International Geology Review, Vol. 44, 2002, p. 479–500.Copyright © 2002 by V. H. Winston & Son, Inc. All rights reserved.

Crustal-Scale Cross-Section of the U.S. Cordillera, California and Beyond, Its Tectonic Significance, and Speculations

on the Andean OrogenyE. M. MOORES,1

Department of Geology, University of California, Davis, California 95616

J. WAKABAYASHI, 1329 Sheridan Lane, Hayward, California 94544-5332

AND J. R. UNRUH

William Lettis & Associates, 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596

Abstract

A cross-section across northern California from the San Andreas fault to central Nevada exhibitsboth major east- and west-vergent structures. East-vergent structures include crustal wedging andfault-propagation folds in the Coast Ranges, emplacement of the Great Valley ophiolitic basementover Sierran basement rocks, early east-vergent structures in the latter, displacement along the east-ern margin of the Sierra Nevada batholith, and thrust faults in western Nevada. West-vergent struc-tures include faults within the Franciscan complex and “retrocharriage” structures in the SierraNevada

A model of evolution of the U.S. Pacific margin emphasizes the role of ophiolites, island arc–continental margin collisions, and subduction of a large oceanic plateau. Early Mesozoic subductionalong the Pacific margin of North America was modified by a 165–176 Ma collision of a major intra-oceanic arc/ophiolite complex. A complex SW Pacific-like set of small plates and their boundariesat various times may have been present in southern California between 115 and 40 Ma. Subductionof an oceanic plateau about 85–65 Ma (remnants in the Franciscan) produced east-vergent tectonicwedging in the Coast Ranges, possible thrusting along the eastern Sierra Nevada batholith margin,and development of Rocky Mountain Laramide structures. The “Laramide orogeny” is herein rede-fined to include all late Cretaceous–Early Tertiary (75–45 Ma) fold-thrust structures from the PacificCoast to the Rocky Mountains. A speculative model for collisional involvement in the Andean orog-eny is also presented, based upon timing of the onset of the Andean orogeny, the presence of oceanicterranes along the western margin of the Andes, and the presence along part of the length of thechain of a remnant marginal basin.

Introduction

A vital lesson of plate tectonics is that there isno validity to any assumption that the sim-plest and therefore most acceptable interpre-tation demands a proximal rather than adistant origin. (Coombs, 1997, p. 763)

RECENT DEVELOPMENTS IN knowledge of the EasternPacific Cordillera suggest that a re-evaluation of itstectonic development is appropriate. We begin ouranalysis with the U.S. Cordillera in general, and

California in particular. We present here a revisedmodel for tectonic development of the region of theU.S. based upon our own work and that of many oth-ers. This model is an elaboration and refinement ofthe collisional model of orogenic development pre-sented, for example, for California, neighboringNorth America, and northern South America byMoores (1970, 1998), and for California and envi-rons by Ingersoll (2000). Although our scenario ismostly by consideration of a cross-section of Califor-nia (south of the Klamath Mountains) and neighbor-ing Nevada, we recognize that strike-slip motion hasaffected and continues to affect the western part ofthe United States. In addition, we present a re-eval-1Corresponding author; email: moores@geology.ucdavis.edu

4790020-6814/02/598/479-22 $10.00

480 MOORES ET AL.

uation of the Andean orogeny, based upon a briefsummary of the literature.

A key element in our analysis is an emphasis onthe importance of ophiolites. They represent oceancrust and mantle formed at spreading centers andemplaced by collision of a continental margin orisland arc with mantle-rooted thrust faults boundingsubduction zones (Moores, 1998, 2002; Moores etal., 2000). These thrust contacts represent the loca-tion of sutures that are major tectonic features of theregion in question.

Western United States

Figure 1 is a generalized map of the western U.S.margin in California and neighboring regions. Twomaps are shown. Figure 1A shows the position ofselected key elements at present. Figure 1B is a pal-inspastic sketch incorporating removal of Basin andRange extension and restoration of approximately200 km of Mesozoic dextral movement on the PineNut/Mojave–Snow Lake fault (Lewis and Girty,2001). Figure 2 shows a generalized cross-section ofthe region of discussion. The cross-section is basedon work by Wakabayashi and Unruh (1995), Godfreyand Dilek (2000), and more recent data, as enumer-ated below. We describe the elements of this cross-section from west to east, approximately along a sec-tor at 39–40° N. Latitude.

The cross-section illustrates major east-directedthrusts beneath the Coast Ranges, the Central Val-ley, the Sierra Nevada, and the eastern part of theSierra Nevada batholith. These thrust faults vary inage. Figure 3 tabulates a listing of selected tectonic/metamorphic/ophiolitic events in the Coast Ranges,Central Valley, Sierra Nevada, Western Nevada, andsouthern California–Baja California. The cross-sec-tion conforms with the hypothesis of major east-directed lithospheric thrusting proposed recently byDucea (2001), and a collisional model presentedearlier by Moores (1970).

Franciscan Complex

The Franciscan complex is a series of complexlyfolded thrust-nappe structures consisting of intact(coherent) thrust sheets and mélange zones (e.g.,Blake et al., 1984, 1988; Wakabayashi, 1992).North of the San Francisco Bay region, the Fran-ciscan complex traditionally has been divided intothree principal belts (e.g., Blake et al., 1988; Fig. 2).Within and south of the greater San Francisco Bay

region, however, the picture is more complex, anddistinctive belts are not discernible. Generally, thestructural position, metamorphic grade, and age ofincorporation of units (individual nappes inferredfrom metamorphic or clastic rock ages) decreasesfrom east to west, consistent with progressive off-scraping and accretion in the accretionary complex(Wakabayashi, 1992), but this relatively simple pat-tern is complicated by displacement on the SanAndreas fault and associated transpressional fold-ing. In the cross-section, the three northern belts areshown modified after the reconstruction of Wakaba-yashi and Unruh (1995).

