Imaging Crustal and Uppermost Mantle Structure Beneath the South Central United States from a Joint Inversion of Receiver Functions and Surface Wave Dispersion

The South Central United States has a long tectonic history with a mosaic of tectonic features dating back to the Paleoproterozic. Imaging the structures at depth of both the passive margin and orogenic belts in this region, would allow us to better understand the tectonic mechanisms and evolution of continents and ocean basins. In spite of numerous geophysical investigations, the lithospheric structure of the region remain enigmatic, which is due largely to the presence of a thick unconsolidated sedimentary layer atop most of the areas that prevents reliable seismic imaging of the underlying structures. In the present study, broadband seismic data recorded by 190 USArray TA stations are mapped into a high-resolution 3-D Vs model for the upper 180 km by joint inversion of receiver functions (RFs) and surface wave dispersions. To suppress the sedimentary effects in imaging, a frequency-domain resonance removal filter is applied to reduce the reverberations of sediments in the RFs at individual stations. This is following by H- (i.e., crustal thickness-Vp/Vs) RF stacking to provide a priori structural constraints for the joint inversion. Rayleigh wave phase velocity dispersions are extracted from ambient noise at short periods (6-24 s) and from teleseismic data at longer periods (28-120 s), and are combined with the processed RFs to invert for the shear velocity (Vs) structure. Our imaging result shows that the coastal plain area has thinner crust (~30 km) than the rest areas (~45 km) with the thinnest crust of 25 km along the coastal line. Besides the thinned crust, the entire lithosphere underlying the coastal plain shows lower than normal shear wave velocities, which are mostly related to the high temperature and decompression melting associated with lithospheric stretching. In the vicinity of the Llano Uplift, normal crustal Vs, thickness, and are observed, suggesting the absence of the previously proposed lower crustal delamination. Beneath the Ouachita Mountains, the observed strong P-to-S conversions from the Moho, when excluding the potential effects caused by a sharp or uneven distributed Moho, are most likely caused by a large velocity contrast. Combined with our relatively small , H, and lower crustal Vs measurements, the strong P-to-S conversions may suggest the occurrence of lower crustal delamination.