EARLY DISRUPTION OF THE SOUTHERN SIERRA NEVADA BATHOLITH, CALIFORNIA, BY THE KERN CANYON FAULT SYSTEM

Disrupted regional spatial variation patterns and structural analyses indicate that significant vertical displacement and horizontal shortening took place across the ~130 km long Proto-Kern Canyon fault (PKCF), a Late Cretaceous ductile shear zone in the southern Sierra Nevada batholith (SNB), early in its history. These observations further suggest that the fault was instrumental in the exhumation and exposure of the deep southern part of the SNB. The disruptions are manifest in igneous emplacement pressures, initial ⁸⁷Sr/⁸⁶Sr (Sr_i), zircon U/Pb ages, and pluton bulk compositions. Disrupted geobarometric gradients across the central part of the PKCF suggest up to ~10±5 km of east side up reverse displacement. In the southern part of the batholith, the PKCF truncates at an oblique angle the petrologically distinct axial zone of the SNB, suggesting that up to ~25 km of normal shortening occurred across this region. The coincidence of the Sr_i = 0.706 isopleth with the PKCF from the origin of the truncation southward further supports normal shortening. Zircon U/Pb ages from plutons emplaced along the shear zone during its activity indicate that this shortening and vertical displacement 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 90–86 Ma, and another 12±1 km of dextral slip occurred along the northern segment of the zone between 86–80 Ma. In its southern reaches, the reverse/thrust displacement increases southward as the PKCF flattens into the Rand fault system, suggesting that it behaved like a lateral ramp. Late-stage dextral ductile shear along the PKCF and the early-phase brittle overprints that document inception of the later, brittle KCF are coeval with tectonic denudation of the southernmost SNB. 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 PKCF, along with the younger KCF, behaved as a transfer system during the extensional phases of tectonic denudation of the southernmost SNB, leading to exposure of the oblique crustal section that we see today.