thogonal Proterozoic fabrics in northwestern Arizona: nltiple orogenic events or progressive deformation ring continental assembly

Deparmnt of Geology, Northern Arizona University, Flagstaf Arizona, 8601 1

- . Gneiss Canyon shear zone appears to be part of the regional boundary between granulite and

terozoic rocks of northwestern Arizona ampwbolite fmies r o c k 1.68 Ga zir-s h.om multiple foliations, a northwesta;triking amphibolige ore interpreted to give the age of foliation (S1) overprinted by northeast- peak metamorphism. foliation (Sa). The Gneiss Canyon shear Tectcvnic relationships across the Gneiss

oblique movements. INTRODUCTIOlY c rocks of nortlrwester~ Ariaono es of presumed supracrustal origin, The Transition Zone of Admna, between the Cslorada rtzofeldspathie schist, amphibolite, Plateau and the highly extended Basin and Range Province, sehist, and pelitic shists. Granitic e x ~ s pPlrt rrf a Proten,mic orogenic belt. North and

and granitoids intrude the para- northeast-striking shear zones within the orogenic belt make up about 60% of the Proitero- sepatate blocks with differing tectonic histories (Fig. 1;

The oldest granitolds are granodio- Karhtrom and Bowring, 1988). A continuing god of re- nalites that contain both S1 and S j seatch is to evaluate tectmic histmy within and across shear yield ages of 1.73 Ga, The Valentine zones in order to better undemtd Proterozaic omgenic 1.715 Ga pluton in tho southern h i i . In spite of models that w-t contbntal growth Cliffs, may have been emplaced late by assembly of once sepanlte Rotemmk temrnes (e.g. Karl-

opment of the S1 fabric. Other gran- stnrm and Bowring, 1988), gecdogic stlldies to date have not the. development of the S2 fabric be- clearly identified suture zones that delineate the margins of

n@@kwest eI the Gneiss Canyon shear daiy is f d in north- krizona Here* aoent isompic

. , . . . I *

of R a m , P.0, Box 1292, Globe, AZ, !35j502

,.K.E., 1991, Proterozoic Oeology and Ore Oepoaits at Ariiam, Arizona G e o l w a l Society Digest I@, p. $7-84,

BRICS IN NC)ETHWSl'ERN ARIZONA 69 1

This study was initiated to help evaluate the wide northeast-sailcing Gneiss Canyon shear m e of QoRhwttstem Arizona, and its possiblerelationship to the MojavcYavapai

na, called the Mojave Province, contains iso- mapped at a scale of 1:24,000 (Albin, 1991% 1991b). nce for crust greater than 2.0 Ga that was incorpo- Mappihg was also done in the Lower Granite Gorge of the

orogenic belt during or before 1.7 Ga tecton- Grand Canyon and Milkweed Canyon (Fig. 3; Fig. 4). en and others, 1988; Wooden and Miller, 1990). Our stmcmal studies documented the presence of do-

e location and origin of the isotopic boundary between mains of northeast-seiling foliation, that overprint domains provinces are not well understood. Various workers, of northwest-striking foliation resulting in a pattern of g different isotopic systems, have placed the boundary nearly orthogonal fabrics Fig. 5). Thus, the study evolved

tern Hualapai block, generally parallel to and to include an examhation of the relationship between the northwest- and northeast-striking foliation domains. This papex has three goals: (1) to descrii Proterozoic lithologies

in part due to the low density of data points, the in northwestem Arizona; (2) to describe the geometry and characteristics of the various isotopic systems and, discuss possible origins of the orthogonal foliation pattern

e case of Pb isotopes, an apparent mixed zone that and; (3) to comment on the tectonic significance of the nds approximately to the Hualapai block (Fig. 1; Gneiss Canyon shear zone. and DeWitt, this volume).

