APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE SOUTHERN APPALACHIAN BASIN - MARYLAND, WEST-VIRGINIA, AND VIRGINIA

Apatite fission-track apparent ages (246 +/- 37 to 95 +/- 18 Ma) for 26 samples of upper Devonian (Hampshire and Chemung Formations) and middle Devonian age (Tioga Ash Bed) from the southern Appalachian Basin of Maryland, Virginia, and West Virginia, along with confined track length distributions for 13 of these samples, suggest that uplift was contemporaneous with triassic-Jurassic extension along the Atlantic continental margin. Uplift, as measured by apatite fission-track analysis, began earliest in the northwestern section on the Cumberland Plateau at approximately 225 +/- 25 Ma. This area probably required the least amount of erosional unroofing (approximately-3.1 km). Samples from the Valley and Ridge Province of northern West Virginia, Virginia, and Maryland yield progressively younger apatite fission-track apparent ages to the east (ranging from 163 +/- 10 to 95 +/- 18 Ma). This is consistent with deeper burial in the eastern Appalachian Basin as indicated by increasing CAI indices and geodynamic modeling. The southwestern Virginia samples yield a mean apatite fission-track apparent age of 176 +/- 11 Ma, which agrees with the Middle Jurassic apatite fission-track ages to the north.