CLIMATIC CONTROLS ON EROSION PATTERNS AND PROVENANCE IN QUATERNARY RIVER TERRACES, ZANSKAR, NORTHWEST INDIAN HIMALAYA

Valley-fill terraces act as sensitive recorders to perturbations in river systems on millennial timescales, where major episodes of aggradation and incision can be documented in cut-and-fill terrace stratigraphy. Investigation into the detrital record of these terrace deposits allows us to consider how climate-landscape interactions moderate the composition of these sediment archives, the evolving erosion patterns and source terrain availability of a river basin.

Prominent Quaternary fill terraces in the Zanskar River basin of the northwest Indian Himalaya provide new OSL ages of ~ 18 ka, ~12 ka and ~8 ka. New detrital zircon and apatite chronologies, from modern river sands and Quaternary terraces demonstrate significant provenance change from the Quaternary to modern day in the central Zanskar River basin as erosional loci shifted with Asian Summer Monsoon precipitation. Detrital zircon populations indicate a shift from Greater Himalayan and > 2.5 Ga material to greater contributions of Tethyan Himalayan material in the eastern part of the basin as the monsoon weakened. At present, more voluminous Greater Himalayan material is eroding from the western portion of the basin and overwhelmingly dominates the detrital signal of the modern Zanskar River. Modern river sand apatite fission track ages are dominated from ages spanning from 7-10 Ma and are consistent with local exhumation ages from the Greater Himalaya and Zanskar Shear Zone. This underlines that the glaciated Greater Himalayan topography sources the majority of bedload material in the modern Zanskar River. This area of maximum sediment production is also that of highest summer monsoon precipitation. We suggest that the Greater Himalaya were even more important as sources to the river before 8 ka because of the stronger monsoon at and before that time.