Late –early Cenozoic tectonic evolution of the southern 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 1Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, USA 2Department of Geological and Environmental Sciences, Green Earth Sciences, Rm. 225, Stanford University, Stanford, California 94305-2115, USA 3Department of Earth Sciences, 723 West Michigan Street, SL118, Indiana University–Purdue University, Indianapolis, Indiana 46202-5132, USA 4Department of Geological Sciences, California State University–Northridge, 18111 Nordhoff Street, Northridge, California 91330- 8266, USA 5Department of Geosciences, 1040 E. 4th Street, Gould-Simpson Building, University of Arizona, Tucson, Arizona 85721, USA 6Department of Geological Sciences, California State Polytechnic University, 3801 W. Temple Avenue, Pomona, California 91768, USA

ABSTRACT Salinian and Nacimiento blocks of the cen- are particularly well-known components of this tral Coast Ranges. This observation is most system (Fig. 1). Similar rocks also characterize During the Late Cretaceous to early Ceno- readily explained if the schists were derived and its displaced correlatives zoic, southern California was impacted by from trench sediments complementary to the within the central Coast Ranges west of the San two anomalous tectonic events: (1) under- forearc basin. The schists and forearc units Andreas. In the latter regions, however, the con- plating of the oceanic Pelona-Orocopia-Rand are inferred to record an evolution from vergent margin belts are severely disrupted by schists beneath North American arc crust normal subduction prior to the early Late the Nacimiento fault, which separates the Salin- and craton; and (2) removal of the west- Cretaceous to fl at subduction extending into ian block on the northeast from the Nacimiento ern margin of the arc and inner part of the the early Cenozoic. The transition from out- block to the southwest (Page, 1970, 1981, 1982; forearc basin along the Nacimiento fault. The board to inboard sediment sources appears Dickinson, 1983; Hall, 1991; Saleeby, 2003; Pelona-Orocopia-Rand schists crop out along to have coincided with removal of arc and Ducea et al., 2009). The is cored a belt extending from the southern Sierra Ne- forearc terranes along the Nacimiento fault, by granitoid plutons similar in age (100–75 Ma) vada to southwestern Arizona. Protolith and which most likely involved either thrusting and composition to those of the central belt of emplacement ages decrease from >90 Ma in or sinistral strike slip. The strike-slip inter- the and regions to the east, yet the northwest to <60 Ma in the southeast. pretation has not been widely accepted but it lacks an analog to the western foothills belt Detrital zircon U-Pb ages imply that meta- can be understood in terms of tectonic escape of the Sierra (Mattinson, 1978, 1990; Silver, sandstones in the older schists originated driven by subduction of an aseismic ridge, 1983; Ross, 1984). The Nacimiento block, in primarily from the western belt of the Sier- and it provides a compelling explanation for contrast, is underlain by the Franciscan Com- ran–Peninsular Ranges arc. Younger units the progressively younger ages of the Pelona- plex and remnants of sedimentary sequences were apparently derived by erosion of pro- Orocopia-Rand schists from northwest to equivalent to the distal parts of the type Great gressively more inboard regions, including southeast. Valley Group, but it is missing forearc units the southwestern edge of the North Ameri- indicative of a proximal setting (Hart, 1976; can craton. The oldest Pelona-Orocopia- INTRODUCTION Hall et al., 1979; Page, 1981; Seiders, 1982; Rand schists overlap in age and provenance Vedder et al., 1983; Hall, 1991). By compari- with the youngest part of the Catalina Schist The geology of California is dominated by son with the Franciscan–Great Valley–Sierran of the southern California inner continental NNW-trending lithotectonic belts produced triad east of the , the juxtapo- borderland, suggesting that the two units are during late –early Cenozoic subduc- sition of the Salinian and Nacimiento blocks broadly correlative. The Pelona-Orocopia- tion of the oceanic Farallon plate beneath the along the Nacimiento fault implies the removal Rand-Catalina schists, in turn, share a com- western edge of North America (Hamilton, of a width of 150 km or greater of formerly mon provenance with forearc sequences 1969; Dickinson, 1970; Ernst, 1970). The intervening westernmost arc and inner to cen- of southern California and the associated Franciscan subduction complex, Great Valley tral forearc basin. This truncation appears forearc basin, and Sierra Nevada batholith of to have occurred sometime between 75 and †E-mail: [email protected] central California east of the San Andreas fault 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 fi gures; 1 table; Data Repository item 2011005.

For permission to copy, contact [email protected] 485 © 2011 Geological Society of America Jacobson et al.

PSP 121°W Faults Pelona-Orocopia-Rand Schists Forearc sampling areas Ef Elsinore BR Blue Ridge AT Atascadero Diablo DP EHf East Huasna CD Castle Dome Mountains BL Ben Lomond Mountain Great Gf Garlock CH SE Chocolate Mountains CA Cambria PP BL Nf Nacimiento EF East Fork CL Coalinga 37°N Rf Rinconada MP Mount Pinos CP Cajon Pass SAf San Andreas NR Neversweat Ridge DP Del Puerto Canyon Range SGf San Gabriel OR Orocopia Mountains IR Indians Ranch SGHf San Gregorio-Hosgri PR Portal Ridge LP PL SS SJf San Jacinto RA Rand Mountains NB Northern Baja California SL PS SYf Santa Ynez SC Santa Catalina Island OR Orocopia Mountains CL SE PL IR Sierra 36°N Valley SP Sierra Pelona PM Mountain Block Rf Nf SS PP Pigeon Point TR Trigo Mountains PS Point Sur Nevada PSP Point San Pedro CA AT SA Northern Santa Ana Mts. SGHf LP RA SC Santa Catalina Island SAf SD San Diego Gf SG San Gabriel block 35°N Salinian block EHf SAf SMM SGHf Nf SE SL NW Santa Lucia Range MP SM Santa Monica Mts-Simi Hills SR Nacimiento blk PM SG PR SMI San Miguel Island SYf SR SMM Sierra Madre Mountains Nf BR SY SY SP SG CP SR CTR WTR EF SY Santa Ynez Mountains SMI SM SGf 34°N ETR McCoy Mts. Fm. WTR SA Peninsular OR Accretionary complex SC N SJ Forearc basin f SAf TR CD Ranges NR Magmatic arc and wall rocks Ef CH CA Pelona-Orocopia-Rand Schists 100 km SD MEX AZ McCoy Mountains Formation 118°WNB 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 defi ned in the fi gure: AZ—Arizona; CA—California; CTR—central ; ETR—eastern Transverse Ranges; MEX—Mexico; WTR—western Transverse Ranges.

involved remain unclear. Previous interpreta- 1978; Ehlig, 1981; Jacobson et al., 1988). These Orocopia-Rand schists has long been debated tions include thrusting of 150–200 km (Silver, rocks broadly resemble the Franciscan Com- (Haxel et al., 2002; Grove et al., 2003), although 1983; Hall, 1991; Barth and Schneiderman, plex, but instead of being positioned outboard most workers now favor a model involving low- 1996; Saleeby, 1997; Ducea et al., 2009), dex- of the forearc basin, they sit in structural contact angle subduction of the Farallon plate during the tral strike slip of 2000 km or more (Page, 1982; directly beneath the Cretaceous marginal batho- Laramide orogeny (Crowell, 1968, 1981; Yeats, Champion et al., 1984), and sinistral strike slip lith and adjoining cratonal areas of southeastern 1968; Burchfi el and Davis, 1981; Dickinson, of 500–600 km (Dickinson, 1983; Seiders and California and southwestern Arizona. Poten- 1981; Hamilton, 1988; May, 1989; Malin et al., Blome, 1988; Dickinson et al., 2005). tially equivalent rocks, recovered as xenoliths, 1995; Jacobson et al., 1996, 2002, 2007; Wood A second key feature of southern and coastal occur still farther east in the Four Corners region and Saleeby, 1997; Yin, 2002; Grove et al., central California is represented by the Pelona- of Arizona, New Mexico, Utah, and Colorado 2003; Saleeby, 2003; Kidder and Ducea, 2006; Orocopia-Rand schists (Fig. 1; Haxel and Dillon, (Usui et al., 2006). The origin of the Pelona- Saleeby et al., 2007).

486 Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California

In the past, detailed understanding of the Luyendyk et al., 1985; Crouch and Suppe, 1993; ridor between the Sierra Nevada batholith to Pelona-Orocopia-Rand schists has been ham- Dillon and Ehlig, 1993; Powell, 1993; Nichol- the north and the Peninsular Ranges batholith pered by a lack of good control on either the son et al., 1994; Dickinson, 1996; Fritsche et al., to the south. The location of the southeastern depositional age of the protolith or the time 2001). To help correct for these complications, extension of the Nacimiento fault is unknown of underthrusting and metamorphism. Recent we utilize a schematic pre-Miocene reconstruc- (queried in Fig. 2), but it likely occurs outboard 40Ar/39Ar thermochronologic analyses and U-Pb tion of southern California and adjoining areas of Proterozoic basement of the central Trans- dating of detrital zircons, however, demon- (Fig. 2) that builds upon a similar analysis pre- verse Ranges and southeasternmost California. strate that sedimentation and underplating oc- sented by Grove et al. (2003; see also the GSA The omission of forearc and western batholithic curred from >90 Ma to <60 Ma (Jacobson et al., Data Repository item for details1). This map rocks and out-of-position outcrops of cratonal 2000, 2002, 2007; Barth et al., 2003; Grove serves as a base for plotting our results and pro- basement are central to understanding the dis- et al., 2003). This point is critical, because it vides context for discussing the regional geo- placement history and tectonic signifi cance of indicates that underthrusting of the Pelona- logic setting of the study area. the Nacimiento fault. Orocopia -Rand schists began at least 15 m.y. In reconstructing southern California at the prior to the 75 Ma or younger initiation of slip level of detail shown here, it is relatively easy to Pelona-Orocopia-Rand and on the Nacimiento fault. Hence, formation restore northwest-southeast–trending strike-slip Catalina Schists of the schists was at least in part a separate faults near the continental margin and to back- phenomenon from the juxtaposition of the rotate the western Transverse Ranges, which lie Pelona-Orocopia-Rand Schists Salinian and Nacimiento blocks. On the other along a “free edge” of the map (Fig. 2). In con- The Pelona-Orocopia-Rand schists consist of hand, both events appear to have been occurring trast, compatibility issues make it far more dif- 90% or more quartzo-feldspathic schist presum- simultaneously sometime between ca. 75 and fi cult to (1) correct for rotations of regions that ably derived from turbidite sandstone (Ehlig, 59 Ma. It is hard to imagine that they were not lie entirely within the map area (e.g., the Sierran 1958, 1981; Haxel and Dillon, 1978; Jacobson linked in some fashion during this period. “tail” or eastern Transverse Ranges); (2) restore et al., 1988, 1996; Haxel et al., 1987, 2002). Detrital zircon ages from the schists also offset on strike-slip faults at a high angle to the The schists also include up to 10% metabasite, reveal a major shift in provenance from rela- San Andreas fault or those parallel to the San along with minor amounts of Fe-Mn meta- tively outboard to inboard source areas at about Andreas but located relatively far inboard; or chert, marble, serpentinite, and talc-actinolite the time of movement on the Nacimiento fault (3) undo extension associated with detachment rock. Metamorphism occurred under conditions (Grove et al., 2003). However, because the faults. Such corrections were not attempted here of moderately high pressure relative to tem- schists are allochthonous, relatively limited in but must be kept in mind when considering the perature. Mineral assemblages lie mostly within geographic distribution, and represent only a paleogeology. the greenschist and albite-epidote amphibolite narrow time range in any given area, they do not A second complication pertains to longstand- facies but locally extend into the epidote-blue- provide tight control on the nature and areal ex- ing controversies regarding the magnitude of schist and upper amphibolite facies (Ehlig, 1981; tent of this event. slip on various branches of the San Andreas Haxel and Dillon, 1978; Graham and Powell, To better understand the interconnections system. Our reconstruction follows interpreta- 1984; Jacobson and Sorensen, 1986; Jacobson between Nacimiento fault slip and underplat- tions of Crowell (1962, 1981), Dillon and Ehlig et al., 1988; Kidder and Ducea, 2006). Inverted ing of the Pelona-Orocopia-Rand schists, we (1993), Hall et al. (1995), Dickinson (1996), and metamorphic zonations are typical. Initial em- conducted a regional detrital zircon and sedi- Dickinson et al. (2005). Contrasting estimates placement of the schists is attributed to a Late mentary petrologic analysis of unmetamor- of slip for a number of major faults within the Cretaceous–early Cenozoic low-angle fault sys- phosed units of the forearc basin that overlap study area have been presented by Sedlock and tem referred to as the Vincent–Chocolate Moun- in age and geographic extent with the schists. Hamilton (1991), Powell (1993), Underwood tains thrust (Haxel and Dillon, 1978). However, As part of this study, we expanded the data set et al. (1995), and Burnham (2009), among most present-day contacts between the schists of Grove et al. (2003) for the Pelona-Orocopia- others. However, these disagreements, while and North American upper plate appear to be Rand schists and treated these analyses along important for a complete understanding of the low-angle normal faults related to exhumation with the results from Grove et al. (2008) for the geology of California, are secondary in nature of the schists, either shortly after underthrust- broadly similar Catalina Schist of the inner con- at the scale considered here and have no signifi - ing or during middle Cenozoic extension (Frost tinental borderland of southern California. The cant impact on our conclusions. et al., 1982; Hamilton, 1988; Jacobson et al., combined schist and forearc data provide im- By restoring middle Cenozoic and younger 1988, 2002, 2007; Malin et al., 1995; Wood and portant constraints on this critical period in the deformations as best we can, Figure 2 more Saleeby, 1997; Haxel et al., 2002). paleogeographic and plate-tectonic evolution of clearly highlights the impact of the Nacimiento Outcrops of the Pelona-Orocopia-Rand southern California and environs. fault on the geologic framework of south- schists defi ne a northwest-southeast–trending ern California. In addition to the ~150 km of belt extending from the southern Sierra Nevada GEOLOGIC BACKGROUND stratigraphic omission of forearc and batho- to southwestern Arizona (Figs. 1 and 2). The lithic rocks that occurs across the fault, note northwestern schists underlie the outboard to Palinspastic Reconstruction that Proterozoic cratonal basement crops out central parts of the Sierran batholith of Early anomalously close to the margin in the cor- to middle Cretaceous age. The southeastern The Late Cretaceous–early Cenozoic assem- schists, in contrast, sit beneath Proterozoic, Tri- blages considered in this study have been 1GSA Data Repository item 2011005, explanation assic, , and latest Cretaceous to early disturbed by younger tectonic events, includ- of the palinspastic reconstruction, geologic summaries Cenozoic rocks situated inboard of the main of forearc sampling areas and notes on individual sam- ing middle Cenozoic extension and middle to ples, supplementary fi gures, and data tables, is avail- Cretaceous arc. As noted by Grove et al. (2003), late Cenozoic San Andreas–related deforma- able at http://www.geosociety.org/pubs/ft2011.htm or and discussed in more detail herein, protolith tion (Crowell, 1962, 1981; Frost et al., 1982; by request to [email protected]. and emplacement ages of the schists decrease

