Thermochronometric Investigation of Fluid Flow and Geothermal Systems in Extensional Settings, Dixie Valley, Nevada

Abstract:

We utilize (U-Th)/He analysis of apatite (AHe) and zircon (ZHe) to resolve cooling ages due to exhumation from those reset by geothermal fluids, and show that structural features play a role in the occurrence of geothermal anomalies. We also test the potential of ⁴He/³He thermochronometry to record fluid-induced reheating of recently exhumed samples. Dixie Valley is located within the Basin and Range province of Nevada where it forms a half graben east of the Stillwater Range normal fault. This region is characterized by extension, thinned crust, elevated heat flow, and high seismicity. Within Dixie Valley the complex interplay between the NE-trending Stillwater Range normal fault and NW-trending dextral faults produced several prominent dilational corners. These corners are hypothesized to result in increased fracture permeability, and often host blind geothermal prospects. Therefore, Dixie Valley is an ideal case study for applied thermochronometry. Samples (n=79) from the footwall of the normal fault yield AHe aliquots ranging from ~0.2-23 Ma and ZHe aliquots from ~1-146 Ma. Three elevation transects of AHe ages record advective cooling of the footwall due to exhumation onset at ~3-5 Ma. AHe data younger than this exhumation age (~0.2-4 Ma) show a strong correlation with previously identified geothermal anomalies and areas of increased strain, thus suggesting that ages are reset by fluids. Additionally, we use the ⁴He/³He method to measure the spatial distribution of ⁴He and to determine continuous t-T paths for individual apatite grains from ~70-20°C. The low-temperature sensitivity of the ⁴He/³He method makes it uniquely suited to address transient reheating by geothermal fluids. Dixie Valley and the Stillwater Range present an ideal opportunity to obtain empirical data regarding the temperature evolution of geothermal systems and develop a novel application of ⁴He/³He thermochronometry.