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, 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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A. K. Chaudhuri ¹, A. B. Pal ¹

Affiliations
1 Dept. of Geology & Geophysics, Indian Institute of Technology, Kharagpur, West Bengal, IN

Abstract

Precambrian metasediments in the northern part of the Gangpur Series, represented by mica schist, phyllite-slate, and schistose quartzite have been investigated over an area of 50 sq km around Lase (22°21' ; 84°45') of Sundargarh district, Orissa. Structural analysis reveals that the earliest folds in bedding (S₁) were reclined in nature, but they have been blurred by the profuse development of axial plane schistosity (S₂). S₂, however, shows upright to slightly overturned folds plunging moderately to E to ESE. The most dominant attitude of S₂ nearly normal to the regional fold axis points to the reclined geometry of the folds in S₁. Non-penetrative fracture cleavage surfaces (S₃) have developed parallel to the axial planes of S₂-folds, generally displacing the folded segments. The S₃-surfaces, however, show appreciable fanning.

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A. K. Chaudhuri ¹, D. Mukherjee ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, IN

Abstract

Near Kishangarh in Rajasthan, Delhi conglomeratic quartzite and quartz mica schist overlie unconformably Aravalli metasediments. The foliation in the concordant nepheline syenites is parallel to the bedding plane foliation of the Aravallis. The Aravalli metasediments and the nepheline syenites show exactly similar development of superposed folds, with the axial surface of early isoclinal, inclined to reclined folds by nearly N-S striking upright folds. Evidence of such superposition is seen in all scales from hand specimens to maps. Spectacular outcrop patterns characteristic of superposed folds are developed. The effects of both the deformations can be seen only in the older Aravalli rocks and the nepheline syenites, while the younger Delhi rocks, in contrast, show only the later folds of the Aravalli rocks, which can be tied up with the first folding of the Delhi rocks.

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A. K. Chaudhuri ¹, A. B. Pal ¹

Affiliations
1 Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur 721302, IN

Abstract

Shape analysis of a large number of pebbles from a 60 km stretch of the basal Raghunathpalli conglomerate in the Gangpur Group in eastern India, indicates that a flattening strain on originally constricted pebbles during the earliest folding episode gave rise to the present shape of the pebbles. The longest and the intermediate axes of pebbles lie invariably on the schistosity developed parallel to the axial planes of the earliest isoclinal folds. Though the Gangpur Group shows a remarkable multideformation plan, the pebbles show mainly the effects of the earliest deformation.

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A. K. Chaudhuri ¹, A. B. Pal ¹, A. Roy ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721302, IN

Abstract

Three phases of folding have been detected in a large exposure along the Sapai lladi near Amaidegri (22°01': 84°08') in the Ghoriajhor manganese belt in the western part of the Gangpur Group. Reclined to inclined folds (F₁) plunging SSE have been coaxially refolded by open upright F₂ folds, and finally, upright folds with ENE-trending axial planes (F₃) cause culmination and depression of axes of open-folded reclined folds. The multiple deformations cause characteristic outcrop patterns and they are reflected in two distinct S-pole girdles around a plane of symmetry which is the axial trend of F₃ folds.

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A. K. Chaudhuri ¹, A. B. Pal ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, IN

Abstract

Gangpur Group, an important Precambrian metasedimentary sequence near Rourkela in eastern India shows an easterly antiformal closure, but the northerly younging towards the core shown by cross beds in the southern limb proves large scale inversion. Three generations of folds F₁, F₂ and F₃ are proved on mesoscopic and macroscopic scales by structural analysis. The limbs and axial planes of low easterly plunging isoclinal reclined folds (F₁) along with schistosity developed parallel to these axial planes are coaxially folded by upright F₂ folds; N-S trending upright F₃ folds cause culminations and depressions in axes of F₁ and F₂ folds. The stratigraphy of the complexly folded Precambrian Group is worked out on the basis of the structural model. One important marker horizon of carbonaceous quartz phyllite and banded quartzite recurs four times because of large scale axial plane folding.

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S. Dastidar ¹, S. Samanta ¹, A. K. Chaudhuri ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur - 721302, IN

Abstract

Structural mapping in this area in Bankura district of West Bengal reveals coaxial easterlyl westerly plunging early isoclinal F₁ folds and later upright F₂ folds giving rise to hook-shaped interference patterns. Later slip and shearing along the F₂ axial plane cause considerable bending of early linear structures in both mesoscopic and macroscopic scales. All rock types including anorthosite, granite gneiss and composite gneiss are affected by multiple deformation plan.

Keywords

Structural Geology, Anorthosites, West Bengal.

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