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Late Cretaceous–early Cenozoic tectonic evolution of the southern California margin inferred from provenance of trench and forearc sediments Carl E. Jacobson1,†, Marty Grove2, Jane N. Pedrick1, Andrew P. Barth3, Kathleen M. Marsaglia4, George E. Gehrels5, and Jonathan A. Nourse6 1 Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, USA Department of Geological and Environmental Sciences, Green Earth Sciences, Rm. 225, Stanford University, Stanford, California 94305-2115, USA 3 Department of Earth Sciences, 723 West Michigan Street, SL118, Indiana University–Purdue University, Indianapolis, Indiana 46202-5132, USA 4 Department of Geological Sciences, California State University–Northridge, 18111 Nordhoff Street, Northridge, California 913308266, USA 5 Department of Geosciences, 1040 E. 4th Street, Gould-Simpson Building, University of Arizona, Tucson, Arizona 85721, USA 6 Department of Geological Sciences, California State Polytechnic University, 3801 W. Temple Avenue, Pomona, California 91768, USA 2 ABSTRACT During the Late Cretaceous to early Cenozoic, southern California was impacted by two anomalous tectonic events: (1) underplating of the oceanic Pelona-Orocopia-Rand schists beneath North American arc crust and craton; and (2) removal of the western margin of the arc and inner part of the forearc basin along the Nacimiento fault. The Pelona-Orocopia-Rand schists crop out along a belt extending from the southern Sierra Nevada to southwestern Arizona. Protolith and emplacement ages decrease from >90 Ma in the northwest to <60 Ma in the southeast. Detrital zircon U-Pb ages imply that metasandstones in the older schists originated primarily from the western belt of the Sierran–Peninsular Ranges arc. Younger units were apparently derived by erosion of progressively more inboard regions, including the southwestern edge of the North American craton. The oldest Pelona-OrocopiaRand schists overlap in age and provenance with the youngest part of the Catalina Schist of the southern California inner continental borderland, suggesting that the two units are broadly correlative. The Pelona-OrocopiaRand-Catalina schists, in turn, share a common provenance with forearc sequences of southern California and the associated † E-mail: [email protected] Salinian and Nacimiento blocks of the central Coast Ranges. This observation is most readily explained if the schists were derived from trench sediments complementary to the forearc basin. The schists and forearc units are inferred to record an evolution from normal subduction prior to the early Late Cretaceous to flat subduction extending into the early Cenozoic. The transition from outboard to inboard sediment sources appears to have coincided with removal of arc and forearc terranes along the Nacimiento fault, which most likely involved either thrusting or sinistral strike slip. The strike-slip interpretation has not been widely accepted but can be understood in terms of tectonic escape driven by subduction of an aseismic ridge, and it provides a compelling explanation for the progressively younger ages of the PelonaOrocopia-Rand schists from northwest to southeast. INTRODUCTION The geology of California is dominated by NNW-trending lithotectonic belts produced during late Mesozoic–early Cenozoic subduction of the oceanic Farallon plate beneath the western edge of North America (Hamilton, 1969; Dickinson, 1970; Ernst, 1970). The Franciscan subduction complex, Great Valley forearc basin, and Sierra Nevada batholith of central California east of the San Andreas fault are particularly well-known components of this system (Fig. 1). Similar rocks also characterize southern California and its displaced correlatives within the central Coast Ranges west of the San Andreas. In the latter regions, however, the convergent margin belts are severely disrupted by the Nacimiento fault, which separates the Salinian block on the northeast from the Nacimiento block to the southwest (Page, 1970, 1981, 1982; Dickinson, 1983; Hall, 1991; Saleeby, 2003; Ducea et al., 2009). The Salinian block is cored by granitoid plutons similar in age (100–75 Ma) and composition to those of the central belt of the Sierra Nevada and regions to the east, yet it lacks an analog to the western foothills belt of the Sierra (Mattinson, 1978, 1990; Silver, 1983; Ross, 1984). The Nacimiento block, in contrast, is underlain by the Franciscan Complex and remnants of sedimentary sequences equivalent to the distal parts of the type Great Valley Group, but it is missing forearc units indicative of a proximal setting (Hart, 1976; Hall et al., 1979; Page, 1981; Seiders, 1982; Vedder et al., 1983; Hall, 1991). By comparison with the Franciscan–Great Valley–Sierran triad east of the San Andreas fault, the juxtaposition of the Salinian and Nacimiento blocks along the Nacimiento fault implies the removal of a width of 150 km or greater of formerly intervening westernmost arc and inner to central forearc basin. This truncation appears to have occurred sometime between 75 and 59 Ma (see following), although the mechanisms GSA Bulletin; March/April 2011; v. 123; no. 3/4; p. 485–506; doi: 10.1130/B30238.1; 10 figures; 1 table; Data Repository item 2011005. For permission to copy, contact [email protected] © 2011 Geological Society of America 485 Jacobson et al. PSP 121°W Faults Ef EHf Gf Nf Rf SAf SGf SGHf SJf SYf DP lo Gr ab Di BL t ea PP n Ra ge PL SL PS SS CL IR Rf f N CA k im ie nt o SMM Nf SR SYf Nf bl k RA f SA bl ac a LP oc N ra N Forearc sampling areas AT Atascadero BL Ben Lomond Mountain CA Cambria CL Coalinga 37°N CP Cajon Pass DP Del Puerto Canyon IR Indians Ranch LP La Panza Range NB Northern Baja California OR Orocopia Mountains PL Point Lobos PM Pine Mountain Block PP Pigeon Point PS Point Sur PSP Point San Pedro SA Northern Santa Ana Mts. SC Santa Catalina Island SD San Diego 35°N SG San Gabriel block SL NW Santa Lucia Range SM Santa Monica Mts-Simi Hills SMI San Miguel Island SMM Sierra Madre Mountains SR San Rafael Mountains SY Santa Ynez Mountains ev ad f EH ian f SA Hf SG lin er AT Hf SG Sa Si y lle Va 36°N Pelona-Orocopia-Rand Schists BR Blue Ridge CD Castle Dome Mountains CH SE Chocolate Mountains EF East Fork MP Mount Pinos NR Neversweat Ridge OR Orocopia Mountains PR Portal Ridge RA Rand Mountains SC Santa Catalina Island SE San Emigdio Mountains SP Sierra Pelona SS Sierra de Salinas TR Trigo Mountains Elsinore East Huasna Garlock Nacimiento Rinconada San Andreas San Gabriel San Gregorio-Hosgri San Jacinto Santa Ynez SMI Gf SE MP SR SY WTR PM SY PR SG SP CTR BR SG CP EF SM 34°N SGf SA Accretionary complex N Pe ni SC ns ul ar SJ f Ra Magmatic arc and wall rocks SD McCoy Mountains Formation 118°W SA f TR es CA MEX NB Fm. CD NR CH Ef ng 100 km y Mts. OR Forearc basin Pelona-Orocopia-Rand Schists McCo ETR WTR AZ 115°W Figure 1. Generalized late Mesozoic–early Cenozoic geology of central to southern California and adjoining areas, based largely on Jennings (1977). Some outcrops of arc rocks and older wall rocks were omitted from southeasternmost California and southwesternmost Arizona to emphasize outcrops of Pelona-Orocopia-Rand schist and McCoy Mountains Formation. Inset delineates the Salinian and Nacimiento blocks. The Nacimiento fault is indicated by a heavier line weight than the other faults. We follow the interpretation of Hall et al. (1995) and Dickinson et al. (2005) that the Nacimiento fault has been offset ~45 km in a dextral sense by slip on the combined Rinconada–East Huasna fault. Sample localities for both Pelona-Orocopia-Rand schists and forearc units are indicated. Repeated labels (SG, SY, and SR) indicate the grouping of samples collected over relatively large areas. Abbreviations not defined in the figure: AZ—Arizona; CA—California; CTR—central Transverse Ranges; ETR—eastern Transverse Ranges; MEX—Mexico; WTR—western Transverse Ranges. involved remain unclear. Previous interpretations include thrusting of 150–200 km (Silver, 1983; Hall, 1991; Barth and Schneiderman, 1996; Saleeby, 1997; Ducea et al., 2009), dextral strike slip of 2000 km or more (Page, 1982; Champion et al., 1984), and sinistral strike slip of 500–600 km (Dickinson, 1983; Seiders and Blome, 1988; Dickinson et al., 2005). A second key feature of southern and coastal central California is represented by the PelonaOrocopia-Rand schists (Fig. 1; Haxel and Dillon, 486 1978; Ehlig, 1981; Jacobson et al., 1988). These rocks broadly resemble the Franciscan Complex, but instead of being positioned outboard of the forearc basin, they sit in structural contact directly beneath the Cretaceous marginal batholith and adjoining cratonal areas of southeastern California and southwestern Arizona. Potentially equivalent rocks, recovered as xenoliths, occur still farther east in the Four Corners region of Arizona, New Mexico, Utah, and Colorado (Usui et al., 2006). The origin of the Pelona- Orocopia-Rand schists has long been debated (Haxel et al., 2002; Grove et al., 2003), although most workers now favor a model involving lowangle subduction of the Farallon plate during the Laramide orogeny (Crowell, 1968, 1981; Yeats, 1968; Burchfiel and Davis, 1981; Dickinson, 1981; Hamilton, 1988; May, 1989; Malin et al., 1995; Jacobson et al., 1996, 2002, 2007; Wood and Saleeby, 1997; Yin, 2002; Grove et al., 2003; Saleeby, 2003; Kidder and Ducea, 2006; Saleeby et al., 2007). Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California In the past, detailed understanding of the Pelona-Orocopia-Rand schists has been hampered by a lack of good control on either the depositional age of the protolith or the time of underthrusting and metamorphism. Recent 40 Ar/39Ar thermochronologic analyses and U-Pb dating of detrital zircons, however, demonstrate that sedimentation and underplating occurred from >90 Ma to <60 Ma (Jacobson et al., 2000, 2002, 2007; Barth et al., 2003; Grove et al., 2003). This point is critical, because it indicates that underthrusting of the PelonaOrocopia-Rand schists began at least 15 m.y. prior to the 75 Ma or younger initiation of slip on the Nacimiento fault. Hence, formation of the schists was at least in part a separate phenomenon from the juxtaposition of the Salinian and Nacimiento blocks. On the other hand, both events appear to have been occurring simultaneously sometime between ca. 75 and 59 Ma. It is hard to imagine that they were not linked in some fashion during this period. Detrital zircon ages from the schists also reveal a major shift in provenance from relatively outboard to inboard source areas at about the time of movement on the Nacimiento fault (Grove et al., 2003). However, because the schists are allochthonous, relatively limited in geographic distribution, and represent only a narrow time range in any given area, they do not provide tight control on the nature and areal extent of this event. To better understand the interconnections between Nacimiento fault slip and underplating of the Pelona-Orocopia-Rand schists, we conducted a regional detrital zircon and sedimentary petrologic analysis of unmetamorphosed units of the forearc basin that overlap in age and geographic extent with the schists. As part of this study, we expanded the data set of Grove et al. (2003) for the Pelona-OrocopiaRand schists and treated these analyses along with the results from Grove et al. (2008) for the broadly similar Catalina Schist of the inner continental borderland of southern California. The combined schist and forearc data provide important constraints on this critical period in the paleogeographic and plate-tectonic evolution of southern California and environs. GEOLOGIC BACKGROUND Palinspastic Reconstruction The Late Cretaceous–early Cenozoic assemblages considered in this study have been disturbed by younger tectonic events, including middle Cenozoic extension and middle to late Cenozoic San Andreas–related deformation (Crowell, 1962, 1981; Frost et al., 1982; Luyendyk et al., 1985; Crouch and Suppe, 1993; Dillon and Ehlig, 1993; Powell, 1993; Nicholson et al., 1994; Dickinson, 1996; Fritsche et al., 2001). To help correct for these complications, we utilize a schematic pre-Miocene reconstruction of southern California and adjoining areas (Fig. 2) that builds upon a similar analysis presented by Grove et al. (2003; see also the GSA Data Repository item for details1). This map serves as a base for plotting our results and provides context for discussing the regional geologic setting of the study area. In reconstructing southern California at the level of detail shown here, it is relatively easy to restore northwest-southeast–trending strike-slip faults near the continental margin and to backrotate the western Transverse Ranges, which lie along a “free edge” of the map (Fig. 2). In contrast, compatibility issues make