| Abstract |
In our inventory of the structural settings of known geothermal systems (>400 total) in the extensional to transtensional terrane of the Great Basin in the western USA, we have found that stepovers or relay ramps in normal fault zones serve as the most favorable structural setting, hosting ~33% of the systems. Other common settings include a) intersections between normal faults and either transversely oriented strike-slip or oblique-slip faults (22%) and b) normal fault terminations or tip-lines (23%). A major subset of fault intersections includes displacement transfer zones (5%) between strike-slip faults in the Walker Lane and north- to northeast-striking normal faults, with geothermal systems commonly focused along the normal faults proximal to their dilational intersections with nearby dextral faults. Other notable structural settings include accommodation zones (i.e., belts of intermeshing, oppositely dipping normal faults; 7%), major range-front faults (3%), major bends in normal faults (2%), and pull-aparts in strike-slip fault systems (4%). Pull aparts and displacement transfer zones are more abundant within or along the margins of the Walker Lane, whereas step-overs and accommodation zones in normal fault systems are more prevalent within the extensional terrane that characterizes much of the Great Basin northeast of the Walker Lane. The more favorable settings correspond to long-term, critically stressed areas, where fluid pathways would more likely remain open in networks of closely-spaced, brecciadominated fractures. Of the 27 power plants in the Great Basin region, however, the proportions differ, with step-overs hosting ~41% and accommodation zones and fault intersections each containing ~26%. Moreover, hybrid settings appear to characterize many of the producing systems, including some of the higher enthalpy systems (e.g., Brady?s and Steamboat). It would appear that the greater the structural complexity, the greater the likelihood of a geothermal system being robust enough to generate electricity. Although relatively few systems have been developed, these findings may help to guide future exploration and estimates of geothermal potential in green-field areas. |