KINEMATICS OF THE WESTERN AGUA BLANCA FAULT AND THE WESTWARD DRIFT OF THE BAJA MICRO-PLATE

The Agua Blanca Fault (ABF) is the southernmost fault in the San Andreas system (SAS) of faults that accommodate the transfer slip around the “Big Bend” domain in the San Andreas in southern California and northern Mexico. The ABF transfers 2-6 mm/yr out of the Gulf of California, across the Baja Peninsula, and into the Continental Borderlands fault system that continue along the Pacific coastline. Most faults of the SAS that strike east-west are transpressional with a significant component of contractional dip-slip and those that strike NW-SE, like the San Andreas itself, are dominated by strike-slip motion. Such kinematics arise from large scale plate motions such that maximum compressive stress is oriented nearly north-south.

From west to east, the ABF can be divided into five segments including the Maneadero, Santo Tomas, Agua Blanca, Canon Delores, and Valle Trinidad segments. From its eastern terminus to the transition between the Santo Tomas and Agua-Blanca segments, the ABF is oriented nearly east-west. West of this point the ABF splits into two branches each trending NW-SE. Slip on the ABF in the Agua Blanca and Canon Delores segments is dominantly strike-slip, while that in the two western segments is characterized by a significant component of normal dip-slip. The most salient evidence of fault-perpendicular extension is the presence of relatively large extensional basins associated with these segments of the fault. The observed departure of the overall kinematics of the ABF from those of the greater SAS suggests that the motion of the Baja micro-plate located south of the ABF is distinct from that of the Pacific Plate and the micro-plate located north of the ABF and south of the Transverse Ranges. Specifically, the kinematics described above suggest that the Baja micro-plate is drifting westward, perhaps as a result of gravitationally-induced stresses at the transition between it and the Pacific Plate.