DESCRIPTION OF EARLY PROTEROZOIC ROCKS

Gneissic rocks

The oldest Proterozoic roclcs in northwestern Arizona are gneisses of probable supracrustal origin. These include quartzofeldspathic gneisses, amphibolite and metapelite. These units are interlayered with tonalitic to granodioritic octb&neiss. Rare ultramafic rocks are also found in associa- tion with s m t a l rocks. Units are generally laterally extensive and range from a few cm to 10's of meters in thickness. These rocks are generally highly strained and metamorphosed at high amphibolite to granulite grade. Bedding and primary structures are rare and facing indicators absent so that stratigraphic relationships are not known.

Metapelitic rocks crop out in only a few localities within , the study area A unit in Black Rock Canyon (Fig. 4)

contains the granulite grade assemblage gmet-cdierite- sillimanite-biotite-potassium feldspar-plagioclaseq~artz~ M-litic rocks in Black Rock Canyon commonly contain deformed blebs and stringers of quartz, K-feldspar and occa- s i d y garnet, which are interpreted as pods of partial melt produced~nretamorphism.

2. Proposed isotopic and geochemical \ Arnphibolites range from hornblende-rich plagioclase s in northwestern Arizoua. Nd = SmINd gneiss to micaceous quartzofeldspathic amphibole schist of Bennett and DePaolo(l987); Pb (dot- with some garnet-bearing amphiblite, to coarse-grained

common lead boundary of Wooden and hornblendite. Amphiilites occut as thin, (cms to m thick), (1988); Bb (dots) = common lead. Wansition laterally extensive units and as massive, pod-& and len- den and Dewitt (this volume); Pb (long soidid bodies (10's of m thick). The cornpasition d the

dash), U-Pb and common lead boundary thin, laterally extensive units is highly changable across erlain and Bowring(1989); 1.7 = bounda- lay&&; these units sre commonly intElayered with meta- d based on the geochemical character- mndames. The protolidrs of the thin units are intexpted

1.7 Ga. granites (Anderson and others, to be ruffs or o&es vol- rocks, volcanically derived ; 1.4 = boundary proposed based on the metasedimentaty rucks, or possibly thin or tectonically ical characteristics of 1.4 Ga granites thinned basaltic lava flows. Relict pillow structure in a n and others, in press). metabasalt was observed in a highly chloritized unit in Big

70 ALBIN AND KARLSTROM

r -

Migmatitic Gneiss - ortho- and para- gneiss

injected by granitoid

1 1 5 O l 1 4 O 1 1 3 O

Figure 3. Generalized geologic map of Precambrian rocks in northwestern Arizona. Nd = Nd boa dary of Bennett and DePaolo (1987); Pb = common lead boundary of Wooden and others (1988). Loc tion of Figure 4 is labeled "study area".

Bend Canyon, about one half mile from the canyon's mouth. nte thick pad-like and lensoidal bodies of amphibolite and

m thick. Some of the qwumfeldspathic gneisses are litho- types with which it is associated. logically and texturally similar to the gneissic margins of nearby plutons and these are interpreted to be orthogneiss, Granitic Rocks

migmatitic g n e h are uncertain but may include lithic and d e f d xenoliths

Me tasedimentary and

, \ \

ALBIN AND KARLSI'ROM

only S2 foliation. The Valentine Granite, a large pluton in the southern

Grand Wash Cliffs and the northern Cottonwood Cliffs, belongs to the second group. It is a pink biotite momgran- ite with some leucocratic biotite-bearing phases. Through- out most of the pluton, the granite is strongly to moderately foliated by SZ; however, it may contain S1 structural ele- ments (disc& below), The Valentine Granite has a U-Pb zircon age of 1.715 Ga (Chamberlain and Bowring, 1990). Numerous small intrusive bodies and dikes in the Grand Wash Cliffs and Peacock Mountains may be related to the Valentine Granite, based on compositional and textural simi- larities.

