Layered Landforms And Drainage Evolution In The Eastern Qilian Mountains Since Late Miocene

The formation of the Tibetan Plateau is one of the greatest geological events in the history of the Earth.The collision between the Indian and Eurasian plates and the continuous north wedge of Indian plate have a significant impact on the landscapes and drainage evolution in the Asia.There have formed a series of layered landforms,just like planation surfaces and river terraces,during the uplift of the Tibetan Plateau.Layered landforms represent a key piece of evidence in the reconstruction of landscape formation and evolution,and they were widely utilized in landscape and neotectonics researches.Layered landforms generally shaped by the fluvial system.Moreover,among orogens,fluvial system is very sensitive to changes in tectonic and climate change and can also record the related information.Thus,the study of drainage evolution provides special insight to clarify the interactions among tectonic,climatic,and surface processes.The Qilian Mountains locates at the northeastern margin of the Tibetan Plateau,where is a key region to respond the uplift and expansion of the Tibetan Plateau.A series of erosion surfaces,such as the planation surfaces and fluvial terraces preserved well in the eastern Qilian Mountains.Thus,it is an ideal area to study the uplift history and landscape evolution by layered landforms,and discuss the interactions among tectonic,climatic,and drainage evolution.But there is a lack of chronological constraints on the whole layered landforms sequence,and a lack of systematic study on the drainage pattern evolution.Therefore,we focused on the Dabanshan planation surface and river terraces of the Datong and Huangshui rivers in the eastern Qilian Mountains.The sedimentary facies and chronology of Cenozoic sediments overlying the Dabanshan planation surface in the eastern Qilian Mountains are studied in detail,also including the sequence,chronology and provenance of Datong River and Huangshui River terraces in the region.The main conclusions achievements are shown as follows:(1)The sediments overlying the thick weathering crust on the Dabanshan planation surface began to deposit at 8.1–7.1 Ma,and the red clay began to deposit at 6.7–6.4 Ma.And these results indicate that the Dabanshan Mountains experienced a long period of relative tectonic stability before ~8 Ma,and the development of the Dabanshan planation surface was terminated at ~8 Ma and started to accelerate uplift after ~6.5 Ma.Accelerated downcutting obtained from ages and elevations of the layered landforms also reveals that the eastern Qilian Mountains experienced accelerated uplift since the Late Miocene.Timing of the Dabanshan planation surface formation and abandonment indicates that the eastern tip of the Qilian Mountains experienced uplift later than the west and central segments.To account for these timing differences,we propose that the Qilian Mountains expanded eastward at this time.(2)We observed that 8 river terraces of the Datong River have developed near Heqiao site in the Babaochuan Basin,and the age of river terrace T8 have been dated to 1081 ka.As for the Menyuan Basin,5 river terraces of the Datong River have developed,and the ages of river terraces T5–T1 have been established as 424 ka(T5),243 ka(T4),130 ka(T3),14 ka(T2),Holocene(T1),respectively.As for the Huangshui River,10 river terraces have been observed near Minhe,and the ages of river terraces T10–T1 have been established as 1405 ka(T10),1081 ka(T9),866 ka(T8),621 ka(T7),424 ka(T6),337 ka(T5),243 ka(T4),130 ka(T3),14 ka(T2),Holocene(T1),respectively.We conclude that the drainage system of the modern Datong River began to form at 1100 ka,and the drainage basin was limited to the area of the lower modern Datong River.Since then,it has gradually extended with headward erosion.The river channelized into the Menyuan Basin between 620 ka and 420 ka,and it captured the upper Datong River between 130 ka and 60 ka.Furthermore,digital geomorphology results indicate that the upper Datong River will be captured by the Heihe River in the future.The provenance of terraces of the Huangshui River at Minhe little has changed,indicating that the Huangshui River has cut through the Laoya Gorge before 1400 ka.(3)The formation of Huangshui River and Datong River terraces may be resulted from the coupled tectonic-climate interaction.Climate change determines the formation time of river terraces,while tectonic uplift provides driving force and space for the long-term river cutting,and appropriate rate of surface uplift is the necessary condition for the terrace formation.The direction of drainage pattern evolution is controlled by tectonics but the time of drainage reorganization is controlled by climate change in the active fold-thrust belt.