Upper mantle anisotropy and transition zone thickness beneath southeastern North America and implications for mantle dynamics

Though the mantle structure beneath western North America has been elucidated using data from EarthScope and other initiatives, very little is known about the dynamics and structure of the mantle beneath the eastern portion of the continent. Several models for mantle flow have been proposed for the region, including those that invoke westward-driven return flow from the sinking Farallon slab, small-scale convective downwelling at the edge of the continental root, or the upwards advective transport of volatiles from the slab through the upper mantle. We evaluate these proposed models for mantle flow beneath southeastern North America using shear wave splitting observations and receiver function analysis from data recorded at permanent broadband seismic stations in the region. Stations near the Atlantic coast show well-resolved null (no splitting) behavior for SKS phases over a range of backazimuths, consistent with either an isotropic upper mantle or with a vertical axis of symmetry. Further inland, fast SKS splitting directions are mainly NE-SW, which is consistent with asthenospheric flow due to absolute plate motion, lithospheric anisotropy aligned with Appalachian tectonic structure, or a combination of these. Results from phase-weighted stacking of receiver functions clearly identifies the 410 and 660 km discontinuities beneath most stations. Overall, the transition zone thickness is consistent with the global average (~ 240 km) throughout the region, though two stations have a slightly thickened transition zone (~ 250 km). Our observations of SKS splitting suggest that the mantle flow is primarily vertical (either upwelling or downwelling) beneath the southeastern edge of the North American continent; however, the receiver function analysis does not necessarily suggest that the transition zone is widely thickened across the region due to the downwelling of relatively cold mantle material. Furthermore, our results do not unequivocally support widespread hydration of the transition zone beneath the region. We anticipate that data collected in the near future from denser seismic arrays, including the Transportable and Flexible Arrays, will be able to constrain the nature of mantle flow.