Moores Et Al..Fm

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

E. 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 exhibits both major east- and west-vergent structures. East-vergent structures include crustal wedging and fault-propagation folds in the Coast Ranges, emplacement of the Great Valley ophiolitic basement over Sierran basement rocks, early east-vergent structures in the latter, displacement along the eastern margin of the Sierra Nevada batholith, and thrust faults in western Nevada. West-vergent structures include faults within the Franciscan complex and “retrocharriage” structures in the Sierra Nevada 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 subduction along 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 boundaries at various times may have been present in southern California between 115 and 40 Ma. Subduction of an oceanic plateau about 85–65 Ma (remnants in the Franciscan) produced east-vergent tectonic wedging 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 Pacific Coast to the Rocky Mountains. A speculative model for collisional involvement in the Andean orogeny is also presented, based upon timing of the onset of the Andean orogeny, the presence of oceanic terranes along the western margin of the Andes, and the presence along part of the length of the chain of a remnant marginal basin.

Introduction California in particular. We present here a revised model for tectonic development of the region of the A vital lesson of plate tectonics is that there is U.S. based upon our own work and that of many oth- no validity to any assumption that the sim- ers. This model is an elaboration and refinement of plest and therefore most acceptable interpre- the collisional model of orogenic development pre- tation demands a proximal rather than a sented, for example, for California, neighboring distant origin. (Coombs, 1997, p. 763) North America, and northern South America by RECENT DEVELOPMENTS IN knowledge of the Eastern Moores (1970, 1998), and for California and envi- Pacific Cordillera suggest that a re-evaluation of its rons by Ingersoll (2000). Although our scenario is tectonic development is appropriate. We begin our mostly by consideration of a cross-section of Califor- analysis with the U.S. Cordillera in general, and nia (south of the Klamath Mountains) and neighboring Nevada, we recognize that strike-slip motion has affected and continues to affect the western part of 1Corresponding author; email: [email protected] the United States. In addition, we present a re-eval-

0020-6814/02/598/479-22 $10.00 479 480 MOORES ET AL. uation of the Andean orogeny, based upon a brief region, however, the picture is more complex, and summary of the literature. distinctive belts are not discernible. Generally, the A key element in our analysis is an emphasis on structural position, metamorphic grade, and age of the importance of ophiolites. They represent ocean incorporation of units (individual nappes inferred crust and mantle formed at spreading centers and from metamorphic or clastic rock ages) decreases emplaced by collision of a continental margin or from east to west, consistent with progressive off- island arc with mantle-rooted thrust faults bounding scraping and accretion in the accretionary complex subduction zones (Moores, 1998, 2002; Moores et (Wakabayashi, 1992), but this relatively simple pat- al., 2000). These thrust contacts represent the loca- tern is complicated by displacement on the San tion of sutures that are major tectonic features of the Andreas fault and associated transpressional fold- region in question. ing. In the cross-section, the three northern belts are shown modified after the reconstruction of Wakaba- yashi and Unruh (1995). Western United States Major events of the Franciscan shown in the Figure 1 is a generalized map of the western U.S. table of Figure 3 include the ages of formation and margin in California and neighboring regions. Two arrival of pelagic sediments in the “Central Belt,” as maps are shown. Figure 1A shows the position of well as periods of major metamorphism and exhu- selected key elements at present. Figure 1B is a pal- mation, and timing of clastic sedimentation. Gener- inspastic sketch incorporating removal of Basin and ally the ages of deposition and incorporation of Range extension and restoration of approximately Franciscan rocks are not the same, as indicated in 200 km of Mesozoic dextral movement on the Pine Figure 3. Some mélanges may have originated as Nut/Mojave–Snow Lake fault (Lewis and Girty, olistostromes and have been subsequently sub- 2001). Figure 2 shows a generalized cross-section of ducted, whereas other mélanges may be solely of the region of discussion. The cross-section is based tectonic origin (e.g., Cowan, 1985). on work by Wakabayashi and Unruh (1995), Godfrey The Coastal Belt rocks represent the youngest and Dilek (2000), and more recent data, as enumer- Franciscan rocks, accreted from Paleocene to ated below. We describe the elements of this cross- Eocene time (Blake et al., 1988). Rocks include section from west to east, approximately along a sec- variably deformed sandstone and shale, subordinate tor at 39–40° N. Latitude. mélange, and minor basalt, limestone, and chert. The cross-section illustrates major east-directed Field relations indicate that this belt is thrust thrusts beneath the Coast Ranges, the Central Val- beneath the Central Belt to the east. Two fresh ley, the Sierra Nevada, and the eastern part of the peridotites, the Leggett and the Cazadero bodies Sierra Nevada batholith. These thrust faults vary in along the eastern margin of the Coastal Belt, may age. Figure 3 tabulates a listing of selected tectonic/ represent the offscraped remnants of high-standing metamorphic/ophiolitic events in the Coast Ranges, domes formed near ridge-transform intersections Central Valley, Sierra Nevada, Western Nevada, and (Coleman, 2000). southern California–Baja California. The cross-sec- The Central Belt Franciscan comprises a belt of tion conforms with the hypothesis of major east- shale-matrix mélange units with blocks of diverse directed lithospheric thrusting proposed recently by lithologies and metamorphic grades. These blocks Ducea (2001), and a collisional model presented include the major pelagic units listed on Figure 3. earlier by Moores (1970). The Central Belt mélange matrix exhibits both prehnite-pumpellyite facies (e.g., Blake et al., Franciscan Complex 1988), and blueschist facies (Terabayashi and Maruyama, 1998) metamorphism. Fossils from The Franciscan complex is a series of complexly clastic rocks in this belt range from Tithonian to folded thrust-nappe structures consisting of intact Campanian age, but radiolaria from cherts are as (coherent) thrust sheets and mélange zones (e.g., old as Pliensbachian (Blake et al., 1988). The dis- Blake et al., 1984, 1988; Wakabayashi, 1992). tribution of Tithonian to Valangian fossils within the North of the San Francisco Bay region, the Fran- matrix clastic rocks indicate considerable tectonic ciscan complex traditionally has been divided into recycling (by mélange plucking and recirculation, three principal belts (e.g., Blake et al., 1988; Fig. 2). e.g., Cloos, 1986; generation of a strike-slip Within and south of the greater San Francisco Bay mélange, e.g., McLaughlin et al., 1988), or exhuma- U.S. CORDILLERA 481

