EXHUMATIONAL HISTORY OF THE PELONA-OROCOPIA SCHIST AND DEVELOPMENT OF UPPER-PLATE STRUCTURES OF THE OROCOPIA FAULT, SOUTHEASTERN CALIFORNIA

The Pelona-Orocopia Schist of southern California represents a graywacke-dominated, oceanic assemblage that was tectonically underplated during low-angle subduction of the Farallon plate beneath the North American craton during the Laramide orogeny. Although the transition from flat-slab subduction to a strike-slip boundary between the Late Cretaceous and late Tertiary provides a general tectonic framework, the structural processes responsible for exhumation of the schist remain debated. Detailed geologic mapping and thermochronologic analysis in the southeastern Orocopia Mountains have been performed to evaluate the timing of deformation and stress regimes in the hanging wall of the Orocopia fault to understand the exhumation of the Pelona-Orocopia schist. The Orocopia fault (OF) is broadly folded with a top-east sense of slip, which separates an antiformal schistose footwall from a northwest-plunging synformal crystalline hanging wall. Footwall rocks yield 40Ar/ 39Ar total-gas ages of 41 to 22 Ma for biotite and 53 to 41 Ma for muscovite. The hanging wall cooled significantly earlier (71 Ma for biotite and 112 Ma for hornblende). Hence, the OF juxtaposes rocks with different cooling histories and truncates all structures in the upper plate. Folded hanging-wall rocks and faults are cut by the Clemens Well fault (CWF), a high-angle northwest-striking fault with kinematics that change from reverse-right-oblique in the southeast to normal-right-oblique slip to the northwest. Cooling ages in basement rocks northeast of the CWF are radically different (653-291 Ma for biotite) than for hanging-wall rocks of the OF, indicating large displacement across the CWF. Rapid cooling of K-feldspar 500 m southwest of the CWF around 23 Ma indicates that the CWF was already active by this time. Because the CWF cuts the terrestrial Diligencia Formation (20 Ma U-Pb zircon age of an ash bed in an upper unit), the fault must have remained active after this time. Exhumation of the Pelona-Orocopia schist shows both early Tertiary cooling, possibly related to emplacement of the schist associated with erosion, and mid-Tertiary cooling driven by normal faulting along the OF. The final exhumation of the schist was related to late Tertiary folding during transpressional motion on the CWF and the San Andreas fault.