Geological Society of America Bulletin, March/April 2011 487 Jacobson et al.

Sierra Gf NV AZ PSP Nevada RA BL CA

SE Mojave Desert PR SGHf SAf SS M o g o l l o n H i g h l a n d s CP PL Rf SL Nf IR ? Nf LP PP SMM AT SG 100 km EHf MP CA SR SGHf SP BR OR SR Igneous Rocks Nf CA PS PM SGf EF AZ 55–85 Ma TR CD NR CH 85–100 Ma ? 100–135 Ma MEX Cretaceous undifferentiated SY 135–300 Ma N SM Proterozoic SA Peninsular SY Sedimentary (±Volcanic) Sequences SC and Metamorphosed Equivalents SJf Cretaceous-Eocene forearc basin Ranges Accretionary complex Pelona-Orocopia-Rand-Catalina Schists Figure 6 Jurassic-Cretaceous McCoy Mountains Fm. SD SMI Early Cretaceous Alisitos arc 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 fi gure 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 more nearly parallel to the strike of the mar- Complex. The unit is most widely exposed on <60 Ma in the southeast. Signifi cant variations gin). Alternatively, A. Chapman and J. Saleeby Santa Catalina Island, where it forms a series in provenance are also observed from one end (2009, personal commun.) argue that rotation of fault-bounded slices of metasedimentary, of the belt to the other. of the upper-plate, batholithic rocks did not af- metavolcanic, and ultramafi c rocks (Woodford, In contrast to the northwest-southeast align- fect the structurally underlying schists and that 1960; Platt, 1975, 1976; Sorensen, 1988; Grove ment of schist exposures as a whole, the north- the current locations of the northwestern schists and Bebout, 1995; Grove et al., 2008). Meta- western bodies defi ne a southwest-northeast closely refl ect their relative distributions at the morphism ranges from upper amphibolite facies subbelt extending from the Sierra de Salinas time of underthrusting. This is a further compli- in the structurally highest unit to lawsonite- to the Rand Mountains. One possibility is that cation for reconstructing the Late Cretaceous– blueschist, lawsonite-albite, and albite-actinolite this deviation is an artifact of our failure to cor- early Cenozoic paleogeography of southern facies in the deepest parts of the section (Grove rect for oroclinal bending of the Sierran tail California and vicinity. et al., 2008). The Catalina Schist also occurs as and adjacent parts of the Mojave Desert and widespread submarine outcrops within the inner Salinian block (Kanter and McWilliams, 1982; Catalina Schist continental borderland, as minor outcrops on the McWilliams and Li, 1985). In this case, the belt The Catalina Schist is a moderately high- Palos Verdes Peninsula, and in the subsurface of northwestern schists would originally have pressure metamorphic terrane typically viewed of the Los Angeles basin (references in Grove been oriented approximately north-south (i.e., as the southern extension of the Franciscan et al., 2008).

