Baylor University & UTIG
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The southern margin of Laurentia has been modified by a wide range of tectonic processes, including deformation due to repeated orogenies and continental collisions (i.e. Llano, Ancestral Rockies, Ouachita, Laramide) as well as ongoing rifting (at the Rio Grande Rift) and completed rifting (which created the Gulf of Mexico, GoM). Artifacts of these processes are likely to remain at lithospheric depths beneath the region but, until recently, the tools needed to examine the 2D and 3D structures at mantle depths were not available. With the passage of the EarthScope's USArray stations and two targeted broadband deployments, new images of the region's lithosphere have emerged. These images reveal lithospheric-scale anomalies that correlate strongly with surface features such as a large fast anomaly that corresponds to the southern extent of the Laurentia (or "Great Plains") craton and a large slow anomaly associated with the Southern Oklahoma Aulacogen. Other features were unanticipated, including slow shear velocities beneath the Texas-GoM margin, a layer that is bounded at its top and bottom by seismic discontinuities, and high levels of seismic anisotropy.
Lithospheric thinning associated with the Rio Grande Rift appears to be propagating eastward and eroding the Great Plains craton. A large, seismically fast anomaly imaged beneath west Texas and southeastern New Mexico has implications for the region's structural evolution. The size, seismic velocity, and location of this anomaly suggests that it may be the result of a "step instability," in which progressive deformation has locally removed mantle lithosphere beneath the eastern flank of the Rio Grande Rift, resulting in thickened lithosphere at the western edge of the Great Plains craton and uplift beneath the rift flank.