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The Blackfoot Volcanic Field, Southeast Idaho: A New Structural Paradigm for Hidden Geothermal Resources in the Northeastern Basin and Range

John A. Welhan, Dean L. Garwood, and Michael O. McCurry
Idaho Geological Survey

The Blackfoot volcanic field (BVF), located on the western margin of the Idaho-Wyoming overthrust belt in southeast Idaho, is a Quaternary bimodal volcanic province consisting of extensive basaltic lava flows and rhyolite lava domes in a north-trending Basin and Range graben whose Tertiary normal faults cross-cut west-dipping Cretaceous thrust faults. During the geothermal exploration boom of the 1970s and 80s, the BVF was considered to have high geothermal resource potential but interest waned after a 2.5 km-deep geothermal exploration well drilled near the 58 Ka China Hat rhyolite dome did not encounter temperatures above 100 oC. The temperature information in that borehole, obtained from intermittent soundings during drilling, suggested that the volcanic heat source was already spent, possibly because of limited magma volume, rapid mid-crustal cooling or both. Recent re-examination of the petrology and geochemistry of BVF lavas (McCurry et al, 2012), however, suggests that a substantial magmatic heat source at depths of 12-14 km is not only likely but now appears to be necessary to explain the low-temperature geothermal activity and 3He and CO2 gas fluxes that characterize the southern end of the BVF (Lewicki et al, 2012). Considering the region's geohydrology and structural setting, we propose a new paradigm for hidden geothermal resources where the overthrust belt and eastern Basin and Range merge. Based on the inferred depths and geometry of large-scale thrust faults in this area and the ubiquitous shows of high-temperature geothermal fluids encountered at depths of 3-5 km in wildcat wells east of the BVF, it is possible that fluids heated by a magmatic body at depths of 10-15 km beneath the BVF may rise and move eastward along one or more thrust faults (or along carbonate strata within the thrust sheets) to feed hidden geothermal reservoirs within the overthrust belt. The acidic nature of magmatically influenced fluids would contribute solution-enhanced porosity and permeability to the host thrust faults / sheets and could promote lateral migration well away from the BVF. Such a mechanism would also explain the presence of high-temperature brines reported in wildcat wells east of the BVF and suggests that the geothermal resource potential of this part of the Basin and Range and overthrust belt needs to be reconsidered. Both legacy information and new heat flow data collected for the National Geothermal Data System will be presented.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013