Response of the Brahmaputra River to Tectonic Deformation and Paleohydrological Events in the Foreland Bengal Basin

Abstract

The Brahmaputra River has constructed an upland fluvial fan delta with an estimated volume of >5 million km^3; of riverine sediments. Much of this stratigraphy was generated during the Quaternary, a period of time characterized globally by cyclic glacioeustatic fluctuations and regionally by ongoing deformation of the Bengal basin associated with Himalayan convergence. This research examines the extent to which these external climatic and tectonic conditions have influenced the paleogeography of the river and the evolution of the basin. Stratigraphic analysis of sediments constructed by the Brahmaputra reveals that the course of the river has evolved in direct response to uplift of the Shillong Plateau, a regional crust block associated with a forward jump of the Himalayan arc. Specifically, the river has been episodically deflected by uplifting terrain and subsequently attracted to topographic subsidence below the overriding thrust, resulting in the anfractuous course that the river follows today. Overprinted upon this tectonic steering of the river, episodic paleohydrological events in the form of ice-dam floods were instrumental in scouring a wide valley paved by the deposition of cobble to boulder-sized gravel during the late glacial to interglacial transition. Presently, the river braidbelt is constrained within this flood-generated paleovalley, reflecting the long-term influence of paleohydrological events that occurred more than 10,000 years ago on the modern course of the river. Importantly, these findings suggest that allogenic influences played a major role in the morphostratigraphic evolution of one of the world’s largest fluvial systems.