ASSESSMENT OF FAVORABLE STRUCTURAL SETTINGS OF GEOTHERMAL SYSTEMS IN THE GREAT BASIN REGION, WESTERN USA

In an effort to refine exploration strategies and reduce the risks in drilling geothermal wells, we have undertaken a thorough inventory of structural settings of geothermal systems (>400 total) in the extensional to transtensional Great Basin region. Of the 200+ geothermal fields catalogued, step-overs or relay ramps in normal fault zones serve as the most favorable setting, hosting ~32% of the systems. Such areas typically have multiple, overlapping fault strands, increased fracture density, and thus enhanced permeability. Other common settings include a) intersections between normal faults and transversely oriented strike-slip or oblique-slip faults (22%), where multiple minor faults connect major structures and fluids can flow readily through highly fractured, dilational quadrants, and b) normal fault terminations or tip-lines (22%), where horse-tailing generates closely-spaced faults and increased permeability. A major subset of fault intersections includes displacement transfer zones (5%) between strike-slip faults in the Walker Lane and N- to NNE-striking normal faults. Other settings for geothermal systems include accommodation zones (i.e., belts of intermeshing, oppositely dipping normal faults; 8%), major range-front faults (5-6%), and pull-aparts in strike-slip faults (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 in the more purely extensional terrane northeast of the Walker Lane. In addition, Quaternary faults typically lie within or near most systems. The relative scarcity of geothermal systems along displacement-maxima or mid-segments of major range-front faults may be due to reduced permeability in thick zones of clay gouge and periodic release of stress in major earthquakes. Step-overs, terminations, intersections, and accommodation zones in normal fault systems correspond to long-term, critically stressed areas, where fluid pathways are more likely to remain open in networks of closely-spaced, breccia-dominated fractures. It is also notable that many of the more highly productive systems are characterized by more than one type of favorable setting at a single locality (e.g., Steamboat and Coso).