Formation of oroclines in the new England Orogen, Eastern Australia

Most of the New England Orogen comprises a convergent margin, in which Middle Devonian to latest Carboniferous continental margin arcs, forearc basin and subduction complexes were developed above a west-dipping subduction zone. The subsequent history is marked by an early and middle Permian hiatus in arc magmatism, followed by a resumption of west-dipping subduction from the late Permian to Triassic. The geometry of the southern New England Orogen is dominated by a northern, well-established oroclinal fold pair, developed in a subduction complex and overlying Permian rocks, and by a southern oroclinal fold pair, more controversial in acceptance. Our data sustain the presence of the two southern oroclines or megafolds, and suggest that they formed by anticlockwise fold rotation over a possible time span of ~40 million years, beginning in the latest Carboniferous and continuing into the middle Permian. By generating an oroclinal model that takes into account the fold-thrust deformation style of the forearc basin, along with multiple deformation, variations in directions and amounts of shortening, as well as vergence variations, we suggest that the Manning and Hastings oroclinal folds in forearc basin and subduction complex rocks developed as amplified buckle folds of large amplitude, the hinges of which can be tracked south-southwest along their axial traces into smaller amplitude folds along the old arc/forearc boundary. Rather than forming in response to either sinistral or dextral simple shear slip of hundred of kilometres on an inferred N-trending onshore or offshore master fault, these oroclines are reflections of changes in directions and amounts of shortening that occurred along the western margin of the New England Orogen during a lull in convergent margin tectonism.