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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. Chandrakala ¹, O. P. Pandey ¹, D. M. Mall ¹, D. Sarkar ¹

Affiliations
1 National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad - 500 007, IN

Abstract

Origin and evolutionary history of the Cuddapah Basin in SE India has remained a subject of considerable speculation whether it was evolved through vertical tectonic movements, extentional stretching or even cometary impact. Based on detailed seismic and other geophysical studies (Gravity, magnetotelluric and heat flow), we have delineated signatures of a possible deep seated mantle plume below southwestern part of the Cuddapah Basin, which may have been responsible for the 1.1 Ga kimberlitic magmatism in the eastern part of the Dharwar craton (EDC). The thermal anomaly associated with this mantle plume appears to have resulted into 15 - 20 km thick magmatic underplating (Vp: 7.10-7.30 km/s; density 3.07-3.16 g/cm³) below the Parnapalle region of the southwestern Cuddapah Basin, which also coincides with the high gravity and high conductivity anomaly. The massive underplating led to thickening of the crust to about 40 - 44 km below southwestern part of the Cuddapah Basin, compared to about 34 ± 2 km in the surrounding regions of EDC, indicating thermal restructuring of the Crust/Mantle boundary. This plume, which was apparently active in an area of about 500 km radius, may have also affected the Closepet granitic region, which is ∼100 km west of Cuddapah Basin.

Keywords

Deep Seismic Sounding, Mantle Plume, Gravity, Underplating, Cuddapah Basin, Dharwar Craton.

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