QUATERNARY RECORD OF INITIATION, DISPLACEMENT, AND EROSION IN THE SAN JACINTO FAULT ZONE

The San Jacinto fault zone (SJFZ) is generally believed to be young, but its actual age of initiation has not been directly constrained in previous studies. Stratigraphy and geomorphology can be used to reconstruct the history of surface deformation, stream reorganization, and erosion that resulted from fault zone initiation. In the Coyote Badlands a well exposed section of sandstone and conglomerate contains the 760-ka Bishop Ash and reveals: (1) complete reversal of paleocurrents from NW-directed (opposed to modern drainages) to SE-directed (consistent with modern drainages); (2) fanning dips and a progressive unconformity bounded by the Coyote Creek and Box Canyon faults; and (3) deep erosion and incision of the entire section (>600 m thick). In the Anza area, the same-age stratigraphy is variably exposed in a region of active headward erosion and ongoing stream captures: Anza and Terwilliger Valleys have not yet been exhumed, but Horse Canyon and Santa Rosa Reservation reveal fine-grained valley-fill deposits that are deeply incised at their edges along fault-controlled Coyote Creek and its steep tributaries. These data permit reconstruction of the regional stream system that existed during early Pleistocene time, prior to initiation of the SJFZ. Low-gradient streams flowed across an irregular landscape of weathered plutonic and metamorphic rocks, originating near the crest of the San Jacinto and Santa Rosa Mts and flowing west to the Pacific coast. Topography was controlled by footwall uplift on the western Salton Trough detachment fault. Initiation of the SJFZ between ~1.0 and 0.8 Ma ruptured the former Peninsular Ranges divide and created steep new drainages to the Salton Sea. This produced rapid drainage diversion followed by erosion and stream captures that are now propagating NW toward Anza. Total offset on the SJFZ is 20-21 km, which gives a time-averaged slip rate of ~20-25 mm/yr. This is faster than previous estimates and may help motivate a re-evaluation of the long-term slip budget for the southern San Andreas fault system.