Exhumation of the Pelona-Orocopia-Rand and related schist

Alan Chapman, Macalester College
Jason Ricketts, University of Texas at El Paso

Abstract

A belt of schist exposed from southern Arizona to central California represents the best-known example of an exhumed shallow-flat subduction complex. 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. Constraints on the low-temperature thermal evolution 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, and Tehachapi and San Emigdio ranges 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 compatible with a single pulse of Late Cretaceous cooling to the uppermost crust. Conversely, samples collected adjacent to these structures require 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 reveals that the pattern of late Cenozoic cooling is far 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).

Session

Session 1: Fault Zones from Top to Bottom