it far more difficult to (1) correct for rotations of regions that lie entirely within the map area (e.g., the Sierran “tail” or eastern Transverse Ranges); (2) restore offset on strike-slip faults at a high angle to the San Andreas fault or those parallel to the San Andreas but located relatively far inboard; or (3) undo extension associated with detachment faults. Such corrections were not attempted here but must be kept in mind when considering the paleogeology. A second complication pertains to longstanding controversies regarding the magnitude of slip on various branches of the San Andreas system. Our reconstruction follows interpretations of Crowell (1962, 1981), Dillon and Ehlig (1993), Hall et al. (1995), Dickinson (1996), and Dickinson et al. (2005). Contrasting estimates of slip for a number of major faults within the study area have been presented by Sedlock and Hamilton (1991), Powell (1993), Underwood et al. (1995), and Burnham (2009), among others. However, these disagreements, while important for a complete understanding of the geology of California, are secondary in nature at the scale considered here and have no significant impact on our conclusions. By restoring middle Cenozoic and younger deformations as best we can, Figure 2 more clearly highlights the impact of the Nacimiento fault on the geologic framework of southern California. In addition to the ~150 km of stratigraphic omission of forearc and batholithic rocks that occurs across the fault, note that Proterozoic cratonal basement crops out anomalously close to the margin in the cor1 GSA Data Repository item 2011005, explanation of the palinspastic reconstruction, geologic summaries of forearc sampling areas and notes on individual samples, supplementary figures, and data tables, is available at http://www.geosociety.org/pubs/ft2011.htm or by request to [email protected]. ridor between the Sierra Nevada batholith to the north and the Peninsular Ranges batholith to the south. The location of the southeastern extension of the Nacimiento fault is unknown (queried in Fig. 2), but it likely occurs outboard of Proterozoic basement of the central Transverse Ranges and southeasternmost California. The omission of forearc and western batholithic rocks and out-of-position outcrops of cratonal basement are central to understanding the displacement history and tectonic significance of the Nacimiento fault. Pelona-Orocopia-Rand and Catalina Schists Pelona-Orocopia-Rand Schists The Pelona-Orocopia-Rand schists consist of 90% or more quartzo-feldspathic schist presumably derived from turbidite sandstone (Ehlig, 1958, 1981; Haxel and Dillon, 1978; Jacobson et al., 1988, 1996; Haxel et al., 1987, 2002). The schists also include up to 10% metabasite, along with minor amounts of Fe-Mn metachert, marble, serpentinite, and talc-actinolite rock. Metamorphism occurred under conditions of moderately high pressure relative to temperature. Mineral assemblages lie mostly within the greenschist and albite-epidote amphibolite facies but locally extend into the epidote-blueschist and upper amphibolite facies (Ehlig, 1981; Haxel and Dillon, 1978; Graham and Powell, 1984; Jacobson and Sorensen, 1986; Jacobson et al., 1988; Kidder and Ducea, 2006). Inverted metamorphic zonations are typical. Initial emplacement of the schists is attributed to a Late Cretaceous–early Cenozoic low-angle fault system referred to as the Vincent–Chocolate Mountains thrust (Haxel and Dillon, 1978). However, most present-day contacts between the schists and North American upper plate appear to be low-angle normal faults related to exhumation of the schists, either shortly after underthrusting or during middle Cenozoic extension (Frost et al., 1982; Hamilton, 1988; Jacobson et al., 1988, 2002, 2007; Malin et al., 1995; Wood and Saleeby, 1997; Haxel et al., 2002). Outcrops of the Pelona-Orocopia-Rand schists define a northwest-southeast–trending belt extending from the southern Sierra Nevada to southwestern Arizona (Figs. 1 and 2). The northwestern schists underlie the outboard to central parts of the Sierran batholith of Early to middle Cretaceous age. The southeastern schists, in contrast, sit beneath Proterozoic, Triassic, Jurassic, and latest Cretaceous to early Cenozoic rocks situated inboard of the main Cretaceous arc. As noted by Grove et al. (2003), and discussed in more detail herein, protolith and emplacement ages of the schists decrease Geological Society of America Bulletin, March/April 2011 487 Jacobson et al. Sierra Nevada PSP BL RA Gf NV AZ CA SE Mojave Desert PR SG H f SA f SS Rf SL ? Nf 100 km M CP PL og ol IR Nf LP SMM PP AT CA SG BR SP OR SR Nf Igneous Rocks MP EH SR f Hf SG PS PM SG 55–85 Ma CA EF f CH ? 100–135 Ma n Hi gh la nd s AZ TR 85–100 Ma lo CD NR MEX Cretaceous undifferentiated N 135–300 Ma SY SM Proterozoic SA Pe nin SY Sedimentary (±Volcanic) Sequences and Metamorphosed Equivalents su SC lar es SD re gu 6 Early Cretaceous Alisitos arc ng Fi Jurassic-Cretaceous McCoy Mountains Fm. f Ra Cretaceous-Eocene forearc basin Accretionary complex Pelona-Orocopia-Rand-Catalina Schists SJ SMI NB Paleozoic-Mesozoic Figure 2. Pre–San Andreas palinspastic reconstruction of southern California, southwestern Arizona, and northwestern Mexico. See text and GSA Data Repository item for explanation (see text footnote 1). Abbreviations for faults and sampling localities are as in Figure 1. Inferred extension of Nacimiento fault west of the San Gregorio–Hosgri fault is based on figure 10 of Dickinson et al. (2005). Truncated rectangular box indicates the approximate area of coverage of the panels in Figure 6. from >90 Ma at the northwest end of the belt to <60 Ma in the southeast. Significant variations in provenance are also observed from one end of the belt to the other. In contrast to the northwest-southeast alignment of schist exposures as a whole, the northwestern bodies define a southwest-northeast subbelt extending from the Sierra de Salinas to the Rand Mountains. One possibility is that this deviation is an artifact of our failure to correct for oroclinal bending of the Sierran tail and adjacent parts of the Mojave Desert and Salinian block (Kanter and McWilliams, 1982; McWilliams and Li, 1985). In this case, the belt of northwestern schists would originally have been oriented approximately north-south (i.e., 488 more nearly parallel to the strike of the margin). Alternatively, A. Chapman and J. Saleeby (2009, personal commun.) argue that rotation of the upper-plate, batholithic rocks did not affect the structurally underlying schists and that the current locations of the northwestern schists closely reflect their relative distributions at the time of underthrusting. This is a further complication for reconstructing the Late Cretaceous– early Cenozoic paleogeography of southern California and vicinity. Catalina Schist The Catalina Schist is a moderately highpressure metamorphic terrane typically viewed as the southern extension of the Franciscan Complex. The unit is most widely exposed on Santa Catalina Island, where it forms a series of fault-bounded slices of metasedimentary, metavolcanic, and ultramafic rocks (Woodford, 1960; Platt, 1975, 1976; Sorensen, 1988; Grove and Bebout, 1995; Grove et al., 2008). Metamorphism ranges from upper amphibolite facies in the structurally highest unit to lawsoniteblueschist, lawsonite-albite, and albite-actinolite facies in the deepest parts of the section (Grove et al., 2008). The Catalina Schist also occurs as widespread submarine outcrops within the inner continental borderland, as minor outcrops on the Palos Verdes Peninsula, and in the subsurface of the Los Angeles basin (references in Grove et al., 2008). Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California Relationship of Pelona-Orocopia-Rand and Catalina Schists Strong lithologic and metamorphic similarities between the Pelona-Orocopia-Rand schists and parts of the Catalina Schist have long been noted (Ehlig, 1958; Woodford, 1960; Platt, 1976; Jacobson and Sorensen, 1986). Nonetheless, the two groups of rocks have generally been treated separately because: (1) the Catalina Schist is lithologically more diverse and exhibits a greater range of metamorphic facies than the Pelona-Orocopia-Rand schists; (2) initial geochronologic studies suggested an Early Cretaceous metamorphic age for the Catalina Schist (Mattinson, 1986) versus a latest Cretaceous– early Cenozoic age for the Pelona-OrocopiaRand schists (Jacobson, 1990); and (3) the Catalina Schist occurs in a more outboard setting than the Pelona-Orocopia-Rand schists. However, subsequent work on both units indicates that the structurally deepest, youngest part of the Catalina Schist (lawsonite-blueschist and lower-grade facies of Grove et al., 2008) overlaps in both depositional and metamorphic age with the oldest known part of the PelonaOrocopia-Rand schists (San Emigdio Mountains) at ca. 95–90 Ma (Grove et al., 2003, 2008). In addition, our work reveals that the coeval parts of the Catalina and Pelona-OrocopiaRand schists include similar assemblages of detrital zircons. (Combined zircon results for the Catalina Schist and Pelona-Orocopia-Rand schist of the San Emigdio Mountains are presented herein. Individual plots for the two units are included in Figure DR1 in the GSA Data Repository item [see footnote 1].) Finally, recent workers have proposed that initial emplacement of the Catalina Schist occurred beneath the northern Peninsular Ranges, implying a tectonic setting analogous to that for the PelonaOrocopia-Rand schists (Crouch and Suppe, 1993; ten Brink et al., 2000; Fritsche et al., 2001; Grove et al., 2008). According to this interpretation, the present location of the Catalina Schist in the borderland is the result of being exhumed from beneath the Peninsular Ranges during Miocene extension. Based on these relations, we conclude that the Pelona-OrocopiaRand schists and youngest parts of the Catalina Schist are broadly correlative. We do not include the older elements of the Catalina Schist in this grouping, because they may have originated in a forearc, rather than accretionary wedge, setting (Grove et al., 2008). Forearc Sedimentary Units We analyzed unmetamorphosed Upper Cretaceous to middle Eocene forearc units extending from northern Baja California to just south of San Francisco. All sampling localities can be classified into one of three tectonic domains: (1) Salinian block and central Transverse Ranges, (2) Nacimiento block, and (3) western Transverse Ranges and Peninsular Ranges. Salinian Block–Central Transverse Ranges The term “Salinian block and central Transverse Ranges” as used here also includes the western fringes of both the Mojave Desert and eastern Transverse Ranges. Basement rocks of this region consist of Proterozoic to Jurassic magmatic and metamorphic units intruded by Late Cretaceous plutons characteristic of the central to eastern parts of the Sierran– Peninsular Ranges arc (Mattinson, 1990; Barth et al., 1995, 1997, 2000, 2001a, 2008a; Kidder et al., 2003; Barbeau et al., 2005). Even the youngest Cretaceous intrusive rocks (ca. 75 Ma) are cut by the Nacimiento fault, thus providing a maximum age for the structure. Cretaceous magmatism was followed by a marine transgression that was in progress by the middle Maastrichtian (time scale of Walker and Geissman, 2009)(e.g., Santa Lucia and La Panza Ranges; Howell et al., 1977; Vedder et al., 1983; Saul, 1986; Seiders, 1986; Sliter, 1986; Grove, 1993) and that advanced southeast and inboard through the early Eocene (e.g., Pine Mountain block and Orocopia Mountains; Crowell and Susuki, 1959; Chipping, 1972; Advocate et al., 1988; Dickinson, 1995). The marine flooding of the Salinian block was presumably related to removal of the western fringe of the arc and adjoining earlier phases of the forearc basin and thus implies that significant slip had accumulated on the Nacimiento fault by the middle Maastrichtian (ca. 68 Ma). The transgressive sequences of the Salinian block and central Transverse Ranges are distinctive in that many show evidence for relatively continuous deposition across the CretaceousPaleocene boundary (Kooser, 1982; Saul, 1983). Furthermore, Paleocene rocks, in general, are widespread and relatively thick, although unconformities occur locally within this interval (Chipping, 1972; Graham, 1979; Ruetz, 1979; Saul, 1986; Nilsen, 1987a). Facies relations indicate that initiation of deposition in individual areas, whether it occurred during the Maastrichtian, Paleocene, or Eocene, was commonly accompanied by rapid subsidence of the basin floor to bathyal depths (Kooser, 1982; Advocate et al., 1988; Grove, 1993). Lower Eocene marine deposits within the southern Salinian block resemble units of similar age in the southernmost Nacimiento block and eastern part of the western