LONG-LIVED STRUCTURAL DISCONTINUITIES AS GUIDES FOR GEOTHERMAL EXPLORATION

Many structural discontinuities in the crust, including those associated with ancient rift systems, cratonic boundaries and terrane boundaries have acted as conduits for robust fluid circulation. As zones of relative weakness caused by deformation, generation of weak alteration materials, and/or high fluid pressures, structural discontinuities are commonly reactivated in subsequent tectonic events throughout their evolution over geologic time. Despite the ephemeral nature of permeability on geologic time-scales, the inherent weakness and reactivation of structural discontinuities indicates that they may act as long-lived fluid circulation pathways. The Great Basin region in the western North American Cordillera has undergone nearly continuous extensional, collisional and strike-slip deformation since the early- to mid-Paleozoic. These deformational events have generated a series of crustal-scale structural discontinuities of Paleozoic, Mesozoic, and Cenozoic age across the Great Basin. Some of these discontinuities have been important in channeling fluids associated with Tertiary epithermal mineral deposits. Crustal fluid circulation is also important for geothermal systems. ³He/⁴He isotopic data suggest that two of the most robust geothermal systems in production in the Great Basin, Dixie Valley and McGinness Hills, are associated with deep, crustal-scale fluid circulation. This suggests that geothermal systems with connectivity to deep crustal and/or upper mantle fluid and heat supply are favorable for sustained geothermal production. Correlation between isotopically defined zones of deep fluid circulation and deep seated structural discontinues beneath Dixie Valley, McGinness Hills, and other known geothermal systems suggests that this deep fluid circulation may occur along structural discontinuities. The near surface expression of deep seated structural discontinuities, therefore may represent a highly prospective ‘play’ for geothermal exploration.