Abstract
Discrepancies in right-lateral strike-slip separation of pre-Tertiary and Tertiary geologic features along the northern San Andreas fault give rise to two contrasting views of the evolution of the fault system. One model entails two stages of right slip on the San Andreas fault, one Late Cretaceous–early Tertiary and one post-Oligocene. The other model requires significant right slip on multiple faults of the Neogene San Andreas fault system. Several lines of evidence now suggest that major Neogene right slip occurred on several faults of the San Andreas transform-fault system in addition to the San Andreas fault proper. Furthermore, these data indicate that the magnitude of proto-San Andreas right slip probably was far less than previously suggested.
Integration of the fault studies and regional geologic relations, especially in the central Salinian block, permits reconstruction of the tectonic evolution of the San Andreas fault system and the central California margin. A relatively modest amount of right slip on a proto-San Andreas fault, possibly the consequence of oblique plate convergence, emplaced granitic basement of the Salinian block west of the fault in Paleocene time. Following an initial rise-trench encounter in the Oligocene, migration of a triple junction from south to north past central California resulted in coordinate termination of subduction and propagation of the San Andreas transform-fault system. Initially, the Salinian block experienced extensional tectonism, perhaps related to the passage of the triple junction. Right slip on the San Andreas fault and on faults cutting the Salinian block probably began in middle Miocene time. Since a late Miocene change of pole of rotation of the Pacific-North American plate pair, motion has been increasingly absorbed on the San Andreas fault proper.