Transverse Ranges (Chipping, 1972; Page, 1981, 1982; Vedder et al., 1983; Dickinson, 1995). We inter- pret these rocks as an overlap sequence, indicating that most or all of the slip on the Nacimiento fault had accumulated by ca. 56 Ma. In a number of locations southwest of the fault, the Eocene section sits conformably on Upper Paleocene Sierra Blanca Limestone (Whidden et al., 1995). This limestone has not been recognized in the southernmost Salinian block northeast of the Nacimiento fault, although the base of the Eocene section in the latter area is exposed only on the inboard side of the basin, where it onlaps crystalline basement. Hence, the overlap sequence could be as old as late Paleocene (ca. 59 Ma). In addition, Hall (1991, p. 29) and Dickinson et al. (2005, p. 15) concluded from unconformities within the Salinian block that slip on the Nacimiento fault was most likely over by ca. 62.5–62 Ma. As a compromise, we use 59 Ma as the minimum age bound (Fig. 3), keeping in mind that the actual age for cessation of fault movement could be somewhat older or younger. In fact, the absolute minimum age of slip is indicated by a distinctive barnaclebearing unit of early Miocene age that overlaps the central part of the Nacimiento fault (Dickinson et al., 2005, p. 15). Nacimiento Block The Nacimiento block (equivalent to the SurObispo belt of Page, 1981, 1982) is underlain primarily by the Franciscan Complex. Locally, the Franciscan rocks are structurally overlain by (1) fragments of Coast Range ophiolite; (2) uppermost Jurassic to Valanginian and Upper Cretaceous (through Campanian) sedimentary rocks deposited nonconformably on Coast Range ophiolite and resembling units of the Great Valley Group east of the San Andreas fault; and (3) sandstone-rich sequences of Campanian(?) age, which may have been deposited directly on the Franciscan accretionary wedge (Hall and Corbató, 1967; Gilbert and Dickinson, 1970; Page, 1970, 1981, 1982; Hart, 1976; Howell et al., 1977; Seiders, 1982; Vedder et al., 1983; Seiders and Blome, 1988; Hall, 1991; Seiders and Cox, 1992; Dickinson et al., 2005). Rocks of Maastrichtian to Eocene age have not been recognized, except at the southernmost end of the block. We view the forearc units of the Nacimiento block as distal analogs of the proximal facies present within the Salinian block. However, the scarcity of forearc units younger than Campanian in the Nacimiento block but older than Maastrichtian in the Salinian block makes it difficult to constrain the exact paleogeography prior to slip on the Nacimiento fault. Most of our samples from the Nacimiento block come from two distinct sequences of Campanian or inferred Campanian age. One of these groups represents the uppermost part Geological Society of America Bulletin, March/April 2011 489 Jacobson et al. of the Jurassic–Cretaceous sequence that sits depositionally upon the Coast Range ophiolite (e.g., samples of the Cachuma and Atascadero Formations and potentially the Pigeon Point Formation; see GSA Data Repository item [see footnote 1]). We also analyzed samples from the Cambria slab, which is widely regarded as a trench-slope-basin assemblage deposited on top of the Franciscan Complex (Howell et al., 1977; Smith et al., 1979; Dickinson et al., 2005), and the Pfeiffer Slab of Hall (1991), which has been interpreted as either a trench-slope deposit (Howell et al., 1977; Smith et al., 1979; Hall, 1991) or part of the Franciscan Complex (Underwood and Laughland, 2001; Dickinson et al., 2005). Despite the potentially contrasting depositional settings of the Cambria and Pfeiffer slabs, our detrital zircon results are consistent with a common provenance. Peninsular Ranges–Western Transverse Ranges This area shows no evidence for the structural dislocation that affected the Nacimiento and Salinian blocks to the north; any Nacimientorelated slip at this latitude is presumed to lie well inboard of the forearc basin. On the other hand, the western Transverse Ranges are widely considered to have undergone 90° or more of clockwise rotation during Neogene development of the San Andreas system (Luyendyk et al., 1985; Nicholson et al., 1994; Dickinson, 1996). Prior to rotation, the western Transverse Ranges are believed to have been positioned alongside the western margin of the Peninsular Ranges. Sedimentary units exposed along the western flank of the Peninsular Ranges were deposited in the relatively inboard part of the forearc basin (Schoellhamer et al., 1981; Bottjer and Link, 1984; Fritsche et al., 2001). The base of the sequence consists of nonmarine units of Turonian age that pass upward into marine formations of Turonian to Campanian or early Maastrichtian age. These units are overlain unconformably by Upper Paleocene to Eocene formations. This contrasts with the relatively more continuous Maastrichtian to Paleocene deposition within the Salinian block (see previous). Units analyzed here from San Miguel Island, the Santa Monica Mountains, and the Simi Hills exhibit strong sedimentologic affinities with, and probably were located close to, the Peninsular Ranges prior to Neogene rotation (Bartling and Abbott, 1983; Bottjer and Link, 1984; Link et al., 1984; Alderson, 1988; Fritsche et al., 2001). The sequences from San Miguel Island, the Santa Monica Mountains, and the Simi Hills, however, appear to have been deposited somewhat farther offshore than coeval units of the Peninsular Ranges (Santa 490 Ana Mountains; Fritsche et al., 2001; Saul and Alderson, 2001). In analogous fashion, the western Transverse Ranges represent an even more distal setting (westerly in palinspastic coordinates; e.g., Fig. 2) within the forearc basin (Dibblee, 1950, 1966, 1991; MacKinnon, 1978; Vedder et al., 1983, 1998; Thompson, 1988; Dickinson, 1995). Sedimentation in the western Transverse Ranges began in latest Jurassic and/or earliest Cretaceous time. The base of the section is depositional upon ophiolite, which in turn sits structurally above the Franciscan Complex. This relationship is similar to that of the Nacimiento block. Some paleomagnetic data have been taken to indicate large-scale northward translation of the Peninsular Ranges prior to slip on the San Andreas system. However, we accept the alternate view that the Peninsular Ranges are essentially in place other than for dextral translation along the San Andreas system and possibly sinistral slip on the Nacimiento fault (Dickinson and Butler, 1998). METHODS Zircon U-Pb Ages Based on the regional scope of the study, we considered it most useful to maximize the number of samples analyzed rather than the density of age data collected per sample. Our study is thus best viewed as an exit poll, with more comprehensive work required to identify all minor populations and answer detailed provenance questions related to individual samples. For the Pelona-Orocopia-Rand schists, our data set includes 1405 zircon ages for 55 samples of quartzo-feldspathic schist (Table DR1 [see footnote 1]) reflecting nine to 55 ages per sample. Of these, 850 ages, representing 40 samples, come from our previous work (Jacobson et al., 2000; Barth et al., 2003; Grove et al., 2003). We also incorporated results from Grove et al. (2008) for the last accreted elements of the Catalina Schist. The latter data set includes 305 zircon ages from 21 samples. For the forearc units, we determined 2983 detrital zircon ages from 100 sandstones of Cenomanian to middle Eocene age (Tables DR2 and DR3 [see footnote 1]). The number of ages per sample ranged from nine to 64. Sixty-eight of the forearc samples were collected specifically for this study (the stratigraphic context of these samples is described in the GSA Data Repository item [see footnote 1]). The remaining forearc samples were collected as part of a more focused study of the Peninsular Ranges by one of us (M. Grove). Zircons were extracted from ~1 kg samples using standard density and magnetic tech- niques. Most zircons from both the schists and forearc units were analyzed by secondary ionization mass spectrometry (SIMS) using the Cameca IMS 1270 ion microprobe at the University of California, Los Angeles. Procedures followed those described in Grove et al. (2003). Ion microprobe analysis is time intensive, and because of our desire to sample a broad geographic and stratigraphic range, we determined a median of only 15–16 SIMS zircon U-Pb ages per sample. Since age distributions for individual samples are not well constrained, we focus upon pooled results from groups of samples. During the latter part of the study, zircon analyses were conducted at the University of Arizona using the laser ablation–multicollector– inductively coupled plasma–mass spectrometer (LA-ICP-MS) approach carried out with a 193 nm excimer laser and a GVI Isoprobe mass spectrometer. Procedures followed those of Gehrels et al. (2008). The LA-ICP-MS technique is highly efficient, which generally led to larger data sets than with the SIMS (although some large data sets were collected even with the SIMS and some small ones with the LAICP-MS). This creates a problem when pooling data, because samples with a large number of analyses will be weighted more heavily than those for which fewer ages were determined. To avoid this complication, we utilized a subsampling procedure in which ages from individual samples with a large number of results were sorted in numeric order. We then selected the oldest and youngest ages and every second, third, etc., age as needed to reduce the number of analyses to a more appropriate level. This approach yielded probability plots for individual “weeded” samples that were virtually indistinguishable from those obtained for the same samples using all analyses. Analytical results generated specifically for this study are included in the GSA Data Repository item (Table DR4 [see footnote 1]). Additional results can be found in Barth et al. (2003) and Grove et al. (2003), or were provided by one of us (M. Grove). Sandstone Petrology Detrital framework modes were estimated using the Gazzi-Dickinson method (e.g., Ingersoll et al., 1984) for 59 of the 68 forearc samples collected specifically for this study (i.e., the samples from the Peninsular Ranges were not included in this part of the study). All samples were stained for potassium feldspar and plagioclase. Three hundred to 400 grains were counted per sample depending on grain size and size of the thin section billet. Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California 10 RESULTS Mio 20 Pelona-Orocopia-Rand and Catalina Schists 30 Protolith Age One of the most critical parameters to determine for any given schist body is the depositional age of the sandstone protolith, which must lie within a time window bounded by the youngest reliable detrital zircon age and oldest reliable metamorphic age (Fig. 3). Grove et al. (2003) referred to this time span as the “cycling interval,” because it encompasses erosion in the source area, transport of that material to the site of deposition, underthrusting beneath the arc, accretion to the overriding plate, and initial stages of exhumation. As observed by Grove et al. (2003), the cycling interval of the PelonaOrocopia-Rand schists becomes younger from northwest to southeast (Fig. 3). Also notable, the duration of the cycling interval increases to the southeast, although this may be at least partly an artifact in the data. For example, we obtained no hornblende 40Ar/39Ar ages from the easternmost schist bodies (Fig. 3) and thus may not have captured the oldest cooling ages in this region. In addition, igneous bodies younger than ca. 70 Ma are not common in the inferred provenance areas (Barth et al., 2008a), making it difficult to constrain the maximum depositional age of sediments younger than latest Cretaceous. For example, in delineating the cycling interval, we ignored three early Cenozoic ages from the Orocopia Schist due to their status as outliers (Fig. 3). However, it should be kept in mind that any or all of these ages could be significant. Despite the uncertainties, the schist protoliths clearly range from Turonian to at least as young as middle Paleocene, representing a time span of 30 m.y. or greater. For purposes of comparison to the forearc units, we divided the schists into four groups based on depositional age and geographic location (Figs. 3 and 4). This contrasts with a threefold division utilized by Grove et al. (2003). Our fourth category includes the Catalina Schist, which was not considered by Grove et al. (2003), and the Pelona-OrocopiaRand schist of the San Emigdio Mountains, which Grove et al. (2003) grouped with the Rand schists. 