridge (Figure 3 C-A).
Whereas middle and late Cenozoic intermediateand silicic volcanism in the southern Basin and Rangeprovince is apparently related directly to post-40Ma interaction of the North America-Farallon platesand Pacific plate, post-20 Ma basaltic volcanism isapparently related to crustal extension. Whethercrustal extension however, is caused by back arcspreading due to the release of compressive stressfollowing the termination of subduction (Scholtzand others, 1971), or by a slowdown in the absoluteplate motion of North America due to interaction withthe Farallon plate (Cross and Pilger, 1979), is un-
Figure 3. Schematic model of plate interactions assuming the constant motion model of Atwater (1970).The coast is approximated as parallel to the SanAndreas fault as in A~wa:er (1970) figure 5. Initials represent cities: S, Seattle; SF, San Francisco; LA, Los Angeles; G, Guaymas; M, Mazatlan.Symbols follow McKenzie and Parker (1967): singlelines are transform faults, double lines are spreading centers, and hatched lines are zones of subduction (usually trenches). Large arrows show motionof plates with respect to the pacific plate beingheld fixed. Captions show the time represented byeach sketch in million of years before present andthe distance that the North American plate must subsequently be displaced to reach its present positionwith respect to the Pacific plate (adopted from Atwater, 1970). Termination of subduction, begin-ning 29 to 21 Ma (Figure D) eventually lead tocessation of arc volcanism back-arc extension and ultimately basaltic volcanism.
uncertain. What is certain, is that the primarilypost-20 Ma basaltic volcanism in the southern Basinand Range province of California is chemically transitional between alkalic and tholeitic associations,typical of the capping basalts of the Great Basinin Nevada (Lesman and Rodgers, 1970; Best and Brimhall, 1974; Vitaliano, 197D). Moreover, experimentalwork (Yoder, 1976), petrographic and chemical studies (Murray, 1981), and geophysical studies (Royand others, 1968; Scholtz and others, 1971) all indicate that the basaltic magma must have risen rapidly to the surface and was extruded with a minimumof differentiation. Due to its low fluid pressure,and high density, these magmas are more capable ofpenetrating the lithosphere only under extensionalregions. The timing, therefore of the transitionfrom intermediate to silicic volcanic activityto fundamentally basaltic associations, as suggestedby the early Lipman model, also constrains the timing of Basin and Range faulting.
Relation of Basin and Range Faulting to NorthAmerican-Farallon Plate Interaction
Formation of the southern Basin and Range province during the late Cenozoic may also be relatedto North American-Farallon plate interactionsas implied above. Atwater (1970) suggested thatextension within the Basin and Range province as awhole, was a result of broadly distributed, rightlateral shear stress originating by the Pacific-North American plate interaction along the San Andreas fault. Scholtz and others (1971), presentedan alternative explanation for Basin and Range tectonism, hypothesizing that the termination of subduction removed the compressional forces actingacross the Cordillera, allowing back-arc extension.In the detailed tectonic model of Christiansen andLipman (1972) the authors adopted Atwater's hypothesis as a genetic relationship between the Basinand Range tectonism, and the northward lengtheningof the San Andreas transform. Similarly, Snyder andothers, 1976, interpreted the Basin and Range torepresent an ensialic interarc basin, accepting therelationship of crustal extension to the lengtheningof the San Andreas transform. In the majority ofthose models, the authors also relate the onset ofbasaltic volcanism with crustal extension. As discussed above, a direct relationship is implied between the termination of subduction, the initiationof strike-slip faulting, Basin and Range extensionand basaltic volcanism. If this implication is valid, then a close temporal and spatial correspondenceshould exist between the various volcanic and structural features manifested by the changing of tectonic regimes. There appears to be well-documentedevidence relating the timing and spatial associationof the onset of basaltic volcanism to Basin and Rangefaulting and northward migration of the Mendocinotriple junction. In the northern Basin and Range,McKee (1971) and Noble (1972) both recognized thatthe onset of Basin and Range faulting was closelyfollowed by the widespread inception of basaltic vol- .canism. Similarly, McKee and Silberman (1975) emphasize a correspondence of major extension and basaltic volcanism at 16 to 17 Ma., throughout muchof the Great Basin at that time. Snyder and others(1976) adopted the Christiansen and Lipman model andarmed with a wealth of new K-Ar age determinations,closely documented the correspondence in the timingof the onset of basaltic volcanism within the GreatBasin with the generally northward migration of theMendocino triple-junction, and concluded that the relationship was too close to be ignored.
~'#I-I.>S~S_F-:Lo=A=-_G_~
I I North American PlateS SF LA G M ::: > constant
1111111111111111111111111111111 mollon
,,~~arallon.Plate"'7L motion
Pacilic Plate
E > 30MY
A PRESENT
D 29-21MY
B 10 MY
C 25-22 MY
80
Direct application of Christiansen and Lipman's1972 model to the southern Basin and Range provincehowever, has led to many disparities in both the timing and spatial association of Basin and Range tectonism and the timing of the onset of basaltic volcanism (Cross and Pilger, 1979; Crowe and others,1979) .
As summarized by Cross and Pilger (1979) rightlateral shear deformation occurs only along thewestern part of the Basin and Range province. Theeastern Basin and Range apparently is deforming byeast-west extension and associated left lateral intercontinental transform faulting. In addition,Basin and Range faults extend farther north than thelandward projection of the Mendocino fracture zone(the northern terminus of the San Andreas fault).Thus, Cross and Pilger suggest that there is no obvious correspondence between contemporary Basin andRange deformation and deformation along the SanAndreas fault. In addition, the Lipman and Christiansen (1972) model based on plate reconstructionsof Atwater (1970), predicts extensional faulting,marked by the transition to fundamentally basalticvolcanism to migrate northwestward into southeasternCa 1iforni a at 26 to 24 Ma. (correspondi ng to thenorthward movement of the Mendocino triple junctionat this time and initiation of Basin and Rangefaulting). Extensional faulting, however, appearsto take place either much earlier (pre-32 Ma.;Crowe and others, 1979) or much later (21-17 Ma.;Murray, 1980, 1981, 1982). Further, the age oftransition to fundamentally basaltic volcanism asdetermined throughout southeastern California (Croweand others, 1979) is clearly much younger varyingfrom 22 to 13 Ma (Fig. 2C). Moreover, as indicatedin Fig. 2C, the transition to fundamentally basalticvolcanism in southeastern California is actually ina southeasterly direction, indicated by the southeastward younging in the ages of the capping volcanicsequence (Fig. 2C).
DISCUSSION
The age of transition to fundamentall basalticvolcanism in southeastern California overlaps withthe probably mid-Miocene age of inception of movementon the San Andreas fault south of the TransverseRanges (Crowell, 1973; Ehlig and others, 1975)and, this is in accord with the general concepts ofthe Lipman model. The Lipman model however was basedon limited sea floor'spreading data and little detailed field mapping, particularly in the southern Basinand Range province of California. Recent mapping ofCenozoic volcanic complexes in southeastern California (Crowe, 1973; Murray and Crowe, 1976; Croweand others, 1979; Murray, 1979, 1980, 1981, 1982 inpress) and more work involving modern plate reconstruction theory (Cross and Pilger, 1979; Pilger andHenyey, 1977, 1979) can be used to refine the Lipmanmodel while still supporting its basic tenets. As anexample, field mapping in the Palo Verde Mountains,Black Hills, and Little Mule Mountains of southeastern California (Murray, 1980, 1981, 1982 in press)have established a close spatial and temporal relationship between termination of intermediate andsilicic composition volcansim, the initiation ofBasin and Range faulting, and the subsequent onsetof basaltic volcanism. These relationships suggestthat Basin and Range faulting was initiated atapproximately 17 Ma. Reconnaissance mapping throughout southeastern California tend to support theserelationships on a regional level. There thusappears to be adequate field evidence to suggest that
81
a close spatial and temporal relationship does existbetween the termination of subduction, initiation ofstrike-slip faulting, Basin and Range extension andbasaltic volcanism.
SUMMARY
An early hypothesis proposed by Christiansen andLipman (1972), suggesting a genetic relationshipbetween termination of middle Cenozoic subduction,formation of the San Andreas fault, inception ofBasin and Range faulting and subsequent onset offundamentally basaltic volcanism still appears tobe valid in southeasternmost California if constraints can be imposed on the timing and locationof the initial interaction between the Farallon andNorth American plates. Recent detailed mapping ofCenozoic volcanic complexes in southeasternCalifornia in concert with plate reconstructions,recent sea-floor spreading data and a palinspasticrestoration of the western United States provides thenecessary constraints to refine the Lipman model. Therefined model results in an initial collision of theFarallon and North American plates occurring oppositethe Transverse Ranges between 29 and 21 Ma. This collision yielded the formation of two triple-pointjunctions, one moving northwestward, the Mendocinofracture zone; the other moving generally southwardcorresponding to the Pacific-Cocos Ridge. It is thesouthward migration of the Pacific-Cocos Ridge andcorresponding progressive southward termination ofarc magmatism that is apparently documented by thechanging volcanic terranes of southeastern California.
REFERENCES CITED
Armstrong, R. L., 1974, Magmatism, orogenic timing,and orogenic diachronism in the Cordillera fromMexico to Canada: Nature, v. 247, p. 348-351.
Armstrong, R. L., Eckel, E. B., McKee, E. H., andNoble, D. C., 1969, Space-time relations ofCenozoic silicic volcanism in the Great Basinof the western United States: Am. Jour. Sci.,v. 267, p. 478-490.
Armstrong, R. L., and Higgins, R. E., 1973, K-Ardatin9 of the beginning of tertiary volcanismin the Mojave Desert, California: Geol. Soc.America Bull., v. 84, p. 1095-1100.
Atwater, T., 1970, Implication of plate tectonicsfor the Cenozoic tectonic evolution of westernNorth America: Geol. Soc. America Bull.,v. 81, p. 3513-3533.
Atwater, T., and Molnar, P., 1973, Relative motionof the Pacific and North America plates deducedfrom sea-floor spreading in the Atlantic, Indian,and South Pacific Oceans, in Kovach, R. L., andNur, A., eds., Proceedings of the Conferenceon Tectonic Problems of the San Andreas faultsystem: Stanford University Pubs. Geol. Sci.,v. 13, p. 136-148.
Best, M. G., and Brimhall, W. H., 1974, Late Cenozoicalkali basalt magmas in the western ColoradoPlateaus and the Basin Range, Transition Zone,U.S.A., and their bearing on mantle dynamics:Geol. Soc. America Bull., v. 85, p. 1677-1690.
Christiansen, R. L., and Lipman, P. E., 1972, Cenozoic volcanism and plate tectonic evolution ofwestern United States, Part 2, Late Cenozoic:
Royal Soc. London Philos. Trans., Ser. A,v. 271, p. 249-284.
Coney, P. J., 1972, Cordilleran tectonics North American plate motion: Am. Jour. Sci., v. 272,p. 603-628.
----, 1976, Plate tectonics and the Laramide Orogeny:New Mexico Geol. Soc. Spec. Pub. 6, p. 5-10.
Cross, T. A., 1973, Implications of igneous activityfor the early Cenozoic tectonic evolution ofwestern United States: Geol. Soc. America Abs.with Programs, v. 6, p. 587.
Cross, T. A., and Pilger, R. H. Jr., 1974, Space/timedistribution of late Cenozoic igneous activityin the western United States, Geol. Soc. AmericaAbs. with Programs, v. 6, p. 702.
-----, 1979, Origin of Basin-Range extension: Backarc spreading controlled by North American platemotion: Geol. Soc. America Abs. with Programs,v. 11, p. 74.
Crowe, B. M., 1973a, Tertairy volcanism east of theSan Andreas fault, southeastern California:Stanford Univ. Publ. v. XIII, Proceedings ofthe conference on tectonic problems of the SanAndreas fault system, Kovach, R. L., and Nur, A.,eds., p. 334-338.
