EVIDENCE FOR TRANSTENSIONAL DEFORMATION AND TRANSFER OF NORMAL SLIP ALONG THE WESTERN SEGMENT OF THE AGUA BLANCA FAULT, BAJA CALIFORNIA, MEXICO

The Agua Blanca fault (ABF) is a ~120-km-long fault traversing Baja California, Mexico near latitude 31º30’N. The fault stretches from the Main Gulf of California Escarpment to the Pacific Ocean, where it connects with faults of the Continental Borderlands. The western third of the ABF trends WNW-ESE and stretches from north of Punta Banda ridge into Valle Santo Tomas. Farther east, the fault trend changes with the ABF oriented roughly E-W. Within the North American-Pacific transform plate boundary, faults with E-W and NW-SE orientations commonly show strong transpression and pure strike-slip displacement, respectively.

Mapping along the western segments of the ABF suggests dextral motion, which is supported by fault scarps, deflected stream channels and shutter ridges. A component of normal dip-slip motion is suggested by offset fluvial terraces and triangular facets. However, these are best developed along the ABF on the north side of Punta Banda ridge and are less well-expressed geomorphologically along in Valle Santo Tomas, where the dip-slip component appears to be primarily accommodated by the Santo Tomas Fault (STF) to the southwest of the ABF. However, a scarp with a maximum height of 2 meters along the ABF in the central part of the Santo Tomas segment suggests the possibility that the last event(s) were characterized by a component of dip-slip motion.

Several ground-penetrating radar grid surveys imaged the ABF in the shallow subsurface in Valle Santo Tomas. Here, various fault strands offset laterally continuous reflectors, which are interpreted as Quaternary deposits and Cretaceous volcanic bedrock, indicating a complex fault structure. Small, normal offsets and a minor negative flower structure support modest normal displacement along the ABF in Valle Santo Tomas.

The observation of normal dip-slip displacement on the ABF along the Santo Tomas segment of the fault suggests the possibility that the extension formerly accommodated by the STF is being transferred to the ABF. This is further supported by a lack of evidence for a recent (<10,000 years?) slip event on the STF. Alternatively, normal displacement along the ABF may be rare and continues to be mostly accommodated at a slow rate along the STF. In that case, the geomorphic expression of dip-slip motion on the ABF is suppressed.