TECTONIC SIGNIFICANCE OF QUASI-CYLINDRICAL S/Z FOLDS IN HIGH-GRADE METAMORPHIC ROCKS OF THE WESTERN GRENVILLE PROVINCE AND THE SOUTHERN APPALACHIANS: A COMPARATIVE STRUCTURAL STUDY
Quasi-cylindrical S/Z folds in the main foliation are commonly decorated by axial mineral-elongation lineations, but usually lack pervasive axial-plane schistosity. Such folds have S-style in the CGB, and their axes make obtuse angles with the northeast trend of southeast-dipping lithotectonic surfaces. Consequently, the DSC are orogen-subparallel within the CGB, and have sinistral-normal sense. In contrast, quasi-cylindrical folds in the WIP are overturned to the northwest, and their horizontal axes trend parallel to the orogen. Therefore, the DSC are orogen-normal and have the sense of reverse faults. The S-folds of the CGB may have formed under vertical thinning and associated horizontal extension, whereas the horizontal folds of the WIP developed in a regime of NW-SE shortening and associated vertical thickening. Both results are novel, and enhance the geological knowledge of the western Grenville Province and the southern Appalachians.