Crowe, B. M., 1973b, Cenozoic volcanic geology ofthe southeasternmost Chocolate Mountains,California (Ph.D. thesis): University ofCalifornia, Santa Barbara, 117 p.
----, 1975 a, Probable age of inception of basinrange faulting in the southeasternmost ChocolateMountains, California: Geol. Soc. AmericaBull., v. 89, p. 259-264.
Crowell, J. C., 1973, Problems concering the SanAndreas fault system in southern California,in Kovach, R. L., and Nur, A., eds., Proceedings of the Conference on Tectonic Problemsof the San Andreas Fault System: StanfordUniversity, Pub. 13, p. 125-235.
Damon, P. E., and Mauger, R. L., 1966, Eperiogenyorogeny viewed from the Basin and Range Province: Soc. Mining Eng. Trans., v. 235,p. 99-112.
Ehlig, E. B., Ehlert, K. W., and Crowe, B. M., 1975,Offset of the Upper Miocene Caliente and MintCanyon formations along the San Andreasfault in southern California: California Div.Mines Spec. Rept. 188, p. 83-92.
Elston, W. E., and Damon, P. E., Coney, P. J.,Rhodes, R. C., Smith, E. I., and Bikerman, M.,1973, Tertiary volcanic rocks, Mogollon-Datrilprovince, New Mexico and surrounding region:Geol. Soc. America Bull., v. 84, p. 973-982.
Green T. H., and Ringwood, A. E., 1958, Genesis ofthe calc-alkaline igneous rock suite: Contr.Mineraology Petrology, v. 18, p. 105-162.
Hatherton, T., Dickinson, W. R., 1969, The relationship between andesite volcanism and seismicityin Indonesia, the lesser Antilles, and other island areas: Jour. Geophys. Research, v. 74,
82
p. 5031-5310.
Jennings, C. W., 1977, Geologic map of California:Calif. Div of Mines and Geology, Californiageologic map series.
Leeman, W. P., and Rogers, J. J. W., 1970, Late Cenozoic alkali-olivine basalts of the Basin-Rangeprovince: Contr. Mineralogy and Petrology,v. 35, p. 1-24.
Lipman, P. W., Prostka, H. J., and Christiansen,R. L. 1971, Evolving subduction zones in thewestern United States, as interpreted fromigneous rocks: Science, v. 174, p. 821.
----, 1972, Cenozoic volcanism and plate tectonicevolution of western United States, Part 1;early and middle Cenozoic: Royal Soc. LondonPhilos. Trans., Ser. A, v. 271, p. 217-248.
Mcl(ee, E. H., 1971, Tertiary igneous chronology ofthe Great Basin of western United Statesimplication of tectonic models: Geol. Soc.America Bull., v. 82, p. 3497.
McKee, E. H., and Silberman, M. L., 1971, Geochronology of the Great Basin of the western UnitedStates-implications of tectonic models: Geol.Soc. America Bull., v. 82, p. 2317-2328.
Murray, K. S., 1979, Cenozoic geology of the PaloVerde Mountain Volcanic Field: Geol. Soc.America Abs. with Programs, v. 12, p. 489.
----. 1980, Implications of Cenozoic volcanism in thePalo Verde Mountain volcanic field, southeasternCalifornia: South Coast Geological societyGuidebook, p. 256-259.
----, 1981, Tectonic implication of space-timepatterns of Cenozoic volcanism in the Palo VerdeMountain Volcanic Field, southeastern California(Ph.D. thesis): University of California,Davis, 132 p.
----,1982, in press, K-Ar Age Bracket on the Initiation of Basin and Range faulting in southeasternCalifornia: Ischron West.
Murray, K. S., and Crowe, B. M., 1976, Geology ofthe northern and central Chocolate Mountains,southeastern California: Geol. Soc. AmericaAbs. with Programs, v. 8, p. 1023-1024.
Noble, D. C., 1972, Some observations on the Cenozoic volcano-tectonic evolution of the GreatBasin, western United States: Earth andPlanetary Sci. Letters, v. 17. p. 142-150.
Pilger, R. J. Jr., and Henyey, T. L., 1977, Plate tectonic reconstructions and the Neogene tectonicand volcanic evolution of the California Borderland the Coast Ranges: Geol. Soc. America Abs.with Programs, v. 9, p. 482.
----, 1979, Pacific-North-American plate interactionand Neogene volcanism in coastal California:Tectonphysics, v. 57, p. 189-209.
Ringwood, A. E" 1977, Petrogenesis in island arcsystems in Talwani, Manik, W. C. III, eds.,Island Deep Sea Trenches and Back Arc Basins:Am. Geophys. Union, Maurice Ewing Ser. 1, p.
311-331.
Rotstein, Y., 1974, Geophysical investigations insoutheastern Mojave Desert, California (Ph.D.thesis), University of California, Riverside,California, 175 p.
Roy, R. F., Blackwell, D. D., and Birch, F., 1968,Heat generation of plutonic rocks and continental heat flow provinces; Earth and PlanetarySci. Letters, v. 5, p. 1-12.
Scholtz, C. H., Barazangi, M., and Sbar, M. L., 1971,Late Cenozoic evolution of the Great Basin,western United States, as an ensialic interarcbasin: Geol. Soc. America Bull., v. 82, p.2979-2990.
Sheridan, J. F., Stuckless, J. S., and Fodor, R. B.,1970, A Tertairy silicic cauldron complex at thenorthern margin of the Basin and Range province,central Arizona. U.S.A.: Bull. Volcan.,v. 34, p. 649-662.
"'-Silberman, M. L., Noble, D. C., and Bonham, H. F.,
1975, Ages and tectonic implications of thetransitions of calc-alkaline andesite to basaltic volcanism in the Great Basin and SierraNevada: Geol. Soc. America Abs. with Programs, v. 7, p. 375.
Snyder, W. S., Dickinson, S. R., and Silberman, M. L.,1976, Tectonic implications of space-timepatterns of Cenozoic magmatism in th~ westernUnited States: Earth and Planetary Sci. Letters,v. 32, p. 91.
Thompson, G. A., and Burke, D. B., 1974, Regionalgeophysics of the Basin and Range province: Ann.Rev. Earth and Planetary Sci. Letters, v. 2, p.213-238.
Vitaliano. C. J., 1970, Capping volcanic rocks: westcentral Nevada: Bull. Volcani. v. 34, No.2,p. 617-635.
Yoder, H. S. Jr., 1976, Generation of Basaltic magma:Washington. D. C., National Academy of Sciences,265 p.
83
PRELIMINARY REPORT ON DIKING EVENTS IN THE MOHAVE MOUNTAINS, ARIZONA
JOHN K. NAKATA
U.S. Geological Survey345 Middlefield Road
Menlo Park, California 94025
INTRODUCTION Young gabbro and andesitic dikes, class 1;
Swarms of per-Tertiary and Tertiary dikes intrudePrecambrian crystalline rocks in the Mohave Mountains,western Arizona. This range lies in a terrane ofOligocene(?) and Miocene detachment faulting describedby Davis et al. (1980), Carr (1981), and Rehrig(1981). The dike swarm records repeated magmatism andapparent northeast-southwest crustal distension. Thispaper briefly summarizes the characteristics andorientation of the dikes.
Six different diking events are currentlyrecognized in the Mohave Mountains Fig.1 ).Radiometric ages for the dikes are not currentlyavailable, but ages can be inferred from similar dikesin adjacent areas. Crosscutting relations establishrelative ages, and dikes are assigned numbers thatincrease with age (Fig. 2). The most abundant aredikes of Tertiary age that represent four intrusiveevents. The pre-Tertiary dikes include Cretaceous(?)grani te intrusions and pre-Cretaceous (Precambrian?)dikes. The following descriptions of dikecharacteristics are based primarily on fieldobservations and are summarized in Table 1.
1 14° 22.5
00'
FIGURE 1. INDEX TO QUADRANGLE MAPS OF THE
MOHAVE MOUNTAINS AREA. THE SHADED AREA
REPRESENTS LIMITS OF MAPPING.
85
The Class dikes appear to be basal tic(gabbroic) and andesitic in composition. Prominentexposures of the gabbro occur in the northeasternportion of the area mapped, where the dikes trend N.20 0 -30 0 W., dip vertically, and truncate both theleucocratic suite 4 and the young mafic dikes 3. Inthis area they are medium-grained, hypidiomorphicgranular mela-gabbros. Fresh surfaces are darkgrayish green but weather to dark brown. The andesiteand andesite porphyries are grayish green on freshsurfaces and weather to light brown to pale orange andoccur on the western side of the range. Both of theserock types truncate the young mafic dikes,3, but theirages relative to each other and to the trachytictextured dikes, 2, have not been established.
Trachytic-textured dikes, class 2;
The traychytic-textured dikes contain prominentplagioclase phenocrysts 2 to 12 rom in length in agrayish andesitic (?) matrix (these may be related tothe "jackstraw porphyry" described by Pike and Hansenthis volume). The most prominent exposures are
southeast of the intersection of the V-shaped fault,where the dikes trend southeastward and truncate largeold ophitic diabase dikes. Elsewhere, they also cutthe young mafic dikes, 3.
Young mafic dikes, class 3;
The young mafic dikes are most abundant east ofthe V-shaped bifurcating fault; they occupy a zone
approximately 7 km wide trending N. 65 0 _88 0 W. and dipmoderately to steeply to the east. They are gray toblack on fresh surfaces but weather to a dark brown.These weathered surfaces are often pockmarked owing todifferentially weathered phenocrysts. The dikes rangein width from 0.3 to 10m; the average width is 2.3m. Dikes less than 0.5 m wide are mostly finegrained. Those of greater width are commonlysymmetrically zoned and have chilled margins, anintermediate porphyritic zone, and a medium-grainedcore •
Leucocratic suite, class 4;
This suite includes a variety of dike types thatcut the old ophitic diabase and are in turn cut by theyoung mafic dikes. Dikes of this suite that cutMiocene volcanic rocks on the west and south sides ofthe Mohave Mountains establish a maximum age. Therocks are aplite, rhyolite, dacite and andesite(?)with textures ranging from hypidiomorphic granular toaphanitic. The granular outcrops weather to knoblikeforms typical of desert areas, while the fine-grainedequivalents form spires. They are cream to light grayon fresh surfaces but weather to pale orange and allshades of brown. Dark-gray chill zones are common andgrade inward toward a light-colored central zone.
N
o
o-<\
+34° 32'
1140
17' 30"
I---~I 1 Kilometer
I-----~I 1 Mile
ALLUVIAL COVER
FIGURE 2. DISTRIBUTION OF DIKES IN THE MOHAVE MOUNTAINS, ARIZONA
86
35' 00" +114
005'
Blocks of
FAULTS
BLOCKS
apparently rotated
EXPLANATION
dikes
87
DIKES
o Young gabbroo and andesitic
~ Trachytic tA - extured dikes
Lib Voung mafic .!3F4'] dIkes, .. l2.; Leucocratic/).. suite dikes
~ Granitic dikes
® Old ophitic .dIabase dikes
dikes
00' I
coCO
TABLE 1. SUMMARY OF DIKE CHARACTERISTICS
(J) (f) W--l
" <0 <!J0 (..) OUTCROP CHARACTERISTICS W(f) zlD U- 0 za: « z::;; 0 a: "W ~
0>- (..)1- (f)(f)
FIELD NAME RELATIVE AGE CRITERIA -w ATTITUDES zW a: i=::;; z<!J « « W
a.. I-« « ..J w::: W (f)::;; RELIEF COLOR ::;; 0::;;
" "(J) Cl IX
Bot h cut I he young mafic & dark gray: weathers Gabbro N. 30 0 WYOUNG DIKES Gabbro: verticle dip
8 bul their rela t ive position
+10 dark brown
Gabbroic rocks with respect 10 class 0 Andesite: gray-green; (3- 5)A n Ii e sit eN. 4 os.. 5 0° W.
