Stratigraphy, structure, and tectonics of part of the Southern Appalachian Inner Piedmont, near Marion, North Carolina

This thesis presents the results of a field and laboratory study of the Marion East 7.5 minute quadrangle and the compilation, reconnaissance mapping, reinterpretation of contacts, and field checking of southern half of the published Marion West 7.5 minute quadrangle. The study area is located within the western Inner Piedmont approximately 50 km (35 mi.) east of Asheville, North Carolina. The Inner Piedmont is the Acadian metamorphic core of the southern Appalachians. A recognizable stratigraphy exists in the study area and in the Inner Piedmont, at least as far east as the Six Mile fault. This stratigraphy consists of the Tallulah Falls (Ashe) Formation unconformably overlain by the Poor Mountain Formation, both of which have been tectonically overridden by the Henderson Gneiss. No Grenville basement has been recognized in the study area. A minimum of three orogenic events have affected the study area: Taconic-Penobscott, Acadian, and Alleghanian. The study area has undergone at least 5 episodes of deformation and a single episode of prograde metamorphism. D₁ is thought to be Taconic; D₂ and D₃ are associated with the Acadian orogeny; and D₄ and D₅ are attributed to the Alleghanian orogeny. Peak metamorphism and penetrative deformation is associated with the D₂ event. Peak metamorphic conditions in the study area correspond to the first sillimanite zone, although most of the study area is at kyanite grade and only the southeast portion is at sillimanite grade. Metamorphic pressures and temperatures inferred from this study are 5-6.5 kb and 535-675 °C, respectively. Cooling curves were created using hornblende and muscovite ⁴⁰Ar/³⁹Ar ages. Results indicate that the peak D₂ event probably occurred between 378 and 386 Ma. To better understand the tectonics of the western Inner Piedmont, a tectonic model is presented to explain both orogen-parallel and orogen-oblique Acadian structures in the Inner Piedmont. The model involves an oblique collisional event between the Inner Piedmont microcontinent and Laurentia followed by clockwise rotation of the Inner Piedmont before final accretion.