No regional anisotropic domains in the northeastern US Appalachians

A fabric "frozen" within the continental lithosphere should lead to "region al anisotropic domains" with close correlation between tectonics, seismic a nisotropy, and shear wave travel time delays. Alternatively, seismic anisot ropy could be maintained by active deformation in the sub-lithospheric mant le. This scenario would predict broad spatial consistency in shear wave spl itting and weak correlation between the splitting, surface tectonics, and t ravel time delays. A coherent pattern of shear wave splitting is observed i n the northeastern United States, covering large parts of New York and New England, spanning the Appalachian Orogen and areas underlain by the Grenvil lian basement. The direction of fast shear wave propagation, as measured fr om core-refracted SKS, SKKS, and PKS phases: varies systematically with eve nt back azimuth at all sites examined, and is fit well by two-layers of ani sotropy with subhorizontal symmetry axes. Relative travel time delays of sh ear waves that sample the region vary over 100 km length scale, and correla te with surface geology. Tomographic images suggest lateral shear velocity variations similar to 3%, including a slow anomaly beneath the Grenvillian basement exposed in the Adirondack Mountains. There is little correlation b etween this horizontally rough seismic velocity and the horizontally smooth anisotropic model consist;ent with shear wave splitting. We therefore conc lude that in the northeastern US the concept of "regional anisotropic domai ns" (i.e. distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation) does not apply.