Cosmogenic ¹⁰Be and ²⁶Al isochron burial dating of Yangtze River terraces in the Yichang Region: Implications for the Middle Pleistocene integration of the Three Gorges

The timing and mechanisms of fluvial terrace formation along the Yangtze River in the Yichang region provide critical insights into the interplay between tectonics, climate, and river evolution. In this study, we present new cosmogenic nuclide isochron burial ages for the fifth (T5) and fourth (T4) terraces, yielding ages of 0.49 ± 0.05 Ma and 0.35 ± 0.04 Ma, respectively. By integrating our data with existing ¹⁴C and ESR ages for lower terraces, we reconstruct a detailed incision history, showing ~85 m of river downcutting since the Middle Pleistocene. Incision rates varied over time, with a notable acceleration after 0.1 Ma coinciding with the Gonghe movement of the Tibetan Plateau, highlighting the role of tectonic uplift in driving fluvial incision. The formation of the fifth terrace is associated with the Kunlun–Huanghe uplift, while the development of the fourth terrace corresponds to the phase of regional uplift in the Qinling Mountains. Sedimentological evidence further indicates that terrace formation was influenced by both climatic and tectonic controls: while T4 aggraded during a glacial period, T5 formed during the interglacial MIS 13 under strong monsoonal conditions—demonstrating that major aggradation can occur during warm phases. However, sustained incision required tectonic forcing, as climatic transitions alone were insufficient to drive deep bedrock erosion. Our results also constrain the long-debated integration of the Three Gorges. The 0.49 Ma age for T5 provides a robust minimum age for the hydrological connection between the Sichuan and Jianghan Basins. This timing, combined with the earlier formation of the "First Bend of the Yangtze" (Eocene–Miocene), contradicts the classical east-to-west headward erosion model. Instead, geomorphic analysis of the gorges—showing diachronous valley development from east to west—supports a progressive, bottom-to-top integration, initiated in the Xiling Gorge and culminating in the incision of the Qutang Gorge. This sequence aligns with knickpoint migration driven by base-level fall and regional tectonic uplift. We conclude that the evolution of the Yangtze River through the Three Gorges is the result of a complex interplay between climate, tectonics, and base-level dynamics, with terraces serving as key archives of landscape response to these forces.