Termination of fluvial-alluvial sedimentation in the Xining Basin, NE Tibetan Plateau, and its subsequent geomorphic evolution

The Xining Basin in northern China lies topographically in the transitional zone between the Tibetan Plateau and the Loess Plateau, and climatically in the transitional zone between humid East Asia and the arid Asian interior. During the Cenozoic, the basin accumulated a thick, continuous sequence of fluviolacustrine-dominated red beds and subsequent eolian loess. The termination of sediment deposition in the Xining Basin is the key to understanding its subsequent evolution, the development of the modern Yellow River, and the history of dust deposition. Here we present the results of a detailed paleomagnetic study of the upper sedimentary sequence of the Xining Basin below the river terraces, and in addition we review previous studies of river evolution development. Our results indicate that the age of the uppermost part of the continuous sequence of fluviolacustrine-dominated red beds in the Xining Basin ranges from ca. 20 Ma to 4.8 Ma, suggesting that basin sedimentation ended at the earliest at 4.8 Ma, and they also provide an earliest age constraint for the subsequent river terrace development and eolian dust deposition. In addition, these results demonstrate that the termination of sedimentation in the Xining Basin and the nearby Linxia Basin was approximately coeval, suggesting the rapid regional tectonic uplift of the NE Tibetan Plateau and resulting reorganization of the landforms and development of the paleo-Yellow River in the late Pliocene.