SEISMICALLY ACTIVE FAULT CONTROLS AT HIGH-TEMPERATURE GEOTHERMAL SITES IN THE WESTERN BASIN AND RANGE: A NEW EXPLORATION TOOL

The association of high-temperature (>100^o C) non-magmatic geothermal resources with geologic structure (faults) in basins of the western Basin and Range is well known, and most exploration is targeted at these structures. Considering that hundreds of Quaternary faults occur in the western Basin and Range, the role of age and most recent fault activity of these structures has been less well understood in evaluating geothermal potential. In this study, we examined the occurrence of seismically active Holocene (<10-13 ka) faults and high-temperature sites in order to evaluate the spatial association of young faults and resources. We utilized existing published data together with newly acquired low-sun-angle aerial photography and high-resolution LiDAR to detect and field map previously unrecognized Holocene faults.

Our results show that out of a total of 37 high-temperature sites in the western Nevada region, 32 sites occur directly on, or within close proximity, to seismically active Holocene faults. Five of these 32 sites are spatially associated with large-magnitude, surface-rupturing, historical earthquakes. New Holocene faults have been found along geothermal sites in the Black Rock Desert, Pyramid Lake region, and San Emidio Valley, and at Kyle, Lee (Allen), Eightmile Flat, Patua, Salt Wells, and Wabuska hot springs. In the lower Walker River basin, an entire new Holocene fault zone that is associated with a known geothermal resource area has been identified, and in Dixie Valley new Holocene and historical fault relations help define two known geothermal resource areas.

These structural associations provided the basis for the development of a simple conceptual model in which recently active faults are considered to be preferred conduits for migration of thermal water from deep crustal depths. Earthquakes nucleating at seismogenic depths (i.e., 10-15 km) can rupture to the surface creating new or re-opened fluid pathways. We suggest that the identification of sites of recent surface-rupturing earthquakes may be an effective exploration tool for geothermal resources.