GEOLOGIC MAPPING AND GROUND-PENETRATING RADAR SURVEYS OF THE VALLE SANTO TOMAS SEGMENT OF THE AGUA BLANCA FAULT, BAJA CALIFORNIA, MEXICO

The Valle Santo Tomás is situated on a segment of the Agua Blanca fault in Baja California, Mexico. The fault is a ~120 km WNW-ESE trending fault within the Pacific-North American plate boundary system. Although there is no historic seismicity along the Agua Blanca fault, the fault is thought to accommodate displacement within the Pacific-North American plate boundary system. In Valle Santo Tomás, geologic, geomorphic and ground-penetrating radar surveys were conducted to evaluate the fault segment for features that demonstrate recent movement. Features such as diverted stream channels, fault scarps and a shutter ridge were observed. Results of the ground-penetrating radar surveys produced profile images of the fault in the shallow subsurface. These data suggest that various fault strands offset Cretaceous bedrock and Quaternary alluvial units. In addition, we document small, rotated hanging wall blocks and a negative flower structure, indicating recent, albeit minor transtensional movement along the Valle Santo Tomás segment of the Agua Blanca fault. In summary, our observations indicate recent movement along the Valle Santo Tomás segment of the Agua Blanca fault. Movement appears to be transtensional, which is unusual for faults within the Pacific-North American plate boundary system trending in a WNW-ESE orientation.