Major events of the Franciscan shown in thetable of Figure 3 include the ages of formation andarrival of pelagic sediments in the “Central Belt,” aswell as periods of major metamorphism and exhu-mation, and timing of clastic sedimentation. Gener-ally the ages of deposition and incorporation ofFranciscan rocks are not the same, as indicated inFigure 3. Some mélanges may have originated asolistostromes and have been subsequently sub-ducted, whereas other mélanges may be solely oftectonic origin (e.g., Cowan, 1985).

The Coastal Belt rocks represent the youngestFranciscan rocks, accreted from Paleocene toEocene time (Blake et al., 1988). Rocks includevariably deformed sandstone and shale, subordinatemélange, and minor basalt, limestone, and chert.Field relations indicate that this belt is thrustbeneath the Central Belt to the east. Two freshperidotites, the Leggett and the Cazadero bodiesalong the eastern margin of the Coastal Belt, mayrepresent the offscraped remnants of high-standingdomes formed near ridge-transform intersections(Coleman, 2000).

The Central Belt Franciscan comprises a belt ofshale-matrix mélange units with blocks of diverselithologies and metamorphic grades. These blocksinclude the major pelagic units listed on Figure 3.The Central Belt mélange matrix exhibits bothprehnite-pumpellyite facies (e.g., Blake et al.,1988), and blueschist facies (Terabayashi andMaruyama, 1998) metamorphism. Fossils fromclastic rocks in this belt range from Tithonian toCampanian age, but radiolaria from cherts are asold as Pliensbachian (Blake et al., 1988). The dis-tribution of Tithonian to Valangian fossils within thematrix clastic rocks indicate considerable tectonicrecycling (by mélange plucking and recirculation,e.g., Cloos, 1986; generation of a strike-slipmélange, e.g., McLaughlin et al., 1988), or exhuma-

U.S. CORDILLERA 481

tion and resedimentation. Most Central Belt accre-tion probably occurred from Cenomanian toCampanian time.

The Eastern Belt Franciscan, structurally thehighest and most uniformly recrystallized of the

Franciscan units, comprises coherent thrust sheetsof mostly metaclastic and metavolcanic rocks, andsubordinate mélange (Worrall, 1981), exhibitingblueschist– and blueschist-greenschist–grade meta-morphism (Blake et al., 1988). Metamorphic cooling

FIG. 1. Tectonic sketch map of part of the U.S. Pacific margin, showing selected principal tectonic features, majorophiolite complexes (dark shading), the Great Valley ophiolite (light shading), and the Salinian block (cross-hatched).Abbreviations: B = Bear Mountains ophiolite; C = Catalina schist; CRO = Coast Range Ophiolite; EF = Excelsior fault;F = Franciscan; FRP = Feather River Peridotite; GM = Grizzly Mountain thrust; GV = Great Valley sequence; GVO =Great Valley ophiolite; JO = Josephine ophiolite; HC = Humboldt complex; K = Klamath Mts; KK = Kings Kaweahophiolite; LFT = Luning Fencemaker thrusts; M = Mojave block; MS = Mojave Sonora megashear; MSLF = Mojave SnowLake Fault; PNF = Pine Nut fault; PrP = Preston Peak ophiolite; S = Salinian block; SC = Santa Cruz Is.; SCC = Smart-ville, Slate Creek, and Jarbo Gap ophiolites; T = Trinity ophiolite. A. Present configuration. B. Palinspastic map withdisplacement on PNF-MSLF removed (Lewis and Girty, 2001). Diagrams modified after Dilek and Moores (1992, 1993)and Moores (1998).

482 MOORES ET AL.

ages of the structurally highest and most recrystal-lized part of the belt (Pickett Peak terrane) are 110–146 Ma (e.g., Wakabayashi, 1992, 1999). The meta-morphic age of part of the structurally lower YollaBolly terrane is probably slightly younger than 120Ma (reviewed in Wakabayashi, 1999). Cenomanianrocks of the Hull Mountain area, probably the struc-turally lowest part of the Eastern Belt, were accretedshortly after 95 Ma.

The 159–163 Ma high-temperature/high-pres-sure metamorphism of the high-grade blocks ofEastern and Central Belt mélanges (Fig. 3) probablyoccurred during the inception of subduction (Waka-bayashi, 1990). The most significant exhumation ofcoherent blueschist-facies Franciscan rocks tookplace from 100 to 70 Ma (Tagami and Dumitru,1996).

The widespread presence of aragonite in Fran-ciscan blueschists and the lack of greenschist-faciesoverprints indicate that east-dipping subductionwas continuous throughout Franciscan accretionaryhistory (160 to 20 Ma; cessation dependent on lati-tude; e.g., Wakabayashi, 1992). Continuous subduc-tion is consistent with uninterrupted but variable-rate pluton emplacement in the Sierra Nevada arcuntil ~80 Ma (e.g., Ducea, 2001).

