2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 9
Presentation Time: 4:00 PM

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


JANECKE, Susanne U.1, KIRBY, Stefan1, LANGENHEIM, Victoria2, STEELY, Alexander N.1, DORSEY, Rebecca3, HOUSEN, Bernard4 and LUTZ, Andrew3, (1)Dept. of Geology, 4505 Old Main Hill, Utah State University, Logan, UT 84322-4505, (2)U.S. Geol Survey, 345 Middlefield Road, Menlo Park, CA 94025, (3)Geological Sciences, Univ of Oregon, Eugene, OR 97403-1272, (4)Geology, Western Washington Univ, Bellingham, WA 98225, sjanecke@cc.usu.edu

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).