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

V. N. Agrawal ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar (M.P.), IN

Abstract

The Precambrian metamorphic rocks of the Sausar Group of the northern Nagpur district, Maharashtra have a polyphase structural history. Three distinct fold phases (F₁-F_s) have affected these rocks. The first folds which are tight to isoclinal structures of general' similar type' are developed throughout the region, while structures related to later deformations show a discontinuous development. The first folds have been refolded into a set of close to tight structures (F₂) on axial surfaces oriented close to earlier axial direction and dipping steeply S. The superposition has given rise to Type 3 interference pattern which is manifest both in the map pattern and on the scale of outcrops. F_s fold phase has produced open, nearly upright folds with axial traces running N/NNE and axes plunging generally steeply S., the steep plunges resulting from the superposition of these folds on steeply dipping limbs of early structures. In contrast to the much simpler pattern shown by the map in the domains of major F_s folds, the outcrops reveal on a small scale a conspicuous development of Type 1 interference pattern largely in the hinge zones of the late structures, resulting from the superposition of minor F_s on minor F₂ structures.

The style of structures of different generations and the pattern of their development in the region seem to suggest that the intensity of deformation has gradually declined from F₁ to F_s and that the concentration of fold movements has varied rather rapidly within small areas of the fold belt during later phases causing discontinuous development of related structures.

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V. N. Agrawal ¹

Affiliations
1 University of Saugar, Sagar, M.P., IN

Abstract

The' Marble Rocks' of the Narbada gorge, Jabalpur, Madhya Pradesh, appear to have a polyphase structural history. Of the three fold phases (F₁ - F₃) recognised in the metamorphic rocks of the Narbada section, west-southwest of Jabalpur town, only two are distinguishable in the Bhera ghat section-an early phase of tight folding on roughly easterly trending axes and a late phase of gentle folding on axes oriented transverse to the earlier fold trend. The early and late folds are, related to second (F₂) and third (F₃) fold phases respectively, in the region as a whole. Basic dykes cutting the marbles of Bhera ghat are shown to belong to two different ages-one highly metamorphosed and deformed and considered to be syntectonic with the early fold phase, and the other, practically unaltered and undeformed, of a post-tectonic, post-metamorphic age. Emplacement of quartz veins associated with the marbles is shown to have begun sometime during the early fold phase, continued through the late fold phase and outlasted it. Special features of the above rocks and those of the Sausar group are compared.

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V. N. Agrawal ¹

Affiliations
1 Department of Geology, Kumaun University, Nainital 263001, IN

Abstract

The Precambrian rocks of the Narmada section, southwest of Jabalpur, which bear imprint of several periods of deformation, are found to have undergone Barrovian type of regional metamorphism with garnet-staurolite-sillimanite assemblages in the higher grades. An increase in the metamorphic grade is indicated by successive development of a higher grade mineral assemblage, as also a coarser metamorphic fabric, as the rocks are followed to the east. Textural evidence is given to show that metamorphism in the area has been a polyphase event, the main episodes of crystallisation having occurred before, during and after the second (F₂) fold phase. The metamorphic climax, marked by the development of sillimanite, is shown to have occurred after the second fold movement. That higher temperatures prevailed in the Tilwaraghat section through successive metamorphic episodes is shown by the development of higher grade minerals and a coarser rock fabric in the above area than anywhere to the west of it. Retrogression was negligible in this area, while extensive retrogression occurred in the western part, particularly between the water falls and Lamhetaghat.

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