Palinspastic restoration of the Anniston transverse zone in the Appalachian thrust belt, Alabama

Abstract

Transverse zones are cross-strike alignments of cross-strike linking structures (lateral ramps, transverse faults, and displacement-transfer zones) in foreland thrust belts. Cross-strike links constitute one component of a three-dimensional system of fault surfaces, connecting strike-parallel structures (frontal ramps) and detachment flats both geometrically and kinematically. Three-dimensional palinspastic restoration provides the basis to consider causes of cross-strike alignments of cross-strike links into transverse zones, as well as the role of transverse zones in kinematic history. The Anniston transverse zone in the Appalachian thrust belt in Alabama (southeastern United States) exemplifies the types and distribution of cross-strike links within a transverse zone. Palinspastic reconstruction of the Anniston transverse zone relies on matching hanging-wall/footwall pairs of frontal and lateral ramps for geometric balance in three dimensions in palinspastic-map and cross-section views. Cross-strike links within the Anniston transverse zone are systematically distributed across strike, from hinterland to foreland, as domains of lateral ramps, transverse faults, and displacement-transfer zones, in response to variations in depth to basement beneath the thrust belt and variations in thickness of décollement-host weak rocks. Contrasting structural profiles characterize the thrust belt on opposite sides of the Anniston transverse zone as a result of abrupt along-strike changes at the cross-strike alignment of cross-strike links. The Anniston transverse zone is aligned with a northwest-striking basement fault that offsets the boundary faults of the Birmingham basement graben, suggesting kinematic partitioning within the advancing thrust sheets at a stress concentrator. The northwest-striking basement fault separates domains of contrasting structural profiles of basement fault systems, differing elevations of top of basement, and differing thicknesses of the regional décollement-host weak layer in the lower part of the sedimentary succession above basement rocks.