Mississippian Exhumation and Tilting of the Ashe Metamorphic Suite above the Fries Fault, NC: Evidence from Amphibole and Muscovite 40 Ar/39Ar Age Spectrum Dating Results

New ⁴⁰Ar/³⁹Ar data from amphibole and muscovite samples in the Ashe Metamorphic Suite of Eastern Blue Ridge, NC, show age gradients that young to the south. Schists and amphibolites were collected along a NW traverse from the Fries fault to the Brevard fault. Five of six amphibole samples give interpretable age spectra through inverse isotope age diagrams that agree with minimum and plateau ages and show a N-S age gradient ranging from ~360 to 327 Ma. Ten of eleven muscovite samples show a N-S age gradient ranging from ~340 to ~325 Ma. These data suggests that the Ashe Metamorphic Suite first passed through the closure temperature of amphibole at about 360 Ma and was subsequently tilted with the SE side up in order to establish the amphibole age gradient. The less prominent muscovite age gradient suggests tilting at ~325 Ma, with more rapid apparent cooling in the south.

The Fries Thrust sheet is the upper most allochthon in the Eastern Blue Ridge and, on the eastern side, marks the boundary with the Inner Piedmont. Northwestward movement along the Fries Fault was initiated during the Taconian Orogeny, but exhumation through the 350˚C isotherm did not occur until the late Mississippian. We envision a slice of Inner Piedmont gneiss SE of the Brevard Zone that wedged with a NW component under the Ashe Metamorphic Suite to have caused the major tilting in the area about the time of the onset of the Alleghanian Orogeny.