STRUCTURAL CONTROLS OF THE BLACK WARRIOR BLIND GEOTHERMAL SYSTEM, TRUCKEE RANGE, NORTHWESTERN NEVADA

The Black Warrior geothermal system lies 20 km east of the southern end of Pyramid Lake in the Truckee Range of northwestern Nevada. 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 also observed with a two-meter 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 and regional relations indicate a NW-trending extension direction (300-310°). The structural framework is characterized by 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 for this geothermal system: (1) a fault termination of the southeastern range-front fault with accompanying ~2 km wide horse-tail splaying producing an area with abundant closely spaced faults and high fracture permeability; and/or (2) a fault step-over ~3-5 km wide between two major west-dipping normal faults, whereby many closely-spaced minor faults produce a zone of high fracture permeability. Thus, the Black Warrior thermal anomaly appears to reside 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, the structural relationships at Black Warrior may help to guide exploration efforts for other blind geothermal systems in the Great Basin region and other extensional settings.