Rayleigh Wave Constraints on Shear-Wave Structure and Azimuthal Anisotropy Beneath the Colorado Rocky Mountains

Abstract

We inverted Rayleigh wave data recorded in the Rocky Mountain Front Broadband Seismic Experiment for shear-wave velocity structure and azimuthal anisotropy. Distinctive structures are imaged beneath the southern Rocky Mountains, the western Great Plains, and the eastern Colorado Plateau. Beneath the southern Rockies, shear velocities are anomalously low from the Moho to depths of 150 km or more, suggesting replacement or delamination of the mantle lithosphere. The lowest velocities are beneath the extension of the Rio Grande rift into southern Colorado and are probably associated with partial melt. Beneath the Colorado Plateau, a thin, high-velocity lid is underlain by a low velocity layer to a depth of at least 160 km. Under the high plains, the velocities are above average down to ~150 km depth, but not as fast as beneath the cratonic core of the continent. A crustal, low-velocity anomaly is observed beneath the high elevations of central Colorado. Elsewhere, inferred crustal thickness correlates with elevation, with the thickest crust beneath the San Juan Mountains in southwestern Colorado. These crustal anomalies suggest that much of the isostatic compensation for the high topography takes place withn the crust. We observe a simple pattern of azimuthal anisotropy in the Rocky Mountain region with fast directions rotated slightly counterclockwise from the absolute plate motion of the North America plate and strength increasing with period. The observed anisotropy can be explained by deep asthenospheric flow dominated by current plate motion and shallower and perhaps laterally variable anisotropy in the upper lithosphere.