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

K. L. Kaila ¹, V. G. Krishna ¹, K. Roy Chowdhury ¹, Hari Narain ¹

Affiliations
1 National Geo-Physical Research Institute, Hyderabad 500007, IN

Abstract

Deep Seismic Sounding (DSS) studies carried out for the first time in the syntaxial bend of the Himalaya in Kashmir region during 1974 and 1975 have revealed the deep crustal and upper mantle structure in the region of the great Himalaya Karakorum-Pamir ranges. The Moho which is at a depth of about 45 km near Sopur changes rapidly to 54 km depth in the region of Wular lake from where it continues to go still deeper towards NNE, attaining a depth of about 64 km near Kanzalwan. Then there is a deep fault displacing Moho to a depth of 76 km, which then rises to about 62 km under the Nangaparbat shot point. Similarly the Moho, along profile 2, is found to be at a depth of about 53 km in the region about 10 km NNW of Tral going still deeper towards NNW and attaining a depth of about 64 km in the region about 17km NE of Srinagar. Further NNW, in the region of Kanzalwan, there seems to be some flattening of the Moho boundary which is at a depth of about 70km. Beyond Kanzalwan both the profiles 1 and 2, reveal a change in the dip direction on the Moho boundary which then starts rising up towards Nangaparbat. This updip of the Moho boundary in the region between Kanzalwan and Nangaparbat is an indication of the upwarp of the Moho in this region, which could be responsible for the uplift of the Nangaparbat massif. The crustal block in the Kashmir Himalayan region between Sopur and Kanzalwan and between Tral and Kanzalwan appears to be bounded by two large-angle deep-faults which extend down almost to the Moho boundary.

A number of deep reflections have been recorded along profile 3 indicating deep reflecting boundary at a depth of about 140km extending right from beneath Nangaparbat to the great Pamir ranges which is inferred as representing the top of the asthenospheric layer.

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K. L. Kaila ¹, K. Roy Chowdhury ¹, P. R. Reddy ¹, V. G. Krishna ¹, Hari Narain ¹, S. I. Subbotin ², V. B. Sollogub ², A. V. Chekunov ², G. E. Kharetchko ², M. A. Lazarenko ², T. V. Ilchenko ²

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN
2 Geophysical Institute of the Ukrainian Academy of Sciences, Kiev, USSR, UA

Abstract

Deep Seismic Sounding (DSS) studies were initiated for the first time in India under a 3-year Indo-Soviet collaboration agreement starting 1972, along a 600km long roughly ENE-WSW profile which lies near 14°N latitude, extending from Kavali on the east coast of India to Udipi on the west coast.

A crustal section is depicted along the Kavali-Udipi profile, showing a large number of reflectors from about 2 km to about 50 km depth. The 600 km long section consists of 17 major blocks, besides a few smaller ones. It is cut up by 15 major deep faults and two major low angle thrusts. In addition there are 5 faults/thrusts extending to intermediate depths. The geological evolution of this part of the shield probably began in Early Proterozoic with the formation of the Dharwar geosyncline between Parnapalle and Agumbe, This was divided into two parts by the uplift of the block where Closepet granites are now exposed. The Dharwars in the eastern part are now almost completely eroded, exposing their basement. The Moho here is at an average depth of 35-36 km, going down to 38 km below Dharmavaram. In the western part, where there is a large thickness of Dharwars still present, Moho is more or less flat at a depth of 38 km, going down to 41 km below Holalkere and rising about 36 km below Chennagiri, The basement of the schist belt here varies from 4 to 7 km in depth.

The Cuddapahs must have been deposited starting near Parnapalle eastward due to en-echelon type faulting. Moho goes down from 34km below Parnapalle to 40 km below Duttaluru. The block between Maidukuru and Malepadu subsided later to create a fresh depression in which the Kurnools were deposited. In general, the basement of the Cuddapahs is at a depth of 8-10 km. The Cuddapahs, on their eastern margin, have been subjected to a major low angle thrust.

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K. L. Kaila ¹, P. R. Reddy ¹, M. M. Dixit ¹, M. A. Lazarenko ²

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN
2 Geophysical Institute of the Ukranian Academy of Sciences, Kiev, USSR, IN

Abstract

Deep Seismic Soundings (DSS) were carried out by the National Geophysical Research Institute, Hyderabad, during December 1975-April 1976, along a profile across Koyna. The studies of the data reveal a number of reflection horizons below the Deccan Traps up to the Moho discontinuity. Below the Deccan Traps, the crustal section along this profile is cut into two blocks by a deep fault west of Koyna. The eastern block is further cut by another deep fault which affects only the deeper horizons including Moho. Recent movements along the first of these deep faults, west of Koyna, appear to be responsible for the major Koyna earthquake of 1967 and subsequent seismicity in this region. The Moho depth in the western block is around 40 km in the vicinity of the deep fault and is of about 30 km at the west coast of India. In the eastern block the Moho boundary lies at an average depth of 36 to 38 km.

The thickness of the Deccan Traps along Koyna profile varies from o.4 km in the east to about 1.5 km near the west coast. The velocity in Deccan Traps is found to vary between 4.7 to 4.9 km/sec, and in the Pre-Trap basement it varies between 5.9 to 6.1 km/sec.

