| Abstract |
We have conducted a detailed structural assessment (detailed geologic mapping of Tertiary bedrock,structural analysis, stress determinations, andintegration with existing geophysical data) to better elucidate the controls on fluid flow in the Reese River geothermal system in the Basin and Range of central Nevada, western USA. The Reese River geothermal area is a ~150°C blind system that occupies a 10 km-long by 5 km-wide unnamed basin (herein referred to as the Manhattan basin) between the Shoshone Mountains and the Shoshone Range. The Manhattan basin is a complexly faulted, NNE-trending graben that is transected by a NE-SW-trending synclinal accommodation zone. Within the basin Tertiary volcanic and sedimentary strata reach 0.4-0.8 km-thick and rest nonconformably on Paleozoic sedimentary and Mesozoic crystalline basement rocks. A horst block composed mostly of Paleozoic sedimentary rock bounds the east side of Manhattan basin and is segmented, with the southern half trending N10°E and the northern half trending N40°E. Throughout the entire basin faults dip 35-80° at the surface, and all observed fault surfaces accommodated normal or slightly oblique normal slip. Slip directions are relatively homogeneous, suggesting that all faults formed in the same stress field with a WNW-trending (297° azimuth) extension direction. The only faults identified as active in the Quaternary are the faults bounding both sides of the horst block. Temperature-gradient holes and exploration wells define a broad NNE-SSW-trending temperature anomaly in the east half of Manhattan basin between and subparallel to both the horst block and the synclinal accommodation zone. The peak temperature gradient among historic temperature gradient holes (<150 m deep) and peak measured temperature in the deep wells (over 1500 m deep) lie directly west-northwest and west, respectively, of the major bend in the horst block. Fault slip data indicate that the extension direction is nearly orthogonal to the bend in the horst block. This part of the fault system would therefore accommodate greater dilation than other segments. Multiple fault strands near the fault bend probably increase permeability. Intersecting WNWand ESE-dipping faults within the synclinal accommodation zone in the Manhattan basin may also contribute to increased permeability within the basin. Considering the relatively thin Tertiary section, the major geothermal reservoir is likely hosted by the Paleozoic and Mesozoic basement. Inherent basement structures and fracture networks in more competent basement lithologies (e.g., granitic or carbonate rocks) may further enhance geothermal potential where intersected by favorable late Cenozoic structures. |