Geothermal significance of magnetotelluric sounding in the Eastern Snake River Plain-Yellowstone Region

Abstract

Magnetotelluric soundings along a profile extending from the Raft River geothermal area in southern Idaho to Yellowstone National Park in Wyoming reveal a highly anomalous crustal structure involving a conductive zone at depths that range from 18 km in the central part of the eastern Snake River Plain to 7 km beneath the Raft River thermal area and as little as 5 km in Yellowstone. Resistivities in this conductive zone are less than 10 ohm m and at some sites less than 1 ohm m. Structural parameters obtained in processing the magnetotelluric data suggest the possibility of a conductive axis along the center of the eastern Snake River Plain, and these parameters also point to very conductive structures beneath the Yellowstone caldera system. A sounding completed in the Island Park caldera can only be modeled with a crustal structure very different from the Yellowstone caldera system, requiring the absence of this conductive zone to depths greater than 25 km in the Island Park caldera. In addition to the deep conductive zone the thickness of extensive surface basalts in the eastern Snake River Plain was mapped geophysically, and units between the basalts and the deep conductive zone were also well defined and fitted to geologic models.