END OF DETACHMENT FAULTING AND INITIATION OF STRIKE-SLIP FAULTS AT 1.0 MA, WESTERN FISH CREEK – VALLECITO BASIN, SOUTHERN CALIFORNIA

The West Salton detachment fault (WSDF) is a corrugated low-angle normal fault that evolved during late Cenozoic transtensional strain in the western Salton Trough. While the age of earliest slip on the WSDF is uncertain, recent studies show that slip on the central WSDF ended during a rapid transition to modern strike-slip faulting at 1.1-1.3 Ma (Lutz et al., 2006; Kirby et al., 2007; Steely et al., 2009). Until now the timing and nature of this transition was not well documented for the southern WSDF. New geologic mapping at the faulted western margin of the Fish Creek – Vallecito basin (FCVB) from the Tierra Blanca Mts to Whale Peak, combined with stratigraphic analysis of deposits in the hanging wall of the WSDF, provides insight into this critical tectonic change in this area. The Canebrake Conglomerate at Whale Peak (an antiformal corrugation in the WSDF) is cut by the WSDF and is age-equivalent to the upper part of the Hueso Formation (sst and pebbly sst), which ranges in age from 2.8 to 1.0 Ma. Coarsening of facies up-section and north toward Whale Peak, and a similar coarsening-up in the adjacent synformal corrugation to the south, records local progradation that may have resulted from accelerated footwall erosion after ~1.5-2 Ma. Paleocurrent data indicate a sediment-dispersal system generally similar to that of the modern drainages. The Hueso Fm and Canebrake Cgl are overlain along a progressive angular unconformity (fanning-dip interval) by a thin unit of calcareous sandy gravel (Mesa Fm) that contains well developed carbonate paleosols and represents a gravel bypass surface. The contact between the Hueso and Mesa formations has been dated with magnetostratigraphy at 0.99 Ma (top of the Jaramillo subchron; Dorsey et al., submitted). This contact records the end of subsidence and sediment accumulation in the FCVB, and thus the end of slip on the WSDF at 1.0 Ma. We infer that the end of detachment faulting and onset of basin tilting, uplift and erosion coincided with initiation of the Elsinore, San Felipe, Fish Creek Mountains, and San Jacinto dextral-oblique fault zones, which cross-cut and locally reactivate the WSDF. These results are consistent with other recent studies that document a rapid transition from detachment to strike-slip faulting at 1.1-1.3 Ma, and suggest that the change may be slightly younger (1.0 Ma) in the south.