NEW U-PB ZIRCON AGES INDICATE MAJOR EXTENSION IN DEATH VALLEY, CA PREDATED 10 MA: IMPLICATIONS FOR MODELS OF CRUSTAL EXTENSION

The Death Valley region has long been considered the type locality for two contrasting and influential models of crustal extension. The rolling hinge model calls for about 80 km of horizontal extension on a low-angle detachment, which in Death Valley has been interpreted as the brittle Amargosa fault and ductile shear zones of the Mormon Point and Copper Canyon turtlebacks. In the context of this model, the Amargosa fault and shear zones of the Mormon Point and Copper Canyon turtlebacks had to be active at about 9.0 Ma, 8.1 Ma, and 6.7 Ma respectively, so that the hinge could "roll" to the northwest in accord with cooling data by Holm et al., 1992. By contrast, in the pull-apart model these shear zones are distinct, crustal penetrating fault zones that were not linked by a rolling hinge.

We report two new U-Pb zircon ages for rhyolitic dikes that intrude and post-date deformation at the Amargosa fault and the ductile shear zone at Mormon Point. The dike that plugs the Amargosa fault yields a 10.08 +/- .03 Ma age, based on a weighted ²⁰⁶Pb/²³⁸U average for four zircon fractions that give concordant and overlapping results. The dike that cuts the Mormon Point turtleback yields a 9.53 + .04 Ma age and is similarly based on a weighted ²⁰⁶Pb/²³⁸U average for three fractions. Zircons from two highly discordant fractions from the latter dike contain inherited cores with an estimated average age of ca. 1.40-1.75 Ga.

These data indicate that the main episode of slip on the Amargosa fault was over by 10 Ma and that the Mormon Point turtleback shear zone was largely inactive by 9.5 Ma. Both of these ages significantly predate major extension as required by the rolling hinge model. Instead, they corroborate the proposal by Miller and Pavlis (2003, in press), that the turtleback shear zones are exhumed fragments of a mid-crustal ductile shear zone. This shear zone accommodated extension prior to emplacement of the 10-11 Ma Black Mountains intrusive suite; the Amargosa fault was likely one of several upper crustal faults that terminated downwards at the shear zone. Cooling and exhumation of the Mormon Point and Copper Canyon turtlebacks then occurred by slip along other distinct fault zones, similar to the pull-apart model.