TRANSTENSION ALONG THE AGUA BLANCA FAULT: IMPLICATIONS FOR BLOCK MOTIONS AND REGIONAL KINEMATIC VARIATIONS WITHIN THE BIG BEND DOMAIN OF THE PACIFIC-NORTH AMERICAN PLATE BOUNDARY

The Agua Blanca Fault (ABF) of northern Baja California, Mexico is a prominent transverse structure through the Baja Peninsula that transfers plate boundary strain from the Gulf of California on the east into the Continental Borderlands (CB) on the west thereby circumventing the “Big Bend” along the San Andreas Fault in southern California. Given its structural context the ABF should be characterized by a significant component of contractional dip-slip motion. However, the ABF is uniquely characterized by nearly pure strike-slip displacements along the east-west trending eastern portion and an increasing component of dip-slip motion along western segments where its trend becomes more northwesterly. These kinematics along the ABF are well-defined in the geologic as well as recent geodetic records, suggesting that the series of fault-bound blocks to the north of the fault are moving eastward relative to a stable Baja reference frame. The net effect is to connect regions of high extension in the Gulf of California with those in the northern CB. The nearly coincident location of the ABF’s active trace with a Cretaceous-age structure suggests that the particular kinematics of the region is heavily influenced by the presence of this pre-existing structure.