Decoding the control of active tectonics on the sedimentary budget of a Himalayan River basin: Insights from Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr, _Nd) of bed sediments

The interaction between climate and tectonics chiefly governs the erosion and sedimentary budget in the tectonically active Himalayan Mountain chain. In the present study, the relative contribution from the various litho-tectonic units of the Himalaya to the sedimentary budget of the Teesta River System (TRS) (a major tributary of the Brahmaputra system) has been deciphered using the Sr and Nd isotopic compositions (Sr-87/Sr-86, epsilon(Nd)) of bed sediments. The possible controlling factors for such observed sedimentary provenance have also been assessed. The discharge-weighted sediment transport amount has been translated into the quantification of the erosion rate in the basin. The Sr-87/Sr-86 and epsilon(Nd) values in the silicate fraction of bed sediments range from 0.74867 to 0.90529 and - 24.3 to -13.9, respectively. In the Teesta main channel, Sr-87/Sr-86 and epsilon(Nd) show large variability ranging from 0.74867 to 0.82288 and -21.8 to -13.9, respectively along its entire course. This large variability indicates changes in sediment sources i.e., Higher Himalaya and Lesser Himalaya. These data sets show that >60 % of TRS sediments are derived from the Lesser Himalaya, indicating a higher physical erosion over the catchments in the Lesser Himalaya. Our assessment suggests that the sedimentary budget of the Teesta Basin is chiefly governed by higher exhumation as well as the vertical uplift rate of the lesser Himalaya sector of the study area (due to the presence of Rangeet duplex) coupled with focused precipitations. Based on the sediment discharge method, we estimate the erosion rate in the Teesta basin to be 1.7 +/- 0.5 mm/yr, which yields a total annual sediment flux of similar to 41.4 +/- 12.4 Mtons. This estimate is comparable to other Himalayan River basins like the Ganga basin, however, lower than the erosion hotspots such as the Brahmaputra basin (Namche Barwa and eastern Syntaxis region) and Indus basin (western Syntaxis region). The obtained erosion rate of the Teesta basin seems to result from the active tectonics in the Sikkim sector of the Himalaya.