Receiver function analysis of the Texas Gulf Coast to better understand the Ouachita Orogeny, opening of the Gulf of Mexico, and current state of the southern margin of North America

For the past two years, Texas Tech and Baylor universities have been operating 21 broadband seismic stations extending from Matagorda Island, crossing the Gulf Coast plain and Balcones fault, ending in the middle of the Llano Uplift at Johnson City, Texas. The goal of this project is to image the basement to better understand the opening of the Gulf of Mexico (GOM) and the stabilization of the southern lithosphere of North America. In this presentation we will present preliminary results of receiver function analysis of data recorded by our broadband array. Using 18 months' of data we performed common conversion point (CCP) stack imaging of the crust and lithosphere along the profile and have produced a cross section showing receiver function derived Vp/Vs ratios along the profile. The strongest phase on the CCP stack is the Ps phase from the base of a 15 to 20-km-deep (at the shoreline) sedimentary basin that shallows to approximately 1 km deep at the Balcones fault. Vp/Vs analysis shows that this sedimentary unit has Vp/Vs ratios between 1.9 and 2.0. This unusually high Vp/Vs ratio is consistent with the basin being dominated by poorly consolidated, relatively young clastic sediment. This layer pinches out to the northwest. No clear Ps phase is observed from base of the Mesozoic carbonates and Ouachita-related sediments that we believe is several kilometers thick, based on observed Vp/Vs ratios of 1.85 to 1.9 (which is more consistent with carbonate rock than granitic basement). The pattern of Vp/Vs implies near-vertical, crust-wide displacement along the Balcones Fault. The Moho appears to be 15 to 20 km deep near the ocean but descends to more than 40 km beneath the Llano uplift. There is a strong Ps phase from about 70 km depth near the coastline that we interpret as remnant depleted mantle from the rifting of the GOM. The region between this P70s phase and the Moho appear to have a low Vp/Vs ratio (~1.75), which would be consistent with mantle depleted in iron as a result of rifting during the opening of the Gulf of Mexico.