Structurally, the Franciscan rocks display surf-icial dips generally to the east, but with a signifi-cant antiform/synform structures affecting theeasternmost rocks and, as shown on Figure 2, theunderlying Central Belt Rocks (Maxwell, 1974).The age of this regional-scale folding may beequivalent to the “exhumation” and formation ofthe Coast Range fault between 70 and 100 Ma (Fig.3), or alternatively to formation of regional-scalefolds of ≤ 70 Ma of the Great Valley group,described below.

In offshore southern California, garnet amphibo-lite and amphibolite metamorphism in the Catalinaschist occurred at 112 Ma (Mattinson, 1986), andlikely marks the inception of subduction there(Platt, 1975). Continued subduction produced blue-schist/greenschist– and blueschist-facies metamor-phic rocks structurally beneath the high-temperature rocks. All Catalina schists cooled totemperatures between 200 and 300°C by 90–100Ma (Grove and Bebout, 1995). The Pelona-Orocopiaschists of southern California and Arizona, a unitthat includes blueschist-greenschist, epidoteamphibolite, and amphibolite metamorphism, wasmetamorphosed at 60–65 Ma (Jacobson, 1990). Thehigher-temperature metamorphism of the Pelona-

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U.S. CORDILLERA 483

Oropia schists may be related to the inception ofanother subduction zone, or to shallowing of east-dipping subduction beneath an active arc (Jacobson,1997). In Baja California, 160–170 Ma amphibolite

metamorphism in tectonic blocks may mark theinception of subduction, and blueschist assem-blages yield dates of 95–115 Ma (Baldwin and Har-rison, 1992).

FIG. 3. Generalized time-space diagram of features along North American Cordilleran margin. Abbreviations are thesame as those in Figures 1 and 2, plus: am = amphibolite-facies metamorphism; bls = blueschist-facies metamorphism;BH = Burnt Hills unit of Franciscan; “CRF” = Coast Range fault; dr = dextral-reverse shear; FFS = Foothill fault systemof Sierra Nevada; H.C. = Humboldt complex; HM = Hull Mountain unit of Franciscan; MH = Marin Headlands terrane;PT = Permanente terrane; SC = Slate Creek complex; SFM = South Fork Mountain schist; SI = Stikine-Intermontanesuperterrane; sr = sinistral-reverse shear; WI = Wrangell-Insular superterrane; WM = white mica; wr = whole rock; YRP =Yuba Rivers pluton.

484 MOORES ET AL.

Coast Range/Great Valley Ophiolite

This ophiolite sequence underlies the Great Val-ley group throughout the Coast Ranges (the CoastRange ophiolite), as well as in the subsurface of theGreat Valley itself (the Great Valley ophiolite)(Godfrey et al., 1997; Godfrey and Klemperer,1998). The magmatic age of the Coast Range ophio-lite is ~165–170 Ma (Hopson et al., 1996), and a“sedimentary hiatus” between formation of the ophi-olite and deposition of overlying volcanopelagicsediments apparently occurred from 165 to 155 Ma(Pessagno et al., 2000).

In outcrop, faults are present everywhere below,and in some places above, the Coast Range ophio-lite. The fault beneath the ophiolite is commonlycalled the Coast Range fault (Worrall, 1981; Jaykoet al., 1987), and north of San Francisco, along theeastern margin of the Coast Ranges, the fault at thebase of the Great Valley Group is the Stony Creekfault (Lawton, 1956; Chuber, 1962). Godfrey andKlemperer (1998) and Godfrey et al. (1997) inter-preted their results to reflect extensive boudinage inthe ophiolite along the western side of the Great Val-ley, an interpretation that agrees with our own obser-vations of outcrop relations and data from thewestern Sacramento Valley.

Most of the Coast Range ophiolite lacks penetra-tive deformation and displays negligible burialmetamorphism (Hopson et al., 1981). In southernCalifornia, however, the ophiolite on Santa CruzIsland exhibits a pronounced foliation and green-schist-facies burial metamorphism (Hopson et al.,1981), as discussed below.

The geophysical data of Godfrey and Klemperer(1998) and Godfrey et al. (1997) indicate thatbeneath the Central Valley the ophiolite includes athick crustal and mantle sequence that in turn over-lies continental crust and mantle. Thus the CentralValley possesses a double Moho, which is congruentwith its low-lying status surrounded by the rapidlyrising Coast Ranges and Sierra Nevada, and mayexplain the long-standing enigma of the Great Val-ley’s existence as an intermontane basin surroundedon all sides by rising regions — the Coast Ranges,the Klamath Mountains, Sierra Nevada, and Teh-achapis. We adopt the thrust interpretation for thisMoho duplication, an interpretation favored by God-frey and Klemperer (1998), Godfrey et al. (1997),and Godfrey and Dilek (2000), and which is consis-tent with the isotopic evidence of Ducea (2001).

Great Valley Group

Deposits in the Central Valley intermontanebasin include the Tithonian-Maastrichtian GreatValley group (e.g. Moxon, 1988) and overlying Cen-ozoic basin fill (e.g. Bartow, 1990). Jurassic andCretaceous rocks are deep-sea fan turbidites depos-ited on oceanic basement represented by the CoastRange ophiolite (described above). Sedimentationin Late Jurassic and Early Cretaceous time mayhave coincided with subsidence and boudinage ofthe Coast Range ophiolite and subsidence of theancestral forearc basin. A pronounced unconformityin the subsurface affects pre-Albian rocks, withUpper Cretaceous strata lapping eastward overdeformed, west-dipping, lower Great Valley rocksand ophiolitic basement. The mid-Cretaceous angu-lar unconformity visible in seismic reflection pro-files from the Sacramento Valley may be due to: (1)west-down subsidence along the eastern margin offorearc basin; or (2) westward tilting of Late Jurassicand Early Cretaceous strata due to shorteningaccommodated by east-vergent thrusting (asdepicted on Figure 2). Reflector geometries in datafrom the northern Sacramento Valley are consistentwith the former, whereas deformed (ophiolitic?)basement visible in data from the southwestern Sac-ramento Valley seem better explained by the latter.

