Geometrical similarity in successively developed folds and sheath folds in the basement rocks of the Northwestern Indian shield

Abstract

An intensely deformed gneiss-migmatite terrane and a relatively undeformed granulite-granitoid terrane constitute the bulk of Precambrian basement in the northwestern Indian Shield. This article traces the structural evolution in the gneiss-migmatite terrane, where traditional methods of structural analysis are difficult to apply, and shows how successively developed folds can assume identical geometry and orientation at an advanced stage of progressive ductile shearing. The gneiss-migmatite terrane exemplifies a regional-scale ductile shear zone that preserves the history of polyphase folding and sheath folding. Geometrical similarity between individual/domain-scale sheath folds and mesoscopic/regional-scale folds implies that sheath folding is common at all scales in the gneiss-migmatite terrane. As the mylonite foliation that traces successive folds is curviplanar, the successively initiated hinge lines were curvilinear from their inception in the shear zone. At the advanced stage of ductile shearing, the hinge line curvatures were accentuated due to their rotation towards subvertically directed maximum stretching (X), and variably oriented fold axial planes were brought into approximate parallelism with the upright principal plane (XY) of the bulk strain ellipsoid. Eventually all the folds, irrespective of their relative order of development, became strongly non-cylindrical, extremely tight, isoclinal and approximately co-planar with respect to each other. It is due to the above geometrical modifications during ductile shearing that folds, irrespective of their order of development, now appear identical with respect to isoclinal geometry, axial plane orientation and hinge line curvilinearity. Evidence from the fold orientations, the deformed lineation patterns and the sheath fold geometry suggest that the shearing occurred in a general shear type of bulk strain, and NNW-SSE-directed subhorizontal compression resulted in subvertically directed stretching in the gneiss-migmatite terrane.