Sheath Fold in Charnockite Around Nekkanamalai, Vellore District, Tamil Nadu

A sheath fold with inverted hinge is recorded within the charnockite exposed around Nekkanamalai in the Southern Granulite Terrain. The area lies within the NNE-trending Gudiyatham-Bhavani Straight Belt (GBSB) bounded on either side by two distinctly different tectono-stratigraphic terranes brought into juxtaposition along major tectonic contacts. Such folds are known in regions of re-folding, with later folding events of high intensity at high angles to the earlier folds, as well as in shear zones, where the direction of shear is subparallel to the sheath axes. The sheath fold in the area is interpreted to be due to superposed folding resulting from the complicated pattern of deformation produced during the welding of different terranes along the GBSB, which had brought about not only plunge reversals but also plunge inversion of the earlier folds.