The Sierra Nevada batholith is an ∼600-km-long, NNW-trending composite arc assemblage consisting of a myriad of plutons exhibiting a distinct transverse zonation in structural, petrologic, geochronologic, and isotopic patterns. This zonation is most clearly expressed by a west-to-east variation from mafic to felsic plutonic assemblages. South of 35.5°N, the depth of exposure increases markedly, and fragments of shallow-level eastern Sierra Nevada batholith affinity rocks overlie deeper-level western zone rocks and subjacent subduction accretion assemblages along a major Late Cretaceous detachment system. The magnitude of displacement along this detachment system is assessed here by palinspastic reconstruction of vertical piercing points provided by batholithic and metamorphic pendant structure and stratigraphy. Integration of new and published U-Pb zircon geochronologic, thermobarometric, (U-Th)/He thermochronometric, and geochemical data from plutonic and metamorphic framework assemblages in the southern Sierra Nevada batholith reveal seven potential correlations between dispersed crustal fragments and the Sierra Nevada batholith autochthon. Each correlation suggests at least 50 km of south- to southwest-directed transport and tectonic excision of ∼5–10 km of crust along the Late Cretaceous detachment system. The timing and pattern of regional dispersion of crustal fragments in the southern Sierra Nevada batholith is most consistent with Late Cretaceous collapse above the underplated accretionary complex. We infer, from data presented herein (1) a high degree of coupling between the shallow and deep crust during extension, and (2) that the development of modern landscape in southern California was greatly preconditioned by Late Cretaceous tectonics.
In zones of convergence, regional gradients in gravitational potential energy can be relaxed through lateral spreading and vertical thinning of orogenic crust (e.g., Dewey, 1988; Rey et al., 2001). Deep-crustal exposures of the southern Sierra Nevada batholith, subjacent subduction accretion assemblages, and flanking shallow-level assemblages of the Mojave Desert and Salinian block represent a multi-tiered regional core complex that formed in response to crustal thickening and subsequent extensional collapse (Glazner et al., 1989; Malin et al., 1995; Wood and Saleeby, 1997; Kidder et al., 2003; Saleeby, 2003; Chapman et al., 2010, 2011). The collapse phase is marked by the structural ascent of presently exposed high-pressure (7–11 kbar) rocks of the southern Sierra Nevada batholith and vicinity (Ague and Brimhall, 1988; Pickett and Saleeby, 1993; Fletcher et al., 2002; Grove et al., 2003; Kidder et al., 2003; Saleeby et al., 2007; Nadin and Saleeby, 2008; Chapman et al., 2010, 2011) and the dispersal of lower pressure (2–4 kbar), southeastern Sierra Nevada batholith affinity assemblages across the entire width of the batholith (May 1989; Malin et al., 1995; Wood and Saleeby, 1997; Saleeby, 2003; Chapman et al., 2010).
Dokka and Ross (1995) speculate that extension in the southern Sierra-Salinia core complex is Neogene in age. Glazner et al. (1996) refute this model, arguing that late Cenozoic structures and structural relief in the core complex resulted instead from crustal shortening. We contribute to this debate by showing that major extension and basement rotations in the southern Sierra-Salinia core complex are Late Cretaceous in age. Furthermore, this effort focuses on specific and elusive issues pertaining to Late Cretaceous disruption and dispersion of the southern Sierra Nevada batholith and vicinity, including: (1) the magnitude and timing of tectonic transport of upper crustal fragments along a regional detachment system; (2) the degree of strain coupling between differentially exhuming levels of the crust; (3) the importance of strike-slip faulting during collapse; and (4) the extent to which the development of the southern Sierra Nevada landscape was influenced by Late Cretaceous tectonism.
Major plutonic units and metamorphic pendants of the southern Sierra Nevada batholith are fairly well characterized in terms of geologic, U-Pb zircon, δ18O zircon, 87Sr/86Sr (Sri), thermobarometric, paleontologic, and bulk compositional data (Kistler and Peterman, 1973, 1978; Chen and Moore, 1979, 1982; Saleeby et al., 1987, 2007, 2008; Ross, 1989; Kistler and Ross, 1990; Pickett and Saleeby, 1993, 1994; Saleeby and Busby, 1993; Lackey et al., 2005, 2008; Nadin and Saleeby, 2008; Memeti et al., 2010). These data sets reveal a regional primary zonation pattern to the batholith and distinct longitudinal belts of Neoproterozoic to lower Mesozoic metamorphic pendant sequences. In aggregate the pendants and plutons of the southern Sierra Nevada batholith are characterized well enough to recognize allochthonous equivalents.
In this study, we present new: (1) field and structural relations to provide the basis for recognizing potential correlations; (2) U-Pb geochronology of plutonic and detrital zircon populations to identify the age and provenance of displaced plutonic and metamorphic assemblages; (3) thermobarometric work to assess pressure differentials between native and displaced assemblages; (4) major and trace element geochemistry and Sr and Nd isotopic ratios to determine the sources of allochthonous metavolcanic rocks; and (5) zircon (U-Th)/He thermochronometry to constrain the timing of extension. These results are integrated with previous geologic studies and thermobarometric, geochronologic, and geochemical databases in order to recognize allochthon-autochthon pairs and to estimate the magnitude of transport that they imply.
“Autochthonous” Sierra Nevada Batholith
The Sierra Nevada block is a NNW-trending composite batholith with juvenile batholithic crust extending to ≥35 km depth (Ruppert et al., 1998; Saleeby et al., 2003, 2007). Gradients in pluton ages, integrated bulk compositions, amounts of recycled crustal components, and the paleogeographic affinities of metamorphic pendants define a distinct west to east zonation (Figs. 1 and 2). These zones may be used to recognize and reconstruct superimposed tectonic disruptions.
For convenience, we refer to the pre-Late Cretaceous architecture of the Sierra Nevada batholith and adjacent southern California batholith of the Mojave Desert and Salinia as autochthonous, although a number of the pre-batholithic elements were deformed prior to and during emplacement of the Sierra Nevada batholith (Kistler, 1990; Dunne and Suczek, 1991; Saleeby and Busby, 1993; Stevens and Stone, 2005; Nadin and Saleeby, 2008; Saleeby, 2011; Fig. 2). The following is a summary of key compositional, geochemical, geochronologic, and structural relations between “autochthonous” longitudinal zones of the Sierra Nevada batholith and its framework.
Cretaceous Batholithic Belts
Across-strike variations in composition, pluton emplacement ages, and geochemistry delineate four distinct zones to the Sierra Nevada batholith (Nadin and Saleeby, 2008 and references therein). From west to east, we define the following zones: (1) the ∼140–115 Ma dioritic zone, a collection of mafic assemblages dominated by quartz diorite, gabbro, and tonalite, with limited outcrop along the westernmost Sierra Nevada and extensive subcrop beneath the San Joaquin Basin (May and Hewitt, 1948; Williams and Curtis, 1976; Ross, 1989; Saleeby et al., 2009a, 2011); (2) the ∼115–100 Ma tonalitic zone with domains of tonalite and lesser amounts of quartz diorite and gabbro; (3) the ∼105–90 Ma granodioritic zone rich in mainly tonalite and granodiorite; and (4) ∼90–80 Ma granitic zone granodioritic and granitic rocks (Figs. 1 and 3). Initial 87Sr/86Sr (Sri) increases eastward across the Cretaceous zones from ∼0.703 to ∼0.709. The boundary between tonalitic and granodioritic zones is generally defined by the Sri = 0.706 isopleth (Nadin and Saleeby, 2008).