FIG. 1. Tectonic sketch map of part of the U.S. Pacific margin, showing selected principal tectonic features, major ophiolite 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 Kaweah ophiolite; LFT = Luning Fencemaker thrusts; M = Mojave block; MS = Mojave Sonora megashear; MSLF = Mojave Snow Lake 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 with displacement on PNF-MSLF removed (Lewis and Girty, 2001). Diagrams modified after Dilek and Moores (1992, 1993) and Moores (1998). tion and resedimentation. Most Central Belt accre- Franciscan units, comprises coherent thrust sheets tion probably occurred from Cenomanian to of mostly metaclastic and metavolcanic rocks, and Campanian time. subordinate mélange (Worrall, 1981), exhibiting The Eastern Belt Franciscan, structurally the blueschist– and blueschist-greenschist–grade meta- highest and most uniformly recrystallized of the morphism (Blake et al., 1988). Metamorphic cooling 482 MOORES ET AL.

ages of the structurally highest and most recrystallized part of the belt (Pickett Peak terrane) are 110– 146 Ma (e.g., Wakabayashi, 1992, 1999). The metamorphic age of part of the structurally lower Yolla Bolly terrane is probably slightly younger than 120 Ma (reviewed in Wakabayashi, 1999). Cenomanian rocks of the Hull Mountain area, probably the structurally lowest part of the Eastern Belt, were accreted shortly after 95 Ma. The 159–163 Ma high-temperature/high-pressure metamorphism of the high-grade blocks of Eastern and Central Belt mélanges (Fig. 3) probably occurred during the inception of subduction (Waka-

can Precambrian crystalline basement; YB = basement; crystalline Precambrian can bayashi, 1990). The most significant exhumation of i belt; F CenB = Franciscan Central belt; GT = an Andreas development. Abbreviations are the are Abbreviations development. Andreas an coherent blueschist-facies Franciscan rocks took

astal place from 100 to 70 Ma (Tagami and Dumitru, 1996). The widespread presence of aragonite in Fran- ciscan blueschists and the lack of greenschist-facies overprints indicate that east-dipping subduction was continuous throughout Franciscan accretionary history (160 to 20 Ma; cessation dependent on latitude; e.g., Wakabayashi, 1992). Continuous subduction is consistent with uninterrupted but variable- rate pluton emplacement in the Sierra Nevada arc until ~80 Ma (e.g., Ducea, 2001). Structurally, the Franciscan rocks display surf- icial dips generally to the east, but with a significant antiform/synform structures affecting the easternmost rocks and, as shown on Figure 2, the underlying Central Belt Rocks (Maxwell, 1974). ong the approximate line indicated in Figure 1, just prior to S to prior just 1, Figure in indicated line approximate the ong it; S.N. = Sierra Nevada; WL = Walker Lane; XB = North Amer North XB = Lane; WLWalker = Nevada; Sierra = S.N. it; The age of this regional-scale folding may be equivalent to the “exhumation” and formation of

B = Sierra Nevada Eastern Belt; F = Coastal Belt Franciscan Co Franciscan Belt Coastal = F Belt; Eastern Nevada Sierra = B the Coast Range fault between 70 and 100 Ma (Fig. ek (2000). 3), or alternatively to formation of regional-scale folds of ≤ 70 Ma of the Great Valley group, described below. In offshore southern California, garnet amphibolite and amphibolite metamorphism in the Catalina schist occurred at 112 Ma (Mattinson, 1986), and likely marks the inception of subduction there (Platt, 1975). Continued subduction produced blueschist/greenschist– and blueschist-facies metamorphic rocks structurally beneath the high- temperature rocks. All Catalina schists cooled to temperatures between 200 and 300°C by 90–100 Ma (Grove and Bebout, 1995). The Pelona-Orocopia schists of southern California and Arizona, a unit

