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

D. Mukhopadhyay ¹

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

Abstract

The effect of compressive strain on the thicknesses of concentrically folded beds is discussed. Two sets of graphs have been given illustrating the thickness variation due to compression. If the fold axis is parallel to one of the principal axes of strain the two dimensional strain on the profile plane can be directly determined from these graphs. When this is not so the numerical value obtained from the graphs will only give a qualitative idea of the two dimensional strain. If the fold axis has got a uniform orientation in space it will not be possible to determine the three dimensional strain from thickness measurements. Equations have been given by which the three dimensional strain can be computed if the fold axis has a variable orientation.

It has been shown that in compressed concentric folds there is a critical value of θ (θ being the angle between the bedding plane and the normal to the axial plane), below which the fold will show a nearly perfect similar geometry. If in natural compressed concentric folds the maximum value of θ observed falls below this critical value the fold will show a similar geometry. Thus, it is possible to produce similar fold by compressing an initially concentric one.

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D. Mukhopadhyay ¹, S. Bhattacharya ¹

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

Abstract

The deformed pebbles in the conglomerate show a large variation in the orientation of their longest axes as well as in the values of the axial ratios a' and b'. This has been interpreted as due to a nonspherical nature of the original pebbles. The effect of deformation has been separated from the effect of the original shape. It has been shown that a superficial analysis of the shape of deformed pebbles may give a wrong idea about the state of strain in the rocks, and the method by which the true amount of strain can be computed from dc:formed pebbles has been discussed.

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A. K. Choudhary ¹, A. Naik ², D. Mukhopadhyay ³, K. Gopalan ⁴

Affiliations
1 Department of Physics, Samastipur College, Samastipur, Bihar, IN
2 Department of Earth Sciences, Sambalpur University, Orissa, IN
3 Department of Geology, Calcutta University, Calcutta, W.B., IN
4 National Geophysical Research Institute, Hyderabad, A.P., IN

Abstract

The wholerock Rb-Sr isochron age of 2380 &#177; 44 Ma (Sr<SUB>i</SUB> = 0.70492  &#177; 14) obtained for the Sambalpur granodiorite indicates an Early Proterozoic granitic activity in this region younger than Singhbhum Granite (ca. 3.3 to 3.1 Ga). Comparable Rb-Sr ages have also been reported for different granites from Mandhya Pradesh.

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D. Mukhopadhyay ¹

Affiliations
1 Department of Geology University of Calcutta 35 Ballygunge Circular Road Kolkatna-70001 9, IN

Abstract

No Abstract.

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A. F. Trendall ¹, J. R De Laeter ¹, D. R. Nelson ², D. Mukhopadhyay ³

Affiliations
1 School of Physical Sciences, Curtin University of Technology, GPO Box 1987, Perth 6000, Western Australia, AU
2 Geological Survey of Western Australia, Mineral House, 1 Plain Street, East Perth 6004, Western Australia, AU
3 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road. Calcutta 700 019, IN

Abstract

SHRIMP U-Pb analyses of zircon grains from two thin dark shales believed to be syndepositional ashfall tuffs close to the base of the banded iron-formation (BIF) within the Mulaingiri Formation of the Bababudan Group (Dharwar Supergroup) of the western Kamataka Craton provide a precise age for the onset of BIF deposition in the basin. Sample 120019C, from the exact base of the BIF, gave an age of 2720 ± 7 Ma, while sample 120018C, 14.5m above the base, gave an age of 2718 ± 6 Ma. The second sample contained younger grains which may have been affected by a late thermal event.

Keywords

Geochronology, Bababudan Schist Belt, SHRIMP, BIF, Karnataka.

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D. Mukhopadhyay ¹

Affiliations
1 Dept. of Geology University of Calcutta, Calcutta - 700 019, IN

Abstract

No Abstract.

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