Cretaceous plutonic rocks of the southern Sierra Nevada batholith between latitudes 35.5°N and 36°N lie in a strategic position that physically links shallow, subvolcanic levels of the batholith to lower-crustal (~35 km deep) batholithic rocks. This region preserves an oblique crustal section through the southern Sierra Nevada batholith. Prior studies have produced large U/Pb zircon data sets for an aerially extensive region of the batholith to the north of this area and for the lower-crustal rocks of the Tehachapi complex to the south. We present a large set of new U/Pb zircon age data that ties together the temporal relations of pluton emplacement and intra-arc ductile deformation for the region. We define five informal intrusive suites in the area based on petrography, structural setting, U/Pb zircon ages, and patterns in initial 87 Sr/ 86 Sr (Sr i ). Two regionally extensive intrusive suites, the 105–98 Ma Bear Valley suite and 95–84 Ma Domelands suite, underlie the entire southwestern and eastern regions of the study area, respectively, and extend beyond the limits of the study area. A third regionally extensive suite (101–95 Ma Needles suite) cuts out the northern end of the Bear Valley suite and extends for an unknown distance to the north of the study area. The Bear Valley and Needles suites are tectonically separated from the Domelands suite by the proto–Kern Canyon fault, which is a regional Late Cretaceous ductile shear zone that runs along the axis of the southern Sierra Nevada batholith. The 105–102 Ma Kern River suite also lies west of the proto–Kern Canyon fault and constitutes the subvolcanic plutonic complex for the 105–102 Ma Erskine Canyon sequence, an ~2-km-thick silicic ignimbrite-hypabyssal complex. The 100–94 Ma South Fork suite lies east of the proto–Kern Canyon fault. It records temporal and structural relations of high-magnitude ductile strain and migmatization in its host metamorphic pendant rocks commensurate with magmatic emplacement. Integration of the U/Pb age data with structural and isotopic data provides insights into a number of fundamental issues concerning composite batholith primary structure, pluton emplacement mechanisms, compositional variations in plutons, and the chronology and kinematics of regional intra-arc ductile deformation. Most fundamentally, the popular view that Sierran batholithic plutons rise to mid-crustal levels (~20–15 km) and spread out above a high-grade metamorphic substrate is rendered obsolete. Age and structural data of the study area and the Tehachapi complex to the south, corroborated by seismic studies across the shallow-level Sierra Nevada batholith to the north, indicate that felsic batholithic rocks are continuous down to at least ~35 km paleodepths and that the shallower-level plutons, when and if they spread out, do so above steeply dipping primary structures of deeper-level batholith. This steep structure reflects incremental assembly of the lower crust by multiple magma pulses. Smaller pulses at deeper structural levels appear to be more susceptible to having source isotopic and compositional signatures modified by assimilation of partial melt products from metamorphic framework rocks as well as previously plated-out intrusives. Higher-volume magma pulses appear to rise to higher crustal levels without substantial compositional modifications and are more likely to reflect source regime characteristics. There are abundant age, petrographic, and structural data to indicate that the more areally extensive intrusive suites of the study area were assembled incrementally over 5–10 m.y. time scales. Incremental assembly involved the emplacement of several large magma batches in each (~50 km 2 -scale) of the larger plutons, and progressively greater numbers of smaller batches down to a myriad of meter-scale plutons, and smaller dikes. The total flux of batholithic magma emplaced in the study area during the Late Cretaceous is about four times that modeled for oceanic-island arcs. Integration of the U/Pb zircon age data with detailed structural and stratigraphic studies along the proto–Kern Canyon fault indicates that east-side-up reverse-sense ductile shear along the zone was operating by ca. 95 Ma. Dextral-sense ductile shear, including a small reverse component, commenced at ca. 90 Ma and was in its waning phases by ca. 83 Ma. Because ~50% of the southern Sierra Nevada batholith was magmatically emplaced during this time interval, primarily within the east wall of the proto–Kern Canyon fault, the total displacement history of the shear zone is poorly constrained. Stratigraphic relations of the Erskine Canyon sequence and its relationship with the proto–Kern Canyon fault suggest that it was ponded within a 102–105 Ma volcano-tectonic depression that formed along the early traces of the shear zone. Such structures are common in active arcs above zones of oblique convergence. If such is the case for the Erskine Canyon sequence, this window into the early history of the “proto–Kern Canyon fault” could preserve a remnant or branch of the Mojave–Snow Lake fault, a heretofore cryptic hypothetical fault that is thought to have undergone large-magnitude dextral slip in Early Cretaceous time. The changing kinematic patterns of the proto–Kern Canyon fault are consistent with age and deformational relations of ductile shear zones present within the shallow-level central Sierra Nevada batholith, and with those of the deep-level exposures in the Tehachapi complex. This deformational regime correlates with flat-slab segment subduction beneath the southern California region batholithic belt and resultant tilting and unroofing of the southern Sierra Nevada batholith oblique crustal section. These events may be correlated to the earliest phases of the Laramide orogeny.

