NEW GEOLOGIC MAP AND GEOCHRONOLOGY OF THE SHENANDOAH NATIONAL PARK REGION, VIRGINIA

Geologic mapping in the Shenandoah National Park region of Virginia was conducted from 1995 to 2008 as part of a cooperative investigation between the USGS, National Park Service, College of William and Mary, and George Washington University, through the National Cooperative Geologic Mapping Program’s Educational Mapping component. The 1:100,000-scale map was compiled from new and existing field data collected at 1:24,000-scale.

Geochronologic analyses of igneous, metamorphic, and detrital zircons using U-Pb SHRIMP and TIMS, ⁴⁰Ar/³⁹Ar analysis of hornblende and white mica, and fission-track analysis of zircon and apatite, delimit the tectonic history. SHRIMP U-Pb zircon ages of 26 Mesoproterozoic granitoid units range from 1,183±11 to 1,028±9 Ma. Rocks older than 1144 Ma were deformed during the Shawinigan phase of the Elzeviran orogeny; less intense deformation and metamorphism associated with the Ottawan phase of the Grenvillian orogeny concluded in the early Neoproterozoic (960 Ma zircon overgrowths and 920 Ma ⁴⁰Ar/³⁹Ar hornblende cooling ages). Newly recognized Neoproterozoic rocks include metasedimentary paragneisses that contain detrital zircons as young as 997, 959, and 812 Ma, and 719-714 Ma volcanic rocks temporally related to the Robertson River Igneous Suite. Five outliers of the Swift Run Formation reflect paleotopographic and extensional basins. U-Pb TIMS analyses of chemically abraded zircons from felsic tuffs intercalated with metabasalt flows in the lower part of the Catoctin Formation, and the uppermost rhyolite flow in Pennsylvania, bracket volcanism between 571 and 563 Ma (errors of the units are 0.1-0.2%, 95% c.l.). ⁴⁰Ar/³⁹Ar and fission-track data support metamorphism, deformation, and uplift in the Mississippian to Permian Alleghanian orogeny. Eight anastomosing Alleghanian high-strain zones in the Mesoproterozoic rocks are as much as 2 km wide, 100 km long, with as much as 3 km of thrust displacement. Using an average effective uplift and denudation rate of about 25 m/m.y., about 600 m of rock have been removed since the proto-Potomac River breached the Blue Ridge from the east in the Oligocene-Miocene. The highlands were, thus, at least twice the current altitude. Neogene alluvial and colluvial fans mantle the lower slopes, whereas stratified slope deposits and colluvium of Pleistocene periglacial origin are in the uplands.