40 Age (Ma) Aside from the fact that we did not previously consider the Catalina Schist, our results are little changed from those obtained by Grove et al. (2003). We thus emphasize only those points most relevant for comparison with the forearc data. Olig Eoc Metamorphic bio mus hbd Detrital zircon al interv 50 60 70 80 90 Pal Maast Cycling Slip on Nac flt Camp Sant Con Tur Cen 100 Age/ Epoch SC SE Cat-San Emig PR RA SS MP SP Rand BR Pelona EF OR CH TR CD NR Orocopia Figure 3. Comparison of 40Ar/ 39Ar cooling ages and zircon U-Pb ages ≤100 Ma for the Pelona-Orocopia-Rand-Catalina schists plotted by area (see Figs. 1, 2, and 4 for locations). Zircon ages are from Barth et al. (2003), Grove et al. (2003, 2008), and this study. Argon ages are from Jacobson (1990), Jacobson et al. (2002, 2007), Barth et al. (2003), and Grove et al. (2003, 2008). The Pelona-Orocopia-Rand schists are organized by the conventional northwest-southeast grouping, i.e., Rand Schists in the northwest Mojave Desert and environs, Pelona Schists in the central Transverse Ranges, and Orocopia Schists in southeastern California–southwestern Arizona. The exception is the Rand Schist of the San Emigdio Mountains, which we group with the Catalina Schist. Within the Pelona and Orocopia Schist groups, individual areas are ordered from northwest on the left to southeast on the right. Northwest-southeast position for the Rand Schists is not clear owing to potential rotation of the Sierran tail; these bodies were instead ordered from left to right by depositional and metamorphic age. Diagonally ruled box indicates time of most likely movement on the Nacimiento fault. Yellow band indicates the cycling interval, which, as discussed in the text, is not well constrained in the southeast. Also note that 40Ar/ 39Ar and zircon ages overlap for the Catalina Schist (the two types of ages have been separated slightly along the x-axis to clarify this relation). The low grade of metamorphism for this unit (Grove et al., 2008) suggests that the 40Ar/ 39Ar ages may be influenced by excess radiogenic argon or lack of complete recrystallization of detrital mica during metamorphism. Abbreviations for schist localities are same as in Figure 1. Other abbreviations are: bio—biotite; Camp— Campanian; Cen—Cenomanian; Eoc—Eocene; hbd—hornblende; Maast—Maastrichtian; Mio—Miocene; mus—muscovite; Olig—Oligocene; Pal—Paleocene; Sant—Santonian; Tur—Turonian. Variation in Detrital Zircon Populations as a Function of Depositional Age and Location Pie diagrams of detrital zircon ages for individual ranges and/or adjacent ranges are plotted in Figure 4 using the palinspastic base of Figure 2. Probability density functions for the four regional groupings are illustrated in Figures 5B–5E. As pointed out by Grove et al. (2003), detrital zircon patterns in the Pelona-OrocopiaRand schists vary systematically from northwest to southeast, and thus with depositional age of the protolith. The northwestern schists, which have protolith ages of Turonian to Campanian, are dominated by Early to middle Cre- taceous zircons, with lesser, but still significant, proportions of Proterozoic and Jurassic ages. In contrast, the central to southeastern schists, which have depositional ages of Maastrichtian and younger, are characterized by progressively decreasing abundances of Early to middle Cretaceous detrital zircons and increasing numbers of Proterozoic and latest Cretaceous–early Cenozoic ages. As concluded by Grove et al. (2003), the patterns imply a shift from outboard to inboard sources with time (e.g., Fig. 2 and following discussion). Compared to the Pelona-Orocopia-Rand schists, the zircon age distribution within Geological Society of America Bulletin, March/April 2011 491 Jacobson et al. San Emigdio Mts. (3/82) Gf Sierra Nevada NV AZ Rand Mts. (5/166) Hf SG SA f Portal Ridge (1/37) Blue Ridge (2/70) Rf Sierra de Salinas (5/138) CA Orocopia Orocopia Mts. (10/253) Mt. Pinos (5/125) Nf Nf Trigo Mts. (4/92) Neversweat Ridge (2/66) f EH Rand Mojave Desert SG SG f H f 100 km Nf l Pe a 55–70 Ma 70–85 Ma 85–100 Ma 100–135 Ma 135–300 Ma >300 Ma on Detrital zircon ages ? Sierra Pelona (7/199) East Fork (4/110) Catalina Schist (21/305) N SE Chocolate Castle Dome Mts. (3/79) Mts. (4/77) Peninsular Ranges Figure 4. Pie diagrams of detrital zircon ages from the Pelona-Orocopia-Rand-Catalina schists plotted on the same palinspastic base as in Figure 2. The Nacimiento fault is indicated by a heavier line weight than the other faults. Present-day outcrops of Pelona-OrocopiaRand schists are shown in black. The Catalina Schist is inferred to restore beneath the northern Peninsular Ranges (Crouch and Suppe, 1993). Mesozoic magmatic rocks and associated wall rocks of the Sierra Nevada and Peninsular Ranges batholiths are shown by the plus pattern. Arc and wall rocks of the Mojave Desert and Salinian block are omitted for clarity. Dashed blue outlines delineate three of the four schist groupings used for plotting zircon results in Figure 5 (see also Fig. 3). The fourth group includes the spatially separated Catalina Schist and schist of the San Emigdio Mountains (see text). Paired numbers in parentheses indicate number of samples and number of zircon ages. the Catalina Schist is geographically distinct (Fig. 4); i.e., despite its southerly location, the Catalina Schist is dominated by detrital zircons of Early Cretaceous age and, as noted previously, appears most similar to the Pelona-OrocopiaRand schist of the San Emigdio Mountains. As we will discuss later, the origin of this pattern is ambiguous owing to the uncertain nature of slip on the Nacimiento fault. Forearc Sedimentary Units Age Divisions of Forearc Units Samples from the forearc units were divided into four age groups: (1) Cenomanian-Turonian, (2) Campanian–early Maastrichtian, (3) middle Maastrichtian–Paleocene, and (4) early to middle Eocene. These subdivisions were guided by structural and stratigraphic breaks in the sedimentary record (see the descriptions of individual sampling areas in the GSA Data Re- 492 pository item [see footnote 1]) and correspond closely in age to the four schist groups, particularly with respect to the three older age divisions (Fig. 3). However, the youngest forearc age group (early to middle Eocene) may have an average age somewhat younger than that of the youngest schist group (Orocopia Schist); i.e., as discussed already, the minimum age of the Orocopia Schists is not well defined and conceivably could be no younger than middle Paleocene (Fig. 3). Despite this uncertainty, it is clear that the schists and forearc units analyzed here represent at least 30 m.y. of overlapping depositional history during a critically important period in the tectonic evolution of southern California. Variation in Detrital Zircon Populations as a Function of Depositional Age and Location Probability plots for the forearc age groups are illustrated in Figures 5F–5J alongside the matching divisions for the schists. The overall similarity between the two sample sets is impressive. By analogy with the schists, the variation from oldest to youngest parts of the forearc section is considered to reflect a transition from outboard to inboard source areas. Pie diagrams of the zircon age distributions plotted on a geographic base (Fig. 6) confirm the temporal progression from marginal to inboard sources and reveal significant alongstrike variations in provenance (see also Figs. DR2–DR4 [see footnote 1]). Material derived from east of the main Sierran–Peninsular Ranges arc first appeared at the continental margin in a few localities in the central part of the study area during Campanian–early Maastrichtian time (Fig. 6B; Cambria and Pfeiffer slabs, Santa Ynez Mountains, Santa Monica Mountains, Simi Hills). With time, detritus originating from inboard sources became the dominant component in the central part of the area and was also delivered in significant quantities both to the northwest and southeast (Figs. 6C and 6D). It is important to keep in mind that Figure 6 does not take into account displacement on the Nacimiento fault. Because movement appears to have been over by the late Paleocene, the Eocene reconstruction (Fig. 6D) should be independent of this event. However, the preceding time frames presumably require varying degrees of correction, depending on the exact nature and age of the Nacimiento fault. If this structure is dominantly a thrust (Page, 1970, 1981; Hall 1991; Saleeby, 2003), then displacement should have been largely normal to the length of the margin. In this case, the along-strike variations evident in Figure 6 would not be significantly altered. On the other hand, for 500–600 km of sinistral slip (Dickinson, 1983; Dickinson et al., 2005), the Nacimiento block would restore approximately outboard of the Diablo Range of central California, without the Salinian block intervening. Further implications of the sinistral case are considered later herein. Figure 6 reveals an impressive similarity in the Cenomanian-Turonian to Eocene patterns from San Miguel Island with those of equivalent age from the vicinity of San Diego and northern Baja California. This is consistent with models involving large-scale rotation of the western Transverse Ranges from an initial position alongside the Peninsular Ranges. One striking anomaly in our results pertains to a single sample of Paleocene age from northern Baja California (Fig. 6C). Work by one of us (M. Grove) suggests that the distinctive character of this sample reflects a highly restricted source region in the vicinity of the Sierra El Mayor of eastern Baja California. Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California POR-Catalina Schists Cumulative prob. 100 50 100 150 200 250 300 600 Forearc Sedimentary Units 900 1200 1500 1800 2100 50 100 A 80 80 100 150 200 250 300 600 900 1200 1500 1800 2100 F Kuu PE 60 60 OR 40 Orocopia (OR) Pelona (PE) Rand (RA) San Emig.-Catalina (SE) RA SE 20 Eoc 40 20 0 Eocene (Eoc) Mid Maast-Pal (KP) Camp-early Maast (Kuu) Cen-Tur (Klu) KP Klu Relative probability 0 Orocopia (23/478) B Eocene (28/827) G Pelona (18/504) C Mid Maast-Pal (30/582) H Rand (11/341) D S. Emigdio-Catalina (24/387) Camp-early Maast (32/853) Cen-Tur (10/333) E Vert exag = 2 for ages>300 Ma 50 100 150 200 250 300 600 900 1200 1500 1800 2100 50 Detrital zircon age (Ma) 100 150 200 250 300 Vert exag = 2 for ages>300 Ma 600 I J 900 1200 1500 1800 2100 Detrital zircon age (Ma) Figure 5. Detrital zircon age distributions for the Pelona-Orocopia-Rand-Catalina schists (left column) and forearc sedimentary units (right column). Zircons older than 2.1 Ga comprise <0.5% of the total and are not included. Sample groupings are explained in the text. Forearc and schist groups of similar depositional age are plotted next to each other and indicated by the same fill pattern. Note x-axis scale break at 300 Ma. Vertical scales also differ to the left and right of the x-axis break such that equal areas represent equal probability throughout the graph, except for plots E and J, for which ages older than 300 Ma are exaggerated by a factor of 2. Black lines in plots G–J indicate schist probabilities from plots B–E, respectively. They are raised above the x-axis so as not to obscure the plots for the forearc units. Abbreviations for geologic time periods are as in Figure 3. Paired numbers in parentheses indicate number of samples and number of zircon ages. Note that some analyses were excluded from these plots in order to avoid overweighting samples with a large number of analyses (see text and explanation for Fig. 6). Sandstone Petrology Summary modal data for the 59 sandstones analyzed petrographically are presented in Table 1. More complete results, including statistical values, are provided in the GSA Data Repository item (Tables DR5 and DR6 [see footnote 1]). The Data Repository item also includes QtFL and QmKP ternary diagrams of the results, along with plots of various compositional parameters shown in geographic coordinates using the same format as Figure 6 (Figs. DR5–DR9 [see footnote 1]). Most (56) of the samples are Campanian to middle Eocene. This group is rich in quartz and feldspar, with low to modest contents of lithic grains (Table 1). Volcanic fragments comprise less than 30% of total lithics. Plagioclase is more abundant than K-feldspar, although generally not by much. Total lithic contents and P/F values show a weak tendency to decrease with decreasing depositional age (Table 1; see also Figs. DR5–DR8 and Table DR6 [see footnote 1]). Other than this, the samples exhibit no systematic trends in composition with time. In contrast to the Campanian to Eocene samples, three of Cenomanian-Turonian age from the San Rafael Mountains and Atascadero area are marked by high total lithics, Lv/Lt, and P/F. COMPARATIVE PROVENANCE Detrital Zircons Sources of Individual Zircon Populations The most distinctive finding of this study is the remarkable parallelism in evolution of detrital zircon suites within the Pelona-OrocopiaRand-Catalina schists and spatially associated forearc basin. As already noted, the evolving patterns suggest a transition from outboard to inboard source areas with time, compatible with previous interpretations based on sandstone petrology and conglomerate clasts (Gilbert and Dickinson, 1970; Lee-Wong and Howell, 1977; Geological Society of America Bulletin, March/April 2011 493 SA f Nf Rf SGHf SGf Nf Geological Society of America Bulletin, March/April 2011 Northern Baja (2/88) A Northern Baja (2/158) San Diego (4/169) Figure 6. B Northern Baja (1/39) C Santa Monica Mts-Simi Hills (4/66) San Diego (2/93) Nf San Diego (1/58) Santa Ana Mts.