METAMORPHIC AND STRUCTURAL EVOLUTION OF THE EASTERN BLUE RIDGE-WESTERN BLUE RIDGE BOUNDARY ("THE HAYESVILLE FAULT") ALONG THE BLUE RIDGE PARKWAY, WESTERN NC

Classical interpretations concerning evolution of the Eastern Blue Ridge (EBR)-Western Blue Ridge (WBR) boundary, or Hayesville fault (HF), involve emplacement of the HF (Taconic suture) by thrusting of an oceanic fragment/accretionary wedge/island arc upon Laurentian basement and metasedimentary cover, followed by Taconian regional metamorphism with an Alleghanian greenschist overprint. Detailed structural and petrologic analysis of exposures along the Blue Ridge Parkway south of Maggie Valley, NC reveals the EBR-WBR boundary is 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 and crenulations; late F₄ open folding is associated with brittle faulting. S₂ transposes to an S₃ axial planar position as a result of F₃. S₃ is characterized by crystallization and reorientation of micas, and deformation of quartz, as seen by lobate grain boundaries, undulose extinction, and subgrain development; this is the youngest penetrative feature in both WBR and EBR assemblages. Two ductile shear zones in the WBR, recognized by 10- and 3-m wide mylonite zones, show amphibolite facies recrystallization of feldspar porphyroclasts; rotation of local S₂ trends, resorbed Grt within the matrix, and S₃ deformation indicate post-kinematic activation relative to S₂, and pre-kinematic to S₃. Ky-St grade metamorphism in the WBR is pre- to syn-kinematic, as almost all are included within S₂. Ky contains numerous inclusions of Qtz and Grt, however, Grt within S₂ also includes Ky; late Sil (fine-grained acicular and/or fibrolite) nucleates on Grt, St, and Ky; coarse Mus overprints S₂ porphyroblasts and Sil, but Mus is deformed by S₃. Mafic EBR rocks attained amphibolite grade (Pl+Grt+Hbl+Bt+Ttn+Ep), consistent with peak WBR grade. Bt+Chl replacement of Grt, Chl replacement of Bt, and sericitization, locally static, of Fs, Mus, 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. Any premetamorphic fabrics and mineralogy must have been obliterated by Taconian deformation and metamorphism.