Late Miocene birth of the Middle Jinsha River revealed by the fluvial incision rate

The Jinsha River has attracted considerable attention for nearly a century due to the morphology of its drainage basin. To explain the evolution of the Jinsha drainage basin, most researchers have considered that the present-day Jinsha (the upper reach of the Yangtze) was once a tributary of a single, southward-flowing river system called the Paleo-Red River, which drained into the South China Sea and was captured by the Upper Yangtze River, after which the flow direction changed to eastwards. The sudden disappearance, at 5.5 Ma, of the Red River submarine fan, which was fed primarily by the Paleo-Red River system, provides an important chronological constraint on the drainage reorganization and the capture event. However, until now, no geomorphic evidence has been presented to support this hypothesized age. Considering this problem, nine well-preserved terraces (T9 to T1) at Jinjiangjie in the Middle Jinsha River were evaluated in this study. To establish the chronological framework of these terraces, a combined dating approach was employed, incorporating electron spin resonance dating, cosmogenic nuclide (26Al/10Be) burial dating, and optically stimulated luminescence dating. We obtained a complete chronological sequence for terraces T9–T1, with ages of approximately 1.07 (T9), 0.70 (T8), 0.65 (T7), 0.51 (T6), 0.47(T5), 0.44(T4), 0.30 (T3), 0.18 (T2), and 0.047 (T1) Ma, respectively. Constrained by this framework, we calculated an average long-term fluvial incision rate of ~147 mm/ka. The reconstruction of the paleo-topography and paleo-drainage, based on digital elevation model data for the re-filling of the deeply incised river valley, indicates that the present drainage pattern in the Middle Jinsha system was established after the disruption of the paleo-landscape surface at an elevation of ~2000 m above sea level. The age of the present-day Middle Jinsha drainage was determined at approximately 5.5 Ma by extrapolating the average valley incision rate, suggesting that the present-day Jinsha was formed at about this time. Our results agree with the previously estimated timing of the disappearance of the Red River submarine fan.