THERMOCHRONOMETRIC CONSTRAINTS ON THE EXTENT AND LONGEVITY OF A GEOTHERMAL SYSTEM IN AN EXTENSIONAL TECTONIC SETTING, WASSUK RANGE, NEVADA

Extensional tectonic settings have long been a focus of geothermal exploration in light of the tectonic heat advection in the footwalls of normal faults coupled with magmatism and fracture permeability in the hanging wall rocks. Recent work on occurrence models for geothermal resources has identified thermochronometric ages as potentially key elements in exploring for areas that contain sufficient heat to constitute a utility-grade geothermal resource. As geothermal resources require advection of heat to the upper crust, cooling ages give critical insight into whether such advection has occurred by faulting, flow of hot fluids, or very rapid erosion. This case study in the Hawthorne-Wassuk Range region, Nevada, reports new results from the footwall of the central Wassuk Range fault block in an attempt to illustrate the utility of this approach. The range is a westward tilted fault block of pre-Cenozoic rocks overlain by Miocene strata that underwent large-magnitude extension in the middle Miocene and renewed range-front faulting since the Pliocene. Detailed mapping and age dating of progressively tilted Miocene and younger volcanic and sedimentary rocks on the tilted (60-70˚) western range flank are coupled with high-density apatite and zircon (U-Th)/He thermochronometry (~90 samples) to reconstruct the exhumation and tilting history and thermal evolution of the central Wassuk Range. The study sheds light on the role of syn-extensional heat-advection, footwall fracture permeability, and magmatism and the longevity and nature of the geothermal anomaly in the central eastern Wassuk Range. New zircon and apatite (U-Th)/He thermochronometric ages along the eastern range front cluster between 3.5-6.5 Ma and are consistent with thermochronometric data from the northern Wassuk Range, indicating renewed faulting at 4 Ma. Preliminary along-strike apatite (U-Th)/He ages from the eastern range front decrease from 8.2 to 1.6 Ma, approaching the Hawthorne geothermal prospect, suggesting the presence of a large, long-lived geothermal anomaly. The grid-like apatite and zircon (U-Th/He) data set will allow a quantitative assessment of the size, magnitude, longevity, and nature of the anomaly and should in a cost-effective fashion help to further pinpoint targets of geothermal exploration.