. 2. Crustal-scale cross-section of the western United States al States United western the of cross-section Crustal-scale 2. . that includes blueschist-greenschist, epidote IG

F amphibolite, and amphibolite metamorphism, was metamorphosed at 60–65 Ma (Jacobson, 1990). The same as in Figure 1 plus: CB = Sierra Nevada Central Belt; E Belt; Central Nevada Sierra = CB plus: 1 Figure in as same Golconda thrust; J = Jurassic rocks; PP = Franciscan Picket Peak un Peak Picket Franciscan = PP rocks; = J Jurassic thrust; Golconda FranciscanBolla Yolla unit. Modified after Godfrey and Dil higher-temperature metamorphism of the Pelona- U.S. CORDILLERA 483

FIG. 3. Generalized time-space diagram of features along North American Cordilleran margin. Abbreviations are the same 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 system of 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-Intermontane superterrane; sr = sinistral-reverse shear; WI = Wrangell-Insular superterrane; WM = white mica; wr = whole rock; YRP = Yuba Rivers pluton.

Oropia schists may be related to the inception of metamorphism in tectonic blocks may mark the another subduction zone, or to shallowing of east- inception of subduction, and blueschist assem- dipping subduction beneath an active arc (Jacobson, blages yield dates of 95–115 Ma (Baldwin and Har- 1997). In Baja California, 160–170 Ma amphibolite rison, 1992). 484 MOORES ET AL.

Coast Range/Great Valley Ophiolite Great Valley Group This ophiolite sequence underlies the Great Val- Deposits in the Central Valley intermontane ley group throughout the Coast Ranges (the Coast basin include the Tithonian-Maastrichtian Great Range ophiolite), as well as in the subsurface of the Valley group (e.g. Moxon, 1988) and overlying Cen- Great Valley itself (the Great Valley ophiolite) ozoic basin fill (e.g. Bartow, 1990). Jurassic and (Godfrey et al., 1997; Godfrey and Klemperer, Cretaceous rocks are deep-sea fan turbidites depos- 1998). The magmatic age of the Coast Range ophio- ited on oceanic basement represented by the Coast lite is ~165–170 Ma (Hopson et al., 1996), and a Range ophiolite (described above). Sedimentation “sedimentary hiatus” between formation of the ophi- in Late Jurassic and Early Cretaceous time may olite and deposition of overlying volcanopelagic have coincided with subsidence and boudinage of sediments apparently occurred from 165 to 155 Ma the Coast Range ophiolite and subsidence of the (Pessagno et al., 2000). ancestral forearc basin. A pronounced unconformity In outcrop, faults are present everywhere below, in the subsurface affects pre-Albian rocks, with and in some places above, the Coast Range ophio- Upper Cretaceous strata lapping eastward over lite. The fault beneath the ophiolite is commonly deformed, west-dipping, lower Great Valley rocks called the Coast Range fault (Worrall, 1981; Jayko and ophiolitic basement. The mid-Cretaceous angu- et al., 1987), and north of San Francisco, along the lar unconformity visible in seismic reflection pro- eastern margin of the Coast Ranges, the fault at the files from the Sacramento Valley may be due to: (1) west-down subsidence along the eastern margin of base of the Great Valley Group is the Stony Creek forearc basin; or (2) westward tilting of Late Jurassic fault (Lawton, 1956; Chuber, 1962). Godfrey and and Early Cretaceous strata due to shortening Klemperer (1998) and Godfrey et al. (1997) inter- accommodated by east-vergent thrusting (as preted their results to reflect extensive boudinage in depicted on Figure 2). Reflector geometries in data the ophiolite along the western side of the Great Val- from the northern Sacramento Valley are consistent ley, an interpretation that agrees with our own obser- with the former, whereas deformed (ophiolitic?) vations of outcrop relations and data from the basement visible in data from the southwestern Sac- western Sacramento Valley. ramento Valley seem better explained by the latter. Most of the Coast Range ophiolite lacks penetra- Franciscan, Coast Range ophiolite and lower tive deformation and displays negligible burial Great Valley rocks along the eastern Coast Ranges metamorphism (Hopson et al., 1981). In southern are deformed in a series of NW-trending folds that California, however, the ophiolite on Santa Cruz are not present in the upper Great Valley rocks (see Island exhibits a pronounced foliation and green- Fig. 4). Given the lack of Late Cenozoic cover in the schist-facies burial metamorphism (Hopson et al., Coast Ranges, interpreting the origin of these folds 1981), as discussed below. is problematic. They may be pre–Late Cretaceous in The geophysical data of Godfrey and Klemperer age. Alternatively, they may be Pliocene-Recent, (1998) and Godfrey et al. (1997) indicate that and reflective of folding of similar orientation and beneath the Central Valley the ophiolite includes a style in the northern Coast Ranges that has affected thick crustal and mantle sequence that in turn over- rocks as young as Pleistocene. If pre-Late Creta- lies continental crust and mantle. Thus the Central ceous, the period of deformation would coincide in Valley possesses a double Moho, which is congruent time with the onset of the Sevier orogeny to the east, with its low-lying status surrounded by the rapidly and approximately with a major east-directed litho- rising Coast Ranges and Sierra Nevada, and may sphere-scale thrusting postulated on the basis of explain the long-standing enigma of the Great Val- geochemistry of the Sierra Nevada batholith and its ley’s existence as an intermontane basin surrounded inclusions by Ducea (2001). on all sides by rising regions — the Coast Ranges, A major east-vergent thrust fault that was rooted the Klamath Mountains, Sierra Nevada, and Teh- in the ancestral subduction zone displaces Fran- achapis. We adopt the thrust interpretation for this ciscan, Coast Range ophiolite, and Great Valley Moho duplication, an interpretation favored by God- Group rocks to the east (Figs. 2 and 3). As discussed frey and Klemperer (1998), Godfrey et al. (1997), by Wakabayashi and Unruh (1995), this thrusting and Godfrey and Dilek (2000), and which is consis- (antithetic to the subduction zone) began in latest tent with the isotopic evidence of Ducea (2001). Cretaceous–Early Tertiary time, just after a period U.S. CORDILLERA 485