A large number of biotitebearing granite dikes in the Grand Wash Cliffs strike n d w e s t and dip 20-30' to the northeast. They cut the steeply dipping S1 and S2 fabrics and are generally unfoliated and are thus late synkinematic to postkinematic. Two such dikes from Gamqt Mountain, in the Grand Wash Cliffs north of the study area, have yielded U-Pb zircon ages of about 1.62 Ga

Pegmatite dikes crop out widely but are most common near the margins of Early Proteromic granites. The dikes are composed of varying percentages of pink K-feldspar, white quartz f muscovite. Most pegmatites are less than a meter thick, but a few are as much as 10 m thick. Some pegma- tite dikes are highly folded suggesting emplacement before

Figure 5. Foliation trajectory map of north- the end of regional deformation. Most dikes, however, are western Arizona based of a compilation of struc- undeformed or only slightly deformed and thus are postkine tural data collected in the course of this study maticor latesynkinematic. and fiom previous studies.

MIDDLE PROTEROZOIC ROCKS gray hornblende-bearing biotite gmnodiorite. Also included in this group are b i o t i t e m g , hornblende tonalites. One A large pluton of K-feldspar megacrystic granite is sample from the Grand Wash Cliffs (ALS-9) has a U-F% located at the northern end of the Peacock Mountains. A zircon age of 1.73 Ga (Albin and others 1991). The primary smaller pluton is exposed in the central Pe8cock Mountains lithologic distinction between the granodiorites and granites and a few small bodies crop out elsewhere in the study area (discussed below) is the K-feldspar content. The most (Fig. 4). Several small (about 2m wide) dikes intrude the K-feldspar rich of the granodiorites is still within the grano- country rock, generally near the larger bodies. The mega- diorite field in the IUGS classification. The most K-feldspar crystic granite of the northem Peacock Mountains contains poor granite is well within the monzogranite field. K-feldspar phenocrysts (up to 8 cm in length) that make up

Biotite momgranites are abundant in the study area. 20% to 70% of the rock. The matrix contains generally less Bodies ranging from large plutons to thin veins intrude the than 20% plagioclase with varying amounts of biotite and su-tal rocks and granodiorites and often contain small quartz. Samples of the megacrystic granite from the north- xenoliths of these lithologies. The granites have been ern Peacock Mountains, AGW-138 and APK-11, have U-Pb divided into two major groups: 1) gray biotite f homblende zifcon ages of 1.37 and 1.39 Ga respectively (Albin and

are characteristically well foliated and commonly become

both S1 and S2 foliation (discussed below). Granites in the margins, there are small-scale open folds of the second group are generally less highly deformed, ranging foliation, sometimes involving the contact itse fiom strongly foliated to unfoliated, and generally containing folds are in- to have formed during the emp

ORTHOGONAL FABRICS IN NORTHWESERN ARIZONA 73

thick, but most are 1 to 10 m thick. Dikes in the Peacock Mountains swarm have a maximum thickness on the order of 10 m. Diabase dikes throughout the study area generally s e e northwest and dip roughly 20' NE. There are also a number of rmrtheast-striking, shallowly southeast-dipping dikes, a few steeply dipping feedex dikes and a few =tabular bodies (Fig. 7). A diabase sample from the Central Peacock Mountains, AHU-70, has a U-Pb sphene age of ca 1.08 Ga that is interpreted to be the emplacement age (Shasai and

apatite and rare orthopyroxene and inherited zircon are also present (Shastri and others 1991). Larger dikes commonly contain zones with knobby, crumbly granules in their in- teriors presumably caused by deuteric alteration. Relatively rare zones of coarse-grain felsic segregations are present usually in the cenrral portions of the thicker dikes. All diabase samples contain greenschist facies minerals such as chlorite, actinolite, clincnoisite and epidote, presumably due to deutenc alteration.

The dike swarm in the northern Peacock Mountains re 6. Equal area projection of preferred intrudes an uncleformed pluton of ca. 1.39 Ga K-feldspar ion of K-spar megacrysts (magmatic foli- megacrystic granite. Diabase dikes are much less common n the megacrystic granite. Points = poles elsewhere in the study area where they intrude highly matic foliation in the northern Peacock deformed granitic and supracrustal gneiss. These relations ins; point in circle = pole to megacrystic suggest that the diabase may have ascended along the same country rock contact in the northern crustal weakness used by the 1.39 Ga granite.