Pre-Cretaceous Plutons and Metamorphic Framework
Cretaceous plutons of the Sierra Nevada batholith intruded a framework of Neoproterozoic to Mesozoic continental shelf, slope, and rise strata, a belt of accreted abyssal lithosphere, and Triassic to Jurassic plutons, now all exposed as metamorphic pendants (Saleeby et al., 2008; Figs. 1–3). Covariation of pendant stratigraphy, age, and provenance track with plutonic zonation patterns discussed above (Figs. 2 and 3). West to east longitudinal zones include: (1) the Paleozoic Foothills ophiolite belt with overlying Permian to Triassic (?) Calaveras complex hemipelagic deposits, and unconformable infolds of suprasubduction mafic volcanic rocks and siliciclastic turbidites (Saleeby, 2011; Fig. 2). (2) A belt, spanning western to eastern Sierra Nevada batholith zones, of Cambrian to Ordovician eugeoclinal (i.e., deep marine sediments of the outer continental margin) quartzite, phyllite, and chert named the Sierra City mélange and Shoo Fly Complex (e.g., Harding et al., 2000) in the north and the remnants of similar strata preserved in pendants of the Kernville terrane (e.g., Saleeby and Busby, 1993) in the south. (3) Neoproterozoic to Cambrian inner shelf facies of miogeoclinal (i.e., shallow marine sedimentary rocks of the inner continental margin) strata of the Death Valley and Mojave Desert regions, and correlative exposures in the Snow Lake terrane, an allochthonous slice that may have been shuffled ∼400 km northward along the cryptic Mesozoic Mojave–Snow Lake fault (Lahren and Schweickert, 1989; Memeti et al., 2010). (4) Neoproterozoic to Cambrian outer shelf miogeoclinal strata of the Inyo facies (Nelson, 1962; Grasse et al., 2001). (5) Cambrian to Devonian eugeoclinal deposits of chert, siliceous argillite, limestone, shale, metaserpentinites, and volcanics belonging to the El Paso terrane (Miller and Sutter, 1982; Carr et al., 1984; Walker, 1988; Martin and Walker, 1995; Gehrels et al., 2000b). The Roberts Mountains allochthon, a similar package of eugeoclinal rocks in Nevada, differs from the El Paso terrane in that the Roberts Mountains allochthon has been thrust over belts (3) and (4) during the Antler orogeny (e.g., Stevens and Greene, 1999; Gehrels et al., 2000a).
Lower Mesozoic sequences of quartzite, carbonate, psammite, pelite, and generally westward-younging Triassic to Cretaceous metavolcanic sequences unconformably overlie Paleozoic strata from western to eastern zones of the Sierra Nevada batholith (Saleeby, 1978, 1987; Walker, 1988; Saleeby and Busby, 1993; Manuszak et al., 2000; Memeti et al., 2010).
Tectonostratigraphy of the Southern Sierra Nevada Region
South of 35.5° N, the depth of exposure increases markedly and the transverse structure of the “autochthonous” Sierra Nevada batholith is disrupted by two major Late Cretaceous systems of parallel low-angle normal faults that juxtapose: (1) subduction accretion assemblages and deep-level exposures of the Sierra Nevada batholith (the Rand fault and Salinas shear zone; Postlethwaite and Jacobson, 1987; Nourse, 1989; Kidder and Ducea, 2006; Chapman et al., 2010, 2011) and (2) eastern and western Sierra Nevada batholith assemblages (the southern Sierra detachment; Wood and Saleeby, 1997; Fig. 1). The resulting generalized tectonostratigraphy of the southern Sierra Nevada batholith, from top to bottom, is that of a multi-tiered core complex consisting of three packages: allochthonous shallow-level mafic rocks and eastern Sierra Nevada batholith affinity rocks, (par)autochthonous deep-level western Sierra Nevada batholith assemblages, and subduction accretion assemblages. Characteristic features of exposed tectonostratigraphic units and bounding structures in the southern Sierra Nevada batholith and southern California batholith are discussed below.
Mafic Complexes and Cover Strata
The western San Emigdio mafic complex consists of hornblende-hornfels–facies basaltic sheeted dikes and pillows intruded by a suite of mid- to Late Jurassic layered to isotropic gabbroids and lesser amounts of ultramafic cumulates, which in turn are intruded by tonalitic assemblages (Hammond, 1958; James, 1986a; Reitz, 1986; Ross, 1989; Chapman et al., 2010). Correlative rocks of Logan Quarry and Gold Hill continue from the southwesternmost Sierra Nevada batholith into adjacent Salinia (Ross, 1970; Ross et al., 1973; James, 1986a; James et al., 1993).
The contact between the western San Emigdio mafic complex and western Sierra Nevada batholith assemblages is concealed beneath a veneer of Tertiary sedimentary rocks that form the southern margin of the San Joaquin basin. Subcrop mapping indicates that the basement surface is a detachment fault, informally named here as the Maricopa detachment (Fig. 1), consisting of northeast-southwest striated mylonitic and cataclastic assemblages derived from the western Sierra Nevada batholith and the western San Emigdio mafic complex (Saleeby et al., 2009a). The Uvas Member of the Tejon Formation (e.g., Critelli and Nilsen, 2000) unconformably overlies the western San Emigdio mafic complex and contains meter-scale subrounded boulders of western San Emigdio mafic complex material (Fig. 4). The proximity of the Uvas Member to the Maricopa detachment and the presence of large boulders within the unit are consistent with inferred deposition in a supradetachment basin (Wood and Saleeby, 1997).
Eastern Sierra Nevada Batholith–Affinity Shallow-Level Granitoids and Metamorphic Framework Rocks
A first-order feature of the southern Sierra Nevada batholith and vicinity is a tectonostratigraphy topped by crustal fragments of shallow-level granitoids and amphibolite to hornblende hornfels–facies metamorphic pendant rocks (Nourse, 1989; Wood and Saleeby, 1997). These crustal fragments lie in the hanging wall of a regional detachment system consisting of “fault II” of Nourse (1989), the Blackburn Canyon, Jawbone Canyon, and Pastoria faults (Wood and Saleeby, 1997), and the cryptic westward continuation of the Pastoria fault into the Gabilan range (e.g., Kistler and Champion, 2001; Fig. 1A and B). These faults are referred to in aggregate as the southern Sierra detachment system. Here we review key temporal, kinematic, and structural relations of the southern Sierra detachment system.
The southern Sierra detachment system is a complex ductile to brittle low-angle structure that has been differentially remobilized, truncated, and folded by Transverse Ranges contractile deformation. Locally preserved ductile fabrics along eastern elements of the southern Sierra detachment system indicate a top to the south or southeast transport direction (Nourse, 1989; Wood and Saleeby, 1997). Kinematic analysis of the western southern Sierra detachment system is prohibited because original ductile fabrics are severely overprinted in the brittle regime.
Upper and lower plate juxtapositions across the southern Sierra detachment system are profound. These consist of: 2–4 kbar versus 7–11 kbar for pluton emplacement pressures, 87–105 Ma versus 102–138 Ma for pluton emplacement ages, and ∼0.708 versus ∼0.705 for Sri (Saleeby et al., 1987; Kistler and Ross, 1990; Pickett and Saleeby, 1994; Wood and Saleeby, 1997).
Deep-Level Exposures of the Western Sierra Nevada Batholith
Deep-crustal exposures of the Cretaceous batholithic belt lie in the footwall of the southern Sierra detachment system and in the hanging wall of the Rand fault–Salinas shear zone. These rocks decrease in thickness and abundance westward from the southern Sierra Nevada batholith into Salinia, where only small remnants are preserved as basement inliers (Kistler and Champion, 2001; Kidder et al., 2003) and conglomerate clasts (Ross et al., 1973; Ross, 1988; James et al. 1993; Schott and Johnson, 1998, 2001; Fig. 1).
Unroofing of deep-level western Sierra Nevada batholith rocks is clearly linked to the transport of upper crustal fragments in the hanging wall of the southern Sierra detachment system. Mated thermochronometric and barometric data for the southern Sierra Nevada batholith, northern Salinia, and the northwestern Mojave Desert show an abrupt Late Cretaceous decompression event coincident with rapid cooling in the deep-level batholithic upper plate (Saleeby et al., 2007; Chapman et al., 2010 and references therein).
