Thermal History of the Central and Southern Appalachians from Apatite and Zircon Fission-Track Analysis

Naeser, Charles W., Naeser, Nancy D., and Southworth, C. Scott

U.S. Geological Survey, Reston, VA

Fission-track (FT) ages have been determined for >160 samples from the Allegheny Plateau, Valley and Ridge, Blue Ridge, and Piedmont provinces in West Virginia, Maryland, Virginia, District of Columbia, Tennessee, North Carolina, and South Carolina.  The data provide an overview of Phanerozoic burial and exhumation over a large region. 

In the eastern Blue Ridge and Piedmont, most rocks were buried to temperatures high enough to totally anneal zircon (>~240°C) prior to cooling during the late Paleozoic Alleghanian orogeny.  In the eastern Blue Ridge, zircons yield remarkably uniform ages, ranging from ~300 Ma in northern Virginia to ~280 Ma in Tennessee-North Carolina, that reflect rapid cooling, most likely during emplacement and rapid denudation of Alleghanian thrust sheets.

In contrast, rocks from the Valley and Ridge and Allegheny Plateau, which yield zircon FT ages equal to or greater than the depositional age of the rocks, have never been buried to temperatures high enough to totally anneal zircon, and some may have undergone little, if any, annealing.    

Apatite FT data indicate that rocks across the region cooled below 90-100°C (the apatite FT closure temperature) between ~90 Ma and ~200 Ma.  Apatite ages are youngest in the eastern Valley and Ridge, and become older east towards Washington, D.C., and west towards Frostburg, Md.  The age distribution is interpreted as reflecting flexural uplift and differential erosion.  In the Blue Ridge, where elevation differences are sufficient to obtain a relief section, apatite data suggest effective denudation rates of ~20 m/m.y. since the late Triassic-early Jurassic.