Andesitic roc l( s d ik es rema in unc Ie a r weathers 10 light moderate to steep
brown and pale-orange dips to the NE
®Gr a y; X ::; 3.5
N. 60~850W
TRAC HY TIC -T E X TURED Cut y Dung mafic ,& + moderate to steepweathers 10 brown (1- 9)dip s tot he NE
MIOCENE t 6.5 % IGTa y 10 b lac k;
N. 70~8cPW.
& X = 2.3Truncate all o Ide r dik es + moderate to steep
YOUNG MAFICweathers 10 dark b Town
(0.3- 1 0) dips 10 the NE
Cream 10 light-gray; N. 60~80oW.X.;:. 6
8]weathers pale-orange 10 moderate dips
LEUCOCRATIC SUITE cuI old ophit ie diabase ® + brown to the NE
( 1-20)
GJContains xenoliths of Light gra y; N. 20 0 W.
CRETACEOUS weathers 10 light-brown ( I-C)steep dips to the NE
IGRANITE old ophitic diabase +
X'" 3.7i
PRE-CRETACEOUS Olive-gray;N. 20°_40° W
\
®OLD Col by leucocratic ~uite
OPHITIC DIABASEPRECAMBRIP,N ? + weathers fo olive-brown high dips to the NE
end young diabase (1-20 )
These dikes are found throughout the Mohave Mountainsbut are most abundant on the west side of the range.They strike N. 50 0 _80 0 W. and dip predominantly to thenortheast, but their strike changes significantly toeast-west at the northern end of the range. The dikesrange in thickness from 1 to 20+ m and average 6 m inwidth. A dense swarm of these dikes in the southwestpart of the map area occupies as much as 90 percent ofthe rock; the densest patch is stippled on the map(Fig.2). This area was not included in the dike-widthcalculations.
Granitic dikes, class 5;
Granitic intrusions ar~ limited to the northernpart of the map area. The medium-grainedhypidiomorphic-granular rock consists of quartz,feldspar, biotite, and hornblende. These granitedikes connec t wi th a small stock (not shown in fig.2). The granite contains xenoliths of the old ophiticdiabase and the Precambrian gneisses that range indiameter from 10 to 30+ em. The granite is cut byboth class 4 and 3 dikes. Lithologic similarity tograni tes to the west in the Mojave Desert suggest aCretaceous age (John, 1981).
Old ophitic diabase, class 6;
Dikes that resemble pre-Cretaceous diabases inthe nearby Whipple Mountains (Davis et al.,1980) andthe Precambrian diabases of the Hualapai Mountains tothe northeast are the oldest in the MohaveMountains. These dikes are characterized by a mediumgrained ophitic and subophitic texture. Localresistant outcrops are patinated a dark purplishbrown, but typical exposures have negative relief andconsist of olive-brown surface float. These diabasesoccur as steeply and shallow dipping dikes and sills,however only the dikes are portrayed in Fig. 2. Theytrend approximately east-west in the eastern part ofthe range, curve northwestward in the west-centralportion, then bend around again to an east-west strikein the northwestern part of the area mapped. Theyhave an average thickness of 3.7 m (with a range of 1to 20 m) and dip at moderate to steep angles to theeast. Small remnants throughout the range suggestthat these dikes may have been more extensive thanrecognized but are obscured by the overprint ofTertiary dikes.
STRUCTURAL SUMMARY
The distribution of the dikes can be used toinfer certain significant structural events in theMohave Mountains. After the intrusion of the preCretaceous (Precambrian?) diabase, the dikes weredeformed into an S-shape. This warping does not seemto affect the Cretaceous(?) granitic intrusions, sothe deformation may be older. The granitic dikes wererecognized only in the northwestern Mohave Mountainswhere they bring up xenoliths of the older diabase andPrecambrian gneiss.
The oldest Tertiary diking event intrudedleucocratic rocks and distended the Mohave Mountainblock in a direction N. 100 -30 0 E. These werefollowed by the intrusions of the class 3,2, and 1dikes that have basically the same trend as theleucocratic suite. In a southwest to northeasttransect across the range, measurements showed thatthe Tertiary dikes occupy 16.5 percent of the area.This amount records NNE-SSW crustal distension of aleast 1.75 km.
89
All the faults portrayed in figure 2 cut the dikeswarm. If these faults are Miocene in age like othermajor faul ts in the region (Davis et al. , 1980;Carr, 1981), this establishes a Miocene age for thefour youngest dike swarms. The dikes are not onlyfaulted, but in places show anomalous trendssuggesting that they are part of rotated landslideblocks.
ACKNOWLEDGMENTS
I would like to express my appreciation to KeithHoward for his helpful comments.
REFERENCES
Carr, W. J., 1981, Tectonic history of the VidalParker region, California and Arizona, in Howard, K.A., Carr, M. D., and Miller, D. M., eds., Tectonicframework of the Mojave and Sonoran Deserts,California and Arizona: U.S. Geological Survey OpenFile Report 81-503, p.18-20.
Davis, G. A., Anderson, J. L., Frost, E. G., andShackelford, T. J. , 1980, Mylonitization anddetachment faulting in the Whipple-Buckskin-RawhideMountains terrane, southeastern California and westernArizona: Cordilleran metamorphic core complexes:Geological Society of America Memoir 153, p.79-129.
John, B. E., 1981, Reconnaissance study of Mesozoicplutonic rocks in the Mojave Desert region, in Howard,K. A., Carr, M. D., and Miller, D. M., eds., Tectonicframework of the Mojave and Sonoran Deserts,California and Arizona: U. S .Geological Survey OpenFile Report 81-503, p.48-50.
Rehrig, W.A., 1981, Principal tectonic effects of themid-Tertiary orogeny in the Sonaran Desert province,in Howard, K A., Carr, M. D., and Miller, D. M., eds.,Tectonic framework of the Mojave and Sonoran Deserts,California and Arizona: U. S. Geological Survey OpenFile Report 81-503, p.90-92.
COMPLEX TERTIARY STRATIGRAPHY AND STRUCTUREMOHAVE MOUNTAINS, ARIZONA: A PRELIMINARY REPORT
by
J.E.N. PikeU. S. Geological SurveyMenlo Park, CA 94025
ABSTRACT
Tertiary volcanic and sedimentary rocks of theMohave Mountains, Arizona are divisible into threesequences: an oldest (early Miocene) unit of mafic andsilicic flows, pyroclastic and sedimentary rocks, amiddle unit of similar rock types, but dominantlysedimentary rocks, and a youngest unit that consistsprincipally of fanglomerate and thin silicic andbasaltic flows. The middle and youngest unitsprobably are middle Miocene in age. Except for theuppermost layer of the oldest unit, individual stratahave little lateral continui ty, and lithologies varywith locality. The oldest unit is complicated furtherby repeated, sometimes voluminous intrusion. Theoldest unit lies unconformably on Precambrian gneissalong the western flank of the range. Where contactrela tions are exposed, the middle unit is separa tedfrom the overlying and underlying sequences by angularunconformi ties. In several localities the youngestunit overlies the oldest unit in marked angularunconformity.
Too few radiometric ages have been determinedfrom the Mohave Mountains to establish the timing oferuptive and tectonic events. A small number of K-Arages from nearby ranges and of two Mohave Mountainsrocks suggest that mafic volcanism and fault-induceddeformation occurred between 21 and 18 m.y. ago.Renewed faul ting, volcanism and rapid sedimentationoccurred following silicic intrusion into the oldestunit, possibly before deposition of the Peach SpringsTuff of Young and Brennan (1974) about 18 m.y. ago.Determination of the timing of these events in theMohave Mountains may aid understanding of low-angleextensional (II detachment") faul ts and relatedstructures in the Mohave and Whipple Mountains, andother nearby mountain ranges.
SETTING
The Mohave Mountains of western Arizona, anorthwest-trending range, is composed largely of preTertiary metamorphic and igneous rocks. Tertiaryvolcanic and volcaniclastic sedimentary rocks comprisea small part of the range. The Mohave Mountains areseparated from the Chemeheuvi and Whipple Mountains,both composed largely of similar pre-Tertiary rocks,by the Colorado River. To the north are extrusiveTertiary volcanic rocks that comprise the southern endof the Black Mountains, and to the northeast are theHualapai Mountains, an outlier of the ColoradoPlateau. A fault separates the Mohave Mountains fromthe Bill Williams Mountains to the southeast. Thisrange too is composed largely of pre-Tertiary gneiss,amphibolite, and granitic rocks.
and
91
V.L. HansenDepartment of GeologyUniversity of Montana
Missoula, MT 59801
STRATIGRAPHYGeneral Relationships
The Tertiary rocks of the Mohave Mountains,Arizona, include several tilted and faulted volcanicand sedimentary sequences, one or more intrusivecenters, and extensive dike swarms that intrude preTertiary and Tertiary rocks (Nakata, 1982, thisvolume). The deformed volcanic/sedimentary unitstrend southeast from The Needles at the Colorado Riverto the Bill Williams Mountains along the western flankof the range (Figure 1). Isolated fault-boundedslivers of Tertiary volcanic and sedimentary rocksoccur north of the gneiss and granite core of therange, but no Tertiary sequences have been found inthe interior of the range, or along its northeastflank.
Our preliminary work indicates that the Miocenevolcanic and sedimentary rocks are divisible intothree units (Figure 2): the oldest is steeply til tedand dips are commonly to the south or southwest. Itlies unconformably on Precambrian gneisses along thewestern and southwestern margin of the range, andconsists dominantly of interbedded mafic and siliciclava flows and tuffs, with subordina te sedimentaryunits. Mafic flows are more common in the lower partof this section and the top is marked in places by ablue-sanidine-bearing tuff. None of the rock types inthis unit has great lateral extent in the MohaveMountains. The middle unit (Figure 2) overlies theoldest sequence in angular unconformity. This unitnormally dips more steeply at the base than at the topof the sequence, although we have measured dips up to45 0 locally in the upper levels of the sequence. Likethe oldest unit, this unit consists of laterallydiscontinuous flows, tuffs, and sedimentary rocks;unlike the oldest unit sedimentary rocks aredominant. The youngest unit (Figure 2) overlies boththe oldest and middle units with marked angularunconformities. The contact of the youngest over theoldest unit is exposed in the areas of the Lake HavasuCity dump and Standard Wash; that of the youngest overthe middle unit is exposed south of Tumarion Peak, andalso in Standard Wash (Figure 1). The youngest unitcomprises alluvial fan materials derived dominantlyfrom the older volcanic-sedimentary rock sequences,and less commonly from Precambrian rocks; dips in thisunit range from 50 to 250 • Locally the fanglomerateis interleaved with and overlain by very gentlydipping silicic tuffs and flows, and capped by thinbasal t flows.
All three sequences can be identified along theentire western flank of the Mohave range, but nosingle lithologic member is present everywhere, sothere is no reliable marker for any of the uni ts.
pGu Precambrian r-oc~~s,undi vi ded
pGg Precambriangranitic F'ock5.
ROCK UNITS
Quaternary deposits
Tvs Tertiary volcanic andsedimem:ary un; ts
Tertiary 'i(l,~rusive
centers
pTu Pre-Tertiary rocks, l1r,div;cJEJ
Q
faults and contactsdashed where a,'proximate
1000
pou
SCALE (meters)
Cl'QBSman Peak
'it pou
157& n
\ ,
EXPLANATION
+ Antiel ine GEOLOGIC SYMBOLS "'- ContactspTu + Syncline '" Stri ke and di p of strata
TVS~ r Nanna 1 faul t - bar on downth rawn s i at:: "" Overturned s tra ta~ Low-angle fault ~
/~ \Vertical strata
<1:>- I Q Faul t ,)f unknm,n character ;;< Altitude location
fla p A,'ea
N
GFigure 1. Sketch map of locations and generalized geologic relationships, Mohave Mountains, Arizona
Lithologies of each rock type vary from northwest tosoutheast, as does the type of alteration. Thus,although the sequences are generally similar, rocks ateach 10ca11 ty have distinct characteristics.Intrusion and accompanying alteration of the sequencesfurther complicates the stratigraphic record.
