Late Pleistocene valley fills source sediment flux of Tibetan Plateau margin rivers, Zanskar, India

Abstract:

The Indus and its tributaries, one of Asia's largest river systems, drain the NW Himalaya and the Transhimalayan ranges that border the western Tibetan Plateau margin. From the internally drained low-relief areas of the Tibetan Plateau, local relief increases towards the Western Himalayan Syntaxis, where it exceeds 7 km. Simultaneously, average denudation rates rise from as little as 10 mm ka^-1 at the Tibetan Plateau margin to rates of >1000 mm ka^-1 close to the western Himalayan Syntaxis. In this rugged bedrock landscape, river valleys frequently alternate between deeply incised gorges and broad alluviated reaches. Vast fill terrace staircases of up to 400 m height above current river levels, and intercalated lake sediments point to repeated phases of incision and aggradation within the region. Despite a broad interest in a better understanding of mechanisms that modulate plateau erosion, age constraints on the generation of these impressive features remain sparse, though indicate mainly Pleistocene formation ages.

Here we present new data from the More Plains section, a vast sedimentary fill, located in the headwaters of the Zanskar River, the largest tributary to the upper Indus. The vast sedimentary successions of the More Plains originally belonged to a former endorheic basin that has been tapped by the Zanskar River, today revealing a sedimentary exposures of >250 m thickness.

We combine morphometric analysis and field based observations with ¹⁰Be surface exposure dating and basin-wide denudation rates to constrain the late Quaternary history of this setting. Analysis of a ¹⁰Be depth profile on top of the More Plains section indicate a surface exposure age of ~125 +/- 15 ka, which is supported by ages from nearby amalgamated surface samples. Grounding on a morphometric approach, we estimate that ~1.65-1.95 km³ were removed from this section by fluvial erosion since aggradation ceased, requiring a specific sediment yield of 85-100 t km^-2 yr^-1 averaged over the past 125 ka. This outweighs the ¹⁰Be-constrained flux of sediment from adjacent catchments by a factor of 2-3, suggesting that dissection of large Pleistocene valley fills might constitute a significant source of sediment in Transhimalayan rivers, potentially offsetting estimates of bedrock denudation in the region.