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Description
The Agua Blanca fault extends across the western two-thirds of northern Baja California. Northwesterly from peninsular California, the Agua Blanca fault continues into the inner borderland where it comprises a major structural element of a late Quaternary zone of deformation that includes the Coronado Bank and Palos Verde Hills fault zones. Slip rate estimates, useful in seismic hazard evaluations, have not been confidently established for these offshore fault zones. The purpose of this study was to develop slip rate information for the Agua Blanca fault that can be used to infer the relative activity of the Coronado Bank fault. This study focused on evaluating the late Quaternary displacement history of a portion of the Agua Blanca fault onland. Mapping of Quaternary alluvial deposits based on soil development and geomorphic relationships indicates progressively greater dextral displacement with increasing age of alluvial deposits along the western reach of the Agua Blanca fault inland from Punta Banda, Baja California, Mexico. A chronosequence for soils developed in alluvium along the fault, calibrated by Carbon-14 dates and correlated to U-series dated soils at nearby Punta Banda, provides age control for laterally offset alluvial fans, terraces, and stream channels. The late Pleistocene-Holocene slip rate, based on the above information, is about 4.1-4.3 mm/yr. A slightly lower mid-Pleistocene to present rate of about 2.3 mm/yr is also suggested by the soils. This rate compares favorably with a previously reported rate of 4-6 mm/yr for the fault in Valle Agua Blanca, 50 km inland from Punta Banda. These data provide estimates of a slip rate for the Coronado Bank fault in the southern California borderland.
