Lateral changes in seismic velocity 100-300 km beneath the Appalachian orogen (eastern North America) do not follow the pattern of its major terranes, suggesting that more recent, and possibly ongoing, geodynamic processes are taking place in the sub-lithospheric mantle. One prominent, sharply delineated, seismically slow feature underlying parts of New England (USA) likely reflects a volume of significantly elevated temperatures in the asthenosphere. Using numerous new observations of splitting in seismic shear waves from distant earthquakes, we show that this upper mantle volume also lacks the systematic directional dependence (anisotropy) of seismic wave speed that is ubiquitous beneath most of northeastern North America. This regional anisotropic fabric, which likely forms as the asthenosphere is sheared by North American plate motion, appears to be locally erased beneath central New England, with changes in its strength occurring over distances on the order of 50 km. Highly localized variation in the strength of seismic anisotropy in a region of strongly elevated asthenospheric temperature suggests the presence of a narrow thermal upwelling in the upper mantle beneath New England. The lack of obvious surface expressions (volcanism or uplift) and the small lateral scale of the hypothesized upwelling suggest a geologically recent phenomenon.
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