POSSIBLE CAUSES OF LOW VELOCITY IN THE UPPER MANTLE BENEATH THE MISSISSIPPI EMBAYMENT

Tomographic images obtained using local and teleseismic earthquake data recorded by the EarthScope transportable array (TA) and the FlexArray Northern Embayment Lithosphere Experiment (NELE) reveal the presence of a thinned lithosphere below the Mississippi Embayment and a region of low P- and S-wave velocities (Vp and Vs) that extends from the near-surface down to at least 300 km. The Vp and Vs models were determined independently and contain Vp and Vs anomalies that have similar magnitudes, reaching ±6%. The similarity of the Vp and Vs anomalies is unusual and places constraints on the origin of the low velocities. We discuss possible contributions from several sources including elevated temperature, mantle compositional variations, the presence of a small percentage of melt, orthopyroxene enrichment, and the presence of fluids. We suggest that the dominant factors influencing the low velocities are elevated temperature, fluids, and orthopyroxene enrichment. We speculate that these conditions are produced by the release of subduction zone fluids from a fragment of the Farallon slab that is present in the transition zone below the Mississippi Embayment. Flat subduction of the Farallon slab during the Late Cretaceous may have resulted from subduction of an oceanic plateau, suggesting that the slab fragment below the Mississippi Embayment contains thick, fluid-rich crust. Dewatering effects of a flat slab can influence the rheology and dynamics of the mantle, resulting in lithospheric thinning and the influx of low velocity, hydrated mantle. The presence of the low velocity upper mantle has important implications for seismic hazard evaluation; numerical modeling suggests that the presence of the associated low mantle viscosities may have a profound effect on earthquake nucleation and rupture processes in the New Madrid seismic zone.