The youngest magmatic event in Eastern North America: A window in the post rift evolution of continents

Abstract:

The rifted Eastern North American Margin (ENAM) provides important clues to the long-term evolution of continental margins. An Eocene (ca. 47-48 Ma) volcanic swarm exposed in the Appalachian Valley and Ridge Province of Virginia and West Virginia, contains the youngest known igneous rocks in the ENAM. These magmas are bimodal in composition, and provide the only window into the most recent deep processes contributing to the post-rift evolution of this margin. We expand on the data presented in Mazza et al., 2014, with new geochemical data that further constrains the magmatic evolution of the ENAM. Using integrated radiogenic isotopic data, petrologic modeling, and regional geomorphology, we determine source domains, melting conditions, and regional implications. Modeling of the melting conditions on primitive basalts yielded an average temperature and pressure of 1412±25°C and 2.32±0.31 GPa, corresponding to a mantle potential temperature of ~1410°C, suggesting melting conditions slightly higher than ambient mantle but not as high as expected from plume activity. When compared with magmas from Atlantic hotspots, the Eocene ENAM samples share isotopic signatures with the Azores and Cape Verde. This similarity suggests the possibility of a large-scale dissemination of similar sources in the upper mantle left over from the opening of the Atlantic. Asthenosphere upwelling related to localized lithospheric delamination is a possible process that can explain the intraplate signature of these magmas that lack evidence of a thermal anomaly. This process can also explain the Cenozoic dynamic topography and rejuvenation of the Central Appalachians. New P- and S-wave tomography using data from EarthScope’s USArray shows that a low-velocity anomaly persists in the upper mantle beneath the Eocene volcanic swarm, indicating that the magmatic event substantially modified regional lithospheric structure. Our geochemical and petrologic constraints will be vital for a comprehensive interpretation of new geophysical data emerging from EarthScope and understanding its implications for the geological evolution of ENAM.