The geology and geothermal setting of the Magic Reservoir Area, Blaine and Camas Counties, Idaho

The Magic Reservoir area straddles the Blaine- Camas county line in south-central Idaho, along the northern boundary of the central Snake River Plain. The rocks exposed at Magic Reservoir include a 5.8- million-year-old rhyolite flow, the Pliocene Square Mountain Basalt, multiple cooling units of a 5.6- million-year-old rhyolite ash-flow tuff, a 4.7-millionyear- old rhyolite dome, and Quaternary basalt flows and sediments. These newly reported ages for the rhyolites at Magic Reservoir reveal that they are the youngest, westernmost silicic volcanic rocks presently known in the Snake River Plain. They represent an anomalously young rhyolitic event that may include the Moonstone Mountain rhyolite dome to the northwest and the previously dated (Armstrong and others, 1975) 3-million-ycar-old Wedge Butte rhyolite dome to the southeast of Magic Reservoir. The area is cut by numerous normal faults trending northwest, northeast, and west. The northwest-trending faults are the dominant structures. They form a horst block at Hot Springs Landing and parallel the regional structural grain. The geothermal resource at Magic Reservoir occurs within the elevated heat flow province at the northern margin of the Snake River Plain. The system is probably controlled by the deep, convectivce circulation of fluids along faults at the intersection of the Hot Springs Landing horst with west- and northeast-trending fractures. Although the volcanic rocks in the Magic Reservoir area are young, they are too old to contribute any magmatic heat to the geothermal system.