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 may be produced throughout ductile rock masses either during tectonic convergence and associated vertical thickening, or during subsequent gravitational collapse and associated vertical thinning. But the attitude and geometric style of quasi-cylindrical S/Z folds can vary significantly with tectonic setting. This can lead to marked differences in the direction and sense of distributed-shear components (DSC) parallel to the fold-enveloping surfaces, as documented by us in the Central Gneiss Belt (CGB) of the western Grenville Province, Ontario, and in the western Inner Piedmont (WIP) of the Carolinas.

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.