A“HIDDEN” FAULT? STRUCTURAL GEOLOGY OF THREE SEGMENTS OF THE CLARK FAULT, SAN JACINTO FAULT ZONE, CALIFORNIA:

We mapped and analyzed complexities of the Clark strand of the San Jacinto fault to better document its southeast extent and style of deformation. The Clark fault is a major fault with a lifetime slip rate of ~14.3± 0.7 mm/yr, yet its surface traces in the Salton Trough are poorly known. The fault's deformation zone widens southeastward from 1-2 km in the crystalline rocks of the Santa Rosa segment to ~ 18-km in the sedimentary rocks of the Salton Trough. The Clark fault continues ~ 25-30 km to the southeast of the termination point postulated by Dibblee (1954) and Sharp (1967), and 18 km beyond Sanders' (1989) Arroyo Salada segment, in agreement with Kirby et al. (2007). The Clark fault splays at the SE end of the Santa Rosa segment. The adjacent Arroyo Salada segment has abundant NW-striking scarps, is ~ 11-12 km long, and forms a complex web of faults and folds. A structural boundary at its southeast end consists of sinistral strike-slip faults that block the simple SE continuation of the fault zone. Southeast of the sinistral boundary we define the 12-13 km long San-Felipe-Hills structural segment with 4 subparallel deformation zones. Microseismicity broadens in plan view to the SE while narrowing with increasing depth from 12 km wide at 3 km depth to roughly 4±1 km wide below ~8 km depth. The Arroyo Salada segment contains two major NW-striking seismic alignments with a right-stepping geometry. Many alignments directly correlate to the central traces of the Clark fault zone. Other well-defined alignments do not correlate with the surface geology and require decollements within 5 km of the surface. Areas of few earthquakes suggest that the Truckhaven, San Felipe Hills, and West Calcite Mine Hill faults are seismically locked or inactive. NW-striking seismic alignments dip steeply towards the northeast whereas surface exposures of dextral faults dip southwest, subparallel to bedding along flats in the faults. We infer a dip reversal along these faults roughly at the basement-cover contact (< 3 km). Better documentation of faults, folds, scarps, microseismicity and surface deformation in the San Jacinto zone provides insights into diffuse deformation within mud-rich sedimentary basins, and illustrates how the high seismic potential and hazard of such fault zones may be “hidden” from conventional methods of analysis.