A critical appraisal of the interrogation of sedimentary archives to investigate the proposed forcing of drainage network reorganisation by plateau uplift in Southeast Tibet.

An often quoted type example of the adaption of a geomorphic system due to external forcing, can be found in south-eastern Tibet. In this area, it is hypothesised that uplift of Tibet resulted in a major drainage reorganisation; prior to plateau uplift, it is proposed that a continental-scale drainage network including the upper Yangtze, Mekong and Salween, used to flow into the palaeo-Red River (Clark et al, 2004). Since documentation of the timing of uplift of Tibet is important to understanding broad research questions related to crustal deformation processes and the impact of the Himalaya-Tibet orogen on climate, the evolution of this geomorphic system is well studied. Yet the timing of proposed drainage reorganisation is still debated, with a range of suggested timings between Eocene and Pleistocene, if indeed major reorganisation occurred at all.

The most commonly used approach to determining the timing of this proposed drainage reorganization involves source to sink provenance studies, with interrogation of sedimentary archives. Using this approach, the majority of studies have used detrital zircon U-Pb dating as a provenance tool. Proposed recognition of “characteristic” zircons of upper Yangtze provenance in the palaeo-Red River archive, and their subsequent disappearance up section, has been used to argue that the upper Yangtze used to flow into the Red River, with subsequent river capture of the upper Yangtze away from the Red River into its present course due to Tibetan plateau uplift.

In order for this approach to document river capture, the detrital zircon U-Pb signature of the upper Yangtze must be identifiable in the palaeo-Red River repository. Previous compilations used to characterise the zircon U-Pb signatures of the various contributing terranes to the upper Yangtze and Red River drainage basins were largely comprised of data from igneous rocks. However, this neglects the contribution of zircons from older sedimentary rocks of these terranes. We compiled all published detrital zircon U-Pb data (n=29,545) from Late Triassic and younger sedimentary rocks from these terranes (Li et al, 2024). Our compilation shows that the zircon U-Pb spectra from these various terranes are similar, and there is no unique characteristic of the upper Yangtze.  Therefore the similarity in zircon U-Pb signature between the upper Yangtze region, and Cenozoic rocks from palaeo-Red River basins may result from similarity in the various local hinterland source regions, rather than requiring that the upper Yangtze used to flow into the Red River.  

This case study highlights the importance of consideration of the adequacy of source region characterisation and the impact of recycling, when using sedimentary archives to document geomorphic evolution.