WHAT IS THE ORIGIN OF THE UPPER MANTLE LOW VELOCITY ZONE BELOW THE MISSISSIPPI EMBAYMENT?

One of the more striking EarthScope discoveries made in the central and eastern United States (U.S.) is the presence of a pronounced upper mantle low velocity zone (LVZ) below the Mississippi Embayment. The LVZ has been detected in several studies using Transportable Array (TA) stations and imaged in detail using the Northern Embayment Lithosphere Experiment (NELE) FlexArray deployment. Tomographic images for Vp and Vs obtained using local and teleseismic earthquake data recorded during the NELE experiment indicate that the LVZ is less than 100 km wide, trends approximately NE-SW and dips at about 45° to the SW. The LVZ is shallowest below the upper ME, in the vicinity of the New Madrid seismic zone, and extends to a depth of at least 250 km. The Vp and Vs models were determined independently and contain anomalies that have similar magnitudes, reaching ±5%. Two competing theories exist for the origin of the LVZ. One theory is that the LVZ is produced by the assent of orthopyroxene rich fluids from a slab fragment trapped in the transition zone. The second theory is that the LVZ is produced by counter-flow of the asthenosphere displaced by lithospheric foundering located below the southeastern U.S. These competing processes can be distinguished by developing new Vp and Vs models that extend to transition zone depths. If the SW dipping LVZ imaged in the upper 250 km in the NELE study continues to these depths, this will be a strong argument for the slab fragment hypothesis. Conversely, if the LVZ can be connected to the high velocity lithospheric drip this would lend support to the displaced asthenosphere hypothesis. Mantle flow patterns will also help distinguish between these possibilities. We suggest that determining the new Vp and Vs models is an important use of existing data collected by EarthScope.