POSTOROGENIC DENUDATION ALONG THE LATE PALEOZOIC OUACHITA TREND, SOUTH CENTRAL UNITED-STATES-OF-AMERICA - MAGNITUDE AND TIMING CONSTRAINTS FROM APATITE FISSION-TRACK DATA

The magnitude and timing of synorogenic and postorogenic denudation al ong fold and thrust belts and their associated foreland basins, of int erest because of both tectonic and economic implications, is generally poorly constrained. Along the late Paleozoic Ouachita trend, a thin v eneer of Cretaceous strata is preserved above a low-relief erosional s urface that beveled the Ouachita orogen and adjacent foreland areas. T his regional erosion surface provides a valuable constraint for interp reting new and previously published apatite fission track (AFT) data o btained from exposed structural highs along the Ouachita trend (Marath on, Llano, Arbuckle, and Benton uplifts). AFT data from sampled locali ties within the deformation belt (Marathon and Benton uplifts) exhibit younger ages and, generally, longer mean lengths than data from local ities on the foreland side of the deformation front (Llano and Arbuckl e uplifts). This observation suggests that erosion of the orogen, rath er than its extensional collapse, was the primary mechanism responsibl e for flexural isostatic unloading of the foreland crust. In addition, all samples show evidence for mild reheating following their pre-Cret aceous cooling history. Specifically, the lack of a significant popula tion of >14.5-mu m tracks in all samples appears to require residence at temperatures of greater than or equal to 55 degrees+/-5 degrees C a fter development of the sub-Cretaceous erosional surface. This implies that similar to 1000 m of Cretaceous-Paleogene(?) strata were deposit ed across the entire Ouachita frontal trend and subsequently removed d uring later Tertiary time. This Tertiary denudation is interpreted to reflect the interplay between regional denudation and isostatic compen sation in response to slow (similar to 10 m/m.y.) epeirogenic uplift o f the southern midcontinent and a long-term drop (similar to 200 m) in eustatic sealevel during this time.