Down-Dip Segmentation of Strike-Slip Fault Zones in the Brittle Crust

Outcrop-scale field evidence from the southern San Jacinto fault zone in southern California indicates that bedding-parallel slip may be an important mechanism for transferring and accommodating slip within strike-slip fault zones that cut across heterogeneous stratigraphy. In outcrop, steeply-dipping strike-slip fault segments exposed in conglomeratic sandstones abruptly terminate at weaker shale layers where slip occurs on bedding-parallel planes. The distribution of seismicity within the youthful southern San Jacinto fault zone is consistent with this compelling outcrop-scale evidence of vertically-restricted fault segments. Cross sections of fault-related seismicity reveal that events generally cluster within tabular, steeply-dipping fault-parallel streaks that are limited in extent in the down-dip direction. The depths of the down-dip terminations of shallow streaks tend to be roughly coincident with the depths of the up-dip terminations of deeper streaks. These configurations define step-over geometries that range in character from sharp to diffuse and imply down-dip fault segmentation and related kinematics. The similarities between geometries observed in the field and inferred in seismicity data suggest that down-dip segmentation within strike-slip fault zones may be scale-independent and may not be uncommon.