Paleomagnetic and structural analyses of the kinematics of the San Gregorio Fault Zone, California

The San Gregorio Fault Zone (SGFZ), a major component of the San Andreas transform boundary in central California, has accommodated over 100 km of right-lateral offset since Miocene time. Sea cliffs provide essentially 100% exposure along two cross sections of the fault zone, and this exposure has allowed for a detailed study of the kinematics of the fault zone.;A paleomagnetic analysis of the Neogene Purisima Formation and Santa Cruz Mudstone within and around the SGFZ was conducted to determine the locations and magnitudes of vertical axis rotations. The paleomagnetic study showed that vertical axis rotations of 40;A study of the faulting within and around the SGFZ showed that the fault zone consists of several distinct zones of deformation, with Quaternary horizontal extension often occurring in close proximity to Quaternary horizontal shortening. The patterns of extension and shortening were compared with the deformation expected at the corners of a rotated block. Such comparisons resulted in the tentative identification of at least two individual blocks. In both cases, the structures are most consistent with blocks that span the width of the fault zone and that are elongate by a factor of roughly 2:1. Eastward-dipping faults at 3-4 km depth within the SGFZ may have effectively detached the blocks from the underlying material, making the rotations possible and accounting for the observed eastward tilts of the identified blocks.;Finally, an analysis of the rotation magnitude as a function of age indicates that the modern SGFZ became active at approximately 3 Ma, suggesting that the SGFZ may have formed in response to a change in the relative motions of the Pacific and North American plates at about 3.5 Ma.