Entrenchment of the Yellow River since the late Miocene under changing tectonics and climate

The distinctive right-angled bend of the Yellow River around the Ordos Block, a large Permo-Mesozoic continental block in East Asia, has attracted much attention from geomorphologists and geologists concerning its formation and evolution for more than a century. The Jinshaan Gorge, an over 600 km long entrenched reach of the Yellow River, currently connecting the Hetao Graben to the north and the Weihe Graben to the south, is considered to have developed in very recent geological time. However, the timing and processes of drainage integration between the two grabens and formation of the Jinshaan Gorge as a deeply incised form are still under debate, owing to limited data and various interpretations of them. In order to better understand the geomorphological evolution of the Yellow River, we investigated the sedimentology, magnetostratigraphy, and detrital zircon U-Pb dating in the upper Neogene and Quaternary deposits along the Jinshaan Gorge. We further integrated the new data with the sedimentary record of the adjacent basins in the northeastern Tibetan Plateau (NTP) and the basins around the Ordos Block, as well as the incision history of the Yellow River inferred from its terrace sequence. We show that the upper Neogene alluvial sediments preserved in the broad Tangxian valley along the Jinshaan Gorge are a mixture of materials from the Lvliang Shan and the Ordos Block. Importantly, they share almost the same provenance and zircon age spectra with the Pleistocene and Holocene Yellow River deposits. The broad Tangxian valley commenced incision during the Late Pliocene, followed by the formation of two Early Pleistocene terraces dated to ~2.2 Ma and ~1.2 Ma. The incision of the Yellow River and the accumulation in the basins were characterized by steady and slow rates before the Late Pleistocene, but an acceleration thereafter.We propose a three-stage evolutionary model for the development of the Yellow River, controlled by changing tectonics and climate during the Neogene. Before the late Miocene, an endorheic river system was developing in the Longzhong Basin. From the late Miocene to Early Pliocene, the Yellow River developed the current drainage system from the Longzhong Basin to the lower reach, constrained by the topographic pattern of the Ordos Block and its periphery, in turn controlled by coeval tectonic movements. Since the Late Pliocene, the Yellow River has undergone rapid incision with integration of the upper reach in the NTP and entrenchment in the upper and middle reaches, which were controlled by the continuously rising NTP and a deteriorating climate. The Tangxian valley developed and was filled between the end of the Miocene and Pliocene, approximately 8 to 3 Ma, and it was abandoned in the Late Pliocene in response to important climatic variations. The next acceleration in both incision and subsidence around the Ordos Block in the Late Pleistocene was also coeval with strong climatic oscillations, suggesting a possible link between climate and tectonics in the Neogene entrenchment history of the Yellow River.