CONSTRAINTS ON THE TIMING OF PALEOZOIC METAMORPHISM AND LARGE-SCALE EXTENSION WITHIN THE EASTERN BLUE RIDGE NORTH OF GRANDFATHER MOUNTAIN WINDOW

The rocks of the southern Appalachian Eastern Blue Ridge (EBR) record deformation from three orogenic events that spanned the Paleozoic. High-pressure rocks interpreted as retrograded eclogite have been identified within the base of the EBR north of Grandfather Mountain window (GMW), but the surrounding area has lacked detailed geologic mapping and metamorphic age determinations to better understand the metamorphic history. Our recent mapping of the EBR north of GMW revealed the existence of two ductile normal faults, the Idlewild and Grassy Creek faults, which reflect large-scale Paleozoic extension. Mylonite within the Idlewild and Grassy Creek shear zones record top-to-the-SSE normal faulting under amphibolite-facies conditions (500-600 ֯C). Hornblende and muscovite ⁴⁰Ar/³⁹Ar ages help constrain the timing of amphibolite through greenschist facies metamorphism within the faults and the surrounding area. Muscovite within the two shear zones and the hanging wall of the Idlewild fault yield ⁴⁰Ar/³⁹Ar ages between 334.25 ± 0.39 Ma and 337.87 ± 0.48 Ma. Hornblende from an epidote-rich amphibolite within the Idlewild fault yields a similar ⁴⁰Ar/³⁹Ar age of 337.95 ± 1.29 Ma, reflecting the timing of greenschist-facies metamorphism during early Alleghanian deformation. Hornblende across the area yield ⁴⁰Ar/³⁹Ar ages between 346.19 ± 0.95 Ma and 359.09 ± 6.27 Ma, indicating that cooling below ~500 ֯C occurred during the Neoacadian. Hornblende from within the Idlewild and Grassy Creek faults yield ⁴⁰Ar/³⁹Ar ages of 352.43 ± 5.12 Ma and 351.52 ± 0.84 Ma, respectively. At amphibolite facies deformation, the hornblende ⁴⁰Ar/³⁹Ar ages within the shear zones should represent the minimum possible age of faulting. An age of ~352 Ma suggests faulting may have occurred during or prior to the Neoacadian orogeny when the EBR was locally deformed under amphibolite-facies conditions. The Idlewild and Grassy Creek faults may be associated with syncollisional extension during the Taconic or Neoacadian orogenies. Synorogenic extensional faults in other orogenic belts are often associated with the exhumation of high-pressure rocks. The extension in the EBR may explain the exhumation of the high-pressure rocks that lie within the footwall of the faults.