DETRITAL THERMOCHRONOLOGY REVEALS TEMPORAL CHANGES IN EOCENE TO MIOCENE EROSION THROUGHOUT THE SOUTHERN SIERRA NEVADA BATHOLITH AND NORTHWEST MOJAVE DESERT REGION

Previous studies of forearc strata along the California margin have revealed that a striking local-to-extraregional provenance shift occurred during Eocene time outboard of the Mojave region. The provenance shift from mid-Cretaceous-batholith-dominated detrital zircon provenance to more cratonal provenance was related to development of a tectonic breach of the mid-Cretaceous batholith between the southern Sierra Nevada and Peninsular Ranges coupled with cratonward displacement of magmatism, deformation, and uplift associated with Laramide shallow subduction. During this time arc crust of the southernmost Sierra Nevada was exhumed from depths as great as 25-30 km. Early Eocene to Early Miocene sandstones deposited atop deeply denuded southernmost Sierran basement in the San Emigdio Mountains record the appearance of extraregionally-derived Eocene detritus and the renewal of local denudation associated with Early Miocene intraplate deformation produced by the onset of dextral Pacific-North American plate interaction. We have recovered this information by performing coupled U-Pb dating of detrital zircon and ⁴⁰Ar/³⁹Ar thermochronology of detrital K-feldspar. The U-Pb zircon age distributions of three samples including the Early Eocene Uvas Conglomerate, Late Eocene basal San Emigdio Fm, and the Early Miocene Temblor Fm record the extraregional to local transition with the percentage of mid-Cretaceous zircon increasing from 40 to 80%. Detrital K-feldspar records progressively deeper erosion from Early Eocene to Late Eocene time with a pronounced peak developing at 78 Ma in the Late Eocene. Based upon the detrital zircon provenance, these bulk cooling ages sample a broad region of the southern Sierra and northern Mojave region. This timing of the 78 Ma cooling episode coincides with underplating of Rand Schist beneath the Mojave and southern Sierran region. While this peak is still manifested by Early Miocene sandstone, a second peak developed at 90 Ma may record erosion of shallower crust in the more local western Mojave region. Work in progress includes incremental heating of individual detrital K-feldspar grains to recover more detailed thermal history information and a broad survey of basement thermochronology throughout the southern Sierra-Mojave Desert region.