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

Abstract

A variety of models for mantle flow beneath southeastern North America have been proposed, 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 upward advective transport of volatiles from the deep slab through the upper mantle. We use shear wave splitting observations and receiver function analysis at broadband seismic stations in the southeastern United States to test several of these proposed mantle flow geometries. Near the coast, stations exhibit well‐resolved null (no splitting) behavior for SKS phases over a range of back azimuths, consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry. Farther inland we identify splitting with mainly NE–SW fast directions, consistent with asthenospheric shear due to absolute plate motion (APM), lithospheric anisotropy aligned with Appalachian tectonic structure, or a combination of these. Phase‐weighted stacking of individual receiver functions allows us to place constraints on the timing of arrivals from the 410 and 660 km discontinuities and on average transition zone thickness beneath individual stations. At most stations we find transition zone thicknesses that are consistent with the global average (∼240 km), with two stations showing evidence for a slightly thickened transition zone (∼250 km). Our results are relevant for testing different models for mantle dynamics beneath the southeastern United States, but due to the sparse station coverage, we are unable to uniquely constrain the pattern of mantle flow beneath the region. Our SKS splitting observations support a model in which mantle flow is primarily vertical (either upwelling or downwelling) beneath the southeastern edge of the North American continent, in contrast to the likely horizontal, APM‐driven flow beneath the continental interior. However, our receiver function analysis does not provide unequivocal support either for widespread hydration of the transition zone or for widespread thickening due to the downwelling of relatively cold mantle material. We expect that the necessary data to constrain such models more tightly can be obtained from the operation of denser seismic networks, including the Transportable Array and Flexible Array components of USArray.