Structural Styles in the Precambrian Metamorphic Terranes of Peninsular India: A Synthesis

Abstract

Abstract Structural features are known in reasonable detail for three Precambrian metamorphic terranes of Peninsular India, namely, Karnataka in South India, Kajasthan in Western India, and Bihar-Orissa in Eastern India. All the rock groups of the Early Precambrian Dharwar tectonic province of Karnataka-the supposed Sargur and Dharwar supracrustal belts, the Peninsular Gneiss and the granulite-have been involved in the same style and sequence of superposed deformations. The folds of the first set are isoclinal with attenuated limbs and thickened hinges. These, together with their axial planar cleavage, have bean affected in selected sectors by near-coaxial upright folds of varying tightness. On these structures a set of open folds has been overprinted. Superposition of folds of three sets has resulted in interference patterns of different types from the scale of hand specimen to map. This structural unity runs counter to any suggestion of two groups of supracrustal rocks now separated by an angular unconformity. Migmatization synkinematic with the first folding in a large part of the Peninsular Gneiss also argues against the gneiss, as we see it now, being the basement for the supracrustal rocks. Evidence has been adduced from relict structures in enclaves in the Peninsuar Gneiss for one episode of deformation prior to the first decipherable folding in the supracrustal rocks. The NNW extension of the supracrustal belts is due to the north-northwesterly strike of axial planes of folds of two systems. The NNE to NE extension of the supracrustal Aravalli and Delhi belts of Rajasthan in Western India is also a reflection of the strikes of axial planes of folds of two generations, whose axes have a nearly orthogonal relation. W- to WNW-trending reclined isoclinal folds of the first generation, which are ubiquitous in the Aravalli Group, are absent in the rocks of the Delhi Group. Structures of the three later generations are, however, common to both the groups. These are the upright folds on N- to NNE-trending axes, overprinted by gravity-induced conjugate folds, which are in turn followed by upright conjugate folds indicating longitudinal shortening. The Banded Gneissic Complex, considered by Heron to be the basement, comprises granites and gneisses of different ages - some older lying below the Aravalli Group, and some others representing the migmatized Aravalli rocks. Similarity in structural style and sequence of even the undoubtedly older gneisses with those in the Aravalli rocks suggests remobi1ization of the basement during the first deformation in the Aravalli Group. In the eastern part of the Indian shield, several distinct tectonic provinces can be recognized, but their interrelation remains obscure. The Older Metamorphic Group of supracrustal rocks and the intrusive tonalitic gneisses show the same sequence of superposed folding, with NE-plunging early folds overprinted by folds plunging SE. The Singhbhum Granite body is a composite batholith with several independent plutons showing swirling patterns of primary foliation. The dominantly chemogenic and volcaniclastic sedimentary rocks of the Iron Ore province are folded into an overturned synclinorium with NNE-plunging axis. Coaxial refolding followed by EW warping are the manifestations of later deformational phases. The North and East Singhbhum province comprises the metaflysch sequence of the Chakradharpur-Tatanagar-Ghatsila belt, the long linear zone of Dalma metavolcanics, and the northern belt of metasediments. The most pervasive planar structure here is an EW-striking schistosity axial planar to large-scale folds of nearly EW axial trend, with northerly axial trend in some sectors. These structures apparently continue northward to the Chhotanagpur Gneiss terrane, where the structural details are imperfectly known. The southern boundary of the Northern and Eastern Singhbhum province is marked by a belt of cataclasis and mylonitization - the Singhbhum shear zone - whose tectonic significance is a matter of debate.