New shear wave splitting measurements (n = 76) from the Mid-Atlantic section of North America, when combined with previously reported measurements, provide improved clarity to patterns of seismic anisotropy across the region, including a rotation of fast polarization directions (φ) from east-west to northeast-southwest in Pennsylvania (USA). We attribute the patterns in φ to frozen-in (i.e., fossil) anisotropy in the lithospheric mantle. Most φ directions parallel the strike of the Appalachian orogen, except in parts of New England, where φ directions are oriented perpendicular to both the strike of the orogen and continental margin. The φ directions parallel to the strike of the Appalachian orogen, including those that rotate from east-west to northeast-southwest in Pennsylvania, are consistent with the expected orientation for fossil anisotropy developed during the Appalachian orogenic cycle. The φ directions in New England, perpendicular to the continental margin, are consistent with the expected orientation for fossil anisotropy developed during the breakup of Pangea, and φ directions at locations west of the Appalachian Mountains in areas not affected by the Appalachian orogeny can be explained by fossil anisotropy developed during the Proterozoic suturing of the Granite-Rhyolite Province and Elzevir block to the eastern margin of Laurentia.
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