QUATERNARY LOW-ANGLE SLIP ON DETACHMENT FAULTS IN DEATH VALLEY, CALIFORNIA

Detachment faults in the Black Mountains of Death Valley, California, have been interpreted as inactive remnants of originally steeply dipping normal faults that have been tilted to gentler dips. Several workers hold that the detachments are cut by range-front high-angle normal faults at depth. Relationships between Quaternary faults confined to the hanging wall of the detachment and the low angle surface indicate that the detachments have accommodated slip near their present-day dip. In a typical exposure of a Death Valley detachment fault, a well-defined principal slip plane separates the hanging wall from a 0.25 m thick zone of gouge and breccia. Above the detachments, hundreds of steeply dipping or listric normal faults cut the hanging-wall sediments. In exposures, these faults sole into or are cut by the principal slip plane, and locally hanging-wall sediment is incorporated into the fault zone. Playa lake sediments cut by the detachments and hanging-wall normal faults contain Bishop (0.77 Ma) and Lava Creek B (0.66 Ma) tephras, and OIS6 gravels (0.12-0.18 Ma). A section of older, upper Tertiary sediments is also cut by the detachments and is affected by a similar, but potentially older, hanging-wall fault system. In many places these sediments are not tilted from their original depositional orientation, and the tilting that is present follows a broad hanging-wall antiform. There is a bi-modal distribution of the dips of the hanging-wall faults; the modes are between 55*-65*E (an antithetic orientation to the detachments) and 55*-65*W (a synthetic orientation to the detachments). These modes are compatible with an unchanging, vertically oriented maximum principal compressive stress typical of the basin and range. The mode of the strikes of the normal-fault populations at each locality is parallel to the local strike of the detachment. The slip sense on the fault system is dominantly normal, except for local deviations controlled by slip parallel to the hinges of the folded landforms that the detachments bound. We estimate a total net slip on the order of 1 kilometer on the hanging-wall fault population yielding extension rates <1-2 mm/yr. Our observations are compatible with the hypothesis that the active range-front faulting is confined to the hanging walls of the detachment faults.