Integrating thermochronometric and geologic data to develop a new structural model for large-magnitude extension in central Death Valley, CA

Death Valley, California, in the Basin and Range province, contains surface exposures of basement rocks that were once 10-20 km deep. These rocks form dome-shaped "turtlebacks" and are only found in highly-extended terranes. Thermochronometry has been used widely throughout the Basin and Range, including in Death Valley, to develop thermal-based exhumation histories of exhumed crustal blocks and for basin provenance studies. In our study, we applied the (U-Th)/He and Ar/Ar methods on 10 samples from the Badwater turtleback and developed a structural model for the Central Death Valley region based on integrating our dataset into a diverse published dataset.

We used our own data as well as existing cooling ages, crystallization ages, mapping, structural analyses, palinspastic reconstructions, and paleomagnetic interpretations to produce a five-stage structural evolution model: 1) >32-16 Ma denudation of Sevier-Laramide overthickened crust, 2) 16-14 Ma onset of Basin and Range extension accommodated mostly by a mid-crustal detachment fault, 3) 14-8 Ma onset of San-Andreas-related oblique-slip with lateral and vertical rotation of the blocks within Death Valley causing the exhumation of the underlying detachment fault to the surface, 4) 8-3 Ma formation and slip of a secondary detachment fault at depth below a pull-apart basin, which accommodates substantial amount of the subsequent motion, and 5) 3 Ma-present transfer of deformation to the west, with lower modern extension rates in Central Death Valley. Current models state that the turtlebacks are either an exhumed detachment surface or the exhumed mylonitic roots of high-angle normal faults; however, both models do not explain the current structural configuration in time-temperature space. We make a case for our model with two sequential, overlapping detachment faults that better fits the data. However, no thermokinematic model exists with two overlapping detachment faults. We invite a discussion on whether such a process could exist from a mechanical perspective and how it could be tested using computer modeling or seismic methods.