Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Dhruba Mukhopadhyay ¹, Sabyasachi Dasgupta ¹, Kunal Dhar ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta 700073, IN

Abstract

A sequence of metasediments with conglomerate at the base, unconformably overlies the Singhbhurn Granite. The direction of younging in the arkose-conglomerate is towards north, away from the granite border. Wedges of basement granite were emplaced as thrust slices within the metasediments at the earliest stage of deformation. The soda granite also occurs as a sheet like concordant body and is highly deformed and in places rnylonitized. The soda granite may also represent a tectonically emplaced slice of the basement.

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Dhruba Mukhopadhyay ¹, Mohan Chand Baral ¹, Debashish Ghosh ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta 700 001, IN

Abstract

The regional structure within the Chitradurga schist belt consists of a central antiform flanked by two synclines on either side. The central antiform is a second generation struc:ture which has refolded an earlier syncline whose axial trace is now exposed on either side of the central antiform. The second generation antiform shows regional plunge culminations and depressions along its axis. The earlier syncline had originally curved fold axis and was a canoe shaped fold. In the southern part of the schist belt a tectonic slide has cut off the lower limb of the early syncline. This came into existence before the second phase of folding. The stratigraphic sequence within the schist belt has been worked out taking into account this structural framework.

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Dhruba Mukhopadhyay ¹, Mohan C. Baral ²

Affiliations
1 Indian School of Mines, Dhanbad 826004, IN
2 Jogamaya Devi College, Calcutta, IN

Abstract

The major structure south of Chitradurga is a southerly closing, very steeply plunging, nearly vertical fold (Chitradurga Fold) defined by the BIF horizons. This is a second generation structure and the dominant schistosity with NNW-SSE strike is parallel to the axial plane of this fold. The first generation structures are represented by minor folds only. They have variably oriented axes and their axial planes are bent by the major Chitradurga Fold. The strike of the third generation axial planes varies from NNE-SSW to ESE-WNW, and the axes are generally steeply plunging.

During the second deformation, folds formed at different stages and their geometry was modified by rotation in the subsequent stages of progressive deformation. The axes of the minor folds show a wider range in orientation than the direction of maximum elongation in the rocks, and there is often an angle between the two.

The distribution of the bedding plane poles in the projection diagrams of most of the sectors indicate steeply plunging β (F2). The major Chitradurga Fold shows a conical geometry with a subvertical cone axis and very small apical angle.

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Abdul Matin ¹, Kamal Nandi ², Dhruba Mukhopadhyay ³

Affiliations
1 Department of Geology, Ashutosh College, Calcutta 700 026, IN
2 Department of Geology, Presidency College, Calcutta 700 073, IN
3 Department of Applied Geology, Indian School of Mines, Dhanbad 826 004, IN

Abstract

Spodumence is recorded from the northern part of the Sandur Schist Belt, near T. B. Dam, Hospel The mineral occurs in a quartz-spodumene vein near the contact of the supracrustal rocks with the granitic gneisses.

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Abdul Matini ¹, Dhruba Mukhopadhyay ²

Affiliations
1 Department of Geology, Asutosh College, Calcutta-700026, IN
2 Department of Geology, Ballygunge Science College, University of Calcutta, Calcutta-700019, IN

Abstract

Banded ferruginous quartzite (BFQ) in the Sandur Schist Belt belongs to the Archaean Dharwar Supergroup of Karnataka and contains primary depositional and diagenetic features. These include banding of different types, ripple marks. syneresis cracks, scour-and-fill structures, globular and nodular structures and penecontemporaneous faults. The significance of these structures in terms of the environment of deposition of BFQ has been considered. The BFQ was deposited as chemical precipitates and passed through a hydroplastic stage of lithification. Shallow water environment of dcposition in a more or less quiet condition with intermittent local disturbances has been envisaged.

Keywords

Sedimentary Structure, Banded Ferruginous Quartzite, Sandur Schist Belt. Kamataka.

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Dhruba Mukhopadhyay ¹, Ashutosh Naik ²

Affiliations
1 Department of Geology, University of Calcutta Kolkata - 700 019, IN
2 Department of Earth Sciences, Sambalpur University Jyott ViharPO Sambalpur - 768 019, IN

Abstract

No Abstract.