Volcanic and Sedimentary RocksOldest unit
Elsewhere, rhyolitic flows and breccia form thelowermost part of this unit. Coarser-grained rocksapparently were derived entirely from granitic andmetamorphic rocks, or from both volcanic andmetamorphic/granitic materials. Clasts in the basalconglomerates invariably resemble the Precambriancrystalline rocks they unconformably overlie. Acommon red-weathered zone in the underlying gneiss,and red matrix in basal Tertiary sedimentary layers,suggest deep pre-Miocene weathering.
The oldest volcanic-sedimentary unit ranges from600 to 1200 m in thickness and comprises interbeddedmafic and silicic flows, silicic (and rarely mafic)tuffs and volcanic breccias, and sedimentary rocksthat range from arkosic sandstone and conglomerate torocks dominantly composed of reworked volcanicmaterials. A whole-rock K-Ar age of 21.1 m.y.(Conoco, unpublished data) has been derived from amafic flow near the base of this sequence. Dips onstrata in the oldest unit range from 45 0 to vertical,and locally are overturned. In general, dips aresteepest near the basal contact, although overturnedstrata are present locally near the top of thesequence.
In some localities the basal strata aresandstones, shales, and conglomerates that range fromseveral to several hundred meters thickness.
Other lithologies in the lower part of the oldestunit are arkosic and tuffaceous sedimentary rocks,boulder conglomerates and megabreccias, abundant maficlava flows, and interbedded platy silicic flows.Chemical analyses of these rock types are not yetavailable, but in the field the flows appear to beandesi te and platy dacite. All the rocks arecharacterized by abundant plagioclase and one or acombina tion of the mafic minerals clinopyroxen'e,hornblende, and biotite. Many of the mafic flows haveplaty plagioclase phenocrysts ("turkey-track" or" jackstraw" porphyry) . Finer-grained equival entscommonly have equant dark-green clinopyroxene, andequant spots of iron clay, probably altered fromolivine. Whether or not these volcanic rocksoriginally had calc-alkaline affinity, or were abasal t-rhyo11 te bimodal association, is difficult toassess because the rocks are extremely altered.
92
QUATER:-iARY DEPOS I TS
TERT JARY UNITS (~liucene)
Fang]\lmeralt2
Unl"llnfnrmity
Youngest Unit
Niddle Unit
{
[
basalt flows--rhyolite flows; welded tuff
~==="'-<----__I__ :::::::e::::;w:::C::::l:::C:::riXUnconformity
I- ,--quartz- and reworked tuff-sandstone
--purplish shale or claystone with thin1-------------; airfall tuff or reworked tuff interbeds
--conglomeratic sandstone
_ ----boulder conglomerate
L-==::::~:=2=====d-_-lahardepos its
Unconformity
Tuff of Peach Springs Unconformity?
Oldest Unit
airf3ll tuff withbasal t interbeds
welded tuff
silicic intrusive rocks of theintrusive centers
tuffaceous volcanic brecciaandesite (altered basalt?) and dacitic
flowsInterbedded airfall tuff and sandstone
_---- dacitic intrusions
andes He (basa 1 t?)and dacitic fJows
----- andesite (altered basalt?)intrusions
_C=:,-!__pyroxene-plagioclase porphyry with flowaligned phenocrysts
rhyolite fluw andvolcanic breccia
aphyric andesite(?)fl{lwS
__basal sedimentary units: conglomerate,arkose, tuffaceous sandstone
PRE-TERTIARY ROCKS
HESOZOJC
PRECAflBR IAN
Unconformity
eremite Plutons
Undifferentiated gneiss
figure 2. Diagrammatic geologic column of pre-Tertiary and Tertiary (Miocene) rock types
Very coarse boulder conglomerates, andmegabreccias are ubiquitous throughout the exposuresof the oldest unit. Sizes of clasts range fromseveral centimeters to 6 m; clasts of the largest sizecommonly are a distinctive unfoliated Precambrianpophyritic granite characterized by large, elongate,flow-aligned feldspars. Potential sources are rare inthe present Mohave Mountains, where this lithologycommonly has foliated structure or is augen gneiss,but the porphyritic granite crops out widely in theWhipple, (Davis and others, 1980), Bill Williams, andHualapai Mountains.
The uppermost part of the oldest unit clearlycomprises a basalt-rhyolite association: silicic (andsilicified) airfall tuffs interbedded with basaltflows and basaltic tuffs (and cinder cones), capped bya blue-sanidine and sphene-bearing welded ash-flowtuff, that we believe to be the Peach Springs Tuff of
93
Young and Brennan (1974). Inclusion of Peach SpringsTuff into the oldest unit is tentative; it mayunconformably overlie the tuff and basalt unit.
Unlike most of the other parts of the oldestvolcanic and sedimentary unit, the Peach Springs Tuffis a distinctive and persistent stratigraphic markerin the field. It occurs in the Mohave Mountains areaonly between The Needles and a major northeasttrending faul t north of the Lake Havasu City dump(provisionally named the "Castle Rock" faul t for easeof description, Figure 1), and in the Aubrey Hills.Samples of the Peach Springs Tuff from two localitieson the Colorado Plateau have been dated at 16.9 and18.3 m.y. (Damon and others, 1964; 1966), and asimilar-appearing unit 32 krn west of Mohave Mountains(Sawtooth Range, adjacent to the Chemehuevi Mountains)yields an age of 18.1 m.y. on sanidine (determined byR.F. Marvin; K.A. Howard, personal communication).
Carr and others (1980) reported whole-rock K-Ar agesfor basalt flows that overlie and underlie PeachSprings Tuff near Pyramid Butte, west of the WhippleMountains. The bracketing ages are 14.5 m. y. on theoverlying and 17 m.y. on the underlying flows.
Throughout the oldest unit, features such ascomplex brecciation and deformation of tuffaceoussedimentary rocks are associated with brecciated andhighly al tered mafic lavas. This associationindicates that the sediments were wet when the lavasoverrode them. Also, syndepositional intrusion withinthe oldest unit is indicated by dismemberment andpartial assimilation of sedimentary uni ts by mafic(andesi te or basal t) lavas. Where sedimentary unitswere assimilated by intrusions, coarser particles suchas conglomeratic layers rarely can be traced, onlyslightly deflected from the original strike direction,wi thin the engulfing red-al tered "andesi te" (al teredbasal t?) or daci te. Most of these intrusions areconformable, bu t some crosscutting mafic and daci ticdikes occur.
Middle unit
Basal strata of the middle unit are dominantlysedimentary, al though locally there are volcanicbreccias and lahars. The sequence of strata in themiddle unit is varied, like the oldest unit. Measuredthicknesses range from 300 m in an area near theColorado River where the Tertiary units are overlainby Qua ternary deposi ts, to 600 m adjacent to theCrossman Peak faul t. Basal sedimentary rocks arelargely derived from mafic volcanic rocks, and includeboulder conglomerate with blue-sanidine tufffragments. Clasts of granitic and metamorphic rocksalso occur, and increase in abundance higher in thesection. The middle unit contains probable landslidedeposits that reflect local source regions. In someareas these deposits are extensive ledges ofbrecciated Precambrian rocks, while in other areaslarge isolated lenses of Peach Springs Tuff areinterbedded with tuffaceous sandstone. Otherli thological members incl ude thin airfall tuffsinterbedded with red shale or claystone (probably lakebed deposits), and large thicknesses of poorlyconsolida ted tuffaceous sandstone layers. Dipsmeasured on this unit range from 12 0 to 45 0 locally.No ages have yet been determined on volcanic rocks ofthe middle unit.
Youngest unit
This unit comprises fanglomerate with adistinctive purplish (volcanic-derived) matrix. Thecontacts of the voungest unit with both the oldest andmiddle units ar'e inconsistently exposed. Near thecity dump the fanglomerate overlies south-dippingstrata of the oldest unit. North of Standard Wash theyoungest unit overlies northeast-dipping rocks of t?eoldest unit, but adjacent to Standard Wash, and Inrocks adjacent to the Crossman Peak fault, theyoungest unit bevels southwest-dipping rocks of themiddle unit; fragments of reworked middle-unitconglomerate occur in the lowest fanglomeratelayers. Dips in the fanglomerate vary from southHestto northeast locally. In both the city dump andStandard Wash areas fanglomerate apparently isinterleaved Hith or overlain by thin floHs. Near thecity dump these include rhyolite to rhyodacite flows,volcanic breccias, airfall tuff, welded ashflow tuff,and a thin cap of basalt (now represented only by lagboulders). The sequence is not as well-knoHn in thearea of Mohave Springs Mesa and Black Mountain, but
94
may be similar; there the basalt floHs are wellpreserved. We believe that this unit may becorrelated with the interbedded fanglomerates andflows of Osborne Wash, 35 km south of Standard Hash(Eberly and Stanley, 1978).
Intrusive Center
Silicic intrusion of the oldest unit dominates inseveral parts of the area north of the Lake HavasuCi ty dump (Figure 1). The mos t abundant intrusivebodies are dikes and irregular masses of latite(?) anddacite(?) porphyry, which are cut by rhyolite orrhyodacite dikes, and locally by thin mafic dikes. Inthe areas of most concentrated intrusion (patterned inFigure 1) the silicic intrusive bodies are pervasivelyaltered, as are the host rocks.
Abundant silicic dikes intrude the gneisses ofthe interior Mohave Mountains (leucocratic suite ofNakata, 1982, this volume), and are in turn intrudedby young mafic dikes. The area of densest intrusionof these dikes lies southeast of the Tertiaryintrusive center along strike. The dikes and intrusivebodies within the oldest unit may represent the sameintrusive event as the leucocratic suite.
Dating of the intrusions is in progress. An ageof 16.5 m.y. was reported by Martin and Frost (1981,unpublished 'data) for plagioclase from a latite thatapparently intrudes the older sequence near the"Castle Rock" fault. He have not seen intrusions inthe middle unit, but thin rhyolitic dikes intrudesilicic flows of the youngest unit near the city dump,where the middle unit is missing. He assume thatthese dikes were feeders for the flows in which theyare found.
STRUCTURE
Two major faults divide the Tertiary volcanic andsedimentary sequences of the Mohave Mountains intothree structural regions. The northern region ischaracterized by faulted folds, the central region byrock units that tilt dominantly to the south, andcontain abundant intrusions, and the southern regionby mul tiple repetitions of southwes t-dipping sectionacross normal faults (Figure 1). The "Castle Rock"faul t separates the northern and central structuralregions. It is unexposed except for a breccia zone atits southwest end (K.A. HOHard, personalcommunication). This fault strikes N65E, andapparently dips about 60 0 to the north but its truenature is unknoHn. The fault displaces a distinctivePrecambrian gneiss unit, as Hell as the overlyingoldest volcanic and sedimentary unit and intrusiveTertiary rocks, by about 3000 m along strike. Thesouthern faul t (Crossman Peak faul t of Howard andothers, 1982, this volume) dips gently southeast andseparates Precambrian gneisses of the central MohaveMountains from contrasting Precambrian granitic rocksthat are overlain by all three Tertiary volcanic andsedimentary units.
The northernmost structural region occupies thearea west and south of Tumarion Peak, and extendsnorthwest through The Needles, and into the ChemehueviMountains west of the Colorado River. At thenortheast boundary of this region, the oldest unituncornfomably overlies Precambrian basement with dipsin the basal strata that are vertical to overturned tothe northeast. Both the oldest and middle units aredeformed into a series of folds, dominantly synclineswi th northwes t- trending axes. The apparent
thicknesses of both units are greater in southwestdipping than in northeast-dipping synclinal limbs.Wavelengths of the folds are in the range of 1000-2000m, but the folds are broken by faults sub-parallel tothe strikes of the fold axes. The faults cut out theanticlines with only one exception; the thicknesses ofthe oldest and middle units exposed in this anticlineare considerably less than those in the southwestdipping limbs of the synclines.
