ZIRCON (U-TH)/HE CONSTRAINTS ON THE EXHUMATION OF THE PELONA-OROCOPIA-RAND SCHIST AND IMPLICATIONS FOR THE LATE CRETACEOUS TO NEOGENE TECTONIC DEVELOPMENT OF SW NORTH AMERICA

A belt of schist exposed from southern Arizona to central California represents the best-known example of an exhumed shallow-flat subduction complex in the world. Geo-/thermochronologic data from southern schist exposures (i.e. the Pelona and Orocopia varieties) suggest that these rocks were exhumed in two major (Late Cretaceous– early Eocene and late Oligocene–early Miocene) pulses. Similar constraints from the northern half of the schist outcrop belt (i.e., the Rand and related schists) are lacking. We present 33 new zircon (U-Th)/He (ZHe) ages from schist exposed in the Sierra de Salinas, Portal Ridge, Tehachapi Range, and San Emigdio Range to compare the low-temperature history of “Rand-type” schist with the remainder of the outcrop belt and to evaluate the mechanism(s) by which the schist ascended from the base of the crust. Samples collected >5 km from major Neogene structures (e.g. the San Andreas and Garlock faults) yield inverse models and age-eU relationships compatible with a single pulse of Late Cretaceous cooling to the uppermost crust. Conversely, samples collected adjacent to these structures show a positive ZHe date-eU correlation requiring significant Late Cretaceous cooling to temperatures of ~150 °C plus a second phase of cooling to near-surface temperatures from 20-10 Ma. These results corroborate earlier data suggesting that the entire schist outcrop belt experienced significant Late Cretaceous – early Cenozoic cooling, previously attributed to some combination of subduction processes, buoyancy, and erosion. This work shows that the magnitude of cooling associated with this uplift pulse was greater than previously thought, with some localities ascending to within a few kilometers of the surface. In contrast, the pattern of late Cenozoic cooling is more localized, with significant cooling adjacent to major Neogene structures and relatively minor cooling at distances >5 km from these structures. We suggest that the Garlock, San Andreas, and related faults originated as early to middle Miocene extensional structures cored by schist and evolved in the late Miocene to crust-penetrating strike-slip faults as the plate boundary evolved from convergent to transform. We further suggest that the coeval development of the schist outcrop belt and Cordilleran metamorphic core complexes ~150 km inboard represent subparallel structural culminations resulting from plate margin processes (e.g. the impingement of the Pacific–Farallon ridge and the North American plate and/or Farallon slab foundering/removal).