MULTI-STAGE EXHUMATION HISTORY OF THE OROCOPIA SCHIST IN THE OROCOPIA MOUNTAINS OF SOUTHEAST CALIFORNIA

The Orocopia Schist of the Orocopia Mountains is part of the enigmatic Pelona, Orocopia, and Rand (POR) Schist terrane of southern California and southwestern Arizona that tectonically underlies North American continental basement. Within the Orocopia Mountains, the schist is separated from upper-plate Proterozoic to Late Cretaceous crystalline rocks by the Orocopia fault. U-Pb ages from detrital zircons indicate that the schist protolith is no older than 80 Ma. Schist far beneath the fault exhibits prograde metamorphism to lower amphibolite facies. That adjacent to the fault is mylonitic and retrograded to greenschist facies. Rocks in the upper plate of the fault (including pervasive leucogranite that yields a U-Pb zircon age of ~75 Ma) are not mylonitic, but exhibit a strong brittle overprint. Currently two interpretations exist for the Orocopia fault: (1) it is a middle Tertiary post-metamorphic normal fault or (2) an early Tertiary passive roof thrust. To help select between these models, we obtained ⁴⁰Ar/³⁹Ar thermal history results from both schist and upper plate. ⁴⁰Ar/³⁹Ar total-gas ages from the schist are 47-56 Ma for hornblende, 34-47 Ma for muscovite, and 18-33 Ma for biotite. Ages from the upper plate are 70-74 Ma for hornblende, 63-70 Ma for biotite, and 59 Ma for K-feldspar. The strong discordance in cooling history between schist and upper plate favors normal faulting over thrusting. Biotite ages from the top of the schist indicate the fault is no older than late Oligocene. In addition to the phase of exhumation of Orocopia Schist implied by movement on the Orocopia fault, hornblende and muscovite ages indicate a prior cooling (exhumation) event in early Tertiary time. We attribute both this early cooling and the mylonitic fabric at the top of the schist to movement along an excised fault that we infer was correlative with the Chocolate Mountains fault in ranges to the southeast. Erosional denudation may also have contributed to the early Tertiary phase of cooling. However, a major regional structure, the Chocolate Mountains anticlinorium, that has been previously linked to early Tertiary denudation must be late Oligocene or younger (within the Orocopia Mtns.) since it folds the Orocopia fault.