Kinematic and Temporal Evolution of the Southern Appalachian Foreland Fold-thrust Belt: Constraints from Structural, Magnetic and Radiometric Analyses.

Abstract

Formed during the Alleghanian orogeny, the southern Appalachian foreland fold-thrust belt displays curvature at the scale of an individual structure to the entire thrust belt. Using an integrated approach, this study shows that secondary rotations are not required to accommodate curvature, which contrasts with observation in curved belts elsewhere. In the Tennessee salient, the southern Appalachian fold-thrust belt displays a significant degree of curvature (~55°). Early, layer-parallel paleostress orientations recorded in calcite twins show a systematically fanned distribution that correlates with the overall strike distribution for limestone sites within both the fold-thrust belt and the “undeformed” foreland, indicating primary curvature of the salient. Moreover, the degree of fanning matches the geometry of the hinterland Blue Ridge front instead of the most forward section of the belt, implying that indentation by the Blue Ridge allochthon produced both the curvature of the Tennessee salient and the radial paleostress pattern. Paleomagnetic data from three lithologic units complement this scenario, as remagnetized directions show no correlation with orogenic strike. The synfolding remagnetization indicates that by the Middle to Late Pennsylvanian, approximately 50% of the folding in the fold-thrust belt was completed and suggests that deformation progressed from the hinterland toward the foreland. Direct constraints on the timing of deformation within the belt are provided by radiometric dating of illitic fault gouge. Illite 40Ar/39Ar ages from fault gouges show that the entire frontal thrust wedge was active simultaneously at approximately 278 Ma, such that the southern Appalachian foreland fold-thrust belt acted as a critically-stressed Coulomb wedge during the waning stages of Alleghanian deformation in the Early Permian. Shale ages from the extended foreland reflect local diagenetic conditions, rather than a far-field fluid-flow event associated with Alleghanian deformation. This study shows that the kinematic and temporal evolution of curved fold-thrust belts can be understood by integrating multiple approaches. Paleomagnetic and structural data give insight into the regional processes driving deformation, with direct fault dating giving temporal constraints. The research also shows that diagenetic grade has a significant effect on the interpretation of 40Ar/39Ar ages, which is significant for future studies of sedimentary basins and fault gouge.