NEW INSIGHT INTO THE LITHOSPHERE OF THE SOUTHERN MARGIN OF NORTH AMERICA FROM EARTHSCOPE SEISMIC DATA

The Gulf Coast of Texas is characterized as a passive margin outboard of what is considered to be stable continental lithosphere. However, seismic models (tomographic, receiver function, and seismic anisotropy) produced from data collected by the EarthScope Transportable Array (a 70 km grid of seismic stations) and a 2-D profile of 26 broadband stations across the Gulf Coastal Plain (GCP) suggest that the lithosphere beneath parts of this region may experience flow or modification due to recent tectonic events. High seismic P and S velocities observed to depths of ~200 km beneath central Texas indicate a stable, traditional ~200 km thick continental lithosphere. Negatively polarized Sp phases (in receiver functions) at depths of about 110 km beneath the GCP and outermost Llano Uplift were interpreted as the Lithosphere-Asthenosphere boundary (LAB). Observations of additional Sp phases (positively and negatively polarized) lead us to believe that the 100 to 200 km depth range beneath the GCP and Llano uplift is characterized by a lithosphere-asthenosphere transition zone (LATZ) that is the result of layered mantle flow in this depth interval. The presence of low S-wave velocities but high P-velocities in tomography models and large delay times in split SKS waves beneath the GCP and Llano uplift support the hypothesis of mantle flow.

Further, the distribution of low velocity anomalies observed in Pn and body wave tomography models in areas beneath the Southern Oklahoma Aulacogen (SOA) suggest flow of warm mantle material beneath the SOA from the Rockies and Rio Grande Rift toward the GCP. Such mantle flow beneath the SOA is consistent with the change in polarization direction of SKS splitting measurements across the SOA.

Lastly, Ps receiver function imaging of the GCP profile reveals remnants of subducted crust beneath the GCP along the central portion of the Texas coast, which suggests that the outboard dipping, late Paleozoic slab associated with the assembly of Pangaea may have been preserved. Its preservation would imply that rifting associated with the opening of the GCP did not completely overprint older features in this region.