The Northern Gulf Anomaly: P- and S-wave travel time delays illuminate a strong thermal feature beneath the Northern Gulf of Mexico

Abstract

Seismic measurements are used in a detailed investigation of a region of extremely low asthenospheric seismic velocities along the US Gulf Coast, first imaged in continental-scale geotomography, which we term the Northern Gulf Anomaly (NGA). Differential P- and S-wave arrival times from teleseisms at a variety of back-azimuths, observed on EarthScope Transportable Array stations near the US Gulf Coast, demonstrate that asthenospheric seismic velocities are 8-10% lower than the neighboring craton, and define the spatial extent and character of the anomaly. Travel time anomalies are calculated relative to the AK135 earth model and corrected to account for the effect of the kilometers-thick sedimentary cover in the region. The NGA is most intense at the southernmost coast of Louisiana and East Texas (with an eastern edge at 89°W) and smoothly tapers away in a triangular wedge that extends inland as far as 300 km. It has sharper edges and a smaller areal extent (by ∼50%) than previously-published geotomography has indicated. Both the magnitude and ratio of delays indicate that the NGA has a thermal origin, which may represent past or present-day small-scale convective upwelling near the southeastern edge of the North American continent. The NGA suggests that large-scale but poorly understood asthenospheric processes are at work beneath the US Gulf Coast, notwithstanding this region's reputation as an aseismic, passively-subsiding continental margin.