TIMING OF RAND DETACHMENT FAULT SLIP IN THE RAND MOUNTAINS, WESTERN MOJAVE DESERT

Laramide flat-slab subduction extinguished Sierra Nevada arc magmatism in southern California as trench-derived sediments were underthrust and accreted beneath arc basement during the early Campanian. In the Rand Mountains south of the Garlock fault in the NW Mojave Desert, the Rand detachment fault juxtaposes subcreted Rand Schist beneath overlying 87 Ma Sierran granitoids. Similar detachment fault exposures elsewhere are proposed to have accommodated large-scale Late Cretaceous displacement that exhumed Rand Schist to shallow crustal levels by late Campanian time. Previous and new ⁴⁰Ar/³⁹Ar and (U-Th)/He thermochronology from the Rand Mountains requires much later schist exhumation. This includes 68-73 Ma phengite cooling ages from the Rand Schist, 68-71 Ma biotite cooling ages from the structurally overlying arc basement, and much younger zircon (U-Th)/He age results that collectively require both schist and basement remained at mid-crustal depth until the Miocene. Slab window(?) magmatism related to the onset of Pacific-North American transform plate motion caused 19 Ma emplacement of the Yellow Aster pluton, coeval rhyolite porphyry volcanism, and hydrothermal circulation/gold mineralization in the Rand Mountains. This 19 Ma event complicates efforts to relate thermochronologic data to detachment faulting. Multi-diffusion domain ⁴⁰Ar/³⁹Ar analysis of K-feldspar was performed to determine whether detachment faulting was synchronous with 19 Ma magmatism or earlier fault displacement was overprinted by Miocene magmatism. Results from granodiorite K-feldspar sampled along the fault are consistent with either magmatic overprinting or differential exhumation of hanging-wall granitoids as a function of distance from the fault. The maximum temperature experienced by samples close to the fault was 200-250°C at 19 Ma. In contrast, thermal histories calculated for K-feldspar sampled up to 1 km from the fault favor monotonic cooling. Collectively, the data seem most consistent with Middle Miocene slip along the Rand detachment where normal faulting was accommodated by differential vertical displacement of tectonic sheets. Minor contrast (< 50°C) in hanging and footwall thermal histories indicates that the vertical component of slip along the Rand detachment was < 3 km.