FAVORABLE STRUCTURAL SETTINGS OF ACTIVE GEOTHERMAL SYSTEMS IN THE GREAT BASIN REGION, WESTERN USA: IMPLICATIONS FOR FLUID FLOW, NORMAL FAULTING MECHANICS, AND GEOTHERMAL AND EPITHERMAL MINERAL EXPLORATION

Active amagmatic geothermal systems of the Great Basin, USA are used to generate electricity and have temperatures overlapping formation of some epithermal mineral deposits (up to and >200°C). We recently completed an inventory of structural settings of known geothermal systems (426 total) in the extensional Great Basin region. Of ~250 categorized fields, step-overs or relay ramps in normal fault zones are the most favorable setting, hosting ~32% of the systems. Such areas have overlapping fault strands, increased fracture density, and thus enhanced permeability. Other common settings include a) normal fault tips (25%), where horse-tailing generates closely-spaced faults and increased permeability; b) fault intersections (22%), where multiple minor faults typically connect major faults and fluids can flow readily through highly fractured, dilational quadrants; and c) accommodation zones (9%), where oppositely dipping normal fault systems intermesh in belts of multiple fault tips and intersections. 3D modeling indicates subvertical conduits of high fracture density in these settings. The primary segments of major faults only host ~1% of the geothermal fields. Quaternary faults lie within or near most fields.

The paucity of geothermal systems along primary fault segments may result from reduced permeability in thick clay gouge and periodic stress release in major earthquakes. Step-overs, terminations, intersections, and accommodation zones represent critically stressed areas, where fluid pathways more likely remain open in breccia-dominated fracture networks. Because stress is not relieved by major earthquakes, abundant microseismicity characterizes fault interaction areas, which precludes pervasive healing of fractures and thus facilitates fluid flow. Increased pore-pressure may also provide a positive feedback mechanism that promotes more frequent but lesser magnitude earthquakes. The association of some young (<5 Ma) epithermal mineral deposits in normal fault footwalls (e.g. Florida Canyon, Wind Mountain, and Hycroft-Lewis) with active geothermal systems in the hanging walls suggests recurrent activity and/or longevity for some hydrothermal systems. These data are relevant to understanding the mechanics of normal faulting and in guiding both geothermal and mineral exploration.