488 Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California

Relationship of Pelona-Orocopia-Rand and south of San Francisco. All sampling localities pret these rocks as an overlap sequence, indicat- Catalina Schists can be classifi ed into one of three tectonic do- ing that most or all of the slip on the Nacimiento Strong lithologic and metamorphic similari- mains: (1) Salinian block and central Transverse fault had accumulated by ca. 56 Ma. In a num- ties between the Pelona-Orocopia-Rand schists Ranges, (2) Nacimiento block, and (3) western ber of locations southwest of the fault, the and parts of the Catalina Schist have long been Transverse Ranges and Peninsular Ranges. Eocene section sits conformably on Upper noted (Ehlig, 1958; Woodford, 1960; Platt, Paleocene Sierra Blanca Limestone (Whidden 1976; Jacobson and Sorensen, 1986). None- Salinian Block–Central Transverse Ranges et al., 1995). This limestone has not been rec- theless, the two groups of rocks have generally The term “Salinian block and central Trans- ognized in the southernmost Salinian block been treated separately because: (1) the Catalina verse Ranges” as used here also includes the northeast of the Nacimiento fault, although the Schist is lithologically more diverse and exhib- western fringes of both the Mojave Desert and base of the Eocene section in the latter area is its a greater range of metamorphic facies than eastern Transverse Ranges. Basement rocks exposed only on the inboard side of the basin, the Pelona-Orocopia-Rand schists; (2) initial of this region consist of Proterozoic to Juras- where it onlaps crystalline basement. Hence, the geochronologic studies suggested an Early Cre- sic magmatic and metamorphic units intruded overlap sequence could be as old as late Paleo- taceous metamorphic age for the Catalina Schist by Late Cretaceous plutons characteristic cene (ca. 59 Ma). In addition, Hall (1991, p. 29) (Mattinson, 1986) versus a latest Cretaceous– of the central to eastern parts of the Sierran– and Dickinson et al. (2005, p. 15) concluded early Cenozoic age for the Pelona-Orocopia - Peninsular Ranges arc (Mattinson, 1990; from unconformities within the Salinian block Rand schists (Jacobson, 1990); and (3) the Barth et al., 1995, 1997, 2000, 2001a, 2008a; that slip on the Nacimiento fault was most likely Catalina Schist occurs in a more outboard set- Kidder et al., 2003; Barbeau et al., 2005). over by ca. 62.5–62 Ma. As a compromise, we ting than the Pelona-Orocopia-Rand schists. Even the youngest Cretaceous intrusive rocks use 59 Ma as the minimum age bound (Fig. 3), However, subsequent work on both units in- (ca. 75 Ma) are cut by the Nacimiento fault, keeping in mind that the actual age for cessa- dicates that the structurally deepest, youngest thus providing a maximum age for the struc- tion of fault movement could be somewhat older part of the Catalina Schist (lawsonite-blueschist ture. Cretaceous magmatism was followed by or younger. In fact, the absolute minimum age and lower-grade facies of Grove et al., 2008) a marine transgression that was in progress by of slip is indicated by a distinctive barnacle- overlaps in both depositional and metamorphic the middle Maastrichtian (time scale of Walker bearing unit of early Miocene age that overlaps age with the oldest known part of the Pelona- and Geissman, 2009)(e.g., Santa Lucia and the central part of the Nacimiento fault (Dickin- Orocopia-Rand schists (San Emigdio Moun- La Panza Ranges; Howell et al., 1977; Vedder son et al., 2005, p. 15). tains) at ca. 95–90 Ma (Grove et al., 2003, et al., 1983; Saul, 1986; Seiders, 1986; Sliter, 2008). In addition, our work reveals that the co- 1986; Grove, 1993) and that advanced south- Nacimiento Block eval parts of the Catalina and Pelona-Orocopia- east and inboard through the early Eocene (e.g., The Nacimiento block (equivalent to the Sur- Rand schists include similar assemblages of Pine Mountain block and Orocopia Mountains; Obispo belt of Page, 1981, 1982) is underlain detrital zircons. (Combined zircon results for Crowell and Susuki, 1959; Chipping, 1972; primarily by the . Locally, the Catalina Schist and Pelona-Orocopia-Rand Advocate et al., 1988; Dickinson, 1995). The the Franciscan rocks are structurally overlain schist of the San Emigdio Mountains are pre- marine fl ooding of the Salinian block was by (1) fragments of Coast Range ophiolite; sented herein. Individual plots for the two units presumably related to removal of the western (2) uppermost Jurassic to Valanginian and are included in Figure DR1 in the GSA Data Re- fringe of the arc and adjoining earlier phases of Upper Cretaceous (through Campanian) sedi- pository item [see footnote 1].) Finally, recent the forearc basin and thus implies that signifi - mentary rocks deposited nonconformably on workers have proposed that initial emplacement cant slip had accumulated on the Nacimiento Coast Range ophiolite and resembling units of of the Catalina Schist occurred beneath the fault by the middle Maastrichtian (ca. 68 Ma). the Great Valley Group east of the San Andreas northern Peninsular Ranges, implying a tec- The transgressive sequences of the Salinian fault; and (3) sandstone-rich sequences of Cam- tonic setting analogous to that for the Pelona- block and central Transverse Ranges are distinc- panian(?) age, which may have been deposited Orocopia-Rand schists (Crouch and Suppe, tive in that many show evidence for relatively directly on the Franciscan accretionary wedge 1993; ten Brink et al., 2000; Fritsche et al., continuous deposition across the Cretaceous- (Hall and Corbató, 1967; Gilbert and Dickin- 2001; Grove et al., 2008). According to this in- Paleocene boundary (Kooser, 1982; Saul, 1983). son, 1970; Page, 1970, 1981, 1982; Hart, 1976; terpretation, the present location of the Catalina Furthermore, Paleocene rocks, in general, are Howell et al., 1977; Seiders, 1982; Vedder et al., Schist in the borderland is the result of being widespread and relatively thick, although un- 1983; Seiders and Blome, 1988; Hall, 1991; exhumed from beneath the Peninsular Ranges conformities occur locally within this interval Seiders and Cox, 1992; Dickinson et al., 2005). during Miocene extension. Based on these re- (Chipping, 1972; Graham, 1979; Ruetz, 1979; Rocks of Maastrichtian to Eocene age have not lations, we conclude that the Pelona-Orocopia- Saul, 1986; Nilsen, 1987a). Facies relations in- been recognized, except at the southernmost end Rand schists and youngest parts of the Catalina dicate that initiation of deposition in individual of the block. We view the forearc units of the Schist are broadly correlative. We do not include areas, whether it occurred during the Maas- Nacimiento block as distal analogs of the proxi- the older elements of the Catalina Schist in this trichtian, Paleocene, or Eocene, was commonly mal facies present within the Salinian block. grouping, because they may have originated in a accompanied by rapid subsidence of the basin However, the scarcity of forearc units younger forearc, rather than accretionary wedge, setting fl oor to bathyal depths (Kooser, 1982; Advocate than Campanian in the Nacimiento block but (Grove et al., 2008). et al., 1988; Grove, 1993). older than Maastrichtian in the Salinian block Lower Eocene marine deposits within the makes it diffi cult to constrain the exact paleo- Forearc Sedimentary Units southern Salinian block resemble units of simi- geography prior to slip on the Nacimiento fault. lar age in the southernmost Nacimiento block Most of our samples from the Nacimiento We analyzed unmetamorphosed Upper Cre- and eastern part of the western Transverse block come from two distinct sequences of taceous to middle Eocene forearc units ex- Ranges (Chipping, 1972; Page, 1981, 1982; Campanian or inferred Campanian age. One tending from northern Baja California to just Vedder et al., 1983; Dickinson, 1995). We inter- 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 Ana Mountains; Fritsche et al., 2001; Saul and niques. Most zircons from both the schists depositionally upon the Coast Range ophiolite Alderson, 2001). In analogous fashion, the and forearc units were analyzed by secondary (e.g., samples of the Cachuma and Atascadero western Transverse Ranges represent an even ionization mass spectrometry (SIMS) using Formations and potentially the Pigeon Point more distal setting (westerly in palinspastic co- the Cameca IMS 1270 ion microprobe at the Formation; see GSA Data Repository item [see ordinates; e.g., Fig. 2) within the forearc basin University of California, Los Angeles. Pro- footnote 1]). We also analyzed samples from (Dibblee, 1950, 1966, 1991; MacKinnon, 1978; cedures followed those described in Grove the Cambria slab, which is widely regarded as Vedder et al., 1983, 1998; Thompson, 1988; et al. (2003). Ion microprobe analysis is time a trench-slope-basin assemblage deposited on Dickinson, 1995). Sedimentation in the west- intensive, and because of our desire to sample top of the Franciscan Complex (Howell et al., ern Transverse Ranges began in latest Jurassic a broad geographic and stratigraphic range, 1977; Smith et al., 1979; Dickinson et al., 2005), and/or earliest Cretaceous time. The base of the we determined a median of only 15–16 SIMS and the Pfeiffer Slab of Hall (1991), which has section is depositional upon ophiolite, which in zircon U-Pb ages per sample. Since age dis- been interpreted as either a trench-slope de- turn sits structurally above the Franciscan Com- tributions for individual samples are not well posit (Howell et al., 1977; Smith et al., 1979; plex. This relationship is similar to that of the constrained, we focus upon pooled results Hall, 1991) or part of the Franciscan Complex Nacimiento block. from groups of samples. (Under wood and Laughland, 2001; Dickinson Some paleomagnetic data have been taken to During the latter part of the study, zircon et al., 2005). Despite the potentially contrasting indicate large-scale northward translation of the analyses were conducted at the University of depositional settings of the Cambria and Pfeiffer Peninsular Ranges prior to slip on the San An- Arizona using the laser ablation–multicollector– slabs, our detrital zircon results are consistent dreas system. However, we accept the alternate inductively coupled plasma–mass spectrom- with a common provenance. view that the Peninsular Ranges are essentially eter (LA-ICP-MS) approach carried out with a in place other than for dextral translation along 193 nm excimer laser and a GVI Isoprobe mass Peninsular Ranges–Western the San Andreas system and possibly sinistral spectrometer. Procedures followed those of Transverse Ranges slip on the Nacimiento fault (Dickinson and Gehrels et al. (2008). The LA-ICP-MS tech- This area shows no evidence for the structural Butler, 1998). nique is highly effi cient, which generally led to dislocation that affected the Nacimiento and larger data sets than with the SIMS (although Salinian blocks to the north; any Nacimiento- METHODS some large data sets were collected even with related slip at this latitude is presumed to lie well the SIMS and some small ones with the LA- inboard of the forearc basin. On the other hand, Zircon U-Pb Ages ICP-MS). This creates a problem when pooling the western Transverse Ranges are widely con- data, because samples with a large number of sidered to have undergone 90° or more of clock- Based on the regional scope of the study, we analyses will be weighted more heavily than wise rotation during Neogene development of considered it most useful to maximize the num- those for which fewer ages were determined. the San Andreas system (Luyendyk et al., 1985; ber of samples analyzed rather than the density To avoid this complication, we utilized a sub- Nicholson et al., 1994; Dickinson, 1996). Prior of age data collected per sample. Our study is sampling procedure in which ages from indi- to rotation, the western Transverse Ranges are thus best viewed as an exit poll, with more com- vidual samples with a large number of results believed to have been positioned alongside the prehensive work required to identify all minor were sorted in numeric order. We then selected western margin of the Peninsular Ranges. populations and answer detailed provenance the oldest and youngest ages and every second, Sedimentary units exposed along the western questions related to individual samples. For third, etc., age as needed to reduce the number fl ank of the Peninsular Ranges were deposited the Pelona-Orocopia-Rand schists, our data set of analyses to a more appropriate level. This ap- in the relatively inboard part of the forearc basin includes 1405 zircon ages for 55 samples of proach yielded probability plots for individual (Schoellhamer et al., 1981; Bottjer and Link, quartzo-feldspathic schist (Table DR1 [see foot- “weeded” samples that were virtually indis- 1984; Fritsche et al., 2001). The base of the se- note 1]) refl ecting nine to 55 ages per sample. Of tinguishable from those obtained for the same quence consists of nonmarine units of Turonian these, 850 ages, representing 40 samples, come samples using all analyses. age that pass upward into marine formations of from our previous work (Jacobson et al., 2000; Analytical results generated specifi cally for Turonian to Campanian or early Maastrichtian Barth et al., 2003; Grove et al., 2003). We also this study are included in the GSA Data Reposi- age. These units are overlain unconformably by incorporated results from Grove et al. (2008) for tory item (Table DR4 [see footnote 1]). Addi- Upper Paleocene to Eocene formations. This the last accreted elements of the Catalina Schist. tional results can be found in Barth et al. (2003) contrasts with the relatively more continuous The latter data set includes 305 zircon ages from and Grove et al. (2003), or were provided by one Maastrichtian to Paleocene deposition within 21 samples. For the forearc units, we determined of us (M. Grove). the Salinian block (see previous). 2983 detrital zircon ages from 100 sandstones of Units analyzed here from San Miguel Island , Cenomanian to middle Eocene age (Tables DR2 Sandstone Petrology the , and the Simi and DR3 [see footnote 1]). The number of ages Hills exhibit strong sedimentologic affini- per sample ranged from nine to 64. Sixty-eight Detrital framework modes were estimated us- ties with, and probably were located close to, of the forearc samples were collected specifi - ing the Gazzi-Dickinson method (e.g., Ingersoll the Peninsular Ranges prior to Neogene rota- cally for this study (the stratigraphic context of et al., 1984) for 59 of the 68 forearc samples tion (Bartling and Abbott, 1983; Bottjer and these samples is described in the GSA Data Re- collected specifi cally for this study (i.e., the Link, 1984; Link et al., 1984; Alderson, 1988; pository item [see footnote 1]). The remaining samples from the Peninsular Ranges were not Fritsche et al., 2001). The sequences from San forearc samples were collected as part of a more included in this part of the study). All samples Miguel Island, the Santa Monica Mountains, focused study of the Peninsular Ranges by one were stained for potassium feldspar and plagio- and the Simi Hills, however, appear to have of us (M. Grove). clase. Three hundred to 400 grains were counted been deposited somewhat farther offshore than Zircons were extracted from ~1 kg samples per sample depending on grain size and size of coeval units of the Peninsular Ranges (Santa using standard density and magnetic tech- the thin section billet.

490 Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California

RESULTS 10 Mio 20 Pelona-Orocopia-Rand and Metamorphic bio Catalina Schists 30 Olig mus hbd Aside from the fact that we did not previ- Detrital 40 ously consider the Catalina Schist, our results zircon Eoc are little changed from those obtained by Grove 50 erval et al. (2003). We thus emphasize only those int points most relevant for comparison with the 60 Age (Ma) Pal forearc data. Cycling Maast Slip on Nac flt 70 Protolith Age Camp One of the most critical parameters to de- 80 termine for any given schist body is the depo- Sant Con sitional age of the sandstone protolith, which 90 must lie within a time window bounded by the Tur Cen youngest reliable detrital zircon age and oldest 100 reliable metamorphic age (Fig. 3). Grove et al. Age/ SC SE PRRA SS MP SP BR EF OR CH TR CD NR (2003) referred to this time span as the “cycling Epoch Cat-San Emig Rand Pelona Orocopia interval,” because it encompasses erosion in 40 39 ≤ the source area, transport of that material to the Figure 3. Comparison of Ar/ Ar cooling ages and zircon U-Pb ages 100 Ma for the site of deposition, underthrusting beneath the Pelona-Orocopia-Rand-Catalina schists plotted by area (see Figs. 1, 2, and 4 for locations). arc, accretion to the overriding plate, and initial Zircon ages are from Barth et al. (2003), Grove et al. (2003, 2008), and this study. Argon stages of exhumation. As observed by Grove ages are from Jacobson (1990), Jacobson et al. (2002, 2007), Barth et al. (2003), and Grove et al. (2003), the cycling interval of the Pelona- et al. (2003, 2008). The Pelona-Orocopia-Rand schists are organized by the conventional Orocopia-Rand schists becomes younger from northwest-southeast grouping, i.e., Rand Schists in the northwest Mojave Desert and envi- northwest to southeast (Fig. 3). Also notable, rons, Pelona Schists in the central Transverse Ranges, and Orocopia Schists in southeast- the duration of the cycling interval increases ern California–southwestern Arizona. The exception is the Rand Schist of the San Emigdio to the southeast, although this may be at least Mountains, which we group with the Catalina Schist. Within the Pelona and Orocopia partly an artifact in the data. For example, we Schist groups, individual areas are ordered from northwest on the left to southeast on the obtained no hornblende 40Ar/39Ar ages from right. Northwest-southeast position for the Rand Schists is not clear owing to potential ro- the easternmost schist bodies (Fig. 3) and tation of the Sierran tail; these bodies were instead ordered from left to right by deposi- thus may not have captured the oldest cool- tional and metamorphic age. Diagonally ruled box indicates time of most likely movement ing ages in this region. In addition, igneous on the Nacimiento fault. Yellow band indicates the cycling interval, which, as discussed in 40 39 bodies younger than ca. 70 Ma are not com- the text, is not well constrained in the southeast. Also note that Ar/ Ar and zircon ages mon in the inferred provenance areas (Barth overlap for the Catalina Schist (the two types of ages have been separated slightly along et al., 2008a), making it diffi cult to constrain the x-axis to clarify this relation). The low grade of metamorphism for this unit (Grove 40 39 the maximum depositional age of sediments et al., 2008) suggests that the Ar/ Ar ages may be infl uenced by excess radiogenic argon younger than latest Cretaceous. For example, or lack of complete recrystallization of detrital mica during metamorphism. Abbreviations in delineating the cycling interval, we ignored for schist localities are same as in Figure 1. Other abbreviations are: bio—biotite; Camp— three early Cenozoic ages from the Orocopia Campanian; Cen—Cenomanian; Eoc—Eocene; hbd—hornblende; Maast—Maastrichtian; Schist due to their status as outliers (Fig. 3). Mio—Miocene; mus—muscovite; Olig—Oligocene; Pal—Paleocene; Sant—Santonian; However, it should be kept in mind that any or Tur—Turonian. all of these ages could be signifi cant. Despite the uncertainties, the schist protoliths clearly range from Turonian to at least as young as Variation in Detrital Zircon Populations as a taceous zircons, with lesser, but still signifi cant, middle Paleocene, representing a time span of Function of Depositional Age and Location proportions of Proterozoic and Jurassic ages. 30 m.y. or greater. Pie diagrams of detrital zircon ages for indi- In contrast, the central to southeastern schists, For purposes of comparison to the forearc vidual ranges and/or adjacent ranges are plot- which have depositional ages of Maastrichtian units, we divided the schists into four groups ted in Figure 4 using the palinspastic base of and younger, are characterized by progressively based on depositional age and geographic Figure 2. Probability density functions for the decreasing abundances of Early to middle Cre- location (Figs. 3 and 4). This contrasts with four regional groupings are illustrated in Figures taceous detrital zircons and increasing num- a threefold division utilized by Grove et al. 5B–5E. As pointed out by Grove et al. (2003), bers of Proterozoic and latest Cretaceous–early (2003). Our fourth category includes the detrital zircon patterns in the Pelona-Orocopia- Cenozoic ages. As concluded by Grove et al. Catalina Schist, which was not considered by Rand schists vary systematically from north- (2003), the patterns imply a shift from outboard Grove et al. (2003), and the Pelona-Orocopia- west to southeast, and thus with depositional to inboard sources with time (e.g., Fig. 2 and Rand schist of the San Emigdio Mountains, age of the protolith. The northwestern schists, following discussion). which Grove et al. (2003) grouped with the which have protolith ages of Turonian to Cam- Compared to the Pelona-Orocopia-Rand Rand schists. panian, are dominated by Early to middle Cre- schists, the zircon age distribution within