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K. L. Kaila ¹, K. M. Tripathi ¹, M. M. Dixit ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Deep Seismic Soundings along Wular lake-Gulmarg-Naoshera profile across Pir Panjal range of the Himalaya have delineated the Srinagar and Jammu synclines, each having a maximum sedimentary thickness of about 18 km. In the Srinagar syncline, the thickness of Karewas (velocity 2.8 km/sec) varies bctween 0.5 km to 1.1 km near Babarishi. Karewas are underlain by Triassic limestones (velocity 4.55 km/sec) having a thickness of 2.0km near Muquam, increasing to 3.6 km south of Sopur. Its thickness decreases to 1.2 km near Babarishi. The third layer with velocity 5.8 km/sec is inferred as Panjal Traps having a thickness of 2.5 to 3.0km. A system of 3 high angle faults is also indicated in the Srinagar valley. In the Jammu syncline, the exposed Murrees and Middle and Lower Siwaliks (velocity 3.8 to 4.6 km/sec) indicate a total thickness of about 3 km near Naoshera overlying the Waishnodevi limestones (velocity 5.8 km/sec). A number of low angle faults have been delineated in the Jammu syncline.

The deeper crustal section is divided into three major blocks bounded by deep faults, the northern block being downfaulted with respect to the southern block. In the southern block, Moho is at a depth of 44km near, Main Boundary Fault, 57km under Pir Panjal range, 51 km under Babarishi. Moho is at a depth of 63 to 69km north of Muquam. Under the Great Himalaya, Moho starts rising from 78 km depth in the deepest part to 67 km under the Nangaparbat shot point.

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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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K. L. Kaila ¹, H. C. Tewari ¹, D. M. Mall ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

A Gondwana graben with a maximum sedimentary thickness of about 2.8 km has been mapped in the Cuttack-Kendrapara area of the Mahanadi basin. This graben is bounded by faults towards its north and south. The velocities which are about 3.9 km/sec around Cuttack change to 3.4 to 3.2 km/sec towards Kendrapara. South of the Gondwana graben lies the shallow basement area, the shallowest part of which is at a depth of about 200 metres under alluvium (velocity 1.6 to 2.0 km/sec). This basement high probably formed the barrier northwest of which the Gondwana graben was formed. Southeast of this, in the coastal area and in the adjacent offshore region, new basins developed during Tertiary period which received Tertiary and younger sediments. Two coastal depressions which may be extending into the Bay of Bengal have been identified: the Konarak depression which received a maximum sedimentary thickness of about 2400 metres on land and the Paradip depression which has a maximum sedimentary thickness of 2200 metres on land. These depressions are fault-controlled and the velocity over the major part is 2.4 km/sec followed by a deeper layer with velocity 4.2 km/sec in the Konarak depression which may be represented by limestone formations or basalt flows.

In the crustal depth sections, there are velocity discontinuities at average depths of 6.0, 17.5, 20.5 and 34.5 km along various profiles which have been mapped fairly weIl. The crustal velocity model in the Mahanadi delta area shows a velocity of 1.6 km/sec at the top which is followed by a velocity of 2.45 km/sec at about 0.3 km depth. The velocity in the basement is 5.9 km/sec at a depth of 1.1 km which changes to 6.45 km/sec at 6.0 km depth. A low velocity layer about 3.0 km thick with velocity 6.0 km/ sec is indicated at 17.5 km, velocity changing to 7.1 km/sec at 20.5 km depth. The velocity at the Moho depth 34.5 km, changes to 8.1 km/sec. The crustal thickness in this area is found to be 30-34 km which is smaller than that in the Indian shield (40 km) and in the Narmada-Son lineament area (37-42 km).

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K. L. Kaila ¹, Kalachand Sain ²

Affiliations
1 2-16-106, Prashantinagar, Uppal Road, Hyderabad - 500 039, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The Indian subcontinent has been divided into six crustal velocity zones based on P-wave velocity structure obtained along 20 Deep Seismic Sounding (DSS) profiles, covered in India from 1972 to 1992. Zone - 1 consists of two parts: zone  - 1.a comprises the Dharwar craton, Cuddapah basin, Godavari graben and the Koyna region, and zone- 1 .b consists of the western margin of the Bengal basin and the Singhbhum craton. Zone - 2 covers the West Bengal sedimentary basin and the Mahanadi coastal basin. Zone - 3 includes Bundelkhand craton and the Vindhyan basin. Zone - 4 covers Cambay basin north of the Narmada lineament and the Aravalli - Delhi fold belt. Zone - 5 lies south of the Nannada lineament covering the Tapti graben extending up to the west coast of India. Zone - 6 comprises the northwest Himalaya up to the Nangaparbat including Jammu and Srinagar sedimentary basins. Detailed crustal velocity structures in these six zones have been described with their bearing on regional tectonics. Velocity structure of the lower crust in some of these zones, suggests that crustal underplating has played a very important role in the evolution of the Indian continental crust and lithosphere. Some petrological inferences have also been drawn for the Indian crust from crustal velocity zonation.

Keywords

Geophysics, Crustal Velocity Structure, Tectonics, Indian Subcontinent.

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