Timing of Cenozoic Extension in the Southern Stillwater Range and Dixie Valley, Nevada

The Dixie Valley fault bounds the east side of the Stillwater Range in west-central Nevada and last ruptured in 1954. Offset basalts indicate that slip began more recently than 14 Ma, and prior work has interpreted the southern segment as an active low-angle normal fault. Oligocene igneous rocks in the southern Stillwater Range were steeply tilted during large-magnitude extension prior to 14 Ma. To refine the timing of early extension and the onset of slip on the Dixie Valley fault, we collected two transects of samples for apatite fission track, apatite and zircon (U-Th)/He (AHe and ZHe), and apatite He-4/He-3 thermochronometry. Apatite fission track ages from the Oligocene IXL pluton indicate rapid cooling 18-14 Ma, and AHe and ZHe ages from the Cretaceous La Plata Canyon pluton indicate rapid cooling 16-19 Ma. We interpret these data to record cooling during rapid extension. AHe ages from the IXL pluton are 6-8 Ma and record cooling during slip on the Dixie Valley fault. We modeled these ages and He-4/He-3 spectra from one sample as the result of cooling during exhumation of a tilted fault block at a constant extension rate. The model predicts slip on the Dixie Valley fault beginning 8 Ma. Although it does not constrain the initial fault dip, the model illustrates how a low-angle fault requires a higher extension rate to reproduce cooling ages. Consequently, we prefer a high-angle southern Dixie Valley fault for strain compatibility with the high-angle northern segment.