Chittagong-Tripura Folded Belt
Chittagong-Tripura Folded Belt the youngest structural province along the western flank of the indo-burmaN ranges. This zone of sub-parallel, arcuate, elongated folds of meridional and submeridional trending stretches from the Arakan coastal area in the south through the chittagong hill tracts, tripura hills and along the eastern margin of the surma basin as far as the shillong massif in the north, where it is truncated by two major structural elements of regional importance, the Dauki Fault and the Haflong Fault. To the northeast, the belt continues through the Kohima Synclinorium and merges in the schuppen belt of Upper Assam.
The Eocene flysch (marine sedimentary rock) sequences of the Indo-Burma Range constitute the eastern limit of this tectonic province. However, based mainly on the structural configuration of the anticlines, some geologists have suggested a subdivision of this zone into two belts, the Mizo folded belt in the east and the Chittagong-Tripura folded belt in the west, both separated by the NNW-SSE striking kaladan fault. A line drawn from the western slope of st martin's island in a NNW direction to the western shore of sandwip island marks approximately the western limit of the Chittagong-Tripura folded belt in the bay of bengal. On shore, this line continues as far as Daudkandi, follows the Meghna river upwards, turns in the Ashuganj area to the northeast and finally ends at the foothills of the Shillong Massif NE of Sylhet.
The age of the sediments outcropping in the folded belt ranges from Lower Miocene (about 24 million years before the present) to Recent age. The Miocene sediments were deposited under marine and predominantly deltaic conditions in a rapidly subsiding, unstable basin, while the depositional environments were governed by subaqueous to subaerial, fluviatile to lacustrine conditions in Plio-Pleistocene times (5 million years to 0.1 million years before the present). The poorly fossiliferous Neogene (24 to 2 million years before the present) succession of this tectonic province consists of alternations of shale, siltstone and sandstone indicating unstable conditions during the deposition.
In response to the eastward subduction of the Indian plate, the molasse sediments (partly marine and partly continental/deltaic sedimentary rock) of this zone were folded into a series of elongated, doubly plunging asymmetric folds arranged en-echelon. The alignment of the folds follows a NNW-SSE trend in the Chittagong Hill Tracts, whereas the structures trend N-S in Tripura and NNE-SSE in the eastern portion of the Surma Basin. A slight convexity to the west characterises the anticlinal structures in tripura and in parts of Mizoram.
The folded belt is a prime zone for exploration, comprising a proven Miocene gas province with 12 onshore Gas Fields and one offshore gas field, leaving, however, the Chittagong Hill Tracts for commercial discoveries; there is still a favourable outlook for oil discovery in the Chittagong Hill Tracts. [AKM Khorshed Alam and Sifatul Quader Chowdhury]
See map in tectonic framework.