Geological Society of America Bulletin, March/April 2011 491 Jacobson et al.

San Emigdio Gf matching divisions for the schists. The overall Mts. (3/82) Sierra NV similarity between the two sample sets is im- Nevada AZ pressive. By analogy with the schists, the varia- CA tion from oldest to youngest parts of the forearc Mojave Rand Mts. section is considered to refl ect a transition from SGHf Desert SAf (5/166) outboard to inboard source areas. Portal Ridge Orocopia Pie diagrams of the zircon age distributions (1/37) Blue Ridge (2/70) plotted on a geographic base (Fig. 6) confi rm Rf Orocopia Mts. the temporal progression from marginal to Mt. Pinos (10/253) inboard sources and reveal signifi cant along- Sierra de Nf (5/125) Salinas (5/138) Nf Trigo Mts. Neversweat strike variations in provenance (see also Figs. (4/92) Ridge (2/66) EHf DR2–DR4 [see footnote 1]). Material derived Rand from east of the main Sierran–Peninsular SGHf SGf Ranges arc fi rst appeared at the continental 100 km margin in a few localities in the central part of N Pelona f the study area during Campanian–early Maas- ? trichtian time (Fig. 6B; Cambria and Pfeiffer Sierra Pelona Detrital zircon ages (7/199) slabs, Santa Ynez Mountains, Santa Monica Mountains, Simi Hills). With time, detritus 55–70 Ma East Fork 70–85 Ma (4/110) SE Chocolate Castle Dome originating from inboard sources became the 85–100 Ma Mts. (3/79) Mts. (4/77) dominant component in the central part of 100–135 Ma Catalina the area and was also delivered in signifi cant 135–300 Ma N Schist Peninsular (21/305) quantities both to the northwest and southeast >300 Ma Ranges (Figs. 6C and 6D). It is important to keep in mind that Figure 6 does not take into account displacement on the Figure 4. Pie diagrams of detrital zircon ages from the Pelona-Orocopia-Rand-Catalina Nacimiento fault. Because movement appears schists plotted on the same palinspastic base as in Figure 2. The Nacimiento fault is indicated to have been over by the late Paleocene, the by a heavier line weight than the other faults. Present-day outcrops of Pelona- Orocopia- Eocene reconstruction (Fig. 6D) should be in- Rand schists are shown in black. The Catalina Schist is inferred to restore beneath the dependent of this event. However, the preceding northern Peninsular Ranges (Crouch and Suppe, 1993). Mesozoic magmatic rocks and asso- time frames presumably require varying degrees ciated wall rocks of the Sierra Nevada and Peninsular Ranges batholiths are shown by the of correction, depending on the exact nature and plus pattern. Arc and wall rocks of the Mojave Desert and Salinian block are omitted for age of the Nacimiento fault. If this structure is clarity. Dashed blue outlines delineate three of the four schist groupings used for plotting dominantly a thrust (Page, 1970, 1981; Hall zircon results in Figure 5 (see also Fig. 3). The fourth group includes the spatially sepa- 1991; Saleeby, 2003), then displacement should rated Catalina Schist and schist of the San Emigdio Mountains (see text). Paired numbers have been largely normal to the length of the in paren theses indicate number of samples and number of zircon ages. margin. In this case, the along-strike variations evident in Figure 6 would not be signifi cantly altered. On the other hand, for 500–600 km of the Catalina Schist is geographically distinct pository item [see footnote 1]) and correspond sinistral slip (Dickinson, 1983; Dickinson et al., (Fig. 4); i.e., despite its southerly location, the closely in age to the four schist groups, particu- 2005), the Nacimiento block would restore ap- Catalina Schist is dominated by detrital zircons larly with respect to the three older age divi- proximately outboard of the of of Early Cretaceous age and, as noted previously, sions (Fig. 3). However, the youngest forearc central California, without the Salinian block appears most similar to the Pelona-Orocopia- age group (early to middle Eocene) may have intervening. Further implications of the sinistral Rand schist of the San Emigdio Mountains. As an average age somewhat younger than that of case are considered later herein. we will discuss later, the origin of this pattern is the youngest schist group (Orocopia Schist); Figure 6 reveals an impressive similarity in ambiguous owing to the uncertain nature of slip i.e., as discussed already, the minimum age of the Cenomanian-Turonian to Eocene patterns on the Nacimiento fault. the Orocopia Schists is not well defi ned and from San Miguel Island with those of equivalent conceivably could be no younger than middle age from the vicinity of San Diego and northern Forearc Sedimentary Units Paleocene (Fig. 3). Despite this uncertainty, it is Baja California. This is consistent with mod- clear that the schists and forearc units analyzed els involving large-scale rotation of the west- Age Divisions of Forearc Units here represent at least 30 m.y. of overlapping ern Transverse Ranges from an initial position Samples from the forearc units were divided depositional history during a critically impor- alongside the Peninsular Ranges. into four age groups: (1) Cenomanian-Turonian, tant period in the tectonic evolution of southern One striking anomaly in our results pertains (2) Campanian–early Maastrichtian, (3) middle California. to a single sample of Paleocene age from north- Maastrichtian–Paleocene, and (4) early to ern Baja California (Fig. 6C). Work by one of middle Eocene. These subdivisions were guided Variation in Detrital Zircon Populations as a us (M. Grove) suggests that the distinctive char- by structural and stratigraphic breaks in the Function of Depositional Age and Location acter of this sample refl ects a highly restricted sedimentary record (see the descriptions of in- Probability plots for the forearc age groups source region in the vicinity of the Sierra El dividual sampling areas in the GSA Data Re- are illustrated in Figures 5F–5J alongside the Mayor of eastern Baja California.

492 Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California

POR-Catalina Schists Forearc Sedimentary Units 50 100 150 200 250 300 600 900 1200 1500 1800 2100 50 100 150 200 250 300 600 900 1200 1500 1800 2100 100 100 A Kuu F 80 80 PE 60 60 OR Eoc Orocopia (OR) 40 40 Eocene (Eoc) RA Pelona (PE) KP Mid Maast-Pal (KP) Rand (RA) 20 SE 20 Klu Camp-early Maast (Kuu) San Emig.-Catalina (SE) Cen-Tur (Klu) Cumulative prob. 0 0 Orocopia (23/478) B Eocene (28/827) G

Pelona (18/504) C Mid Maast-Pal (30/582) H

Rand (11/341) D Camp-early Maast (32/853) I Relative probability

S. Emigdio-Catalina (24/387) E Cen-Tur (10/333) Vert exag = 2 J Vert exag = 2 for ages>300 Ma for ages>300 Ma

50 100 150 200 250 300 600 900 1200 1500 1800 2100 50 100 150 200 250 300 600 900 1200 1500 1800 2100 Detrital zircon age (Ma) 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 fi ll 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. Abbrevia- tions 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 of lithic grains (Table 1). Volcanic frag- COMPARATIVE PROVENANCE Summary modal data for the 59 sandstones ments comprise less than 30% of total lithics. analyzed petrographically are presented in Plagio clase is more abundant than K-feldspar, Detrital Zircons Table 1. More complete results, including sta- although generally not by much. Total lithic tistical values, are provided in the GSA Data contents and P/F values show a weak ten- Sources of Individual Zircon Populations Repository item (Tables DR5 and DR6 [see dency to decrease with decreasing deposi- The most distinctive fi nding of this study is footnote 1]). The Data Repository item also tional age (Table 1; see also Figs. DR5–DR8 the remarkable parallelism in evolution of de- includes QtFL and QmKP ternary diagrams of and Table DR6 [see footnote 1]). Other than trital zircon suites within the Pelona-Orocopia- the results, along with plots of various compo- this, the samples exhibit no systematic trends Rand-Catalina schists and spatially associated sitional parameters shown in geographic coordi- in composition with time. forearc basin. As already noted, the evolving nates using the same format as Figure 6 (Figs. In contrast to the Campanian to Eocene patterns suggest a transition from outboard to DR5–DR9 [see footnote 1]). samples, three of Cenomanian-Turonian age inboard source areas with time, compatible with Most (56) of the samples are Campanian to from the San Rafael Mountains and Atasca- previous interpretations based on sandstone middle Eocene. This group is rich in quartz dero area are marked by high total lithics, petrol ogy and conglomerate clasts (Gilbert and and feldspar, with low to modest contents Lv/Lt, and P/F. Dickinson, 1970; Lee-Wong and Howell, 1977;

Geological Society of America Bulletin, March/April 2011 493 Jacobson et al.

Gf

N Orocopia Mts. (2/129) Mts. (3/39) Eocene Sierra Madre

SGf (3/65) Early-Mid Mts.-Simi Hills Nf Santa Monica SAf ?

EHf (4/212) San Diego (2/50) Indians Ranch (3/146) Nf Rf Northern Baja SGHf Pine Mt. Block (2/26) Santa Ana Mts. (2/184) Mt. (1/28) Ben Lomond (1/53) Mts. (5/74)

SGHf Santa Ynez S. Miguel Is. Pass Cajon (1/13) (4/66) San Gabriel Block (5/73) Mts-Simi Hills SGf Santa Monica (3/35) ? NW Santa Lucia Range

Nf (2/93) Paleocene Mid Maast- SAf San Diego EHf Rf (1/39) Nf Northern Baja SGHf Mts.(2/82) Santa Ana La Panza Range (4/50) Pt. San Pedro (4/62) (2/29) (2/40) Ranch Indians Pt. Lobos (4/119) (4/119) Tur-Camp Barth et al. Figure 6. Figure Upper McCoy (4/55) (4/85) Atascadero SGf Camp- Santa Monica Mts.-Simi Hills Nf ? (4/169) San Diego Early Maast Rf EHf 300 km SAf (2/158) (3/39) Cyn (1/36) Del Puerto Northern Baja Mts. (3/55)

Rf Nf San Rafael Pigeon Point Mts. (2/56) Santa Ana (2/64) Mts. (2/65) Santa Ynez Slab SGHf (3/64) (3/43) 120 km Cambria S. Miguel Is. Pfeiffer Pfeiffer (2/112) Coalinga Point Sur - DeGraaff- Surpless et al.

