• Dhruba Mukhopadhyay

      Articles written in Journal of Earth System Science

    • Structural pattern in the Precambrian rocks of Sonua-Lotapahar region, North Singhbhum, eastern India

      Dhruba Mukhopadhyay Tapas Bhattacharya Tapan Chakraborty Arun Kanti Dey

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      In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL2lineation is mostly downdip on theS2surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.

    • Structural and textural development in Singhbhum shear zone, eastern India

      Dhruba Mukhopadhyay Gautam Kumar Deb

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      The rocks within the Singhbhum shear zone in the North Singhbhum fold belt, eastern India, form a tectonic melange comprising granitic mylonite, quartz-mica phyllonite, quartz-tourmaline rock and deformed volcanic and volcaniclastic rocks. The granitic rocks show a textural gradation from the least-deformed variety having coarse-to medium-grained granitoid texture through augen-bearing protomylonite and mylonite to ultramylonite. Both type I and type II S-C mylonites are present. The most intensely deformed varieties include ultramylonite. The phyllosilicate-bearing supracrustal rocks are converted to phyllonites. The different minerals exhibit a variety of crystal plastic deformation features. Generation of successive sets of mylonitic foliation, folding of the earlier sets and their truncation by the later ones results from the progressive shearing movement. The shear sense indicators suggest a thrust-type deformation. The microstructural and textural evolution of the rocks took place in an environment of relatively low temperature, dislocation creep accompanied by dynamic recovery and dynamic recrystallization being the principal deformation mechanisms. Palaeostress estimation suggests a flow stress within the range of 50–190 MPa during mylonitization.

    • Preface

      Vinod K Gaur Dhruba Mukhopadhyay

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    • Structures in the banded iron-formation of the southeastern Bababudan Hills, Karnataka, India

      Dhruba Mukhopadhyay Mohan C Baral Ranjan K Niyogi

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      The banded iron-formation in the southeastern Bababudan Hills display a macroscopic synformal bend gently plunging towards WNW. The bedding planes in smaller individual sectors show a cylindrical or conical pattern of folding. The dominant set of minor folds has WNW-ESE trending axial planes and the axes plunge towards WNW at gentle to moderate angles, though there is considerable variation in orientation of both axes and axial planes. A later set of sporadically observed folds has N-S trending axial planes. The macroscopic synformal bend within the study area forms the southeastern corner of a horseshoe shaped regional synformal fold closure which encompasses the entire Bababudan range.

      The minor folds are buckle folds modified to a varying extent by flattening. In some examples the quartzose layers appear to be more competent than the ferruginous layers; in others the reverse is true. The folds are frequently noncylindrical and the axes show curvature with branching and en echelon patterns. Such patterns are interpreted to be the result of complex linking of progressively growing folds whose initiation is controlled by the presence of original perturbations in the layers. Domes and basins have at places developed as a result of shortening along two perpendicular directions in a constrictional type of strain. Development of folds at different stages of progressive deformation has given rise to nonparallelism of fold axes and axial planes. The axes and axial planes of smaller folds developed on the limbs of a larger fold are often oriented oblique to those of the latter. Progressive deformation has caused rotation and bending of axial planes of earlier formed folds by those developed at later stages of the same deformational episode. Coaxial recumbent to nearly reclined fold locally encountered on the N-S limb of the macroscopic fold may belong to an earlier episode of deformation or to the early stage of the main deformation episode.

      The E-W to ESE-WNW strike of axial plane of the regional fold system in the Bababudan belt contrasts with the N-S to NNW-SSE strike of axial planes of the main fold system in the Chitradurga and other schist belts of Karnataka.

    • Anasagar gneiss: A folded granitoid pluton in the Phanerozoic South Delhi Fold Belt, central Rajasthan

      Dhruba Mukhopadhyay Tapas Bhattacharyya Nandini Chattopadhyay Robert Lopez Othmar T Tobisch

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      The Anasagar gneiss was emplaced as a concordant sheet like body along the contact of quartzite and pelitic/semipelitic schist horizons in the northern part of the South Delhi Fold Belt. It is typically a granite gneiss containing megacrysts of K-feldspar set in a recrystallised foliated matrix. The megacrysts are in general converted to granular aggregates, often retaining their crystal outline. Garnet, sillimanite (fibrolite) and rarely staurolite are the metamorphic minerals in the gneiss; these are also present in the enveloping supracrustal rocks. Both the gneiss and the supracrustal rocks are involved in polyphase deformation. F1 isoclinal folds are present only on minor scale in the supracrustal rocks. F2 major and minor folding have affected both the gneiss and the supracrustal rocks. These are asymmetrical folds with alternate flat and steep, locally overturned, limbs and have consistent easterly vergence. F3 folds are upright and coaxial with F2. F4 puckers and large scale warps have E-W to ESE-WNW subvertical axial planes. The gneiss is exposed in the core of an F3 arch on the flat limb of a major F2 antiform whose axial trace is bent by an F4 fold. The intrusion was pre-F2 and late-tectonic with F1. U-Pb zircon dating suggests a crystallization age of 1849 ± 8 Ma. Hence the Anasagar gneiss is older than the late- to post-tectonic ``Erinpura-type'' granite in the South Delhi Fold Belt.

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