Palinspastic Reconstruction Around a Recess in the Appalachian Thrust Belt in Georgia

Regionally, the Appalachian thrust belt includes the gradually curved Tennessee salient, convex toward the craton in the direction of thrust translation, and the more angular bend of the Alabama recess, concave toward the craton. In northwestern Georgia, north-northeast-striking thrust faults and related folds in the southern arm of the Tennessee salient intersect east- northeast-striking thrust faults and related folds of the Alabama recess (Fig. 1). The primary scientific objective of this project is to determine the structural history of the intersection between two regionally extensive structural trends in the southern Appalachian thrust belt. Two end-member alternative solutions may be suggested for such a junction of structural trends. One involves two or more temporally successive episodes of deformation, each with a different translation direction that corresponds to the observed structural trends, such that the younger set of structures overprints the other with compressional interference structures. In contrast, the other solution involves a single episode of deformation in which thrust-translated structures are bent around a pre-existing footwall structure. Variations in strain distribution, deformation style, and nature of structural overprinting should distinguish the two alternatives. Moreover, variations in strain accommodation should occur not only laterally, but also vertically as a result of contrasting mechanical properties between stratigraphic units. We hope to produce a restored three-dimensional model that will delineate domains of strain accommodation. The proposed geologic mapping area in Georgia (Fig. 2) is selected to help address questions of kinematics and mechanisms of thrust translation in the Appalachian thrust belt. Brian Cook, a Ph.D. student at the University of Kentucky, will conduct the field mapping. In interpreting the cause (mechanism) of curvature, the most fundamental alternative is the relation of slip direction to the shape of curvature. The construction of palinspastically restorable, balanced structural cross sections (e.g., Dahlstrom, 1969; Thomas and Bayona, 2005) relies on the assumption of slip direction approximately perpendicular to strike of frontal thrust ramps and trends of compressional fold axes. A long-standing problem is the recognition that palinspastic restoration in map view of a bend in a thrust belt commonly results in "gaps" or "overlaps" in the area of any particular stratigraphic horizon, and that imbalance in map view underscores the question of the orientation of slip direction with respect to strike of thrust-belt structures. An array of cross sections around a structural bend may be restored and balanced individually; however, restorations perpendicular to strike along intersecting thrust faults yield an imbalance in the along-strike lengths of frontal ramps. The restoration leads to a similar imbalance in the surface area of a stratigraphic horizon, reflecting a volume imbalance in three dimensions.