Developmental Model for the Southern Appalachians

Abstract

A developmental model is proposed for the southern Appalachians. This model integrates current ideas on structure, sedimentation, metamorphism, and igneous activity with the concept of plate tectonics. Progressive metamorphism culminated about 1,000 m.y. ago (middle Precambrian) and again about 400 m.y. ago (Silurian-Devonian). Later, localized retrogressive metamorphism affected the rocks of the Blue Ridge and Piedmont. Igneous activity occurred during the middle Precambrian (1,000 m.y. ago), prior to, during, and after Paleozoic regional metamorphism, and during the Mesozoic. The Valley and Ridge and Cumberland Plateau structures are treated as areas deformed by thin-skinned tectonics generated during the late Paleozoic, as is the Blue Ridge thrust sheet. The Blue Ridge anticlinorium, Murphy syncline, Chauga belt, mobilized Inner Piedmont, Kings Mountain belt, Charlotte and Carolina slate belts developed during the deformational event accompanying early-mid-Paleozoic regional metamorphism. The Blue Ridge anticlinorium, mobilized Inner Piedmont and Charlotte belts are interpreted as anticlinoria composed of middle Precambrian basement and later Precambrian metasedimentary and metavolcanic rocks mobilized to a greater or lesser degree. The Murphy, Chauga, Kings Mountain, and Carolina slate belts are inferred to be synclinoria composed of younger (late Precambrian to early-mid-Paleozoic) metasedimentary and metavolcanic rocks. The development of the sedimentary, deformational, metamorphic, and intrusive history of the southern Appalachians may be directly related to the Paleozoic history of movement of the North American and African continents as portions of major Paleozoic lithospheric plates. This developmental scheme is divisible into several phases: (1) An early (late Precambrian to mid-Ordovician) phase of continental margin sedimentation, igneous activity, and initial compression occurred. (2) This was followed by an intermediate (late Precambrian to late Devonian) phase of compression producing isoclinal folding, regional metamorphism, and intrusive activity with deposits from a rising tectonic source land. These phenomena are the direct result of westward underflow of a proto-Atlantic plate beneath an eastward-moving proto-North American plate with concomitant development of a subduction zone along the continental margin. (3) Next a later (mid-late Paleozoic) phase of compression and igneous activity accompanied by continued deposition from the rising mountain system occurred. The Brevard Zone, Towaliga, Goat Rock, and Gold Hill faults developed early in phase 3 and experienced renewed movement during formation of the thrusts and folds in the Valley and Ridge, Cumberland Plateau, and the late structures in the Blue Ridge. Compressional stresses that formed these structures were generated during the collision and suturing of Africa with southeastern North America. (4) Finally, there was a tensional phase (Triassic-Jurassic) accompanied by normal faulting, igneous activity, and deposition related to the decoupling of Africa and North America and the formation of the present Atlantic Ocean and continental margin.