Back to list of UTIG abstract submissions, Fall Agu 2003

HR: 0830h
AN: S31E-0810

AU: * Gao, W
EM: gao@maestro.geo.utexas.edu
AF: Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712 United States
AU: Grand, S P
EM: steveg@maestro.geo.utexas.edu
AF: Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712 United States
AU: Baldridge, W S
EM: sbaldridge@lanl.gov
AF: Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AU: Wilson, D
EM: davew@ees.nmt.edu
AF: Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States
AU: West, M
EM: west@nmsu.edu
AF: Department of Physics, MSC 3D, New Mexico State University, Las Cruces, NM 88003 United States
AU: Ni, J F
EM: jni@nmsu.edu
AF: Department of Physics, MSC 3D, New Mexico State University, Las Cruces, NM 88003 United States
AU: Aster, R
EM: aster@ees.nmt.edu
AF: Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States

Abstract
We present new images of the P and S wave seismic structure of the upper mantle beneath a transect extending from the Colorado Plateau across the Rio Grande Rift to the Great Plains of western Texas. The upper mantle structure was determined using travel times from the LA RISTRA passive seismic array deployed from July 1999 to May 2001. Large seismic anomalies are seen (up to 8% variation in S wave speed and up to 5% in P wave speed) in the shallow mantle beneath the array. The seismically slow shallow mantle lies beneath the Rift as well as to the west beneath Mount Taylor, part of the Jemez lineament of volcanic activity. Relative P to S variations indicate the cause of the slow shallow mantle is primarily higher temperature with possibly a small degree of partial melt. The mantle deeper than 150 km beneath the rift is not anomalously slow. The deeper upper mantle does show several eastward dipping seismic structures. Beneath the western Colorado Plateau a fast anomaly is seen near 500 km depth that we interpret to be the trailing edge of the Farallon plate as proposed by Van der Lee and Nolet (1998). To the east of that anomaly is a slow seismic anomaly at 400 km depth that appears to connect to the slow shallow anomaly beneath Mount Taylor. Beneath the Great Plains a narrow fast structure is imaged throughout the upper mantle from 200 to 600 km depth. We interpret the deep slow anomaly to be an upwelling, possibly with higher than normal concentrations of volatiles, and the fast Great Plains anomaly to be an associated downwelling. Petrologic data indicate the lithosphere has been thinned within the past 10 Ma beneath the Rift. There has also been uplift, increased extension within the Rift, and increased volcanic activity during that time period. A possible cause of this recent tectonic activity is the removal of lithosphere from beneath the Rift and Mount Taylor. The removed lithosphere may be entrained in the imaged downwelling beneath the Great Plains.