A NEW STRUCTURAL MODEL FOR THE DEVELOPMENT OF THE CLEMENS WELL FAULT IN THE SAN ANDREAS FAULT SYSTEM

The NW-striking Clemens Well fault in the Orocopia Mountains is regarded as an early strand of the San Andreas system and may have accommodated up to 110-km right-slip motion between the North America and Pacific plates. However, controversies exist involving its kinematics and spatial extent. It has been reported that the fault records normal, reverse, and right-slip motion, leading to different interpretations for its origin (i.e., detachment vs. right-slip fault). Past geologic studies and our own analysis of Aster images also fail to trace the fault southward across the east-striking left-slip Salton Creek fault into the Chocolate Mountains. The latter observation has raised the question of whether the fault is truly a major strike-slip structure in the San Andreas system. Our recent detailed field mapping and kinematic analysis of both Cenozoic right-slip and left-slip faults in the Orocopia region indicate coeval development of the NW-striking Clemens Well fault, east-striking left-slip Salton Creek fault, and E-trending folds in the Eocene-Miocene strata sandwiched between the two strike-slip faults. However, field relationships also indicate that left-slip faulting outlasts right-slip faulting. To explain these field relationships, we propose a two-stage model. In the first stage, the Clemens Well fault is a conjugate structure of the Salton Creek fault. Motion on the two faults generated N-S contraction as expressed by E-trending folds. Because the folded block was deforming while the two strike-slip faults were active, it produces complex kinematics on both strike-slip systems (i.e., dip slip, strike-slip, and various partition of the two components). In the second stage, the Salton Creek fault extends further west and connects with the currently active San Andreas fault. It was during this stage that motion along the right-slip Clemens Well fault ceased. This model explains the complex kinematics of the Clemens Well fault, its abrupt termination at the Salton Creek fault, and proximal sources of conglomerates (within the Miocene sediments) across the Clemens Well fault. The model implies the Clemens Well fault has a similar amount of slip to that on the Salton Creek fault, which is on the order of 5-10 km.