MESOZOIC-CENOZOIC UNROOFING OF THE SOUTHERN APPALACHIAN BASIN - APATITE FISSION-TRACK EVIDENCE FROM MIDDLE PENNSYLVANIAN SANDSTONES

Apatite fission track ages (137 +/- 8 to 92 +/- 7 Ma) and mean etchable track lengths (12.1 +/- 1.8 to 12.9 +/- 1.3 mum) are reported for 10 sandstones from the Breathitt Formation (middle Pennsylvanian) at the northern end of the Pine Mountain Overthrust in Kentucky and Virginia. Because all of the apatite fission track ages are 150-200 m.y. younger than the depositional age, fission tracks that formed in the detrital apatite prior to deposition have been totally annealed. Thus, maximum temperatures attained by these samples exceeded 110 +/- 20-degrees-C. The length distribution of etchable, horizontally confined fission tracks precludes rapid (> 10-degrees-C/m.y.) cooling through the zone of partial annealing (60-125-degrees-C). Assuming a geothermal gradient of 25 +/- 5-degrees-C/km and a mean surface temperature of 15-degrees-C, sandstones at the northern end of the Pine Mountain Overthrust underwent slow erosional unroofing of 4 +/- 1 km since the Early Cretaceous (average rate of 28 +/- 6 m/m.y.). The age-track length relationships require a Miocene acceleration of unroofing, a result that corresponds to increased rates of clastic sediment accumulation along the Mid-Atlantic continental margin and the northern Gulf of Mexico. We speculate that a shift from a tropical morphogenetic regime to colder and drier conditions in the Miocene promoted mechanical weathering over chemical weathering, resulting in increased erosions rates for the central Appalachians and increased rates of clastic sedimentation in offshore basins.