Deformation and metamorphic history of the Western Blue Ridge–Eastern Blue Ridge terrane boundary, southern Appalachian Orogen

Abstract

The Western–Eastern Blue Ridge boundary involved formation by premetamorphic thrusting of an oceanic fragment/accretionary wedge/island arc upon Laurentian basement and metasedimentary cover. Detailed outcrop‐scale mapping, structural and petrographic analysis reveal both terranes experienced the same deformation at the same metamorphic grade. Early F1 and F2 isoclinal folds are overprinted by F3 open to tight folds; F3 transposes S2 to an axial planar cleavage (S3), the youngest penetrative feature. Prekinematic to synkinematic (relative to S2) Barrovian metamorphism attained kyanite grade in the Western Blue Ridge (WBR), upper amphibolite facies in the Eastern Blue Ridge (EBR), and anatexis in both terranes. Postkinematic M2B is characterized by appearance of fibrolite throughout the kyanite zone, coarsening to granular sillimanite and identification of a kyanite‐sillimanite isograd in the WBR. D3 strain was accommodated by dynamic recrystallization of quartz, neocrystallization, folding, and basal slip of micas, pressure solution of plagioclase, folding of kyanite, and fracturing of garnet. Scattered lower greenschist‐facies M4 static retrograde metamorphism resulted in replacement of kyanite and garnet by sericite and chlorite, respectively. Although the juxtaposition of two lithologically distinct terranes implies a fault relationship, polymetamorphism and deformation obliterated any microfabrics, mineral assemblages, or structures that might have developed during premetamorphic thrusting. The boundary cannot be an extension of a continent‐scale, dextral strike‐slip fault system inferred to have developed along Laurentian margin in the Devonian.