SPATIALLY VARIABLE ALLEGHANIAN EXHUMATION OF THE CENTRAL APPALACHIAN MOUNTAINS FROM ZIRCON (U-TH)/HE THERMOCHRONOLOGY

Syn- and post-orogenic rates of exhumation convey key information about the structural and lithospheric dynamics of an evolving orogen. In the Central Appalachian Mountains, however, limited data exist to evaluate the rates and spatial trends of exhumation during the late Paleozoic Alleghanian orogeny. Here we present targeted detrital and crystalline mid-temperature zircon (U-Th)/He (ZHe; closure temperature = 150-200°C) data along an orogen-perpendicular transect spanning the Appalachian Plateau, Valley and Ridge, Blue Ridge, and Piedmont physiographic provinces of West Virginia and Virginia. ZHe data are consistent with prior maximum thermal constraints, and display an eastward younging trend, from ~425 Ma in the Appalachian Plateau to ~165 Ma in the Piedmont. Pennsylvanian Appalachian Plateau rocks record ZHe dates that pre-date deposition, which limits post-depositional burial temperatures to <160°C, and reflects mid-Paleozoic exhumation signatures indicative of provenance from early Alleghanian uplift and re-working of Taconic or Acadian strata. Silurian through Cambrian rocks in the Valley and Ridge and western Blue Ridge yield fully reset Permian ZHe dates, which suggests maximum burial temperatures exceeded 180°C, and indicates rapid Alleghanian exhumation, the magnitude of which decreased towards the west. ZHe dates within the Valley and Ridge are >100 Ma older than previously reported regional apatite fission track dates, suggesting a protracted period of stable post-Alleghanian thermal conditions within the foreland, during which average cooling rates did not exceed 0.5°C/Ma. In contrast, a mid-Jurassic average ZHe date in the Piedmont documents rapid post-rift cooling, likely the result of the relaxation of an elevated geothermal gradient and exhumation during rift-flank uplift. Overall, results illustrate the across-strike evolution of exhumation rates from Alleghenian fold-thrust deformation to the onset of Atlantic rifting, and refine existing thermal and structural reconstructions in the Central Appalachians.