Ni et al., Shear Wave Splitting and Mantle Flow Beneath the Ristra Array

Data from LA RISTRA (Colorado Plateau/Rio Grande Rift/Great Plains Seismic Transect) array, a linear array from the Great Plains (GP) through the Jemez Lineament (JL) and the Rio Grande Rift (RGR) to the Colorado Plateau (CP), are used to determine polarization anisotropy beneath these regions. The results allow us to study mantle deformation, and help to constrain mantle flow beneath the RGR and surrounding regions. The seismic anisotropy beneath the RGR, JL and adjacent regions shows a remarkably consistent pattern with a mean fast direction of about 40 +- 6 and large delay time of 1.4 +- 0.2 seconds. The uniform NE-SW trend of fast split SKS and SKKS waves that traverse the upper mantle beneath the Rio Grande Rift and adjacent regions is oblique to the Rio Grande Rift axis but sub-parallel to the North American absolute plate motion (~62 degrees). A low velocity zone, presumable hot upwelling mantle down to 250 km, underlies this region. Geological data suggest that the lithosphere is thin beneath the RGR and adjacent regions, thus, the anisotropy resides predominantly in the asthenosphere. The observation also argues against an E-W uniform extensional origin for the formation of the RGR. The differential horizontal motion between the North American lithosphere and westerly flow of the asthenospheric mantle best explains seismic anisotropy below the lithosphere. The approximately N-S fast direction found beneath western Texas is similar to that observed beneath the southern RGR. The difference in fast direction between northern and southern RGR can simply be explained by a larger magnitude westerly flow (in the hot spot reference frame) of the asthenosphere beneath the southern RGR. Our results show that the interior of the CP is where lithospheric anisotropy can be significant.

For more information on LA RISTRA shear wave splitting contact:

Jim Ni jni@nmsu.edu,
New Mexico State University, Department of Physics (MSC 3D), Las Cruces, NM 88003

Eric Sandvol sandvole@missouri.edu,
Department of Geological Sciences, University of Missouri, Columbia, MO 65211