Structural analysis of the Tablerock thrust sheet, Grandfather Mountain window, northwestern North Carolina: Emplacement kinematics of a large horse in a major thrust system

The Tablerock thrust sheet is exposed along the southwestern margin of Grandfather Mountain window in northwestern North Carolina, where it separates basement and cover rocks inside the window from basement thrust sheets of the overriding Blue Ridge-Piedmont megathrust sheet. It is a complex of footwall-derived horses of rifted-margin metasedimentary rocks, including Neoproterozoic to Early Cambrian Chilhowee Group quartzite and phyllite, and Shady Dolomite. Penetrative deformation throughout the Tablerock thrust sheet is defined by an extensively transposed foliation, and strong colinearity between well developed transport lineations and SE/NW-trending tight, isoclinal, and sheath folds. Centimeter- to meter-scale sheath folds are common throughout the interior of the thrust sheet, and not just within fault zones. NE-trending/NW-vergent crenulations and mesoscale open folds overprint these fabric elements, and represent one to two later phases of deformation. The strong colinearity between transport lineations and the hinges of tight to isoclinal folds, prevalence of mesoscale sheath folds, and outcrop patterns on Bryant and Reed’s 1970 map suggest SE/NW-trending megascopic folds at Linville Falls waterfall, Woodlawn quarry, and in the Linville Gorge Wilderness Area are map-scale sheath folds. Deformation temperatures estimated from metamorphic mineral assemblages, quartz lattice-preferred orientations, dynamic recrystallization microstructures, feldspar deformation microstructures, and the opening angles of quartz [c]-axis fabrics indicate the Tablerock thrust sheet reached 550–595 °C during lower–middle amphibolite facies conditions, and was later overprinted by upper greenschist to lower amphibolite facies conditions at 465–550 °C. These estimates are at least 120 °C higher than all previously reported temperatures, and are consistent with a model in which the Tablerock thrust sheet was significantly deformed during coupled transport with the Blue Ridge-Piedmont megathrust sheet prior to being emplaced at its present location.