ABSTRACT The Sierra Nevada batholith is an ~600-km-long, NNW-trending continental arc generally exposed from epizonal to mesozonal levels and showing a distinct strike-perpendicular zonation in structural, lithologic, petrologic, geochronologic, and isotopic patterns. South of 35.5° N, in the southern Sierra Nevada–northern Mojave Desert region, the depth of exposure increases markedly and a tectonostratigraphy consisting of three distinct, fault-bounded assemblages is observed. From high to low structural levels, these units are (1) fragments of shallow-level eastern Sierra Nevada batholith affinity rocks, (2) deeper-level western to axial zone rocks, and (3) subduction accretion assemblages (e.g., the Rand schist). This multi-tiered core complex is the product of shallow subduction that occurred over ~500 km of the plate margin in Late Cretaceous time. Slab shallowing was accompanied by intense contractile deformation within the crust and along the subduction megathrust; crustal thickening, uplift, and denudation of the residual arc to midcrustal levels; removal of the forearc and frontal arc by subduction erosion; and replacement of sub-batholithic mantle with underplated subduction assemblages. As the slab reverted to a “normal” trajectory, previously thickened crust no longer compensated at depth by a shallowly dipping slab became gravitationally unstable and underwent a profound phase of extensional collapse. Two subparallel shear zones, one separating assemblages 1 and 2 (the southern Sierra detachment) and the other juxtaposing units 2 and 3 (the Rand fault), comprise an integrated Late Cretaceous detachment system that accommodated extensional collapse. These Late Cretaceous events preconditioned the southern California crust for imprints of subsequent tectonic regimes. For example, subduction of the Pacific-Farallon slab window in early Neogene time created an extensional stress regime in the overriding plate, facilitating high-angle normal faulting across the previously extended region and volcanism associated with upwelling astheno-spheric material. The invasion of hot and buoyant asthenosphere destabilized dense sub-batholithic root material still affixed beneath the central Sierra Nevada batholith, leading to Pliocene–Quaternary delamination of the high-density rocks. Replacement of dense sub-batholithic root materials with asthenosphere has led to ~1 km of uplift across the southern Sierra Nevada and into the eastern San Joaquin Basin. The purpose of this trip is to highlight structural and petrologic records of multiple phases of tectonism in the southern Sierra Nevada–Mojave Desert region, illustrating the profound and lasting effect that shallow subduction may have on a continental margin.

Regional spatial variation patterns in igneous emplacement pressures, initial 87 Sr/ 86 Sr (Sr i ) values, zircon U/Pb ages, and pluton bulk compositions of the Sierra Nevada batholith are disrupted by the ~130-km-long proto–Kern Canyon fault, a Late Cretaceous ductile shear zone in the southern Sierra Nevada batholith. Vertical displacement and horizontal shortening across the proto–Kern Canyon fault in its early history are roughly constrained by the disruption of a regional primary batholithic structure that is recorded in petrologic and geochemical spatial variation patterns. The disruption of these patterns suggests that the proto–Kern Canyon fault underwent (1) subvertical west-directed reverse faulting that was instrumental in the exhumation and deep exposure of the southern part of the Sierra Nevada batholith, and (2) southward-increasing reverse/thrust displacement. The disruption of otherwise smoothly varying geobarometric gradients across the central part of the proto–Kern Canyon fault suggests up to ~10 ± 5 km of east-side-up reverse displacement across the shear zone. Southward from this area, the proto–Kern Canyon fault truncates, at an oblique angle, the petrologically distinct axial zone of the Sierra Nevada batholith, which suggests that up to ~25 km of normal shortening occurred across the southern part of the proto–Kern Canyon fault. Normal shortening is further supported by the coincidence of the Sr i = 0.706 isopleth with the proto–Kern Canyon fault from the point of initial truncation southward. Zircon U/Pb ages from plutons emplaced along the shear zone during its activity indicate that this shortening and vertical displacement had commenced by 95 Ma and was abruptly overprinted by dominantly dextral displacement with small east-side-up reverse components by 90 Ma. Conventional structural and shear fabric analyses, in conjunction with geochronological data, indicate that at least ~15 km of dextral shear slip occurred along the zone between 90 and 86 Ma, and another 12 ± 1 km of dextral slip occurred along the northern segment of the zone between 86 and 80 