APPALACHIAN OROGENIC BELT

an old mountain range that extends for more than 3,000 km (1,860 miles) along the eastern margin of North America from Alabama in the southern United States to Newfoundland, Canada, in the north. The geosynclinal theory of mountain building was first worked out in the Appalachians by James Dana and James Hall in the late 19th century; today a plate tectonic theory is invoked. The earliest Appalachian sediments were deposited near the start of the Cambrian Period (540 million years ago) on the shores of the opening Iapetus Ocean. Subduction of the Iapetus led to its destruction and the collision of different continental blocks and island arcs. Those collisions gave rise to three Appalachian orogenies: Taconic in the Middle Ordovician (about 460 million years ago); Acadian in the Early to Late Devonian (at 390 to 370 million years); and Alleghenian in the Late Carboniferous to Permian (300 to 250 million years ago). The age of these orogenies decreases eastward across the orogenic belt, demonstrating that it was formed by the progressive eastward addition of arcs and continental fragments to the continental margin of North America. The Appalachian belt continues to the east in the form of the Caledonian and Hercynian orogenic belts in western Europe. The Alleghenian orogeny led to the formation in the Permian Period (286 to 245 million years ago) of the Pangaea supercontinent. Geophysical seismic studies show that the southern Appalachian Mountains comprising the Ridge and Valley region, the Blue Ridge Mountains, and the Piedmont region belong to a crustal slab some 615 km (3.7 to 9.3 miles) thick that has been thrust 260 km (162 miles) westwards over the former continental margin.