Mountains; plus = pole to foliation in tral Peacock pluton; plus in circle = pole STRUCTURAL GEOLOGY OF THE a1 Peacock plnton-country rock contact. GNEISS CANYON SHEAR ZONE

During the 1988 Northern Arizona University summer felsic dikes are relatively common in the field camp, the Gneiss Canyon shear zone was discovered in Wash Cliffs but are rare elsewhere in the the Lower Granite Gorge of the Grand Canyon (Karlstrom

The dikes consistently strike north-northeast and and Morgan, 1988). Deformation related to the zone is y to steeply to the northwest. They are rarely approximately 11 km wide, located between river miles 236 a few meters thick. They are commonly and 243. The Gneiss Canyon shear zone was subsequently

c, containing plagioclase, biotite, and rare identified in Milkweed Canyon, the Grand Wash Cliffs and , Phenocrysts are usually not larger than 3 mm the Peacock Mountains for a minimum strike length of 60 matrix is very fine-grained to microcrystalline. A km. Thus, the Gneiss Canyon shear zone appears to be a

dikes have a trachytic texture, a magmatic regional-scale deformation zone. The shear zone is character- fined by the alignment of euhedral plagioclase ized by relatively discrete northeast-striking, steeply north- parallel to the dike margins. These dikes are west-dipping zones of intense deformation that overprint less

h dike from the southern highly deformed northwest-striking structural domains. This a U-Pb zircon age of ca. overprinting relationship produces a pam of nearly orthog-

, suggesting that the felsic onal fabrics that appears to be characteristic of the Prom

exposed at the northern end Domains of Northeast Striking S2 e diabase dikes also crop out Foliation: Zones of Focused Shortening

Valentine, nearly on and Northwest-Side Up Shearing Peacock Mountains.

study area diabase occurs as local, individu- 'Ihe northeast-striking zones of the Gneiss Canyon shear 4). The dikes range from a few cm to 30 m zone are of strongly foliated and lineated gneisses.

ALBlN AND KARLSTROM

Figure 7. Equal area net projection of diabase dike orientations mapped in the study area. Poles to dikes (dots) show a concentration of dikes striking northwesterly and dipping f 20" NE.

Foliation dips steeply to the no@west with stretching linea- tions plunging steeply to the west Intense deformation has produced finescale sheet-like tectonic interlayering of supra- crustal rocks, orthogneisses, and intrusive granites and Figure 8. Intimately interlayered granitic and

supracrustal gneisses in the northern Peacock psgnatitestes Intimate has some mig- Mountains, view to the northeast. Foliation mati- within thw mnes* granik was as* drikn 0350 and dips ,ooNW. ~ ~ ~ ~ i t i ~ roe.S,

into, ihiaS to a few shew lUs light htrude the darker rockG in length, parallel to foliation (Fii. 8).

Asymmetric fabrics with kinematic indicators are com- mires in the sauly area md by .a Ga mite i. me mon within the shear zone. Kinematic indicators dominant- Garnet Mountain area, which constrains the end of S2 to ly show northwest-side-up with a right lateral oblique com- 1.69-1.a Ga (Chamberlain Bowring, 19901. ponent. Types of kinematic indicators include: shear bands, asymmetrical folds of gneissic layering and pegmatite veins, and numerous sigma and a few delta type porphyroclasts

Domains of Northwest-Striking (S1) Foliation

S1 structural domains contain well-foliated and common- Boudinaged layem of mphibolite, with @k injected ly well-line8ted meis& oEh with folson smg crP-

stretching lineations in amphibolite (Fig. 1W). Boudinage pig. 4). In mDas -, foktion dips bth in nderlY features record shortening perpendicular to foliation and subvertical extension. Vein arrays are compatible with ~mrthwest-sideup movement

1.69 Ga. S2 foliation is cross cut at high angles by 1.62 Ga

JORTHWESI'ERN ARIZONA

9B. Asymmetricallp folded vein of pegmatitic granite in gmeh from a horizontal surface im- the Grand Wash Cgffs; suggest a component of right lateral movement. The pencil points north.

