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Moores Et Al..Fm 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 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 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 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 orog- eny 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 neighbor- ing 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
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