Sierra Nevada/Klamaths (SK) :

The Sierra Nevada and Klamath mountains together exhibit a complex set of pre-batholithic rocks, generally loosely correlated with each other and with the Stikine-Intermontane superterrane of the northern Cordillera (e.g., Moores, 1998). Although various units of the Sierra Nevada and Klamath mountains have been correlated with one another, in detail, significant differences exist between the correlated units and their tectonic his- tories. One possible explanation for some of the differences is that there was a trench-trench transform fault between the Sierra and the Klamaths during part of Mesozoic (e.g. Dilek and Moores, 1992). We restrict our comments to the Sierra Nevada, where pre-batholithic rocks are grouped into four generally recognized belts. The Western Jurassic belt consists of a belt of andesitic and related extrusives, shallow intrusives, and plutonic rocks that extends some 200 km southward from the northern end of the Sierra Nevada (e.g., Schweickert, 1981). In the north it chiefly consists of the Smartville complex, a rifted volcanic arc constructed on 200–220 Ma oceanic basement (Bickford and Day, 1988, 2001; Dilek, 1989a, 1989b; Saleeby et al., 1989). In the Smartville com- FIG. 4. Generalized map of Great Valley–Franciscan, plex itself, folded pillow lavas and oceanic andesite Coast Range ophiolite relations along western side of the Sac- volcaniclastic deposts are cut by dikes that in turn ramento Valley, California. Note that folds in the Coast Range intruded and are intruded by plutons ranging in age fault, Coast Range ophiolite, Stony Creek fault, and lower from 164 to 152 Ma (Beard and Day, 1987; Bickford Great Valley sequence do not affect upper Cretaceous Great and Day, 1988, 2001; Saleeby et al., 1989). Valley rocks. After Jennings (1977). The Central belt consists of the 225–175 Ma Jarbo Gap and Slate Creek ophiolites, the youngest units of which are correlative with the oldest Smart- of thick accumulation in the Great Valley forearc ville rocks, and a Mesozoic chert-argillite chaotic basin, and it corresponds to a major unconformity in unit containing blocks of Carboniferous–Permian the Great Valley group (Peterson, 1967a, 1967b). Panthalassa limestones (Edelman et al., 1989). Main-stage exhumation of blueschist-facies rocks of Ophiolitic rocks sit in thrust contact above the the Franciscan (100–70 Ma) was likely completed chert-argillite sequence in places, but elsewhere the prior to the inception of this east-vergent thrusting, latter are intruded by ophiolitic lithologies. The because significant differential exhumation of the thrust contact between the ophiolitic rocks (Slate Franciscan relative to the unmetamorphosed Great Creek and related terranes) and chert-argillite Valley Group and Coast Range ophiolite cannot sequence is intruded by a 165 Ma pluton (Edelman occur with this fault geometry (Wakabayashi and and Sharp, 1989). Although ophiolitic remnants are Unruh, 1995). The east-vergent thrusting was reac- common within and along the western margin of the tivated in Pliocene–Pleistocene time, resulting in Central Belt, a tectonic root zone is not present on steep dips in the Plio-Pleistocene strata along the the eastern margin of the belt. We interpret this to western Central Valley margin, and significant seis- mean that the Central belt rocks at least in part rep- micity including the 1892 Winters-Vacaville and resent an accretionary prism that was formed in a 1983 Coalinga earthquakes (Unruh and Moores, west-dipping subduction zone. Bickford and Day’s 1992; Unruh et al., 1995). (2001) data indicate that Smartville magmas incor- 486 MOORES ET AL. porated Precambrian zircons, indicating a conti- (?)