HR: 1330h
AN: S22A-0420    [PDF]
TI: Lithospheric and Upper Mantle Structure of the Rio Grande Rift: Implications for Pure Shear Extension
AU: * Wilson, D C
EM: davew@nmt.edu
AF: New Mexico Institute of Mining and Technology, Dept. of Earth and Environmental Science, Socorro, NM 87801 United States
AU: West, M
EM: west@nmsu.edu
AF: New Mexico State University, Department of Physics (MSC 3D), Las Cruces, NM 88003 United States
AU: Aster, R
EM: aster@ees.nmt.edu
AF: New Mexico Institute of Mining and Technology, Dept. of Earth and Environmental Science, Socorro, NM 87801 United States
AU: Ni, J
EM: jni@nmsu.edu
AF: New Mexico State University, Department of Physics (MSC 3D), Las Cruces, NM 88003 United States
AU: Gao, W
EM: gao@speer.geo.utexas.edu
AF: University of Texas, Austin, Department of Geological Sciences, Austin, TX 78712 United States
AU: Grand, S
EM: steveg@speer.geo.utexas.edu
AF: University of Texas, Austin, Department of Geological Sciences, Austin, TX 78712 United States
AU: Baldridge, W S
EM: sbaldridge@lanl.gov
AF: Los Alamos National Laboratory, Earth and Environmental Sciences Division, MS D462, Los Alamos, NM 87545 United States
AU: Semken, S
EM: semken@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287 United States
AB: Results from the Colorado Plateau-Rio Grande Rift-Great Plains seismic transect (LA RISTRA) experiment are consistent with a pure shear extension mechanism for the Rio Grande rift (RGR). LA RISTRA was a 950 km-long PASSCAL broadband seismic line with approximately 18 km station spacing deployed during 1999-2001 along a great circle from Lake Powell, UT to Pecos, TX, crossing the RGR near 34.5 degrees N. We image crust and uppermost mantle discontinuity structure along the transect by migrating receiver functions to produce a high resolution P-to-S converted phase image. Receiver function results show crustal thickness ranging from 45 to 50 km beneath both the Colorado Plateau and the Great Plains, thinning to a minimum of approximately 37 km centered beneath the RGR axis. The centering of the thinnest crust on the rift axis indicates that the deep crust has undergone primarily pure shear extension. Inversion of LA RISTRA surface wave data and tomographic inversion of teleseismic body-wave delay times for upper-mantle structure show a broad low velocity region, also centered beneath the rift axis. This low-velocity region is interpreted as rift-centered lithospheric thinning, indicating that lithospheric deformation, like that of the deep crust, is also primarily pure shear. A pure shear extensional mechanism for the RGR is consistent with geochemical evidence suggesting that the source region for rift related magmatism has evolved from predominantly lithospheric to asthenospheric mantle sources. This geochemical evolution of magmatism has been interpreted as lithospheric thinning, with the greatest thinning centered beneath the rift axis. Pure shear extension is further supported by processing regional gravity data to produce the decompensative gravity anomaly, which again shows symmetric thinning of the lithosphere closely tracking the rift axis. We conclude that while the upper crust along the RGR has undergone brittle deformation expressed as a series of asymmetric grabens, the lower crust and mantle lithosphere of the RGR have thinned symmetrically about the rift axis, indicating a pure shear mode of lithospheric deformation.
UR: http://www.ees.nmt.edu/Geop/Ristra/ristra.html
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
DE: 8010 Fractures and faults
DE: 8020 Mechanics
SC: Seismology [S]
MN: 2003 Fall Meeting