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

摘要

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 F_1 and F_2 isoclinal folds are overprinted by F_3 open to tight folds; F_3 transposes S_2 to an axial planar cleavage (S_3), the youngest penetrative feature. Prekinematic to synkinematic (relative to S_2) 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 M_(2B) is characterized by appearance of fibrolite throughout the kyanite zone, coarsening to granular sillimanite and identification of a kyanite-sillimanite isograd in the WBR. D_3 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 M_4 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.