–Jurassic rocks crop out that show affinities with nent-derived source component for this oceanic arc, sequences in the Klamath Mountains (Harwood, consistent with a west-dipping subduction zone. 1992; Jayko, 1988, 1990; Hannah and Moores, The Feather River complex and associated Devils 1986). Gate ophiolite contain mafic and ultramafic rocks Three major fault blocks separated by east-ver- that formed in at least two magmatic events, a poorly gent thrust faults are present in the Eastern belt defined one of Devonian age, and a later one at (Fig. 1). A cleavage fan in the northern Sierra passes about 300–320 Ma (Saleeby et al., 1989). Horn- westward to the south and incorporates the Feather blendes, which date amphibolite-grade metamor- River peridotite. In this cleavage fan, NE-dipping phism, range in age from 240 to 390 Ma (reviewed in foliation and axial surfaces of folds in the west pass Hacker and Peacock, 1990). This complex is a 6–10 eastward into SW-dipping foliation and axial sur- km wide zone that extends over 100 km from the faces. We interpret this structure to reflect the “ret- north end of the Sierra to the south, where it may rocharriage” that has affected rocks of the northern merge with the Calaveras–Shoo Fly thrust (Schwe- Sierra Nevada. ickert, 1981). Deformation in the Eastern belt also increases in The Eastern belt is a thick, polydeformed complexity from east to west. In the east, post–Shoo sequence of Paleozoic–Jurassic rocks. The basal Fly rocks are generally unfoliated and only slightly unit is the Lower Paleozoic Shoo Fly complex, con- metamorphosed, but become progressively isocli- taining fault-bounded units of quartzose turbidite, nally folded and finally multiply folded as one mafic volcanics, a 15-km long serpentinite lens dis- approaches the contact with the Feather River continuously developed in the northern Sierra, and a peridotite. mélange containing exotic blocks of chert and The general structural relations described by Ordovician limestone (Hannah and Moores, 1986; Day et al. (1985) suggest that the main Early Meso- Harwood, 1992). Early ENE-trending isoclinal folds zoic structures in the Sierra Nevada consist of east- are present in this sequence (Varga and Moores, vergent thrust faults and folds, subsequently modi- 1981). The serpentinite lens is only a few hundred fied by west-dipping faults. Accordingly, structures meters wide in outcrop, but gravity and magnetic are depicted on the cross-section as both east-dip- data indicate that it becomes several kilometers ping and west-dipping (Fig. 2). The early east-ver- thick at depth (Griscom, in Blake et al., 1989). gent faults are interpreted to dip westward beneath These rocks are interpreted as an off-continent tur- the Central Belt, Western Belt, and Great Valley, bidite sequence deposited upon unknown, but pre- consistent with geophysical and geochemical data sumably oceanic, crust. The mélange and (Dilek and Moores, 1993; Godfrey et al., 1997; God- serpentinite may reflect an ophiolite emplacement frey and Klemperer, 1998; Godfrey and Dilek, 2000; event in mid–Late Devonian time (Varga and Bickford and Day, 2001; Ducea 2001). These fault Moores, 1981). Three volcanic-arc complexes geometries also conform with those displayed by a unconformably overlie the Shoo Fly rocks : (1) COCORP traverse across the northern Sierra Devonian–Mississippian units of the Sierra Buttes Nevada that indicated both west- and east-dipping and Taylor, Elwell, Keddie, and Peale formations, reflections (Nelson et al., 1986). interpreted as an oceanic volcanic arc; (2) Permian– The Sierra Nevada batholith intrudes the rocks Triassic units of the Robinson, Reeve, Arlington, described above. On Figure 3, the batholith is and possibly Cedar formations, also possibly repre- shown modified from Godfrey and Dilek (2000). senting an oceanic arc; and (3) a Jurassic sequence Significant tectonic and magmatic events for the of the Mount Jura and Milton sequences that is Sierra Nevada on Figure 3 include: the age of the interpreted to represent a continental or near-conti- Slate Creek and Smartville complexes, as well as nental arc (e.g., Hannah and Moores, 1986). For the Yuba River “stitching” pluton (Bickford and much of the length of their exposure, the rocks are Day, 2001); gold mineralization (Böhlke, 1999); the primarily east-facing and overturned to the east. Humboldt complex ages (Dilek and Moores, 1993); Near the northern end of the Sierra Nevada, how- age of Eastern Belt thrusts (Hannah and Moores, ever, NW-trending, tight to isoclinal, overturned to 1986); plutonic activity and lithosphere-scale recumbent east-vergent folds and thrust faults thrusting (Ducea 2001); apparent times of arrival of deform the rocks. Along the western margin of the the Smartville/Slate Creek (i.e., the Stikine-Inter- Eastern Belt, an enigmatic sequence of Devonian montane terrane; Moores, 1998); backfolding and U.S. CORDILLERA 487