Franciscan, Coast Range ophiolite and lowerGreat Valley rocks along the eastern Coast Rangesare deformed in a series of NW-trending folds thatare not present in the upper Great Valley rocks (seeFig. 4). Given the lack of Late Cenozoic cover in theCoast Ranges, interpreting the origin of these foldsis problematic. They may be pre–Late Cretaceous inage. Alternatively, they may be Pliocene-Recent,and reflective of folding of similar orientation andstyle in the northern Coast Ranges that has affectedrocks as young as Pleistocene. If pre-Late Creta-ceous, the period of deformation would coincide intime with the onset of the Sevier orogeny to the east,and approximately with a major east-directed litho-sphere-scale thrusting postulated on the basis ofgeochemistry of the Sierra Nevada batholith and itsinclusions by Ducea (2001).

A major east-vergent thrust fault that was rootedin the ancestral subduction zone displaces Fran-ciscan, Coast Range ophiolite, and Great ValleyGroup rocks to the east (Figs. 2 and 3). As discussedby Wakabayashi and Unruh (1995), this thrusting(antithetic to the subduction zone) began in latestCretaceous–Early Tertiary time, just after a period

U.S. CORDILLERA 485

of thick accumulation in the Great Valley forearcbasin, and it corresponds to a major unconformity inthe Great Valley group (Peterson, 1967a, 1967b).Main-stage exhumation of blueschist-facies rocks ofthe Franciscan (100–70 Ma) was likely completedprior to the inception of this east-vergent thrusting,because significant differential exhumation of theFranciscan relative to the unmetamorphosed GreatValley Group and Coast Range ophiolite cannotoccur with this fault geometry (Wakabayashi andUnruh, 1995). The east-vergent thrusting was reac-tivated in Pliocene–Pleistocene time, resulting insteep dips in the Plio-Pleistocene strata along thewestern Central Valley margin, and significant seis-micity including the 1892 Winters-Vacaville and1983 Coalinga earthquakes (Unruh and Moores,1992; Unruh et al., 1995).

Sierra Nevada/Klamaths (SK) :

The Sierra Nevada and Klamath mountainstogether exhibit a complex set of pre-batholithicrocks, generally loosely correlated with each otherand with the Stikine-Intermontane superterrane ofthe northern Cordillera (e.g., Moores, 1998).Although various units of the Sierra Nevada andKlamath mountains have been correlated with oneanother, in detail, significant differences existbetween the correlated units and their tectonic his-tories. One possible explanation for some of the dif-ferences is that there was a trench-trench transformfault between the Sierra and the Klamaths duringpart of Mesozoic (e.g. Dilek and Moores, 1992). Werestrict our comments to the Sierra Nevada, wherepre-batholithic rocks are grouped into four generallyrecognized belts.

The Western Jurassic belt consists of a belt ofandesitic and related extrusives, shallow intrusives,and plutonic rocks that extends some 200 km south-ward from the northern end of the Sierra Nevada(e.g., Schweickert, 1981). In the north it chiefly con-sists of the Smartville complex, a rifted volcanic arcconstructed on 200–220 Ma oceanic basement(Bickford and Day, 1988, 2001; Dilek, 1989a,1989b; Saleeby et al., 1989). In the Smartville com-plex itself, folded pillow lavas and oceanic andesitevolcaniclastic deposts are cut by dikes that in turnintruded and are intruded by plutons ranging in agefrom 164 to 152 Ma (Beard and Day, 1987; Bickfordand Day, 1988, 2001; Saleeby et al., 1989).

The Central belt consists of the 225–175 MaJarbo Gap and Slate Creek ophiolites, the youngestunits of which are correlative with the oldest Smart-ville rocks, and a Mesozoic chert-argillite chaoticunit containing blocks of Carboniferous–PermianPanthalassa limestones (Edelman et al., 1989).Ophiolitic rocks sit in thrust contact above thechert-argillite sequence in places, but elsewhere thelatter are intruded by ophiolitic lithologies. Thethrust contact between the ophiolitic rocks (SlateCreek and related terranes) and chert-argillitesequence is intruded by a 165 Ma pluton (Edelmanand Sharp, 1989). Although ophiolitic remnants arecommon within and along the western margin of theCentral Belt, a tectonic root zone is not present onthe eastern margin of the belt. We interpret this tomean that the Central belt rocks at least in part rep-resent an accretionary prism that was formed in awest-dipping subduction zone. Bickford and Day’s(2001) data indicate that Smartville magmas incor-

FIG. 4. Generalized map of Great Valley–Franciscan,Coast Range ophiolite relations along western side of the Sac-ramento Valley, California. Note that folds in the Coast Rangefault, Coast Range ophiolite, Stony Creek fault, and lowerGreat Valley sequence do not affect upper Cretaceous GreatValley rocks. After Jennings (1977).

486 MOORES ET AL.

porated Precambrian zircons, indicating a conti-nent-derived source component for this oceanic arc,consistent with a west-dipping subduction zone.