Subduction Accretion Assemblages
The Rand, San Emigdio, and Sierra de Salinas schists (referred to in aggregate as “the schist”) and similar early Tertiary Pelona and Orocopiaschists of more southerly California crop out along detachment structures beneath older crystalline rocks of the southwest Cordilleran batholithic belt (Graham and England, 1976; Haxel and Dillon, 1978; Ehlig, 1981; Jacobson, 1983, 1995; Jacobson et al., 1988, 2007, 2011; Simpson, 1990; Kidder and Ducea, 2006; Ducea et al., 2009; Chapman et al., 2010, 2011). Most workers agree that the deposition and emplacement of the schist occurred during an episode of shallow subduction related to the Laramide orogeny (Jacobson et al., 2007 and references therein).
The deposition, subduction, and structural ascent of the schist are temporally and spatially associated in plate reconstructions with the subduction of a large igneous province (LIP; Saleeby, 2003; Liu et al., 2008, 2010). Subduction of a LIP beneath the southernmost Sierra Nevada batholith and adjacent southern California batholith is hypothesized to have driven slab flattening, leading to the tectonic removal of subbatholithic mantle lithosphere, the cessation of arc magmatism, abrupt crustal thickening in the overriding batholithic plate (Malin et al., 1995; Ducea and Saleeby, 1998; House et al., 2001; Saleeby, 2003; Nadin and Saleeby, 2008) and decompression of batholithic assemblages from deep- to midcrustal levels (Saleeby et al., 2007; Chapman et al., 2010). The resultant high-elevation mountain belt segment shed detritus as the schist protolith into the trench, which was immediately underthrust beneath the recently extinguished arc (Kidder and Ducea, 2006).
Late Cretaceous Tectonic Shuffling
Here we review temporal and kinematic relations between the principal members of the integrated proto–Kern Canyon–Kern Canyon–White Wolf fault system (Nadin and Saleeby, 2008) and the Owens Valley shear system (Bartley et al., 2007), because they are important for both the contractile and extensional phases of Late Cretaceous regional deformation in the southern Sierra Nevada batholith. Beginning at ca. 95 Ma, the proto–Kern Canyon fault functioned as a west-directed deep crustal reverse fault with southward increasing throw of 10–25 km and a northward increasing component of dextral shear (Nadin and Saleeby, 2008).
At ca. 88 Ma, the Kern Canyon fault branched from the proto–Kern Canyon fault to the southwest and merged with the proto–White Wolf fault (abbreviated in aggregate as KWF below). Displacement along the KWF increases southward from zero at 36.7° N to 40 km of dextral slip and 15 km of dip slip near the Tejon embayment (Nadin and Saleeby, 2008).
Bartley et al. (2007) show that ∼65 km of dextral slip along the Owens Valley shear system occurred between Late Cretaceous and Paleogene time. Coeval shear along the Owens Valley shear system, KWF, and southern Sierra detachment system suggests that these structures may have been cogenetic, with the Owens Valley shear system and KWF representing transfer faults flanking the southern Sierra-Salinia core complex.
Analytical techniques for U-Pb geochronology, (U-Th)/He thermochronometry, thermobarometry, and geochemistry are available in the Supplemental File1. Also included in the Supplemental File are complete data sets, sample petrography, representative zircon cathodoluminescence (CL) images and notes, details of the multidimensional scaling algorithm used, a discussion of correlative granitoids, and U-Pb zircon, Sri, and igneous and metamorphic pressure databases.
U-Pb Zircon Geochronology
This geochronologic investigation focuses on batholithic suites adjacent to the southern Sierra detachment system and Maricopa detachment: hanging-wall granitoids of the Pastoria and southern Tehachapi plates, footwall deep-level plutonic assemblages of the San Emigdio Mountains (Tehachapi–San Emigdio complex of Chapman et al., 2010, 2011; Chapman and Saleeby, 2012), and the White Ridge tonalite of the western San Emigdio mafic complex.
Deep-level rocks from the San Emigdio Mountains include: (1) the Antimony Peak tonalite, dated at 131 Ma in reconnaissance U-Pb zircon studies by James (1986a, 1986b); (2) the San Emigdio tonalite, a more felsic phase of the Antimony Peak body; (3) the San Emigdio gneiss (Chapman et al., 2011), an undated hornblende quartz diorite orthogneiss that crops out north of the San Emigdio schist and likely correlates with similar rocks of the 99–105 Ma intrusive suite of Bear Valley (Saleeby et al., 1987; Pickett and Saleeby, 1993; Saleeby et al., 2007); (4) the Digier Canyon diorite gneiss, the western continuation of the White Oak diorite gneiss (Saleeby et al., 2007), a tectonic mixture of amphibolite to locally retrograde greenschist-facies dioritic and subordinate tonalitic, gabbroic, and mylonitic gneisses and cataclasites at the base of the Tehachapi–San Emigdio complex; and (5) the western continuation of the Grapevine Canyon paragneiss (Pickett and Saleeby, 1993, 1994).
Hanging-wall granitoids include: (1) the Lebec granodiorite of Crowell (1952), which hosts the Salt Creek pendant; (2) the Claraville granodiorite of the Blackburn plate, which yields U-Pb zircon ages of 91 ± 1 Ma ∼50 km north of our sample site (Saleeby et al., 1987, 2008); (3) the granite of Brush Mountain, assigned a 98 Ma U-Pb zircon age by James (1986b); (4) the granodiorite of Gato Montes, which yields a Rb-Sr isochron age of 96.3 ± 8.7 Ma (Kistler and Ross, 1990); and (5) a hornblende granodiorite in contact with the Tylerhorse Canyon pendant, informally named here the granodiorite of Gamble Spring Canyon. Sample locations and interpreted U-Pb ages of plutonic as well as metavolcanic rocks are given in Table 1 and shown in Plate 1. U-Pb results are shown in Figure 5.
Results from our geochronologic work are combined with all known U-Pb zircon ages from the southern Sierra Nevada and Salinia to produce a color contour map (Plate 1), showing regional variations in pluton emplacement age, using the spatial analyst extension of Arcmap 9 and employing “barriers” such as the proto–Kern Canyon fault, KWF, and other faults shown on Figure 1. Sri (Plate 2) and pluton emplacement and metamorphic equilibration pressure (Plate 3) compilation maps were also produced in an identical manner.
Metamorphic Pendant Rocks
A single sample of dacitic metatuff from the Bean Canyon pendant (07TM10) yields a total of 24 concordant analyses with an interpreted age of 273.0 ± 2.4 Ma (Fig. 5). A concordia age of ca. 102.6 ± 1 Ma was determined using two multigrain zircon fractions from a metamorphosed pumice lapilli silicic tuff (sample 91TH140) from the Oak Creek Pass complex. Thirty-five concordant zircon grains were analyzed from a felsic segregation within the basal amphibolite of the Bean Canyon pendant (11BC1) and give a weighted mean age of 487.4 ± 3.2 Ma.
Samples of metamorphosed siliciclastic rock from the Salt Creek (08SE258), Tylerhorse (08TC44), and Bean Canyon (07BC60) pendants yielded a total of 286 concordant grains suitable for provenance analysis. A normalized probability plot comparing detrital zircon age spectra of samples analyzed here with strata of Death Valley, the Snow Lake terrane, the Salinian block, the Kernville terrane, the El Paso terrane, the Roberts Mountains allochthon, the Sierra City mélange, and the Shoo Fly complex is shown in Figure 6. Sample 08SE258 has major age peaks at ca. 1100 Ma, 1400 Ma, and 1700 Ma. Sample 07BC60 has a major age peak at ca. 1800 Ma, with scattered ages between 1000–1700 Ma and 2300–2900 Ma. Sample 08TC44 contains scattered ages between ca. 200–600 Ma, 1000–1800 Ma, and 2000–2900 Ma.
Figure 7 compares the age spectra of samples from this study with those of a suite of samples from the terranes listed above using multidimensional scaling (MDS). Multidimensional scaling offers an advantage over traditionally used Kolmogorov-Smirnov (K-S) statistical tests in that MDS can be used to produce visual representations of statistical distances between arrays of age data. In other words, MDS calculates a matrix of statistical distances between samples of detrital zircon ages and plots the samples on a map such that the samples that contain similar age spectra are placed close to each other on the map. Multidimensional scaling mapping of data from the El Paso terrane–Roberts Mountains allochthon, Death Valley–Snow Lake terrane, and northern and southern Sierra produces clusters (Fig. 7) that can be exploited to test provenance hypotheses for unknown samples.
