BASEMENT, BASINS, AND TECTONICS: EARLY EVOLUTION OF SAN ANDREAS FAULT SYSTEM IN SOUTHERN CALIFORNIA

Evolution of the San Andreas Fault system in southern California led to disruption of a basement terrain comprising an array of distinctive crystalline rock bodies. Deforming basement blocks shed debris into adjacent syn-tectonic basins that later were deformed, inverted, and subsequently replaced by successor basins as blocks shifted along the evolving plate-margin fault system. Earliest plate-margin fault-related basins formed during Oligocene and early Miocene and accumulated predominantly volcanic-volcaniclastic sequences in basins in southeasternmost California and predominantly sedimentary sequences in the Plush Ranch, Diligencia, and western Vasquez basins farther northwest. Paleogeologic reintegration achieved by simultaneous restoration of crystalline basement units, basinal lithostratigraphic packages, and clast provenance connections reveals much about deformational style, magnitude, and evolution of faulting phases. Sequential restoration of San Andreas displacements—including 160-180 km on the San Andreas Fault, 40-45 km on the San Gabriel-San Andreas Fault northwest of Saugus, and 100 km on the Clemens Well-Fenner-San Francisquito-San Andreas (CW-SA) Fault (Powell, 1993)—is only part of this history. Prior to and during movement along the CW-SA Fault, a structural domain between the proto-eastern Transverse Ranges block and the proto-San Gabriel Mts-Frazier Mtn block defined a broad zone, eventually including the CW-SA Fault, that underwent greater dextral shear than the bounding proto-blocks. In this zone, transtension was accompanied by significant clockwise rotation and internal disruption. The age of volcanism indicates that transtension, accompanied by hyperextension in the Chocolate-Orocopia Mts region, began before inception of the CW-SA Fault. Coevolving NNW-trending right-oblique faults likely contributed to this transtensional deformation and continued moving after slip on the CW-SA Fault ceased. The earliest evidence for right slip is represented by a displacement of about 25 km that occurred along a pre- to syn- Diligencia-Plush Ranch fault, which projects northwestward toward the Russell Fault (Yeats and others, 1989) and may be the southeastward continuation of that fault.