Seismic constraints on crustal thickness and Moho structure of the northwestern Gulf of Mexico margin

Abstract:

The Gulf of Mexico has undergone two Wilson cycles of continental rifting and collision. Structural artifacts associated with these events are difficult to investigate due to the thick layer of sediments in the northwestern Gulf. The crust beneath the sediments is challenging to model using seismic techniques because of sparse monitoring and low rates of seismicity in the region. To overcome these shortcomings we use a combination of different data types to create a 2D model in which the various data types provide strong constraints on the model parameters with distinct and complementary sampling of the subsurface. The data used in this study comes from teleseismic and regional earthquakes, postcritical SsPmp arrivals, and direct P wave energy identified with seismic interferometry.

The data used in this study was recorded by a 107-km-long array consisting of 412 4.5 Hz vertical component geophones with 125-A Reftek Texan digitizers) and a 290-km-long broadband seismometer deployment with 22 stations (X4 2010-13; 22 Reftek 130 digtizers and 6 Guralp CMG-3T, 4 CMG-3ESP, 2 Guralp CMG-40T, and 10 Nanometrics Trillium Compact broadband sensors). The two arrays partly overlapped each other and formed a linear transect across the northwest margin of the Gulf of Mexico from Matagorda Island to Johnson City, TX.

The Moho discontinuity along the transect is constrained by Pn and the postcritical SsPmp phase, where a previous receiver function study had suggested that the Moho was absent beneath the seaward portion of the transect, and the discontinuity varies from a depth of ~28 km near the coast to ~40 km beneath the Llano Uplift. An important feature observed in the 2D model was a high velocity body in the crust, ~60 km in width, and collocated with the Houston Magnetic Anomaly (HMA). The HMA has been interpreted to be an ophiolite-bearing zone indicating rocks from the Alleghenian orogeny. The sediment-basement contact is at a depth of ~15 km at the coast in the southeast, grading to zero depth to the northwest. The thickness of the crust decreases from northwest to southeast, indicating that most of the margin’s extension occurred outboard of the Ouachita orogeny.