Broken bivalve remains were discovered by undergraduate assistant S. Peek in a hornfelsic calcareous sandstone from the Tylerhorse Canyon pendant (Fig. 8). These remains strongly resemble Early Jurassic bivalves of the genus Weyla that were recovered from two locations in the same stratigraphic horizon of the Isabella pendant (Saleeby and Busby, 1993; identified by J.W. Durham and D.L. Jones). In each pendant, bivalve fragments are located in a centimeter- to meter-scale layered quartzite unit characterized by thin, calc-silicate and psammitic interbeds that sit stratigraphically above a thick highly recrystallized gray marble. The Isabella pendant fossils are from ∼10 and ∼50 m above the marble, and the Tylerhorse Canyon fossils are from ∼40 m above the marble. These relations suggest stratigraphic equivalence of marble-layered quartzite units in Isabella and Tylershorse Canyon pendants.
(U-Th)/He zircon ages were determined along a traverse across the footwall of the southern Sierra detachment system from a subset of samples studied by apatite He (see Mahéo et al., 2009 for details and sample locations). Equal elevation and vertical transect sampling traverses are included in the data (Table 2).
Figure 9A shows apatite He ages plotted against sample elevation and zircon He ages adjusted to a “pseudo-elevation” calculated by: (1) taking the difference in apatite He and zircon He ages, (2) multiplying this difference by the Cenozoic regional erosion rate (0.05 ± 0.01 mm/yr; Clark et al., 2005; Cecil et al., 2006; Mahéo et al., 2009) to calculate the amount of erosion that took place between apatite He and zircon He closure, and (3) adding the eroded amount to the sample elevation for zircon He determinations. The age-(pseudo)elevation relationships shown in Figure 9A indicate that the footwall of the southern Sierra detachment system cooled through the ∼180 °C zircon He closure temperature (Reiners, 2005) rapidly at 77 ± 5 Ma.
K/Ar and Ar/Ar ages on biotite and hornblende scattered across the autochthon are in the 89 to 75 Ma range (Kistler and Peterman, 1978; Dixon, 1995). These ages, when coupled with zircon U-Pb igneous and He cooling ages, further indicate cooling from solidus conditions to He zircon closure at ∼100 °C/m.y. scale (Fig. 9B).
The rationale behind our thermobarometric work is that correlative native and displaced sites should show pressure differentials that reflect the relative upper and lower plate positions. To investigate igneous emplacement pressures as well as peak metamorphic conditions within pendant rocks, a suite of samples was collected from the southern Tehachapi Mountains, San Emigdio Mountains, and the Santa Lucia Range. Pressure-temperature conditions for metapelitic pendant rock were calculated from garnet + biotite + plagioclase + quartz ± sillimanite/andalusite ± cordierite ± K-feldspar (Table 3) assemblages interpreted to have equilibrated during peak metamorphism. Calculated temperatures and pressures for metapelitic pendant rock range from ∼550 to 700 °C and 2.5 to 4 kbar.
Host plutons containing the equilibrium assemblage hornblende + plagioclase + K-feldspar + quartz + sphene + iron titanium oxide were collected for aluminum-in-hornblende (Al-in-hbl) igneous barometry (Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1989; Schmidt, 1992; Anderson and Smith, 1995; Ague, 1997). We report hornblende-plagioclase temperatures as well as Al-in-hbl pressures for Hammarstrom and Zen (1986), Hollister et al. (1987), Johnson and Rutherford (1989), Schmidt (1992), and Anderson and Smith (1995) calibrations in Table 4. Thermobarometric results are appended to a recent compilation by Nadin and Saleeby (2008) to produce a color contour map (Plate 3), showing regional variations in igneous and metamorphic pressure for the southern Sierra Nevada batholith and Salinia. We refer below to the Schmidt (1992) determinations to remain consistent with the Nadin and Saleeby (2008) compilation.
Western San Emigdio Mafic Complex
Major and trace element geochemistry and Sr and Nd isotopic ratios were determined from allochthonous metavolcanic rocks to evaluate the most likely native sites of these rocks. Major and trace element data indicate that sheeted dikes and pillow basalts from the western San Emigdio mafic complex are of normal mid-ocean ridge basalt (N-MORB) affinity (Sun and McDonough, 1989), with SiO2 of ∼49 wt%. Trace element data for these rocks are normalized to N-MORB on Figure 10A and shown with the compositional trend of enriched MORB (E-MORB) as well as the range of data from the Paleozoic Kings-Kaweah ophiolite of the Foothills belt (Saleeby, 2011) abundances underlain for comparison. Compositional trends of western San Emigdio mafic complex basalts lie between N-MORB and E-MORB with positive compositional spikes in Cs, Ba, U, K, Pb, and Sr.
The relationship between 143Nd/144Nd and 87Sr/86Sr supports a MORB origin for the western San Emigdio mafic complex basalts. Initial ɛNd and 87Sr/86Sr (Sri) values, age corrected for 160 Ma (the intrusion age of tonalite and gabbro of the western San Emigdio mafic complex) and 484 Ma (the age of basalt-gabbro blocks of the Kings-Kaweah ophiolite belt), range from: ɛNd160 = +9.3 to +13.7, ɛNd484 = +11.3 to +15.9, 87Sr/86Sr160 = 0.7033 to 0.7035, and 87Sr/86Sr484 = 0.7032 to 0.7033 (Supplemental Table 11 in the Supplemental File [see footnote 1]), indicating that western San Emigdio mafic complex basalts were derived from a source with long-term large ion lithophile element depletion, like that of convecting mantle beneath the Pacific Ocean basin (Hofmann, 2003). This is consistent with the trace element patterns of Figure 10A, and with primitive mantle normalized La/Sm values of 0.67–0.72 at high initial ɛNd, which also plot within the field of modern Pacific MORB (Hofmann, 2003) and reflect long-term large-ion lithophile element depletion of the mantle source.
Bean Canyon Metavolcanic Rocks
Approximately 150 m of silicic metavolcanic rock crop out near the top of the Bean Canyon pendant section. Geochemical analysis of Bean Canyon metavolcanic rocks indicates that they are medium- to high-K dacite with ∼66 wt% SiO2. Figure 10B is an N-MORB–normalized spidergram comparing the trace element patterns of Bean Canyon pendant metatuffs and dacitic to rhyolitic metatuffs of the Kennedy pendant (Dunne and Suczek, 1991). Compositional trends of metatuff suites overlap, with prominent troughs in Nb, P, and Ti and a peak in K. An additional peak in Pb is detected in Bean Canyon metavolcanic rocks. Subduction-related volcanic rocks commonly exhibit similar Nb and Pb spikes, as well as spikes in mobile elements such as K (e.g., Hofmann, 2003). AFM and Ti versus Zr diagrams (Pearce et al., 1981) support the view that Bean Canyon and Kennedy pendant metavolcanic rocks resulted from suprasubduction zone magmatism, with data plotting within calc-alkaline (Fig. 11A) and arc volcanic (Fig. 11B) fields.
Palinspastic Restoration of Vertical Piercing Points
Integrated field mapping, petrography, geochronology, and geochemistry reveal seven distinctive allochthonous assemblages that may correlate with similar autochthonous rocks of the southern Sierra Nevada and vicinity. Wood and Saleeby (1997) argued on the basis of geologic evidence for an additional nine correlations, two of which are supported by this study (correlations 2 and 3, described below) and two are modified slightly (correlations 1 and 5). All correlations are summarized in Table 5 and shown on Figure 12A.
