LATE PALEOZOIC OROGENESIS IN THE VIRGINIA BLUE RIDGE AND PIEDMONT: A VIEW FROM THE SOUTH

In the central and southern Appalachians the Blue Ridge forms a large basement massif at the hinterland edge of the Alleghanian fold-and-thrust belt. Blue Ridge thrust sheets in north-central Virginia were emplaced during Paleozoic contractional deformation, however the age of deformation has remained uncertain. Some workers posit that significant ductile deformation occurred during the Taconic orogeny whereas other workers suggest that foliations and thrust faults developed in the Alleghanian orogeny. Metamorphic mineral assemblages, geothermometry, and diagnostic microstructures indicate that during the Paleozoic: (1) the Mesoproterozoic basement rocks reached maximum temperatures of ~500° C, (2) the Neoproterozoic cover rocks in the eastern Blue Ridge experienced temperatures of ≤400° C, and (3) Neoproterozoic and Cambrian cover rocks in the western Blue Ridge never exceeded ~350° C. ⁴⁰Ar/³⁹Ar data from white mica reveal that the Mesoproterozoic basement and Neoproterozoic cover rocks in the eastern Blue Ridge cooled through ~350° C prior to ~350 Ma. In the western Blue Ridge white mica growth ages are typically 330 to 310 Ma. These data suggest that high-strain zones, penetrative foliations and folds developed from the Devonian to Pennsylvanian. Penetrative deformation and metamorphism in the Virginia Blue Ridge occurred after the classic Devonian Acadian orogeny and before the terminal part of the Alleghanian orogeny in the Permian. Although the foreland rock sequence to the west of the Blue Ridge is testament to a Taconic orogeny, the new geochronology provides little information about Ordovician/Silurian tectonics in the Blue Ridge. The Blue Ridge massif was emplaced onto sedimentary rocks of the Valley & Ridge province along a brittle thrust fault system that cuts older ductile structures and penetrative fabrics in the Blue Ridge and likely occurred during the late Alleghanian (305 to 280 Ma). Greenschist facies metasedimentary rocks of the western Piedmont were juxtaposed onto the eastern Blue Ridge rocks along the Mountain Run fault zone. ⁴⁰Ar/³⁹Ar age-spectra of ~400 Ma from muscovite in western Piedmont rocks indicate that Acadian or older deformation occurred in the western Piedmont.