LATE DEFORMATION FABRICS NEAR THE WESTERN AND EASTERN BLUE RIDGE BOUNDARY, EASTERN SMOKY MOUNTAINS

At least four orogenies (Grenvillian, Taconian, Acadian, and Alleghanian) potentially produced petrofabrics and microstructures that are preserved in Laurentian basement of the Western Blue Ridge (WBR). Only the youngest three events could produce fabrics and microstructures preserved in Ocoee Supergroup cover rocks. Although most mineral assemblages and microstructures in the easternmost WBR have been assumed to be Taconian, most microstructures of the WBR have not been dated. Some structures of the Great Smoky Mountains can be correlated with the tectonic episode that caused them: the Great Smoky and Miller Cove faults and related fabrics are probably Alleghanian; the Greenbrier fault is interpreted to be pre-Taconian. Acadian microstructures are possible but have yet to be confirmed. Laurentian basement and Great Smoky Group (GSG) cover rocks in the WBR near Maggie Valley, NC exhibit deformation microstructures that overprint all regional metamorphic mineral assemblages and fabrics. These microstructures are not obvious in outcrop or hand sample. Late deformation modified older metamorphic fabrics by (1) shearing, extending, and rotating quartz- and feldspar-ribbons; (2) kinking or attenuating metamorphic index minerals (kyanite: Ky); and (3) deflecting foliation adjacent to Grt and Ky porphyroblasts. The event resulted in new muscovite and biotite growth around kyanite and garnet, respectively. Shearing produced mica-, feldspar-, and quartz-fish structures, as well as mosaics of polycrystalline quartz adjacent to quartz exhibiting subgrain boundaries (recrystallization textures adjacent to recovery textures). In Laurentian basement rocks, quartz and feldspar porphyroclasts and sphene experienced recrystallization. Although the fabric has not been dated, it is the youngest penetrative fabric in the WBR and EBR. If regional metamorphism is Taconian, as suggested by monazite chemical ages from GSG rocks, then the youngest event could be a late Taconian feature or an expression of Devonian deformation. The regional extent and pervasiveness of these structures, and their effect on resetting of isotopic systematics and mineral equilibria, have yet to be determined.