Tholeiitic diabase dikes of Late Triassic and Early Jurassic age occur in swarms throughout the Appalachians. The intrusions exhibit regional variations in magnetic and chemical parameters. Aeromagnetic and ground-magnetic anomalies are highest over dikes in the Carolinas. High amplitude is due primarily to a stronger magnetization (the induced component dominates and is nearly constant within individual intrusions). Effects of host-rock magnetization, thermoremanence, and dike width, attitude, and orientation are insignificant. Chemically the Carolinas dikes are characterized by higher mean values for total iron and relatively low TiO2 content. The magnetic and chemical patterns are reminiscent of those found for tholeiitic complexes over oceanic hotspots (Galapagos, Iceland), and suggest that the Carolinas may have been the site of such a feature in Late Triassic—Early Jurassic time. Crustal arching, rifting, and volcanism probably originated in the Carolinas. Tectonic-volcanic activity gradually migrated northeast (and possibly southwest) along the Appalachian axis with enlargement of the crustal bulge. In Jurassic time, spreading began farther east; the crustal stress configuration changed, and the tectonic pattern was dominated by the formation of new sets of tensional fractures and sinistral strike slip along pre-existing faults.