Late Quaternary Fluvial Geomorphologic Evolution Of The Sikeshu River In The Northern Chinese Tian Shan Foreland

Fluvial geomorphology is an important hotspot in geoscience field.As one of the most basic and widely distributed geomorphic types-river terrace,naturally becomes the research object of many scholars.Through the research of river terraces,not only can we recover the history of fluvial geomorphology evolution,but also can obtain information of regional climate change and the tectonic movement.The age of ri-ver terrace is the basis of studying the river terraces,especially the emergence of new dating technology in recent years,for example,cosmogenic nuclide method and so on,laid the foundation for the research of river terraces,which greatly promoted the development of the fluvial geomorphology.The Tian Shan,as one of the largest and most active mountain ranges in Central Asia,has long been one important research area in tectonic geomorphology.Since the Cenozoic,due to the control from the collision of Indian plate with Eurasian plate,the Tian Shan uplift again.The northern piedmont of the Tian Shan Mountains,which is characterized by three parallel ridges of anticlines,known as Fold-and-Thrust zones?-? from the mountains towards the foreland basin.At the same time,many rivers cut through the Fold-and-Thrust zones,formed a variety of fluvial geomorphology types.Previous studies on the faults' activities in the northern piedmont of the Tian Shan Mountains are still incomplete,the activity of the first Fold-and-Thrust zone does not give a clear reply.In addition,the forcing mechanism behind river incision in a fore land setting generally characterized by tectonic migration remains an issue of debate,combined with the vast area of Tian Shan,more research is helpful to better reveal the forcing mechanism behind river incisioa Therefore,this paper chooses the Tuostai anticline as the research background,a fold of the proximal structure Belt I in the northern Chinese Tian Shan foreland,where the Sikeshu River incised deeply into the bedrock strata comprising the fold and the overlying alluvial conglomerates.Based on the filed survey and indoor analysis,we determine the fluvial geomorphology sequence of the Sikeshu River.According to the longitudinal profile of river terraces,we try to estimate the late quaternary tectonic activity of the Tuostai anticline,and combining with the method of chronology,discuss the forcing mechanism behind river incision,the main conclusions are as fllows:(1)The Sikeshu River develOps five river terraces(designated as T1 to T5 decreasing systematically in elevation).Because we do not find the right samples of the T4 and T5,so we only get the highest three terraces(T1 to T3)ages at about 77.271±6.92ka?15.925±3.04 ka?14.98±1.62ka,respectively.Also,based on the morphology and distribution of alluvial fan of the Sikeshu River,we divide the alluvial fan into three period(designated as F1 to F3 decreasing systematically in elevation).Due to the serious erosion,the F1 scattered in the mountain pass;F2 and F3 are widely distributed in the middle and lower reaches of rivers.The three alluvial fans,in turn,correspond to the river terraces T1.T2,T5.(2)Based on the filed survey and indoor analysis,we map the longitudinal profiles of terraces T2,T3 and T4 across the Tuostai anticline.According to the analysis of the fongitudinal profile of river terraces,the three river terraces are gradually converge in the downstream,but did not show obvious deformation.In addition,we do not find any obvious deformation on the Ti surface.So we s-uspect the Tuostai anticline has been tectonically inactive since the formation of these terraces.(3)According to the height and dating of the river terrace,we obtain the incised rate between T3 and river bed:6mmyr-1.The incised rate between T1 and T2 is 1.14mmyr-1.Clearly,the incised rate of the latest Pleistocene-Holocene is significantly higher than that of late Pleistocene.(4)Based on the dating of the terrace and compared with late quaternary paleo-climate records in Tian Shan regions,we think that river incision and terrace formation over the Tuostai anticline are largely forced by climate control.We interpret that incision began during the glacial-interglacial transition.During the time,the climatic condition is more progressively warmer and wetter.