9D. Delta-type porpbyroclast in gneiss fndi- cates top to the right (northwest-side-up) move- ment (view to the northeast).

76 ALBIN AND KARLSTROM

Figure 10A. Boudinaged layers of amphibolite within a unit of quartzofeldspathic gneiss (meta- sandstone (?), view to the northeast, The granite between the boudins indicate that the granite intruded during deformation. 10B. Granite injected into a small scale duc- -

tile shear and betkeen foliation planes in gneiss. In this view to the northeast, shearing is top to the rinht (northwest side UD).

016" and d i d 49"SE, the poles ti the tension gashes plunge 41" toward 286", and are nearly parallel to L2 stretching lineations (Fig. 14).

10C. Outcrop drawing of a brittle offset in gneiss and amphibolite. Pegmatitic granite in- trudes the fracture (view to the northeast). I

UORTHWE!3IZRN ARIZONA 77

Foliation trajectory map of the ing domains of the northwest-

foliation, northeast-striking (S2) he curved and abrupt transitions

be part of an S1 domain. The amphibolite

ng, alligned amphiboles, and sills

S1 foWons apparently developed during or after crystal- lization of 1.73 Ga granodiorites (Albin and others, 1991) as these rocks contain northwest-saiking foliation. Shallow- dipping. initially s u b h ~ t a l , foliation in the 1.715 Ga Valentine Granite suggests that those fabrics developed during or after 1.715 Ga.

S1-S2 Overprinting Relationships

Overprinting relationship indicate that the northwest- striking SI foHation(s) predates the development of the northeaststriking % foliation. M-pic S2 shear zones oveqrhtiqg Si wem obsmed in a large body of gmadiaite in the Grand Wash Ciiffs, 8 km north of Hack- @g. 13a). An abrupt aursition beoween structural domains is shown in Pi- 1% which schematically summarizes rewonships obmvd lvlong the contact between a domain of northwest-striking foliation, S1, and a mylonitized granite, dominated by S2, S1 foliations are tightly folded within a meter or two of the m3"lolritbd granite. The northeast- striking F2 axial plane% are parallel ta mylonitic (Sz) foliation, and both folds and the shear zone oleaaly ovetprint S1. Asymmetric porphyroclasts in the mylonite indicate southeast-side-up movement. ?he u n d norbasf plunge of the stretching lineation, 60' toward 045'; M m e s a right lateral component of movement acnws the lmybnite.

Macmmpic scale noRhwest-srriking sOructmes W i n t o northeast striking m e s (Pig. 11). The largest bends am open, concave to the east and naaheast, but bends concave to the southeast are also present. The fold axes of these bends, with few exceptions, plunge steeply west or west-southwest, subparallel to LZ stretching lineations and memscopic F2 fold hingelines (Fig. 14). These d a t i d p s suggest that the hingelines of the minor folds and those of the major structures were both rotated into parallelism with L2 stretch- ing heations as a result of high % strain. In several areas, S1 foliations bend in opposite directions into the same northeast-strilring deformation zone, similar to millipede geometries obscmed in some porphmblasts (Bell 1980).

The large open bend in the Peacock Mountains and the smaller, more numerous and more complex F2 bends in the Grand Wash Cliffs are very similar in style and orientation (Fig. 15). The large domain of S1 foliation in the central Peacock Mountains appeats to be largely unaffected by %. However, near the western front of the range, the S1 fabric in amphibolites, gneisses and metasedhentary rocks becomes hawsingly folded. Large open F2 folds reach amplitudes in the 100's m and plunge moderately to the west southwest. To the north, along the western front of the range, the d e of the folds decrease rapidly to low amplitude (f lm), rela- tively long wavelength (10 m), and plunge moderately to the southwest.