Correlation #1: Area between Lake Isabella and Kern Plateau Pendants (A) and Bean Canyon and Tylerhorse Canyon Pendants (A′)
The Bean Canyon pendant is characterized by a succession of marble, quartzite, calc-silicate hornfels, pelitic schist, and silicic metatuff, unconformably underlain by amphibolite and associated ultramafic rock (Rindosh, 1977; Ross, 1989; Wood and Saleeby, 1997; and this study). We report an Early Ordovician (487.4 ± 3.4 Ma) U-Pb zircon age from the basal amphibolite unit. An overlapping Sm-Nd isochron age of 484 ± 18 Ma from the Kings River ophiolite (Saleeby, 2011) suggests a possible relationship, currently under investigation, between mafic rocks at the base of the Bean Canyon section and the Kings River ophiolite. The correlation of metamorphosed mafic to ultramafic bodies in Bean Canyon and Salinia with similar rocks at the apparent base of the Kennedy pendant section in the El Paso terrane is discussed below in “correlation #5.”
Detrital zircon geochronologic data reported herein indicate that quartzite (sample 07BC60) from the Bean Canyon pendant contains an age distribution similar to that of eugeoclinal deposits of the El Paso terrane (J. Saleeby, 2011, personal commun.) and Roberts Mountains allochthon (Gehrels et al., 2000a) and miogeoclinal rocks of the Snow Lake pendant (Figs. 6 and 7). A U-Pb zircon age of 273.0 ± 2.4 Ma for dacitic metatuff from near the top of the Bean Canyon section overlaps in age with andesitic flows and breccias from the El Paso Mountains (Walker, 1988; Martin and Walker, 1995) and indicates that the Bean Canyon pendant is entirely Paleozoic. Trace element abundances of undated silicic metavolcanic rocks from the Kern Plateau resemble those of the Bean Canyon pendant (Fig. 10B). Age, geochemical, petrographic, and stratigraphic relations lead us to suggest that volcanic and metavolcanic rocks exposed in the El Paso Mountains, and Bean and Kern Plateau pendants are correlative.
The Tylerhorse Canyon pendant lies stratigraphically above the Bean Canyon pendant and consists of pelitic and psammitic schist, calc-silicates, and marble. The U-Pb detrital zircon age spectrum from metapsammitic sample 08TC44 partially overlaps that of sample 07BC60 from the Bean Canyon pendant for grains older than ca. 1400 Ma (Fig. 6), but abundant late Paleozoic to early Mesozoic grains indicate that strata of the Tylerhorse Canyon pendant were deposited into Jurassic time. The Tylerhorse Canyon section is, therefore, interpreted as a Mesozoic overlap sequence deposited across Bean Canyon pendant protoliths.
A similar depositional setting is envisaged for the Isabella pendant, based on the presence of Paleozoic amphibolite overlain by Triassic to Jurassic metavolcanic and metasedimentary rocks. Fossils that resemble the Early Jurassic pectinid bivalve Weyla (Fig. 8) corroborate a Mesozoic depositional age for Tylerhorse Canyon strata and permit correlation to equivalent strata found in the Lake Isabella pendant.
Two igneous suites of distinct age and composition intrude the Bean Canyon–Tylerhorse Canyon package. First is a series of east-striking tonalitic, gabbroic, and granodioritic dikes and lenticular masses distributed throughout and along the margins of the pendants. The granodiorite of Gamble Spring Canyon represents one phase of the suite, and with a U-Pb zircon emplacement age of 146.8 ± 0.4 Ma, is similar in age to the quartz diorite of Long Valley (J. Saleeby, 2011, personal commun.) and to the Late Jurassic Independence dike swarm of the eastern Sierra Nevada–Owens Valley–Mojave Desert region (Chen and Moore, 1979; Carl and Glazner, 2002; Glazner et al., 2002; Bartley et al., 2007; Hopson et al., 2008; Fig. 1). The granodiorite of Gato Montes and granite of Bean Canyon, both Late Cretaceous in age (Evernden and Kistler, 1970; Kistler and Ross, 1990, and this study), comprise a second plutonic suite that intrudes Bean Canyon and Tylerhorse Canyon pendants and the Jurassic granodiorite of Gamble Spring Canyon. High Sri ratios (>0.707) characterize these plutons and autochthonous equivalents. An upper plate (a′)–lower plate (a) pressure differential of ∼2.4–2.8 kbar (samples 08TC27a and 08TC29; Tables 3 and 4) versus 4.1–5.5 kbar (Nadin and Saleeby, 2008, samples 03SS1, 04SS31, and 51) suggests the removal of 5–10 km of crust along the southern Sierra detachment system during faulting.
Correlation #2: Silicic Metavolcanic Rocks of the Erskine Canyon Sequence (B) and the Oak Creek Pass Complex (B′)
Small screens of finely recrystallized laminated tuffs with remnants of quartz phenocrysts and pumice lapilli crop out in the Oak Creek Pass complex (Wood, 1997; Wood and Saleeby, 1997). The Oaks metatuffs are similar in petrography, texture, and field setting to the 102–105 Ma Erskine Canyon rhyolitic to andesitic metavolcanic sequence, which has been transposed along the proto–Kern Canyon fault over ∼40 km adjacent to Lake Isabella (Nadin and Saleeby, 2008; Saleeby et al., 2008). We hypothesize that Oaks metavolcanic rocks represent Erskine Canyon rocks formerly situated east of the proto–Kern Canyon fault that were displaced southward by detachment faulting. U-Pb zircon geochronology reported here supports this view, with an interpreted eruption age of 102.6 ± 1.0 Ma for Oaks metavolcanic rocks.
Pressure determinations from plutons and metamorphic assemblages in contact with Erskine Canyon metavolcanic rocks range from 6.6 to 4.5 kbar (Nadin and Saleeby, 2008, Plate 3), in contrast to 3 to 4.4 kbar determinations from the Claraville granodiorite in the hanging wall (Nadin and Saleeby, 2008, sample RB020601, and samples 93TH417 and 91TH181; Table 4), which intrudes Oaks metavolcanic rocks, corresponding to ∼0–12 km of missing crust.
Correlation #3: Monolith and Back Canyon Pendants (C) and Quinn Ranch and Aqueduct Tunnel Pendants (C′)
The Quinn Ranch and Aqueduct Tunnel pendants are characterized by sequences of chiefly marble interleaved with calc-silicate and minor quartzite and siliceous meta-argillite (Ross, 1989). The granodiorite of Gato Montes intrudes these pendants and yields a U-Pb zircon age of 92.1 ± 1.0 Ma. The Aqueduct Tunnel and Quinn Ranch pendants, neighboring small marble septa, and host plutons share similar pendant-pluton relations with Monolith and Back Canyon pendants and intrusive Claraville granodiorite of the south-central Sierra Nevada batholith. A similar range of lithologies to that of Aqueduct Tunnel and Quinn Ranch pendants is seen in Monolith and Back Canyon pendants (Wood and Saleeby, 1997). We suggest, on the basis of distinctive marble successions found in pendants at c and c′, that these pendants belong to the Death Valley facies of the passive margin as exposed in the Shadow Mountains of the western Mojave Desert (Martin and Walker, 1995; Fig. 1). Al-in-hbl pressures of 5.7–6.5 kbar are calculated from the Claraville granodiorite near Kelso Valley in the footwall of the southern Sierra detachment system (Ague and Brimhall, 1988, sample 420; Dixon, 1995, samples 49 and Th275; Nadin and Saleeby, 2008, sample 04SS34). The presence of prograde andalusite as the only stable aluminosilicate phase in pelitic assemblages near c′ (Ross, 1989) suggests that these rocks were intruded and contact metamorphosed at pressures no greater than ∼4 kbar (Spear, 1993). These relations suggest the excision of at least 5.5 km of crust along the southern Sierra detachment system.
Correlation #4: Antelope Canyon Group (D) and Salt Creek Pendant (D′)
The Salt Creek pendant of the San Emigdio Mountains is composed principally of layers of marble, quartzite, and quartzofeldspathic gneiss, with lesser amounts (<5%) of pelitic to psammitic schist. These lithologies suggest that the protoliths of the Salt Creek pendant were deposited in a slope to inner shelf environment and potentially correlate to similar assemblages found in the Sierra City mélange–Shoo Fly–Kernville terrane (Fig. 2). Our detrital zircon geochronologic data from quartzite sample 08SE258 support this view, with major age peaks corresponding to those of slope strata from the Kernville terrane (Saleeby, 2011; Figs. 6 and 7).
