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

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Crustal Structure Across the Narmada-Son Lineament, Central India from Deep Seismic Soundings


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1 National Geophysical Research Institute, Hyderabad 500007, India
     

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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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  • Crustal Structure Across the Narmada-Son Lineament, Central India from Deep Seismic Soundings

Abstract Views: 208  |  PDF Views: 5

Authors

K. L. Kaila
National Geophysical Research Institute, Hyderabad 500007, India
P. R. Reddy
National Geophysical Research Institute, Hyderabad 500007, India
M. M. Dixit
National Geophysical Research Institute, Hyderabad 500007, India
P. Koteswara Rao
National Geophysical Research Institute, Hyderabad 500007, India

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.