Lithospheric Structure Above the Northern Appalachian Anomaly: Initial Results From the NEST Array

Seismic tomography observations show a low‐velocity feature in the upper mantle beneath eastern North America known as the Northern Appalachian Anomaly (NAA). Proposed models for the formation of the NAA include a remnant high‐temperature feature resulting from the passage of the Great Meteor Hotspot, edge‐driven convection, and ongoing asthenospheric upwelling. We investigate the structure of the lithosphere above the central portion of the NAA using data from the New England Seismic Transects (NEST) experiment. Ps receiver functions reveal two consistent interfaces beneath the dense northern line of NEST: the Moho (the base of the crust) and a deeper negative velocity gradient (NVG) feature located at depths between 60 and 110 km. We consider several potential explanations for this NVG feature; based on comparisons with previous results, we propose that it likely corresponds to the lithosphere‐asthenosphere boundary. Our results indicate that the lithosphere beneath New England is nonuniform and has likely been thinned. Plain Language Summary Eastern North America has a rich geologic history. One puzzling aspect of the seismic structure of New England is that it overlies a prominent region of anomalously low seismic velocities in the upper mantle, known as the Northern Appalachian Anomaly (NAA). The origin of this feature remains poorly understood; furthermore, it is not clear whether and how the upper mantle anomaly has affected the tectonic plate above it. Here we use data from seismic stations of the New England Seismic Transects (NEST) array to study the thickness of the continental lithosphere above the NAA. Specifically, we examine P waves from distant earthquakes and look for evidence of P wave to S wave conversions at the base of the lithosphere. We find evidence for particularly thin lithosphere, indicating that it has been thinned over geologic time. Key Points We use P‐to‐S receiver function analysis to identify negative velocity gradients beneath New England We find evidence for prominent negative velocity gradients between ∼60 and 110 km depth across stations of the NEST array We infer relatively thin continental lithosphere above the Northern Appalachian Anomaly (NAA) in the upper mantle