Abstract

The modern erosion rate of continental-scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains. These two parameters are very difficult to measure directly in continental-scale basins. In this paper we examine the chemical composition of the suspended load, bedload, and dissolved load of the Ganga and Brahmaputra Rivers and compare them with the average composition of Himalayan source rocks. A mass-balance equation of erosion fluxes shows that a Si-rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock. It corresponds to bedload sediment and flood-plain deposits, which are enriched in quartz by mineral sorting during transport. The combined budget of Si, Al, and Fe in the river system allows us to estimate this Si-rich flux. By this method, the total Himalayan erosion is estimated to be twice the measured flux of suspended load. The comparison between the Brahmaputra and the Ganga shows that the eastern Himalaya has a higher erosion rate (2.9 mm/yr) than the western Himalaya (2.1 mm/yr). This is likely the result of the higher runoff in the Brahmaputra basin. The intensity of the monsoon acts as a first-order control of the erosion rate in the range.

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