LARGE-MAGNITUDE PALEOZOIC EXTENSION IN THE EASTERN BLUE RIDGE OF NORTHWESTERN NORTH CAROLINA

The southern Appalachian Blue Ridge experienced multiple episodes of contractional deformation during the Paleozoic that are reflected in the structures that dominate the region. During our recent mapping in the Eastern Blue Ridge (EBR) north of Grandfather Mountain window, we identified two ductile normal faults that indicate large-magnitude Paleozoic extension. The Idlewild and Grassy Creek faults are the first identified extensional faults in the southern Appalachians unrelated to a known rifting event. The Grassy Creek fault consists of a shear zone less than 200 meters thick that is discontinuous along strike. The Idlewild fault is ~5 km east and consists of a shear zone that is locally 700 meters thick and continuous along the currently mapped extent of 10 km. The Idlewild and Grassy Creek shear zones consist of mylonite associated with top-to-the-SSE normal faulting under amphibolite-facies conditions (500-600 ˚C). Muscovite from the Idlewild and Grassy Creek shear zones yield ⁴⁰Ar/³⁹Ar ages of 336.27 ± 0.39 Ma and 337.87 ± 0.48 Ma, respectively. Hornblende from the Idlewild fault yields an ⁴⁰Ar/³⁹Ar age of 352.98 ± 5.12 Ma. The Idlewild and Grassy Creek faults both experienced amphibolite-facies normal faulting, which indicates they are likely related to the same tectonic event. The thickness and extent of the Idlewild shear zone implies it was likely the principal shear zone and experienced more displacement. For amphibolite-facies deformation, the hornblende ⁴⁰Ar/³⁹Ar age should represent the minimum possible age of faulting along the Idlewild fault. An age of ~352 Ma indicates that the faulting may have occurred during or prior to the Acadian/Neoacadian orogeny when the EBR was locally deformed under amphibolite-facies conditions. Extensional faults in other orogenic belts are often associated with the exhumation of high-pressure rocks, and the extension in the EBR may explain the exhumation of high-pressure rocks that lie within the footwall of the faults.