Quantitative tectonic geomorphic study from bedrock rivers of the Fold and Thrust Belt in the Bengal Basin

Abstract

Research on the Bengal Basin, the most important sedimentary basin in the world that supports ~200 million people, is primarily focused on its geologic evolution and anthropogenic effects, yet the basin is poorly studied in terms of geomorphic response to tectonic activity. The eastern part of the Bengal Basin is characterized by the Chittagong Tripura Fold Belt (CTFB) and the Indo‐Burman Range (IBR), which have structurally deformed into a series of mostly compressional fold and thrust belts including strike–slip faults because of the ongoing subduction‐related convergence between the Eurasian Plate and the Indian Plate. A network of river channels incising through thick sedimentary rocks and crossing and/or flowing parallel to this fold and thrust Belt. The active tectonic deformation caused by the structures and ongoing compression‐related activity remains enigmatic due to the sparse dataset, thick vegetation cover, outcrop paucity, and the Basin's complex geology. In this paper, we investigate river response to ongoing tectonics in the CTFB and western IBR by analysing channel geomorphic metrics (ksn, χ, and knickpoints) coupled with river profile forms derived from 30 m resolution NASADEM. Our results reveal (1) several convex and double‐concave reaches of major rivers and their tributaries that primarily indicate the transient state of the rivers; (2) a total of 219 upstream propagating knickpoints including distinct knickzones in profile reaches, suggesting river profile adjustment; (3) two different ksn zones in the CTFB and IBR with high ksn zone to the east and low ksn zone to the west, where ksn values range from 0 to 4905. These zones are thought to be associated with the differential uplift rate of the two east‐dipping thrust fault systems, the Kaladan Fault and Chittagong Cox's Bazar Fault (CCF); and (4) the χ‐ map (values ranging from 0.47 to 19,631.75) shows an occasional transient state of drainage divides where divides are migrating to the east, west, and north‐east, and χ versus elevation plots exhibiting disequilibrium state of the rivers suggesting that these drainage basins are likely adjusting with tectonic activities. Thus, our empirical observation suggests that the major rivers and their tributaries are in a transient state related to the region's overall ongoing tectonics.