Thermochronometric investigation of structural evolution and geothermal systems in extensional settings, Dixie Valley, Nevada
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This study utilizes (U-Th)/He analysis of apatite (AHe) and zircon (ZHe) to develop a new model for the structural and thermal evolution of the Stillwater Range and to resolve cooling ages due to exhumation from those reset by geothermal fluids. Dixie Valley is located within the Basin and Range and Walker Lane Belt provinces in Nevada where it forms a half graben east of the normal fault-bound Stillwater Range. This region is characterized by ongoing extension, thinned crust, elevated heat flow, and high seismicity. A new structural model is proposed for the study area based on recent mapping. Middle Miocene Basin and Range-related normal faults were reactivated as strike- and oblique-slip faults in the Pliocene at the same time the Dixie Valley normal fault (DVF) initiated. The complex interplay between the NE-striking DVF and NW-striking dextral faults produced localized zones of high strain or dilational corners. These corners are hypothesized to cause increased fracture permeability, and to host both manifested and blind geothermal prospects. Therefore, Dixie Valley is an ideal case study for applying thermochronometry to geothermal systems. Samples (n=79) from the Dixie Valley Fault footwall yield AHe aliquots ranging from ~0.2-16 Ma and ZHe aliquots from ~1-163 Ma. Three elevation transects of AHe ages record advective cooling of the footwall due to exhumation onset at 3-5 Ma. Thermochonology combined with structural data corroborate the style and timing of the proposed structural model. AHe data significantly younger than the DVF exhumation age (~0.2-4 Ma) show a strong correlation with previously identified geothermal anomalies and areas of increased strain. Evidence suggests younger, disparate AHe ages are reset by fluids and that intersecting strike-slip and normal faults play a role in the occurrence of geothermal anomalies. The low-temperature sensitivity of the (U-Th)/He method makes it uniquely suited to address identify areas affected by geothermal fluids. Dixie Valley and the Stillwater Range present an ideal opportunity to obtain data regarding the temperature evolution of geothermal systems and develop a novel application of (U-Th)/He thermochronometry.