Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography

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Read Full Article (file size: 949068 bytes) Cited by JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B03305, doi:10.1029/2003JB002743, 2004 Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography Wei Gao Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA Stephen P. Grand Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA W. Scott Baldridge Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA David Wilson Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA Michael West Department of Physics, New Mexico State University, Las Cruces, New Mexico, USA James F. Ni Department of Physics, New Mexico State University, Las Cruces, New Mexico, USA Richard Aster Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA Abstract We present models for upper mantle P and S velocity structure beneath a southwestern United States transect extending from near the center of the Colorado Plateau across the Rio Grande rift to the Great Plains. The models were derived from travel times of compressional and shear seismic phases recorded by the La Ristra passive seismic array deployed from July 1999 to May 2001. Large variations in seismic velocity (up to 8% in S and 5% in P) are seen across the transect in the upper 200 km of the mantle. Seismically slow mantle underlies the Rio Grande rift and Jemez lineament and relatively slow mantle is seen beneath the Navajo volcanic field within the Colorado Plateau. The relative variations of P and S velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out. Sharp boundaries in mantle seismic velocity are coincident with boundaries of Proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region. Weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images. Two of the structures have fast seismic anomalies, beneath the central Colorado Plateau and the Great Plains respectively, and the third has anomalously slow seismic wave speed. We interpret the western deep seismic anomaly to be foundering Farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking Farallon slab. Beneath the Great Plains, there is also downwelling in the upper mantle. The combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift. Received 13 August 2003; accepted 22 January 2004; published 19 March 2004. Index Terms: 7218 Seismology: Lithosphere and upper mantle; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 8180 Tectonophysics: Tomography. Read Full Article (file size: 949068 bytes) Cited by Citation: Gao, W., S. P. Grand, W. S. Baldridge, D. Wilson, M. West, J. F. Ni, and R. Aster (2004), Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography, J. Geophys. Res., 109, B03305, doi:10.1029/2003JB002743. Copyright 2004 by the American Geophysical Union.

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