DISTRIBUTION AND TECTONIC SIGNIFICANCE OF PALEOZOIC FAULTS AND HIGH-STRAIN ZONES OF THE BLUE RIDGE, SOUTHERN VIRGINIA AND NORTHWESTERN NORTH CAROLINA

Structural and isotopic studies across the Blue Ridge thrust system from Mount Rogers, VA, southeast to the Brevard fault zone reveal Late Devonian to Mississippian deformation and a cooling history associated with movement along the Fries-Gossan Lead high-strain zone (F-GLZ). Near Fries, VA, the 1 km thick Gossan Lead high strain zone (GLZ) overrides the Fries high strain zone (FZ) and juxtaposes rocks of the Ashe Fm on basement mylonite. To the northeast, Mesoproterozoic basement truncates the southeast limb (Unicoi Fm) of a footwall anticline along the 2 km-thick FZ. Southwestward they merge into the broad F-GLZ in basement that locally contains paragneisses. Amphibolite-grade rocks of the paraconformable Wills Ridge Fm (WRF) and pelitic schist and wackes and mafic and ultramafic rocks of the overlying Ashe Fm are carried above the F-GLZ. Clasts of basement and ~730-696 Ma detrital zircon in the lower WRF suggest that these rocks are not exotic to Laurentia. The contact between ultramafic rocks in the overlying Ashe Fm may define terranes above a thrust. A 7 km- wide belt of Mesoproterozoic basement with mappable, anastomosing, 0.2-2 km thick, greenschist-facies mylonite zones occurs NW of the F-GLZ. Structural kinematics are typically a top-to-the-NW system containing mafic dikes transposed into the foliation. Low-strain domains contain Mesoproterozoic structures, including extensional features that likely formed during emplacement of the Striped Rock pluton (~690 Ma). SHRIMP U-Pb zircon ages of Mesoproterozoic basement of the French Broad massif indicate continuity of protoliths across the high-strain zones. These high strain zones die out to the NE as the basement-cover contact is not cut. Similar ⁴⁰Ar/³⁹Ar white mica cooling ages of 330 Ma across the F-GLZ record latest motion. ⁴⁰Ar/³⁹Ar cooling (500-150°C) age gradients from the Brevard (325-175 Ma) west to the F-GLZ (360 Ma hbl, 330 Ma mu, 275 Ma ksp) support a late Devonian to Permian thrust system. 335 Ma plutons decapitated by 325 Ma thrusts in the eastern BR terranes suggest rapid emplacement. The rocks and high-strain zones were folded and truncated during 325 Ma doming and thrusting of one of the largest structural culminations of the Alleghanian orogeny. The Grandfather Mountain window and surrounding sheets were unroofed in the Mesozoic.