The Salt Creek pendant is intruded by the ca. 90 Ma Lebec granodiorite. Ross (1989) notes lithologic similarities between the Salt Creek pendant and pendants north of Tehachapi Valley, including the Brite Valley, Tehachapi, Monolith, and Back Canyon pendants. However, the Lebec granodiorite-hosted Salt Creek pendant does not likely correlate with Brite Valley and Tehachapi pendants since they are intruded by 99–105 Ma tonalites, diorites, and gabbroids of the intrusive suite of Bear Valley (Saleeby et al., 2007, 2008). In contrast, similar siliciclastic and carbonate rocks of the Antelope Canyon group are intruded by a border phase of the ca. 90 Ma Claraville granodiorite (Wood, 1997), representing the most likely native site for similar assemblages of the Salt Creek pendant and adjacent rocks.
Correlation #5: Area Southwest of the Rand Mountains (E) and Salinian Framework of the Gabilan Range, Santa Lucia Range, and Southern Ben Lomond Mountain (E′)
The Sur Series (Trask, 1926) of the Santa Lucia and Gabilan Ranges, and smaller exposures in the Ben Lomond and Point Reyes areas, contains abundant interbedded marble and calc-silicate, leading several workers to suggest that metasedimentary rocks of the Salinian block correlate with strata of the Paleozoic Cordilleran miogeocline (e.g., Ross et al., 1973; Ross, 1977; Kidder et al., 2003). However, pure quartzite and marble units that resemble the Stirling and Zabriskie quartzites and the Bonanza King Formation of the Death Valley–Mojave Desert region are conspicuously absent from the Salinian block.
Instead, stratigraphic relations and detrital and igneous zircon ages and pluton geochemistry from northern Salinia and the western Mojave Desert suggest that the El Paso terrane is the most likely parent for “orphan” Salinia. Metasedimentary assemblages of the Salinian terrane share several similarities with eugeoclinal strata of the El Paso terrane. First, thinly-bedded Sur Series assemblages are lithologically similar to strata of the Kern Plateau pendants and the Bean Canyon pendant (Rindosh, 1977; Ross, 1977; Dunne and Suczek, 1991). Second, Sur Series assemblages exhibit broad Late Archean and Proterozoic U-Pb detrital zircon age peaks that roughly correspond to those of the El Paso terrane, including the Bean Canyon pendant (Barbeau et al., 2005; Fig. 6). While Sur Series age peaks are too broad to distinguish between miogeoclinal and eugeoclinal sources within the Mojave Desert, potentially due to zircon isotopic disturbance during high-grade metamorphism, the presence of Permian zircons in Upper Cretaceous cover strata suggests derivation from, or at least proximity to, the El Paso terrane (Barbeau et al., 2005). Lastly, small mafic to ultramafic bodies crop out entirely within, and are elongated parallel to the foliation of, Sur Series metasedimentary rocks (Wiebe, 1966, 1970; Nutt, 1977; Bush, 1981), the Lake Isabella pendant (Saleeby and Busby, 1993), the Bean Canyon pendant (Rindosh, 1977; this study), and the Kern Plateau pendants (Dunne and Suczek, 1991).
The central Santa Lucia and Gabilan Ranges and southern Ben Lomond Mountain contain 93–76 Ma plutons with high Sri (typically >0.708; Kistler and Champion, 2001; Kidder et al., 2003; Dickinson et al., 2005) that intruded the Sur Series at pressures of 3.4–4 kbar (John, 1981, and this study). A zone of platformal sequences characterized by quartzite, marble, and psammitic schist (Miller et al., 1995) and high Sri and young plutons (Plates 1 and 2) southwest of the Rand Mountains appears to be the most likely native site for the central Santa Lucia and Gabilan Ranges. However, Permian metavolcanic rocks and Independence dikes, two striking yet volumetrically minor constituents of the El Paso terrane, are not reported from the Salinian block. These rock types may be absent from the Salinian block or obscured due to poor exposure.
The Coast Ridge Belt, an exposure of orthogneiss and subordinate marble and quartzite (Kidder et al., 2003), probably represents the midcrustal (∼7.5 kbar) equivalent of the central Santa Lucia Range. The belt crops out ∼5 km to the west of our 3.4 kbar determination and 4–6 kbar estimates of peak metamorphic pressures in the Sur Series by Wiebe (1966, 1970). The pressure gradient between the Coast Ridge Belt and the central Santa Lucia Mountains is explained by Kidder et al. (2003) as the result of regional ∼30° to the NE tilt. However, this tilt can only account for an ∼3 km (i.e., ∼1 kbar) difference over 5 km. Therefore, the pressure difference between the Coast Ridge Belt and the central Santa Lucia Range must have resulted either from structural attenuation or faulting, potentially along the Coast Ridge and/or Palo Colorado faults (Ross, 1976).
We suggest that the Coast Ridge Belt restores to a position along the west flank of the central Mojave metamorphic core complex, possibly correlating with the Johannesburg gneiss in the hanging wall of the Rand fault, with shallow exposures of the central Santa Lucia Range, Gabilan Range, and southern Ben Lomond Mountain lying in fault contact above the belt. Pressure determinations from footwall assemblages southwest of the Rand Mountains are sparse, yet values of ∼10 kbar are reported from the central Mojave metamorphic core complex (Henry and Dokka, 1992). A pressure differential of ∼2.5 kbar between the Coast Ridge Belt and the central Mojave metamorphic core complex implies the removal of ∼8 km of crust along a structure that has not yet been recognized.
Correlation #6: Cummings Valley (F) and Salinian Framework of Montara Mountain, Northern Ben Lomond, and Bodega Head (F′)
Tonalitic plutonic rocks of Montara Mountain, northern Ben Lomond Mountain, and Bodega Head range in age from 99 to 104 Ma (James and Mattinson, 1985; James, 1992; Kistler and Champion, 2001) and have similar Sri (0.707–0.705) to the 99–105 Ma intrusive suite of Bear Valley (Saleeby et al., 1987; Pickett and Saleeby, 1993; Saleeby et al., 2007). Furthermore, similarities between Sur Series metasedimentary rocks of northern Salinia and the Brite Valley and Tehachapi pendants of the southern Sierra Nevada batholith (Wood, 1997) suggest that rocks found at f′ originated near Cummings Valley. Plutons and framework rocks near f′ have not received detailed thermobarometric study, although pressures of ∼5 kbar are reported from metapelitic assemblages from Ben Lomond Mountain (Leo, 1967; DeCrisoforo and Cameron, 1977). Pressure determinations of 8.3–8.7 kbar are reported from the area near Cummings Valley (Pickett and Saleeby, 1993, samples GC-16 and GC-2).
Correlation #7: Southwestern Sierra Nevada Foothills (G) and Western San Emigdio Mafic Complex (G′)
Reitz (1986) suggests that the western San Emigdio mafic complex is a unique exposure of a primitive intraoceanic arc and that it does not correlate with (1) the Coast Range ophiolite because the mafic complex lacks ophiolite stratigraphy, or (2) Sierran Foothills belt peridotitic to dioritic intrusive complexes (Snoke et al., 1982) and associated ophiolitic wall rocks. This assertion is based on slight differences in crystallization sequence, geochemistry, and the presence of minor orthopyroxene in the western San Emigdio mafic complex, which is rare in peridotitic to dioritic complexes. New geochemical data and field relations call this interpretation into question.
Major and trace element geochemistry and 143Nd/144Nd versus 87Sr/86Sr systematics indicate that sheeted dikes and pillow basalts from the western San Emigdio mafic complex were generated through MORB magmatism (Fig. 10A). The geochemical similarity between basaltic rocks of the western San Emigdio mafic complex and the Kings-Kaweah segment of the Foothills ophiolite belt suggests that the two bodies are correlative.
