REMNANTS OF PROTEROZOIC FLAT-SLAB SUBDUCTION BENEATH THE MID-CONTINENT, USA

Employing seismic tomography techniques and combining local with regional waveform data, we studied the S-velocity structure beneath the Mid-continent United States, specifically beneath the intracratonic Illinois basin by utilizing continental scale waveform data of seismic S and surface waves, enhanced by regional earthquakes located near the basin. Our 3D tomographic model, IL05, confirms the existence of a slow S-velocity structure in the uppermost mantle beneath the Illinois basin region. This anomalously slow region exists from the base of the crust to depths of ~90 km, and is slower than the North American cratonic average by about 200 m/s. This anomalous uppermost mantle beneath the Illinois basin is underlain by a faster lithosphere, typical of the surrounding craton, to depths of ~200 km. This low S-velocity anomaly is resolved and is not found beneath other major North American interior cratonic basin.

This seismically slow region cannot be explained through elevated mantle temperatures, and while we cannot rule out a potential deep-mantle plume, we do not deem the scenario likely in the context of geological, geochemical, and other geophysical studies of the region. Currently we believe that the anomaly is most straightforwardly explained by an unusual crust-mantle mixture, where either 1.) oceanic crust became incorporated in the mantle of an over-riding plate during a Proterozoic episode of flat-slab subduction, or 2.) a relatively cool mantle wedge is preserved with hydrous minerals. In either case a flat dip-angle is necessary to avoid slab dehydration and associated partial melting of the mantle, which would deplete the mantle and eventually result in a seismically fast lithosphere. A receiver function analysis could further differentiate between the mantle plume and flat-slab hypotheses.