The Northern Appalachian Anomaly is a Modern Asthenospheric Upwelling

The eastern North American coast is the site of significant seismic velocity heterogeneities. They are a record - albeit an ambiguous one - of lithospheric and asthenospheric processes operating at the continental margin. We focus on the Northern Appalachian Anomaly (NAA), a particularly strong slow velocity feature in the shallow mantle located in a westward indentation (or divot) of the continental lithosphere in southern New England. The NAA has been explained as a relic feature associated with the Great Meteor hotspot (GMHS), which traversed southern New England at 130-100 Ma. Here we consider the alternative hypothesis that it is a modern feature associated with small-scale asthenospheric upwelling unrelated to any hotspot. We show that the NAA is a narrow (400 km wide) columnar feature and that its travel time delays are consistent with an extremely strong ( 700K ) asthenospheric temperature anomaly. After analyzing several previously-published tomographic images and a new one described here, we conclude that it is most consistent with a strong local upwelling associated with the eastern edge of the Laurentian (pre-Cambrian) continental lithosphere.