Characterization of a blind geothermal prospect through LiDAR analysis and shallow temperature survey, Gabbs Valley, Nye and Mineral Co., NV
AdvisorBell, John W
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The Gabbs Valley geothermal prospect is located in west-central Nevada within the Basin and Range province at the transition between NW directed dextral shear in the Walker Lane and WNW-ESE extension in the Basin and Range. The tectonics at the prospect site were investigated using Quaternary fault mapping from LiDAR and low sun-angle aerial photos. Reconnaissance geochemistry from a spring seep on a Holocene fault scarp, and a two-meter temperature survey were used to investigate geothermal activity. Active Holocene faulting has been associated with geothermal systems in the Basin and Range province and forms the basis for a conceptual model of geothermal systems: repeated and recent fault motions increase permeability in faults and associated fractures allowing for the circulation of fluids to geothermal heat sources and back to shallow depths. Gabbs Valley is bounded by inwardly dipping east and west dipping range bounding normal faults. These faults along with others in the basin form an anticlinal accommodation zone. Major Quaternary fault trends in the study area are the East Monte Cristo fault zone, Phillips Wash fault zone, and southern Gabbs Valley faults. These faults have discontinuous rupture patterns that span the width of Gabbs Valley between the Monte Cristo Mountains and Paradise Range and have been involved in two historical large magnitude surface rupturing earthquakes. These earthquake events record ongoing deformation in the accommodation zone. Ambiguous lateral slip indicators are inconsistent with Riedel simple shear models for the central Walker Lane, and long lived Quaternary normal faulting in Gabbs Valley indicate deformation is due to Basin and Range style extension.The Cobble Cuesta anticline is a broad up-right fold within the anticlinal accommodation zone. The Phillips Wash and southern Gabbs Valley faults breach the anticline near its axis, on the west side of Cobble Cuesta. Regional attitude data depict north striking moderately east dipping late Tertiary strata and are similar to the dip of the eastern fold limb. Western dips in the west limb of the fold are inferred to form as a result of reverse drag on an east dipping fault that shallows at depth. The fold axis (~N20°E) is parallel to the major bounding normal faults and perpendicular to the late-Tertiary and contemporary extension direction (N69°W). This geometry is consistent with formation as an extensional fault bend fold. Two-meter temperature survey results indicate a shallow thermal anomaly west of Cobble Cuesta. This anomaly is oriented in a WNW direction and is 2.5 km long and 1.5 km wide. The anomaly is bounded on the east by a critically stressed north striking, west dipping normal fault. Three complementary structural models are consistent with the location of the thermal anomaly: 1) Nested west dipping normal faults control permeability. 2) A gap in Quaternary faulting is due to a long lived fault rupture termination that may have increased stresses in the area, promoting micro-fractures and permeability. 3) A dilatant fault intersection exists between the mapped N striking fault and a concealed NW striking fault. The faults in these models lie within the actively deforming anticlinal accommodation zone, and suggest a genetic link between geothermal activity and active faulting within the accommodation zone.