CONSTRAINTS ON THE SLIP MAGNITUDE OF THE AMARGOSA FAULT: IMPLICATIONS FOR EXTENSION IN DEATH VALLEY, CALIFORNIA, USA

Two models of continental extension are applied to the Death Valley region: extension by slip on distinct, crustal-penetrating fault zones, and extension by slip on a single detachment fault of regional extent, the "Rolling Hinge model". If the Rolling Hinge model is correct, then the Amargosa fault, which separates Proterozoic sedimentary rock from basement rock in the Black Mountains, must be an exposure of the detachment fault, with slip in excess of 80 km. New mapping, however, shows that NW-directed slip on the Amargosa fault did not exceed 15 km and was probably much less.

Earlier mapping by Wright and Troxel (1984) showed that the Amargosa fault consists of two distinct segments. A southern segment displays a highly faulted, but intact hanging wall, and a northern segment contains a chaotic upper plate. They also noted many localities along both segments where Proterozoic sedimentary rocks rest depositionally on basement.

New mapping extends those observations to limit the slip magnitude of the fault. Instead of one continuous fault surface, the southern segment consists of at least three shorter surfaces along its length, two of which offset the basal unconformity of the Crystal Spring Fm. Fault slip analyses, using the unconformity as a marker and imposing a N60°W slip vector, yield normal-oblique slip of less than 1 km for each fault. Along the northern segment, the Noonday Dolomite locally rests depositionally on basement, showing that the underlying Crystal Spring Fm must pinch out between there and exposures of its unconformity ~4 km to the south.

If the southern and northern segments of the Amargosa fault are continuous, then slip on the entire fault is limited by the minor offset of the basal unconformity in the south. If they are not continuous, then northwest translation on the northern segment must be less than 15 km. Otherwise, the Noonday-basement unconformity would restore to a position above the along-strike projection of the exposed Crystal Spring-basement unconformity.