A COMPLEX FAULT NETWORK IN AN ACTIVE ZONE OF DISTRIBUTED RIGHT-LATERAL SHEAR, COYOTE MOUNTAINS, SOUTHERN CALIFORNIA

The Coyote Mountains, located 16 km north of the Mexican border, form a NW-trending hook-shaped range at the interface of the Peninsular Ranges and Colorado Desert geomorphic provinces. The steep linear SW-facing range front was created by uplift along the active right-lateral Elsinore fault (EF), a strand of the San Andreas system. All published geologic maps of the range depict the trace of the EF and interpret its displacement in essentially identical ways. But the complex network of faults within the range has been inconsistently mapped and interpreted, in part because the abundant steep buttress unconformities are easily mistaken for faults and in part because the patchy original distribution of sedimentary and volcanic units makes it difficult to determine the sense of fault slip.

Nevertheless, new detailed geologic mapping reveals a coherent pattern of faulting that is consistent with NW-striking distributed right-lateral shear. The northeastern flank of the range is dominated by the Painted Gorge fault (PGF), first discovered by Dibblee in 1943 (Dibblee, 1996) and named by Christensen (1957), who interpreted it as a normal fault. We reinterpret it as a right lateral strike-slip fault with ~1 km of displacement, as evidenced by an offset hornblende andesite body. This magnitude of displacement is similar to that of the EF (1-2 km; Dorsey et al, 2012). Fault scarps on alluvial deposits reveal that the PGF is active. It is aligned with the Earthquake Valley fault zone, located farther NW, and is thus likely to be a section of this fault zone.

Minor mutually cross-cutting faults abound in the Coyote Mountains. The EF and PGF zones have complex comb-like and ladder-like geometries within and between them. NW-striking right lateral faults are concentrated near the EF and PGF; but NE-striking left-lateral faults dominate the intervening region, linking the two major fault zones. N-striking normal faults are also common; most are oblique with a component of left-lateral motion. Many faults have slickenlines with varying rakes on the same fault surface. These types of fault patterns produce distributed faulting that results in abrupt lateral changes in slip magnitude and rate along active faults.

Christensen, 1957, MA Thesis, UCLA

Dibblee, 1996, South Coast Geol Soc Annual Field Trip Guidebook

Dorsey et al, 2012, Tectonics