H. C. Tewari

Evolution of Proterozoic Aravalli Delhi Fold Belt in the northwestern Indian Shield from seismic studies

A deep seismic reflection study across the Aravalli Delhi Fold Belt, situated in the northwestern part of the Indian Shield, has revealed a deep penetrating 25-km-wide crustal-scale thrust fault, dipping reflections from the upper crust to the Moho and a divergent reflection fabric. Paleo-subduction zones and island-arc signatures are identified from the present study. Seismic images of the crust reveal tectonics of the region with two distinct episodes of rifting, sedimentation, collision and suturing corresponding to the Aravalli and Delhi orogenies. Plate tectonic processes were responsible for the evolution of the Paleoproterozoic Aravalli and Mesoproterozoic Delhi Fold Belts with the juxtaposition of the Bundelkhand craton in the east and the Marwar craton in the west. A 50-V m electrical conductor, extending to a depth of 25 km, and a steep gradient gravity anomaly of 70 mGal, extending all along the strike of the fold belt with conspicuous lows on either side, correlate well with the deepseated dipping reflections/sutures inferred from the seismic reflection data. The present study suggests that a high-velocity, thick crust was produced in the Proterozoic orogens of the region. q 2000 Elsevier Science B.V. All rights reserved.

Structure and tectonics of the Proterozoic Aravalli-Delhi Fold Belt in northwestern India from deep seismic reflection studies

Abstract Seismic imaging of the crust along a 400-km-long deep seismic reflection profile across the Palaeo/Mesoproterozoic Aravalli-Delhi Fold Belt, in the northwestern Indian Shield, brings into focus its complex structure and provides clues to understand the geological processes involved in the evolution of this belt. The reflectivity pattern varies considerably for different crustal units along the profile. The deep-crustal reflection data image two sets of oppositely dipping strong reflection bands, from upper- to lower-crustal levels. These are identified as the signatures of the collision corresponding to Aravalli and Delhi orogeny. The data also exhibit a clear Moho and strong lower-crustal reflections near the collision boundaries. A stack of dipping reflections from the top of the Moho to the surface is identified as a major thrust fault indicating that the Proterozoic collision and deformation were primarily thick-skinned in nature

Velocity Structure of the Indian Crust

The velocity structure of the continental crust is one of the most important parameters in identification of different regions within the crust. Several attempts have been made to obtain the velocity structure of the Indian continental crust, particularly its shield region. Iso-velocity lines, basement and Moho depth contours and average velocity images of the crust explain the general structure of the Indian peninsular shield. These show that different units of the shield region differ from each other in several details, probably due to different phases of tectonic activity.

East Coast Sedimentary Basins of India

The east coast of India consists of three deltaic sedimentary basins, the Bengal, Mahanadi, and Godavari. These basins were created due to drainage of the major rivers into the Bay of Bengal. Basic volcanism has played a great part in formation of these basins. Except for the Bengal basin, which is highly tide dominated, the other deltas have more or less similar formation. These are basically Gondwana basins and have the possibility of large hydrocarbon reserves. Knowledge of the basement and crustal configuration of these basins is of great help as, despite large-scale hydrocarbon exploration, sufficient success has not yet been achieved. Most of the Bengal basin has a deep basement and shallower crust, as compared to the other two basins.

Central Indian Region

The central part of the Indian subcontinent consists of several tectonic units, the Narmada-Son lineament and the central Indian suture zone being the most prominent features. Deep seismic studies across the Narmada-Son lineament suggest that major crustal disturbances are confined to the upper crust. The Narmada north and south faults have been active at different times and acted as fissure zones through which molten magma erupted and was emplaced on both sides. The Barwani-Sukta fault divides the region into two parts; east of this fault the upper crust is upwarped between the two Narmada faults. The subcrustal lithosphere in the central Indian region indicates varying structural properties. The central Indian suture represents an obducted oceanic crust due to collision of two microcontinents, represented by the present-day Satpura mobile belt and Bastar craton.