Disruption of regional primary structure of the Sierra Nevada batholith by the Kern Canyon fault system, California

Abstract

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.

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