Sheeted dikes and pillow basalts of the western San Emigdio mafic complex are intruded by a mid- to Late Jurassic suite of ultramafic, gabbroic, and tonalitic assemblages (Hammond, 1958; Ross, 1970, 1989; Dibblee and Nilsen, 1973; James, 1986a; Reitz, 1986) that resemble 170–150 Ma intrusive complexes of the western Sierra Nevada (Snoke et al., 1982). Zircons from tonalitic segregations in gabbros yield concordant U-Pb ages of ca. 161 Ma (James, 1986a). These gabbros are intruded by the White Ridge tonalite, which yielded a U-Pb age of 154.2 ± 3.5 Ma. Similar U-Pb zircon ages of 170–156 Ma are reported from gabbroic to tonalitic assemblages of the Mill Creek complex, which intrudes the Kings-Kaweah ophiolite belt (Saleeby and Sharp, 1980; Snoke et al., 1982; Wolf and Saleeby, 1995; Saleeby, 2011).
Exposures and basement cores of variably mylonitized and highly altered mafic to ultramafic rocks occur in the footwall of the Maricopa detachment near g (Dibblee and Chesterman, 1953; Ross, 1989; Saleeby et al., 2009a; Fig. 12A). Estimating the amount of crust that was removed by the Maricopa detachment is difficult, because the western San Emigdio mafic complex does not appear to contain assemblages conducive to thermobarometry. However, the preservation of albite-epidote and hornblende hornfels–facies assemblages in pillow basalts and sheeted dikes of the western San Emigdio mafic complex (Hammond, 1958; this study) suggest that metamorphic pressures of equilibration did not exceed ∼2 kbar (Spear, 1993). Al-in-hbl pressure determinations of 4.5–5.8 kbar from footwall assemblages (Nadin and Saleeby, 2008, samples 48 and 49) suggest that restoration of g′ to g implies the removal of at least 8 km of formerly intervening crust.
Magnitude and Direction of Displacement
The correlations discussed above each imply the removal of ∼5–10 km of crust along detachment faults with a north-bounding breakaway zone in the Isabella basin area. The KWF bounds the western margin of the Isabella breakaway and transfers the zone ∼50 km to the southwest to the Kern range front, the headwall for the Maricopa detachment (Figs. 1 and 12).
The average magnitude of transport is 71 ± 28 km (Table 5), calculated by measuring the map view distance between native and displaced rocks. Transport azimuths, with an average of 205 ± 28°, were determined by measuring the map view heading of the displaced site from the native site after restoring slip along San Andreas and Garlock faults. Plunge angles, with an average of 6.5 ± 3.1°, were calculated by multiplying the pressure differential between sites by 3.3 km/kbar, dividing this value by the lateral distance between sites, and taking the arc tangent of this quotient.
The determination of these values critically depends on recognizing superimposed increments of post-Cretaceous deformation that may have contributed to the transport of allochthonous fragments. The biggest challenge in “seeing through” post-Cretaceous deformation in the southern Sierra Nevada batholith and vicinity, after restoring slip along Neogene faults, is accounting for apparent clockwise vertical axis rotations of up to 90° in the region (Kanter and McWilliams, 1982; McWilliams and Li, 1983; Dokka and Ross, 1995; Wood and Saleeby, 1997; Nadin and Saleeby, 2008; Hopson et al., 2008). In short, a significant fraction of observed clockwise rotation in the southern Sierra Nevada batholith is thought to have taken place during Late Cretaceous extension (Malin et al., 1995; Saleeby, 2003; Chapman et al., 2010) and post-Cretaceous vertical axis rotation probably did not result in significant additional dispersion of upper crustal fragments in the southern Sierra Nevada batholith, with the exception of the San Emigdio Mountains, which accommodated ∼7 km of north-south shortening since late Pliocene time (Davis, 1983). The reader is referred to Chapman et al. (2010) for a thorough review of rotations in the Sierran “tail.”
Time-Transgressive Detachment Faulting
Integration of Al-in-hbl igneous barometry and thermochronometric data indicates that the area south of the Isabella breakaway cooled and decompressed from >650 °C and ∼6 kbar (Nadin and Saleeby, 2008) at ca. 90 Ma through ∼180 °C (i.e., shallow crustal levels) at 77 ± 5 Ma (Fig. 9). Such profound cooling and decompression at 77 ± 5 Ma requires a tectonic driving mechanism and is interpreted to reflect denudation related to detachment faulting along the southern Sierra detachment system at this time.
Structural data from the southern Sierra detachment system suggest, however, that detachment faulting was time-transgressive. The calculated average transport azimuth of 205 ± 28° is oblique to SSE-trending and moderately plunging lineations along the Blackburn Canyon and Jawbone Canyon faults (Wood, 1997; Wood and Saleeby, 1997) and fault II of the Rand fault complex (Nourse, 1989; Fig. 12). Two potential explanations for this difference are that Cretaceous to recent deformation has led either to remobilization of the eastern southern Sierra detachment system with a top to the SSE sense of shear, or systematic clockwise rotation of allochthonous fragments without concurrent rotation of lineations along the eastern southern Sierra detachment system. Small increments of post-Cretaceous tectonism may account for some of the discrepancy between the lineation orientation and the actual transport direction along the southern Sierra detachment system.
Our preferred explanation for the discrepancy between the lineation orientation and inferred transport direction along the southern Sierra detachment system, however, is that extension began in the southwestern Sierra Nevada batholith and propagated eastward with time. To better articulate this model, we define the following allochthonous regions of similar inferred paleogeographic affinity and timing of transport: the Logan–western San Emigdio allochthon, the northern Salinia allochthon, the Gabilan-Pastoria allochthon, the Santa Lucia allochthon, and the Jawbone-Rand allochthon (Fig. 12B). Hornblende and biotite 40Ar/39Ar and K-Ar cooling ages from the northern Salinia allochthon cluster around 90 Ma (Kistler and Champion, 2001), probably reflecting the timing of tectonic transport and upper plate attenuation from above the Cummings Valley area. The Logan–western San Emigdio allochthon was derived from a nearby area and has a similar dispersal pattern to the northern Salinia allochthon. In contrast, K-Ar hornblende and biotite ages from the Gabilan-Pastoria, Santa Lucia, and Jawbone-Rand allochthons lie in the 85 to 75 Ma range (Evernden and Kistler, 1970; Huffman, 1972), overlapping with He zircon results indicating ca. 77 Ma rapid cooling in the autochthonous southeastern Sierra Nevada batholith. We suggest that detachment and transport of these allochthons occurred at roughly 80 Ma with a more southerly transport direction than Logan–western San Emigdio and northern Salinia allochthons (Fig. 12B; Movie SD1 in the Supplemental File [see footnote 1]).
A similar outboard to inboard time-transgressive model invoking oblique collision of an aseismic ridge with North America is envisioned by Barth and Schneiderman (1996) and Chapman et al. (2010) to explain an ∼20 Myr time lag in schist cooling ages between the southwestern Sierra Nevada batholith, northwestern Mojave Desert, and Salinia. We speculate that south- and SSE-directed transport of upper crustal fragments in the Jawbone-Rand allochthon resulted as schist eduction created a void at the lateral ramp in the subduction megathrust, approximately centered along the trace of the superimposed Garlock fault, causing superjacent batholithic crust to breach southward across the inflection zone (Fig. 13).
Late Cretaceous Gravitational Collapse of the Southern Sierra Nevada Batholith
A thorough review of crosscutting relationships between dated plutons, detachment faults, and Late Cretaceous to Eocene nonconformities by Wood and Saleeby (1997) loosely brackets tectonic unroofing along the southern Sierra detachment system and the transport of upper crustal fragments to between ca. 90 and 50 Ma, suggesting that extension may have taken place over as much as 40 Myr. However, thermochronometric work presented here and a recent review of available geochronologic, thermochronometric, and thermobarometric data (Chapman et al., 2010) indicate that the majority of extension, while diachronous across the southern Sierra Nevada, occurred entirely within the Late Cretaceous.
