LATEST CRETACEOUS THROUGH CENOZOIC EXHUMATION HISTORY OF THE SOUTHERN SIERRA NEVADA AND NORTHWESTERN MOJAVE DESERT REGION USING BASEMENT AND DETRITAL THERMOCHRONOLOGY

Laramide flat-slab subduction extinguished Sierra Nevada arc magmatism as trench-derived sediments (i.e., Rand and related schists) were underthrust and accreted at 25-30 km depth beneath the southernmost Sierra Nevada and NW Mojave Desert during the Late Cretaceous. The timing and mechanisms of latest Cretaceous-early Cenozoic underthrusting and subsequent exhumation, whether by erosion and/or tectonic denudation, of these deep rocks remains controversial. The distinctive structural and thermal evolution of the Mojave region has produced unique isotopic and thermochronologic fingerprints that can be used to decipher the Cenozoic slip history of the North American-Pacific Plate boundary. We have acquired coupled ⁴⁰Ar/³⁹Ar age and Pb isotopic data from single detrital K-feldspar grains from latest Cretaceous to Early Miocene strata from the Santa Lucia Range (SLR) in the Salinian block and the San Emigdio Mountains (SEM) in the southernmost Sierra Nevada batholith (SNB) to constrain basement thermal evolution and source region provenance of the region affected by shallow subduction. ⁴⁰Ar/³⁹Ar total fusion of 805 detrital K-feldspars from the SEM yield tightly overlapping age distributions between 72-93 Ma. Step-heated single K-feldspars from these sandstones confirm that regionally extensive rapid cooling of the southern Sierra Nevada and Mojave crust occurred at ca. 80 ± 5 Ma. To constrain the spatial distribution of basement rocks that contributed sediment to the SLR and SEM forearc, we have measured K-feldspar Pb isotope values from a regional suite of 150 plutonic samples. These new results combined with those of prior studies indicate that our single grain K-feldspar Pb isotope data from SLR and SEM forearc strata were transported by Paleocene to Eocene rivers from the eastern SNB and eastern Mojave Desert. In addition, the SLR and SEM forearc records an Eocene-Miocene extraregional-to-local provenance shift associated with the transition from Laramide shallow subduction to slab gap magmatism in the western Mojave. This newly expanded K-feldspar Pb isotopic database for the southern SNB and Mojave region can be used to evaluate models that identify this region as the source for sediments present in southern British Columbia and northwestern Washington (e.g., Mojave-BC).