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Dhruba Mukhopadhyay ¹, Krishnapriya Basak ¹

Affiliations
1 Department of Geology, University of Calcutta, Kolkata - 700 019, IN

Abstract

The Eastern Ghats Belt is a polycyclic granulite terrain along the east coast of India whose western boundary is marked by a shear zone along which the granulites are thrusted over the cratonic units of the Indian shield, and its northern margin is marked by the presence of a number of fault-bounded blocks. Recent work has convincingly brought out that there are domains within the belt having different evolutionary histories. The segment south of the Godavari Rift went through a high grade thermo-tectonic event at ∼1.6-1.7 Ga. North of the Godavari Rift in a narrow zone along the western boundary the last high-grade metamorphic event is of late Archaean age. A series of alkaline plutons along the western boundary zone testifies to a rifting episode at ∼1.3-1.5 Ga. In the major part of the EGB the metamorphism is broadly of Grenvillian age, with two major thermo-tectonic pulses at ∼1.1-1.2 Ga and ∼0.95-1.0 Ga. But high grade conditions persisted for a long period and younger thermal events of ∼0.65 Ga to ∼0.80 Ga are locally recorded. There are differences in the tectonometamorphic histories of different domains, but the tectonic significance of these differences remains uncertain. Pan-African (0.50-0.55) thermal overprints are common and become conspicuous along the western boundary zone. The thrusting of the Eastern Ghats granulites in a hot state over the cratons to the west is of Pan-African age. In the Rodinia assembly (∼0.9 Ga) the Eastern Ghats and the Rayner-Napier Complexes of Antarctica were contiguous, but the pre-Rodinia configuration of these terrains remains unclear. At ∼0.8 Ga during the Rodinia break up Greater India rifted apart from East Antarctica, and only later it docked with Australia-East Antarctica at 530-550 Ma. The continuation of the East Antarctic Pan-African orogenic belts into the Eastern Ghats is yet to be ascertained.

Keywords

Granulite Terrain, Rodinia Assembly, Pan-African Events, Eastern Ghats.

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Dhruba Mukhopadhyay ¹

Affiliations
1 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata- 700 019, IN

Abstract

No Abstract.

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Dhruba Mukhopadhyay ¹, Nandini Chattopadhyay ², Tapas Bhattacharyya ¹

Affiliations
1 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata-700 019, IN
2 Department of Geological Sciences, Jadavpur University, Kolkata-700 032, IN

Abstract

The structural geometry of the Anasagar gneiss dome in the axial zone of the South Delhi Fold Belt is controlled by polyphase folding. It is classified as a thrust-related gneiss dome and not as a metamorphic core complex. Four phases of deformation have affected both the gneiss and the enveloping supracrustal rocks. D₂ and D₃ deformations probably represent early and late stages of a progressive deformation episode in a simple shear regime combined with compression. The contact between the gneiss and the supracrustal rocks is a dislocation plane (thrust) with top-to-east sense of movement which is consistent with the vergence of the D₂ folds. The thrust had a ramp-and-flat geometry at depth. At the present level of exposure it is a footwall flat (that is, parallel to the gneissosity in the footwall), but it truncates the bedding of the hanging wall at some places and is parallel at others. The thrusting was probably broadly coeval with the D₂ folds and the thrust plane is locally folded by D₂. D₂ and D₃ folds have similar style and orientation as the first and second phases respectively of major folds in the Delhi Supergroup of the South Delhi Fold Belt and these are mutually correlatable. It is suggested that D₁ may be Pre-Delhi in age. Available geochronological data indicate that the emplacement of the Anasagar gneiss predated the formation of volcanic rocks in the Delhi Supergroup and also predated the main crust forming event in the fold belt. The Anasagar gneiss and its enveloping supracrustal rocks are probably older than the Delhi Supergroup.

Keywords

Anasagar Gneiss Dome, Polyphase Folding, Thrust-Related Gneiss Dome, South Delhi Fold Belt, Rajasthan.

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