The eastern margin of North America has been shaped by several cycles of supercontinent assembly. These past episodes of orogenesis and continental rifting have likely deformed the lithosphere, but the extent, style, and geometry of this deformation remain poorly known. Measurements of seismic anisotropy in the upper mantle can shed light on past lithospheric deformation, but may also reveal contributions from present‐day mantle flow in the asthenosphere. Here we examine SKS waveforms and measure splitting of SKS phases recorded by the MAGIC experiment, a dense transect of seismic stations across the central Appalachians. Our measurements constrain small‐scale lateral variations in azimuthal anisotropy and reveal distinct regions of upper mantle anisotropy. Stations within the present‐day Appalachian Mountains exhibit fast splitting directions roughly parallel to the strike of the mountains and delay times of about 1.0 s. To the west, transverse component waveforms for individual events reveal lateral variability in anisotropic structure. Stations immediately to the east of the mountains exhibit complicated splitting patterns, more null SKS arrivals, and a distinct clockwise rotation of fast directions. The observed variability in splitting behavior argues for contributions from both the lithosphere and the asthenospheric mantle. We infer that the sharp lateral transition in splitting behavior at the eastern edge of the Appalachians is controlled by a change in anisotropy in the lithospheric mantle. We hypothesize that beneath the Appalachians, SKS splitting reflects lithospheric deformation associated with Appalachian orogenesis, while just to the east this anisotropic signature was modified by Mesozoic rifting.

Subduction, terrane accretion, and continental rifting are fundamental plate tectonic processes. Geologic features such as igneous rocks produced during arc magmatism, terrane boundaries separating regions with different origins, and rift basins filled with sedimentary units reflect such tectonic processes. It is likely

The ca. 1 Ga Grenville orogeny was a protracted mountain-building event that culminated in the collision of Laurentia and Amazonia and the formation of the Rodinia supercontinent. While the expression of Grenville orogenesis in present-day crustal structure has been extensively investigated in eastern Canada, evidence for contemporaneous crustal deformation is less well established beneath the eastern United States. Furthermore, the interpretation of a geophysical lineament through the U.S. midcontinent, typically inferred to be the Grenville deformation front, has recently been called into question; an alternative hypothesis is that this feature actually corresponds to an eastern arm of the Midcontinent Rift. Here we present P-to-S receiver functions computed for stations of the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment, a dense array of broadband seismometers across the central Appalachians and midcontinent. We see evidence for a crustal negative velocity gradient that dips gently (dip angle <10°) to the southeast and extends east from a location near the putative Grenville front, terminating near the Appalachian Mountains. While we cannot date this feature, its location and characteristics are consistent with a shallowly dipping, seismically anisotropic intracrustal shear zone associated with collisional deformation, perhaps during Grenville orogenesis. The similarity between this feature and similar mid-crustal detachments in other orogens, both ancient (Appalachians) and modern (Himalayas), suggests that this style of crustal deformation has been common in continental collisional orogens.