Analysis of the Piedmont in Northwest South Carolina

Abstract

Two major autochthonous nappes and a deep synformal structure occur in the Piedmont of northwest South Carolina. The cataclastic Brevard zone, which borders the Blue Ridge geologic belt on the southeast, is associated with the synformal belt of partially cataclastic and retrograded rocks at low amphibolite facies grade. This Non-migmatitic belt contains tight cylindroidal isoclines and tectonic slides with a northwest vergence. Refolded isoclines in the Non-migmatitic belt occur near the Brevard zone as a result of recurring displacements there. Immediately southeast lies the Walhalla nappe, which contains kyanite-grade rocks complexly folded into a northeast-plunging conical system. The migmatite front lies within this nappe and in places is coincident with the major slide boundary between the Walhalla nappe and the underlying synformal Non-migmatitic belt. The major slide that separates the overlying Six Mile nappe from the Walhalla nappe is subhorizontal; however, in the broad root zone of both nappes in the southeast part of the area, northwest dips and steep dips represent the attitudes of s-surfaces in many locations. The autochthonous nappe complex probably formed in a continental rise assemblage subjected to intense thermal effects in the early Paleozoic. Sedimentary and (or) volcanic-volcaniclastic assemblages sited along the continental margin were squeezed as a detachment zone between the northwest-driving Piedmont nappes and the activated craton margin. These assemblages became tectonically related to a detachment zone overlying the rising infrastructural nappes, which is now eroded away with the overlying supra-structure. Annealed textures indicate that metamorphism outlasted penetrative deformation, except along the Brevard zone, which continued as a zone of dislocation while thermal metamorphism waned. Late Paleozoic Alleghanian movements may be recorded as microbreccia zones within the Brevard zone and Inner Piedmont belt. This event was not penetrative; instead, it was one of vertical en bloc uplift primarily centered in the Inner Piedmont, and perhaps it accompanied a weak retrogressive metamorphism.