CONSTRAINTS ON THE CENOZOIC EXHUMATION HISTORY OF THE SOUTHERN SIERRA NEVADA AND NW MOJAVE DESERT REGION USING BASEMENT AND DETRITAL THERMOCHRONOLOGY

Early Eocene to Early Miocene forearc strata of the San Emigdio Mountains (SEM) were deposited outboard of the southernmost Sierra Nevada and Mojave Desert region. We have acquired coupled ⁴⁰Ar/³⁹Ar age and Pb isotopic data from single detrital K-feldspar grains from three stratigraphic levels (Uvas, Basal San Emigdio, and Temblor Fms.) to constrain basement thermal evolution and source region provenance. Detrital K-feldspar ⁴⁰Ar/³⁹Ar total fusion of 805 single grains reveal tightly overlapping age distributions with 94% of the results between 72-93 Ma. Additional step-heated single grains from these samples confirm that the total fusion ages reflect rapid cooling throughout the source region at ca. 86-73 Ma. The plutonic and cratonal rocks of the southwestern United States that potentially contributed sediment to the SEM can be subdivided into at least three Pb isotope provinces. To determine the regions from which the sediments derive, Pb isotopic compositions were measured from fused K-feldspars via LA-ICP-MS. All three formations are dominated by detritus with Pb isotopic values consistent with derivation from the Sierra Nevada and western Mojave province (Pb values plot on linear trend with ²⁰⁸Pb/²⁰⁶Pb between 1.98-2.12; ²⁰⁷Pb/²⁰⁶Pb between 0.8-0.86). However, the two Eocene samples also contain a number of K-feldspars that yield elevated ²⁰⁸Pb/²⁰⁶Pb values, characteristic of the eastern Mojave and west-central Arizona. The fact that the extraregional K-feldspars tend to yield ca. 80 Ma cooling ages demonstrates the large spatial dimensions of the region affected by Laramide cooling. These age distributions show that the southernmost Sierra Nevada and greater Mojave Desert region was deeply denuded by Early Eocene time. We interpret the K-feldspar ⁴⁰Ar/³⁹Ar age and Pb isotopic results to record an extraregional-to-local provenance shift from the Eocene to Miocene that accompanied the transition from Laramide shallow subduction to slab gap magmatism. Work in progress will determine the extent of the extra-regional sources, including a broad survey of basement feldspar thermochronology and common Pb throughout southern Sierra-Mojave Desert region.