ABC D

Nevada

SGf Ranges Sierra Sierra Peninsular Peninsular Mojave Desert (1/26) Nf ? Atascadero Cen-Tur (1/58) Mts. (2/61) Santa Ana San Diego (2/88) SAf Rf Northern Baja Mts. (2/31) San Rafael Nf SGHf 55–70 Ma 70–85 Ma 85–100 Ma 100–135 Ma 135–300 Ma >300 Ma 100 km (2/69)

120 km SGHf (3/166) Coalinga DeGraaff- S. Miguel Is. Surpless et al. Detrital zircon ages

494 Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California

Figure 6. Pie diagrams of detrital zircon ages from the forearc sedimentary units plotted on (Fig. 5). The latter assemblage likely refl ects a the same palinspastic base as in Figures 2 and 4. Present-day outcrops of Pelona-Orocopia- source in southeastern California, southwestern Rand schists (black) and the Sierra Nevada and Peninsular Ranges batholiths (pluses) are Arizona, and northwestern Sonora, where the included for reference. See Figures 1 and 2 for distributions of the forearc units. The four Jurassic arc lies inboard of the Early to middle panels correspond to the four age groups plotted in Figure 5. Note that the left and right Cretaceous arc (Fig. 2; Tosdal et al., 1989; Barth boundaries are positioned in slightly different locations in each panel, the average of which et al., 2008b). is indicated in Figure 2. Individual sampling localities are described in the GSA Data Re- Permian–. Grains of Permian and pository item (see text footnote 1). Results from the Great Valley Group of the Coalinga area Triassic age comprise a small but distinctive (panels A and B) are from DeGraaff-Surpless et al. (2002). Those for the McCoy Mountains component of both the schists and forearc units Formation (panel B) are from Barth et al. (2004). Paired numbers in parentheses indicate with depositional ages of middle Maastrichtian number of samples and number of zircon ages. Note that the total number of analyses rep- and younger (Fig. 5). Triassic plutons are mi- resented in this fi gure is larger than that shown in Figures 5G–5J. This relates to the fact nor in abundance but widely distributed within some analyses were excluded from the plots of Figure 5 in order to avoid overweighting the inboard side of the Cordilleran arc (Barth samples with an exceptionally large number of zircon results (see text). Fault abbreviations et al., 1997; Barth and Wooden, 2006). Permian are as in Figure 1. igneous rocks are less common, but have been reported from the northwestern Mojave Desert and vicinity (Martin and Walker, 1995, and ref- erences therein). This distribution is consistent TABLE 1. AVERAGE DETRITAL MODES OF SELECTED SANDSTONES with the fact that Permian (and Triassic) grains Qt F L M are most common in samples from the north- Sample group n (%) (%) (%) (%) Lv/Lt P/F western part of the study area (Figs. DR3 and This study Early–mid-Eocene 14 36 52 12 3 0.24 0.59 DR4 [see footnote 1]). Mid-Maastrichtian–Paleocene 23 41 47 12 7 0.28 0.56 Pre-Permian. Both the schists and forearc Campanian–early Maastrichtian 19 36 46 18 7 0.23 0.67 units provide evidence for two distinct associa- Cenomanian–Turonian 3 25 3 5 40 2 0.50 0.90 tions of pre-Permian ages. That present within Great Valley Group (Ingersoll, 1983) the San Emigdio–Catalina schists and Ceno- Rumsey (Santonian–Maastrichtian) 34 40 36 24 10 0.60 0.55 Grabast (Cenomanian–Turonian) 17 30 30 41 7 0.37 0.68 manian-Turonian forearc units is characterized Stony Creek (Tithonian–Barremian) 33 25 22 53 2 0.60 0.92 by a comparatively wide spread of ages from Note: n—number of samples; Qt—monocrystalline and polycrystalline quartz relative to total QtFL; F—feldspar Paleo zoic to Paleoproterozoic (Figs. 5E and 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 5J). This distribution is similar to that exhibited relative to total feldspar; 300 to 400 grains were counted per sample (mainly the latter); see the GSA Data by pre-Mesozoic detrital zircons within Lower Repository item (see text footnote 1) for more complete results, including standard deviations and standard errors. Mesozoic sedimentary framework rocks of the western belt of the Peninsular Ranges batho- lith (Fig. 7C; Grove et al., 2008). Such grains Kies and Abbott, 1983; Bachman and Abbott, 2000; McDowell et al., 2001). Grains within the are considered to have a wide range of ultimate 1988; Abbott and Smith, 1989; Seiders and Cox, latter age range form a distinctive component of sources within North America (e.g., Dickinson 1992; Grove, 1993; Dickinson, 1995; Dickinson some of the younger samples of both the schists and Gehrels, 2003, 2009). Also noteworthy, this et al., 2005). Here, we evaluate in more detail and forearc units (Figs. 3–6). assemblage occurs in units marked by a strong the potential source areas for individual zircon Jurassic. Grains of Jurassic age are present in concentration of Early to middle Cretaceous de- age populations (Fig. 5). modest abundances throughout the sample suite trital zircons (Figs. 5E and 5J), which likewise Cretaceous–Early Cenozoic. Early to early (Figs. 4–6). Those in the oldest units occur in point to a source in the western Sierra Nevada Late Cretaceous grains characteristic of the association with abundant Early to middle Cre- (see previous). Hence, the lower Upper Creta- older depositional units are thought to have taceous ages (Figs. 5E and 5J). This same corre- ceous deposits can be explained entirely by ero- a source in the western to central Sierran– lation is also evident within the type Great Valley sion of the western fl ank of the Cordilleran arc, Peninsular Ranges arc (Fig. 2; Silver et al., 1979; Group of central California (Figs. 6A and 6B), including its older wall rocks. Saleeby and Sharp, 1980; Stern et al., 1981; as represented by one sample of Campanian age The second pre-Permian association is char- Chen and Moore, 1982). In contrast, latest Cre- that we collected from Del Puerto Canyon in acterized by a main peak centered at ca. 1.7 Ga taceous grains that typify the younger units were the northern Diablo Range and fi ve of Cenoma- and variably developed peaks at ca. 1.4 and presumably derived from widespread Laramide nian to Campanian age from the Coalinga area 1.2 Ga. These three peaks are broadly equiva- plutons of the Mojave Desert, southwestern of the southern Diablo Range from the work of lent to the Yavapai-Mazatzal, anorogenic, and Arizona , and northwestern Sonora (Fig. 2; DeGraaff-Surpless et al. (2002). This pattern Grenville populations, respectively, identifi ed Barth et al., 1995, 2001a, 2004, 2008a; is consistent with a source in the western fl ank by Dickinson and Gehrels (2009) within Juras- McDowell et al., 2001; Wells and Hoisch, of the central to northern Sierra Nevada, where sic eolianites and related sequences of the Colo- 2008). Laramide intrusive rocks younger than the Cretaceous and Jurassic arcs overlap (Stern rado Plateau region. However, for the relatively ca. 70 Ma have not been reported from southern et al., 1981; Chen and Moore, 1982; Irwin and outboard setting considered here, we assume California, although igneous rocks with ages of Wooden, 2001; DeGraaff-Surpless et al., 2002). that the ca. 1.7 Ga peak includes a substantial ca. 70–50 Ma occur locally in southern Arizona In contrast, Jurassic grains in younger samples contribution from Mojave sources in addition to and northwestern Sonora (Haxel et al., 1984; tend to be associated with high proportions of those from the Yavapai and Mazatzal terranes. Spencer et al., 1995) and are relatively common Proterozoic and latest Cretaceous ages and low The size of the anorogenic peak is consistent in north-central Sonora (González-León et al., abundances of Early to middle Cretaceous ages with the proportion of intrusive rocks of this age

Geological Society of America Bulletin, March/April 2011 495 Jacobson et al.

50 100 150 200 250 300 600 900 1200 1500 1800 2100 2400 2700 3000 100 creasing average age of the Cretaceous–early Cenozoic population and increasing abundance F-POR-C 80 A of Proterozoic grains with decreasing deposi- tional age is somewhat more systematic within 60 the forearc sequence than the schists (Fig. 5). MC This is probably due to the fact that the forearc 40 samples represent a greater geographic range, 20 array of depositional environments, and total PR ERG number of samples and analyses than the Cumulative prob. 0 schists; i.e., the forearc groups likely refl ect a Forearc-POR-Catalina (F-POR-C) (172/4305) B more complete averaging of the margin than the schists.

Sandstone Petrology

Lower Mesozoic wall rocks - C Results from the sandstone modal analysis are consistent with the detrital zircon data but Peninsular Ranges (PR) (4/433) not as diagnostic. For example, the high total lithics, Lv/Lt, and P/F for the Cenomanian- Turonian forearc units (Table 1; see also fi g. 13 Upper McCoy Mountains D in Dickinson, 1995) confi rm a source in the Formation (MC) (4/119) western side of the Sierran–Peninsular Ranges arc (cf. Ingersoll, 1983), as concluded based on the zircon data. For the younger samples, Relative probability however, detrital zircon and sand composi- Jurassic ergs (ERG) (10/890) E tions are not strongly coupled. Specifi cally, the Campanian to Eocene units exhibit a dramatic shift in detrital zircon populations with deposi- tional age refl ecting progressively more inboard source regions (Figs. 5G–5I). In contrast, sand 50 100 150 200 250 300 600 900 1200 1500 1800 2100 2400 2700 3000 compositions for these same units show only Detrital zircon age (Ma) subtle variations with age (Table 1). The detrital zircons thus provide a reliable indication of Figure 7. Detrital zircon age distributions for various sedimentary sequences in Califor- ages of rocks within the source areas, whereas nia and adjacent areas of the southwestern United States. (A) Cumulative probabilities for modal mineralogy refl ects only their lithology. sample groups plotted in B–E. (B) Combined data for the Pelona-Orocopia-Rand-Catalina The latter is apparently relatively uniform both schists and forearc units (this paper). (C) Lower Mesozoic sedimentary framework rocks of spatially and temporally within the eastern side the western belt of the Peninsular Ranges batholith (Morgan et al., 2005; Grove et al., 2008). of the arc and southwestern part of the craton. (D) Upper McCoy Mountains Formation in the McCoy Mountains (Barth et al., 2004). The sandstones analyzed here are broadly (E) Colorado Plateau eolianites (Dickinson and Gehrels, 2003, 2009). See Figure 5 for expla- similar in detrital modes to those from the type nation of scale break at 300 Ma. Paired numbers in parentheses indicate number of samples Great Valley Group of central California east and number of zircon ages. of the San Andreas fault (Table 1; Ingersoll, 1983). Overall, our samples tend to be some- what lower in lithics and P/F than the Great within the Mojave-Yavapai-Mazatzal basement orogen, are common in Neoproterozoic to Cam- Valley rocks, suggesting a more inboard and of the Southwest United States (Karlstrom et al., brian cratonal and miogeoclinal sections of this deeply denuded source area (cf. Dickinson, 1987; Anderson and Bender, 1989; Wooden and region (Gehrels, 2000; Stewart et al., 2001; 1985; Marsaglia and Ingersoll, 1992; Ingersoll Miller, 1990; Barth et al., 2000, 2001a, 2001b; Gehrels et al., 2002; Barth et al., 2009). This im- and Eastmond, 2007). This probably refl ects Anderson and Silver, 2005; Farmer et al., 2005) plies that zircons of Grenville age in the schists the younger average age of our samples and and with predicted zircon “fertility” of the in- and forearc units are recycled. The cratonal and their proximity to the strongly disrupted seg- ferred source rocks (Dickinson, 2008). In con- miogeoclinal sequences also include substantial ment of the arc at the latitude of the Mojave trast, whereas zircons of Grenville age are not populations of anorogenic and Mojave-Yavapai- Desert (see also Dickinson, 1995). abundant in either the schists or forearc units, Mazatzal ages; consequently, some of these they nonetheless appear to be overrepresented grains may be recycled, as well. Comparison to Other Depositional Systems compared to the known distribution of Grenville rocks in the southwestern United States and Contrasts between the Schists and Upper McCoy Mountains Formation northwestern Sonora (Barth et al., 1995, 2001b; Forearc Units The McCoy Mountains Formation of south- Anderson and Silver, 2005; Farmer et al., 2005). Despite the striking similarities between the eastern California and southwestern Arizona On the other hand, detrital zircons of Grenville schists and forearc units, there are some differ- (Fig. 1) is a weakly metamorphosed Upper Ju- age, apparently derived from the Appalachian ences. For example, the overall pattern of de- rassic to Upper Cretaceous fl uvial sequence with

