Landscape evolution of the Dabanshan planation surface: Implications for the uplift of the eastern tip of the Qilian Mountains since the Late Miocene

Abstract

A series of erosional surfaces, including a planation surface (PS) and several fluvial terraces are well preserved in the Dabanshan Mountains, northeastern (NE) Tibetan Plateau. The Dabanshan planation surface has been divided into several platform patches by river incision, including the Dongping PS and the Halagu PS, and is overlain by Late Cenozoic sediments. Furthermore, a series of fluvial terraces are inset into the Dabanshan PS. In order to reconstruct the formation and evolution of the Dabanshan PS, we analysed two drill cores of Late Cenozoic sediments from the uplifted Dongping PS and Halagu PS, respectively. First, we analysed the sedimentary facies to investigate the sedimentary environment during deposition using various integrated methods, including the sediment properties, grain size, and provenance. To constrain the termination time of Dabanshan PS development, we used micromammalian fossil assemblages and magnetostratigraphy. Based on the overlying strata and the morphometric features, we propose that this low-relief upland landscape along the eastern tip of the Qilian Mountains represents preserved remnants of elevated planation surface. Our sedimentological and geochronological results show that the stable weathering conditions of the Dongping PS and the Halagu PS were ended at ~8.1 Ma and ~7.5 Ma, respectively, when they changed to a fluviolacustrine sedimentary environment. This result reveals that the development of the Dongping PS and the Halagu PS were terminated at ~8.1 Ma and ~7.5 Ma, respectively. The uplift of the Dabanshan PS at ~8–6 Ma suggests that the initial uplift of the eastern tip of the Qilian Mountains occurred later than in the western and central segments, indicating eastward growth of the Qilian Mountains over time. In addition, the accelerated downcutting recorded by the planation surface and its inset river terraces indicates that the eastern tip of the Qilian Mountains may have experienced accelerated uplift since the Late Miocene.