TWO-PHASE CENOZOIC EXTENSION IN THE 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 slip began more recently than 13 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 inferred to have begun ca. 24 Ma. To refine the timing of both early extension and the onset of slip on the Dixie Valley fault, we collected two transects of samples for apatite fission-track (AFT), apatite and zircon (U-Th)/He (AHe & ZHe), and apatite 4He/3He analysis. AFT ages from the Oligocene IXL pluton indicate rapid cooling ca. 14–18 Ma, and AHe and ZHe ages from the Cretaceous La Plata Canyon pluton indicate rapid cooling ca. 16–19 Ma. We interpret these data to record cooling during rapid extension. AHe ages from the IXL pluton are ca. 7-8 Ma and record cooling during slip on the Dixie Valley fault. We modeled these ages and 4He/3He spectra (from one sample) as the result of cooling during exhumation of a fault block tilting in response to a constant extension rate. We used a Markov chain Monte Carlo model to infer the credible geothermal gradients, block geometry, and deformation rates that describe this cooling history. The model predicts Miocene faulting and tilting on the Dixie Valley fault beginning 7.75 - 8.75 Ma. Although the model does not constrain the initial fault dip, it illustrates how a low-angle fault requires a higher extension rate to reproduce the observed cooling ages. Consequently, we prefer a high-angle southern Dixie Valley fault for strain compatibility with the high-angle northern segment. Early extension in the Stillwater Range took place ca. 19–14 Ma and was partitioned into variably east and west-tilted domains separated by strike-slip accommodation zones. The Dixie Valley fault began to form at 8 Ma, and initially propagated as two segments, a northern horst bounded to the west by a fault, and a southern half graben tilted gently west. Shortly after inception, these segments coalesced along an old east-west domain boundary near Alameda Canyon, reactivating it and forming the prominent bend in the Dixie Valley fault. The integrated Dixie Valley fault continued to slip into historic time and formed the modern Stillwater Range.