UNROOFING OF THE PELONA-OROCOPIA SCHIST AND PALINSPASTIC RECONSTRUCTION OF SOUTHERN CALIFORNIA

Useful palinspastic reconstruction of southern California is dependent on successful cross-fault correlations between source rocks and derived sedimentary clasts. Deposition of oldest strata with known clasts of Pelona-Orocopia Schist (POS) was coincident with capture of the Monterey microplate (20Ma) and initiation of transrotation of the western Transverse Ranges. Thus, unroofing of the POS provides important paleogeographic constraints on locations of extensional terranes that were later offset by younger segments of the San Andreas fault system.

Unroofing of the POS provides useful piercing lines for several reasons: 1. the stratigraphically lowest clasts to highest clasts are: the Vasquez and San Francisquito formations, which overlie plutonic/gneissic basement of the upper plate, chloritic breccia and mylonite from the detachment fault, and POS clasts; 2. conglomerate clasts of these rock types are readily identifiable in the field; 3. sandstone petrofacies act as proxies for these rock types in the absence of conglomerate; 4. stratigraphic intervals containing these unroofing sequences are geographically and chronostratigraphically limited; 5. unroofing events happened within a narrow time frame (20-17Ma).

Sites chosen for study range from the Chocolate Mountains to the big bend in the San Andreas fault. Preliminary results show that the Punchbowl Formation should be differentiated from the Paradise formation, found near the hamlet of Paradise. The Paradise formation probably correlates with the Tick Canyon Formation of the Soledad basin, based on unroofing of the POS and stratigraphic position. The Paradise formation consists of 210m of alluvial conglomerate and sandstone. Both sandstone petrofacies and conglomerate clast counts confirm the unroofing sequence. Conglomerate imbrications indicate transport from the NW.

Reversal of 42 km of dextral slip on the Punchbowl fault places the Paradise formation SE of an area of granitoid basement overlain by San Francisquito formation. The granitoid basement structurally overlies the Pelona Schist along the San Francisquito fault. Thus, the unroofing sequence and paleocurrents of the Paradise formation are consistent with its derivation from terrane across the Punchbowl fault.