CENOZOIC EXTENSION AND STRUCTURAL CONTROLS OF GEOTHERMAL SYSTEMS IN THE HOT SPRINGS MOUNTAINS, WESTERN NEVADA

The northern Great Basin (NGB) hosts abundant geothermal activity even though volcanism generally ceased ~7 Ma. This region lies directly east of the northwest end of the Walker Lane, which currently accommodates ~20% of the Pacific-North America dextral plate motion. Significant geothermal exploration is ongoing in the NGB, but controls on the amagmatic geothermal systems are generally poorly understood. Thus, targeting of new and blind geothermal resources can be difficult. We have therefore undertaken integrated structural and geophysical studies of several geothermal fields, including two in the Hot Springs Mountains (HSM) that contain operating geothermal power plants (Bradys Hot Springs and Desert Peak). Interestingly, the Desert Peak system is blind, with essentially no surficial expression of hot spring activity.

The HSM are dominated by a thick (> 2km) section of Miocene volcanic and sedimentary rocks resting on Mesozoic plutonic-metamorphic basement. The strata are cut by NNE-striking en echelon normal faults and deformed into a series of NNE-trending, moderately tilted fault blocks and folds. Tilt fanning in the Miocene section, new 40Ar/39Ar dates, and tephrochronologic correlations bracket the major episode of extension between ~13 and 9 Ma. However, fault scarps indicate significant Quaternary extension in the area. The NNE-trending folds parallel normal faults and are extensional in origin, resulting from either fault drag or reversals in the tilt direction of fault blocks.

Both geothermal fields occupy small stepovers in the en echelon, steeply dipping NNE-striking normal fault zones. Multiple fault strands in the stepovers provide subvertical conduits of high fracture density that probably enhance fluid flow and facilitate the rise of deep-seated thermal plumes. The NNE-striking faults are orthogonal to regional WNW extension and are thus favorably oriented for fluid flow. Two factors that may further enhance dilation and fluid flow on the NNE-striking faults include mild left-lateral shear in the ENE-trending Humboldt structural zone and a transfer of NW-directed dextral shear in the Walker Lane to WNW extension in the NGB. These relations suggest that stepovers along steep NNE-striking normal faults may represent promising exploration targets throughout the NGB.