LATE CRETACEOUS TO PALEOGENE THERMAL AND TECTONIC EVOLUTION OF SOUTHERN CALIFORNIA-SOUTHWESTERN ARIZONA RELATED TO LOW-ANGLE SUBDUCTION

Recent thermochronologic results (particularly K-feldspar ⁴⁰Ar/³⁹Ar MDD analysis) from the southern Sierra Nevada and northwest Mojave Desert of California indicate eastward-prograding, rapid cooling of magmatic arc crust at 90–75 Ma (Shulaker, 2019). This cooling is attributed to refrigeration during underplating of the Rand Schist subduction complex, exposed beneath the arc crust by early Miocene detachment faults. The Rand Schist exhibits maximum cooling ages close to those in adjacent upper plate and to the inferred time of underplating. Subduction refrigeration of North American arc crust (hereafter “upper plate”) also seems likely in the central and eastern Transverse Ranges of southern California (TR) and in southeasternmost California-southwestern Arizona (SEMCA-SWAZ) related to underplating of younger equivalents of the Rand Schist (Pelona and Orocopia Schists). White mica and biotite cooling ages from mid-crustal levels of the upper plate in the TR are commonly ca. 70 Ma, nominally coeval with underplating of the Pelona Schist. In contrast, the oldest white mica ages from the Pelona Schist are ca. 60 Ma, with biotite and K-feldspar ages from the schist as young as late Oligocene to early Miocene. The schist ages imply some conductive reheating of the deepest crustal levels following flat subduction. In SEMCA-SWAZ, underplating of Orocopia Schist occurred at ca. 70–65 Ma; i.e., about the same time as emplacement of Pelona Schist in the TR. However, cooling ages for SEMCA-SWAZ are significantly younger than those in the TR, typically 60 Ma for white mica, biotite, and K-feldspar from mid-crustal parts of the upper plate and <45 Ma for the same minerals from Orocopia Schist and structurally deep upper plate. Moreover, some schist samples from SEMCA-SWAZ exhibit metamorphic zircon growth at 65–40 Ma. These results indicate either lesser subduction refrigeration or greater reheating in SEMCA-SWAZ. This region may have been situated above a hinge in the Farallon plate, where the plate descended from 30–40 km depth beneath the former magmatic arc to mantle depth beneath the craton. During emplacement of the older Rand Schist, the hinge was likely situated farther outboard, beneath the western to central Sierra Nevada.