The Role of the Sheephead Fault Zone In Bounding Neogene Extension and Magmatism In the Death Valley Area, California

Death Valley, California, has served as a type locality for both pull-apart basins and continental extensional tectonics in general. Its core area of greatest Neogene extension and magmatism – the Central Death Valley Extended Zone, including much of the Black Mountains and Greenwater Range – is distinguished lithologically by an abundance of mostly Miocene igneous rocks and a paucity of Neoproterozoic-Paleozoic sedimentary rocks. The terrain is bounded to the north by the right-lateral Northern Death Valley-Furnace Creek fault zone. Offset along that structure has provided a key constraint on the magnitude of extension in the Central Death Valley Extended Zone. To the south, the Sheephead fault zone demarcates a similar geologic boundary. Yet the history of this structure, including its own constraint on Death Valley extension, is largely unknown. Hypotheses for the regional tectonic role of the Sheephead fault zone range from a bounding fault on a central Death Valley pull-apart basin, to a component of the larger Garlock fault system, to a part of a regional detachment system floored by the low-angle Amargosa fault.

Here we present new data on both the geographic extent of the Sheephead fault zone and the sense, magnitude, and timing of motion along it. We interpret it as a relatively young, immature, bookshelf-style, right-lateral shear zone. It appears to have functioned as an important tectonic boundary, but not as a true counterpart to the Northern Death Valley-Furnace Creek fault zone across a pull-apart basin. Left-separation planes within the Sheephead system are consistent with deformation related to the Garlock fault, but the overall right-lateral shear implies a more complex regional strain picture. The style and timing of deformation in the Sheephead fault zone suggest that it does not sole into or merge with the Amargosa fault system.