Lineations from domains along the large bend in the Peacock Mountains are generally parallel to b, but some subareas have lineations that diverge from (Fig. 15). This suggests that early L1 lineations were not completely rotated into parallelism with during % s b n i n g . It

ALBIN AND KARLSTROM

Figure 12A. Equal area net of structural data from the roof pendant in the Valentine Granite. Points =poles to S1 foliation; plus = L1 stretch- ing lineations; arc = F2 axial plane; dot in tri- angle =F2 fold axis.

also suggests penetrative, but partitioned, shortening during & defonnation. Where D2 strains are highest, near the large northeast-striking shear zones, F1 and F2 fold axes and stretching lineations are parallel; for example in a refolded fold in amphibolite from the northern Peacock Mountains (Fig. 13c).

S3 Structures

In the Big Bend Canyon area, S2 foliation is deformed and overprinted by minor east-west striking mylonites and shear zones, S3, that dip moderately to steeply to the north. The sense of movement associated with these zones is both right and left lateral. Near the mouth of Big Bend Canyon, S2 foliation bends into an Sg shear, the bends plunge 39' toward 298'. The S3 features may represent deformation during the latter stages of progressive regional deformation. They might also be due to Proterozoic, post-deformational readjustments in the crust.

Microstructure

Microsaucture in metapelite sampled from Black Rock Canyon suggests that the &formational history is at least locally more complex than the D I P 2 history presented above. Figure 16a is a photomicrograph cut parallel to the stretching lineation and perpendicular to what is interpreted

Figure 12B. When the Fz fold axis is rotated to horizontal and the limbs of those folds re- stored to horizontal, the resultant lineations (+%I are oriented eastwest to northwestsouth- east.

to be the regional northwest-striking S1 foliation. Straight inclusion trails within the core of the large euhedral garnet are composed of allignment of quartz blebs and small grains of biotite. The inclusion trails are at a high angle to the S1 foliation in the section. The large whebl garnet shown is typical of the 12 other garnets in the thin section (not shown in Fig. 16a) with inclusion trails that range from 18' to 80' from the foliation. The inclusion trails apparently represent a pre-Sl tectonic layering. The diversity in dK orientations of the inclusion mails indicates that the trace of the early layering does not lie on a single plane, but instead may represent a folded early surface. If so, our "S1" postdates an earlier defonnation and may be axial planar to earlier folds. Alternatively, the various orientations of the inclusion trails might also be due to rotation of the garnets during progres- sive Dl deformation.

Figure 16b is a microscopic example that may be ado- gous to the style of macroscapic partitioning of deformation. This metapelite from Black Rock Canyon has planar zones of aligned sillimanite that appear to be accommodating the simple shear component of deformation. The well developed S-shaped bends defined by the sillimanite needles in the quar&z rich zones are not consistent with folds formed during dextral shearing. Instead, these folds appears to have fonned by flattening, as the quartz-rich zones accommodated the pure shear component of deformation (Bell, 1985).

NORTHWE!SI'ERN ARIZONA 79

Figure 13B. Northeast~striking F2 axial planes and parallel S2 mylonitized zone ooer- print a domain of northwest striking, S1, fol i - tion.

Figure 13C. A refolded fold in amphibolite. The F1 and F2 fold axes are parallel

Peacock Mountains. The thermal history in the Peacock Mountains is inferred to be similar to that of the nearby Hualapai Mountains based on the similarity of lithologies in the two areas. Peak m e q c temperatures of f 7WC.

- N

F2 axis -

O n I

ALBIN AND

Figure 14. Equal area net projection of struc- from the Black Rock Canyon area,

Cliffs: dots = pole to foliation; plus =stretching lineation; dot in square =minor F2 fold axis; dot in triangle = mesoscopic F2 fold

are i n f d hm the granulite grade mineral assemblage (dis- cussed below) and the evidence of parthl melting found in the Grand Wash M s .

