aulacogen

aulacogen In plate tectonics, a rift arm along which movement has ceased; a failed arm—for example, the Southern Oklahoma Aulacogen. At typical plate junctions, three intersecting plates are separated by ‘arms’, which may be diverging, converging, or transform faults. New oceanic basins may form as a result of spreading or rifting along the other two arms of the triple junction, while the aulacogen can become a sediment‐filled graben.