Mesozoic rift basins in eastern North America formed during continental extension associated with the separation of North America and Africa. These basins locally overprint the Appalachian orogen and involve the extensional reactivation of Paleozoic faults. Half graben are thought to have formed where Mesozoic extension was subperpendicular to orogenic strike. Transtensional basins formed where the extension was more oblique. Segmented border fault systems and predominantly synthetic intrabasinal faults characterize the half graben. These basins resemble elongate synclines in longitudinal section; this geometry resulted from border-fault displacement that was greatest near the center of the fault and decreased toward both ends. Large-scale segmentation of some border fault systems resulted in the formation of multiple synclinal subbasins separated by transverse anticlines at segment boundaries, where fault displacement was less. As displacement increased on individual fault segments, the faults and associated basins grew in length, perhaps linking originally isolated basins. Smaller-scale fault segmentation resulted in the formation of relay ramps, rider blocks, and transverse folds. Some transverse synclines are located near the centers of fault segments, and related anticlines are located at segment boundaries. Adjacent half graben units within larger rift zones do not alternate polarity along strike and are generally not linked via accommodation zones, as in the East African rift system. Strike-slip-dominated basins are characterized by a network of strike-slip and normal faults, and are shallower and narrower than dip-slip-dominated basins. ¿ American Geophysical Union 1993