The transport of upper crustal fragments from above the southern Sierra Nevada batholith at 77 ± 5 Ma coincides with: (1) structural ascent of deep batholithic rocks in the Tehachapi, San Emigdio, Santa Lucia, and Rand Mountains from ca. 95 to 70 Ma (Malin et al., 1995; Fletcher et al., 2002; Kidder et al., 2003; Saleeby et al., 2007); (2) rapid eduction of the schist from beneath the autochthon from ca. 90 to 70 Ma (Barth et al., 2003; Grove et al., 2003; Kidder and Ducea, 2006; Chapman et al., 2010); (3) dextral strike-slip transfer motion along the KWF (Nadin and Saleeby, 2008); (4) dextral shear across Owens Valley (Bartley et al., 2007); and (5) marine transgression and deposition of supradetachment basin deposits across the highly attenuated crust of the southern Sierra Nevada batholith and adjacent areas (Grove, 1993; Wood and Saleeby, 1997; Saleeby, 2003).
Structural and kinematic relations from the Rand fault and Salinas shear zone suggest that the schist moved from deep to shallow crustal levels with a lower plate to the SSW (210 ± 10°) sense of shear (Chapman et al., 2010), overlapping with the 205 ± 28° transport direction of allochthonous upper crustal assemblages (Fig. 12). These relations indicate that coeval SSW-directed transport of the entire crustal column, from upper plate shallow-level assemblages of the southern Sierra Nevada batholith, Mojave Desert, and Salinian block, through middle plate deep-crustal exposures of the southern Sierra Nevada batholith, and into the lower plate schist, occurred in Late Cretaceous time. Late Cretaceous trenchward flow of Rand and related schists (Malin et al., 1995; Saleeby, 2003; Chapman et al., 2010) and strain coupling between the educting schist and upper plate(s) provide an explanation for the overlapping transport directions along the southern Sierra detachment system, Rand fault, and Salinas shear zone. We speculate that strain coupling between the schist and deep batholithic plate along the Rand fault and Salinas shear zone and, in turn, between the deep batholithic plate and upper crustal fragments along the southern Sierra detachment system, reflects gravitational collapse of the southern Sierra Nevada batholith and adjacent northwest Mojave and Salinia.
Mass transfer associated with lateral spreading and vertical thinning of the Sierran crust leads to a space problem. Rey et al. (2001) address this space problem by defining two end-member modes of gravitational collapse in which gravitational potential energy is (“fixed-boundary collapse”) and is not (“free-boundary collapse”) transferred from hinterland to foreland regions. South- to southwestward-directed extension of the entire crustal column of the southern Sierra Nevada batholith without synchronous shortening in the foreland best fits the criteria for free-boundary gravitational collapse. However, Late Cretaceous transcurrent faulting along the KWF and Owens Valley shear system has partitioned the highly extended and exhumed core of the southern Sierra Nevada batholith from less extended and exhumed adjacent regions. These relations imply that SSW-directed extrusion toward the unconfined continental margin probably accompanied crustal attenuation in the southern Sierra Nevada batholith (Fig. 13). Subduction of a LIP beneath the southern Sierra Nevada batholith and adjacent areas (Liu et al., 2010) and associated transient horizontal compressional stresses, basal shear stresses, and lithospheric strength (i.e., the replacement of mantle wedge material with underplated schist) is posited to have preconditioned the southern Sierra Nevada batholith for lateral extrusion-modified, free-boundary collapse.
Southern Sierra Landscape Development
Apatite He data (Mahéo et al., 2009) from the zircon He sample suite can be further utilized to approximate the position of the southern Sierra detachment system surface relative to the modern topographic surface and to relate the topography of the detachment surface to the early landscape development of the greater Sierra Nevada. Following the termination of large volume arc magmatism in the Sierra Nevada batholith at ca. 84 Ma (Chen and Moore, 1982), the topographic surface of the arc underwent slow regional erosion at a rate of 0.05 ± 0.01 mm/yr throughout much of Cenozoic time (Clark et al., 2005; Cecil et al., 2006; Mahéo et al., 2009). Thus the constructional topographic surface of the greater Sierran arc, commonly interpreted to have constituted the western margin of a Cordilleran-wide orogenic plateau locally termed the Nevadaplano (DeCelles, 2004), began to slowly erode at virtually the same time as the tectonically denuded lower plate regime of the southern Sierra detachment system.
Apatite He age-elevation relations are used to adjust each data point of our zircon He suite to a position (“virtual distance”) that would correspond to the freshly denuded detachment surface (Fig. 14). The reconstructed detachment surface is subhorizontal and projects at ∼2 ± 1 km above the modern landscape. We have also calculated “virtual pressures” of igneous equilibration by dividing calculated “virtual distances” by 3.3 km/kbar and adding this quotient to existing Al-in-hbl pressure determinations, where available. Calculated “virtual pressures” reveal the crustal depth of the footwall of the southern Sierra detachment system at the moment of large magnitude detachment faulting.
Figure 14 shows that the detachment surface in the area of the sample traverse sat at ∼4 ± 1 kb levels of the crust, which corresponds well to the typical 3 ± 1 kb crustal levels determined for the allochthons. These features are synthesized in Figure 15, a regional N-S cross section crossing the restored Rand Mountains, extending the schist under the southern Sierra Nevada batholith, and passing north of footwall assemblages of the southern Sierra detachment system, with the system continuing northward as the reconstructed detachment surface up to the Isabella breakaway zone. The breakaway zone ramps up to the Kern plateau and continues to the Nevadaplano margin with the Late Cretaceous–early Tertiary paleodrainage of the Kings River region shown at the northern end of the section (House et al., 2001; Saleeby et al., 2009b). This cross section represents a snapshot of southern Sierra paleolandscape and large-scale crustal structure immediately following large-magnitude detachment faulting.
Integration of new field and structural relations, U-Pb zircon geochronology, thermobarometry, major and trace element chemistry, Sr and Nd isotopic ratios, and zircon (U-Th)/He thermochronometry with existing databases reveals temporal and spatial overlap between: (1) tectonic transport of allochthonous fragments of shallow-level eastern Sierra Nevada batholith affinity rocks; (2) structural attenuation and ascent of deep-level western Sierra Nevada batholith assemblages; and (3) trench-directed flow in the schist. These relationships suggest that the entire crustal column of the southern Sierra Nevada batholith and vicinity collapsed in Late Cretaceous time due to excess gravitational potential energy.
This work places several constraints on the timing and magnitude of extension attending gravitational collapse of the southern Sierra Nevada batholith. First, zircon (U-Th)/He data presented herein reveal a rapid cooling event at 77 ± 5 Ma, probably reflecting the time of large-magnitude detachment faulting in the southern Sierra Nevada batholith autochthon. A comparison of this data set with existing apatite (U-Th)/He thermochronometry from the same sample suite (Mahéo et al., 2009) suggests that the development of modern landscape and arrangement of tectonic elements in southern California was greatly preconditioned by Late Cretaceous tectonics. Most notably is the southward-sloping topographic gradient off the modern southern Sierra, which reflects elevation loss in that region due to extension and detachment faulting. Second, palinspastic restoration of seven presumably correlative allochthon-autochthon pairs implies 50–70 km of lateral transport and the removal of ∼5–10 km of crust along the southern Sierra detachment system. Finally, the timing and the kinematics of dispersal of upper crustal fragments and ascent of deep-level batholithic and subduction accretion assemblages overlap, suggesting that the shallow and deep crust were highly coupled during gravitational collapse. The observations presented here clarify several issues pertaining to Late Cretaceous orogenesis and subsequent collapse of the southern Sierra Nevada batholith and vicinity, and are consistent with the free-boundary gravitational collapse mode of Rey et al. (2001).
This research was supported by National Science Foundation grant EAR-0739071 and by the Gordon and Betty Moore Foundation. This is Caltech Tectonics Observatory Contribution 188. The manuscript benefitted from enlightening discussions with M. Robinson Cecil, George Dunne, Vali Memeti, Elisabeth Nadin, and Scott Paterson and from thoughtful reviews by Jade Star Lackey, John Bartley, and Associate Editor Keith Putirka. We thank Pieter Vermeesch for providing a MATLAB script for multidimensional scaling of U-Pb detrital zircon ages. Ronald Kistler graciously provided samples of north and south tonalites of Vergeles (Gabilan Range).