REGIONALLY EXTENSIVE 85-70 MA COOLING OF THE SOUTHERN SIERRA NEVADA AND NORTHWEST MOJAVE DESERT BATHOLITH: IMPLICATIONS FOR THE INITIATION OF SHALLOW SUBDUCTION

Late Cretaceous shallow subduction extinguished mid-Cretaceous arc magmatism close to the California margin, refrigerated the batholith, and underplated its southern extent with trench-derived sediment (Rand Schist). We have acquired a regionally extensive K-feldspar thermal history data set (n= 115) from the southern Sierra Nevada and western Mojave basement. Collectively, this extensive database of K-feldspar ⁴⁰Ar/³⁹Ar thermal histories constrains the magnitude and timing of Laramide subduction refrigeration over a > 18,000 km² area. Modeled thermal histories document that the southwestern Sierra Nevada experienced rapid cooling from 85–80 Ma at rates that approached 70 °C/m.y. The southeastern Sierra Nevada/western Mojave cooled later (75–70 Ma) at maximum rates of 50 °C/m.y. Near-unimodal detrital K-feldspar ⁴⁰Ar/³⁹Ar total fusion age distributions and thermal histories measured from Eocene-Miocene forearc strata derived from the southern Sierra Nevada and broader Mojave region confirm subduction refrigeration extended into the shallow crust and affected a broad region of the Mojave Desert province. Exhumation of Rand Schist, exposed in the deepest structural depths throughout the region, was also diachronous. For example, deep-seated plutonic rocks and Rand Schist in the southwesternmost Sierra Nevada in the San Emigdio Mountains were rapidly exhumed between 90–80 Ma and reached the surface by the early Eocene. Alternatively, Rand Schist and upper plate granitoids exhumed along the Garlock fault up to 85 km east (i.e., Rand Mountains) cooled slowly throughout the early Cenozoic and persisted at mid-crustal depths (10–20 km) until 20 Ma. Exhumation of the schist in this easterly domain was accommodated by slip along the Rand detachment fault between 20 to 10 Ma. This extensional faulting appears to have been temporally and kinematically linked to initiation of the Garlock fault. Final exhumation during the Miocene was broadly related to Basin and Range extension and slab-window volcanism along the Garlock fault that collectively was triggered by northwestward migration of the Mendocino triple junction along the California margin.