THERMOCHRONOLOGIC AND FAULT KINEMATIC CONSTRAINTS ON THE TIMING OF DEVELOPMENT OF THE EASTERN CALIFORNIA SHEAR ZONE IN SOUTHEASTERN CALIFORNIA

Relative transform motion across the Pacific-North America plate boundary is predominantly accommodated by the San Andreas fault system, with a lesser but substantial portion occurring within a broad zone of distributed deformation known as the Eastern California shear zone (ECSZ). The ECSZ can be divided into a western zone of active deformation (modern ECSZ) and an eastern belt that is largely inactive (paleo-ECSZ). The modern ECSZ currently accommodates 20-25% of the relative Pacific-North America plate motion, but there are few constraints on the onset of faulting and kinematic evolution of the paleo-ECSZ, where previous models indicate both early (>10 Ma) and late (<5 Ma) initiation of deformation in this region. In and around Picacho State Recreation Area in southeastern California, metamorphic basement rocks include, from structurally lowest to highest, the Orocopia schist, a mafic orthogneiss unit, and the Winterhaven Formation. These units are in turn overlain by 23 Ma volcanic rocks and the <23 Ma to <9.8 Ma Bear Canyon conglomerate. Collectively these units were emplaced and deformed during evolution and development of the paleo-ECSZ. This study will use a combination of mapping, geo/thermochronology, and fault kinematic analysis to document the timing and nature of transition from NE-SW extension during early evolution of the transform margin to NS shortening and EW extension that is characteristic of the ECSZ. Samples were collected from the three metamorphic basement units for apatite and zircon (U-Th)/He thermochronology and modeling. These data and modeling efforts will be used to constrain the latest pulse of extensional exhumation in this region. Preliminary fault kinematic data from the overlying Bear Canyon conglomerate indicate that this unit was predominantly shortened in a NS direction and extended EW. Although work is ongoing, these data thus far suggest that early development of the paleo-ECSZ in southeastern California occurred prior to 9.8 Ma and support previous models for early initiation of the ECSZ. Newly-collected thermochronologic data will also be compared to existing zircon (U-Th)/He data from other exposures of Pelona-Orocopia-Rand schist for a regional analysis on the timing and mechanisms of exhumation.