FROM CHAOS TO APPARENT CALM:THE SAGA OF TECTONICS AND SEDIMENTATION IN ANDAMAN BASIN

Following breakup of Gondwanaland, the swift northward flight of the Indian plate, its anticlockwise rotation and oblique subduction beneath the SE Asian plate led to the evolution of the Andaman basin. Since late Cretaceous to present, each increment of subduction, witnessed scrapping of sediment floor of the Indian plate at the trench and its accretion to the SE Asian plate as under thrusted fold packets, one beneath the other, from east to west. This plate convergence led to outgrowth of the accretionary prism and a formation of a narrow ridged forearc .By Late Oligocene time, step up in the rate of convergence resulted in the emergence of the Andaman Islands, basement uplift through thrusting and the development of a regional Pre Neogene unconformity in the forearc. The principal stress axis till this time was N-S expressed by a series of dextral strike slip faults . Transtensional stresses developing in the Back arc by Miocene led to the development of a pull apart basin. Seamounts and volcanic islands defined the volcanic arc. By Miocene time, NW-SE wrenching coupled with late Miocene NE- SW compression led to development of enechelon folds , shifting depocentres in Neogene in the forearc. The Bengal fan started accreting at the trench in Mid Miocene.Late Pliocene saw the emergence of the Ritchies Archipelago and some islands in the Nicobar area.

During initial subduction ,scrapping of the Ophiolite from the sea floor and its plastering to the accretionary prism occurred. Oceanic pelagic sediments constituted Late Cretaceous sedimentation. Paleocene –Eocene sedimentation represented a chaos in the form of ophiolite derived coarse clastics overlying a pelagic shale. Conglomerates and coarse grained sandstones with outsized clay and rock clasts and minor yet definitive sub bituminous coals defined early Paleogene sedimentation. The Oligocene witnessed flysch formation in the form of coarse to fine grained, concretion and clast filled thick sandstone sequences alternating with Sand shale intercalations. While the Clastics dominated the Paleogene, the Neogene is Carbonate rich with some ash beds and clastics in the forearc. During mid Miocene, finer clastics of the Bengal fan contributed to the accretionary prism . Contribution to the back arc in Neogene, was guided by the Sagaing fault and Irrawaddy delta from the North.