EXTENT AND SIGNIFICANCE OF ALLEGHANIAN FAULTING NEAR THE GRANDFATHER MOUNTAIN WINDOW, NORTHWESTERN NORTH CAROLINA

Geologic mapping in the vicinity of the Grandfather Mountain window reveals data for evaluating the extent and significance of deformation in the Blue Ridge of western North Carolina. Around the Grandfather Mountain window, the Fries, Fork Ridge, and Linville Falls faults and their associated shear zones merge, resulting in a deformation zone greater than a kilometer thick. Late Paleozoic deformation in this area is dominated by northwest-directed thrust faulting accompanied by widespread greenschist-facies mylonitization which overprints (or obliterates) older fabrics and assemblages in basement and cover rocks. Macro- and micro-scale kinematic indicators and evidence of operative deformation mechanisms document shear sense and conditions of deformation. The Alleghanian age of the deformation has been historically inferred by structural/stratigraphic evidence and confirmed by isotopic analyses of mylonites and sheared intrusive rocks. Along the western margin of the window, Grenville-age basement gneisses and Neoproterozoic eastern Blue Ridge rocks (Ashe Metamorphic Suite – AMS) record greenschist facies, retrograde metamorphism overprinting kyanite-grade assemblages for several hundred meters into the hanging wall. On the northwestern side of the window, basement gneisses and Neoproterozoic intrusive rocks (Crossnore and Bakersville Suites) are strongly sheared from the Linville Falls–Fork Ridge-Fries faults to the Stone Mountain fault along the edge of the Mountain City window. Locally, these deformed rocks show evidence of a sequence of ductile-brittle-ductile-brittle overprinting deformation. The alternating styles of deformation exhibited in these rocks indicate a change in strain rate or a change in effective confining pressure during deformation caused by either an increase in fluid pressure or movement into relatively shallower crustal levels. Northeast of the window, the Alleghanian Fries fault appears to have cut through structurally higher levels and has overthrust rock units and structures that are exposed west of the window. As a result, the same sequence of structural features and lithotectonic units exposed on the southwestern side of the Grandfather Mountain window is not repeated on the northeastern side.