STRUCTURAL AND METAMORPHIC EVOLUTION OF THE OROCOPIA SCHIST AND RELATED ROCKS, SOUTHERN CALIFORNIA - EVIDENCE FOR LATE MOVEMENT ON THE OROCOPIA FAULT

The Pelona, Orocopia, and Rand Schists (FOR schists) of southern Calif ornia and southwesternmost Arizona are late Mesozoic or early Tertiary subduction complexes that underlie Precambrian to Mesozoic continenta l basement along the low-angle Vincent-Chocolate Mountains (VCM) fault system. The VCM faults are often considered to be remnants of the ori ginal subduction zone, but recent work indicates that many have underg one substantial postsubduction reactivation. In the Orocopia Mountains , for example, the Orocopia Schist exhibits an exceptionally complex s tructural and metamorphic history due to multiple periods of movement along the Orocopia fault. Structures in the schist include isoclinal f olds with axial-planar schistosity, open-to-tight folds that fold schi stosity, penetrative stretching lineations, and crenulation lineations , all of which show a nearly 360 degrees range in trend. Folds and lin eations that trend approximately NE-SW occur throughout the schist and are thought to be part of an early phase of deformation related to su bduction. Folds of this orientation show no consistent vergence. Folds and lineations that trend approximately NW-SE are concentrated near t he Orocopia fault and are interpreted to have formed during exhumation of the schist. The NW-SE trending folds, and shear indicators in late -stage mylonite at the top of the schist, consistently verge NE. The e xhumation event culminated in emplacement of the schist against brittl ely deformed upper plate. Exhumation of the Orocopia Schist was accomp anied by retrograde replacement of garnet, biotite, epidote, and calci c amphibole by chlorite, calcite, and sericite. Matrix amphibole has a lower Na/Al ratio than amphibole inclusions in albite, consistent wit h a late-stage decrease in pressure. As NE vergence in the Orocopia Mo untains is associated with exhumation of the schist, the NE movement a long other segments of the VCM fault may also be late and therefore ha ve no bearing on the facing direction of the VCM subduction zone, cont rary to past interpretations.