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.
- Received 11 November 2016.
- Revision received 9 February 2017.
- Accepted 7 March 2017.
- © Geological Society of America