The Evolution Of The Yulin River And Its Defomation Related To The Northwestward Growth Of The Altyn Tagh Fault

The Altyn Tagh fault is a large-scale left-lateral strike-slip fault zone on the northern margin of the Tibetan Plateau.As a boundary fault on the northwestern side of the Tibetan Plateau,it has undergone multiple periods tectonic activity since the Triassic,and it is still active in the Holocene.A large number of studies have shown that the Altyn Tagh fault plays an important role in controlling the growth of the plateau.The extended deformation of the northern margin of the Tibetan Plateau is mainly absorbed by the Altyn Tagh fault.However,it remains controversial about the pattern of continued northward deformation and what kind of structural features and topographic features are caused by the expansion of deformation.Published results show that the Altyn Tagh fault has profoundly affected the sedimentary processes and tectonic deformation of the basins on the both side since Cenozoic.Since the late Pleistocene,the left-lateral strike-slip rate along the Altyn Tagh fault has gradually decreased eastward.On both sides of the fault,many secondary faults parallel or oblique with high angle to the Altyn Tagh fault are distributed.The development of the secondary fault is affected by the lateral extension of the Altyn Tagh fault,and may be characterized as a flower-like structure in depth.Dun Huang basin,to the east of the Altyn Tagh fault,is consisted of the Daba depression,Dongbatu eminentia,Tashi depression,Sanwei Shan eminentia and Anxi depression from south to north.The Dongbatu fault and Sanwei Shan fault,which are nearly parallel to the Altyn Tagh fault,were formed because of the northwestward extension of the Altyn Tagh fault.The Yulin river originated from the Daxue Shan,flows through the Shibaocheng basin,Altyn Tagh mountains,Dongbatu mountains,and Sanwei Shan,and finally merges with the downstream of the Shule River.It can be inferred that the expansion of the Altyn Tagh fault will profoundly affect the evolution of the Yulin river and relevant landform development.Multible river terraces that experienced apparent deformation have developed since the late Pleistocene in both the north and south sides of the Dongbatu mountain.These terraces are important media for us to study the northward expansion of the plateau.Therefore,we chose the terraces along the Yulin river in Dongbatu mountain to study on the response of fluvial geomorphology to tectonic movement.This paper conducts detailed topographic surveys,measurements,and chronological study of the river's landforms in the study area.The evolution history of the Yulin River was discussed through the sedimentary structure,the age of the terraces and the established terrace sequence.Using the deformation of landforms to quantitatively study the structural deformation of the Yulin River in the middle and late Pleistocene in Dong Batu mountain.In addition,we discussed the evolution of the river's geomorphology and its response to tectonic movements.The main methods used in the study are: 10 Be cosmogenic radionuclide dating,high-precision image analysis and interpretation,unmanned aerial vehicle photogrammetry and GPS field measurements.The main progresss made in this paper are as follows:1.We established a complete terrace sequence of the Yulin River.Field surveys and interpretations of high-precision images show that at least 10 major terraces,including strath terrace,bedrock terrace,and constructional terrace,are developed along the Yulin River in the Dongbatu mountain.Based on geomorphological expression and location,the Yulin river can be divided into three different sections,upstream section,middle section,and downstream section.Using 10 Be cosmogenic nuclide dating,we dated the terrace's exposure ages in each section: Q2 fan(166.5 ± 15.2 ka),T4(23.2 ± 2.4 ka),T5(26.2 ± 2.6 ka)in the upstream,T6(22.9 ± 2.7 ka),T7(50.0 ± 4.9 ka),T9(56.5 ± 5.3 ka)in the middle section,T5(20.6 ± 2.4ka),T9(38.7 ± 3.9ka)in the downstream section.2.We build the deformation pattern of the river terraces.The deformation in the area is concentrated near several faults and folds.The vertical slip rates of fault along the northern flank of the Dongbatu mountain is estimated to be 0.1±0.01 mm/a and the horizontal short rate across the fault is 0.07±0.01mm/a since 166.5±15.2 ka.This fault is not active since the late Pleistocene.The fault along the southern flank of the Dongbatu mountain derived a displacement of ~5.3 m for T8 and ~7.5 m for T10 in the middle section.The uplift rate is 0.09-0.10 mm/a and short rate is 0.09-0.10 mm/a since the abandonment of T8.Moreover,the uplift rate is 0.11~0.13 mm/a and short rate is 0.10~0.13 mm/a for T10.3.We obtained the incision rate of the Yulin River near the Dongbatu Mountain.Combining abandoned ages of the terraces and terrace elevation above the active channel,we obtain the incision rate in different section of the river to be 0.63-1.42 mm/a in the upstream section,0.31-0.58 mm/a in the middle stream,0.62-0.67 mm/a in the downstream.Based on these rates,the highter rate of the upstream section may represent the southern flank of the Dongbatu fault are more active,the lower rate of the middle stream may represent the expansion process of Dongbatu mountain.Using the age of Q2 fan surface we calculate the incision rate to be 0.63 ± 0.06 mm/a since 166 ka,which is similar to rate of the downstream section that may represent an average down-cut rate of the Yulin River over a longer period of time.4.Preliminary discussion of the causes of formation of the river terraces.Combining the characteristics of regional tectonics and comparing the abandoned ages of terraces with 18 O global climate change curves,we argue that the concentrated ages around ~20 ka of the Yulin river terraces are mainly caused by severe climate fluctuation that lead to large-scale flood events.The older terraces abandonment ages are corresponded to the period of glacial-interglacial when it is easy for rivers to cut down.The incision rate is significantly higher than the uplift rate which presumably means that terraces are mainly controlled by climate change.According to the geomorphological features of the Yulin river terraces,the special feature of the terrace is controlled by the slope of alluvial fan and kickpoints.The higher slope of the alluvial fan compared to the river bed forms the terrace converges towards the alluvial fan.The formation of terraces is controlled by the continuous retreat of the kickpoints and the kickpoints is caused by the lithology.5.Combining river incision rate,topographic profile,fault activity and the active history of the Altyn Tagh fault since the late Cenozoic,we established the process of geomorphic evolution of the Yulin River since the Quaternary.The pattern of the evolution of the fluvial landform in the Dongbatu Mountain and infer that the growth process of the Dongbatu Mountain is both lateral and vertical,suggestive of the defomation to the northwestward growth of the Altyn Tagh Fault.