(2/82) SGHf S. Miguel Is. (2/64) SGf Santa Monica Mts.-Simi Hills (4/85) EHf S. Miguel Is. (2/69) Nf ? SGf Santa Ana Mts. (2/56) Nf ? S. Miguel Is. (1/53) D Orocopia Mts. (2/129) Santa Monica Mts.-Simi Hills (3/65) San Diego (4/212) ? Northern Baja (3/146) Santa Ana Mts. (2/184) Santa Ynez Mts. (5/74) SGHf Santa Ana Mts. (2/61) EHf Pine Mt. Block (2/26) N Sierra Madre Mts. (3/39) f SG 55–70 Ma 70–85 Ma 85–100 Ma 100–135 Ma 135–300 Ma >300 Ma Rf Santa Ynez Mts. (2/65) San Gabriel Block (5/73) Cajon Pass (1/13) Gf Early-Mid Eocene Indians Ranch (2/50) Ben Lomond Mt. (1/28) SGHf ? Upper McCoy Tur-Camp Barth et al. (4/119) Nf La Panza Range (4/50) f San Rafael Mts. (3/55) Atascadero (4/55) NW Santa Lucia Range (3/35) Mid MaastPaleocene Nf Detrital zircon ages SGHf Indians Ranch (2/40) Pt. Lobos (2/29) Pt. San Pedro (4/62) Rf Point Sur Pfeiffer Slab (3/43) Rf Pigeon Point (3/39) f EHf San Rafael Mts. (2/31) SGHf Cambria (3/64) Del Puerto Cyn (1/36) CampEarly Maast 300 km SA Atascadero (1/26) Mojave Desert Coalinga DeGraaffSurpless et al. (2/112) 120 km Rf f 100 km Coalinga DeGraaffSurpless et al. (3/166) Cen-Tur Si N err e va a da 494 SA SA Nf Pe R ni n a ng sula es r 120 km Jacobson et al. Late Cretaceous–early Cenozoic tectonic evolution of southern California Figure 6. Pie diagrams of detrital zircon ages from the forearc sedimentary units plotted on the same palinspastic base as in Figures 2 and 4. Present-day outcrops of Pelona-OrocopiaRand schists (black) and the Sierra Nevada and Peninsular Ranges batholiths (pluses) are included for reference. See Figures 1 and 2 for distributions of the forearc units. The four panels correspond to the four age groups plotted in Figure 5. Note that the left and right boundaries are positioned in slightly different locations in each panel, the average of which is indicated in Figure 2. Individual sampling localities are described in the GSA Data Repository item (see text footnote 1). Results from the Great Valley Group of the Coalinga area (panels A and B) are from DeGraaff-Surpless et al. (2002). Those for the McCoy Mountains Formation (panel B) are from Barth et al. (2004). Paired numbers in parentheses indicate number of samples and number of zircon ages. Note that the total number of analyses represented in this figure is larger than that shown in Figures 5G–5J. This relates to the fact some analyses were excluded from the plots of Figure 5 in order to avoid overweighting samples with an exceptionally large number of zircon results (see text). Fault abbreviations are as in Figure 1. TABLE 1. AVERAGE DETRITAL MODES OF SELECTED SANDSTONES Qt F L M n (%) (%) (%) (%) Lv/Lt Sample group This study Early–mid-Eocene Mid-Maastrichtian–Paleocene Campanian–early Maastrichtian Cenomanian–Turonian 14 23 19 3 36 41 36 25 52 47 46 35 12 12 18 40 3 7 7 2 0.24 0.28 0.23 0.50 P/F 0.59 0.56 0.67 0.90 Great Valley Group (Ingersoll, 1983) Rumsey (Santonian–Maastrichtian) 34 40 36 24 10 0.60 0.55 Grabast (Cenomanian–Turonian) 17 30 30 41 7 0.37 0.68 Stony Creek (Tithonian–Barremian) 33 25 22 53 2 0.60 0.92 Note: n—number of samples; Qt—monocrystalline and polycrystalline quartz relative to total QtFL; F—feldspar relative to total QtFL; L—lithic fragments relative to total QtFL; M—framework mica relative to total framework grains; Lv/Lt—fraction of volcanic lithic fragments relative to total lithic fragments; P/F—fraction of plagioclase relative to total feldspar; 300 to 400 grains were counted per sample (mainly the latter); see the GSA Data Repository item (see text footnote 1) for more complete results, including standard deviations and standard errors. Kies and Abbott, 1983; Bachman and Abbott, 1988; Abbott and Smith, 1989; Seiders and Cox, 1992; Grove, 1993; Dickinson, 1995; Dickinson et al., 2005). Here, we evaluate in more detail the potential source areas for individual zircon age populations (Fig. 5). Cretaceous–Early Cenozoic. Early to early Late Cretaceous grains characteristic of the older depositional units are thought to have a source in the western to central Sierran– Peninsular Ranges arc (Fig. 2; Silver et al., 1979; Saleeby and Sharp, 1980; Stern et al., 1981; Chen and Moore, 1982). In contrast, latest Cretaceous grains that typify the younger units were presumably derived from widespread Laramide plutons of the Mojave Desert, southwestern Arizona, and northwestern Sonora (Fig. 2; Barth et al., 1995, 2001a, 2004, 2008a; McDowell et al., 2001; Wells and Hoisch, 2008). Laramide intrusive rocks younger than ca. 70 Ma have not been reported from southern California, although igneous rocks with ages of ca. 70–50 Ma occur locally in southern Arizona and northwestern Sonora (Haxel et al., 1984; Spencer et al., 1995) and are relatively common in north-central Sonora (González-León et al., 2000; McDowell et al., 2001). Grains within the latter age range form a distinctive component of some of the younger samples of both the schists and forearc units (Figs. 3–6). Jurassic. Grains of Jurassic age are present in modest abundances throughout the sample suite (Figs. 4–6). Those in the oldest units occur in association with abundant Early to middle Cretaceous ages (Figs. 5E and 5J). This same correlation is also evident within the type Great Valley Group of central California (Figs. 6A and 6B), as represented by one sample of Campanian age that we collected from Del Puerto Canyon in the northern Diablo Range and five of Cenomanian to Campanian age from the Coalinga area of the southern Diablo Range from the work of DeGraaff-Surpless et al. (2002). This pattern is consistent with a source in the western flank of the central to northern Sierra Nevada, where the Cretaceous and Jurassic arcs overlap (Stern et al., 1981; Chen and Moore, 1982; Irwin and Wooden, 2001; DeGraaff-Surpless et al., 2002). In contrast, Jurassic grains in younger samples tend to be associated with high proportions of Proterozoic and latest Cretaceous ages and low abundances of Early to middle Cretaceous ages (Fig. 5). The latter assemblage likely reflects a source in southeastern California, southwestern Arizona, and northwestern Sonora, where the Jurassic arc lies inboard of the Early to middle Cretaceous arc (Fig. 2; Tosdal et al., 1989; Barth et al., 2008b). Permian–Triassic. Grains of Permian and Triassic age comprise a small but distinctive component of both the schists and forearc units with depositional ages of middle Maastrichtian and younger (Fig. 5). Triassic plutons are minor in abundance but widely distributed within the inboard side of the Cordilleran arc (Barth et al., 1997; Barth and Wooden, 2006). Permian igneous rocks are less common, but have been reported from the northwestern Mojave Desert and vicinity (Martin and Walker, 1995, and references therein). This distribution is consistent with the fact that Permian (and Triassic) grains are most common in samples from the northwestern part of the study area (Figs. DR3 and DR4 [see footnote 1]). Pre-Permian. Both the schists and forearc units provide evidence for two distinct associations of pre-Permian ages. That present within the San Emigdio–Catalina schists and Cenomanian-Turonian forearc units is characterized by a comparatively wide spread of ages from Paleozoic to Paleoproterozoic (Figs. 5E and 5J). This distribution is similar to that exhibited by pre-Mesozoic detrital zircons within Lower Mesozoic sedimentary framework rocks of the western belt of the Peninsular Ranges batholith (Fig. 7C; Grove et al., 2008). Such grains are considered to have a wide range of ultimate sources within North America (e.g., Dickinson and Gehrels, 2003, 2009). Also noteworthy, this assemblage occurs in units marked by a strong concentration of Early to middle Cretaceous detrital zircons (Figs. 5E and 5J), which likewise point to a source in the western Sierra Nevada (see previous). Hence, the lower Upper Cretaceous deposits can be explained entirely by erosion of the western flank of the Cordilleran arc, including its older wall rocks. The second pre-Permian association is characterized by a main peak centered at ca. 1.7 Ga and variably developed peaks at ca. 1.4 and 1.2 Ga. These three peaks are broadly equivalent to the Yavapai-Mazatzal, anorogenic, and Grenville populations, respectively, identified by Dickinson and Gehrels (2009) within Jurassic eolianites and related sequences of the Colorado Plateau region. However, for the relatively outboard setting considered here, we assume that the ca. 1.7 Ga peak includes a substantial contribution from Mojave sources in addition to those from the Yavapai and Mazatzal terranes. The size of the anorogenic peak is consistent with the proportion of intrusive rocks of this age Geological Society of America Bulletin, March/April 2011 495 Jacobson et al. Cumulative prob. 100 50 100 150 200 250 300 600 900 1200 1500 1800 2100 2400 2700 3000 F-POR-C 80 60 A MC 40 20 ERG PR 0 Forearc-POR-Catalina (F-POR-C) (172/4305) B creasing average age of the Cretaceous–early Cenozoic population and increasing abundance of Proterozoic grains with decreasing depositional age is somewhat more systematic within the forearc sequence than the schists (Fig. 5). This is probably due to the fact that the forearc samples represent a greater geographic range, array of depositional environments, and total number of samples and analyses than the schists; i.e., the forearc groups likely reflect a more complete averaging of the margin than the schists. Relative probability Sandstone Petrology 50 Lower Mesozoic wall rocks Peninsular Ranges (PR) (4/433) C Upper McCoy Mountains Formation (MC) (4/119) D Jurassic ergs (ERG) (10/890) E 100 150 200 250 300 600 900 1200 1500 1800 2100 2400 2700 3000 Detrital zircon age (Ma) Figure 7. Detrital zircon age distributions for various sedimentary sequences in California and adjacent areas of the southwestern United States. (A) Cumulative probabilities for sample groups plotted in B–E. (B) Combined data for the Pelona-Orocopia-Rand-Catalina schists and forearc units (this paper). (C) Lower Mesozoic sedimentary framework rocks of the western belt of the Peninsular Ranges batholith (Morgan et al., 2005; Grove et al., 2008). (D) Upper McCoy Mountains Formation in the McCoy Mountains (Barth et al., 2004). (E) Colorado Plateau eolianites (Dickinson and Gehrels, 2003, 2009). See Figure 5 for explanation of scale break at 300 Ma. Paired numbers in parentheses indicate number of samples and number of zircon ages. within the Mojave-Yavapai-Mazatzal basement of the Southwest United States (Karlstrom et al., 1987; Anderson and Bender, 1989; Wooden and Miller, 1990; Barth et al., 2000, 2001a, 2001b; Anderson and Silver, 2005; Farmer et al., 2005) and with predicted zircon “fertility” of the inferred source rocks (Dickinson, 2008). In contrast, whereas zircons of Grenville age are not abundant in either the schists or forearc units, they nonetheless appear to be overrepresented compared to the known distribution of Grenville rocks in the southwestern United States and northwestern Sonora (Barth et al., 1995, 2001b; Anderson and Silver, 2005; Farmer et al., 2005). On the other hand, detrital zircons of Grenville age, apparently derived from the Appalachian 496 orogen, are common in Neoproterozoic to Cambrian cratonal and miogeoclinal sections of this region (Gehrels, 2000; Stewart et al., 2001; Gehrels et al., 2002; Barth et al., 2009). This implies that zircons of Grenville age in the schists and forearc units are recycled. The cratonal and miogeoclinal sequences also include substantial populations of anorogenic and Mojave-YavapaiMazatzal ages; consequently, some of these grains may be recycled, as well. Contrasts between the Schists and Forearc Units Despite the striking similarities between the schists and forearc units, there are some differences. For example, the overall pattern of de- Results from the sandstone modal analysis are consistent with the detrital zircon data but not as diagnostic. For example, the high total lithics, Lv/Lt, and P/F for the CenomanianTuronian forearc units (Table 1; see also fig. 13 in Dickinson, 1995) confirm a source in the western side of the Sierran–Peninsular Ranges arc (cf. Ingersoll, 1983), as