『Abstract
In this study we applied 40Ar/39Ar dating
methods to muscovite grains in sands from the modern Red and Yangtze
Rivers in order to constrain palaeo and modern erosion patterns
and sediment transport processes. Micas in the headwaters of the
Red River and its southern Da River tributary are dominantly Cenozoic,
while the northern Lo River tributary shows a typical 160-220
Ma population. The lower reaches of the Red River comprises 53%
of such Triassic grains, indicating that the Lo River is the most
important net contributor to the modern delta. However, this contrasts
with zircon U-Pb data indicating that the upper reaches of the
Red River dominate that mineral group. We suggest that the mismatch
reflects the rapid transport of mica relative to zircon in the
river and the fact that the Red River is not in equilibrium. We
hypothesize that the zircons were eroded >8 ka under a regime
of stronger monsoon that enhanced erosion in the northern drainage
basin, while modern erosion is focused in the south, especially
in the Song Chay Massif. Micas from the upper Yangtze River are
resolvably older than those in the Red River (230-250 Ma), so
that this method could be used to test for drainage capture in
palaeo-delta sediments. Micas in the modern Yangtze delta are
generally much younger than those in the upper reaches and indicate
that erosion in the Longmen Shan (Sichuan) and neighbouring regions
is more important than the upper Yangtze in supplying sediment,
reflecting the stronger monsoon rains in those areas.
Keywords: Red River; Yangtze River; Ar-Ar muscovite dating; provenance』
1. Introduction
2. Sampling and analytical methods
3. Results
4. Discussion
4.1. Provenance of the Yangtze River
4.2. Provenance of the palaeo-Red River
4.3. Provenance of the Modern Red River
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References