Turtlebacks are smooth, curved surfaces, which form north-northwestward-plunging elongate domes on the east side of Death Valley. These surfaces are roughly parallel to bedding or foliation of anticlines in Precambrian schist, gneiss, and marble. Late Cenozoic fan and playa deposits are faulted over these surfaces along the turtleback faults. Previously the turtleback faults have been interpreted as part of a thrust fault, perhaps the Amargosa thrust fault, that was arched after thrusting. They are interpreted here as individual normal faults younger than the thrust fault and, contrary to previous interpretations, much younger than the formation of the anticlines in the Precambrian rocks. The tectonic history of this unusual area is here considered to include the following events: (1) Precambrian folding of the Precambrian rocks; (2) post-Paleozoic and pre-middle(?) Tertiary Amargosa thrusting; (3) uplift and erosion of Paleozoic strata and the Amargosa thrust fault, down to the folded Precambrian rocks in the Black Mountains block; (4) Middle (?) Tertiary rhyolite extrusions and the accumulation of later Tertiary fan and playa deposits; (5) Pliocene or Pleistocene uplift of the Black Mountains relative to Death Valley, along the Black Mountains fault system, with consequent removal of support for the Tertiary deposits on the turtleback surfaces, and the development of the turtleback faults by normal faulting, or sliding, of the Tertiary sedimentary rocks down the turtleback surfaces toward Death Valley; and, (6) Pleistocene to Recent renewal of movement on the Black Mountains fault system.
