Inherited Upper Mantle Structures in the Southeastern United States

Abstract:

The southeastern United States has experienced repeated cycles of terrane accretion, continental suturing, and extensive rifting over the past 1 Ga. Recent data collected from the EarthScope Transportable Array and the EarthScope FlexArray deployment SESAME (South Eastern Suture of the Appalachian Margin Experiment) allow us to image in detail the upper mantle structures associated with these tectonic events using both Rayleigh waves and teleseismic body waves.

Of particular interest is the mantle expression of the Suwannee suture. Located across southern Georgia and Alabama, the Suwannee suture represents the boundary between the Gondwanan Suwannee terrane and the Laurentian margin. Previous authors have speculated that this accretion occurred due to southward dipping subduction of the Laurentian lithosphere, though more recent work suggests a possible dextral transpressional plate boundary. We find that the mantle lithosphere across the suture may dip slightly to the south, but the dip angle is much shallower than that of crustal structures seen in scattered wave and active source images. Ongoing work will help to constrain the precise dip angle and will also investigate the nature of lower velocities located between the mantle lithosphere and the overriding crust.

Intriguingly, we do image a distinct dipping high velocity feature further to the west. This slab-like structure dips to the east, and has a surface projection located near the 1 Ga Grenville front. While the provenance of this high velocity anomaly is still unclear, a few intriguing observations can be made. Given the location of this anomaly in a tectonically stable area, it is unlikely to be due to an ongoing lithospheric process. This then raises the question of how such a dipping structure can persist over time despite over-riding plate motions and mantle flow patterns. The increase in shear velocities is unlikely due to any remnant thermal effects. Our ability to image this structure at all therefore suggests that this feature is compositionally distinct from the surrounding upper mantle material.