Huge Displacements of the Mantle Lithosphere?

Bird [1992a; 1994] modeled the simultaneous formation of the Rocky Mountains and the Basin/Range province by a single flat subduction event, which he suggested had sheared away and displaced the entire tectonic mantle lithosphere of the western U.S.. According to this model, the only tectonic mantle lithosphere (defined as cold and strong) remaining in the western U.S. should be a 40-km layer which has formed by cooling since mid-Tertiary times. Geochemical objections have been raised to this model [e.g., Livaccari and Perry, 1993; 1994], especially that geochemical lithosphere (defined by certain element and isotope concentrations) is still present. But several recent studies have recently shown that the seismic lithosphere (defined by high velocity and low attenuation) has roughly the predicted structure. Humphreys and Dueker [1994] performed a regional tomographic inversion which confirmed that upper-mantle seismic velocities are systematically slower in the western U.S. than in the east, with the differences confined to the uppermost 300 km. In a profile from Utah to Kansas, PS conversions at the Moho show that the crust thickens eastward from the Colorado Plateau to the Great Plains, so that the high topography of the former must be compensated in the mantle [ Sheehan et al., 1992]. An inversion of teleseismic data [ Halderman and Davis, 1991] shows mantle lithosphere is 80 km thicker on the east side of the Rio Grande rift than on the west. This compares well with analysis of gravity data by Cordell et al. [1991] which shows mantle lithosphere thicknesses of 200 km to the East, but only 50-125 km to the west. Also, Beghoul et al. [1993] used teleseismic travel times to show that mantle lithosphere is typically 20-50 km under the Basin/Range and Colorado Plateau, but 150-190 km under the Great Plains. A tomographic image of uppermost-mantle (P) velocity in the western U.S. [ Hearn et al., 1991] shows that within the low-velocity region, local seismic velocity is lowest in areas of Neogene extension, and along the Yellowstone plume track.

Teleseismic shear wave splitting and polarization provide an exciting new tool to determine the stretching direction of the upper mantle fabric. At 3 sites in the west-central U.S., these directions are east/northeast-west/southwest [ Silver and Chan, 1991]. If these fabrics are in the lithosphere, they are inconsistent with Bird's model; but if they are in the asthenosphere below, they are entirely consistent with past shallow-angle subduction. Improved depth resolution should be a priority.