Detachment Faulting in the Western Basin and Range: New Geometric, Thermal, and Temporal Constraints From the Bare Mountain Region in Southwestern Nevada

Zircon and apatite fission-track cooling ages for 50 samples taken from Bare Mountain and surrounding areas of southern Nevada, analyzed in conjunction with structural and paleomagnetic data and calcite deformation geothermometry data, provide new constraints on the timing and distribution of detachment faulting in the western Basin and Range. Our results show that: (i) Bare Mountain was tilted to the east or northeast, probably during Middle Miocene extension, prior to development of the Bullfrog Hills detachment system. (ii) Bare Mountain cooled through the fission-track closure temperature for fluorine-rich apatite (115-125 C) more or less as a unit at 8 to 17 Ma. (iii) Northwest Bare Mountain cooled through the zircon closure temperature (250 C) at 8 to 17 Ma, whereas the rest of the mountain cooled through this temperature between the Late Paleozoic and the Eocene. The combination of tilting at Bare Mountain and the apatite and zircon fission-track cooling ages indicates the presence of a west-dipping breakaway fault at Bare Mountain at around 15 Ma. New apatite fission-track cooling ages from Yucca Flat, Frenchman Flat, Mount Sterling, the Striped Hills, the Resting Springs Range, and the Funeral Mountains, when combined with published apatite ages, constrain the regional position of a west-dipping breakaway fault and exhumed footwall. The current position of the trailing edge of the hanging wall of this system is the Death Valley - Furnace Creek fault system. Migration rates of the cooling front in the footwall of this system range from 4.0 mm/yr at the latitude of Bare Mountain to 7.3 mm/yr at the latitude of central Death Valley. * Work performed at the CNWRA for the U.S. NRC under contract number NRC-02-97-009. This is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.