Results of apatite fission track analyses on 29 Ordovician through Permian sandstones from the Appalachian Basin in Pennsylvania are presented. Ages range from 111¿17 to 184¿10 Ma. Mean track lengths of 10.71¿0.29 to 13.10¿0.17 &mgr;m with unimodal, negatively skewed length distributions are indicative of slow cooling. The data separate into two groups on an age versus mean length plot. The younger group (111--144 Ma) is found in the structural depressions of the Anthracite Basin and the Broad Top Basin and adjacent Appalachian Plateau. The older group (144--184 Ma) is found in the structurally higher southwestern Appalachian Plateau and Juniata Culmination and adjacent central plateau. Fission track data suggest that the basin cooled slowly after the Alleghanian Orogeny, with culminations cooling earlier than depressions. Cooling histories modeled from apatite fission track data, with maximum temperatures constrained by vitrinite reflectance, indicate cooling beginning soon after the Alleghanian Orogeny except in the Juniata Culmination, which apparently experienced synorogenic cooling and unroofing during formation of the underlying duplex.

Model cooling histories and available geologic information indicate that the foreland basin did not experience Mesozoic reheating. Unroofing histories were modeled from fission track cooling histories using heat flow estimates and burial depths derived from vitrinite reflectance profiles. The models suggest that the unroofing history of the Appalachian Basin in Pennsylvania can be divided into three episodes. An initial episode of relatively rapid cooling and unroofing (Late Permian-Early Jurassic) is attributed to flexural rebound of the foreland in response to erosional removal of Alleghanian topographic load. Initial unroofing rates are higher in eastern Pennsylvania than in the west, consistent with a flexural model. An episode of little to no unroofing (Middle Jurassic-late Oligocene) began contemporaneously with the inception of drift at the Atlantic continental margin. At this time, unloading of the orogen was replaced by subsidence and sedimentation on the new margin. Without flexural rebound the driving force for unroofing of the basin was removed and unroofing slowed greatly. An episode of rapid unroofing over the full width of the basin occurred from the Miocene to the present. Although the driving mechanism for unroofing at this time has not been identified, it is consistent with increased sedimentation rates in the middle Atlantic offshore basins for the same period. ¿ American Geophysical Union 1994