Drainage Pattern Changes in Southeastern Tibet Inferred from Eocene–Modern Sediment Provenance of the Yanyuan Basin

Abstract

The geometry and evolution of the rivers originating from the Tibetan Plateau have been influenced by topography and climate changes during the India–Asia collision. One notable example is the Jinsha (upper Yangtze) River, which expanded its drainage area at the expense of other rivers through several river captures, the timing of which remains controversial. This study focuses on the Yalong River, a major tributary of the Jinsha River, characterised by deep gorges, sharp elbows, where the river exhibits abrupt changes in channel orientation, and interfluves with high-elevation, low-relief areas that contain riverine sediments. One such area north of the Yalong–Jinsha confluence is the Yanyuan Basin, which is filled with mainly Eocene alluvial fan deposits and Pliocene fluvial conglomerates that preserve part of the detrital history of the Yalong River. We use detrital rutile U–Pb, muscovite ⁴⁰Ar/³⁹Ar, zircon U–Pb, and fission-track double dating to constrain the provenance of the Cenozoic detritus of the Yanyuan Basin. Our combined data suggest that the Eocene clastic infill of the Yanyuan Basin was mainly supplied by the Paleo-Yalong River, which, like the modern Yalong, originated from the Yidun Arc but flowed southward into the Yanyuan Basin, in contrast to the river's current course that flows around the basin and then southward. In the Pliocene, the presence of a small population of young (Cenozoic) zircon grains alongside a dominant population of Neoproterozoic zircons, rutiles, and muscovites shows that the Paleo-Yalong catchment expanded eastward, including a river originating from the Gongga batholith that flowed southwestward into the Yanyuan Basin. The absence of Cenozoic and Late Cretaceous detrital ages in the modern Yalong River, alongside the presence of the Late Cretaceous ages in the modern Jinsha River, indicates that between the Pliocene and the Present, the Yalong basin experienced a reduction in area on both its western and eastern margins. Finally, during this time, drainage reorganisation resulted in the isolation of the Yanyuan Basin from distant sediment sources, as the Yalong entrenched into its modern gorge.