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

H. B. Srivastava ¹, V. K. Gairola ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi 221 005, IN

Abstract

The arkosic quartzites overlying the rocks of Aravalli Supergroup along an unconformity cum thrust contact, exhibit strong deformation resulting in preferred dimensional orientation of the pebbles. The three dimensional strain is of constriction type where the mean value of k varies from 1.33 to 6.52 from locality to locality. The shapes of the pebbles prior to deformation were mainly of constriction type. The path of pebble deformation has been traced. The strain in these pebbles is related to folding and later thrusting (gliding) during the first phase of deformation.

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V. K. Gairola ¹, D. Ackermand ²

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi 221005, IN
2 Mineralogy and Petrography Institute, 40-60, Olshausen Str., 2300 Kiell, West Germany, DE

Abstract

Pelitic schists, gneisses and amphibolites from the Central Crystallines of the Garhwal Himalaya display evidence of multiple deformation (D₁, D₂ and D₃) and polymetamorphism (M₁, M₂ and M₃)' During the M₁ episode of regional metamorphism which represents a thermal peak, the pelitic schists and gneisses developed the assemblage: sillimanite-kyanite-garnet- biotite-plagioclase-K-feldspar-quartz±garnet. Widespread development of garnet and sillimanite (locally kyanite) in muscovite-bearing schists and gneisses, hornblende in amphibolites, and diopside in calcschists indicate prograde metamorphism in the sillimanite-muscovite zone of the amphibolite facies. During the M₂ and M₃ episodes the higher grade rocks of the M₁episode retrograded under PT conditions of greenschist facies.

The electron microprobe analyses of coexisting minerals from the schists. gneisses and amphibolites have been used to estimate the PT conditions of metamorphism during the Ml episode through the pertinent models of geothermobarometry. These models of geothermobarometry suggest temperatures of 645°C±50°C and pressures of 7.5±0.5 kb during the thermal peak of metamorphism (M₁ episode).

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V. K. Gairola ¹, Deepak Hatwal ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi 221005, IN

Abstract

A deformed pebbly horizon has been observed in the quartzitic phyllite member of the Garhwal Group rocks exposed along a ENE-WSW striking fault, south of village Pharsaun, District Chamoli, U. P. The directions and ratios of the principal strain axes of tbe finite strain ellipsoids have been determined. The strain data have been plotted on Flinn triangular and polar graphs, respectively. The strain analysis reveals tbat the mean shape of the pebbles prior to deformation was flattening type and with progressive deformation, the shape of the pebbles changed to constriction type, then to flattening type, and finally to constriction type. The deformation of the pebbles bas been related to the first tectonic movement. Shearing along the fault of later generation was responsible for the rotation and reorientation of the principal axes of tbe deformed pebbles.

Keywords

Strain analysis, Garhwal Group, Chamoli Dist, Uttar Pradesh.

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V. K. Gairola ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi 221 005, IN

Abstract

An experiment was carried out to develop ptygmatic fold in a competent agate layer embedded in an incompetent marble matrix in a triaxial press at a temperature of 650°C. The layer was deformed up to 21 per cent bulk-shortening under an initial confining-stress of 1 kb and at a strain rate of 7.95 x 10^-7sec^-1. The deformed layer exhibits the initial stages of ptygmatic folding. The stress-strain and time-strain relationships have been discussed. Thin-section study of the deformed layer reveals that recrystallisation of cryptocrystalline silica was the dominant mechanism of deformation. The low confining pressure and high strain rate was responsible for the development of tension and shear fractures in the agate layer.

Keywords

Ptygmatic Folding, Structural Geology.

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