Abstract

Two models are typically offered to explain severe continental extensional tectonism: low-angle detachment faults with alloch thonous sedimentary basins above a crustal “rolling hinge,” or differential displacement on systems of distinct, deeply rooted normal faults with in-place sedimentary basins. Structural and stratigraphic relationships between the Badwater turtleback and the adjacent Furnace Creek basin in Death Valley provide a test of these contrasting models. There, ductile extensional deformation of basement rock at the Badwater turtleback coincided with brittle extensional deformation and local uplift in the adjacent Furnace Creek basin between 13 and 18 Ma. In the Furnace Creek basin, crustal extension of this age is evident as (1) greater fault density and offset in pre–13 Ma rocks relative to post–13 Ma rocks and (2) the presence of laterally variable sedimentary breccias and conglomerates, which indicate a syndepositional paleotopographic high. This high is approximately on line with the likely projection of an early Badwater turtleback fault, the surface expression of the turtleback mylonites. These relationships indicate that the Badwater turtleback and Furnace Creek basin were contiguous prior to 13 Ma and that the basin formed in place relative to the Black Mountains basement. These findings support arguments for extension in the Death Valley area by distinct, deeply rooted fault zones rather than by a rolling-hinge detachment fault.

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