The Feather River complex and associated DevilsGate ophiolite contain mafic and ultramafic rocksthat formed in at least two magmatic events, a poorlydefined one of Devonian age, and a later one atabout 300–320 Ma (Saleeby et al., 1989). Horn-blendes, which date amphibolite-grade metamor-phism, range in age from 240 to 390 Ma (reviewed inHacker and Peacock, 1990). This complex is a 6–10km wide zone that extends over 100 km from thenorth end of the Sierra to the south, where it maymerge with the Calaveras–Shoo Fly thrust (Schwe-ickert, 1981).

The Eastern belt is a thick, polydeformedsequence of Paleozoic–Jurassic rocks. The basalunit is the Lower Paleozoic Shoo Fly complex, con-taining fault-bounded units of quartzose turbidite,mafic volcanics, a 15-km long serpentinite lens dis-continuously developed in the northern Sierra, and amélange containing exotic blocks of chert andOrdovician limestone (Hannah and Moores, 1986;Harwood, 1992). Early ENE-trending isoclinal foldsare present in this sequence (Varga and Moores,1981). The serpentinite lens is only a few hundredmeters wide in outcrop, but gravity and magneticdata indicate that it becomes several kilometersthick at depth (Griscom, in Blake et al., 1989).These rocks are interpreted as an off-continent tur-bidite sequence deposited upon unknown, but pre-sumably oceanic, crust. The mélange andserpentinite may reflect an ophiolite emplacementevent in mid–Late Devonian time (Varga andMoores, 1981). Three volcanic-arc complexesunconformably overlie the Shoo Fly rocks : (1)Devonian–Mississippian units of the Sierra Buttesand Taylor, Elwell, Keddie, and Peale formations,interpreted as an oceanic volcanic arc; (2) Permian–Triassic units of the Robinson, Reeve, Arlington,and possibly Cedar formations, also possibly repre-senting an oceanic arc; and (3) a Jurassic sequenceof the Mount Jura and Milton sequences that isinterpreted to represent a continental or near-conti-nental arc (e.g., Hannah and Moores, 1986). Formuch of the length of their exposure, the rocks areprimarily east-facing and overturned to the east.Near the northern end of the Sierra Nevada, how-ever, NW-trending, tight to isoclinal, overturned torecumbent east-vergent folds and thrust faultsdeform the rocks. Along the western margin of theEastern Belt, an enigmatic sequence of Devonian

(?)–Jurassic rocks crop out that show affinities withsequences in the Klamath Mountains (Harwood,1992; Jayko, 1988, 1990; Hannah and Moores,1986).

Three major fault blocks separated by east-ver-gent thrust faults are present in the Eastern belt(Fig. 1). A cleavage fan in the northern Sierra passeswestward to the south and incorporates the FeatherRiver peridotite. In this cleavage fan, NE-dippingfoliation and axial surfaces of folds in the west passeastward into SW-dipping foliation and axial sur-faces. We interpret this structure to reflect the “ret-rocharriage” that has affected rocks of the northernSierra Nevada.

Deformation in the Eastern belt also increases incomplexity from east to west. In the east, post–ShooFly rocks are generally unfoliated and only slightlymetamorphosed, but become progressively isocli-nally folded and finally multiply folded as oneapproaches the contact with the Feather Riverperidotite.

The general structural relations described byDay et al. (1985) suggest that the main Early Meso-zoic structures in the Sierra Nevada consist of east-vergent thrust faults and folds, subsequently modi-fied by west-dipping faults. Accordingly, structuresare depicted on the cross-section as both east-dip-ping and west-dipping (Fig. 2). The early east-ver-gent faults are interpreted to dip westward beneaththe Central Belt, Western Belt, and Great Valley,consistent with geophysical and geochemical data(Dilek and Moores, 1993; Godfrey et al., 1997; God-frey and Klemperer, 1998; Godfrey and Dilek, 2000;Bickford and Day, 2001; Ducea 2001). These faultgeometries also conform with those displayed by aCOCORP traverse across the northern SierraNevada that indicated both west- and east-dippingreflections (Nelson et al., 1986).

The Sierra Nevada batholith intrudes the rocksdescribed above. On Figure 3, the batholith isshown modified from Godfrey and Dilek (2000).

Significant tectonic and magmatic events for theSierra Nevada on Figure 3 include: the age of theSlate Creek and Smartville complexes, as well asthe Yuba River “stitching” pluton (Bickford andDay, 2001); gold mineralization (Böhlke, 1999); theHumboldt complex ages (Dilek and Moores, 1993);age of Eastern Belt thrusts (Hannah and Moores,1986); plutonic activity and lithosphere-scalethrusting (Ducea 2001); apparent times of arrival ofthe Smartville/Slate Creek (i.e., the Stikine-Inter-montane terrane; Moores, 1998); backfolding and

U.S. CORDILLERA 487

formation of the Foothill fault system; arrival ofWrangellia; and the arrival of Salinia (Wakabayashiand Moores, 1988). Arrival of the Smartville/SlateCreek ophiolites, widespread penetrative deforma-tion and folding, as well as Eastern belt thrusting,together constitute the “Nevadan” orogeny of Sch-weickert (1981), although they occurred earlierthan originally envisaged (Edelman and Sharp,1989). The well-known Foothill fault system formedlater in the early–mid Cretaceous, associated with

gold mineralization (e.g., Böhlke, 1999; Ducea,2001).