Ma. This later 12 ± 1 km of dextral slip branched southwestward as the ductile-brittle Kern Canyon fault, abandoning the main trace of the shear zone near its central section. Dextral shearing in the ductile regime was replaced by brittle overprinting by 80 Ma. The timing of initiation and the duration of reverse-sense displacement along the proto–Kern Canyon fault correspond closely with the shallow flat subduction of the Franciscan-affinity Rand schist along the Rand fault beneath the southernmost Sierra Nevada batholith. In its southern reaches, the proto–Kern Canyon fault flattens into the Rand fault system, suggesting that it behaved like a lateral ramp. Post–90 Ma dextral shear along the proto–Kern Canyon fault is suggested to have partitioned at least part of the Farallon plate’s tangential relative displacement component during an increase in subduction obliquity. Late-stage dextral ductile shear and early phase brittle overprints on the Kern Canyon fault system are coeval with tectonic denudation of the southernmost Sierra Nevada batholith. Geometric relations of the system’s terminal ductile and early brittle history with orthogonal extensional structures pose the possibility that the southern segment of the proto–Kern Canyon fault, along with the younger Kern Canyon fault, behaved as a transfer system during the extensional phases of tectonic denudation of the southernmost Sierra Nevada batholith, leading to exposure of the oblique crustal section we see today.

ABSTRACT Hafnium isotope ratios in Late Jurassic zircon from the Summit Gabbro provide geochemical evidence for rifting at ca. 148 Ma through the southern Sierra Nevada arc crust. Previous evidence for intra-arc extension includes a linear distribution of early Mesozoic volcano-sedimentary deposits located within the regional footprint of the ~600-km-long Independence dike swarm. In this study, latest Jurassic rifting through the entire thickness of the arc crust is supported by an abrupt, 40-εHf i -unit isotopic pull-up that records the incorporation of depleted-mantle partial melts into basaltic magmas that rapidly migrated into the upper arc crust. Isotopic context for this latest Jurassic pull-up is provided by Permian through Cretaceous zircon xenocrysts sampled by ca. 85–78 Ma dikes and sills that sampled the crust underlying the eastern Sierra Nevada range crest. Summit Gabbro bodies define a northwest-trending lineament paralleling the Kern Plateau shear zone, a geometry consistent with previous interpretations that the Kern Plateau shear zone originated as a late Paleozoic sinistral transform fault that evolved to a normal fault accommodating early Mesozoic intra-arc extension. New zircon U-Pb geochronology data coupled with field observations and whole-rock geochemistry serve as the basis to reclassify Summit gabbros and diorites as latest Jurassic and to motivate definition of the bimodal Summit igneous complex (151.0 ± 1.7–146.1 ± 1.2 Ma; N = 15) as also including coeval gabbro-granite dikes of the Osa Creek ring complex and the newly defined Tübatulabal hypabyssal-volcanic series. Tight age constraints support correlation of the Summit igneous complex to the regionally extensive, ca. 148 Ma Independence dike swarm. Multiple lines of evidence support latest Jurassic Sierran arc magmagenesis within a north-northwest–trending, sinistral transtensional regime, including: (1) subparallel orientations of the Independence dike swarm and aligned Summit Gabbro intrusions; (2) overlapping and bimodal compositions in both suites; and (3) the dramatic Hf isotope pull-up in zircons from Summit gabbro-diorites, recording a short-lived increase in the proportion of isotopically primitive mafic magmas intruding into the shallow arc crust. Though clearly differentiated, the composition of one gabbroic dike is the most primitive composition reported in the Sierra Nevada arc to date. The footprint of an intra-arc graben produced by early Mesozoic extension in eastern California is constrained by the Independence dike swarm, the Kern Plateau shear zone, and the Summit igneous complex, along with distributions of Triassic–Jurassic volcano-sedimentary deposits. Coupling the history of motion along the Kern Plateau shear zone with evidence for local extension beginning in the Permian–Triassic arc, we hypothesize that Mesozoic intra-arc faulting was strongly influenced by north-northwest–trending structures inherited from a mid- to late Paleozoic sinistral transpressional-transtensional plate boundary that extended for thousands of kilometers along the western boundary of Laurentia. We assemble a Permian–Jurassic time line of intra-arc extension within the plate margin locally transitioning from sinistral transform to convergent, one that culminated in latest Jurassic rifting through the entire crust underlying the Kern Plateau.