Structural Controls of the Black Warrior Blind Geothermal System, Washoe-Churchill Counties, Truckee Range, Nevada, USA
The Black Warrior geothermal system lies 20 km east of the southern end of Pyramid Lake in the Truckee Range of northwestern Nevada on the Washoe-Churchill county line. It is an amagmatic blind geothermal system, lacking hydrothermal surface manifestations (no fumaroles, hot springs, sinter deposits, or high temperature alteration) and also lacks recent (<5 Ma) volcanism. The system was discovered by shallow temperature gradient drilling (100-600 m, max temp: 128°C) by Phillips Petroleum Company in the 1980s and observed with a 2-m shallow temperature survey circa 2011. The thermal anomaly resides in a structurally complex zone that has not been previously characterized. Detailed geologic mapping in the area has identified faults and stratigraphic relationships between successive and interfingering Tertiary volcanic sequences that nonconformably overlie Mesozoic plutonic and metamorphic basement. Inversion of kinematic fault data shows the direction of local extension trends southeast (120-130°), and is compatible with the direction of late Miocene to recent extension in the region. The structural framework is characterized by young, north-northeast-striking, moderately to steeply west-dipping normal faults that terminate and step in the vicinity of the thermal anomaly. This suggests two possible favorable structural settings: (1) a fault termination of the southeastern range-front fault with accompanying ~2 m wide horse-tail splaying producing an area with abundant closely spaced faults and high fracture permeability; and/or (2) a narrow fault step-over ~3-5 km wide between two major normal faults, whereby many closely-spaced minor faults provide hard linkage and a zone of high fracture permeability. Thus, the study area resides in a hybrid structural setting in which the faults are favorably oriented for dilation in the local stress field. These structural characteristics are advantageous for geothermal activity and suggest that Black Warrior may host a viable geothermal system with suitable permeability at depth. However, additional exploration is recommended to better define the system and select drilling sites. Ultimately, other blind or unexplored areas that share the structural characteristics of Black Warrior may host viable geothermal systems.
Basin and Range, blind geothermal, exploration, Nevada, Pyramid sequence, structural geology
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