Himalayan drainage patterns and the origin of fluvial megafans in the Ganges foreland basin

Abstract

Large transverse rivers draining the southern flank of the Nepal Himalaya commonly show abrupt diversions to an axial course immediately north of the trace of the Main Boundary thrust. A diverted river may gather other transverse rivers, before merging with a river diverted in the opposite direction and breaking through the mountain front along a single transverse segment. The drainage patterns show a distinctive gridiron geometry. In the presently active Sub-Himalaya, transverse rivers are commonly laterally diverted at Siwalik ranges anticlines. River deviation is interpreted as a response to progressive lateral growth of the anticlines above laterally propagating blind thrust tips. This model provides a possible analogue for diversion of transverse Himalayan rivers at the incipient Main Boundary thrust. Gridiron drainage patterns were created either by pinning deviated rivers between two linked structures or where deviated rivers merged with a river that maintained its course across growing structures. Drainage reorganization by the growth of structurally controlled topography in this manner influences the location and number of river outlets at mountain fronts, and as a result has considerable implications for sediment dispersal into the Ganges foreland basin. The recent development of fluvial megafans in the Ganges basin can be directly linked to the merger of drainage basins as a result of river diversion and subsequent constriction of drainage to a series of point-source outlets during frontal propagation and topographic growth of the Main Boundary thrust.