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

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Structural Evolution of a Gneiss Dome in the Axial Zone of the Proterozoic South Delhi Fold Belt in Central Rajasthan


Affiliations
1 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata-700 019, India
2 Department of Geological Sciences, Jadavpur University, Kolkata-700 032, India
     

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The structural geometry of the Anasagar gneiss dome in the axial zone of the South Delhi Fold Belt is controlled by polyphase folding. It is classified as a thrust-related gneiss dome and not as a metamorphic core complex. Four phases of deformation have affected both the gneiss and the enveloping supracrustal rocks. D2 and D3 deformations probably represent early and late stages of a progressive deformation episode in a simple shear regime combined with compression. The contact between the gneiss and the supracrustal rocks is a dislocation plane (thrust) with top-to-east sense of movement which is consistent with the vergence of the D2 folds. The thrust had a ramp-and-flat geometry at depth. At the present level of exposure it is a footwall flat (that is, parallel to the gneissosity in the footwall), but it truncates the bedding of the hanging wall at some places and is parallel at others. The thrusting was probably broadly coeval with the D2 folds and the thrust plane is locally folded by D2. D2 and D3 folds have similar style and orientation as the first and second phases respectively of major folds in the Delhi Supergroup of the South Delhi Fold Belt and these are mutually correlatable. It is suggested that D1 may be Pre-Delhi in age. Available geochronological data indicate that the emplacement of the Anasagar gneiss predated the formation of volcanic rocks in the Delhi Supergroup and also predated the main crust forming event in the fold belt. The Anasagar gneiss and its enveloping supracrustal rocks are probably older than the Delhi Supergroup.

Keywords

Anasagar Gneiss Dome, Polyphase Folding, Thrust-Related Gneiss Dome, South Delhi Fold Belt, Rajasthan.
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  • Structural Evolution of a Gneiss Dome in the Axial Zone of the Proterozoic South Delhi Fold Belt in Central Rajasthan

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Authors

Dhruba Mukhopadhyay
Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata-700 019, India
Nandini Chattopadhyay
Department of Geological Sciences, Jadavpur University, Kolkata-700 032, India
Tapas Bhattacharyya
Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata-700 019, India

Abstract


The structural geometry of the Anasagar gneiss dome in the axial zone of the South Delhi Fold Belt is controlled by polyphase folding. It is classified as a thrust-related gneiss dome and not as a metamorphic core complex. Four phases of deformation have affected both the gneiss and the enveloping supracrustal rocks. D2 and D3 deformations probably represent early and late stages of a progressive deformation episode in a simple shear regime combined with compression. The contact between the gneiss and the supracrustal rocks is a dislocation plane (thrust) with top-to-east sense of movement which is consistent with the vergence of the D2 folds. The thrust had a ramp-and-flat geometry at depth. At the present level of exposure it is a footwall flat (that is, parallel to the gneissosity in the footwall), but it truncates the bedding of the hanging wall at some places and is parallel at others. The thrusting was probably broadly coeval with the D2 folds and the thrust plane is locally folded by D2. D2 and D3 folds have similar style and orientation as the first and second phases respectively of major folds in the Delhi Supergroup of the South Delhi Fold Belt and these are mutually correlatable. It is suggested that D1 may be Pre-Delhi in age. Available geochronological data indicate that the emplacement of the Anasagar gneiss predated the formation of volcanic rocks in the Delhi Supergroup and also predated the main crust forming event in the fold belt. The Anasagar gneiss and its enveloping supracrustal rocks are probably older than the Delhi Supergroup.

Keywords


Anasagar Gneiss Dome, Polyphase Folding, Thrust-Related Gneiss Dome, South Delhi Fold Belt, Rajasthan.

References