We have been investigating the potential for commercial high-temperature geothermal development in the Snake River Plain (SRP) in Idaho. The SRP is a very large basaltic province that was formed following the passage of the Yellowstone mantle plume. Although the province is characterized by high heat flow in general, the presence of high-temperature geothermal systems within reasonable drilling depths will require relatively shallow magmatic heat sources. Several studies now relate the geometry of mafic heat sources to the rates of magma supply versus tectonic extension. High extension with respect to magma supply results in feeder dikes and rapid magma ascent to the surface whereas high magma supply with respect to extension produces sills or plutons that have sufficient residence time to be an effective heat source. Mountain Home Air Force Base (MHAFB) is located on the southern margin of a gravity and magnetic high that is associated with a large layered sill complex. The initial geothermal drilling (MH-1) at MHAFB in 1986 reached a depth of 1342 m, and continuous core was collected below a depth of 305 m. The maximum temperature measured was 93ºC at a depth of 1207 m, the maximum depth to which the temperature log was run. In 2012, we cored hole MH-2 to a depth of 1821 m 4 km away from MH-1 and measured a maximum temperature of 150o C. Analysis of the geologic data from these holes shows that the hydrothermal activity is hosted by a normal fault zone associated with extension over the top of the sill complex.

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