PS RECEIVER FUNCTIONS AND VP/VS ANALYSIS OF THE TEXAS GULF COAST PLAIN AND LLANO UPLIFT

A deployment of 23 broad-band seismometers were installed, during the summer of 2010, along a line from Matagorda Island, Texas onto the Llano Uplift near Johnson City, Texas. The goal of this investigation is to image the basement and upper mantle beneath the Gulf Coast Plain (GCP), the Balcones fault (BF) and the portion of the Llano Uplift deformed by the Ouachita orogeny. A single station Vp/Vs ratio method that aligns the Ps, PPs, PSs, and PPp arrivals from selected interfaces from each station was used to produce a 2-D cross section of Vp/Vs ratios to a depth of 80 km along the profile. A P-velocity model along the profile was constructed using regional velocity values derived from well log data for the top 3-5 km and for greater depths the velocities were taken from refraction profiles shot near the line in the 1960s. The Vp/Vs velocity profile was used to convert this Vp model into a Vs model. These Vp and Vs models were then used to compute moveout corrections to preform CCP stacking of RFs to produce a 2-D image along the transect.

The Vp/Vs ratio cross-section CCP stack produced from the Ps receiver functions revealed a number of significant features. The sediment-basement contact was the strongest horizon these images. The Moho appears at depths of 30-40 km across most of the profile but the Moho phase beneath the Balcones faults (BF) appears to be missing or may have a negative polarization. There was a positive anomaly beneath the Balcones escarpment at 20 km depth, which we interpret as a mid-crustal horizon rather than the Moho. Low Vp/Vs ratios beneath the Moho, at depths of 30-50 km, at the southernmost part of the image are suggestive of an iron depleted region that is typical of a “rift pillow”, which would have been emplaced during rifting that opened the Gulf of Mexico. The base of this body is also imaged in the CCP stack of RFs. A broadly negative region in the mantle directly outboard of the Balcones escarpment may indicate mantle flow resulting from sediment loading, and is a possible mechanism for the Llano Uplift and erosion of the Moho beneath the BF. There appears to be a wedge shape split in the Moho phase inboard of the BF beneath the Llano uplift. This may to be Ouachita material, possibly delaminated from the colliding terrain or under-plating associated with emplacements during Late-Cretaceous volcanism.