STRUCTURAL GEOLOGY OF THE LINVILLE FALLS SHEAR ZONE ALONG THE WESTERN MARGIN OF THE GRANDFATHER MOUNTAIN WINDOW, NORTHWESTERN NORTH CAROLINA

The Linville Falls fault frames the Grandfather Mountain window, and separates thrust sheets of the Blue Ridge thrust complex from basement and metasedimentary rocks within the window. The fault is named for an exposure at Linville Falls, NC, where basement gneisses of the Fork Ridge thrust sheet overlie Paleozoic metasedimentary rocks of the Tablerock thrust sheet. Previous workers described the fault at this exposure as a meter-scale mylonite zone, and attempted to explain thickness variations between Linville Falls and exposures north of the Grandfather Mountain window. Geologic mapping shows that in the vicinity of Linville Falls, a ductile shear zone several hundred meters thick occurs immediately above the meter-scale mylonite zone. Mylonitization extends structurally upward to the base of the overlying Ashe Metamorphic Suite, where it overprints older regional amphibolite-facies assemblages and strike-slip fabrics of the Burnsville fault. Field relationships indicate that the thin mylonite zone exposed at Linville Falls represents the base of the shear zone and the locus of major displacement. There is no significant variation in shear zone thickness between this exposure and exposures along the northern margin of the Grandfather Mountain window. At Linville Falls, the hanging-wall rock above the meter-scale shear zone is a tectonic block of alkali feldspar granite structurally overlain by basement-derived mylonite of the Linville Falls shear zone. Shear zone mylonites record crystal-plastic deformation under greenschist-facies conditions, and kinematic indicators are consistent with top-to-northwest shear sense. At the base of the shear zone, cataclastic deformation overprints ductile fabric within the Linville Falls granite. Structural relationships in the Linville Falls area confirm ductile overprinting of earlier structures and metamorphic fabrics during Alleghanian shearing, followed by brittle deformation during late movement of the Blue Ridge thrust complex.