concluded based on the zircon data. For the younger samples, however, detrital zircon and sand compositions are not strongly coupled. Specifically, the Campanian to Eocene units exhibit a dramatic shift in detrital zircon populations with depositional age reflecting progressively more inboard source regions (Figs. 5G–5I). In contrast, sand compositions for these same units show only subtle variations with age (Table 1). The detrital zircons thus provide a reliable indication of ages of rocks within the source areas, whereas modal mineralogy reflects only their lithology. The latter is apparently relatively uniform both spatially and temporally within the eastern side of the arc and southwestern part of the craton. The sandstones analyzed here are broadly similar in detrital modes to those from the type Great Valley Group of central California east of the San Andreas fault (Table 1; Ingersoll, 1983). Overall, our samples tend to be somewhat lower in lithics and P/F than the Great Valley rocks, suggesting a more inboard and deeply denuded source area (cf. Dickinson, 1985; Marsaglia and Ingersoll, 1992; Ingersoll and Eastmond, 2007). This probably reflects the younger average age of our samples and their proximity to the strongly disrupted segment of the arc at the latitude of the Mojave Desert (see also Dickinson, 1995). Comparison to Other Depositional Systems Upper McCoy Mountains Formation The McCoy Mountains Formation of southeastern California and southwestern Arizona (Fig. 1) is a weakly metamorphosed Upper Jurassic to Upper Cretaceous fluvial sequence with Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California a stratigraphic thickness of over 7 km (Harding and Coney, 1985; Tosdal and Stone, 1994; Barth et al., 2004; Spencer et al., 2005). The Upper Cretaceous part of the formation was deposited on the foreland side of the Sierran–Peninsular Ranges arc (Barth et al., 2004) and provides an important reference section compared to the coeval forearc units analyzed in this study. Detrital zircon ages (Barth et al., 2004) for four samples of upper McCoy Mountains Formation with depositional ages younger than Turonian but older than middle Campanian are plotted in Figures 6B and 7D (based on these depositional ages, the McCoy data could also have been included in Fig. 6A, but were omitted for reasons of space). Results for the upper McCoy samples contrast sharply with those from contemporaneous parts of both the forearc basin and Pelona-Orocopia-Rand-Catalina schists. In particular, the McCoy Mountains Formation is dominated by Proterozoic detritus (Figs. 6B and 7D), whereas coeval forearc units and Pelona-Orocopia-Rand-Catalina schists were derived overwhelmingly from Cretaceous sources (Figs. 5D, 5E, 5I, 5J, 6A, and 6B). This confirms that the arc served as a topographic barrier between forearc and retroarc basins at this time. The forearc basin was apparently supplied with detritus derived largely from the west side of the arc. In contrast, the retroarc basin received sediment from both the east flank of the arc and nearby cratonal areas to the north and east of the basin that were experiencing foreland shortening and uplift (see also Harding and Coney, 1985; Barth et al., 2004). Colorado Plateau Sequences Recent studies of Permian and Jurassic eolianites from the Colorado Plateau (Dickinson and Gehrels, 2003, 2009) provide important constraints on the eastern limit of the source area for the Pelona-Orocopia-Rand-Catalina schists and forearc units. The Colorado Plateau sequences exhibit some peaks in common with the units analyzed here (e.g., in the range of ca. 1.8–1.0 Ga; Figs. 5, 7B, and 7E). However, the relative sizes of those peaks differ greatly between the two groups. The Colorado Plateau sequences also include substantial proportions of late Neoproterozoic and early to middle Paleozoic ages. Grains of the latter ages are notably rare in the schists and forearc units, particularly in the youngest parts, which are those derived from the most easterly source areas. This indicates the persistence of a topographic divide between coastal and interior regions, even following the removal of the western belt of the arc and inner forearc basin. This is consistent with longstanding inferences of a topographic high, the Mogollon Highlands, in southwestern Ari- zona during the Paleogene (Young and McKee, 1978; Peirce et al., 1979; Potochnik and Faulds, 1998; Spencer et al., 2008). TECTONIC INTERPRETATIONS The Pelona-Orocopia-Rand-Catalina schists and Nacimiento fault represent first-order tectonic elements in the Late Cretaceous–early Cenozoic evolution of the Southwest United States. While it has long been suspected that these two features are in some way tied together, the exact nature of this relationship has been difficult to discern (Page, 1982; Hall, 1991; Barth and Schneiderman, 1996; Barth et al., 2003; Saleeby, 2003). One point that is now evident, however, is that underthrusting of the Pelona-OrocopiaRand-Catalina schists began significantly earlier than movement on the Nacimiento fault (>90 Ma versus <75 Ma, respectively). Furthermore, slip on the Nacimiento fault was probably over by the late Paleocene, whereas emplacement of the schists may have extended into the Eocene (Fig. 3). Because of this timing relationship, we focus the following discussion around the evolution of the schists, and treat the Nacimiento fault and truncation of arc and forearc as one element within that broader context. Preferred Model for Underplating of the Pelona-Orocopia-Rand-Catalina Schists Our preferred model for the origin of the Pelona-Orocopia-Rand-Catalina schists is based on the common, although not universal, view that these units represent a subduction complex (see references cited in the Introduction). This interpretation is compatible with the turbidite sandstone-basalt-chert protoliths and the relatively high-P metamorphism. It is also supported by the extended time frame (>30 m.y.) of emplacement of the schists combined with the short cycling interval (Fig. 3). In fact, we find it difficult to imagine a tectonic setting other than an Andean-style subduction zone that could produce this pattern of ongoing deposition followed almost immediately by underplating and progressive exhumation. Prior to initiation of the Pelona-OrocopiaRand-Catalina event in the early Late Cretaceous, subduction geometry was presumably similar to that commonly invoked to explain the Franciscan–Great Valley–Sierra Nevada triad of central California east of the San Andreas fault (Fig. 8A; for details of the Late Jurassic–Early Cretaceous behavior of the California convergent margin, see Dumitru et al., 2010). By ca. 95–90 Ma, however, the Farallon plate had apparently transitioned, at least in southern California, to a shallow mode of subduction, perhaps related to the presence of an aseismic ridge or oceanic plateau (Livaccari et al., 1981; Henderson et al., 1984; Barth and Schneiderman, 1996; Saleeby, 2003). This geometry would have favored subduction erosion of the overriding North American plate (cf. von Huene and Scholl, 1991; Clift and Vannucchi, 2004; Scholl and von Huene, 2007), inboard migration of the axis of arc magmatism, and underplating of Pelona-Orocopia-Rand-Catalina schist, although continued accretion within the outboard Franciscan wedge is not precluded. Figure 8B depicts the inferred geometry for the early phase of this event, prior to inception of Nacimiento slip at ca. 75–68 Ma. This panel is relevant to the emplacement of the Catalina and San Emigdio Schists and most, if not all, of the Rand Schists (Fig. 3). Note that the western part of the arc would still have been in place at this time, consistent with detrital zircon results for the Catalina– San Emigdio Schists (Fig. 5E) and zircon and sandstone petrologic data for CenomanianTuronian forearc units (Fig. 5J; Table 1). For the most part, the Campanian–early Maastrichtian forearc units (Fig. 5I) suggest a similar paleogeography. The mode of the Cretaceous detrital zircon age peak in the latter group is somewhat younger than for the Cenomanian-Turonian units (100 Ma versus 110 Ma, respectively). However, this could be the result of eastward retreat of the arc front, as described by Linn et al. (1992) for central California, without necessarily requiring any major structural reorganization of the forearc basin and arc. On the other hand, some Campanian–early Maastrichtian sedimentary sequences (Cambria and Pfeiffer slabs, Santa Ynez Mountains, Santa Monica Mountains, Simi Hills; Fig. 6B; Fig. DR10 [see footnote 1]) and the coeval Rand Schists (Fig. 5D; Fig. DR10 [see footnote 1]) include a fraction of sediment that appears to have been derived from sources east of the main axis of the Sierran–Peninsular Ranges arc. It is not clear whether this denotes local erosional breaching of the arc edifice prior to the initiation of slip on the Nacimiento fault or the earliest stages of fault movement. A second transition in the nature of the margin coincided with the removal of the western belt of the arc and associated inner forearc basin beginning in late Campanian or early Maastrichtian time. Considering the uncertain nature of slip on the Nacimiento fault, we present two alternatives for this time period, one involving thrusting (Fig. 8C), and the other strike slip (Fig. 8D). The thrust option (equivalent to the Sur thrust of Hall, 1991) has been supported by numerous workers (Page, 1970, 1981; Yin, 2002; Barth et al., 2003; Saleeby, 2003; Kidder and Ducea, 2006; Ducea et al., 2009) but in our opinion is at odds with the fact that movement Geological Society of America Bulletin, March/April 2011 497 Jacobson et al. A ca. 100 Ma Forearc basin Franciscan Future Colorado Plateau Upper McCoy Mountains Fm. Pre-Cret. crust Top o f Fa Continental lithosphere ral 100 km lon sla b B ca. 80 Ma Franciscan Upper McCoy Mountains Fm. Forearc crust: Jur. and E. Cret? Future Colorado Plateau Pre-Cret. crust Top of Farallon Continental lithosphere sla b Rand/Catalina Schist C ca. 50 Ma—thrust option Rand/Catalina Schist Franciscan Forearc basin Mogollon Highlands Upper McCoy Mountains Fm. NF Future Colorado Plateau Pre-Cret. crust Top Pelona Schist of Orocopia Schist Peraluminous crustal melts Fa ra llo n D ca. 50 Ma—strike-slip option Rand/Catalina Schist Franciscan Forearc basin slab Mogollon Highlands Upper McCoy Mountains Fm. Future Colorado Plateau NF Pre-Cret. crust Top Pelona Schist SW of Orocopia Schist Peraluminous crustal melts Fa ra llo n slab NE Figure 8. Tectonic model for underplating of the Pelona-Orocopia-Rand-Catalina schists and development of the Nacimiento fault. (A) Geometry prior to the onset of flat subduction and emplacement of the Pelona-Orocopia-Rand-Catalina schists. (B) Early phase of flat subduction preceding initiation of slip on the Nacimiento fault. (C–D) Relations following cessation of slip along the Nacimiento fault (NF), assuming thrusting and sinistral strike slip, respectively. For simplicity, slip on the Nacimiento fault is shown in both C and D as postdating emplacement of the Pelona Schist but predating emplacement of the Orocopia Schist. In actuality, movement on the Nacimiento fault is likely to have overlapped with the underthrusting of either or both of those schist groups. Inset in C shows an enlargement of our interpretation of relations adjacent to the Nacimiento fault system in the case of thrusting. The North American craton is held fixed in all panels. Fill patterns for the igneous rocks correspond to the color scheme of Figure 2 with the addition that yellow represents ages of ca. 70–50 Ma. 498 on the Nacimiento fault must have occurred during emplacement of the Pelona-OrocopiaRand schists (Fig. 3). The missing parts of the arc and forearc basin represent a substantial volume of material. Whereas much of this mass could have been completely subducted, it seems reasonable to expect that at least some fraction would have been interleaved with the schists (Fig. 8C). However, as pointed out by Dickinson et al. (2005, p. 14), the Pelona-OrocopiaRand schists bear little resemblance to the omitted units. The schists consist predominantly of coherent meta-sandstone with small amounts of meta-basalt, meta-chert, marble, and ultramafic rock. In contrast, the inferred missing units include parts of the Franciscan mélange, Coast Range ophiolite, uppermost Jurassic through Cretaceous Great Valley Group, and western belt of the Sierran arc, including its pre–Late Jurassic metamorphic framework rocks. The only component of this assemblage that reasonably matches the Pelona-OrocopiaRand-Catalina schists is the Upper Cretaceous part of the Great Valley Group. However, it is not clear how this part of the missing section could have been incorporated within the schists while completely excluding the remainder. Mafic rocks of the Coast Range ophiolite bear superficial compositional resemblance to metabasites within the schists, but they differ in detail. Specifically, the Coast Range ophiolite exhibits an arc signature (Shervais and Kimbrough, 1985; Giaramita et al., 1998; Shervais et al., 2005), whereas mafic rocks in the Pelona-Orocopia-Rand-Catalina schists were derived overwhelmingly from ocean-floor basalts (Haxel et al., 1987, 2002; Dawson and Jacobson, 1989; Moran, 1993). Consequently, we conclude that the Pelona-Orocopia-RandCatalina schists can be explained entirely as subducted trench materials and off-scraped fragments of oceanic crust from the Farallon plate (Figs. 8B and 8D) without any admixture of materials excised from along the Nacimiento fault. Nonetheless, we cannot rule out the possibility that fragments of the missing terranes are hidden within unexposed parts of the PelonaOrocopia-Rand-Catalina schists (Fig. 8C). In contrast to thrust models, strike-slip models for the Nacimiento fault imply that the excised parts of the arc and forearc basin remained at the surface but were translated out of the plane of margin-normal cross sections at the latitude of the Mojave Desert (Fig. 8D). Omission of units by strike-slip motion along the Nacimiento fault can be reconciled with either dextral or sinistral movement, as long as the fault cuts in the appropriate sense across the strike of the margin. Nonetheless, for reasons discussed by Dickinson et al. (2005), we favor the sinistral option. Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California In this case, the missing units would have been transported southward and would be represented by the Peninsular Ranges batholith, the Cretaceous forearc sequence along its western margin, and the more distal, rotated units of the Channel Islands and western Transverse Ranges (see following). The sinistral slip model for the Nacimiento fault avoids some of the issues of the thrust model, but it may be problematic in other ways. Correction for dextral slip on the San Andreas fault (310–320 km; Matthews, 1976) and Rinconada fault (45 km; Graham, 1978) and sinistral slip on the Nacimiento fault (500–600 km; Dickinson et al., 2005) would restore the main body of the Nacimiento block ~150–250 km to the north of its present location and place it directly against the Franciscan Complex and Great Valley Group east of the San Andreas fault (see following and Dickinson, 1983). This is consistent with the work of Gilbert and Dickinson (1970), who demonstrated a parallelism in temporal evolution of petrofacies between forearc units of the Nacimiento block and the Great Valley Group east of the Salinian block. On the other hand, Long and Wakabayashi (2009) argued that accretionary rocks of these two regions do not make a good match. In particular, distinctive sequences within the Franciscan Complex east of the San Andreas fault, such as the Marin Headlands and Permanente terranes and Skaggs Springs Schist (Wakabayashi, 1999), have not been recognized within the Nacimiento block (Wakabayashi, 2010, personal commun.). Considering the extreme spatial variability characteristic of accretionary wedges, these relations do not, in our view, preclude the sinistral slip model. Nonetheless, they do suggest the need for further investigations. A second potential problem with the sinistral slip model relates to longstanding views that relative motion between the Farallon and North American plates was approximately head-on to dextral during the Late Cretaceous–Paleocene (Engebretson et al., 1985; Stock and Molnar, 1988). Dickinson et al. (2005, p. 15), however, suggested that this interpretation may need to be revised in light of recent advances in the understanding of hotspot reference frames. In addition, we propose that sinistral slip may have been an expression of “escape” or “extrusion” tectonics (cf. Tapponnier et al., 1982; Burke and Şengör, 1986) related to subduction of the aseismic ridge or plateau previously called upon to explain the anomalous nature of the southern California margin (Livaccari et al., 1981; Henderson et al., 1984; Barth and Schneiderman, 1996; Saleeby, 2003). Westward translation of the Wrangell terrane of southeast Alaska along the Denali fault due to subduction of the oceanic Yakutat block may serve as a modern analog (Eberhart-Phillips et al., 2006; Fuis et al., 2008). Irrespective of whether the Nacimiento fault operated in a thrust or strike-slip sense, movement on this structure appears to correlate broadly in time with the provenance shift exhibited by the Pelona-Orocopia-Rand-Catalina schists and forearc units. This is not surprising considering that excision of the western arc and inner forearc basin resulted in narrowing of the margin by 150 km or more, thus decreasing the transport distance from the former retroarc (cratonal) region to the coastline (Figs. 8C and 8D). This process can account for the influx of cratonal detritus to the continental margin at the latitude of the Mojave Desert beginning in the latest Cretaceous (Fig. 6). To the north and south of the Mojave, material from inboard sources comprises a smaller fraction of the total sediment volume and first appeared during the latest Paleocene to early Eocene (Figs. 6C and 6D). Regional drainages in the latter areas probably developed across the full width of the Sierran–Peninsular Ranges batholith following arc extinction associated with the Laramide event (Kies and Abbott, 1983; Abbott and Smith, 1989). Telescoping of the arc and forearc basin along the Nacimiento fault helps explain the location of the southeastern (Orocopia) schists within the cratonal province of southwestern Arizona, i.e., inboard of the primary axis of Cretaceous magmatism. Without the removal of the western belt of the arc and inner forearc basin, emplacement of schist this far from the trench requires an extreme degree of flat subduction. In fact, Haxel et al. (2002, p. 124) used this line of reasoning to argue against correlating the Orocopia Schists with the Franciscan Complex and Catalina Schist. However, because the Orocopia Schists postdate much or even all of the truncation of the forearc region (Fig. 3), the required transport distance is not exceptional (Figs. 8C and 8D). In particular, all flat-slab configurations illustrated in Figure 8 fall well within the range of geometries inferred from the modern record (e.g., fig. 5 in Gutscher et al., 2000). The subduction model for the PelonaOrocopia-Rand schists is typically framed in the context of the Laramide orogeny. However, it is worth noting that emplacement of the oldest Pelona-Orocopia-Rand-Catalina schists at ca. 95–90 Ma significantly predates the ca. 80 Ma extinction of the main Sierran arc and ca. 75 Ma onset of the primary pulse of Laramide tectonism in the Rocky Mountain foreland (Dickinson et al., 1988; DeCelles, 2004). This could reflect the time lag between impingement of the flat slab at the continental margin and its propagation to a position beneath the foreland (Lawton, 2008). Alternatively, it might be an indication of tectonic erosion even during normal subduction (cf. von Huene and Scholl, 1991; Clift and Vannucchi, 2004; Scholl and von Huene, 2007). Additional evidence of modest pre-Laramide subduction erosion may be indicated by the 2.7 km/m.y. eastward migration of arc magmatism within the Sierra Nevada between 120 Ma and 90 Ma (Stern et al., 1981; Chen and Moore, 1982). Furthermore, Dumitru et al. (2010) argued for subduction erosion, or at least nonaccretion, in northern California and central California east of the Salinian block prior to ca. 123 Ma. Hence, shallow subduction during Laramide time may simply have enhanced existing erosive processes. A corollary is that rocks like the Pelona-Orocopia-Rand-Catalina schists could be relatively commonplace beneath the outboard edges of Andean-style arcs. Possible analogs of the Pelona-Orocopia-RandCatalina schists in this sense might be the Condrey Mountain Schist of the Klamath Mountains (Brown and Blake, 1987), the Swakane Gneiss of the North Cascades, Washington (Matzel et al., 2004), and the Qiangtang metamorphic belt of northern Tibet (Kapp et al., 2000). Alternative Models for Underplating of the Pelona-Orocopia-Rand-Catalina Schists Forearc Model In the previous section, we interpreted the Pelona-Orocopia-Rand-Catalina schists as a subduction complex formed along the contact between the Farallon and North American plates. Alternatively, Barth and Schneiderman (1996, their fig. 5) proposed that the schists were derived from the Great Valley forearc basin by thrusting beneath the Sierran arc along a fault sympathetic to the subduction boundary, yet contained entirely within the North American plate. As argued already herein, however, we consider the rock types of the Pelona-OrocopiaRand schists to be incompatible with derivation from the forearc basin. Additional difficulties posed by the forearc model are discussed by Haxel et al. (2002) and Grove et al. (2003). Backarc Model This category includes several variants, all of which involve suturing of an intraoceanic continental fragment to North America (Haxel and Dillon, 1978; Ehlig, 1981; Vedder et al., 1983; Haxel et al., 2002). One major obstacle for such models is the lack of compelling evidence for a suture zone (Burchfiel and Davis, 1981; Crowell, 1981; but see counterargument by Haxel et al., 2002). In addition, backarc models imply that the sandstone protoliths of the Pelona-Orocopia-Rand schists and coeval Geological Society of America Bulletin, March/April 2011 499 Jacobson et al. Ra 50 70 na-O Ma (N pia .S vs E op n tio s) A Ma 90 Ra nd 70 W (N Ma Pelo na-O roco pia vs .S ) ns tio op Continental margin E 500 roco W Continental margin ina tal Progressive Subduction Erosion Grove et al. (2003) proposed that buzz-saw removal of the base of North America could allow younger schists to “leap-frog” beneath Pelo Ca One of the most distinctive features of the Pelona-Orocopia-Rand schists is the >30 m.y. decrease in depositional and emplacement ages from northwest to southeast (Fig. 3). Because of the relatively linear distribution of the schist outcrops, it is not clear whether this age variation relates more to distance inboard from the continental edge (Fig. 9A) or to position along strike of the margin (Fig. 9B). Consideration of the Catalina Schist provides some additional constraints but does not fully solve the problem. Assuming no sinistral slip on Nacimiento fault, the Catalina Schist would initially have been located outboard of the Pelona and Orocopia Schists (southeast option in Figs. 9A and 9B). In view of the ca. 95–90 Ma age of the Catalina Schist, this reconstruction is consistent with the isochrons of Figure 9A, but not those of Figure 9B. In other words, the southeast option for the Catalina Schist requires at least some component of decreasing age from outboard to inboard within the schist terrane. Alternatively, for the case of 500– 600 km of sinistral strike slip on the Nacimiento fault, the Catalina Schist would restore to the northwest end of the Pelona-Orocopia-Rand schist belt. This location is consistent with either decreasing age outboard-inboard (Fig. 9A) or northwest-southeast (Fig. 9B). These uncertainties require that we consider several mechanisms to explain the observed variation of age within the schist terrane. Ma Ma 90 ina tal Northwest-Southeast Decreasing Age of the Pelona-Orocopia-Rand Schists would migrate in the same direction, leading to progressively younger packets of accreted material away from the trench. We refer to the predicted geometry as a “tectonic onlap” based on its resemblance to an upside-down sedimentary onlap (clearest in Fig. 8D). nd Ca forearc sequences would have been deposited on opposite sides (east and west, respectively) of the intraoceanic terrane. However, it seems highly unlikely that the detrital zircon age patterns of the schists and forearc units would be so strongly correlated over >30 m.y. (Fig. 5) if their protoliths had been deposited in isolated basins. The McCoy data, in particular, imply that a backarc basin would be highly enriched in cratonal detritus compared to the forearc region (Figs. 6B and 7D). Consequently, we interpret the zircon results to indicate that the forearc units and schists represent the proximal (forearc basin) and distal (trench) facies, respectively, of a single depositional system on the outboard side of the Sierran–Peninsular Ranges arc. Between the previously identified weaknesses (e.g., Burchfiel and Davis, 1981; Crowell, 1981) and zircon data, we see little justification for retaining the backarc models. 