Lithospheric structure of the eastern flank of the Rio Grande Rift via receiver function velocity analysis

To better delineate a seismic anomaly beneath the eastern flank of the Rio Grande Rift identified by seismic tomography, we depth-migrated Ps and Sp receiver functions using data from the SIEDCAR (Seismic Investigation of Edge Driven Convection Association with Rio Grande Rift) and USArray Transportable Array (TA) deployments. We performed Common Conversion Point (CCP) stacking to improve the S/N ratio of receiver functions. Using an incorrect velocity model for depth migration of a stacked CCP image may generate an inaccurate picture of the subsurface. To find sufficiently accurate P- and S-velocity models for migration, we optimize the average correlation value of common receiver gathers for target features - in this case the Moho and the LAB - while perturbing the shear wave velocities in a process driven by simulated annealing. The technique simultaneously finds depths to major discontinuities (in this case the Moho and LAB) and S and P velocity profiles beneath each seismic station in a manner that is similar to velocity analysis in reflection seismology. An application to data acquired in southeastern New Mexico and west Texas, at an average station spacing of 35 km, reveals an abrupt increase in lithospheric thickness from west to east, from the Rio Grande Rift to the Great Plains craton. Previous studies found an elongated high velocity anomaly that extends to depths approaching 500 km in southeastern New Mexico and west Texas that is distinct from the thick Great Plains lithosphere. Our stacked 3-D image confirms the anomaly's existence and shows that it is more laterally extensive than was previously indicated. Recent numerical modeling suggests that an abrupt change in lithospheric thickness, which creates a step change in densities, may produce a gravitational instability that leads to thicker mantle lithosphere dripping off into the lower density asthenosphere. As the mantle deforms it alternately thickens and thins the crust, producing topographic anomalies that correlate with the disappearing mantle lithosphere. The northward time progression of rifting, variable width of the rift, and varying elevations and locations of uplifted mountain ranges in the study area allow us to evaluate this mechanism's role in the evolution of this rift-craton transition.