496 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 zona during the Paleogene (Young and McKee, related to the presence of an aseismic ridge or and Coney, 1985; Tosdal and Stone, 1994; Barth 1978; Peirce et al., 1979; Potochnik and Faulds, oceanic plateau (Livaccari et al., 1981; Hen- et al., 2004; Spencer et al., 2005). The Upper 1998; Spencer et al., 2008). derson et al., 1984; Barth and Schneiderman, Cretaceous part of the formation was deposited 1996; Saleeby, 2003). This geometry would on the foreland side of the Sierran–Peninsular TECTONIC INTERPRETATIONS have favored subduction erosion of the overrid- Ranges arc (Barth et al., 2004) and provides an ing North American plate (cf. von Huene and important reference section compared to the co- The Pelona-Orocopia-Rand-Catalina schists Scholl, 1991; Clift and Vannucchi, 2004; Scholl eval forearc units analyzed in this study. and Nacimiento fault represent fi rst-order tec- and von Huene, 2007), inboard migration of Detrital zircon ages (Barth et al., 2004) for tonic elements in the Late Cretaceous–early the axis of arc magmatism, and underplating four samples of upper McCoy Mountains For- Cenozoic evolution of the Southwest United of Pelona-Orocopia-Rand-Catalina schist, al- mation with depositional ages younger than States. While it has long been suspected that these though continued accretion within the outboard Turonian but older than middle Campanian are two features are in some way tied together, the Franciscan wedge is not precluded. Figure 8B plotted in Figures 6B and 7D (based on these exact nature of this relationship has been diffi cult depicts the inferred geometry for the early phase depositional ages, the McCoy data could also to discern (Page, 1982; Hall, 1991; Barth and of this event, prior to inception of Nacimiento have been included in Fig. 6A, but were omit- Schneiderman, 1996; Barth et al., 2003; Saleeby, slip at ca. 75–68 Ma. This panel is relevant to the ted for reasons of space). Results for the upper 2003). One point that is now evident, however, emplacement of the Catalina and San Emigdio McCoy samples contrast sharply with those is that underthrusting of the Pelona-Orocopia- Schists and most, if not all, of the Rand Schists from contemporaneous parts of both the fore- Rand-Catalina schists began signifi cantly earlier (Fig. 3). Note that the western part of the arc arc basin and Pelona-Orocopia-Rand-Catalina than movement on the Nacimiento fault (>90 Ma would still have been in place at this time, consis- schists. In particular, the McCoy Mountains versus <75 Ma, respectively). Furthermore, slip tent with detrital zircon results for the Catalina– Formation is dominated by Proterozoic detritus on the Nacimiento fault was probably over by San Emigdio Schists (Fig. 5E) and zircon and (Figs. 6B and 7D), whereas coeval forearc units the late Paleocene, whereas emplacement of sandstone petrologic data for Cenomanian- and Pelona-Orocopia-Rand-Catalina schists the schists may have extended into the Eocene Turonian forearc units (Fig. 5J; Table 1). For the were derived overwhelmingly from Cretaceous (Fig. 3). Because of this timing relationship, we most part, the Campanian–early Maastrichtian sources (Figs. 5D, 5E, 5I, 5J, 6A, and 6B). This focus the following discussion around the evolu- forearc units (Fig. 5I) suggest a similar paleo- confi rms that the arc served as a topographic tion of the schists, and treat the Nacimiento fault geography. The mode of the Cretaceous detrital barrier between forearc and retroarc basins at and truncation of arc and forearc as one element zircon age peak in the latter group is somewhat this time. The forearc basin was apparently sup- within that broader context. younger than for the Cenomanian-Turonian units plied with detritus derived largely from the west (100 Ma versus 110 Ma, respectively). However, side of the arc. In contrast, the retroarc basin re- Preferred Model for Underplating of the this could be the result of eastward retreat of the ceived sediment from both the east fl ank of the Pelona-Orocopia-Rand-Catalina Schists arc front, as described by Linn et al. (1992) for arc and nearby cratonal areas to the north and central California, without necessarily requir- east of the basin that were experiencing fore- Our preferred model for the origin of the ing any major structural reorganization of the land shortening and uplift (see also Harding and Pelona-Orocopia-Rand-Catalina schists is based forearc basin and arc. On the other hand, some Coney , 1985; Barth et al., 2004). on the common, although not universal, view Campanian–early Maastrichtian sedimentary that these units represent a subduction complex sequences (Cambria and Pfeiffer slabs, Santa Colorado Plateau Sequences (see references cited in the Introduction). This Ynez Mountains, Santa Monica Mountains, Recent studies of Permian and Jurassic eolia- interpretation is compatible with the turbidite Simi Hills; Fig. 6B; Fig. DR10 [see footnote 1]) nites from the Colorado Plateau (Dickinson sandstone-basalt-chert protoliths and the rela- and the coeval Rand Schists (Fig. 5D; Fig. DR10 and Gehrels, 2003, 2009) provide important tively high-P metamorphism. It is also sup- [see footnote 1]) include a fraction of sediment constraints on the eastern limit of the source ported by the extended time frame (>30 m.y.) of that appears to have been derived from sources area for the Pelona-Orocopia-Rand-Catalina emplacement of the schists combined with the east of the main axis of the Sierran–Peninsular schists and forearc units. The Colorado Plateau short cycling interval (Fig. 3). In fact, we fi nd Ranges arc. It is not clear whether this denotes sequences exhibit some peaks in common with it diffi cult to imagine a tectonic setting other local erosional breaching of the arc edifi ce prior the units analyzed here (e.g., in the range of than an Andean-style subduction zone that could to the initiation of slip on the Nacimiento fault or ca. 1.8–1.0 Ga; Figs. 5, 7B, and 7E). However, produce this pattern of ongoing deposition fol- the earliest stages of fault movement. the relative sizes of those peaks differ greatly be- lowed almost immediately by underplating and A second transition in the nature of the mar- tween the two groups. The Colorado Plateau se- progressive exhumation. gin coincided with the removal of the western quences also include substantial proportions of Prior to initiation of the Pelona-Orocopia- belt of the arc and associated inner forearc ba- late Neoproterozoic and early to middle Paleo- Rand-Catalina event in the early Late Creta- sin beginning in late Campanian or early Maas- zoic ages. Grains of the latter ages are notably ceous, subduction geometry was presumably trichtian time. Considering the uncertain nature rare in the schists and forearc units, particularly similar to that commonly invoked to explain the of slip on the Nacimiento fault, we present two in the youngest parts, which are those derived Franciscan–Great Valley–Sierra Nevada triad of alternatives for this time period, one involving from the most easterly source areas. This in- central California east of the San Andreas fault thrusting (Fig. 8C), and the other strike slip dicates the persistence of a topographic divide (Fig. 8A; for details of the Late Jurassic–Early (Fig. 8D). The thrust option (equivalent to the between coastal and interior regions, even fol- Cretaceous behavior of the California con- Sur thrust of Hall, 1991) has been supported lowing the removal of the western belt of the arc vergent margin, see Dumitru et al., 2010). By by numerous workers (Page, 1970, 1981; Yin, and inner forearc basin. This is consistent with ca. 95–90 Ma, however, the Farallon plate had 2002; Barth et al., 2003; Saleeby, 2003; Kidder longstanding inferences of a topographic high, apparently transitioned, at least in southern Cali- and Ducea, 2006; Ducea et al., 2009) but in our the Mogollon Highlands, in southwestern Ari- fornia, to a shallow mode of subduction, perhaps opinion is at odds with the fact that movement

Geological Society of America Bulletin, March/April 2011 497 Jacobson et al.

on the Nacimiento fault must have occurred A ca. 100 Ma Future Upper McCoy Colorado during emplacement of the Pelona-Orocopia- Franciscan Forearc basin Mountains Fm. Plateau Rand schists (Fig. 3). The missing parts of the arc and forearc basin represent a substantial Pre-Cret. crust volume of material. Whereas much of this mass could have been completely subducted, it seems Top of Farallon Continental reasonable to expect that at least some fraction lithosphere would have been interleaved with the schists 100 km slab (Fig. 8C). However, as pointed out by Dickin- son et al. (2005, p. 14), the Pelona- Orocopia- Rand schists bear little resemblance to the omitted units. The schists consist predomi- B ca. 80 Ma Upper McCoy Future nantly of coherent meta-sandstone with small Mountains Fm. Franciscan Forearc crust: Jur. and Colorado amounts of meta-basalt, meta-chert, marble, E. Cret? Plateau and ultramafi c rock. In contrast, the inferred missing units include parts of the Franciscan Pre-Cret. crust mélange, Coast Range ophiolite, uppermost Ju- rassic through Cretaceous Great Valley Group, Top of Farallon slab Continental and western belt of the Sierran arc, including lithosphere its pre–Late Jurassic metamorphic framework Rand/Catalina Schist rocks. The only component of this assemblage that reasonably matches the Pelona-Orocopia- Rand-Catalina schists is the Upper Cretaceous C ca. 50 Ma—thrust option Mogollon part of the Great Valley Group. However, it Future Upper McCoy Highlands is not clear how this part of the missing sec- Forearc basin Mountains Fm. Colorado Rand/Catalina Franciscan tion could have been incorporated within the Schist NF Plateau schists while completely excluding the remain- Pre-Cret. crust der. Mafi c rocks of the Coast Range ophiolite bear superfi cial compositional resemblance to Peraluminous Top meta basites within the schists, but they differ in of crustal melts detail. Specifi cally, the Coast Range ophiolite Farallon Pelona Schist exhibits an arc signature (Shervais and Kim- Orocopia Schist brough, 1985; Giaramita et al., 1998; Sher- slab vais et al., 2005), whereas mafi c rocks in the D ca. 50 Ma—strike-slip option Pelona-Orocopia-Rand-Catalina schists were Mogollon derived overwhelmingly from ocean-fl oor ba- Future Upper McCoy Highlands salts (Haxel et al., 1987, 2002; Dawson and Forearc basin Mountains Fm. Colorado Rand/Catalina Franciscan Plateau Jacobson, 1989; Moran, 1993). Consequently, Schist NF we conclude that the Pelona-Orocopia-Rand- Pre-Cret. crust Catalina schists can be explained entirely as subducted trench materials and off-scraped Peraluminous fragments of oceanic crust from the Farallon Top of crustal melts Farallon plate (Figs. 8B and 8D) without any admixture Pelona Schist of materials excised from along the Nacimiento Orocopia Schist fault. Nonetheless, we cannot rule out the possi- slab SW NE bility that fragments of the missing terranes are hidden within unexposed parts of the Pelona- Figure 8. Tectonic model for underplating of the Pelona-Orocopia-Rand-Catalina schists Orocopia-Rand-Catalina schists (Fig. 8C). and development of the Nacimiento fault. (A) Geometry prior to the onset of fl at subduction In contrast to thrust models, strike-slip models and emplacement of the Pelona-Orocopia-Rand-Catalina schists. (B) Early phase of fl at for the Nacimiento fault imply that the excised subduction preceding initiation of slip on the Nacimiento fault. (C–D) Relations following parts of the arc and forearc basin remained at the cessation of slip along the Nacimiento fault (NF), assuming thrusting and sinistral strike surface but were translated out of the plane of slip, respectively. For simplicity, slip on the Nacimiento fault is shown in both C and D as margin-normal cross sections at the latitude of postdating emplacement of the Pelona Schist but predating emplacement of the Orocopia the Mojave Desert (Fig. 8D). Omission of units Schist. In actuality, movement on the Nacimiento fault is likely to have overlapped with the by strike-slip motion along the Nacimiento fault underthrusting of either or both of those schist groups. Inset in C shows an enlargement of can be reconciled with either dextral or sinistral our interpretation of relations adjacent to the Nacimiento fault system in the case of thrust- movement, as long as the fault cuts in the ap- ing. The North American craton is held fi xed in all panels. Fill patterns for the igneous rocks propriate sense across the strike of the margin. correspond to the color scheme of Figure 2 with the addition that yellow represents ages of Nonetheless, for reasons discussed by Dickin- ca. 70–50 Ma. son et al. (2005), we favor the sinistral option.