Parts of the western Sierra Nevada wereaffected by penetrative deformation associatedwith a sinistral-reverse sense of shear (east overwest) from 151 to 123 Ma (Paterson et al., 1987;Tobisch et al., 1989), possibly as a consequenceof partitioning of strain associated with obliqueFranciscan subduction (Wakabayashi, 1992).Dextral-reverse (east over west) ductile shear

FIG. 5. Generalized (A) sketch map and (B) cross-section illustrating possible configuration in Middle Jurassic time,just prior to collision of oceanic island arc with the North American continent. Abbreviations: CGO = Coast Range–GreatValley ophiolites; EK = Eastern Klamath belt; ESK = Eastern Klamath, Eastern Sierra Nevada belts; FRP = FeatherRiver peridotite; GU = Guerrero terrane, Mexico; HW = Walker-Humboldt basin; JG = Jarbo Gap ophiolite; NA = NorthAmerican continent. Mescalera plate after Dickinson and Lawton (2001).

488 MOORES ET AL.

zones that cut the Sierra Nevada batholith(approximate deformation age 100–85 Ma; e.g.,Renne et al., 1993; Tobisch et al., 1995) may alsobe related to strain partitioning of oblique Fran-ciscan subduction.

Basin and Range

At the latitude of the cross-section, the SierraNevada batholith extends into the Basin and Range.The eastern side of the batholith is shown involvedin overthrusts, which we associatie with west-dip-ping crustal-scale reflectors of Allmendinger et al.(1987). West-dipping thrust faults along the WalkerLane and other parts of western Nevada are modi-fied after Dilek and Moores (1993), Dilek et al.(1988), and Godfrey and Dilek (2000). The timing ofthese thrusts is not clear, although they must bepost–Jurassic, pre–Late Tertiary.

Archipelago Style of Orogeny

Moores (1998) proposed a model for an “archi-pelago” style of orogenic development, involvingconvergence and collision of already complexlydeformed oceanic island arcs during Mesozoic andCenozoic time along the western margins of NorthAmerica and northern South America. Figure 5shows a generalized tectonic sketch map and cross-section for Early Jurassic time, just prior to this col-lision. A volcanic arc on the North American cratongives way to the northwest to an Alaska Peninsula–like extension in the northern Sierra Nevada andwestern Nevada, separated from the craton by theoceanic Walker-Humboldt basin in western Nevada.This configuration accounts for the total inferreddisplacement on the Mojave–Snow Lake fault(Lewis and Girty, 2001) and the Lower Mesozoicbasinal sediments in thrust contact beneath theHumboldt complex. An oceanic island arc repre-

FIG. 6. Schematic map of eastern Pacific basin at 180 Ma (Middle Jurassic). Abbreviations: C = Cuba; CC = Cordil-lera Central of Colombia; ESK = Eastern Sierra and Klamath belts; FRP = Feather River peridotite; G = Guerreroterrane; GCO = Great Valley-Coast Range ophiolites; H = Hispaniola; HW = Humboldt-Walker basin; PR = Puerto Rico;SI = Stikine Intermontane superterrane; SK = Sierra Nevada central and western belts and related rocks in KlamathMountains; VC = Venezuelan Coast Ranges; WI = Wrangell-Insular superterrane. Modified after Moores (1998, Fig. 10).See text for discussion.

U.S. CORDILLERA 489

FIG. 7. Schematic map of the eastern Pacific basin at 160 Ma (Late Jurassic) after collision of SI and SK terranesalong western North America, and development of west-facing Franciscan subduction zone (outboard of GCO). Abbrevi-ations are the same as those in Figure 6.

FIG. 8. Schematic map of the eastern Pacific basin at 140 Ma (Early Cretaceous). Abbreviations are the same as inFigure 6, plus: C = Catalina schist.

490 MOORES ET AL.

sented by the Jarbo Gap, Smartville–Slate Creekophiolites, Guerrero terrane, and intervening ophi-olitic/island-arc rocks is separated from the NorthAmerican margin by a plate that was consumed onboth its margins—the Mescalera plate of Dickinsonand Lawton (2001). This plate has essentially disap-peared.

We interpret the thrust-fault soles of these islandarc-ophiolite complexes to represent major sutures.The polarity of the colliding arc was along a west-dipping subduction zone (east-facing arc), becauseof the geometry of the associated structures and theinternal pseudostratigraphy of the collided blocks.The deformed and partly chaotic metasedimentaryrocks of the Central Belt of the northern SierraNevada may partly represent material derived fromthis plate.

The Feather River peridotite may either be aremnant of the Mescalera plate preserved by out-of-sequence thrusting during the island arc–continen-

tal arc collision, or part of the North American plate.The Feather River peridotite must also represent amajor suture of an as-yet unknown nature or age.

West of the oceanic arc is an entirely oceanicplate called the Americord plate (this is the sameplate referred to by Moores [1998] as the Cordilleriaplate). The name has been changed because of priorusage of the term “Cordilleria” by Chamberlain andLambert, 1985, and Lambert and Chamberlain,1988). This model for plate evolution is similar tothat proposed by Ingersoll (2000).

Tectonic Reconstructions of North American–NorthernSouth American Margins

We present here a series of approximate cartoons(schematic maps) modified after Moores (1998) thatelaborate on the “archipelago” style of deformation.Figure 6 shows a possible reconstruction at ~180

FIG. 9. Schematic map of the eastern Pacific basin at 100 Ma (later Early Cretaceous) showing possible positions ofWI superterrane and hypothetical oceanic plateau. Abbreviations are the same as in Figures 6 and 7 plus: CH = Chortisblock; P = Piñon sequence of western Colombia and Ecuador; PE = Franciscan Permanente terrane; PO = Pelona andOrocopia basins; V = Venezuelan basin; Y = Yucatan basin.

