METAMORPHIC AND STRUCTURAL EVOLUTION OF THE EASTERN BLUE RIDGE-WESTERN BLUE RIDGE BOUNDARY AS EXPOSED BETWEEN SOCO AND BALSAM GAP, WESTERN NC

Interpretations regarding the evolution of the Eastern Blue Ridge (EBR)-Western Blue Ridge (WBR) boundary, variously referred to as the “Hayesville fault” (HF) or “Taconic suture”, involve its establishment by collision of an oceanic fragment/accretionary wedge/island arc with Laurentian basement and metasedimentary cover, presumably followed by Taconian regional metamorphism with an Acadian/Alleghanian greenschist overprint. Detailed structural and petrologic analysis of exposures along the Blue Ridge Parkway south of Maggie Valley, NC reveal the boundary to be a folded contact between WBR paragneisses and EBR orthogneisses. Two stages of early isoclinal folding (F₁ and F₂) are overprinted in both gneisses by F₃ open to tight folds; late F₄ open folding is associated with brittle faulting. S₂ (defined in part by folded leucosomes) is the dominant regional foliation that transposes to an incipient, anastomozing S₃ axial planar position. Leucosomes are both S₂-parallel and crosscutting, often existing as intrafolial folds parallel to S₂; deformation is variable, including recrystallization of feldspars locally producing mortar textures. S₃ is characterized by moderate T deformation and dynamic recrystallization of quartz, as seen by lobate grain boundaries, undulose extinction, subgrain formation, and localized quartz ribbons, and recrystallization and rotation of micas parallel to S₃; this is the youngest penetrative feature in both WBR and EBR assemblages. Pre- to syn-kinematic M_2a metamorphism reached Ky grade in WBR pelites and upper amphibolite facies in mafic EBR assemblages (Grt + Bt + Ep + Aln + Ttn + Ap). Syn-(?) to post-kinematic M_2b metamorphism attained Sil grade in the WBR and upper amphibolite facies in the EBR (Grt + Hbl + Bt + Ttn + Ep + Al + Ep). Coarse Ms overprints M_2a and M_2b porphphyroblasts, however Ms is deformed by S₃. Bt + Chl replacement of Grt, Chl replacement of Bt, and sericitization, locally static, of Fs, Ms, and Ky characterize a late greenschist event. We observe no structures at the contact indicative of premetamorphic faulting, and cannot discern any evidence, at this locality, that the contact is a fault. Premetamorphic fabrics and mineralogy must have been obliterated by Taconian deformation and metamorphism.