Mantle Velocity Structure and Anisotropy beneath the Gulf of Mexico and Adjacent Areas: Constraining the Geometry of the North American Continental Keel and Possible Asthenospheric Flow Field around It

The 3-D geometry of the continental keel and the existence, direction and strength of asthenospheric flow around it provide critical constraints on models of driving mechanism of plate motion, and on those of continental formation and evolution. Unfortunately, mostly because of the artificial division of continental and marine geophysical experiments, the continental margins were poorly studied relative to the continents and ocean basins.

This presentation summarizes existing seismic tomographic models obtained using surface and P- and S-waves for the GoM region. Most of the models show that the edge of the North American continent has a NW-SE trending beneath Texas, and approximately parallel to the continental margin from Louisiana to Florida. Due to the low resolution and limited amount of data used in the studies, important discrepancies understandably exist between the velocity models, such as whether there is a plume-like low-velocity structure beneath the coastal area in Texas, and if the mantle velocities beneath the GoM oceanic crust are significantly lower than those beneath adjacent continental areas. Recent shear-wave splitting measurements using data from 8 seismic stations beneath TX, OK, and AR found large (up to 1.7 s) splitting times and fast directions that are parallel to the GoM continental margin (Gao et al., 2008). One of the causes of such anisotropy is asthenospheric flow around the keel of the North American continent. This is consistent with the dominantly E-W fast directions observed in the eastern part of the northern continental margin of the GoM.

Breakthroughs in the understanding of the formation and evolution of the North American continent and the flow field around its keel are anticipated with the arrival of the USArray, and the successful completion of a number of Flexible Array experiments that are being proposed (as of June 2008) to study the GoM continental margin.