TRANSROTATIONAL STRATA OF THE TOPANGA GROUP, SOUTHERN CALIFORNIA: KEY TO UNDERSTANDING EXHUMATION OF THE CATALINA SCHIST AND ROTATION OF THE WESTERN TRANSVERSE RANGES

The Lower to Middle Miocene Topanga Group (TG) of southern California provides important constraints for models of microplate capture and transrotation of the western Transverse Ranges (wTR). The TG accumulated in fault-controlled basins east of, south of and on top of the transrotating wTR. Part of the previously accreted late Mesozoic-Paleogene subduction complex (Catalina Schist), which was attached to the rotating block, was pulled from beneath the Peninsular Ranges batholith along the east-dipping Oceanside detachment fault (Odf). The northern part of the breakaway zone for the Odf was along what is now the south side of the wTR; the southern part of the breakaway zone was along the East Santa Cruz Basin fault zone, the present boundary between the Inner Borderland and the Outer Borderland. The surface expression of major slip switched from convergence at the trench during subduction to divergence at the breakaway zone. All of the Coast Ranges ophiolite and overlying forearc strata that were part of the upper plate during subduction were transferred to the lower plate during exhumation along the Odf. Synextensional complexly interstratified siliciclastic and rift-related volcanic strata define multiple depocenters in sub-basins characterized by rapid lateral and vertical facies changes. Palinspastic reconstruction of TG sub-basins suggest a network of coeval normal and transfer zones that define multiple deforming blocks that moved along complex arc paths during clockwise transrotation. The magnitude of extension increased southward away from the pivot zone near the northern (now eastern) edge of the wTR. East of the Odf, Peninsular Range batholithic basement overlain by the inner part of the Cretaceous-Paleogene forearc basin was extended, intruded and subsided as part of the upper plate (hanging wall). West of the surface expression of the detachment fault, the lower plate (footwall) consisting of middle-to-outer-forearc strata with underlying ophiolite and subduction complex was isostatically uplifted as the crust was thinned. Local footwall uplift resulted in sequential exposure and erosion of forearc strata, their underlying greenschist-facies arc-related basement and/or ophiolitic basement, and finally blueschist facies Catalina Schist.