TECTONIC EVOLUTION OF NORTHEASTERN INDIA AND THE ADJOINING AREA WITH SPECIAL EMPHASIS ON CONTEMPORARY GEODYNAMICS

摘要

Northeastern India and the adjoining region consist of 5 important and distinct tectonic domains, though they are related through the unified global tectonic process. They are (i) the eastern Himalaya and Tibetan domain; (ii) the Mishmi block and transverse mountain ranges; (iii) the Patkoi-Chin-Arakan Yoma mobile belt (outer arc ridge) with associated Central Myanmar forearc and backarc basins, and the Neogene accretionary complex in upper Assam; Tipam molasse basin in the north and the Surma basin in the south; (iv) the Meghalaya plateau and the Mikir Hills, representing northeastern extension of Indian shield elements, with their associated platform sediments and (v) the Bengal basin. The eastern Himalaya and the Tibetan plateau have evolved mostly through post collisional tectonics, between the Indian and the Eurasian plates and the closure of Neotethys along the Yarlung-Zangbo (Indus-Tsangpo) suture zone, whereas the Mishmi block and the transverse mountain range are shaped by the intense tectonic forces acting along the northeastern margin of the Indian plate due to its post collisional counterclockwise rotation.The arcuate Patkoi-Naga-Chin-Arakan Yoma mobile belt represents the Palaeogene outer arc ridge. developed during the Eocene-Oligocene time in response to subduction of the Indian lithosphere below the Myanmar-Andaman plate. Western margin of the Sino-Myanmar continent was involved in oblique rifting approximately along the present Shan-Sagaing fault system during the Jurassic-early Cretaceous period, thus forming the Central Myanmar basin, constituted by the arcuate forearc and backarc troughs separated by the median Tertiary volcanic arc. At the close of Oligocene the Palaeogene outer arc ridge became positive element of the Indo-Myanmar mobile belt. to the west of which the Neogene Tipam basin developed to the north of the Meghalaya plateau and the Surma basin to its south. Continued eastward subduction of the Indian plate resulted in development of fold-thrust belt along the Indo-Myanmar mobile belt which became very intense in the upper Assam area due to the resistance put forward by the submerged basement blocks of the "foreland spur" whereas in the south, in Surma basin the Neogene outer arc sediments were only folded and faulted with reduced intensity towards west.The Indian shield elements represented by the Meghalaya plateau, Mikir Hills and the submerged "foreland spur" extend northeastward from Garo-Rajmahal gap to upper Assam. Though there is record of formation of Proterozoic-early Palaeozoic granite within the plateau. late Mesozoic and early Cretaceous tectonism is marked by the effusion of Sylhet trap volcanics and emplacement of carbonatite-ultramafic complex. Buttressed in between the two mobile belts the Meghalaya Plateau has been rising with its platform and shelf sediments along its southern and eastern margins since upper Cretaceous to the Recent, mainly along the Dauki fault system which is relatively dormant at present. The Bengal basin, comprising complete Teritary sequence of sediments concealed under the Quaternary alluvium of Ganga-Brahmaputra river system. is an asymmetrical pericratonic basin, the western margin of which is marked by submerged N-S Gondwana rifted trough with Rajmahal trap, and the Tertiary basin margin faults. To the north the basin is represented by E-W raised continental crust basement ridge and its slopes on either sides. The NE to ENE hinge zone of Eocene shelf sediments demarcates the boundary between shallow water facies to the west and deep water sediments to the east. The eastern deeper basin contains about 20 km thick sedimentary column having oceanic crust basement which extends up to the Surma basin, plunging eastward in response to the continuous subduction of Indian plate below the Sino-Myanmar plate.Seismotectonic analyses show that besides the subducting Benioff zone and its related faults in the Indo-Mynmar (Burma) region, oblique faults cutting the A