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A. M. Bhola ¹, K. K. Saberwal ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi 110007, IN

Abstract

The metamorphic rocks occurring around Nibhor, northwestern part of the Alwar district, Rajasthan, bear evidence of three phases of folding and one period of regional metamorphism of green-schist facies. The first deformation phase, F₁ produced the principal, foliation S₁ and the later deformation phases, F₂ and F₃, produced crenulation cleavages, S₂ and S₃ respectively. Kinking by the rotation mechanism emerges as the dominant structure supported by kinks with tabular dilatant zones between parted laminae. Some kinks may also have developed by simple shear mechanism.

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A. M. Bhola ¹, S. Varadarajan ¹, Virender Khosla ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi 110007, IN

Abstract

Pegmatite. aplite and quartz veins of varying dimensions occur in association with pelitic schists, calcareous rocks, quartzites, amphibolites belonging to Ajabgarh Formation, Delhi Group in the Narnaul area and act as important structural markers.

The polyphase nature of deformation of the rocks with which these veins occur, is shown by rotation of boudins, presence of overturned tight isoclinal folds resembling F₁, presence of F₂ folds and refolded folds shown by quartz veins, refolding of limbs of folded pegmatites by folds resembling F₃. Features like boudins, pinch and swell structures shown by quartz veins and pegmatites at the limbs and along the axial planes of F₁ folds with S₁ development in the host rock and attenuation and disruption of limbs of tight folds resembling F₁ shown by these veins also point to significant amount of flattening during F₁ folding phase and longitudinal stretching in the late stage of their development. Slippage in the later stages of deformation phase (F₃) is also distinctly borne dy displaced aplite veins.

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A. M. Bhola ¹, S. Varadarajan ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi 110007, IN

Abstract

The regional metamorphic rocks of Precambrian age belonging to the Delhi Group, around Narnaul, show complex deformational-crystallization history. The first deformation phase, F₁ of Delhi orogeny produced isoclinal folds overturned towards southeast plunging in NNE-SSW directions associated with most pervasive foliation S₁. The later deformation phases, F₂, tight to open folds, and F₃, open folds produced crenulation cleavages S₂ and S₃ (rare). These three deformational events have been correlated with the crystallization periods of the more important minerals in pelitic schists and amphibolites.

The study reveals that mineral assemblages were identical during the first and second deformation phases. The index minerals garnet, staurolite, kyanite, hornblende started forming either during F₁ folding or immediately after F₁ and continued till the close of F₂ folding with incipient crystallization of micas during F₃ phase. Mineral assemblages in the pelitic schists display a single staurolite-kyanite zone. Mineralogical reactions ,indicate metamorphism under amphibolite condition below 650°C with minimum temperature 540°C and pressure 5.5 kb (P_L =P_total) in this part of the Aravalli belt.

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A. M. Bhola ¹, D. V. Lazarus ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi 110007, IN

Abstract

The geometry, angular relationship and strain in kink-bands developed on earliest fabric, S₁, in slate/phyllite sequence of Chamba area, are studied. The field study substantiatcd by microscopic study reveals that gliding is not the only mechanism for their development, instead, more significant has been the dilation and rotation as suggested by the presence of parted larninae-dilatant zones having quartz filling in many kink-bands. Shearing along kink-band boundaries also played a role in the later stages of kink-band development.

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A. M. Bhola ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi-110007, IN

Abstract

The geometric and tectonic significance of crenulation cleavage in Pukhri Formation (mainly pelitic sequence with interbedded thin quartzitic layers) of Chamba area, is studied by taking into account modifications taking place in original class 1-3 fold layers by three theoretical models viz. i) Homogeneous flattening ii) Shearing along the axial plane of folds iii) Volume reduction in multilayers. Theoretical model studies supported by the study of cleavage microstructure and microlithons in microfolds of Class 1-3 pairs point out that volume reduction model is most appropriate in explaining the development of crenulation cleavage, its intensity and frequency in the Chamba area.

Keywords

Crenulation Cleavage, Pukhri Fonnation, Chamba Area, Himachal Pradesh.

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A. M. Bhola ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi -110 007, IN

Abstract

No Abstract.

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B. K. Sharma ¹, A. M. Bhola ², A. E. Scheidegger ³

Affiliations
1 Physical Research Laboratory, Navrangpura, Ahmedabad - 380 009, IN
2 Department of Geology, University of Delhi, Delhi 110 007, IN
3 Section of Geophysics, Tech.University of Vienna, A-1040 Vienna, AU

Abstract

A study of the possible correlation between kink-band,.jointing and drainage patterns in the Chamba nappe (Himachal Himalaya) shows a correspondence between the valley trends and the joints, indicating a neotectonic control of both features. However, no correlation exists between the Precambrian to Upper Paleozoic conjugate kink-bands and the present joint/valley orientations, showing that there was a fundamental change in the tectonic pattern between the Upper Palaeozoic and the recent ongoing crustal adjustment in the Himachal Himalaya.

Keywords

Neotectonics, Joints, Rivers, Kink Bands, Himalaya, Himachal Pradesh.

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