VELOCITY STRUCTURE IN THE LITHOSPHERE BENEATH THE MISSISSIPPI EMBAYMENT AND NEW MADRID SEISMIC ZONE

A recent tomographic inversion of local earthquake and teleseismic data provides a detailed image of a prominent low velocity anomaly in the upper mantle below the northern Mississippi Embayment (ME). Excellent station coverage is provided by the New Madrid Seismic Network, the Earthscope Transportable Array, and the FlexArray Northern Embayment Lithospheric Experiment. The low velocity anomaly dips to the southwest and extends to a depth of at least 300 km. Low velocities extend to shallow depths (about 50 km) below the northern end of the Reelfoot Rift. The presence of Vp and Vs anomalies with comparable magnitudes suggests that the low velocities are not due to elevated temperatures alone. We attribute the low velocities to the presence of hot fluids upwelling from a flat slab segment stalled in the transition zone below the central U.S.; the thinned and weakened ME lithosphere provides an optimal pathway for the ascent of the fluids. A region of high velocity in the lower crust separates the low velocities from the seismogenic upper crust hosting the NMSZ. Regions of high velocity with comparable Vp and Vs anomalies are present above and to the sides of the low velocity region. The high velocity anomalies are attributed to the presence of mafic rocks emplaced beneath the ME during initial rifting in the early Paleozoic and to remnants of the depleted, lower portion of the lithosphere. An analogous situation is present below the North China Craton where a stalled segment of the Pacific slab is present in the transition zone; distinct regions of high and low Vp and Vs anomalies are present in the upper mantle that are similar in appearance to those observed below the ME.