LITHOSPHERIC MODIFICATION AT THE SOUTHERN EDGE OF LAURENTIA REVEALED BY BROADBAND SEISMOLOGY

The southern margin of Laurentia has witnessed a wide range of tectonic processes, including deformation due to orogeny and continental collision 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 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 that would not have been expected based on surface tectonics include a slow layer beneath the Texas-GoM margin that we interpret to be a shear zone at the base of the craton and transitional continental lithosphere, a zone that is bounded at its top and bottom by discontinuities and high levels of seismic anisotropy. Addionally a high velocity body underlying the Gulf Coast Plains may mark delaminating lower crust. If true it provides indirect evidence that active rifting best describes the process that lead to the opening of the GoM.

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 and location of this anomaly suggests that it was eroded from the Great Plains craton’s lithosphere, although lithospheric or crustal delamination from the thinned lithosphere beneath the eastern flank of the Rio Grande Rift could explain its origin.