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Ma. From west to east, the features are the Farallonplate, the Wrangell-Insular (WI) superterrane withan active island arc in the approximate position atthis time given by Debiche et al. (1987), the Ameri-cord plate separating the Wrangell-Insular from theStikine/Intermontaine (SI) terrane and its possiblecontinuations to the south, the Mescalera plate, andthe North American plate. Note that two plates sep-arate North and South America from the Farallonplate. The spreading center producing the GreatValley/Coast Range ophiolite within the Americordplate may have extended to the north or south.

Figure 7 shows a possible scenario at 160 Ma.The SK terranes have attached to the north, but isstill outboard of the continent to the south, therebypossibly setting up a transform fault between two

trenches of opposite polarity. Subduction is presentoff northern South America.

At 140 Ma (Fig. 8), the Guerrero terrane hasattached (Dickinson and Lawton, 2001), but itssouthern equivalents are still oceanward of the con-tinents. The future position of the Catalina schist(i.e., future C) is shown east of the WI superterrane.

At 100 Ma (Fig. 9), a possible scenario has a sub-duction zone along the entire margin of westernNorth America. The WI terrane has possibly arrivedin a “compromise” position (Stamatakos et al.,2001), although its position could be further to thewest as shown, based upon the lack of paleolongi-tude constraint from paleomagnetic data. A west-dipping subduction zone has formed to produce theCatalina schist (C); this zone begins at 115 Ma and

FIG. 10. Schematic map of the eastern Pacific basin at 60 Ma (Paleocene), showing development of the Laramideorogeny from the Pacific coast to the Rocky Mountains, re-attached Salinian block. Abbreviations are the same as inprevious figures plus: PR = Peninsular Ranges terranes.

492 MOORES ET AL.

collides at 90–100 Ma. An oceanic plateau west ofWI contains the Permanente Terrane (PE). Subduc-tion of this oceanic plateau beginning in latest Cre-taceous–Early Tertiary time (75–45 Ma) will causethe Laramide orogeny (Henderson et al., 1984), ashere understood to include all crustal shortening ofthe same age west to the continental margin. Loca-tion of possible future rifting of the Salinian blockand development of the Pelona-Orocopia (PO)basins are shown.

By 60 Ma (Fig. 10), subduction of the oceanicplateau is inferred to have produced a flattening ofthe slab (now the Farallon plate). Increased tractionfrom this flattening would have produced compres-sional structures from the continental margin to theRocky Mountains, including thrust wedging in theCoast Ranges, possibly thrusting along the easternedge of the Sierra and Klamaths, possibly renewedmovement on Sevier thrusts in Nevada-Utah, and

development of the Laramide structures in theRocky Mountains. At a 5–10 cm/year subductionrate, the front of a subducted wide oceanic plateauwould take approximately 10–20 million years tosweep eastward 1000 km from the coast.

By 40 Ma (Fig. 11), subduction has been re-established along part of the North American mar-gin. The Pelona-Orocopia schists have beenemplaced along an east-vergent thrust, and Saliniamay have moved slightly northward by pre-SanAndreas strike-slip faulting.

Figures 9–11 reflect our interpretation that thistime in eastern Pacific history was exceptionallycomplex, more than commonly envisioned.Although we acknowledge that these figures are onlyschematic, they point toward a complexity that isreminiscent of contemporary reconstructions of theSoutheast Asia/Southwest Pacific region (e.g. Hall,1996).

FIG. 11. Schematic map of the eastern Pacific at 40 Ma (Middle Eocene). Abbreviations are the same as in Figures6, 7, and 8.

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Discussion: The North American Cordillera and a

Speculative Reconstruction of the Andes

Advances yet to be made in geology at firstwill be regarded as outrages.

– W. M. Davis, 1926

The Andes of South America have long been con-sidered as a type example of subduction beneath thecontinental margin paired with antithetic thrusting,

in part associated with “flat slab subduction”(i.e.,“Andean-style orogeny). Gutscher et al. (2000) con-vincingly demonstrated, however, that modern areasof flat slabs correspond to the subduction of aseismicridges. Here we present a short synopsis of a possiblerevision in some of the ideas on tectonic develop-ment of the Andes. This is meant not as a compre-hensive review, but as a provocative essay in thecontext of our model for western North America.

Figure 12 shows a generalized sketch map of theAndes (modified after Moores and Twiss, 1995, Fig.

FIG. 12. Generalized map of Andes, showing possible allochthonous terranes and marginal basins. Symbols: lightshading = allochthonous terranes of Colombia and Ecuador, inferred oceanic arc in western Chile; intermediate shading= aborted marginal basins in Chile; dark shading = Rocas Verdes ophiolites of southern Chile; cross ruling = Pampeanranges of western Argentina. Note location of cross-sections of Figure 13. After Moores and Twiss (1995, Fig. 12.12),Mégard (1989), Mpdozis and Ramos (1989).

494 MOORES ET AL.

FIG. 13. Generalized cross-sections of Andes. Symbols: light shading = accreted terranes in sections A, B, C, D, E,and G; intermediate shading = aborted marginal basin in sections C, and D; dark shading in section G = Rocas Verdesophiolites. After Moores and Twiss (1995, Fig. 12.13), Roeder (1988), Vicente (1989), and Mpodozis and Ramos (1989).