Exposure of one fault surface together with thestratigraphic pattern of displacements suggest thatall the northwest-striking offsets may be northeastdipping normal faults. Outliers of the oldest unitnortheast of the folds probably are erosional remnantsof blocks tilted to the southwest and bounded bysimilar normal faults. Just south of Tumarion Peakthe faul ted folds are truncated by a low-angle faul tthat strikes northeast and dips southeast under theTertiary rocks. This fault can be traced northeastalong strike into pre-Tertiary gneisses and granites.
Southeast of the "Castle Rock" fault in thecentral structural region are steeply-tilted strata ofthe oldest unit that unconformably overlie quartzbioti te and granitic gneisses along the entire westflank of the range to the Crossman Peak faul t. Theunconformable contacts locally are faulted andintruded by dacite porphyry dikes. This sequence hereis the host for the silicic rocks of the intrusivecenter. The basal sediments and flows strikedominantly east - west and dips are very steep (to thesouth) to overturned at the basal contact; the middleunit is not exposed. Here the youngest unit dips veryshallowly southwest, overlapping both the steeplydipping oldest unit and intrusive bodies, but thefanglomerate apparently does not cont'3.in clastsderived from rocks of the intrusive suite.
In the southern structural region, south of theCrossman Peak fault, relations among Precambriangrani tic and diori tic rocks and all three Tertiaryunits are repeatedly exposed in a series of southwesttil ted fault blocks. As in the Tumarion Peak area,these are probably normal faults that dip to thenortheast, and merge into the low-angle Crossman Peakfault at depth. Here, steeply-dipping to overturnedstrata of the oldest unit are unconformably overlainby a thick section of the steep- to moderately-dippingmiddle unit At the mouth of Standard Wash theyoungest unit locally overlies both the oldest andmiddle units. Here, the middle unit is bevelled bynearly-horizontal layers of fanglomerate that formsthe base of the youngest unit. Toward the ColoradoRiver, units that are equivalent to the oldest andmiddle units in Standard Wash are repeated in theAubrey Hills, probably by a normal fault buried underalluvium.
INTERPRETATION
Tertiary volcanic and sedimentary sequencesunconformably overlie Precambrian metamorphic andgrani tic rocks all along the western flank of theMohave Mountains. Our work in Miocene rocks and thatof others in the gneisses and granitic rocks of therange indicate that the Tertiary units overliePrecambrian rocks on southeast-dipping low-anglefaults at the north and south ends of the range (K.A.Howard, personal communication; Howard and others,1982, this volume). The variations in lithologies,structural style, and the occurrence of angulardiscordances within the Tertiary section, noted aboveoccur within a distance of only 6 kID along the western
95
flank of the Mohave Mountains. This seems to requirelocalized sediment sources as well as localizedtectonic events during accumulation of the volcanicand sedimentary sequences.
Rocks of the oldest Tertiary unit appear to havebeen deposited in an interval between about 21 and 18m.y. ago. During this time volcanism alternated withrapid sedimentation, probably along active faultmargins where debris flows and landslides werecommon. The abundance of granite boulders in thesedeposits requires that the source was an unroofedcrystalline terrane. In some localities sedimentaryrocks in the basal part of the oldest unit werederived dominantly from volcanic sources and areassociated with some of these coarse granite-clastconglomerates. For the deposition of thisassociation, multiple sources, including a source ofearlier volcanic materials, must have been available.
Intrusion was an important part of volcanismthroughout deposition of the oldest unit. Some timefollowing formation of the airfall tuff-basaltassociation that lies beneath the Peach Springs Tuffof Young and Brennan (1974) there was abundantcentralized intrusion of mostly silicic rocks withinthe oldest unit. These events probably occurred atthe same time as voluminous silicic diking within theunderlying crystalline rocks.
The existence of a distinct angular unconformitybetween the oldest and middle units indicates thattilting began during or after deposition of the oldestunit, and continued during accumulation of the middleunit. As a result the early Miocene rocks are tilteddominantly to the southwest or south (at some placesoverturned), with dips becoming gentler toward the topof the sequence. The presence of overturned bedslocally in the upper strata of the oldest unit may bedue to intrusion or to local faults, rather thanregional deformation. Near the city dump, tilting ofthe oldest sequence apparently occurred beforeintrusion of dacite and la ti te porphyries. Theseintrusions did not extend north into the Turnarion Peakarea, and may have been emplaced before deposition ofthe Peach Springs Tuff.
Tertiary units in the central structural region,outside of the intrusive center, are the leastcomplexly deformed. The oldest unit is simply til tedto the southwest, with minor tear faulting, and thereare few observed repetitions of the section alongnormal faul ts. In contrast, both the northern andsouthern structural regions are more complexlydeformed. Although the northern structural region isboth folded and faul ted and the southern region isunfolded, the dominant structures in both aresouthwest-dipping Tertiary units that occur in faultedblocks separated by northeast-dipping normal faults.
The oldest Tertiary unit and its unconformablecontacts in the central structural region are steeplytilted. There is no indication of any major faultsthat separate these volcanic and sedimentary unitsfrom the central core of the Mohave Mountains. Thisimplies that the underlying crystalline rocks also aretilted to the southwest, and broken on high-anglenormal faults. John and Howard (1982) report evidencethat rocks of the central and eastern Mohave Mountains("Crossman Peak plate") represent progressively deepercrustal levels exposed by this til ting. If theirinterpreta tion is correct, the Precambrian rocks ofthe Mohave range that lie between the "Castle Rock"faul t and the Crossman Peak fault have been til tedsteeply southwest as a block.
The northern and southern structural regions bothare underlain by shallow southeast-dipping faults.Precambrian rocks are exposed where the "Castle Rock"fault displaces them; we presume that the steep"Castle Rock" fault and the underlying low-angle faultintersect at depth. Similarly, we assume that thenortheast-dipping normal faul ts of the southernstructural region merge at depth with the illlderlyingCrossman Peak fault. Thus, both the "Castle Rock"fault and the Crossman Peak fault are zones thatseparate blocks or plates of Tertiary and underlyingPrecambrian rocks of the northern and southernstructural regions from the rocks of the "CrossmanPeak plate". The plates overlying the low-anglefaul ts are internally broken on steep normal faultsthat allowed differential til ti ng of individualblocks. Thus, south of the Crossman Peak fault thePrecambrian and Tertiary sequences are repeatednumerous times by normal faul ts, but a traverse fromwest to east on the north side of the Crossman Peakfault crosses progressively deeper crustal levels,possibly as much as 9 km of original crustal thickness(Howard and others, 1982, this volume).
In the northern structural region folding of theTertiary rocks produced a more complicated structurethan that in the southern or central structuralregions. The deformational history is not yet clear,but we tentatively assume that folding followed theinitial tilting and intrusion of the oldest unit, anddepositon of the middle unit. The folding may berelated to drag from recurrent movements along the"Castle Rock" fault. Apparent movement along thefault is not large, and the disappearance of the PeachSprings Tuff to the south of it, and of the middleunit in the central structural region, remain aproblem.
The time of deposition of the youngest volcanicand sedimentary unit is not yet well-known. Ourcorrelation of this unit with similar rocks at OsborneWash implies ages that range from about 8 - 21 m.y.,with most clustered in the range 12 - 16 m.y. (Eberlyand Stanley, 1978; Martin and Frost, 1981, unpublisheddata). In the Whipple Mountains, like the Mohaverange, rocks associated with fanglomerate of OsborneWash are only slightly tilted, and thus postdate majordeformational events. La ter rhyolite or rhyodaciteintrusions probably provided feeders for the silicicflows of the youngest unit. Following extrusion ofthese rocks, and a few thin basalts, tilting ceased inthe middle Miocene.
In the Whipple-Rawhide-Buckskin Mountains to thesoutheast, the style of mid-Tertiary deformation issimilar to what we have observed in the MohaveMountains. In the Whipples Mountains the deformationis ascribed to transport of the overlying rocks alonglow-angle "detatchment" faul ts that presumably resultfrom regional extension (Davis and others, 1980).These faults and the rocks overlying them haverelations similar to those at the south end of theMohave Mountains. It is not yet clear how detatchmentfaulting in the Whipple Mountains is related todeformation in the Mohave -Mountains as a whole, butdating of Mohave flows and intrusions may help todefine the timing of such events.
96
ACKNOv~EDGEMENTS
He thank Keith Howard, Jon Spencer, and HowardHilshire for stimulating discussions in the field, andfor helpful reviews of this paper. Thanks also toRichard Knox, John Goodge, John Nakata, Steve Reneau,and Tova Diamond for observations of and opinionsabout the complex field relations we have allconfronted in the Mohave Mountains.
REFERENCES
Carr, H.J., Dickey, D.D., and Quinlivan H.D., 1980,Geologic map of the Vidal NH, Vidal Junction andparts of the Savahia Peak quadrangles, SanBernardino County, California: US GeologicalSurvey Miscellaneous Investigations Series, 11126, 1:24,000.
Eberly, L.D.and Stanley, T.B. Jr., 1978, Cenozoicstratigraphy and geologic history of southwesternArizona: Geological Society of America Bulletin,v. 89, p. 921-940.
Damon, P.E., and others, 1964, Correlation andchronology of ore deposits and volcanic rocks Annual Progress Report No. COO-689-42, ContractAT(11-1)-689, to US Atomic Energy Commission:Tucson, Arizona, Geochronology Labs, Universi tyof Arizona, p. 1-28.
Damon, P.E., and others, 1966, Correlation andchronology of ore deposi ts and volcanic rocks Annual Progress Report No. COO-689-60, ContractAT( 11-1 )-689, to Research Div., US Atomic EnergyCommission: Tucson, Arizona, Geochronology Labs,University of Arizona, p. 26-29.
Davis, G.A., Anderson, J .L., Frost, E.G., andShackelford, T.J., 1980, Mylonitiz~tion anddetachment faul ting in the Whipple-RawhideBuckskin Mountains terrane, southeasternCalifornia and western Arizona, in Crittenden,M.D. Jr., Coney, P.J., and Davis, G.A. ,eds.,Cordilleran Metamorphic Core Complexes:Geological Society of America Memoir 153, p. 79130.
Howard, K.A., Goodge, J., and John, B.E., 1982,Detached crystalline rocks in the Mohave, Buck,and Bill Williams Mountains, this volume.
John, B.E. and Howard, K.A., 1982, Multiple low-angleTertiary faults in the Chemehuevi and MohaveMountains, California and Arizona (abs.),Geological Society of America Abstracts withPrograms, Cordilleran Section, in press.
Martin, D.M. and Frost, E.G., 1981, Compilation of KAI' isotopic ages in the Hhipple Mountains andadjoining ranges wi thin a portion of theCalifornia Arizona detachment terrane: SanDiego State University Institute of Geochronology(unpublished data).
Nakata, J .K., 1982, Preliminary report on dikingevents in the Mohave Mountains, Arizona: thisvolume.
Young, R.A. and Brennan, H.J., 1974, Peach SpringsTuff: Its bearing on structural evolution of theColorado Plateau and development of Cenozoicdrainage in Mohave County, Arizona: GeologicalSociety of America Bulletin, v. 85, p. 83-90.
DIKE SWARMS AND LOW-ANGLE FAULTS, HOMERMOUNTAIN AND THE NORTHWESTERN SACRAMENTO MOUNTAINS,
SOUTHEASTERN CALIFORNIA 1
Jon E. Spencer and Ryan D. TurnerU.S. Geological Survey
345 Middlefield RoadMenlo Park, CA 94025
ABSTRACT
Homer Mountain and the Sacramento Mountains are within an extensive terrane alongthe lower Colorado River characterized bynumerous low-angle (detachment) faults ofTertiary age. Two major 10li-angle faults atHomer Mountain displace crystalline rocks aswell as Tertiary conglomerates, whereas thetwo low-angle faults mapped in the northwestern Sacramento Mountains displace primarilyTertiary volcanic and sedimentary rocks.Tilt directions of allochthonous rocks aregenerally westward. At Homer Mountain, adike is 'offset one to three kilometers eastward by movement on the lowest detachmentfault. The middle detachment fault at HomerMountain juxtaposes rocks of almost completely different lithologies, indicating majordisplacement. Two detachment faults in thenorthwestern Sacramento Mountains are parallel or subparallel to bedding in the lowerallochthon.
