Geological Framework

India is a cratonic shield area that has drifted northeast from an original position near the S W coast of Africa and Madagascar since the break-up of Pangea at the end of the Paleozoic. The northward overall movement of the oceanic plates in the Indian Ocean have driven the Indian craton into the Asian cratonic landmass and thrust it beneath, tectonically elevating the collision zone and the underplated zone and creating the highest mountains on Earth, the Himalayan mountains, that pass in an arc across the northern margin of India. Erosion of the mountains is extreme and the bulk of the sediments are carried to the south. Most of the major rivers, including the Ganges and Brahmaputra, flow into the Bay of Bengal from the Himalayan Mountains. The Indus River system flows into the Arabian Sea. Both the mouths of these major systems have formed huge deltas and submarine sedimentary fans which are the repository of these Himalayan sediments (Wetzel, 1993).

It is believed that the Indian Plate collided with the Eurasian Plate at about 45 Ma. Major rivers which had started flowing from the Tibetan side toward the south became antecedent rivers flowing across the now lofty Himalayas. Once the collision began and the Himalayas rose, the drainage area of the stream system increased dramatically and the sediments carried by the Brahmaputra system entered the Bay of Bengal. Major clastic wedges began to prograde into the Bengal Basin. Significant volumes of sediments began to bypass the delta and, after merging with the deltas of the Ganges, contributed to the growth of the Bengal deep sea fan. Turbidity currents carried the increased sediment load into deeper waters (Lindsay et al., 1991) as they did in the Indus Fan in the northern Arabian Sea (Kenyon, 1987). The Indus fan appears to contain less sediment volume than the Ganges fan.

The two coasts of India are superficially similar passive margin depositional environments, but are tectonically different in detail. The deep water offshore basins along the east and west coast of India both evolved through a rift and then drift phase in a passive divergent framework (Naini and Talwani, 1982) associated with the northward drift of the Indian ocean plate (Biswas and Singh, 1991). The initial phase of rifting formed a system of NNW-SSE horsts and grabens lying parallel to the western continental margin along a NE-SW trend about parallel to the east coast of India. In the post rift stage, massive carbonates were deposited along the west coast while a series of step faults down throwing toward the basins resulted in growth faults and shale diaprism along the east coast of India. After the mid-Miocene, at which time there was a major tectonic episode and uplift, large scale complexes of prograding clastics began to be deposited along both the east and west Indian paleoshelf margins. There is still active uplift in the Himalayas, which yields heavy sedimentation.

The Andaman-Nicobar region is tectonically very different from the coastal areas of mainland India . This region is associated with an east-dipping subduction margin and island arc system at the junction between the eastern Indian Ocean plate and the complex of plates in the SW pacific. This island arc-plate margin system is the eastward continuation of the Himalayan (Tethyian) confrontation zone to the north of India, which continues from the SW Pacific Ocean north of Australia and along a number of tectonic zones into Europe in the west. Whereas in the passive margin area tectonic folding of the upper several kilometers, and usually more, of the sediment is absent, active tectonism in the Andaman-Nicobar area has formed large folds that provide the possibility of complex hydrate-geological gas traps (Max and Lowrie, 1996; Max and Chandra, 1998).

The continental shelf along the west coast of India is much more broad than the eastern shelf. In addition, the water depth increases sharply at the eastern shelf edge up to water depth of 3,000m along the east coast whereas the western shelf margin is geographically complex owing to presence of islands and bathymetric reentrants. The western margin of the Andaman-Nicobar island arc is sharply defined bathymetrically, but less steep than the eastern, and the northern part of the western Indian continental slope. The Andaman sea itself is a back-arc basin with relatively complex bathymetry reflecting complex internal structure and sedimentation.

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