SP RECEIVER FUNCTION IMAGING OF A TRANSECT ACROSS TEXAS’S GULF COAST PLAIN

The nature of the rifting that opened the Gulf of Mexico (GoM) and led to the formation of Texas’ Gulf Coastal Plain (GCP) is not clear. Seismic data that are capable of imaging the deep crustal and upper mantle structure beneath the GCP, and thereby inform models of rifting, are limited and sparse. We therefore deployed a broadband seismic array across the GCP for 2.5 years in an effort to clarify the structure beneath the region.

The seismic array consists of 23 three-component broadband seismometers that were deployed from July 2010 until January 2013. The seismic stations trend in a northwesterly direction from Matagorda Island to Johnson City, Texas, spaced 16-20 km apart. We will present images of the deep crustal structure beneath the seismic array computed from Ps and Sp receiver functions from teleseismic events of magnitude five or greater, at epicentral distances 30-95 degrees from the stations.

Receiver functions were computed using a Matlab program called "Medbow" (Hansen and Dueker, 2011). The Sp receiver functions’ CCP stacked image shows a large-amplitude negative event at ~100 km depth at the southeastern end of the transect that deepens to ~120 km depth at the northeastern end (beneath the Llano Uplift). Events of this polarity at this depth have been interpreted elsewhere as the lithosphere-asthenosphere boundary (LAB; indeed this observation is consistent with the results of Abt et al., 2009, for this region), and a number of mechanisms by which the LAB might produce a sufficiently sharp discontinuity have been proposed. However, a second event, also with negative polarity, appears in the Sp CCP image that extends from beneath the Llano, at 150 km depth, to roughly Cuero, TX, at 170 km depth. An event of the same polarity at similar depths was observed beneath the Kalahari Craton by by Savage and Silver (2008); this second event is likely to have a different mechanism (one that must explain its abrupt end beneath the GCP). Possible mechanisms for both of these events will be reviewed.