INVESTIGATING CONTINENTAL RIFTING IN THE WESTERN US WITH SEISMIC METHODS

Continental rift zones are structures of major geological significance and play host to processes that rupture of continental lithosphere. Many factors are known to contribute to the initiation and evolution of these rifts, including variations in temperature and composition, zones of existing structural weakness, and lithosphere-asthenosphere interactions. However, which features and processes govern the strain localization of any particular segment or stage in a rift system, remains poorly understood. The well-instrumented Rio Grande Rift, located at the boundary between the wide-mode extending Basin and Range province, the Colorado Plateau, and the stable North American craton to the east, provides an excellent opportunity to study active rifting using geophysical methods. The surface expression of the rift is a series of basins extending from Colorado south to Mexico. The basins tend to be smaller and more focused in the north, becoming more broadly distributed in the south. At depth, past seismic imaging has revealed a symmetric, distributed mode of deformation through the crust. We relate structural observations at the surface with deeper processes in the crust and upper mantle by applying a battery of seismological analysis including imaging with converted and multiply reflected waves, body wave tomography, and surface wave dispersion. Particular focus is placed on estimating the uncertainty in measurements of velocity, interface depth, and Vp/Vs ratio in order to better constrain possible temperature variations. We quantify the lateral extent of the rift at depth, as a means of constraining the strain-localization at depth. Finally, we discuss the connection of lithospheric structures observed with processes occurring below the lithosphere.