Constraining Age and Locations of Active and Paleofluid Flow Systems in Dixie Valley, Nevada, with Apatite (U-Th)/He Thermochronometry

Abstract:

We present the first constraints on transient fluid flow using the novel application of apatite (U-Th)/He thermochronometry (AHe) in the geothermally active Dixie Valley area of central Nevada, western United States. The valley is bound to the west by a high angle normal fault, along which the adjacent Stillwater Range has been recently exhumed in the footwall. Zones of elevated shallow geothermal gradients (geothermal anomalies) occur in dilational corners along the range front and therefore make Dixie Valley an ideal site to test the sensitivity of the AHe thermochonometer to fluid flow in the shallow crust. Apatites yield (U-Th)/He ages ca. 0.2-16 Ma and three elevation transects record advective cooling of the footwall during exhumation commencing at 3-5 Ma. Many AHe ages are significantly younger (<4 Ma) and do not overlap exhumational cooling ages within error. Consequently, the younger AHe ages are distinguishable as hydrothermally reset and demonstrate the ability of this method to resolve conductive cooling ages from overprinted, fluid-reheated cooling ages. Interpolation of AHe ages shows that the youngest ages correspond with remarkable accuracy to the spatial extents of previously mapped geothermal anomalies. The capability of the AHe thermochronometer to constrain the timing and location of paleofluid flow recommends this technique as a powerful and cost-effective tool in geothermal exploration and petroleum systems evaluations.