HIGH GEOLOGIC SLIP RATES ON THE SAN JACINTO FAULT ZONE IN THE SW SALTON TROUGH, AND POSSIBLE NEAR-SURFACE SLIP DEFICIT IN SEDIMENTARY BASINS

New data from the Coyote Creek and Clark strands of the San Jacinto fault zone, SW Salton Trough, indicate average geologic slip rates of ~5 mm/yr and ~10 mm/yr, respectively, and a minimum summed slip rate for the fault zone of ~15 mm/yr during the last 0.5 Ma. Preliminary data further suggest that near surface estimates of slip in Plio-Quaternary basinal deposits may be systematically lower than estimates from offset crystalline rocks. Analysis of fault-related folds in Pliocene to Pleistocene sedimentary rocks in the San Felipe Hills shows that the Clark strand slipped at least 5.6 km since 0.5 to 0.6 Ma for a slip rate of ~10 mm/yr. This slip amount may reflect only a third of the total right separation of 15 to 18 km measured in offset marbles and mylonites 15-20 km farther NW. Mapping along the Coyote Creek fault suggests a similar discrepancy between offset crystalline rocks and offset late Cenozoic basin fill deposits. At Borrego Mountain, a tilted marker bed low in the Pliocene Diablo Formation shows 1-2 km of right separation across the Coyote Creek fault zone. This is less than half the value determined from gravity analysis of offset crystalline rocks in the core of the E-W trending San Felipe anticline in the same area. Gravity data show similar right separations on the north and south limbs of the anticline of 2.8 to 4.7 km, with a preferred value of 4.1 km. This right separation is nearly identical to a revised estimate based on displaced crystalline rocks west of Coyote Mtn, 20 km to the NW. There the margin of a distinctive hornblende-bearing mafic tonalite coincides with a thin marble, overlying thick migmatite and mylonite. This assemblage is displaced 3.5 to 4.8 km across the Coyote Creek fault with a right separation of 4 km. Reinterpretation of sedimentary deposits within the Coyote Creek fault zone (Dorsey 2002) show that this fault has been active since ~ 0.8 to 1.0 Ma, for a slip rate between 4 and 6 mm/yr. The summed slip rate of ~15 mm/yr across the San Jacinto fault zone may be underestimated if there is a significant near-surface slip deficit, as has been documented along other strike-slip faults (Fialko et al., 2005). Our geologic data sets also show that cross cutting dextral strike-slip faults initiated in the SW Salton trough at ~1 Ma (Kirby 2005; Lutz, 2005; Steely et al., this meeting; this study).