Azimuthal Anisotropy in Northeastern America From Joint Inversion of Rayleigh Waves and Shear Wave Splitting Data

Abstract We have constructed a 3D high‐resolution anisotropic velocity model in northeastern America by jointly inverting Rayleigh wave phase velocities and shear wave splitting measurements. In the Appalachians, anisotropy with a range‐parallel NE‐SW fast direction is dominant in the crust. The mantle lithosphere is characterized by weak anisotropy with substantial lateral variations, while anisotropy in the asthenosphere is stronger and more coherent. A near NS fast direction is prevalent in the top asthenosphere under the Grenville Province, likely caused by asthenospheric flow channeled by the eastern edge of the Superior cratonic keel. The distinctive anisotropy across the lithosphere‐asthenosphere boundary is observed when a high‐velocity lithosphere is well defined, such as the northern part of the Grenville Province. A circular azimuthal anisotropy is imaged at depths of 100–160 km in the eastern Great Lakes, and a semicircular pattern is revealed in southern New England, overlapping with the northern Appalachian low‐velocity anomaly. Both places of circular anisotropy correlate with a relatively thin lithosphere, suggesting that they may originate from asthenospheric flow guided by the indented lithosphere. On the other hand, both anomalies coincide with an ancient hotspot: the Cape Verde hotspot under the eastern Great Lakes about 215 Ma and the Great Meteor hotspot under southern New England 115 Ma. A circular azimuthal anisotropy fits the typical pattern in a plume head. Considering the circular patterns are well below the lithosphere, they are more likely to be the remaining anisotropy from the plume head of the two ancient hotspots.