Sp receiver function imaging of a passive margin: Transect across Texas's Gulf Coastal Plain

The Gulf Coast of Texas has been the subject of intensive geological and geophysical investigation in pursuit of hydrocarbons but studies that penetrate beyond the upper crust are limited to a few refraction profiles and regional surface wave investigations. The passing of EarthScope's Transportable Array has facilitated regional investigations of the lithosphere but its 70-km station spacing does not allow many important tectonic features to be imaged. A broadband seismic transect across the Texas Gulf Coastal Plain was therefore performed in order to image deep structure beneath this passive margin and the transition to the neighboring craton. A 2D Sp receiver function common conversion point (CCP) stacked image produced for this transect reveals several discontinuities in the sub-crustal lithosphere.The region nearest the shoreline is underlain by an anomalous ~18 km thick low velocity layer that produces a strong negative pulse in the Sp receiver functions. The drop in velocity is too large to be due to any reasonable change in Fe or Mg content but could be produced by partial melt or mantle hydration. It is unlikely that partial melt would still be found in a 160-180-year-old passive margin, such as the Gulf Coast, but hydration, possibly introduced by a through-going Balcones fault system, and resulting serpentinization could produce the observed anomaly.An event with negative polarity appears at a depth of ~110 km, which we interpret to be the lithosphere-asthenosphere boundary (LAB). Thermal variations alone would not produce a sufficiently sharp discontinuity to be imaged by Sp converted phases. Recent shear-wave splitting studies revealed unusually large delay times in this region, along with fast polarization directions that differ from measurements on the Laurentian craton. Large delay times may imply significant flow, which could also produce frictional heating, due to shearing, and partial melt, which would steepen the velocity gradients.An additional layer is bounded by discontinuities at 145-165 km depth (with negative polarity) and 185-205 km depth (with positive polarity). The appearance of these discontinuities suggests that the mantle beneath the LAB is more complicated here than elsewhere and that the transition from lithosphere to asthenosphere occurs over a broad (85-100-km-thick) depth interval.