The Yellowstone-snake River Plain Seismic Profiling Experiment: Crustal Structure of the Eastern Snake River Plain

Seismic refraction profiles recorded layer. Crustal subsidence and periodic basaltic along the eastern Snake River Plain (ESRP) in southeastern Idaho during the 1978 YellowstoneSnake River Plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation (Q-i) structure of the ESRP. Travel-time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the ESRP is highly anomalous. Approximately 3 to 6 km of volcanic rocks (with some interbedded sediments) overlie an uppercrustal layer (compressional velocity •6.1 km/s) which thins southwestward along the ESRP from a thickness of 10 km near Island Park Caldera to 2 to 3 km beneath the central and southwestern portions of the ESRP. An intermediatevelocity (•6.5 km/s) layer extends from •10 to •20 km depth. A thick (•22 km) lower crust of compressional velocity 6.8 km/s, a total crustal thickness of •42 km, and a Pn velocity of m7.9 km/s is observed in the ESRP, similar to the western Snake River Plain and the Rocky Mountains Provinces. High attenuation is evident on the amplitude corrected seismic data due to lowQ values in the volcanic rocks (Q = 20 to 200) and throughout thecrust (Qp = 16• to 300). Based on these characteristics of the crustal structure and volcanic-age progression data, it is suggested that the ESRP has resulted from an intensive period of intrusion of mantlederived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse. This activity began about 15 m.y. ago in southwestern Idaho and has migrated northeast to its present position at Yellowstone. Subsequent cooling of the intruded upper crust results in the 6.5 km/s velocity intermediate 1Department of Geosciences, Purdue University, West Lafayette, Indiana 47907. aDepartment of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112. •Institute of Geophysics, Swiss Federal Institute of Technology, CH-8093 Zurich, Switzerland. •Contribution 341, ETH-Geophysics, Zurich, Switzerland. SNow at Exxon Production Research Company, Houston, Texas 77001. 6Now at Arco Oil and Gas Company, ARB, Dallas, volcanism as represented by the ESRP complete the sequence of crustal evolution.