Modification of Crust and Mantle Lithosphere Beneath the Southern Part of the Eastern North American Passive Margin

The eastern North American passive margin was modified by Mesozoic rifting. Seismic data from recent deployment of onshore and offshore stations offer a unique opportunity for studying the signature of syn‐rifting and postrifting in lithospheric structures. Using full‐wave ambient noise tomography, we construct a new seismic velocity model for the lithosphere of the southeastern United States. Our model confirms an oceanic‐continental transitional crust over a ∼70 km wide zone across the coastline. Our model reveals (a) a patch of lower‐than‐average mantle lithospheric velocities underlying this transitional crust and (b) a low‐velocity column in the mantle lithosphere beneath the Virginia volcanoes. We propose that anomaly 1 represents cooled enriched mantle that underplated the thinning crust during the initial stages of rifting around 230 Ma. Anomaly 2 likely has a more recent origin in the Eocene and may result from an asthenospheric upwelling induced by a localized lithospheric delamination. Plain Language Summary The eastern North American margin is a typical passive margin that was modified during the extensive Mesozoic rifting. How the crust and mantle lithosphere beneath the passive margin were modified during and after rifting remains as a fundamental question to be answered. Using full‐wave ambient noise tomography, we construct a detailed crustal and upper mantle model beneath the southeastern United States. We identify a clear change in the crustal thickness over a transitional zone from ocean to continent. Beneath the transitional crust, the upper mantle has relatively lower seismic speeds in comparison with its surroundings. We also discover a low‐velocity column within the upper mantle beneath West Virginia where two volcanos are located. We propose that the transitional oceanic‐continental crust was formed due to crustal extension during rifting. The mantle lithosphere underlying the transitional crust could have been consequently modified during the rifting process. The rising flow of asthenosphere (a weak layer beneath the rigid lithosphere) induced by a localized lithospheric delamination might result in partial melting of the upper mantle beneath the continental interior, which can explain the presence of volcanoes in West Virginia. Key Points A high‐resolution lithospheric velocity model is constructed in the southern part of the eastern North American passive margin A transitional crust and low‐velocity mantle lithosphere are observe