TWO-STAGE EVOLUTION OF DEATH VALLEY

Death Valley is a rift basin located in the Walker Lane-Eastern California Shear Zone (WL). The area includes some of the most extreme topography in North America. It is evolving into the plate boundary between the Pacific and North American plates, with 20% of this motion already within the WL. Death Valley is bounded to the west by the Panamint Range, composed of Proterozoic metamorphic basement, a Neoproterozoic through Paleozoic sedimentary section, and younger Tertiary sedimentary and volcanic rocks. On the east side of Death Valley is the Black Mountain Range, remarkable in that the mountain flank facing Death Valley includes three large northwest-plunging antiformal structures known as turtlebacks. These structures expose mid-crustal level rocks and are in fault contact with Miocene and younger sedimentary and volcanic rocks. The Black Mountain Front, the western side of the range that faces Death Valley and includes the turtlebacks, is commonly interpreted as a regional low-angle normal fault surface. In these models, the Panamint Range is interpreted as the hanging wall of this fault system, with as much as 80 km of translation of the Panamints with respect to the Black Mtns in some models. Analysis of recent data on age of tectonism and sedimentation in the WL shows that these models should be re-examined. East of Death Valley, extension ceased in the Late Miocene to Early Pliocene. Within Death Valley, exhumation of the extensional fault footwalls is dated as Mid to Late Miocene. Pliocene sedimentation in the region consists mostly of fluvial to lacustrine section displaying little evidence of syn-depositional structuring. The past 2 to 3 my, in contrast, have been tectonically extremely active, with major deformation of the Pliocene section. This tectonic activity includes strike slip faults and what are here interpreted as associated high-angle transtensional normal faults. This faulting re-faulted the older extensional surfaces, forming Death Valley as a pull-apart basin as originally proposed by Burchfiel and Stewart in 1966. This recent strike slip faulting has resulted in the extreme topography of the WL. In this model the turtlebacks form as mullions in the footwalls of transtensional normal faults.