EVIDENCE FOR LATE MIOCENE-QUATERNARY LOW-ANGLE OBLIQUE STRIKE-SLIP FAULTING ON THE WEST SALTON DETACHMENT FAULT, SOUTHERN CALIFORNIA

Low rakes of slickenlines, fault-strike-parallel anticlines, and basin fill that thins toward the West Salton detachment fault (WSDF) all show that the WSDF is not an ordinary low-angle normal fault near Yaqui Ridge in the western Salton Trough. Detailed geologic mapping along the western margin of the Salton Trough suggests that the WSDF was actually a low-angle oblique strike-slip fault during the final and perhaps main phases of deformation. The fault strikes N66ºE and dips 29º ±4.7º NE (n=24) with slickenlines that rake 27º E with an average trend of N89ºE ±7.1º (n=23). Overall the fault and slickenlines are parallel to fabrics of the Late Cretaceous Eastern Peninsular Ranges mylonite zone, and their geometry probably reflects crustal inheritance. Four to five W to NW-trending crystalline-cored anticlines are present in the area and at least two appear to have been active since earliest deposition of the Upper Miocene Hawk Canyon beds. At Borrego Mountain a large NW-trending anticline deformed the hanging wall of the WSDF and produced a progressive unconformity during deposition of the Hawk Canyon beds and Canebrake Formation of Imperial age. These growth strata are folded about a younger anticline in the same location. South of Borrego Mountain the detachment was folded into several NW-trending folds and the Diablo Formation was uplifted, eroded, and redeposited as clasts in a conglomerate. Multiple fault-strike-parallel anticlines are difficult to interpret as monoclinal rollovers above a simple detachment fault, but may be part of a complex, recurrently active fold belt created by oblique slip. Overall, Cenozoic stratigraphic units thin and/or pinchout towards the WSDF near Yaqui Ridge, which suggests that the immediate hangingwall was a paleohigh until just before Diablo time. These stratigraphic and structural relationships are inconsistent with simple extensional detachment slip. Our data suggest the WSDF was a low-angle oblique strike-slip fault that produced fault-strike-parallel folds throughout its history and may have accommodated more strike-slip than dip-slip deformation. If the slickenlines capture most of the fault’s history, then roughly half of the slip on the WSDF was parallel to the plate boundary and half accommodated extension perpendicular to the boundary.