Temporal and spatial variations in erosion rate in the Sikkim Himalaya as a function of climate and tectonics

The Tista River, a major tributary of the Brahmaputra drainage system (Eastern Himalaya -Sikkim) has recently incised its megafan at the topographic front of the mountain range by 30 meters. Neither the timing of deposition/incision of the megafan sediments, nor the erosion rates of the source areas have yet been investigated in detail.

To constrain erosion rates in the hinterland at different temporal scales, we report cosmogenic nuclide (¹⁰Be) and thermochronological (apatite fission-track) data on modern river sands and map the results to evidence spatial variations of erosion/exhumation rates in Sikkim. Millennial erosion rates are significantly higher than geological exhumation rates, display stronger spatial variability and a contrasting pattern, suggesting that the processes controlling these rates are decoupled. Strong exhumation rates at geological timescales in southwest Sikkim (1.2 mm.yr-¹) may be structurally controlled by uplift of the Lesser Himalayan duplex, while strong erosion rates at millennial scales in north Sikkim (5-6 mm.yr-¹) suggest a climatic control.

Cosmogenic nuclides were also used to date the onset of incision of the megafan. In addition, isotope geochemistry (ε_Nd, ⁸⁷Sr/⁸⁶Sr) on modern river sands and late-Quaternary megafan sediments allows characterizing the isotopic signature of the different source areas and constraining variations in provenance of the Tista megafan deposits through time. Results show that the Tista fan deposits are mainly sourced from the High Himalayan Crystalline domain with excursions more influenced by the Lesser Himalaya domain. These results are consistent with the higher erosion rates identified in north Sikkim at millennial timescale.

These data provide a new comprehensive view on modern erosion and long-term exhumation of the Sikkim Himalaya. This study will help our knowledge and understanding of erosional processes and sediment fluxes in mountainous environments as a function of climate and tectonics.