Exploration beneath volcanics; Snake River plain, Idaho

Abstract Coincident gravity, magnetotelluric (MT), and seismic reflection-refraction surveys were conducted along a line traversing the boundary between the volcanic-covered Eastern Snake River plain, Idaho, and the sediment-covered Western overthrust belt. One-dimensional MT and seismic refraction analysis away from the immediate boundary between sediments and volcanics successfully maps the depths to units of igneous and sedimentary--or metasedimentary--rock. Substantially different models of the volcanic-covered terrain and of the sediment-covered terrain are linked by a gravity anomaly and by an elongation and rotation of MT anisotropy ellipses which indicate a fault. Fault dip, depth extent, and motion are constrained by marked changes in the first-arrival apparent velocities and by an electrical marker layer. The analysis shows that metasediments extend laterally at least 16 km beneath the volcanic cover. The two terrains are connected by a boundary zone which is a major normal fault.