In the basement under the Appalachian basin, a northeast-trending lineament extending more than 1,600 km from the Mississippian embayment to the Green Mountains has been found from aeromagnetic mapping augmented by regional gravity data. This lineament, here called the New York–Alabama lineament, is marked by a series of linear magnetic gradients that bound areas of magnetic rocks in the basement and implies strike-slip displacement along a profound crustal break. The lineament tends to coincide with the west side of the regional Appalachian gravity low and separates a province of dominantly north-trending gravity anomalies on the northwest from a province of prevailing northeast trends on the southeast. The lineament seems to mark the southeast edge of a stable crustal block that acted as a buttress for the strong deformation of the Appalachian foldbelt, so that the arcuate salients in Pennsylvania and Tennessee are tangential to it. Present-day seismic activity shows a correlation of the location of the lineament with an active area to the southeast in the eastern part of the Appalachian basin and a seismically inactive block along the northwest side of the lineament. A parallel active zone to the west extends from New Madrid, Missouri, to the valley of the St. Lawrence River. The lineament is in the subsurface extension of the Grenville province, which may mark a region of a former continental collision analogous to the India-Asia region. Strike-slip faults of great length, linearity, and profound horizontal displacement, such as the Altyn Tagh fault of Tibet, typify the region north of India and may have their counterpart in the New York-Alabama lineament, which records probable strike-slip displacement of great magnitude in the basement rocks under the Appalachian basin.