East-west trending, subvertical dikesare numerous in autochthonous rocks at HomerMountain and the northwestern SacramentoMountains, and are cut by several gently eastdipping, north-south striking dikes at HomerMountain. Both dike sets are cut by the lowangle faul ts. The east-west orientation ofthe oldest dike set indicates that the leastcompressive stress was in the north-southdirection at the time of dike emplacement.This state of stress is consistent with inferred maximum east to northeast directedcompression during much of Mesozoic and earlyTertiary time. Detachment faults may be interpreted as products of middle or late Tertiary extensional tectonics. The subhorizontal orientation of the younger dike set,indicating minimal overburden and absence ofany tensional stress, may be interpreted inthe context of either tectonic regime.
rocks, over Phanerozoic and Precambrian plutonic and metamorphic rocks (e.g., Davis andothers,1980). Allochthonous rocks commonlyare cut by numerous listric and/or planarnormal faUlts which merge with or are cut bya basal detachment fault. These normalfaults may accommodate as much as 50% to 100%extension of rocks above the detachment fault(Anderson, 1971).
Alloch thonous rocks commonly are ti 1 tedin one dominant direction over areas thatrange from hundreds to thousands of squarekilometers. The transport direction of allochthonous rocks usually is inferred to bein the direction opposite to the tilt direction of allochthonous fault blocks. Based onthis and other criteria, detached rocks appear to have moved westward in the EldoradoMountains, and northeastward in the WhippleMountains. Tilting of detached fault blocksand other indicators of movement directionare not as well-known in the interveningranges, but available evidence seems to indicate that a reversal in tilt direction occursin the southern Eldorado Mountains nearSearchlight, Nevada (Longwell, 1963; Volborth, 1973), and that allochthon transportin most areas has been toward the east ornortheast.
Within the detachment fault terrane, thedip direction and form ·of the detachmentfaul ts is highly variable, as is the structural complexity of allochthonous rocks. Thevariable and locally highly irregular form ofdetachment faul t surfaces may be the resultof folding or warping (Davis and others,1980; Frost, 1981; Cameron and Frost, 1981)and/or of irregularities in detachment faultsurfaces at the time of formation. Our present lack of understanding about the natureand origin of detachment faults is compoundedby the great structural compleXity of detachment fault terranes.
INTRODUCTION
Low-angle (detachment) faults are exposed in every mountain range along the westside of the Colorado River area between theEldorado Mountains, 100 km north of the studyarea (Anderson, 1971), and the Whipple Mountains, 80 km southeast of the study area (Davis and others, 1980). Throughout this area,these enigmatic subhorizontal faults typically place tilted middle Tertiary volcanic andsedimentary rocks, and older crystalline
1This reportformity withstandards or
has not been reviewed for conU.S. Geological Survey editorialstratigraphic nomenclature.
97
Detachment faults are often thought torepresent upper crustal accommodation ofcrustal extension, although some workers havesuggested that these faults represent slipsurfaces at the base of gigantic gravityslides (e.g., Davis and others, 1980). Ifthe crustal extension hypothesis is correct,we would expect, at least initially, a regional drop in the magnitude of subhorizontalcompression at the time of extensional faulting. The gravitational sliding hypothesisrequi res no parti~ular stress regime in autochthonous rocks. Thus, determination ofthe orientation and relative magnitude ofstresses in· autochthonous rocks, at the timeof detachment faulting, should provide in-
sight into the cause of faulting.
Dikes can be useful indicators of theorientation of stresses in country rock atthe time of dike emplacement. Experimentaland theoretical studies indicate that, duringma gm a em p 1 ace men t, d ike 13 13 h 0 u 1 dad jus t the i 1"
course of propagation so that the minimumprincipal stress (Cl 3 ) remains perpendicularto the walls of the dike. Application ofthis principle to ancient dike swarms mayyield information about the nature and orientation of past stress regimes. For example,Muller and Pollard (1977), following the original analysis of Ode (1957), were able todetermine the orientation of a regionalstress field, the origin and orientation of alocal stress field, and the relative magnitude of the stresses in each, by fi tting thepattern of the Spanish Peaks dike swarm tocalculated stress trajectories.
In detail, features such as dikes mustbe analyzed carefully to distinguish betweenthe effects of regional stresses and of locally generated stresses or other factorsinfluencing dike orientation. Stresses generated locally, for example by a stock orvolcanic neck, will be less effective atgreater distances. Consistency of dike orientation over a large area is thus a goodindication that regional stresses were theonly significant factor controlling dike orientation.
Previous geologic field mapping at HomerMountain (Bonham and others, 1960) and thenorthwestern Sacramento Mountains (Bonham andTischler, 1960; Collier, 1960; Spurck, 1960)consists of a reconnaissance survey by theSouthern Pacific Land Company. As indicatedby their maps, and supported by this study,Homer Mountain and the northwestern Sacramento Mountains are similar in that both areascontain detachment faults of Tertiary age andnumerous dikes that locally are cut by thesefaults. This is a report on an ongoing studydirected at determining the form of the detachment faults, the structure of the allochthons, and the age, abundance, and composition of the dikes. Ul tima tely, we hope touse dike swarms in the study area as paleostress indicators, and possibly to clarifyunderstanding of the nature of detachmentfaulting.
DIKE SWARMS IN THE STUDY AREA
East-West Trending Dikes
East-west trending dikes are numerous inthe autochthon and lowest allochthon of HomerMountain (Fig. 1, Fig. 2) and over severalsquare kilometers immediately south of Interstate Highway 40 in the northwestern Sacramento Mountains (Fig. 3). These subverticaldikes are remarkably consistent in orientation. They trend about N80W ±5° in the autochthon of Homer Mountain, and N75W ±10 o inthe northwestern Sacramento Mountains. Dikestypically are 2 to 15 meters thick and 200 to2,000 meters long. Field mapping and aerialphotograph analysis of a 5 kilometer long,north-south transect across Homer Mountain
98
Figure 1. Sketch map showing location ofstudy areas. Detachment faults placingTertiary or primarily Tertiary rocks overolder crystalline rocks, based primarily onmapping by the Southern Pacific Land Company,are represented by hatchured lines (hatchureson upper plate). Detachment faults at HomerMountain and west of the Little PiuteMountains, and connecting dashed lines,represent the westernmost extent of knowndetachment faults in this part of the lowerColorado River trough. There are no knowndetachment faults within 30 to 50 kilometerswest of this boundary, which may beinterpreted as a breakaway for detached faultblocks to the east.
Figure 2. Simplified geologic map and crosssection of Homer Mountain.
/'
A'
: .....
=
..
gr-s
Qa
rh ~;f. -:.' ...:..': . '. ' ..:.~--
gr-m
... . ... . ...' .r
-------~----:,
".....:=:::::--
gr-m
~
""
gr-m
'~+:-.-.;:.•
gr-m
-.:::-
AYgr-s
.~.A
/~~
Qa
-~HIGH-ANGLE
FAULT
~~ LOW-ANGLEFAULT
N
IQUATERNARY
ALLUVIUM
UNMAPPED
/
gr-m
EJ
KM.o I 2 3~ ! , !
o i 2MI.
<0<0
c:J TERTIARY
VOLCANIC
ROCKS8 RHYOLITIC
DIKE ROCKC'lltiJ
GRANITIC AND SEDIMENT ARY ROCKS,
UNDIFFERENTIATED, OF THE MIDDLE
AND UPPER ALLOCHTHONS ~dike
GRANITIC AND METAMORPHIC ROCKS,
UNDIFFERENTIATED, AND MAFIC AND
FELSIC DIKES, UNDIFFERENTIATED,
OF THE LOWER ALLOCHTHON AND
AUTOCHTHON
Qa
LOW-ANGLE FAULT,
DOTTED WHERE INFERRED
N
o KM.Io MI.
...~.
2I
3I
2
UNMAPPED
UNMAPPED
Qa
~~gr-m
B QUATERNARY ALLUVIUM
[,'T'VS' ",] TERTIARY VOLCANICAND SEDIMENTARY ROCKS,
. ',. UNDIFFERENTIATED
~GR. ANITIC AND METAMORPHICgr-m
. ROCKS, UNDIFFERENTIATED
dike (mafic and felsic, undiff.)
Figure 3. Simplified geologic map of dike swarms anddetachment faults on the west side of the northwesternSacramento Mountains.
100
indicates that 11% of the autochthon is composed of fairly evenly distributed dikes. Asimilar 5 kilometer transect through the Sacramento Mountains indicates that 13% of thebasement is composed of dike rock. Sharp,planar intrusive contacts, chilled margins,and virtual lack of inclusions within dikesindicate that dilation of country rock ratherthan assimilation was the primary mechanismof dike emplacement.
At Homer Mountain, the dikes intrude anassemblage of unfoliated, pegmatitic graniticrock of inferred Mesozoic or Precambrian ageand older pendants and inclusions of dark,variably foliated, porphyritic granite similar to Precambrian porphyritic granite mappedby Volborth (1973) in the nearby NewberryMountains. Country rocks in the northwesternSacramento Mountains include a variety ofgneissic and metamorphic rocks of unknownprotolith. There is no evidence that structural anisotropies in these rocks had anyinfluence on dike orientations.
Although dike compositions are highlyvariable, several basic lithologic types canbe distinguished. These include fine-grainedto aphanitic, dark green mafic dikes and tanto brown weathering, aphanitic, silicic dikeswith less than 40% phenocrysts. Silicicdikes with greater than 40% phenocrysts aresparse in the northvrestern Sacramento Mountains but numerous at Homer Mountain vrherethey include several suites. Based on modalmineralogy, these suites include biotite rhyolite, biotite quartz latite, biotite (±hornblende) andesite, and basalt.Biotite from anandesite dil<e yielded a K-Ar age of 11.5 ±1.8m.y. (Donna Martin, pers. commun., 1982).
Mapping by the Southern Pacific LandCompany and examination of aerial photographsindicates that east-vrest trending dikes alsoare exposed over a several square kilometerexposure of granitic rock located about 10kilometers north of Homer Mountain. Thus,east-vrest trending dikes are exposed discontinuously over a 40 kilometer long northsouth transect. The consistency of dike orientation over this larp;e area may be interpreted as indicating that regional stressesvrere the only significant factor controllingdike orientation.
North-South Trending Dikes
Several rhyolitic dikes vrith north-southstriking, gently east dipping (20 0 ±5°) contacts are present in the autochthon and lovrest allochthon at Homer Mountain. Unlike theeast-vrest trending dikes, these dikes changegreatly in thickness along strike and haveirregular contacts (Fig. 2). Hovrever, thenorth-south trending dikes also have sharpcontacts, chilled margins, and generally lackinclusions, suggesting that assimilation vrasnot an important process in dike emplacement.These dikes clearly crosscut east-west trending dikes, and therefore are younger. Animpure hornblende separate from one of thesedikes yielded a K-Ar age of 41.0 ±16.0 m.y.(Donna Martin, pers. commun., 1982).
101
DETACHMENT FAULTS IN THE STUDY AREA
Detachment Faults at Homer Mountain
At least three detachment faults areexposed on the south and east flanks of HomerMountain. There appears to be only limitedoffset across the upper and lovrer detachmentfaults, but a major lithologic contrastacross the middle detachment fault indicatesthat it has undergone significant displacement. The lowest allochthon pinches out tothe west so that the middle allochthon restsdirectly on the autochthon on the south flankof Homer Mountain (Fig. 2). The lovrest detachment fault offsets both the middle andupper detachment faults.
Lovrest detachment fault
The lowest detachment fault has an arcuate trace that is concave to the southeast,and is inferred to be synformal and gentlysoutheast plunging (Fig. 2). The limbs ofthis synformal detachment fault dip inward atabout 10 0 to 30°. Dikes in the autochthonbeneath the limbs of the synformal fault donot appear to be warped, suggesting that theshape of the lovrest detachment fault originated at the time of formation and vras notproduced by later folding or vrarping.