Metamorphism

A metapelite containing the granulite grade assemblage sillimanite-garnet-biotite-cordierite-potassium feldspar- plagioclase-qua& (sample AGW-99a) was collected in Black Rock Canyon. Granulite grade rocks are also present in southeastern California (Thomas and others 1988, Young and others 1989), the Garnet MounEain-Gold Butte area and the Cerbat Mountains (David Palais, pers. comm.), and the Gneiss Canyon Shear Zone in the Grand Canyon (Williams, pers. comm.). This is in contrast to the upper amphibolite grade, stamlite-andalusite bearing assemblages in the Huala- pai Mountains (Williams, this volume) and lower a m p h i b lite grade rocks of the Cottonwood Cliffs (Beard, 1985; Williams, this volume). The metamorphic decrease h m northwest to southeast across the Gneiss Canyon shear zone is compatible with kinematic data for northwest-side-up movement on the shear zone. The contrast in metamorphic grade suggests that the Gneiss Canyon shear zone experi-

deformation during or after peak meta-

A boundary between granulite and amphibolite grade provinces in northwestern Arizona was proposed by Andex- son (1989) and Thomas and others (1988). to be approxi- mately 50 km northwest of the Gneiss Canyon shear zone (Fig. 2). Although there may be amphibolite grade rocks northwest of the Gneiss Canyon shear zone, a regional granulite-amphibolite boundary might be better placed at the shear zone, as it is the most southeasterly Imown occurrence of granulite grade rocks in the region. I Models for Development of Domains of Northwest-and Northeast-Striking Foliation

ment of the present stnrctural

formation in the Big Bug 1989). In model 2, F1 folds are depicted as

with the predominance Model 2, local shallow

and local scale shear zones. S1, even if initially dipping as m model 1, was generally rotated to steeper in response to rhe vertical extension during shortening 17-lc,2~).

southeast cross section (because of the oblique, steepl plunging stretching lineation). The shear zones are northwest-dipping, with northwest-side-up mov Smaller scale northwest-side-up and southeast-sideup

mation, and this partitioning was likely influenced aopies that existed after the early defonnation(s). bodies in the deforming crust and granitic bodies during deformation may well have infl

the crust, not deformation 8chlally caused by placement (e.g. Reed and others, 1987).

ALBIN AND KARCS'I'ROM

small grains of Biotite which are also aligned with the quartz blebs. Note also the larger grains plrodueedduri of biotite which are at high angles to both the deformation, inclusion trails and foliation in the matrix.

was assembled Erom rnkmpktes, bland ws

shoaening fabric pig. 17-2c), and/- bomdaries themselves may represent

CONCLUSION

99), 9 mm on the long dimension.

83

, . . m i . . ;.

MAP VIEW I

BE shortening (1.72-1.70 Ga) -

b) NW-SE shortening (1.7Ga)

bse shortening and 61 Intense shl

4'Itianing (1.70 - 1.68 Ga)

.. . ortening and partitioning (1.70-1.68 Gal

1. PROGRESSIVE DEFORMATION: la thrusting Model 2. SEPARATE OROGENIC EVENTS: 2a. generally east-west lines: S1 is subsequently folded

Northwest-trending fabric forms in response to northeast- southwest shoJnening, which may have involved progressive

b a d antiforms and synforms. lc. northwest-southeast thrusting and shortening defamation of which only late I mes of D2 progressive s l w i n ~ surface of S* stays

stages &e depicted. 2 b . ~ separate orogenic event &sed northwest-southeast shortening. 2c. Intense deformation

py &&west- trending but Sl bcoolass rotated dsbx& partitioning, as in model 1, ~&s~oses and obscures cadiex @& and paogrwive shearing in which the northwest side history. ki dominanly up and dextrally relative to southeast ii, Ihe location and extent of the shear zones as well as Ayle and orientation of folds was probably controlled in by crustal anisotropy.

