Date of Award
Master of Science
Geological and Atmospheric Sciences
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. Within the Orocopia Mountains, the schist is separated from upper-plate Proterozoic to Late Cretaceous crystalline rocks by the Orocopia Mountains detachment fault (OMDF). The upper plate consists of two (leucogranite-gneiss and anorthosite-syenite) plates separated by the upper plate detachment fault (UPDF). U-Pb ages from detrital zircons indicate that the schist protolith is no older than 83 Ma. A U-Pb zircon age from leucogranite within the upper plate suggests that the schist is younger than 76 Ma. Currently, three interpretations exist for the OMDF: (1,2) it is a middle or early Tertiary normal fault or (3) an early Tertiary passive roof thrust. To help select between these models, we obtained 40Ar/39Ar ages from both schist and upper plate. 40Ar/39Ar total-gas ages from the schist are 50-54 Ma for hornblende, 34-52 Ma for muscovite, and 21-33 Ma for biotite. Ages from leucogranite-gneiss plate are 70-75 Ma for hornblende, 63-70 Ma for biotite, and 59 Ma for K-feldspar, while these from anorthosite-syenite plate are 136 Ma for hornblende and 158 Ma for biotite. The strong discordance in cooling history between schist and upper plate favors normal faulting over thrusting. The age discordance between two plates within the upper plate indicates that the UPDF represents a major structural discontinuity within this plate. Biotite ages from the schist indicate that the OMDF is no older than late Oligocene. In addition to the phase of exhumation of Orocopia Schist implied by movement on the OMDF, hornblende and muscovite ages indicate a prior cooling event in early Tertiary time. We attribute this early exhumation to movement along an excised fault that we correlate with the Chocolate Mountains fault in southeastern most California, but some of it could have occurred by erosion. Regionally, early Tertiary erosion has been linked to formation of the Chocolate Mountains anticlinorium. In the Orocopia Mountains, however, the OMDF is folded by this structure. Hence, in this area, the anticlinorium must be late Oligocene or younger, which is inconsistent with the passive roof thrust interpretation.
Vucic, Ana, "Multi-stage exhumation history of the Orocopia Schist in the Orocopia Mountains of southeast California" (2002). Retrospective Theses and Dissertations. 21347.