Correlation of a north-south trendingrhyolitic dike in the lovrest allochthon vritheither of two dikes of identical compositionin the autochthon indicates about 1 to 3 kilometers of eastvrard displacement (Fig. 2).This amount of inferred displacement is consistent vrith the fact that rocks of the lovrerallochthon are indistinguishable in composition from those of nearby parts of the autochthon. This similarity in rock typesvrould prohibit recognition of the fault traceif not for the presence of chloritic brecciasalong the fault which, although locally absent, may be as much as 5 to 10 meters thick.
The lower allochthon is primarily represented by two elongate, north-south strikingfaul t blocks. Field evidence suggests thatat least three of the four north-south trending faults that bound these two fault blocksdip gently to moderately outward from thefaul t blocks, as shovrn in the cross-sectionof Figure 2. These three faul ts are truncated by the lowest detachment fault, whereasthe easternmost north-south striking faultcuts the lovrest detachment fault.
Our interpretation of this fault patternis shown in the cross-section of Figure 2.We consider the two faults on the west flankof the each fault block to be tilted segmentsof the middle detachment fault. The fault onthe east flank of the western fault block isinterpreted as a lovr-angle normal fault accommodating extension above the lovrest detachment faul t, vrhereas the normal faul t onthe east flank of the east faul t blo.:lk isinterpreted as a high-angle normal fault thatpost-dates movement on the lower and middledetachment faults.
Middle detachment fault
Exposures of the middle detachment faultinclude an east-west trending segment on thesouthwest flank of Homer Mountain \oIhere themiddle allochthon rests directly on the autochthon, and two north-south trending segments along the west sides of two west-tiltedfault blocks where the middle allochthonrests on the lower allochthon (Fig. 2).Brecciation is a characteristic feature ofthe rocks along the middle detachment fault,in places extending for tens or even hundredsof meters into the middle allochthon abovethe fault. Chloritic alteration of faultbreccias may be as well-developed above thefault as below it.
The middle allochthon is composed primarily of dark, variably foliated, porphyritic ~ranite and poorly-sorted conglomeratewith abundant cobbles and boulders of thedark, porphyritic granite. Similar darkgranite occurs locally as pendants and inclusions in the leucocratic ~ranitic rocks ofthe autochthon and lower allochthon. A majorlithologic contrast thus occurs across themiddle detachment fault, suggesting thatlarge net displacement has occurred on thisfault. This inference is supported by thefact that dikes, which are abundant in theautochthon and lower allochthon, are absentin the middle allochthon.
Some moderate sized pendants of darkgranite and its gneissic equivalents occur onthe west side of Homer Mountain and near Signal Hill in the southern Piute Range. Thesemay represent remnants of a once continuousor semi-continuous roof of dark granite aboveyounger, leucocratic plutonic rocks. Thisroof is the inferred source area of the middle allochthon. The middle detachment mighthave carried part of this roof down onto itsfoundation of younger plutonic rocks by perhaps 5 to 20 km of horizontal displacementand perhaps .5 to 3 km of vertical displacement.
Upper detachment fault
A low-angle fault above the middle allochthon is exposed over a small area on thesoutheast flank of Homer Mountain, where itis truncated by the lower detachment fault(Fig. 2). The upper allochthon is composedof conglomerate with cobbles and boulders ofdark granite, and is lithologically indistinguishable from conglomerates in the middleallochthon. Based on this lithologic similarity, we infer that net movement on thisfaul t is small in comparison to net movemen ton the middle detachment fault.
Detachment Faults in the SacramentoMountains
Detailed mapping in the area betweenEagle Peak and Flattop Mountain in the northwestern Sacramento t40untains indicates thatat least two major allochthons, composed almost entirely of Tertiary volcanic and sedimentary rock, are present above an autochthonof Precambrian crystalline rock. The presence of, fault gouge and well-developedchloritic breccia in Precambrian rocks immediately below the lower allochthon, and the
102
almost total lithologic dissimilarity betweenthe autochthon and lower allochthon, supportthe interpretation that the Tertiary rocksare separated from older crystalline rocks bya major detachmen t faul t, as indicated byCollier (1960). Both lower and upper detachment faults are parallel or subparallel tobedding in the lower allochthon (Fig. 4).
The lower allochthon is a sequence ofinterbedded volcanic flows, tuffs, and volcaniclastic sandstone that dips gently to thewest and is about 150 meters thick (Fig. 4B).The youngest unit in the sequence is a volcaniclastic sandstone (Ts S ) that is exposedonly at the southern end of the map area(Fig. 4). At other locations the sandstoneis missing and is inferred to have been truncated by the upper detachment fault, \oIhichusually occurs at the top of a cliff-formingtuff unit (Tv4)'
The upper allochthon is composed ofpoorly bedded to massive cobble- and boulderconglomerate and sedimentary breccias. Theconglomerate contains clasts of gneiss anddark granite, whereas the breccia is derivedfrom gneiss and dark granite, as well as dark(basaltic?) volcanic rocks. Sandstone iscompletely absent at lower stratigraphiclevels, but is increasingly abundant at higher levels.
The contact at the base of the conglomerate locally cuts across section in the lower allochthon, and is also a major lithologicboundary between underlying volcanic rocksand their fine-grained sedimentary derivati ves, and overlying conglomerate and sedimentary breccias that are generally poor involcanic clasts. We interpret this contactas a low-angle fault, and not an unconformity, because volcanic clasts are rare in thebasal part of the conglomerate. In addition,preliminary data indicates that the conglomeratic rocks are tilted much more steeplythan the underlying volcanic and volcaniclastic rocks of the lower allochthon. Theupper cliff forming part of TV4' where located immediately below the inferred low-anglefault, is not brecciated or chloritized, although locally it is pervasively fracturedand has slickenside lineations on fracturesurfaces.
A system of northwest trending highangle faults cuts the upper detachment faultand forms a graben within the allochthonousrocks (Fig. 4). Total stratigraphic throw onthe graben-bounding faults is about 50 to 70meters. Mapping by the Southern Pacific LandCompany (Collier, 1960), and interpretationof aerial photographs suggest that thesefaults do not cut the basal detachment fault.The graben may have formed by extension abovethe basal detachment fault. If this interpretation is correct, it suggests that movement on the upper detachment fault ended before the last movement on the 10Her detachment fault.
DISCUSSION
Dike SHarms
Rehrig and Heidrick (1972,1976) usedthe geometl'Y of magmatic bodies as paleostress indicators to clarify undel'standing ofMesozoic and Cenozoic tectonics in southel'nand we s tel' n Al' i z 0 n a . The y s tat i s tic a 11 y an"11yzed the orientations of a lal'ge number ofsubvertical dikes, veins, and elongate stocksemplaced during the late Cretaceous-earlyTertiary Laramide orogeny and during middleand late Tertial'y time. They interpretedthis data as indicating that the least compressive stress «() 3) was perpendicular tothe planar or tabular form of these magmaticbodies at the time of emplacement. Laramideage magmatic features typically strike N70E±20o , whereas late Tertiary magmatic featurestypically strike N20W ±20°. This indicatesthat principal stress orientations changedapproximately 90° during early or middle Tertiary time. This interpretation is consistent with the widely held notion, based onstyles of faulting, that the maximum compressive stress «() 1) was oriented east-west tonortheast-southwest during the Laramide orogeny, and that () ~ was oriented east-west tonortheast-southwes~ during middle and lateTertiary basin and range extension.
In the study area, the east-west trending dikes are inferred to represent a stateof stress in the upper crust in which theminimum compressive stress (Cl3 ) was orientedperpendicular to a vertical, roughly eastwest striking plane. The subhorizontal dikesare inferred to represent a state of stressin which the minimum compressive stress (Cl
3)
was subvertical. Both dikes sets are thusconsistent with an inferred east directedmaximum compressive stress (°1 ) for much ofMesozoic and early Tertiary time based on thewidespread existence of east to northeastdirected thrust faults of this age (Millerand McKee, 1971; Burchfiel and Davis, 1971,1977; Burchfiel and others, 1974; Howard andothers, 1980; Reynolds and others, 1980).
However, the age of dikes in the studyarea is uncertain. An 11.5 ±1.8 m.y. K-Arage on biotite from one of the east-westtrending dikes is suggestive of a Miocene agefor dike emplacement, yet hornblende from anorth-south trending dike that clearly isyounger based on cross-cutting relationshipsyielded a K-Ar age of 41.0 ±16.0 m.y.. Bothages are Tertiary, indicating that Tertiaryheating and/or c~oling affected the area.However, it is not known if, for example, thehornblende contains excess argon and bothdikes are Miocene in age, or perhaps bothdikes are Mesozoic but have undergone a Mio-cene thermal disturbance causing partial(hor'nblende) or complete (biotite) argonloss. Four biotite separates from graniticrocks from the Newberry Mountains, located 25km northeast of Homer Mountain, all yieldedmiddle Miocene ages (Anderson and others,1972, Volborth, 1973). Thus, Miocene thermaleffects have been significant in this area,and may have caused resetting of K-Ar ages atHomer Mountain, although, if both dike setsat Homer Mountain are Mesozoic, there are noknown intrusions exposed at Homer Mountainthat could be related to a Miocene thermalevent.
103
Low-Angle (Detachment) Faults
The inference of regional northeast toeast directed transport of allochthonousfault blocks along the west side of the lowerColorado River, as determined from tilt directions, is supported by offset indicators atHomer Mountain. Offset of a north-southtrending dike indicates that the lowest allochthon was displaced 1 to 3 km eastward bymovement on the lowest detachment faul t, although the north-south component of movementis not discernable from this offset marker.However, tilted fault blocks of the lowerallochthon strike north-south, suggestingthat the north-south component of displacement was negligible, and that offset was dueeast.
Inferred east to northeast sense oftransport of allochthonous fault blocks isfurther supported by the presence of a breakaway along the west margin of this detachmentfault terrane, as first recognized by Davisand others (1980) in the Mopah Range west ofthe Whipple Mountains. A breakaway is geometrically similar to the scarp at the uphillend of a landslide, and is the fault tracethat separates detached and displaced allochthonous fault blocks from their adjacent autochthonous equivalents. The Mopah Rangebreakaway, as depicted by Davis and others(1980), is a concave upward fault that dipsmoderately at the surface and flattens withdepth toward the northeast where it becomes abasal detachment fault. If this geometry isapplicable to breakaway faul ts in general,then more deeply eroded and exhumed breakawayfaults will have gentler dips.
The trace of the Mopah Range breakawayis inferred to extend northward under Quaternary alluvium toward the east flank of theOld Woman and Piute Mountains where the Little Piute Mountains are detached and displaced eastward (Keith Howard, pel's. commun.,1981). From the east flank of the PiuteMountains, the breakaway may extend northwardunder Quaternary alluvium along the westfl3.nk of the northern Sacr3.mento Mountains,and may be represen ted farther nOY'th by themain (middle) detachment fault at Homer Mountain (Fig. 1). Alth9ugh correlation ofbreakaway faults and the inference of regional northeast to east directed movement ondetachment faults are both speculative, it iscertain that every range between this regional breakaway and the Colorado River containsmajor detachment faults, whereas no detachment faults are known to exist in any of theranges immediately to the west.
The main, or middle detachment fault atHomer Mountain is inferred to have accommodated eastward displacement of allochthonousrocks based on (1) demonstrable eastward displacement of a lower (and related?) detachment fault, and (2) westward tilts of conglomerates in the middle allochthon. Thepresent southward dip of the detachment faultmay be the result of warping and foldingabout an east-west trending axis during faultmovement, similar to folding of detachmentfault surfaces in areas to the southeast
A~
Teg
NT53
/'
/
--- /'I/
IQo/
IUNMAPPED/
I/
/KM
1.0T5 3/ / a 0.5I
I•1.0
I I0.50\ \
MI'')
104
I
1000pCu
J'f __,-I
~~~~r;-~TV~2~==;;;:t,;;lI........-r 2000
400ena::wr-w O.......--------------------...J:... I~ 0
WEST
800
:.:i "j,.' i> -.;;.;.'0 0- (, '0' (io
200 "-.;"";;""~"";;"~.."~:o.:~"~"~~~
FA ULT .:::~:::_::.:.:.::',:', ' .
