PALEOZOIC TO RECENT TECTONIC AND DENUDATION HISTORY OF ROCKS IN THE BLUE RIDGE PROVINCE, CENTRAL AND SOUTHERN APPALACHIANS--EVIDENCE FROM FISSION-TRACK THERMOCHRONOLOGY

Rocks of the Blue Ridge province record a complex history related to Paleozoic plate convergence, Triassic-Jurassic rifting, and construction of the U.S. Atlantic passive margin. Zircon and apatite fission-track (FT) thermochronology from three areas in the Blue Ridge help elucidate this evolution.

Rocks from the upper plates of the Great Smoky fault in Tennessee and North Carolina, the Blue Ridge fault in central Virginia, and an unnamed thrust fault in the Blue Ridge of northern Virginia yield remarkably uniform zircon FT ages of ≈280-305 Ma that do not vary with elevation, over sections with a vertical relief of up to 1.25 km. The zircon data indicate that rocks from the upper plates of these thrust faults, from Tennessee-North Carolina to northern Virginia (a distance of >550 km ), underwent rapid cooling through the zircon FT closure temperature (≈235 °C) in mid-Pennsylvanian to early Permian time. The time and required rate of the cooling suggest rapid uplift and denudation of upper plate rocks during emplacement of the thrust sheets in the late Paleozoic Alleghanian orogeny.

Apatite FT ages record the more recent, lower temperature history of rocks in the Blue Ridge. Apatite ages range from ≈160 to 95 Ma and define very similar trends of younger age with decreasing elevation in the three study areas. Modelling indicates that from Late Triassic or Early Jurassic time to the present, cooling rates (and, by inference, denudation rates) in the Blue Ridge have been, on average, very low. From Middle Jurassic to Late Cretaceous time when the rocks were cooling through the apatite FT closure temperature (≈90 - 100 °C in these rocks), the average effective denudation rate was about 20 m/m.y., very similar to denudation rates over the last 10² to 10⁵ years previously estimated for the Great Smoky Mountains from cosmogenic isotope (¹⁰Be) and sediment-load data.