LATE CRETACEOUS THROUGH PALEOGENE DATES AND DEFORMATIONAL FABRICS FROM THE DEATH VALLEY REGION: GLOBAL IMPORTANCE

The Black Mountains that bound the eastern margin of Death Valley are bounded by low angle normal faults that juxtapose Pliocene through Recent clastic against basement rocks. Below the fault contact a group of gneissic units are folded and cut by shear zones that mix plagioclase and quartz rich lithologies with more intermediate units of the gneissic basement. Previous limited Uranium-Lead and Potassium-Argon dating of these basement rocks have generally found them to be of Proterozoic age. Yet, field relations, interpreted to be due to deformation during Miocene extension and exhumation, would suggest that most of the fabric and late-stage magmatic cross-cutting relations would be late Miocene in age. A recent study of the Black Mountains found that zircons from the Black Mountain basement rocks underneath the Miocene and younger detachment faults are in fact Late Cretaceous through Paleogene in age. Placed in the context of western US core complexes, the new dates point to a protracted heating of the crust over approximately thirty-five million years. To the north, across the Northern Death Valley Fault Zone, the Funeral Range preserves fabrics and metamorphic assemblages more completely than the Black Mountains. In the Funeral Range, a complete textural history of decompression melting and syn-metamorphic fabric development is well preserved. Together, the Black Mountains and Funeral Range show that most of the detachment footwall geologic history is Late Cretaceous to Paleogene in age. Seismic tomography shows that a hot, melt-present, and weak lithosphere persists to the modern day, but this state has persisted since the last days in which dinosaurs walked the earth.