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–Great Valley ophiolites; EK = Eastern Klamath belt; ESK = Eastern Klamath, Eastern Sierra Nevada belts; FRP = Feather River peridotite; GU = Guerrero terrane, Mexico; HW = Walker-Humboldt basin; JG = Jarbo Gap ophiolite; NA = North American continent. Mescalera plate after Dickinson and Lawton (2001). formation of the Foothill fault system; arrival of gold mineralization (e.g., Böhlke, 1999; Ducea, Wrangellia; and the arrival of Salinia (Wakabayashi 2001). and Moores, 1988). Arrival of the Smartville/Slate Parts of the western Sierra Nevada were Creek ophiolites, widespread penetrative deforma- affected by penetrative deformation associated tion and folding, as well as Eastern belt thrusting, with a sinistral-reverse sense of shear (east over together constitute the “Nevadan” orogeny of Sch- west) from 151 to 123 Ma (Paterson et al., 1987; weickert (1981), although they occurred earlier Tobisch et al., 1989), possibly as a consequence than originally envisaged (Edelman and Sharp, of partitioning of strain associated with oblique 1989). The well-known Foothill fault system formed Franciscan subduction (Wakabayashi, 1992). later in the early–mid Cretaceous, associated with Dextral-reverse (east over west) ductile shear 488 MOORES ET AL.

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 = Guerrero terrane; 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 Klamath Mountains; VC = Venezuelan Coast Ranges; WI = Wrangell-Insular superterrane. Modified after Moores (1998, Fig. 10). See text for discussion. zones that cut the Sierra Nevada batholith Archipelago Style of Orogeny (approximate deformation age 100–85 Ma; e.g., Moores (1998) proposed a model for an “archi- Renne et al., 1993; Tobisch et al., 1995) may also pelago” style of orogenic development, involving be related to strain partitioning of oblique Fran- convergence and collision of already complexly ciscan subduction. deformed oceanic island arcs during Mesozoic and Cenozoic time along the western margins of North Basin and Range America and northern South America. Figure 5 shows a generalized tectonic sketch map and cross- At the latitude of the cross-section, the Sierra section for Early Jurassic time, just prior to this col- Nevada batholith extends into the Basin and Range. lision. A volcanic arc on the North American craton The eastern side of the batholith is shown involved gives way to the northwest to an Alaska Peninsula– in overthrusts, which we associatie with west-dip- like extension in the northern Sierra Nevada and ping crustal-scale reflectors of Allmendinger et al. western Nevada, separated from the craton by the (1987). West-dipping thrust faults along the Walker oceanic Walker-Humboldt basin in western Nevada. Lane and other parts of western Nevada are modi- This configuration accounts for the total inferred fied after Dilek and Moores (1993), Dilek et al. displacement on the Mojave–Snow Lake fault (1988), and Godfrey and Dilek (2000). The timing of (Lewis and Girty, 2001) and the Lower Mesozoic these thrusts is not clear, although they must be basinal sediments in thrust contact beneath the post–Jurassic, pre–Late Tertiary. Humboldt complex. An oceanic island arc repre- U.S. CORDILLERA 489

FIG. 7. Schematic map of the eastern Pacific basin at 160 Ma (Late Jurassic) after collision of SI and SK terranes along 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 in Figure 6, plus: C = Catalina schist. 490 MOORES ET AL.

FIG. 9. Schematic map of the eastern Pacific basin at 100 Ma (later Early Cretaceous) showing possible positions of WI superterrane and hypothetical oceanic plateau. Abbreviations are the same as in Figures 6 and 7 plus: CH = Chortis block; P = Piñon sequence of western Colombia and Ecuador; PE = Franciscan Permanente terrane; PO = Pelona and Orocopia basins; V = Venezuelan basin; Y = Yucatan basin. sented by the Jarbo Gap, Smartville–Slate Creek tal arc collision, or part of the North American plate. ophiolites, Guerrero terrane, and intervening ophi- The Feather River peridotite must also represent a olitic/island-arc rocks is separated from the North major suture of an as-yet unknown nature or age. American margin by a plate that was consumed on West of the oceanic arc is an entirely oceanic both its margins—the Mescalera plate of Dickinson plate called the Americord plate (this is the same and Lawton (2001). This plate has essentially disap- plate referred to by Moores [1998] as the Cordilleria peared. plate). The name has been changed because of prior We interpret the thrust-fault soles of these island usage of the term “Cordilleria” by Chamberlain and arc-ophiolite complexes to represent major sutures. Lambert, 1985, and Lambert and Chamberlain, The polarity of the colliding arc was along a west- 1988). This model for plate evolution is similar to dipping subduction zone (east-facing arc), because that proposed by Ingersoll (2000). of the geometry of the associated structures and the internal pseudostratigraphy of the collided blocks. Tectonic Reconstructions of The deformed and partly chaotic metasedimentary North American–Northern rocks of the Central Belt of the northern Sierra South American Margins Nevada may partly represent material derived from this plate. We present here a series of approximate cartoons The Feather River peridotite may either be a (schematic maps) modified after Moores (1998) that remnant of the Mescalera plate preserved by out-of- elaborate on the “archipelago” style of deformation. sequence thrusting during the island arc–continen- Figure 6 shows a possible reconstruction at ~180 U.S. CORDILLERA 491

