Spatially variable syn- and post-Alleghanian exhumation of the central Appalachian Mountains from zircon (U-Th)/He thermochronology

Abstract

Abstract To assess spatial and temporal patterns of Phanerozoic orogenic burial and subsequent exhumation in the central Appalachian Mountains, we present mid-temperature zircon (U-Th)/He (ZHe; closure temperature [TC] = 140–200 °C) dates for 10 samples along a 225 km, strike-perpendicular transect spanning the Appalachian Plateau, Valley and Ridge, Blue Ridge, and Piedmont physiographic provinces in West Virginia and western Virginia. Ranges of single-grain ZHe dates exhibit an eastward younging trend from 455–358 Ma in the Pennsylvanian Appalachian Plateau to 336–209 Ma in the Valley and Ridge, 298–217 Ma in the Blue Ridge, and 186–121 Ma in the Piedmont. Within the Pennsylvanian Appalachian Plateau, detrital ZHe dates are older than corresponding depositional ages, thus limiting postdepositional burial temperatures to less than 160 °C. These ZHe dates capture predepositional mid-Paleozoic cooling signatures, indicating provenance from either recycled Taconic or Acadian basin strata or mid-Paleozoic Appalachian terranes. Across the Valley and Ridge and western Blue Ridge provinces, reset Permian detrital ZHe dates feature flat date-effective uranium correlations that suggest rapid Alleghanian cooling initiating prior to 270 Ma. ZHe dates within the Valley and Ridge are more than 100 m.y. older than previously reported regional apatite fission-track dates, reflecting a protracted period of stable post-Alleghanian thermal conditions within the foreland. By contrast, post-Triassic single-grain ZHe dates in the interior Piedmont document rapid postrift cooling, likely resulting from both the relaxation of an elevated geothermal gradient and exhumation from rift-flank uplift. The spatial discontinuity between stable synrift thermal conditions in the Valley and Ridge and rapid cooling in the Piedmont suggests that rift-flank uplift and cooling were concentrated outboard of the foreland within the Piedmont province.