Detrital U-Pb Zircon Ages as a Sediment-Mixing Tracer in the Central Nepal Himalaya

Tectonic deformation and climate interact and compete to control exhumation in the Himalaya. Geomorphic indicators such as hillslope angle, river gradient and concavity suggest higher rock-uplift rates above the Main Central Thrust (MCT) zone. Long and short term erosion rates calculated from Ar/Ar ages, apatite fission track and cosmogenic nuclides show consistently higher erosion rates above the MCT over variable time scales. To test the tectonic control of modern erosion rates in the Himalaya, this study employs a new technique using the downstream mixing of U-Pb zircon ages in Himalayan river sands as a proxy for sediment fluxes. U-Pb dating is well suited for this application in the Himalaya because sedimentary source rocks contain a range of U-Pb zircon ages providing a lithology-specific fingerprint used to trace mixing of sediment from different sources once in the modern river system. Relative erosion rates are calculated for three sites at which a river draining exclusively from above the MCT adjoins a river draining below the MCT. The rivers are small enough that source regions can be accurately defined and climatic factors are constant for reaches both above and below the MCT. 70-100 zircon grains were randomly selected from river sand samples for U-Pb dating by LA-MC-ICPMS and the age distribution of zircon grains below the confluence was statistically deconvolved to estimate the relative contributions of sediment from the two contributing rivers at all three sites. Samples were also collected from tributaries and trunk sediments throughout the Marsyandi drainage. Understanding the downstream evolution of zircon age distributions and heavy mineral content from the headwaters to the foreland improves understanding of erosional patterns and sediment transport dynamics on a drainage scale. These results have implications for the use of U-Pb and fission track ages in detrital grains extracted from foreland basins to interpret the orogenic evolution of the central Himalaya.