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

Ma. From west to east, the features are the Farallon trenches of opposite polarity. Subduction is present plate, the Wrangell-Insular (WI) superterrane with off northern South America. an active island arc in the approximate position at At 140 Ma (Fig. 8), the Guerrero terrane has this time given by Debiche et al. (1987), the Ameri- attached (Dickinson and Lawton, 2001), but its cord plate separating the Wrangell-Insular from the southern equivalents are still oceanward of the con- Stikine/Intermontaine (SI) terrane and its possible tinents. The future position of the Catalina schist continuations to the south, the Mescalera plate, and (i.e., future C) is shown east of the WI superterrane. the North American plate. Note that two plates sep- At 100 Ma (Fig. 9), a possible scenario has a sub- arate North and South America from the Farallon duction zone along the entire margin of western plate. The spreading center producing the Great North America. The WI terrane has possibly arrived Valley/Coast Range ophiolite within the Americord in a “compromise” position (Stamatakos et al., plate may have extended to the north or south. 2001), although its position could be further to the Figure 7 shows a possible scenario at 160 Ma. west as shown, based upon the lack of paleolongi- The SK terranes have attached to the north, but is tude constraint from paleomagnetic data. A west- still outboard of the continent to the south, thereby dipping subduction zone has formed to produce the possibly setting up a transform fault between two Catalina schist (C); this zone begins at 115 Ma and 492 MOORES ET AL.

FIG. 11. Schematic map of the eastern Pacific at 40 Ma (Middle Eocene). Abbreviations are the same as in Figures 6, 7, and 8. collides at 90–100 Ma. An oceanic plateau west of development of the Laramide structures in the WI contains the Permanente Terrane (PE). Subduc- Rocky Mountains. At a 5–10 cm/year subduction tion of this oceanic plateau beginning in latest Cre- rate, the front of a subducted wide oceanic plateau taceous–Early Tertiary time (75–45 Ma) will cause would take approximately 10–20 million years to the Laramide orogeny (Henderson et al., 1984), as sweep eastward 1000 km from the coast. here understood to include all crustal shortening of By 40 Ma (Fig. 11), subduction has been re- the same age west to the continental margin. Loca- established along part of the North American mar- tion of possible future rifting of the Salinian block gin. The Pelona-Orocopia schists have been and development of the Pelona-Orocopia (PO) emplaced along an east-vergent thrust, and Salinia basins are shown. may have moved slightly northward by pre-San By 60 Ma (Fig. 10), subduction of the oceanic Andreas strike-slip faulting. plateau is inferred to have produced a flattening of Figures 9–11 reflect our interpretation that this the slab (now the Farallon plate). Increased traction time in eastern Pacific history was exceptionally from this flattening would have produced compres- complex, more than commonly envisioned. sional structures from the continental margin to the Although we acknowledge that these figures are only Rocky Mountains, including thrust wedging in the schematic, they point toward a complexity that is Coast Ranges, possibly thrusting along the eastern reminiscent of contemporary reconstructions of the edge of the Sierra and Klamaths, possibly renewed Southeast Asia/Southwest Pacific region (e.g. Hall, movement on Sevier thrusts in Nevada-Utah, and 1996). U.S. CORDILLERA 493

FIG. 12. Generalized map of Andes, showing possible allochthonous terranes and marginal basins. Symbols: light shading = 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 = Pampean ranges 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).

in part associated with “flat slab subduction”(i.e., Discussion: “Andean-style orogeny). Gutscher et al. (2000) con- The North American Cordillera and a vincingly demonstrated, however, that modern areas Speculative Reconstruction of the Andes of flat slabs correspond to the subduction of aseismic ridges. Here we present a short synopsis of a possible Advances yet to be made in geology at first revision in some of the ideas on tectonic develop- will be regarded as outrages. ment of the Andes. This is meant not as a compre- – W. M. Davis, 1926 hensive review, but as a provocative essay in the The Andes of South America have long been con- context of our model for western North America. sidered as a type example of subduction beneath the Figure 12 shows a generalized sketch map of the continental margin paired with antithetic thrusting, Andes (modified after Moores and Twiss, 1995, Fig. 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 Verdes ophiolites. After Moores and Twiss (1995, Fig. 12.13), Roeder (1988), Vicente (1989), and Mpodozis and Ramos (1989). U.S. CORDILLERA 495

FIG. 14. Generalized time-space diagram showing principal depositional and tectonic events for the Andes. Broad line 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).

