The New York–Alabama (NY-AL) lineament, recognized in 1978, is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern North America that is deeply buried beneath the Appalachian basin. Data not in the original aeromagnetic data set, particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of Tennessee, left the source of the lineament open to conjecture. We use modern digital aeromagnetic maps to fill in these data gaps and, for the first time, constrain the southern termination of the NY-AL lineament. Our analysis indicates that the lineament reflects a crustal-scale, right-lateral strike-slip fault that has displaced anomalies attributed to Grenville orogenesis by ∼220 km. Palinspastic restoration of this displacement rearranges the trace of the Grenville belt in southern Rodinia and implies only passive influence on later-formed Appalachian structures. The precise timing of dextral movement on the NY-AL structure is not resolvable from the existing data set, but it must have occurred during one of, or combinations of, the following events: (1) a late, postcontractional (post-Ottawan) stage of the Grenville orogeny; (2) late Neoproterozoic to Cambrian rifting of Laurentia; or (3) right-slip reactivation during the late Neoproterozoic–Cambrian rifting of Laurentia, or during Appalachian movements. Our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the Eastern Tennessee Seismic Zone are metasedimentary gneisses, and it provides an explanation for the spatial location and size of the seismic zone.