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12.12 and Mpdozis and Ramos, 1990), togetherwith possible allochthonous features. The latterinclude the “western terranes” of Cretaceous andJurassic age in western Ecuador and Colombia andnorthwestern Peru, which include blueschist andeclogite (Feininger, 1980, 1987; Aspden and Lith-erland, 1992), the Paracas arc of Colombia, thevolcanic-rich “Western Mesozoic Series” of Peru(Mégard, 1989), and possibly the allochthonousPrecambrian Arequipa massif of southern Peru(Vicente, 1989) and the very thick Jurassic–LowerCretaceous oceanic-affinity island-arc rocks of theCoast Range of Chile (Vergara, et al., 1995, Buch-elt and Cancino, 1988). These oceanic-affinityrocks possibly developed when separated fromSouth America by a marginal basin represented bythe “aborted” marginal basin of Chile (Mpdozisand Ramos, 1990, Ramos and Aleman, 2000,Dalziel, 1986), the West Peruvian Trough (Dalziel,1986), and the “Rocas Verdes” ophiolite basin ofPatagonia and Tierra del Fuego (e.g., Dalziel,1986; Stern and deWit, 1997).

Figure 13 shows cross-sections of the Andes forthe various sectors. These cross-sections display thevarious rocks to the west, and the presence (or

absence) of the Andean fold belt to the east. Note-worthy is the fact that Sector F lacks thrust-relateddeformation. Only orogen-parallel strike-slip fault-ing is present.

Figure 14 is a time-space diagram, modifiedafter Moores and Twiss (1995, Fig. 12.14), thatshows the main lithologies in the various sectors andthe onset of the Andean orogeny. This onset ismarked by a transition from basinal and/or arcdevelopment and major crustal shortening. In par-ticular, this crustal shortening involves east-vergentfolds and thrusts in the north and south (sectors Aand G) where a recognized suture is present, as wellin other parts of the mountain belt where a suturehas not been identified. The similarity of structuresalong the length of the Andes implies a similar tec-tonic history in sectors B through E, but not F.

Note that the timing of onset of the Andean orog-eny is diachronous, being early Late Jurassic in thenorth and south, and becoming progressively later tothe center. This variation in onset of orogeny arguesagainst an orogenic event driven by far-field effectsof spreading from the Mid-Atlantic Ridge or byincrease in movement in a so-called “absolute”(hotspot) frame of reference.

FIG. 14. Generalized time-space diagram showing principal depositional and tectonic events for the Andes. Broadline indicates onset of “Andean orogeny.” After Moores and Twiss (1995, Fig. 12.14) and Mpodozis and Ramos (1989).Abbreviations: bls-ec = blueschist-eclogite–facies metamorphism (after Feininger, 1980).

496 MOORES ET AL.

These data and the orogen-wide prevalence ofeast-vergent thrusting suggest that the Andes orogenalso may include a role for Mesozoic collision of anoffshore island arc, perhaps previously rifted fromthe South American continent by back-arc rifting,reversal of subduction, “collapse” (subduction) ofthe marginal basin, and collision. Figure 15 is aschematic tectonic sketch for Late Mesozoic time,showing an offshore island arc composed of the pos-sibly allochthonous units described above. The“quiet zone” (C. Mpdozis, pers. commun., 1989) ofsector F is represented by a gap in the arc. The platebetween South America and the postulated offshoreisland arc may have been equivalent to the Mescal-era plate of Dickinson and Lawton (2001).

This reconstruction may seem outrageous, but itfits a number of intriguing and heretofore unex-plained features of the Andes. Of course, weacknowledge the major continental-arc nature ofmuch of the Late Mesozoic and Cenozoic history ofthe Andes (e.g., Lamb, et al., 1997). At least onesuture is present from northern Colombia to south-ern Ecuador (e.g., Aspden and Litherland, 1992)and from Tierra del Fuego to the South Patagonianice field (S. Harambour, pers. commun., 1989).Blueschist and eclogite (132 Ma white mica coolingage) in southern Ecuador is certainly associatedwith a suture (Feininger, 1980).

We are aware that no suture has been identifiedalong much of the rest of the length of the Andes. If

FIG. 15. Hypothetical configuration of the western margin of South America in Late Mesozoic time, showing inferredpreorogenic edge of South America, and proposed offshore island arc and inferred components. See text for discussion.

U.S. CORDILLERA 497

one is present, its lack of exposure or recognitionmay possibly have resulted from subsequent short-ening, strike-slip faulting, juxtaposition of volcanicrocks of similar character across the suture, orbecause the suture is overlain by younger rocks orobscured by younger intrusions. We suggest thatthe Andean fold-thrust belt had its inception withthe arc-continent collision, and has been reactivatedtoday because of major South American–Cocos–Nazca–Antarctic plate interactions. Thus, therevised tectonic history of the Mesozoic westernUnited States suggested in this paper may find itscounterpart in the Andes.

Acknowledgments

EMM thanks I. W. D. Dalziel, C. Mpdozis, F.Hervé, and V. Ramos for introducing him to theAndes, W. G. Ernst and S. Klemperer for organizingthe Thompson Symposium, and George Thompsonfor fruitful discussions about many topics. We havealso benefited from helpful discussions with J. F.Dewey, R. J. Twiss, Y. Dilek, and W. D. Sharp. Com-ments from J. F. Dewey and Y. Dilek on an earlierversion of this manuscript helped to improve it.

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