Thermal evolution and exhumation of deep-level batholithic exposures, southernmost Sierra Nevada, California

Abstract

The Tehachapi complex lies at the southern end of the Sierra Nevada batholith adjacent to the Neogene-Quaternary Garlock fault. The complex is composed principally of high-pressure (8-10 kbar) Cretaceous batholithic rocks, and it represents the deepest exposed levels of a continuous oblique crustal section through the southern Sierra Nevada batholith. Over the southern ~100 km of this section, structural/petrologic continuity and geochronological data indicate that ≥35 km of felsic to intermediatecomposition crust was generated by copious arc magmatism primarily between 105 and 99 Ma. In the Tehachapi complex, these batholithic rocks intrude and are bounded to the west by similar-composition gneissic-textured high-pressure batholithic rocks emplaced at ca. 115-110 Ma. This lower crustal complex is bounded below by a regional thrust system, which in Late Cretaceous time tectonically eroded the underlying mantle lithosphere, and in series displaced and underplated the Rand Schist subduction assemblage by low-angle slip from the outboard Franciscan trench. Geophysical and mantle xenolith studies indicate that the remnants of this shallow subduction thrust descend northward through the crust and into the mantle, leaving the mantle lithosphere intact beneath the greater Sierra Nevada batholith. This north-dipping regional structure records an infl ection in the Farallon plate, which was segmented into a shallow subduction trajectory to the south and a normal steeper trajectory to the north. We combine new and published data from a broad spectrum of thermochronometers that together form a coherent data array constraining the thermal evolution of the complex. Integration of these data with published thermobarometric and petrogenetic data also constrains the tectonically driven decompression and exhumation history of the complex. The timing of arc magmatic construction of the complex, as denoted above, is resolved by a large body of U/Pb zircon ages. High-confi dence thermochronometric data track a single retrogressing path commencing from widely established solidus conditions at ca. 100 Ma, and traversing through time-temperature space as follows: (1) Sm/Nd garnet ~770-680 °C at ca. 102-95 Ma, (2) U/Pb titanite

Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system, central California

“…Late Cretaceous rotation from Chapman et al (2010). Figure 9B is adapted from Wentworth et al (1983), Hall and Loomis (1992), Wentworth et al (1995Wentworth et al ( , revised 2012, Saleeby et al (2007Saleeby et al ( , 2008, Nadin and Saleeby (2008), and the references cited for Figure 9A. In addition, the following references were considered in the development of this figure: Crowell (1952), Dibblee and Chesterman (1953), Burchfiel and Davis (1981), Silver (1982Silver ( , 1983Silver ( , 1986, Ross (1984Ross ( , 1988Ross ( , 1989.…”

“…Several factors strongly support this interpretation: (i) neither the higher grade blueschist blocks nor the more widespread lower grade blueschist overprint formed in a position proximal to the upper plate Salinia rocks as evident from the thermal state of the upper plate Salinia batholithic rocks during their low-angle subduction thrusting (Kidder and Ducea 2006), and its observed retrogression path, and it is highly unlikely that such Franciscan metamorphism formed updip to the west; (ii) upper plate Salinia rocks subsided to marine conditions in an extensional setting during the displacement history of Nacimiento fault (Grove 1993); (iii) upper plate Salinia basement rocks are everywhere highly shattered and retrograded, typical of extensional basement nappes; and (iv) Sur-Obispo Franciscan rocks structurally record late-stage extension (Platt 2013). In this model, the Sur-Obispo Franciscan Complex and SdS schist form an extensive E-dipping subduction duplex structurally beneath Salinia batholithic rocks, as implied by Ducea et al (2009), with an inverted metamorphic gradient that resulted from the primary heat of the solidus to hot sub-solidus state of the batholithic upper plate during thrusting (after Saleeby et al 2007;Chapman et al 2011;Kidder et al 2013). This model requires that the schist of SdS was tectonically eroded from the base of Salinia along the exposed trace of the Nacimiento fault either during low-angle subductionrelated thrusting, or during the Franciscan-extrusion phase.…”

“…and west of Bakersfield to ∼35 • N Lat., and to near the White Wolf fault at the southern end of the San Joaquin Valley, are truncated west of an inferred discontinuity between the Franciscan Complex and associated metamorphic rocks and the Sierra Nevada batholith (Wentworth et al 1995 Figure 9A is adapted primarily from Wood and Saleeby (1998), Saleeby et al (2007), Kellogg et al (2008), Saleeby et al (2010), and. The light-green structure contour lines, reflecting the nonconformity between the Sierra Nevada batholith and Cenozoic strata north of the White Wolf fault (WWF) in the southwestern part of the San Joaquin Valley, are adapted from Wentworth et al (1995Wentworth et al ( , revised 2012.…”

“…For the locations of Figures 9A and 9B, see Figure 1. (1997,2003), and Saleeby et al (1987Saleeby et al ( , 2007Saleeby et al ( , 2008Saleeby et al ( , 2009aSaleeby et al ( , 2009bSaleeby et al ( , 2010Saleeby et al ( , 2013 are paramount to understanding the region.…”

“…The fault has been considered a thrust fault (May 1989;Pitchett and Saleeby 1993;Kellogg et al 2008) and/or a regional latest Cretaceous, extensional detachment fault (Wood and Saleeby 1997; Basement dextral offset appears to greatly exceed late Cenozoic sinistral offset -critical for palinspastic restoration of the basement. The White Wolf fault has also been considered to be a sinistral fault [Stein and Thatcher (1981), late Miocene to Quaternary movement; Ross (1986Ross ( , 1989) with a SE dip; Bartow (1991); Bawden (2001) with a 75 • SW dip; Saleeby et al 2007, figure 1, and E side-up (Nadin and Saleeby 2010)]. …”