Structural and Temporal Constraints of Buffalo Valley Hot Springs and Proximal Young Volcanics, North-Central Nevada

Abstract

The Humboldt Structural Zone (HSZ), located within a southwest-to-northeast band that stretches from west-central to north-eastern Nevada and Idaho, is noted for abundant high temperature (>100° C) geothermal systems. This is generally attributed to the unique structural setting formed by the transfer of tectonic stresses from the Walker Lane right-lateral strike-slip regime into Basin and Range extension. The resultant faults are both permeable and preferentially oriented for fluid flow in the northwest-directed extensional strain field. Within the Basin and Range, a handful of structural settings have been found to control the majority of geothermal systems. These settings are a) fault step-overs b) fault terminations c) fault intersections d) accommodation zones. Fault step-overs are the most common geothermal structural setting, controlling ~33% of known geothermal systems within the boundaries of the Great Basin. Hybrids of these settings are even more accommodating to geothermal system generation. Increased heat gradient from extensional crustal thinning is the accepted source of calefaction for these systems and very few sites have demonstrated a mantle fluid interaction within the system. Buffalo Valley hot spring (BVHS) is a high temperature geothermal system located within the boundaries of the HSZ. This spring sits on the eastern side of the Buffalo Valley playa. Uniquely, this site is located proximal to a north-east-trending chain of morphologically young basalt and trachybasalt cones and flows that parallel the western Fish Creek Mountains range front and have been previously dated to 1.4-0.95 Ma using ⁴⁰Ar/39Ar methods. Prior geophysical surveys have interpreted Buffalo Valley as an asymmetrical graben with no major fault bounding the eastern basin boundary. Detailed geologic mapping of both bedrock and surficial units was completed at 1:24,000 scale, covering the area surrounding BVHS. Petrographic thin sections were utilized for delineating felsic volcanic units, and elemental analyses were performed on basalt phenocrysts using a scanning electron microscope. Faults were mapped using both field and aerial photography methods, and slip and dilation tendency calculations were performed on these faults to identify potential fluid conduits. An eighty-two hole shallow temperature survey was conducted to identify the geothermal upwelling zone of this system. Nonlinear diffusive numerical modeling was used to simulate the erosion of the Buffalo Valley cinder cones and estimate the eruption age of four cones. Finally, isotopic ratios of He3/He4 were analyzed to assess potential interaction of mantle fluids with the Buffalo Valley geothermal system. Geologic mapping has revealed that Buffalo Valley hot spring is the surface expression of a geothermal system housed within a right-stepping normal fault that bounds the west side of the Fish Creek mountains. The right-stepping nature of this fault system has resulted in a diffuse zone of faulting rather than a single discrete fault. This fault zone is referred to as the Fish Creek Mountains fault zone (FCMFZ). Within the FCMFZ, variations in fault strike orientations cause secondary fault intersections within the larger fault step-over. This creates a hybrid fault step-over/ fault intersection structural setting. Individual fault slip and dilation tendency calculations have pinpointed faults preferentially oriented for slip and dilation in the current north-northwest directed extensional regime, however shallow temperature measurements indicate that primary geothermal upwelling is controlled by a fault intersection rather than dilation of an individual fault. Nonlinear diffusion erosion modeling used to estimate the age of four cinder cones within the Buffalo Valley volcanic field yields ages of 700 ka to 1.5 Ma with the oldest cone located in the north. Younging of cones proceeds southward sequentially. He3/He4 isotopic measurements, acquired at BVHS (1.48), indicate a local mantle derived helium anomaly. However, mantle helium ratios at BVHS are not anomalous regionally.