498 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 Yakutat block may serve as a modern analog (Lawton, 2008). Alternatively, it might be an in- transported southward and would be repre- (Eberhart-Phillips et al., 2006; Fuis et al., 2008). dication of tectonic erosion even during normal sented by the Peninsular Ranges batholith, the Irrespective of whether the Nacimiento fault subduction (cf. von Huene and Scholl, 1991; Cretaceous forearc sequence along its western operated in a thrust or strike-slip sense, move- Clift and Vannucchi, 2004; Scholl and von margin, and the more distal, rotated units of the ment on this structure appears to correlate Huene, 2007). Additional evidence of modest Channel Islands and western Transverse Ranges broadly in time with the provenance shift ex- pre-Laramide subduction erosion may be in- (see following). hibited by the Pelona-Orocopia-Rand-Catalina dicated by the 2.7 km/m.y. eastward migration The sinistral slip model for the Nacimiento schists and forearc units. This is not surprising of arc magmatism within the Sierra Nevada be- fault avoids some of the issues of the thrust considering that excision of the western arc and tween 120 Ma and 90 Ma (Stern et al., 1981; model, but it may be problematic in other ways. inner forearc basin resulted in narrowing of the Chen and Moore, 1982). Furthermore, Dumitru Correction for dextral slip on the San Andreas margin by 150 km or more, thus decreasing et al. (2010) argued for subduction erosion, or fault (310–320 km; Matthews, 1976) and Rinco- the transport distance from the former retroarc at least nonaccretion, in northern California and nada fault (45 km; Graham, 1978) and sinistral (cratonal) region to the coastline (Figs. 8C and central California east of the Salinian block prior slip on the Nacimiento fault (500–600 km; Dick- 8D). This process can account for the infl ux to ca. 123 Ma. Hence, shallow subduction dur- inson et al., 2005) would restore the main body of cratonal detritus to the continental margin ing Laramide time may simply have enhanced of the Nacimiento block ~150–250 km to the at the latitude of the Mojave Desert beginning existing erosive processes. A corollary is that north of its present location and place it directly in the latest Cretaceous (Fig. 6). To the north rocks like the Pelona-Orocopia-Rand-Catalina against the Franciscan Complex and Great Val- and south of the Mojave, material from in- schists could be relatively commonplace be- ley Group east of the San Andreas fault (see fol- board sources comprises a smaller fraction of neath the outboard edges of Andean-style arcs. lowing and Dickinson, 1983). This is consistent the total sediment volume and fi rst appeared Possible analogs of the Pelona-Orocopia-Rand- with the work of Gilbert and Dickinson (1970), during the latest Paleocene to early Eocene Catalina schists in this sense might be the Con- who demonstrated a parallelism in temporal (Figs. 6C and 6D). Regional drainages in the drey Mountain Schist of the Klamath Mountains evolution of petrofacies between forearc units latter areas probably developed across the full (Brown and Blake, 1987), the Swakane Gneiss of the Nacimiento block and the Great Valley width of the Sierran–Peninsular Ranges batho- of the North Cascades, Washington (Matzel Group east of the Salinian block. On the other lith following arc extinction associated with the et al., 2004), and the Qiangtang metamorphic hand, Long and Wakabayashi (2009) argued Laramide event (Kies and Abbott, 1983; Abbott belt of northern Tibet (Kapp et al., 2000). that accretionary rocks of these two regions do and Smith, 1989). not make a good match. In particular, distinc- Telescoping of the arc and forearc basin along Alternative Models for Underplating of the tive sequences within the Franciscan Complex the Nacimiento fault helps explain the location Pelona-Orocopia-Rand-Catalina Schists east of the San Andreas fault, such as the Marin of the southeastern (Orocopia) schists within the Headlands and Permanente terranes and Skaggs cratonal province of southwestern Arizona, i.e., Forearc Model Springs Schist (Wakabayashi, 1999), have not inboard of the primary axis of Cretaceous mag- In the previous section, we interpreted the been recognized within the Nacimiento block matism. Without the removal of the western belt Pelona-Orocopia-Rand-Catalina schists as a (Wakabayashi, 2010, personal commun.). Con- of the arc and inner forearc basin, emplacement subduction complex formed along the con- sidering the extreme spatial variability charac- of schist this far from the trench requires an ex- tact between the Farallon and North American teristic of accretionary wedges, these relations treme degree of fl at subduction. In fact, Haxel plates. Alternatively, Barth and Schneiderman do not, in our view, preclude the sinistral slip et al. (2002, p. 124) used this line of reasoning (1996, their fi g. 5) proposed that the schists were model. Nonetheless, they do suggest the need to argue against correlating the Orocopia Schists derived from the Great Valley forearc basin by for further investigations. with the Franciscan Complex and Catalina thrusting beneath the Sierran arc along a fault A second potential problem with the sinistral Schist. However, because the Orocopia Schists sympathetic to the subduction boundary, yet slip model relates to longstanding views that postdate much or even all of the truncation of contained entirely within the North American relative motion between the Farallon and North the forearc region (Fig. 3), the required transport plate. As argued already herein, however, we American plates was approximately head-on to distance is not exceptional (Figs. 8C and 8D). In consider the rock types of the Pelona-Orocopia- dextral during the Late Cretaceous–Paleocene particular, all fl at-slab confi gurations illustrated Rand schists to be incompatible with derivation (Engebretson et al., 1985; Stock and Molnar, in Figure 8 fall well within the range of geom- from the forearc basin. Additional diffi culties 1988). Dickinson et al. (2005, p. 15), however, etries inferred from the modern record (e.g., posed by the forearc model are discussed by suggested that this interpretation may need to be fi g. 5 in Gutscher et al., 2000). Haxel et al. (2002) and Grove et al. (2003). revised in light of recent advances in the under- The subduction model for the Pelona- standing of hotspot reference frames. In addi- Orocopia-Rand schists is typically framed in Backarc Model tion, we propose that sinistral slip may have the context of the Laramide orogeny. However, This category includes several variants, all been an expression of “escape” or “extrusion” it is worth noting that emplacement of the old- of which involve suturing of an intraoceanic tectonics (cf. Tapponnier et al., 1982; Burke est Pelona-Orocopia-Rand-Catalina schists at continental fragment to North America (Haxel and Şengör, 1986) related to subduction of the ca. 95–90 Ma signifi cantly predates the ca. 80 Ma and Dillon, 1978; Ehlig, 1981; Vedder et al., aseismic ridge or plateau previously called upon extinction of the main Sierran arc and ca. 75 Ma 1983; Haxel et al., 2002). One major obstacle to explain the anomalous nature of the southern onset of the primary pulse of Laramide tectonism for such models is the lack of compelling evi- California margin (Livaccari et al., 1981; Hen- in the Rocky Mountain foreland (Dickinson dence for a suture zone (Burchfi el and Davis, derson et al., 1984; Barth and Schneiderman, et al., 1988; DeCelles, 2004). This could 1981; Crowell, 1981; but see counterargument 1996; Saleeby, 2003). Westward translation of refl ect the time lag between impingement of by Haxel et al., 2002). In addition, backarc the Wrangell terrane of southeast Alaska along the fl at slab at the continental margin and its models imply that the sandstone protoliths of the Denali fault due to subduction of the oceanic propagation to a position beneath the foreland the Pelona-Orocopia-Rand schists and coeval

Geological Society of America Bulletin, March/April 2011 499 Jacobson et al. forearc sequences would have been deposited would migrate in the same direction, leading to on opposite sides (east and west, respectively) Rand progressively younger packets of accreted mate- 70 50 Ma of the intraoceanic terrane. However, it seems Catalina (NW Ma rial away from the trench. We refer to the pre- highly unlikely that the detrital zircon age pat- 90 Pelona-Orocopia dicted geometry as a “tectonic onlap” based on terns of the schists and forearc units would be Ma its resemblance to an upside-down sedimentary so strongly correlated over >30 m.y. (Fig. 5) if v onlap (clearest in Fig. 8D). s. their protoliths had been deposited in isolated SE Continental options) basins. The McCoy data, in particular, imply margin Migrating Aseismic Ridge that a backarc basin would be highly enriched in As indicated already, Barth and Schneider- cratonal detritus compared to the forearc region A man (1996) proposed that the Pelona-Orocopia- (Figs. 6B and 7D). Consequently, we interpret Rand schists were derived by thrusting of the the zircon results to indicate that the forearc arc over the forearc basin. They attributed this Ma 90 units and schists represent the proximal (forearc Rand event to subduction of an aseismic ridge (cf. basin) and distal (trench) facies, respectively, Livaccari et al., 1981; Henderson et al., 1984) Catalina of a single depositional system on the outboard with its long dimension oriented slightly more Pelo side of the Sierran–Peninsular Ranges arc. Be- na-Orocopia northerly than the convergence vector between ( 70 Ma tween the previously identifi ed weaknesses NW the Farallon and North American plates. In this vs.

