Characterizing Structural Controls of Geothermal Reservoirs in the Great Basin, USA, and Western Turkey: Developing Successful Exploration Strategies in Extended Terranes

Although conventional geothermal systems have been successfully exploited for electrical production and district heating in many parts of the world, exploration and development of new systems is commonly hampered by the risk of unsuccessful drilling. A major problem in selecting drill sites is that existing geothermal systems are generally poorly characterized in terms of favorable settings and structural-stratigraphic controls. In order to characterize the structural controls on geothermal systems in active extensional settings, we have analyzed numerous fields in the western Great Basin (USA) and western Turkey through integrated geologic and geophysical investigations. Methods include detailed geologic mapping, structural analysis of faults, detailed gravity surveys, studies of surficial geothermal features (e.g., travertine, sinter, springs, and fumaroles), shallow temperature surveys, and geochemical analyses. Our findings suggest that many fields occupy a) discrete steps in normal fault zones (e.g., Desert Peak, Brady's, Nevada, USA; and Simav, Turkey); b) intersections between normal faults and transversely oriented oblique-slip faults (Astor Pass, Nevada, and Salihli, Turkey); c) overlapping oppositely dipping normal fault zones (e.g., Salt Wells, Nevada, USA), d) terminations of major normal faults (e.g., Gerlach, Nevada, USA; Germencik and Kizildere, Turkey), or e) transtensional pull-apart zones (e.g., Lee-Allen, Nevada, and Canby, California, USA; Pamukkale, Turkey). These settings are typically associated with steeply dipping faults, involving conduits of highly fractured rock oriented approximately perpendicular to the least principal stress and commonly along or near Quaternary fault zones. General topographic features indicative of these settings include: 1) major steps in range-fronts, 2) interbasinal highs, 3) mountain ranges consisting of relatively low, discontinuous ridges, and 4) lateral terminations of mountain ranges. Surficial features, such as tufa towers, travertine spring mounds, and sinter deposits, are also associated with many systems. These structural, topographic, and surficial features may indicate hidden or blind geothermal fields, which have no surface thermal waters or steam (e.g., hot springs or fumaroles).