No abstract

The upper-middle Yangtze River drains the Qiangtang Block, the Songpan-Ganzi, the Yangtze Craton, and the Qinling-Dabie orogenic belt. These tectonic units have been shown to have heterogeneous Pb isotopic compositions, which allow this isotope system to be used as a sediment provenance tool. In this study we have employed laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) to measure Pb isotope compositions of sand-sized K-feldspar grains from the upper-middle Yangtze River. Data are presented from four major tributaries: Yalongjiang, Minjiang, Jialingjiang, and Hanjiang, as well as from the main Yangtze River near Yichang. A portion of K-feldspar grains in the Yalongjiang shows an ultraradiogenic character ( 206 Pb/ 204 Pb > 20), which is unique in the upper-middle Yangtze. Moreover, these ultraradiogenic grains were transported as far as Yichang, just downstream from the Three Gorges, suggesting that the Pb-in-K-feldspar method could be applied to the sediments within the Jianghan Basin to date the formation of the Three Gorges. Pb isotopic data from Yichang indicate that erosion in the Longmen Shan and neighboring regions is more important than the Jinshajiang in supplying sediment. The grains in Hanoi Basin have little overlap with the Songpan-Ganzi, but show a good match with the Yangtze Craton in its range of lower 206 Pb/ 204 Pb ratios. These observations support the idea that the ''Middle Yangtze'' used to be a tributary of the paleo-Red River and that there has been no drainage linking the SongpanGanzi and the Red River since the Eocene.

The unusual drainage pattern of the Jinsha River (upper Yangtze) is closely linked to the uplift of the Tibetan Plateau, but when this pattern became established is controversial. In this study we compare the Pb isotopic composition of detrital K‐feldspars from the modern Jinsha River with those from the Plio‐Pleistocene Xigeda Formation, a sequence of fluvial and lacustrine deposits widely distributed along the major rivers draining the Tibetan Plateau, to investigate their relationship and to constrain the drainage evolution of the region. Our results demonstrate that the Jinsha River had achieved its current sediment‐transport pattern by the late Pliocene, indicating a drainage character similar to that operating at present. Our findings not only suggest an upper age limit for formation of the Jinsha River but also shed new light on the evolution of the whole Yangtze system. K‐feldspar grains from the Jinsha River have less radiogenic Pb than those in the Red, Mekong, and Salween Rivers, providing a key test as to whether the Jinsha River was previously connected to the paleo‐Red River. Comparison with published Pb isotopic data shows that these characteristically unradiogenic feldspars are absent in the onshore Cenozoic archive from the Hanoi Basin, suggesting that there has been no connection between the lower Jinsha River and the Red River since the Eocene.

The geometry and evolution of rivers originating from the Tibetan plateau are influenced by topography and climate change during the India‐Asia collision. The Yangtze River is the longest among these rivers and formed due to capturing many rivers on the eastern Tibetan Plateau by the middle Yangtze. The timing of these capture events is still controversial. Here, we use detrital muscovite 40Ar/39Ar and zircon U–Pb ages to constrain the provenance of late Cenozoic sediments in the Jianghan Basin in the middle reaches of the Yangtze River. The combined data suggest that late Pliocene sediments were mainly derived from a local source in the Jianghan Basin including the Dabie Shan. The middle Pleistocene sediments were derived from the Min River west of the Three Gorges. This implies that at least one river, perhaps the palaeo‐Han River, originating from the Dabie Shan region, flowed through the centre of the Jianghan Basin during the late Pliocene. The appearance of sediment from the Min River in the Jianghan Basin somewhere between late Pliocene and middle Pleistocene suggests that the Three Gorges section of the Yangtze River was formed somewhere between late Pliocene and middle Pleistocene (N2– Q2).