Intense dilution of the Eastern Himalayan syntaxis detrital signal as recorded by thermochronology in the Yarlung-Siang-Brahmaputra river sediments

Abstract:

The distribution of a detrital mineral’s cooling age signal in modern river sediments yields useful constraints about the exhumation history of an active collisional belt and it can provide an average of the exhumation signal of the region (Brewer et al., 2006). Nevertheless, the potential downstream dilution of such a signal can profoundly affect interpretations, but is a poorly researched topic that is often not considered. In the eastern Himalaya, the syntaxis (Namche Barwa) is exhuming and eroding anomalously fast compared to the rest of Himalaya (Zeitler et al., 2001; Zeitler et al., 2014), providing a significant amount of material drained into the major modern fluvial system, the Yarlung-Siang-Brahmaputra river. It is therefore the ideal location to study dilution effects, given the syntaxis distinctive (young) signal and the volume of detritus eroded from it.

Major questions concern how the distribution of the syntaxis signal is evolving in the main Siang- Brahmaputra drainage system at different position upstream and downstream, and how this aspect is related to the influence of the main Himalayan tributaries.

We sampled fifteen modern rivers samples from the Yarlung-Brahmaputra river system upstream of the syntaxis, at an increased distance downstream from the syntaxis and from the Brahmaputra tributaries draining the Himalaya and the Lohit plutonic suite. We tested two different thermo-chronometers, ⁴⁰Ar/³⁹Ar on white mica and detrital zircon fission tracks, to evaluate the effect of the dilution of the syntaxis signal downstream. Our results show a youngest age peak distinctive of the syntaxis in both the mica and zircon datasets. However, whilst the distinctive young zircon peak endures many kms downstream, the young mica population becomes heavily diluted, presumably either due to hydrodynamic and transport effects, or due to input from tributaries draining the Higher Himalaya.