POST-OROGENIC THERMAL HISTORY AND EXHUMATION OF THE APPALACHIAN LANDSCAPE: LOW-TEMPERATURE THERMOCHRONOLOGIC CONSTRAINTS FROM THE NORTHERN APPALACHIAN BASIN AND THE CATSKILLS

Apatite fission track thermochronology (AFT) and (U-Th)/He dating is applied to Devonian strata to constrain the post-orogenic thermal history and exhumation of the Paleozoic Northern Appalachian Foreland Basin (NAB) and the Catskills. AFT ages across southern NY and northern PA range from ~185-120 Ma with the oldest ages in the west, decreasing to the east. Mean track lengths span from 10.6 to 13 µm, with unimodal, slightly negatively skewed distributions. On the eastern edge of the NAB, samples collected over ~1000 m of relief in the Catskills have AFT ages from 132-96 Ma and mean lengths from 12.5 to 13 µm with unimodal, slightly negatively skewed distributions. Except for the western most samples, all samples reached temperatures higher than the retentivity of fission tracks in apatite (i.e. ~110-120 °C). Thermal maximum was reached after the Alleghanian orogeny and roughly coincident with the cessation of deposition in the NAB and onset rifting in the Late Triassic to Early Jurassic. Combined with existing stratigraphic and maximum paleo-temperature indicators, inverse thermal models constrain best-fit time-temperature paths: (1) Cooling from maximum temperatures beginning in the Early Jurassic across the NAB. (2) In the Catskills, rapid cooling from the base of the partial annealing zone (~120 °C) occurs in the Early Cretaceous, coeval with rapid cooling documented further south in the Appalachian Basin. (3) Western samples constrain slower cooling rates, maximum temperatures of ~100-150 °C and longer residence times at depth compared to eastern samples with simple, relatively rapid cooling paths from maximum temperatures of ~140-190 °C. (4) Initial cooling is likely due to erosion of the orogenic overburden, which was initiated at similar times across the basin with greater erosion in the east as reflected in the stratigraphy and AFT ages. (5) Western samples stabilize and reside within the partial annealing zone from the Early Cretaceous to mid-Eocene while eastern samples constrain continuous cooling during this period, due to differential amounts of overburden. (6) The western samples and the Catskills vertical profile record the onset of more rapid cooling in the Late Cenozoic, but this signal is not seen in most eastern samples. The signal is likely due to landscape rejuvenation across the region.