(e.g., Burchfi el and Davis, 1981; Crowell, 1981) SE case, the intersection point between ridge and and zircon data, we see little justifi cation for re- Continental options) trench would have migrated southward with margin taining the backarc models. 50 Ma time, presumably causing the thrust between arc 100 km and forearc basin to propagate in the same direc- Northwest-Southeast Decreasing Age of the B tion. As discussed previously, we consider that Pelona-Orocopia-Rand Schists the schists were derived from trench materials Figure 9. End-member possibilities for age rather than from the underthrust forearc basin. One of the most distinctive features of the variation within a sheet of Pelona-Orocopia- Nonetheless, the geometric argument of ridge Pelona-Orocopia-Rand schists is the >30 m.y. Rand schist underplated beneath North migration seems applicable to either situation. decrease in depositional and emplacement ages American crust. Northwest and southeast A similar interpretation was also proposed by from northwest to southeast (Fig. 3). Because of locations of the Catalina Schist refl ect the Dickinson et al. (1988, p. 1036) to explain north the relatively linear distribution of the schist out- cases of 500–600 and 0 km of sinistral slip to south migration of Laramide deformation in crops, it is not clear whether this age variation on the Nacimiento fault, respectively. The the foreland region. relates more to distance inboard from the conti- fi gure is highly schematic and does not take nental edge (Fig. 9A) or to position along strike into account the narrowing of the margin Sinistral Slip on the Nacimiento Fault of the margin (Fig. 9B). Consideration of the that would have resulted from displacement The model of a migrating ridge-trench in- Catalina Schist provides some additional con- on the Nacimiento fault, whether by thrust- tersection seems particularly effective in ex- straints but does not fully solve the problem. As- ing or strike slip. plaining the northwest-southeast decrease suming no sinistral slip on Nacimiento fault, the in age of the schists, assuming sinistral slip Catalina Schist would initially have been located on the Nacimiento fault. This is illustrated in outboard of the Pelona and Orocopia Schists older schists to be underplated at increasingly Figure 10, where panel A represents the paleo- (southeast option in Figs. 9A and 9B). In view of greater distances inboard (Fig. 8). In this case, geography prior to fault slip. At this time, the the ca. 95–90 Ma age of the Catalina Schist, this age contours would parallel the strike of the northwestern schist localities would have been reconstruction is consistent with the isochrons of margin (Fig. 9A), and the young age of the situated relatively close to the trench, requir- Figure 9A, but not those of Figure 9B. In other southeastern schists would be a consequence ing only modest fl at subduction to emplace words, the southeast option for the Catalina of their position relatively far inboard. We en- trench materials beneath the former arc (Figs. Schist requires at least some component of de- vision this process involving transfer, due to 8B and 10A). At the same time, the future sites creasing age from outboard to inboard within the buoyancy, of some fraction of trench materi- of the Pelona and Orocopia Schists would have schist terrane. Alternatively, for the case of 500– als from the subducting (Farallon) to overrid- resided far inboard, in a region of arc magma- 600 km of sinistral strike slip on the Nacimiento ing (North American) plate above the hinge at tism (Fig. 10A; Mattinson, 1990; Barth et al., fault, the Catalina Schist would restore to the which the subducting plate descended toward 1995, 1997, 2001a, 2008a; Kidder et al., 2003; northwest end of the Pelona-Orocopia-Rand the mantle. With time, such subcreted materi- Barbeau et al., 2005). With commencement schist belt. This location is consistent with either als could become quite thick, leading to tectonic of Nacimiento slip, the latter regions would decreasing age outboard-inboard (Fig. 9A) or and/or erosional denudation of the overlying have been drawn toward the continental mar- northwest-southeast (Fig. 9B). These uncertain- plate (cf. Platt, 1986; Yin, 2002). In this fashion, gin (Fig. 9B) in the same way that slip on a ties require that we consider several mechanisms underplated materials would be driven upward, low-angle normal fault will translate rocks of to explain the observed variation of age within consistent with thermochronologic evidence the lower plate closer to the surface. As a con- the schist terrane. for exhumation of the Pelona-Orocopia-Rand sequence, progressively more southeastward schists concurrent with subduction (Jacobson (inboard) regions would have been transferred Progressive Subduction Erosion et al., 2002, 2007). This would allow permanent into the zone of underplating near the conti- Grove et al. (2003) proposed that buzz-saw incorporation of some trench materials within nental edge. In this scenario, the schists need removal of the base of North America could the overriding plate. As the fl at slab continued not defi ne more than a relatively narrow band allow younger schists to “leap-frog” beneath to erode inboard, the locus of underplating (becoming younger to the southeast) beneath

500 Geological Society of America Bulletin, March/April 2011 Late Cretaceous–early Cenozoic tectonic evolution of southern California

Diablo workers have interpreted this “borderland-style” A ca. 75 Ma blk Nacimiento B ca. 60 Ma range geometry as direct evidence that strike-slip faulting, whether in a dextral or sinistral sense, Farallon–North p l a t e played an important role in emplacement of the America Salinian block (Nilsen and Clarke, 1975; How-

Nevada Nevada ell and Vedder, 1978; Vedder et al., 1983; Dick- inson et al., 2005). However, whereas strike-slip

F a Nf faulting can generate local topographic highs Nf 100 km r a l l o n p F a r a l l o n Sierra Sierra and lows, it is not clear that this process, by CS Gf Gf itself, can account for the substantial net sub- N sidence implied by the regional transgression

Salinian blk Sal of the Salinian block. Saleeby (2003, Fig. 4C) Mojave NV l a t e Mojave NV Nacimiento bl

inia attributed this event to extension within the up-

SAf block SA block Peninsular

n blk f per plate of the Vincent–Chocolate Mountains CA CA AZ AZ thrust system. Alternatively, or in addition, we Aseismic observe that forearc subsidence is a common k feature in regions of inferred subduction erosion ridge CA Ranges MEX as an isostatic response to removal of material Nf from the base of the overriding plate (Clift and Nf Vannucchi, 2004). By analogy, we propose that marine transgression of the Salinian block may POR-Catalina Schists CS have been related, at least in part, to tectonic Prot. basement w/55–85 Ma plutons erosion of North American mantle lithosphere Aseismic Peninsular Late Cret. batholith (85–100 Ma) and lowermost crust during underplating of the Early Cret. batholith (100–135 Ma) ridge Pelona-Orocopia-Rand-Catalina schists. This interpretation implies that the age of the Foothills belt CA transgression in any given area should correlate N Range X Forearc basin ME with the time of local schist underplating. At 100 km Accretionary complex s least to a fi rst approximation, this appears to be the case for the Salinian block and central Trans- Figure 10. Origin of Nacimiento fault by sinistral strike slip (modifi ed from Dickinson verse Ranges. The correlation does not hold true [1983] using more recent estimates for age of faulting). Faulting is assumed to have been in the San Emigdio Mountains, where the schist driven by subduction of an aseismic ridge. Ridge scale is based on analogy to the Yakutat is early Late Cretaceous in age, but the base of block (Fuis et al., 2008) and ridges on the Cocos and Nazca plates (Gutscher et al., 2000). the marine section in the upper plate is Eocene Present-day outcrops of Pelona-Orocopia-Rand-Catalina schists are shown in black. Only (Nilsen, 1987b). However, because this region the northwesternmost schist bodies would have been in place at the time represented by is at the northwesternmost end of the schist belt, panel A. Outlined area labeled “Nacimiento Blk” is based on present-day outcrop of Fran- it is conceivable that subsidence was limited by ciscan Complex between the Nacimiento and San Gregorio–Hosgri faults southeast of Point the fl exural rigidity of a full thickness of litho- Sur (Fig. 1). Panel A indicates inferred location of this body prior to sinistral slip on the sphere beneath the adjacent Sierra Nevada. 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 CONCLUSIONS spatial relationship (Fig. 1). Note that the reconstruction of panel B is a simplifi ed equiva- lent of that in Figure 2. Heavy arrow shows Farallon–North America relative plate motion. Similar detrital zircon patterns exhibited by CS—Catalina Schist; Nf—future (panel A) and actual (panel B) traces of Nacimiento fault; the Pelona-Orocopia-Rand-Catalina schists and Gf—future trace of Garlock fault; SAf—future trace of San Andreas fault; other abbrevia- coeval sedimentary units of the forearc basin tions are as in Figure 1. provide evidence that these two sequences com- prise parts of a single depositional system on the outboard side of the Sierran–Peninsular Ranges the outer edge of the continent, in contrast to posite the Diablo Range to the current position arc. The >30 m.y. time frame for emplacement the sheet-like geometry typically envisioned adjacent to the Salinian block (Fig. 10; see pre- of the schists and the short cycling interval are (e.g., Fig. 9). This could explain the relatively vious discussion). most consistent with a subduction origin. The linear nature of the schist exposures, which sandstone protoliths of the schists are best in- seems fortuitous if the distribution at depth is Marine Sequences of the Salinian Block terpreted as trench sediments complementary to more laterally persistent. In the sinistral slip the forearc basin sequence. model, extraregional detritus is supplied to the The Maastrichtian to Paleogene marine trans- Emplacement of the schists directly beneath coast via a gradually enlarging “window” be- gression of the Salinian block and adjacent areas the Cordilleran magmatic arc requires the re- tween the southern Sierra Nevada and northern has long been viewed as an important tectonic moval of North American mantle lithosphere Peninsular Ranges (Fig. 10B). Sinistral slip on signal. Clast distributions in conglomerates indi- and lowermost crust. The initial stages of this the Nacimiento fault would also translate the cate a number of localized basins, at least during underplating may have occurred at the end of a Nacimiento block from an initial location op- early phases of deposition (Grove, 1993). Some period of modest subduction erosion associated

Geological Society of America Bulletin, March/April 2011 501 Jacobson et al.

with slow, eastward migration of Sierran mag- Johnston, Alex Pullen, and Victor Valencia provided continental margin: Tectonics, v. 16, p. 290–304, doi: matism during the middle Cretaceous. Later critical assistance with use of the inductively coupled 10.1029/96TC03596. Barth, A.P., Wooden, J.L., Coleman, D.S., and Fanning, C.M., schist emplacement, however, may have been plasma mass spectrometer (ICP-MS) at the University of Arizona. We are deeply indebted to many work- 2000, Geochronology of the Proterozoic basement of expedited by fl at subduction associated with the southwesternmost North America, and the origin and ers who assisted our sampling efforts by suggesting evolution of the Mojave crustal province: Tectonics, classic Laramide orogeny of the Rocky Moun- localities, providing maps, and/or accompanying us v. 19, p. 616–629, doi: 10.1029/1999TC001145. tain foreland. in the fi eld. These include Pat Abbott, Bill Bartling, Barth, A.P., Jacobson, C.E., Coleman, D.S., and Wooden, Initial underplating of the schist (ca. 95– Alan Chapman, Mark Cloos, Ivan Colburn, Brett Cox, J.L., 2001a, Construction and tectonic evolution of Gene Fritsche, Steve Graham, Karen Grove, Clarence Cordilleran continental crust: Examples from the San 75 Ma) was probably not associated with any Hall, Gordon Haxel, Dave Howell, Ray Ingersoll, Gabriel and San Bernardino Mountains, in Dunne, G., major changes in the paleogeography of the Dave Kimbrough, Marilyn Kooser, and Jason Saleeby. and Cooper, J., compilers, Geologic Excursions in the arc and forearc basin. Sediment supplied to Our understanding of the problems discussed herein California Desert and Adjacent Transverse Ranges: Los Angeles, Pacifi c Section, SEPM (Society of Eco- the forearc basin and trench during this stage has also benefi ted from discussions with Kim Bishop, nomic Paleontologists and Mineralogists), Book 88, was derived by erosion of the western fl ank of Bill Dickinson, Mihai Ducea, and Steve Kidder. Ray 17–53. Ingersoll provided helpful comments on an early draft Barth, A.P., Wooden, J.L., and Coleman, D.S., 2001b, the Sierran–Peninsular Ranges arc and associ- of the manuscript. We also thank Luke Beranek, Bill SHRIMP-RG U-Pb zircon geochronology of Meso- ated wall rocks. The arc formed a continuous Dickinson, Cees van Staal, and John Wakabayashi for proterozoic metamorphism and plutonism in the south- topographic barrier that separated the forearc their important insights during the Geological Society westernmost United States: The Journal of Geology, of America review process. v. 109, p. 319–327, doi: 10.1086/319975. region from an area of retroarc sedimentation Barth, A.P., Wooden, J.L., Grove, M.G., Jacobson, C.E., that included the McCoy Mountains Formation. and Pedrick, J.N., 2003, U-Pb zircon geochronology REFERENCES CITED Beginning as early as ca. 75 Ma, but no later of rocks in the region of California: A reevaluation of the crustal structure and origin of the than ca. 68 Ma, slip on the Nacimiento fault led Abbott, P.L., and Smith, T.E., 1989, Sonora, Mexico, source Salinian block: Geology, v. 31, p. 517–520, doi: 10.1130/ to progressive removal of the western belt of the for the Eocene Poway Conglomerate of southern 0091-7613(2003)031<0517:UZGORI>2.0.CO;2. 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