REAPPRAISAL OF THE DEATH VALLEY TURTLEBACKS FROM ANALYSIS OF METAMORPHISM AND DUCTILE DEFORMATION IN THE TURTLEBACKS AND PANAMINT MOUNTAINS

Wright and Troxel ‘s work on the Amargosa Chaos and low angle normal fault systems of the Black Mountains were a landmark in extensional tectonics, but the connections between the brittle faults of the Chaos and ductile structure of the turtlebacks has remained contentious. Recent work on the turtlebacks as well as their hanging wall, the Panamint Mountains, provides new insights on this problem. Geochronology by Lima et al. (2018, Geology) together with our analysis of metamorphic assemblages at Mormon Point turtleback indicate a long-lived, moderate pressure migmatite assemblage developed during Mesozoic contraction with preliminary estimates of PT conditions at 695C and 0.72GPa based on garnet cores with bt+plg +/- ky inclusions. Garnet rims record comparable temperatures (660-685C) but lower pressures of 0.53-0.61GPa which are consistent with observed sill +kfeld and migmatitic texture. These data are broadly consistent with the interpretation that the Black Mountains lay beneath the Panamint Mountains, prior to extension, with some caveats. New 3D mapping and geochronology in the central Panamint Mountains indicates exposure of a partial crustal section across a Mesozoic, low P/highT metamorphic assemblage with a three-phase ductile history developed under greenschist-amphibolite facies conditions. The two earliest phases produced the main continuous cleavage and a younger, third phase produced the prominent NS trending, upright folds systems long recognized in the Panamints. The third phase is demonstrably post 89Ma and represents a Laramide contraction superimposed on the main LS fabric that we interpret as a dextral strike-slip system based on geometry and scattered shear sense indicators. Collectively these data suggest that during the Mesozoic the Black Mountains were deeply buried via basement involved thrust systems that involved a component of transpression when the Panamints were emplaced atop what is now the turtlebacks. Both the Panamint and Black Mountains cooled during erosional exhumation in the Laramide based on cooling ages in the Panamint Mountains, but were probably reheated to melting under lower-P conditions when the turtleback were covered by the ~2km thick intrusive sheet of the ~11Ma Black Mountains intrusive complex.