No regional anisotropic domains in the northeastern U.S. Appalachians

Abstract

A fabric “frozen” within the continental lithosphere should lead to “regional anisotropic domains” with close correlation between tectonics, seismic anisotropy, and shear wave travel time delays. Alternatively, seismic anisotropy could be maintained by active deformation in the sub‐lithospheric mantle. This scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting, surface tectonics, and travel time delays. A coherent pattern of shear wave splitting is observed in the northeastern United States, covering large parts of New York and New England, spanning the Appalachian Orogen and areas underlain by the Grenvillian basement. The direction of fast shear wave propagation, as measured from core‐refracted SKS, SKKS, and PKS phases, varies systematically with event back azimuth at all sites examined, and is fit well by two‐layers of anisotropy with subhorizontal symmetry axes. Relative travel time delays of shear waves that sample the region vary over 100 km length scale, and correlate with surface geology. Tomographic images suggest lateral shear velocity variations ∼3%, including a slow anomaly beneath the Grenvillian basement exposed in the Adirondack Mountains. There is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting. We therefore conclude that in the northeastern US the concept of “regional anisotropic domains” (i.e. distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation) does not apply.