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

T. M. Ganesan ¹, K. Gopalakrishnan ¹, R. Renganathan ¹, S. S. Sivasankaran ¹

Affiliations
1 Geological Survey of India, Inner Ring Road, Guindy Industrial Estate, Chennai 600032, IN

Abstract

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.

Keywords

Structural Geology, Sheath Fold, Granulite Belt, Tamil Nadu.

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K. Gopalakrishnan ¹, S. Badrinarayanan ¹, K. S. Subramanian ¹

Affiliations
1 Geological Survey of India, IN

Abstract

No Abstract.

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K. Gopalakrishnan ¹, K. S. Subramanian ²

Affiliations
1 '"Harikesa", 30/6, (Old No.41/6), 3rd main Road, Raja Annamalai Puram, Chennai - 600 028, IN
2 plot 283, 17 East Street, Kamaraj Nagar, Tiruvanmiyur, Chennai - 600 041, IN

Abstract

In a Palaeo-Tectonic-Domain based classification involving an evolutionary model of '"Terrane-Terrane amalgamation through "Palaeo-sutures" for the Southern Granulite Terrain (SGT), India, the alkaline-carbonatite and related rocks fall within three distinct domain groups viz., Group 1 - "Within Micro-terrancs", Group 2 - "Along the Margins of Micro-Terranes with palaeo-sutures" and Group 3 - "Within Palaeo-sutures". These Late Proterozoic to Early Palaeozoic alkaline and related rocks of SGT falling within the above three groups have inherited the geochemical attributes of their respective Archaean Palaeo-Domains and fall in the fields of such domains in many of the tectonic discriminant plots used and studied by various workers in the different sectors of SGT. Such inheritance of Palaeo-Domain characteristics points to the validity of both the terrane amalgamation through Palaeo-Sutures evolutionary model for SGT as well as the Tectanic-Domain-Based classification of the alkaline related rocks of SGT. It also adds further support to the interpretation of crustal contamination in the genesis and emplacement of alkaline and related rocks in SGT, as deduced from other sources such as field relationships, petrographic criteria etc., while evidences from isotopic signatures are inconclusive.

Keywords

Alkaline Complexes, Genesis, Southern Granulite Terrain, Inherited Geochemistry, Crustal Contamination, South India.

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K. S. Subramanian , K. Gopalakrishnan

Abstract

No Abstract.

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K. S. Subramanian ¹, K. Gopalakrishnan ²

Affiliations
1 Plot 283, 17th East Street, Kamraj Nagar, Tiruvanmiyur, Chennai - 600 041, IN
2 30,3rd Main Road, Chennai - 600 028, IN

Abstract

The earth tremors experienced in Tamil Nadu and Pondicherry on the 26^th September 2001, is discussed on the basis of the evolution of the geological and structural setup in the coastal region, consequent on the separation of Antarctica from India in the geological past. Subsurface data obtained by drilling in the Cauvery basin bear out the definition of a number of NE-SW to NNE-SSW fault bounded ridges and depressions extending from land into the offshore area. The stupendous thickness of sediments in the structures is suggestive of the sagging of the Precambrian crystalline basement and the release of strain in the process is ostensibly manifested as earth tremors. The reported occurrence of volcanic activity off the coast and the occurrence of thermal water in deep boreholes drilled in the coastal region, both falling within the NE-SW to NNE-SSE fault system, suggest that the weak fault planes in the crust provided channel ways for the eruption of magma and dissipation of subsurface heat.

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