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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 160–220Ma population. The lower reaches of the Red River comprise 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 >8ka 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 (230–250Ma), 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. [Copyright &y& Elsevier]