Assessment of Favorable Structural Settings of Geothermal Systems in the Great Basin, Western USA

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Conference Paper: Assessment of Favorable Structural Settings of Geothermal Systems in the Great Basin, Western USA

Abstract
We have undertaken a thorough 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. Of the more than 200 geothermal fields catalogued to date, we found that step-overs or relay ramps in normal fault zones served as the most favorable structural setting, hosting ~32% of the systems. Such areas are characterized by multiple, commonly overlapping fault strands, increased fracture density, and thus enhanced permeability. Other common settings include a) intersections between normal faults and either transversely oriented strike-slip or oblique-slip faults (22%), where multiple minor faults typically 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 a myriad of closely-spaced faults and thus 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, with geothermal systems commonly focused along the normal faults proximal to their dilational intersections with nearby dextral faults. Other notable structural settings for geothermal systems in the Great Basin include accommodation zones (i.e., belts of intermeshing, oppositely dipping normal faults; 8%), major range-front faults (3%), salients or apices of major normal faults (3%), 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. In addition, Quaternary faults typically lie within or near most of the geothermal systems. However, geothermal systems appear to be rare along the displacement-maxima zones or mid-segments of major normal faults (i.e., major range-front faults), possibly due to both 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 would tend to correspond to long-term, critically stressed areas, where fluid pathways would more likely remain open in networks of closely-spaced, breccia-dominated fractures. It is also important to note that many of the higher enthalpy systems are characterized by more than one type of favorable setting at a single locality (e.g., Steamboat, Brady’s, and Salt Wells).

Authors 
James E. Faulds, Nicholas H. Hinz, Mark F. Coolbaugh, Patricia H. Cashman, Christopher Kratt, Gregory Dering, Joel Edwards, Brett Mayhew and Holly McLachlan






Conference 
GRC Anual Meeting; San Diego, CA; 2011/10/23


Published 
Geothermal Resources Council, 2011





DOI 
Not Provided
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Online 
Internet link for Assessment of Favorable Structural Settings of Geothermal Systems in the Great Basin, Western USA

Citation

James E. Faulds,Nicholas H. Hinz,Mark F. Coolbaugh,Patricia H. Cashman,Christopher Kratt,Gregory Dering,Joel Edwards,Brett Mayhew,Holly McLachlan. 2011. Assessment of Favorable Structural Settings of Geothermal Systems in the Great Basin, Western USA. In: Transactions. GRC Anual Meeting; 2011/10/23; San Diego, CA. Davis, CA: Geothermal Resources Council; p. 777–783