Tectonic Framework of Southeastern United States

Abstract

This paper reviews the relations in space and time of the mountain system formed during Paleozoic time which dominates the tectonics of the southeastern states. The most conspicuous representatives of the system are the structures of the Appalachian Highlands, but other structures occur in the Ouachita Mountains and other ranges west of the Mississippi Embayment, and still others are concealed by the post-orogenic deposits of the Atlantic and Gulf Coastal plains. From northwest to southeast, the Appalachian Highlands are divisible into the following. 1. The gently deformed Cumberland and Allegheny plateaus, which constitute a foreland area on the northwest flank of the system. 2. The Valley and Ridge province, made up of more strongly folded and faulted Paleozoic rocks but whose structures are nevertheless marginal to the zones farther southeast. 3. The Blue Ridge province, composed of older Paleozoic and pre-Cambrian rocks, which are not only strongly folded, but are also more or less metamorphosed. 4. A metamorphic and plutonic zone, forming the greater part of the Piedmont province, made up of gneisses and other thoroughly metamorphosed rocks, invaded by granite plutons. This zone is believed to lie along the central axis of the Appalachian system. 5. A belt of less metamorphosed sedimentary and volcanic rocks forming the southeast part of the Piedmont province, and lying southeast of the central axis. In addition to these exposed rocks, unmetamorphosed and little folded sedimentary and volcanic rocks occur still farther southeast, beneath the Mesozoic rocks of Florida. The Appalachian system of the southeastern states has been assumed to have been deformed mostly during the Appalachian revolution, toward the end of Permian and Paleozoic time, but there is evidence of earlier movements during the Paleozoic era, beginning with an orogeny of probable Middle Ordovician age. Late Paleozoic movements may well have been merely the concluding phases of the orogeny. The relations between the Appalachian system and the Ouachita system farther west have long been a puzzle. Further evidence on the problem has been obtained as a result of deep drilling in Mississippi and Alabama, but evidence is as yet insufficient to establish the relation. It is tentatively suggested that the Ouachita system is a westward extension of an interior belt of the Appalachian system, which has been thrust northward to a moderate extent over the westward extension of the belt of the Valley and Ridge province. End_Page 635------------------------------ Fig. 1. Tectonic sketch map of southeastern United States to show mountain systems of Paleozoic time, and their relations to other structural features. End_Page 636------------------------------ Fig. 1. Continued. See caption on page 636. End_Page 637------------------------------