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

K. Naha ¹, A. K. Chaudhuri ¹, A. C. Bhattacharya ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, IN

Abstract

The metamorphic and migmatitic rocks comprising the Banded Gneissic Complex, the Aravallis and the Raialos around Sangat (25° O5': 73° 45'), central Rajasthan, maintain a structural unity with no decipherable unconformity within them. The rocks have been involved in two generations of folding with the earlier set dominant in the eastern part. The early folds are isoclinal and reclined, with their axial planes dipping and fold axes plunging W to WNW. On these is superimposed a set of open, upright, NNE-trending folds resulting in a dispersal of the earlier axial orientation near Sangat. Further west, with increase in the intensity of the second deformation, the later folds become isoclinal, the earlier axial planes are rotated to vertical attitude, and the traces of the earlier folds are lost except for occasional sharply bent reclined fold hinges. Because of the diverse orientation of the foliation surfaces already folded by the first movement, the later fold axes also show a great variation in plunge.

Disharmonic folding, curving foliation around the fold hinges, absence of planar surfaces oblique to the folded foliation, and folds following the competency rule prove that both the sets of folds were formed essentially by flexural slip. Early mullions unrollable on later axes also point to a flexural slip origin for the later folds.

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K. Naha ¹, R. V. Halyburton ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, IN

Abstract

Folds of different style and orientation, bent hinges of folds, curved lineations on folded or planar foliation, folds torn to form boudins and tectonic inclusions of diverse shapes, chocolate-tablet boudins, interference patterns like mirror-image type folds, irregular domes-and-basins and eyed folds-all these small-scale structures provide ample evidence for superposed deformations in the Precambrian terrain around Jawad in the Udaipur district, Rajasthan. Variation in the attitude of foliation, axial planes and axes of small folds of different styles and the related lineations finds a rational explanation in the tectonic model drawn from the geometry of directly observable structures.

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K. Naha ¹, A. K. Chaudhuri ¹, P. Mukherji ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, IN

Abstract

Large scale mapping of the metasedimentary bands bordered by and partially engulfed in migmatites of the Banded Gneissic Complex in the 'hammer-head syncline' between Sangat and Kelwa, along with tectonic analysis in all scales, has led to the recognition of structures of three phases. Very tight to isoclinal folds on westery axial trend have been coaxially folded, with the formation of reclined, inclined and locally upright folds. Overprinting of NNE-trending upright folds of varying tightness on these homoaxial isoclinal folds with diverse axial surfaces has resulted in an intricate foldgeometry which is brought out most dramatically by the marble band. Superposition of gentle, upright folding with NNE trend on inclined or upright isoclinal folds with westerly trend has led to broad, arcuate map patterns of the metasedimentary bands. Mirror image type interference patterns have developed where reclined isoclinal folds have been involved in upright folding on NNE trend. Overprinting of tight to isoclinal upright folds of NNE trend on reclined isoclinal folds plunging westward has resulted in book-shaped patterns with double closures. Marble bands detached completely from the main band in the present topographic surface are also due to fold-interference. The apparently erratic variation in the thickness of the metasedimentary bands is the result of tight folding of different generations, with the limbs of the folds coalesced.

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K. Naha , S. Sengupta

Abstract

No Abstract.

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S. Mohanty ¹, K. Naha ²

Affiliations
1 Department of Applied Geology, Indian School of Mines, Dhanbad 826004, IN
2 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302., IN

Abstract

Around Salumbar in the Udaipur district, Rajasthan, the Banded Gneissic Complex (BGC). the Aravalli metasedimentary rocks, the Dudar Gneiss and the Lakapa Granite Gneiss have been involved in the same style and sequence of muhiple deformation. The oldest rocks are the migmatites forming the BGC, which underlie the Aravalli metasediments with an unconformity marked by a conglomerate. Rare relict pre-Aravalli structures in the BGC, and the conglomerate containing gneissic pebbles provide incontrovertible evidence for a part of the BGC being the original basement. Identical structural style and sequence of the basement gneisses wilh those of the metasedimentary cover rocks, however, point to mobilization so that they could take part in the earliest deformation affecting the metasediments in a ductile manner. The Dudar Gneiss has formed by migmatization of the Aravalli rocks synkinematically with the first deformation, whereas the Lakapa Granite Gneiss has been emplaced as a syn-to late-tectonic intrusion.

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K. Naha ¹

Affiliations
1 Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur 721 302, West Bengal, IN

Abstract

No Abstract.

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