The role of the Haiyuan Fault in accelerating incision rate of the Yellow River at the Mijia Shan Area, northeastern Tibetan Plateau, as revealed by in-situ 10Be dating

Abstract

In this study, we employed cosmogenic nuclide 10Be analysis to determine the formation ages of the Yellow River terraces in the Mijia Shan and the Laolongwan Basin at the northeastern periphery of the Tibetan Plateau. The aim of this study is to determine local fluvial incision rates and the effect from tectonic activities. The two sets of terraces were surveyed in field and dated by cosmogenic 10Be exposure dating method. The results show that the best-fitting terrace ages in the Mijia Shan are respective 68.0-9.4+11.2 ka for T11 and 174.7-38.4+52 ka for T14. In addition, in the Laolongwan basin, sample S3 constrains an age of 123.9 ± 2.7 ka and S2 yields an age of 45.0 ± 1.8 ka. Combining the strath heights of the sampled terraces, the fluvial incision rates were determined. The results showed that the fluvial incision rate of the Yellow River is ∼1.94 mm/a in the Mijia Shan over the last 175 ka, while a much lower incision rate of ∼0.73 mm/a is yielded in the Laolongwan Basin since the last ∼120 ka. The incision rate in the Mijia Shan is almost twice more than that in the Laolongwan Basin. Considering the same climatic condition in both the Mijia Shan and the Laolongwan Basin, the difference between the two incision rates (∼1.21 mm/a) might be due to local tectonic deformation which is derived from the Haiyuan active strike-slip fault. Consequently, our results showed that the Haiyuan fault plays an important role in accelerating the incision rate in the Mijia Shan. Hence, it is suggested that the role of the main geological structures across the main river should also be considered when studying the evolution of terraces of big rivers.