Offset along the San Andreas fault of Eocene strata from the San Juan Bautista area and western San Emigdio Mountains, California

Abstract

Analyses of Eocene strata that rest on gabbroic basement rocks in the western San Emigdio Mountains and northernmost Gabilan Range indicate that the 2 sequences have been offset right-laterally 305–320 km along the San Andreas fault. In both areas, basal shallow-marine conglomerate of Penutian age grades upward into shale of Ulatisian and Narizian age that was deposited at bathyal water depths in a basin with access to open ocean. Additional similarities in the upper Eocene and Oligocene stratigraphic units suggest original contiguity of the depositional basins. The inferred offset indicates that the Salinian block was attached to western California by at least early Eocene time. The longer distances of northward transport of the Salinian block that are suggested by preliminary paleomagnetic data from upper Mesozoic granitic rocks and Upper Cretaceous and Paleocene strata must have occurred during Paleocene or earlier time.