North-East India: Structural Framework

Abstract

Geologically, North-East India presents a stratigraphic sequence which ranges from pre-Cambrian to Quaternary with huge hiatuses in between. The oldest geological formation of the region is represented by the pre-Cambrian gneissic complex of Meghalaya plateau, a craton, and the Karbi-Anglong plateau, both of which are parts of the old Gondwanaland. The Himalayas, occupying the northern border of the region, ranging in height from 1,500 to 7,000 m ASL consist of formations ranging in age from Protozoic to early Palaeozoic in age. These consist of low-grade metamorphics in the southern section to high-grade schists towards the crest of the mountains. The foot zone of the Himalayas is formed by the Tertiary rocks, largely Mio-Pliocene deposits of post-orogenic phase. The rest of the region is formed by Tertiary rocks with different marine facies, ranging in age from Eocene to Pliocene. The movement of the Indian plate to the north as well as to the north-east and east has caused a number of thrusts. The Himalayan Main Boundary Thrust (MBT) and the Naga–Disang thrust separate the Himalayas and the eastern hilly region from Brahmaputra valley. The most notable fault of the region is the Dauki fault, running west–east and separating the Meghalaya plateau from Bangladesh, by a vertical displacement of over 1,000 m. This fault, with a few patches of Cretaceous deposits, at 1,200 m ASL, suggests a post-Cretaceous uplift of the plateau. The north–south ridges and valleys of Mizoram, formed in the Neogene sediments, represent the anticlines and synclines following a balancing compressional movement from the east. The Indo-Myanmar plate boundary is characterised by hills resulting from the underthrusting of the Indian plate and the appearance of ophiolite in the region.