Monsoon and earthquake controls on sediment flux in an arid bedrock landscape, Zanskar, India
Final Report Abstract
The greater context for project ZANSKAR was to decipher the controls, patterns, and rates of sediment flux along the western Tibetan Plateau margin. Specifically, we targeted valley fills such as fluvial terraces, lacustrine sediments, and mass-wasting deposits for absolute age dating and field inspection in order to learn more about whether the South Asian summer monsoon circulation or earthquakes were the prime underlying controls on eroding the arid high-mountain desert of Zanskar and Ladakh, NW India. The main findings in this regard can be summarised as follows: A pronounced order-of-magnitude increase of denudation rates can be traced across the 10 western Tibetan Plateau margin from both river sand petrology and detrital cosmogenic Be concentrations. These postglacial denudation rates consistently remain below those of longterm exhumation, and indicate higher denudation rates in the Pleistocene. In this context, seasonality and aridity may override topography as the key controls on denudation rates along the western Tibetan Plateau margin. Sediment may be liberated, but also retained by damming, through large rock-slope failures that occur also in the drier parts of the Himalaya-Tibet orogen. Recognition of these deposits is far from straightforward, as landslide sedimentology offers confusion potential with glacial and impact-related sediments even at the particle scale. Multiple phases of massive Pleistocene aggradation and incision along the Indus and Zanskar Rivers seem to result from paraglacial instead of monsoonal forcing. In particular, Pleistocene lakes in the western Tibetan Plateau margin are much older and more long-lived than previously thought. An unprecedented quantification of Himalayan valley-fill sediment storage indicates a decisive tectonic trapping control, with the bulk of material being sequestered for up to >10 yr upstream of the Himalayan syntaxes, i.e. near or above the Tibetan Plateau margins. This pattern of sediment retention may eventually induce millennial lag times between intramontane and foreland/deep-sea fan sedimentary archives.