17. Alternate models for the development of orthogonal fabrics in northwestern Arizona, 3-D drawings and corresponding map views of different stages in the deformational

I

I

f : whether the Gneiss Canyon shear zone might be the tectonic history of the western United States as determined by

boundary corresponding to the Mohave-Yavapai isotopic neodymium isotopic mapping: Geological Sociew of America Bulletin, v. 99, p. 674-685.

bolmdary. Structural studies indicate a complex history of Bell, T. H., 1985, Deformation partitioning and mrph3;l

I deformation in northwestern Arizona that resulted in a rotation in metamorphic &ks: a radfcal reG&etation: pattern of nearly orthogonal foliation domains. Early, J o d of Metamorphic Geology, v. 3, p, 109-118.

northWat-s@il&g foliatiom affect 1.73-1 -715 Ga plumns Bell* T. H., 1980, Crenulation cleavage deve lo~enf Pgres-

indicating that early defonnation(s) outlasted 1.715 Ga. The sive bulk inhomogeneous shortening: Tectonophysics, v. 68. p. 9-15.

dominant northeast-striking foliation affects rocks as young Bergh, S. c., and Karhtrom, K. E., The chapanal as 1.69 Ga and is cross-cut bv undeformed 1.68-1.62 Ga Deformation d t i o n i n ~ and heteroneneous bulk (

nlnttnna Thia rlpfnrmatinn waa rhamrt~ri7eA hv the nnrth, shoaenin~ &P ~roterozoic O~OE~G in ~en t rd -a: yru.u..". -..w wr"..uuuv.. .r- r.....rr.r- "J "." r . - r r

Geologi&i ~ocit& of America ~ G l e h , in press. wst-side-up reverse movement on the Gneiss Canyon shear K. R , and Boa, S. A , Roterod zone and penetrative, but partitioned shorn-'-- ------ "- - .

orogenic belt. The growth of met :1Il11g aGruus: UlG chronologic and isotopic boundary in Northwest &ne:

amorphic zitcons at 1.69 Journal of Geology. v. 98, p. 399416. Ga indicates that peak metamorphism coincided with the Condie, IC. C.. and DeMalas, J. D., 1985. The Pinal Schist-- latter shortening event. early Proterozoic quark wacke association in southeastem

Arizona. Precambrian Research, v. 28, p. 337-356. The Gneiss 'One is One of a of C o p s l d P., and C&, K. C., 1986, GeochemiSry rro-

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marks the most southeasterly extent of granulite grade rocks G&1ogiA s&ey Professional Paper 1078, 343 p.

in the regim and genemy eoinci . - . ' A ~ - -L ---- '- nL KarLaom. K. E., 1989, M ~ Y r-kt folding dming R

isotope signatures. The geometry LUGS W~UI a cnange m r o zoic orogeny in central Arizona, in: Geological society of and kinematic history of America Special Paper 235, p. 155-171.

early structures, including possible sutures, remain obscure. Karlstrom, K. E., and Bowring. S. A., 1988, Early Proterozo~c: assemblv of tectonostratinra~hic terranes in

ACKNOWLEDGEMENTS N o d k e r i c x Journal 2 &logy, v. 96, Karlstrom, K. E. and Morgan, P.. 1988,

uate education: cha;lging direction in geology tr We thank Steve Reynolds and Dave Miller for helpful during low enrollments: Geotimes. v. 33. p. 8-10.

reviews of this paper. Research was funded by National Reed, J. C: ~r.. Beckford. M. E.. Premo. W. R., c r i ~ n r e Foundation grant EAR 8916373 to Karl E. Karl- N., and Pallister, J. S., 1987, Evolution

T -..,,I FL..,L- ,,H,L,,~,A ,, c-l~...,-l- -,A I.,, Proterozoic Colorado movince: Constrain SbOm. L ~ U L G I auaaul ~ul~sluwaicu UII IIG~UWULA, arlu IIGI

help is gratefully acknowledged. geochronology: Geology, v. 15, p. 861-865.

Shastri, L. L., Chamberlain, K. R., and Bowring, S. A.,

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