Late Pleistocene history of aggradation and incision, provenance and channel connectivity of the Zanskar River, NW Himalaya

The Zanskar River, one of the largest tributaries of the upper Indus catchment, drains transversely northward from the Higher Himalaya, dominated by the Indian summer monsoon, to flow through the arid, westerlies-dominated, highly folded and thrusted Zanskar ranges in Ladakh. The Doda and the Tsarap Lingti Chu join to form the Zanskar, which in turn joins the Indus at Nimu. With an average gradient of ~ 4 m/km, the Zanskar has a gradient ~2.5 times lower than that of rivers like the Ganga and Brahmaputra, which flow through the southern wet Himalaya. Based on Stream Length (SL) gradient index and valley width and height ratio, the Zanskar valley can be divided into upper and lower divisions, separated by a gorge of nearly 60 km length. The river channel in both the divisions is flanked by 10–30 m thick valley-fill deposits that in the upper part are amalgamated with fan and paleolake deposits. Using these fills and incorporating morpho-stratigraphy, Optically Stimulated Luminescence (OSL) dating and provenance analysis based on U—Pb Zircon chronology, the study show that the Zanskar valley aggraded in three phases: (i) the oldest phase during ~43 to ~32 ka (cool and wet MIS 3), (ii) during 20–12 ka, a climatic transition from the dry LGM to the wet early Holocene and (iii) the youngest aggradation phase commenced between 9 and 6 ka, corresponding with the strengthened monsoon phase of the early–mid Holocene. The study implies that, during the oldest aggradation phase, the wider Padam basin stored 3.25 ± 0.11 km3 of sediment, which, in the present geomorphic setup is 0.96 ± 0.10 km3. The provenance analysis suggests that, despite the presence of the deep narrow gorge and a low gradient, the upper and lower Zanskar valleys remained connected throughout their aggradational history. Unlike in the southern wetter Himalaya, where catchment-wide exhumation is the main source of sedimentation, valley filling in the Zanskar basin has been overwhelmed by sediment derived from headward erosion.