REGIONAL-SCALE DETACHMENT FAULTING AS A MECHANISM FOR CRUSTAL EXTENSION IN THE NORTHERN PORTION OF THE EASTERN CALIFORNIA SHEAR ZONE

Palinspastic reconstructions show that the northern portion of the Eastern California Shear Zone (north of the Garlock fault and west of the Spring Mountains) has undergone ~200% crustal extension since 16 Ma. However, the specific structures accommodating this extension are still unclear. Strain-rate models based on published fault slip rates fall far short of matching either the strain rates inferred from the geological reconstructions or those indicated by contemporary geodetic velocities. We use fault outcrop data, geodetic data, and earthquake hypocenter distributions to evaluate three conceptual models for northern ECSZ faulting. These are (1) traditional high-angle ‘Basin-and-Range’ block faulting, (2) a regional low-angle detachment, and (3) a two-episode model of low-angle faulting in the Miocene subsequently replaced by a modern system of high-angle strike-slip faults. Block normal faulting requires much deeper basins than are observed. Both geodetic and geological analyses indicate that dilation is currently underway and thus contradicts the modern shear-faulting model. However, hypocenter distributions are consistent with displacement on a regional low-angle detachment that crops out along the Death Valley fault system and dips northwest underneath the Sierra Nevada. This conceptual model is compatible with the various lines of evidence that we considered. Modern crustal extension in the region thus appears to be a perpetuation of the faulting style initiated in the Miocene, rather than an evolution from low-angle normal faulting to deep-seated strike-slip faulting.