50 Ma 100 km B Figure 9. End-member possibilities for age variation within a sheet of Pelona-OrocopiaRand schist underplated beneath North American crust. Northwest and southeast locations of the Catalina Schist reflect the cases of 500–600 and 0 km of sinistral slip on the Nacimiento fault, respectively. The figure is highly schematic and does not take into account the narrowing of the margin that would have resulted from displacement on the Nacimiento fault, whether by thrusting or strike slip. older schists to be underplated at increasingly greater distances inboard (Fig. 8). In this case, age contours would parallel the strike of the margin (Fig. 9A), and the young age of the southeastern schists would be a consequence of their position relatively far inboard. We envision this process involving transfer, due to buoyancy, of some fraction of trench materials from the subducting (Farallon) to overriding (North American) plate above the hinge at which the subducting plate descended toward the mantle. With time, such subcreted materials could become quite thick, leading to tectonic and/or erosional denudation of the overlying plate (cf. Platt, 1986; Yin, 2002). In this fashion, underplated materials would be driven upward, consistent with thermochronologic evidence for exhumation of the Pelona-Orocopia-Rand schists concurrent with subduction (Jacobson et al., 2002, 2007). This would allow permanent incorporation of some trench materials within the overriding plate. As the flat slab continued to erode inboard, the locus of underplating Migrating Aseismic Ridge As indicated already, Barth and Schneiderman (1996) proposed that the Pelona-OrocopiaRand schists were derived by thrusting of the arc over the forearc basin. They attributed this event to subduction of an aseismic ridge (cf. Livaccari et al., 1981; Henderson et al., 1984) with its long dimension oriented slightly more northerly than the convergence vector between the Farallon and North American plates. In this case, the intersection point between ridge and trench would have migrated southward with time, presumably causing the thrust between arc and forearc basin to propagate in the same direction. As discussed previously, we consider that the schists were derived from trench materials rather than from the underthrust forearc basin. Nonetheless, the geometric argument of ridge migration seems applicable to either situation. A similar interpretation was also proposed by Dickinson et al. (1988, p. 1036) to explain north to south migration of Laramide deformation in the foreland region. Sinistral Slip on the Nacimiento Fault The model of a migrating ridge-trench intersection seems particularly effective in explaining the northwest-southeast decrease in age of the schists, assuming sinistral slip on the Nacimiento fault. This is illustrated in Figure 10, where panel A represents the paleogeography prior to fault slip. At this time, the northwestern schist localities would have been situated relatively close to the trench, requiring only modest flat subduction to emplace trench materials beneath the former arc (Figs. 8B and 10A). At the same time, the future sites of the Pelona and Orocopia Schists would have resided far inboard, in a region of arc magmatism (Fig. 10A; Mattinson, 1990; Barth et al., 1995, 1997, 2001a, 2008a; Kidder et al., 2003; Barbeau et al., 2005). With commencement of Nacimiento slip, the latter regions would have been drawn toward the continental margin (Fig. 9B) in the same way that slip on a low-angle normal fault will translate rocks of the lower plate closer to the surface. As a consequence, progressively more southeastward (inboard) regions would have been transferred into the zone of underplating near the continental edge. In this scenario, the schists need not define more than a relatively narrow band (becoming younger to the southeast) beneath Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California Nevada Nevada plate ra CA AZ Nf Nf Ranges CA MEX NV SAf ge rid lk nb lar lk ob ent AZ Mojave block inia Sal CA Gf i cim Na plate ic sm NV SAf Peninsu lk nb inia Sal Mojave block Sie r ra Sie r a F Gf Nf CS N ei As Farallon 100 km Nf ra ll o n –North Farallon a c ri e Am B ca. 60 Ma Diablo range Nacimiento blk A ca. 75 Ma POR-Catalina Schists Prot. basement w/55–85 Ma plutons Early Cret. batholith (100–135 Ma) A lar Peninsu Late Cret. batholith (85–100 Ma) CS c mi s sei ge rid Foothills belt CA MEX es ng 100 km Accretionary complex Ra N Forearc basin Figure 10. Origin of Nacimiento fault by sinistral strike slip (modified from Dickinson [1983] using more recent estimates for age of faulting). Faulting is assumed to have been driven by subduction of an aseismic ridge. Ridge scale is based on analogy to the Yakutat block (Fuis et al., 2008) and ridges on the Cocos and Nazca plates (Gutscher et al., 2000). Present-day outcrops of Pelona-Orocopia-Rand-Catalina schists are shown in black. Only the northwesternmost schist bodies would have been in place at the time represented by panel A. Outlined area labeled “Nacimiento Blk” is based on present-day outcrop of Franciscan Complex between the Nacimiento and San Gregorio–Hosgri faults southeast of Point Sur (Fig. 1). Panel A indicates inferred location of this body prior to sinistral slip on the Nacimiento fault. Panel B shows its location outboard of the Salinian block immediately following Nacimiento fault slip in the early Cenozoic, which closely matches the present-day spatial relationship (Fig. 1). Note that the reconstruction of panel B is a simplified equivalent of that in Figure 2. Heavy arrow shows Farallon–North America relative plate motion. CS—Catalina Schist; Nf—future (panel A) and actual (panel B) traces of Nacimiento fault; Gf—future trace of Garlock fault; SAf—future trace of San Andreas fault; other abbreviations are as in Figure 1. the outer edge of the continent, in contrast to the sheet-like geometry typically envisioned (e.g., Fig. 9). This could explain the relatively linear nature of the schist exposures, which seems fortuitous if the distribution at depth is more laterally persistent. In the sinistral slip model, extraregional detritus is supplied to the coast via a gradually enlarging “window” between the southern Sierra Nevada and northern Peninsular Ranges (Fig. 10B). Sinistral slip on the Nacimiento fault would also translate the Nacimiento block from an initial location op- posite the Diablo Range to the current position adjacent to the Salinian block (Fig. 10; see previous discussion). Marine Sequences of the Salinian Block The Maastrichtian to Paleogene marine transgression of the Salinian block and adjacent areas has long been viewed as an important tectonic signal. Clast distributions in conglomerates indicate a number of localized basins, at least during early phases of deposition (Grove, 1993). Some workers have interpreted this “borderland-style” geometry as direct evidence that strike-slip faulting, whether in a dextral or sinistral sense, played an important role in emplacement of the Salinian block (Nilsen and Clarke, 1975; Howell and Vedder, 1978; Vedder et al., 1983; Dickinson et al., 2005). However, whereas strike-slip faulting can generate local topographic highs and lows, it is not clear that this process, by itself, can account for the substantial net subsidence implied by the regional transgression of the Salinian block. Saleeby (2003, Fig. 4C) attributed this event to extension within the upper plate of the Vincent–Chocolate Mountains thrust system. Alternatively, or in addition, we observe that forearc subsidence is a common feature in regions of inferred subduction erosion as an isostatic response to removal of material from the base of the overriding plate (Clift and Vannucchi, 2004). By analogy, we propose that marine transgression of the Salinian block may have been related, at least in part, to tectonic erosion of North American mantle lithosphere and lowermost crust during underplating of the Pelona-Orocopia-Rand-Catalina schists. This interpretation implies that the age of the transgression in any given area should correlate with the time of local schist underplating. At least to a first approximation, this appears to be the case for the Salinian block and central Transverse Ranges. The correlation does not hold true in the San Emigdio Mountains, where the schist is early Late Cretaceous in age, but the base of the marine section in the upper plate is Eocene (Nilsen, 1987b). However, because this region is at the northwesternmost end of the schist belt, it is conceivable that subsidence was limited by the flexural rigidity of a full thickness of lithosphere beneath the adjacent Sierra Nevada. CONCLUSIONS Similar detrital zircon patterns exhibited by the Pelona-Orocopia-Rand-Catalina schists and coeval sedimentary units of the forearc basin provide evidence that these two sequences comprise parts of a single depositional system on the outboard side of the Sierran–Peninsular Ranges arc. The >30 m.y. time frame for emplacement of the schists and the short cycling interval are most consistent with a subduction origin. The sandstone protoliths of the schists are best interpreted as trench sediments complementary to the forearc basin sequence. Emplacement of the schists directly beneath the Cordilleran magmatic arc requires the removal of North American mantle lithosphere and lowermost crust. The initial stages of this underplating may have occurred at the end of a period of modest subduction erosion associated Geological Society of America Bulletin, March/April 2011 501 Jacobson et al. with slow, eastward migration of Sierran magmatism during the middle Cretaceous. Later schist emplacement, however, may have been expedited by flat subduction associated with the classic Laramide orogeny of the Rocky Mountain foreland. Initial underplating of the schist (ca. 95– 75 Ma) was probably not associated with any major changes in the paleogeography of the arc and forearc basin. Sediment supplied to the forearc basin and trench during this stage was derived by erosion of the western flank of the Sierran–Peninsular Ranges arc and associated wall rocks. The arc formed a continuous topographic barrier that separated the forearc region from an area of retroarc sedimentation that included the McCoy Mountains Formation. Beginning as early as ca. 75 Ma, but no later than ca. 68 Ma, slip on the Nacimiento fault led to progressive removal of the western belt of the arc and eastern to central portions of the forearc basin. As a result of the truncation event, basement terranes formerly within the eastern flank of the arc or adjoining craton were relocated much closer to the continental margin, i.e., in a position where they could more easily shed sediment to the continental margin. However, the apparent absence of detritus in either the schists or forearc units derived from the region of the present-day Colorado Plateau implies the continued presence of a high-standing mountain range (Mogollon Highlands) relatively close to the coast. Displacement on the Nacimiento fault most likely involved either thrusting or sinistral strike slip, as opposed to dextral strike slip. The thrust option has been most heavily favored, but it is difficult to reconcile with the lack of exposure of the missing complements of the Salinian and Nacimiento blocks in windows through southern California arc and cratonal crust. The model of sinistral strike slip raises questions regarding the correlation of accretionary rocks between the Nacimiento block and central California east of the San Andreas fault. It is also not obviously consistent with current interpretations of relative plate motions during the Laramide orogeny, but this could be explained by escape tectonics related to collision of an aseismic ridge with the Farallon subduction zone. ACKNOWLEDGMENTS Our work was supported by National Science Foundation grants EAR-0106123, EAR-0106881, EAR0408580, and EAR-0408730. The ion microprobe facility at the University of California–Los Angeles (UCLA) and the LaserChron Center at the University of Arizona are partly supported by grants from the Instrumentation and Facilities Program, Division of Earth Sciences, National Science Foundation. Ana Vućić is thanked for providing substantial support in sample preparation and analysis at UCLA. Scott 502 Johnston, Alex Pullen, and Victor Valencia provided critical assistance with use of the inductively coupled plasma mass spectrometer (ICP-MS) at the University of Arizona. We are deeply indebted to many workers who assisted our sampling efforts by suggesting localities, providing maps, and/or accompanying us in the field. These include Pat Abbott, Bill Bartling, Alan Chapman, Mark Cloos, Ivan Colburn, Brett Cox, Gene Fritsche, Steve Graham, Karen Grove, Clarence Hall, Gordon Haxel, Dave Howell, Ray Ingersoll, Dave Kimbrough, Marilyn Kooser, and Jason Saleeby. 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