12.12 and Mpdozis and Ramos, 1990), together absence) of the Andean fold belt to the east. Note- with possible allochthonous features. The latter worthy is the fact that Sector F lacks thrust-related include the “western terranes” of Cretaceous and deformation. Only orogen-parallel strike-slip fault- Jurassic age in western Ecuador and Colombia and ing is present. northwestern Peru, which include blueschist and Figure 14 is a time-space diagram, modified eclogite (Feininger, 1980, 1987; Aspden and Lith- after Moores and Twiss (1995, Fig. 12.14), that erland, 1992), the Paracas arc of Colombia, the shows the main lithologies in the various sectors and volcanic-rich “Western Mesozoic Series” of Peru the onset of the Andean orogeny. This onset is (Mégard, 1989), and possibly the allochthonous marked by a transition from basinal and/or arc Precambrian Arequipa massif of southern Peru development and major crustal shortening. In par- (Vicente, 1989) and the very thick Jurassic–Lower ticular, this crustal shortening involves east-vergent Cretaceous oceanic-affinity island-arc rocks of the folds and thrusts in the north and south (sectors A Coast Range of Chile (Vergara, et al., 1995, Buch- and G) where a recognized suture is present, as well elt and Cancino, 1988). These oceanic-affinity in other parts of the mountain belt where a suture rocks possibly developed when separated from has not been identified. The similarity of structures South America by a marginal basin represented by along the length of the Andes implies a similar tec- the “aborted” marginal basin of Chile (Mpdozis tonic history in sectors B through E, but not F. and Ramos, 1990, Ramos and Aleman, 2000, Note that the timing of onset of the Andean orog- Dalziel, 1986), the West Peruvian Trough (Dalziel, eny is diachronous, being early Late Jurassic in the 1986), and the “Rocas Verdes” ophiolite basin of north and south, and becoming progressively later to Patagonia and Tierra del Fuego (e.g., Dalziel, the center. This variation in onset of orogeny argues 1986; Stern and deWit, 1997). against an orogenic event driven by far-field effects Figure 13 shows cross-sections of the Andes for of spreading from the Mid-Atlantic Ridge or by the various sectors. These cross-sections display the increase in movement in a so-called “absolute” various rocks to the west, and the presence (or (hotspot) frame of reference. 496 MOORES ET AL.

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

These data and the orogen-wide prevalence of This reconstruction may seem outrageous, but it east-vergent thrusting suggest that the Andes orogen fits a number of intriguing and heretofore unex- also may include a role for Mesozoic collision of an plained features of the Andes. Of course, we offshore island arc, perhaps previously rifted from acknowledge the major continental-arc nature of the South American continent by back-arc rifting, much of the Late Mesozoic and Cenozoic history of reversal of subduction, “collapse” (subduction) of the Andes (e.g., Lamb, et al., 1997). At least one the marginal basin, and collision. Figure 15 is a suture is present from northern Colombia to south- schematic tectonic sketch for Late Mesozoic time, ern Ecuador (e.g., Aspden and Litherland, 1992) showing an offshore island arc composed of the pos- and from Tierra del Fuego to the South Patagonian sibly allochthonous units described above. The ice field (S. Harambour, pers. commun., 1989). “quiet zone” (C. Mpdozis, pers. commun., 1989) of Blueschist and eclogite (132 Ma white mica cooling sector F is represented by a gap in the arc. The plate age) in southern Ecuador is certainly associated between South America and the postulated offshore with a suture (Feininger, 1980). island arc may have been equivalent to the Mescal- We are aware that no suture has been identified era plate of Dickinson and Lawton (2001). along much of the rest of the length of the Andes. If U.S. CORDILLERA 497 one is present, its lack of exposure or recognition ______, 2001, Tectonic setting of the Smartville and Slate may possibly have resulted from subsequent short- Creek complexes, northern Sierra Nevada, California: ening, strike-slip faulting, juxtaposition of volcanic Evidence for zircon geochronology and common Pb rocks of similar character across the suture, or studies: Geological Society of America Abstracts with because the suture is overlain by younger rocks or Programs, v. 33, no. 6, p. A-208. obscured by younger intrusions. We suggest that Blake, M. C., Bruhn, R. L., Miller, E. L., Moores, E. M., Smithson, S. B., and Speed, R. C., 1989, Continental- the Andean fold-thrust belt had its inception with ocean transect C-1 Mendocino triple junction to North the arc-continent collision, and has been reactivated American craton: Boulder, CO, Geological Society of today because of major South American–Cocos– America Decade of North American Geology. Nazca–Antarctic plate interactions. Thus, the Blake, M. C., Jr., Howell, D. G., and Jayko, A. S., 1984, revised tectonic history of the Mesozoic western Tectonostratigraphic terranes of the San Francisco Bay United States suggested in this paper may find its Region, in Blake, M. C., Jr., ed., Franciscan geology of counterpart in the Andes. northern California: Pacific Section, Society of Eco- nomic Paleontologists and Mineralogists, v. 43, p. 5– 22. Acknowledgments Blake, M. C., Jr. Jayko, A. S., McLaughlin, R. J., and Underwood, M. B., 1988, Metamorphic and tectonic EMM thanks I. W. D. Dalziel, C. Mpdozis, F. evolution of the Franciscan Complex, northern Califor- Hervé, and V. Ramos for introducing him to the nia, in Ernst, W. G., ed., Metamorphism and crustal Andes, W. G. Ernst and S. 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