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

K. L. Kaila ¹, P. R. Reddy ¹, M. M. Dixit ¹, P. Koteswara Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

The crustal structure along Ujjain-Mahan profile reveals a layered structure in the vertical direction and block structure in the lateral direction. This profile from north to south is divided into four crustal blocks: I (Ujjain-Sanwer), II (Indore-Dorwa), III (Dorwa-Tapti) and IV (Tapti-Mahan) which were relatively displaced up or down during different times along deep faults bounding them and extending up to the Moho discontinuity. The depth to Moho varies between 37 to 42 km along the entire length of the profile with a velocity jump of 7.3 to 7.8 km/sec across the boundary. It is concluded that during Precambrian, blocks I and II north of Dorwa were downthrown with respect to block III leading to the development of the Vindhyan basin in that region. Blocks III and IV being uplifted at that time, formed the land part and hence no Vindhyan sedimentation took place there. Subsequently during Gondwana times, reverse tectonic movement resulted in downfaulting of block III where Gondwana sedimentation took place. During this period, blocks I and II formed the land part and hence no Gondwanas were deposited there. A shallow refraction boundary at a depth of 8 to 12 km has also been observed along this profile with a velocity jump from 6.0 to 6.9 km/sec, which may represent the Conrad discontinuity in this region.

In the Ujjain-Dorwa section comprising blocks I and II, under a thin cover of about 100 m of Deccan Traps, there lie Cretaceous Lameta beds (200m thick) and Vindhyan quartzites (lOa m thick) and Bijawars (200 to 300 m thick). Thus the total sedimentary thickness above the crystalline basement is hardly 600metres in this region. On the other hand, the Dora-Mahan section, consisting of blocks III and IV, has a maximum thickness of about 1.7 km Gondwana sediments underlying 400 metres of Deccan Traps. This hidden Gondwana basin is believed to be a northwestward continuation under the Deccan Trap of the exposed Gondwana-Godavari graben.

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P. R. Reddy ¹, P. Koteswara Rao ¹, N. Venkateswarlu ¹, A. S. S. S. R. S. Prasad ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

A systematic analysis of refraction data obtained in West BengaI basin, has been carried out to bring into focus the utility of the seismic refraction data in delineating fault zones. The study clearly points out the different conspicuous signatures present in the refraction travel time data, associated with the shallower fault zones affecting the top sedirnentry column and the crystalline basement.

Keywords

Seismology, Geophysics, Structure, West Bengal.

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P. R. Reddy ¹, P. Koteswara Rao ¹, A. S. S. S. R. S. Prasad ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Skips/shadow zones in the first arrival travel seismic time data (refraction) help in delineating hidden low velocity formations sandwiched between two high velocity zones. Efficacy of this processing methodology has been successfully tested utilising analog refraction data acquired along a profile located in the Deccan Trap country. The 2D velocity depth details also suggest probable presence of hidden Sub-Trappean Gondwana formations.

Keywords

Seismic Refraction, Skips/Shadow Zones, Low Velocity, Gondwana.

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