.~.~. ~'~'r~r..... . , .' ... "
""/Ib',Ps'''''~'.~'~ j; .~............ , ... ..
.. - - ..
\:;;:~·~:.??~F.
IT TERTIARY CONGLOMERATE ANDJ cg S DIM NTARY SR CCIA
}TsS T RTIARY SANDSTONE
}TV TERTIARY TUFF
}TS3 TERTIARY SANDSTON"/:> _Do ,g" ~",,4 <!!>A l'"
, .
TV2 TERTIARY TUFF AND
TUFF BRECCIA
:::::::..... :.:;II!! ilJ;> ... <td d~ .Q> ". df!J c)........... - '
:O:~·~·;'·4:";..... " .
::::: : :
:: :.',",",., : ::: } TSj TERTiARY SANDSTONE
1p£u PRECAMBRIAN IGNEOUS AND META-
I MORPHIC ROCKS, UNDIFFER NTIATED
Figure 4A. (other page) Simplified geologic map ofthe central part of the northwestern Sacramento Mountains between Eagle Peak and Flattop Mountain. Geologic unit symbols are keyed in Figure 4B, exceptTb = Tertiary basalt, and Qa = Quaternary alluvium.
Figure 4B. Cross-section and columnar section of areashown in Figure 4A.
105
(Frost, 1981; Cameron and Frost, 1981).The middle detachment fault is inter
preted as a low-angle normal fault becausepart of an approximate vertical section ofupper crustal rocks is missing across thisfault; i.e., that part represented by theint~usive contact between Mesozoic and/orTertiary leucocratic granitic rocks and over-lying, intruded Precambrian granite andgneiss. In some places, the Precambrianrocks are missing as well, and Tertiary conglomerates are juxtaposed against underlyingleucocratic granite rocks.
The lowest detachmen t faul t in thenorthwestern Sacramento Mountains is interpreted as a low-angle normal fault becausethe depositional contact at the base of theTertiary section is omitted across the fault.The nature of the upper detachment fault isuncertain because we do not know the relativeages of the rocks in each allochthon. Theseinferences are all consistent with regionaleastward to northeastward displacement ondetachment faults in and around the studyarea, and with the existence of a headwall orbreakaway along the west side of this area.
Relationship of Dikes to Detachment Faults
In an extensional tectonic environment,the least compressive stress (6""3) should be,at least initially, in the direction of extension. Rehrig and Heidrick (1976), forexample, found that dikes of mi ddle and lateTertiary age were generally subvertical andnorth to northwest striking, consistent withno~mal fault orientations indicating east tonortheast directed crustal extension insouthern and western Arizona. In detachmentfault terranes, the direction of extension isgenerally inferred to be the direction ofmovement on detachment faults, which is eastwest at Horner Mountain and approximately inthe same direction in surrounding areas.
However, secondary effects of detachmentfaulting may include major changes in thesta te of stress in au tochthonous rocks. Thestress due to overburden «()"7"" v) is clearlyreduced by tectonic denudation, while thesubhorizontal stress in the direction of allochthon transport (C:;-E in the study area)may increase due to isostatic uplift and attendant flexure of the aut'ochthon (Spencer,this volume). Both of these effects may leadto conditions favoring subhorizontal dikeorientations instead of north-south, subvertical dike orientations.
East-west trending dikes at Homer Mountain and the northwestern Sacramento Mountains indicate 01 was oriented north-south,at the time of diRe emplacement, which is 90degrees from the inferred orientation of (J 3based on movement directions of nearby detachment faults (assuming detachment faultsreflect crustal extension). While the increase in 0E due to autochthon flexure mayfavor emplacement of dikes with a subvertical, east-west trending orientation, the reduction of 0 v would not. The east-west tonortheast-southwest trending folds documentedby Frost (1981) and Cameron and Frost (1981),
106
and suggested as structural features of thestudy area, indicate conditions in which C>Nwas greater than 0-v' This condi tion wouldnot favor emplacement of dikes with an eastwest, subvertical orientation.
CONCLUSION
Detachment faults in the study area represent the westernmost extent of the detachmen t faul t terrane along the lower ColoradoRiver trough. Movement directions of allochthons are uncertain in the northwestern Sacramento Mountains, although west tilted conglomerates in the upper allochthon and anorthwest trending graben in both allochthonssuggest east to northeast translation andextension. Unlike allochthons in the northwestern Sacramento Mountains, allochthons atHomer Mountain contain crystalline rocks, andclear evidence of eastward translation.
Structural features of Homer Mountainand the northwestern Sacramento Mountains areinterpreted as representing two contrastingtectonic regimes. The orientation of eastwest trending dikes is interpreted as theresult of east-west directed compression inan Andean type orogenic province of Mesozoicand early Tertiary age. The subhorizontalorientation of the younger di.kes is consistent with strong, east-west directed compression during Mesozoic or early Cenozoic time,but a similar state of stress, resulting fromautochthon flexure following low-angle faulting and tectonic denUdation, may have developed during middle and late Tertiary time(Spencer, this volume). The development ofdetachment faults may be interpreted as theresult of crustal extension during middle andlate Tertiary time. However, the interpretation that the study area is part of a denUded, uphill end of a mega-gravity slide cannot be refuted by our data.
In Arizona, two regional dike sets, theolder oriented N70E ±200 and the younger oriented N20W ±200, are both rotated about 20 to30 degrees in a counterclockwise di rectionrelative to the strike of the two dike setsin the study area. Tilt directions and otheroffset indicators suggest northeastwardtranslation of allochthons in areas to thesoutheast of the study area (Davis andothers, 1980; Rehrig and others, 1980; Rehrigand Reynolds, 1980), a direction about 20° to40° more northerly than inferred eastwarddisplacement of allochthonous rocks in thestudy area. These differences in orientationmay reflect the arcuate trace of both theMesozoic magmatic arc and the Tertiary basinand range province around the western cornerof the Colorado Plateau.
ACKNOWLEDGEMENTS
We especially thank Keith Howard for hiscontinued support and interest in this project, and Donna Martin for K-Ar analyses. Wealso thank Bill McClelland and Jane Pike forhelpful discussions in the field, and HenriWathen for assistance in the field and fortyping this paper. This paper was reviewedby Robert Powell and Jane Pike.
REFERENCES CITED
Anderson, R. E., 1971, Thin-skinned distension of Tertiary rocks of southeasternNevada: Geological Society of AmericaBulletin, v. 82, p. 43-58.
Anderson, R. E., Longwell, C. R., Armstrong,R. L., and Marvin, R. F., 1972, Significance of K-Ar ages of Tertiary rocksfrom the Lake Mead region, Nevada-Arizona: Geological Society of AmericaBulletin, v. 83, p. 273-288.
Bonham, H. F., Jr., and Tischler, M. S.,1960, Geology and mineral resources oftownship 9 north, ranges 21 and 22 east,San Bernardino base and meridian, SanBernardino County, California: SouthernPacific Land Company unpublished report,29 pages.
Bonham, H. F., Jr., Tischler, M. S., andSpurck, W. H., 1960, Geology and mineralresources of township 11 north, ranges19 and 20 east, San Bernardino base andmeridian, San Bernardino County, California: Southern Pacific Land Companyunpublished report, 24 pages.
Burchfiel, B. C., and Davis, G. A., 1971,Clark Mountain thrust complex in theCordillera of southeastern California,Geologic summary and field trip guide:University of California at RiversideMuseum Contributions, v. 1, p. 1-28.
Burchfiel, B. C., and Davis, G. A., 1977,Geology of the Sagamore Canyon-Slaughterhouse Spring area, New York Mountains, California: Geological Societyof America Bulletin, v. 88, p. 16231640.
Burchfiel, B. C., Fleck, R. J., Secor, D. To,Vincelette, R. R., and Davis, G. A.,1974, Geology of the Spring Mountains,Nevada: Geological Society of AmericaBulletin, v. 85, p. 1013-1022.
Cameron, T. E., and Frost, E. G., 1981, Regional development of major anti formsand synforms coincident with detachmentfaulting in California, Arizona, Nevada,and Sonora: Geological Society of America Abstracts with Programs, v. 13, no.7, p. 421-422.
Collier, J. T., 1960, Geology and mineralresources of township 8 north, ranges 21and 22 east, San Bernardino base andmeridian, San Bernardino County, California: Southern Pacific Land Companyunpublished report, 32 pages.
Davis, G. A., Anderson, J. L., Frost, E. G.,and Shackelford, T. S., 1980, Mylonitization and detachment faulting in theWhipple-Buckskin-Rawhide Mountains terrane, southeastern California and western Arizona, in Crittenden, M. D., Jr.,Coney, P. J.-,-and Davis, G. H., eds.,Cordilleran metamorphic core complexes:Geological Society of America Memoir153, p. 79-130.
Frost, E. G., 1981, Mid-Tertiary detachmentfaulting in the Whipple Mtns., Calif.,and Buckskin Mtns., Ariz., and its relationship to development of major antiforms and synforms: Geological Societyof America Abstracts with Programs, v.13, no. 2, p. 57.
Howard, K. A., Miller, C. F., and Stone, P.,
107
1980, Mesozoic thrusting in the easternMojave Desert, California (abs.): Geological Society of America Abstractswith Programs, v. 12, p. 112.
LongHell, C. R., 1963, Reconnaissance geologybetween Lake Mead and Davis Dam, Arizona-Nevada: U. S. Geological SurveyProf. Pap. 374-E, 51 p.
Miller, F. K., and McKee, E. H., 1971, Thrustand strike-slip faulting in the PlomosaMountains, southHestern Arizona: Geological Society of America Bulletin, v.82, p. 717-722.
Muller, D. H., and Pollard, D. D., 1977, Thestress state near Span Lsh Peaks, Colorado, determined from a dike pattern:PAGEOPH., v. 115, p. 69-86.
Ode, H., 1957, Mechanical analys is of thedike pattern of the Spanish Peaks area,Colorado: Geological Society of AmericaBulletin, v. 68, p. 567-576.
Rehrig, W. A., and Heidrick, T. L., 1972,Regional fracturing in Laramide stocksof Arizona and its relationship to porphyry copper mineralization: EconomicGeology, v. F,7. n. 198-213.
Rehrig, W. A., and Heidrick, T. L., 1976,Regional tectonic stress during the Laramide and late Tertiary intrusive periods, basin and range province, Arizona:Arizona Geological Society Digest, volume X, p. 205-228.
Rehrig, W. A., Shafiqullah, M., and Damon,P. E., 1980, Geochronology, geology, andlistric normal faulting of the VultureMountains, Maricopa County, Arizona:Arizona Geological Society Digest, v.12, p. 89-11"
Rehrig, W. -A., and Reynolds, S. J., 1980,Geologic and geochronologic reconnaissance of a northwest-trending zone ofmetamorphic complexes in southern Arizona, in Crittenden, M. D., Jr., Coney,P. J.,-and Davis, G. H., eds., Cordilleran metamorphic core complexes: Geological Society of America Memoir 153,p. 131-158.
ReynoldS, S. J., Keith, S. B., and Coney, P.J., 1980, Stacked overthrusts of Precambrian crystalline basement and invertedPaleozoic sections emplaced over Mesozoic strata, Hest-central Arizona: Arizona Geological Society Digest, v. 12,p.45-5"
Spencer, J. E., 1982, Origin of folds of Tertiary lOH-angle faul t surfaces, southeastern California and western Arizona:this volume.
Spurck, W. H., 1960, Geology and mineral resources of township 9 north, ranges 19and 20 east, San Bernardino base andmeridian, San Bernardino County, California: Southern Pacific Land Companyunpublished report, 23 pages.
Volborth, A., 1973, Geology of the granitecomplex of the Eldorado, Newberry, andnorthern Dead Mountains, Clark County,Nevada: Nevada Bureau of Mines and Geology, Bulletin no. 80,40 p.
