Interaction Of Late Quaternary Tectonic Deformation With Sedimentation And Erosion Along The Manas River,Northern Tianshan

The interaction between tectonic,climate and surface processes has always been the focus of geoscience research and the core of tectonic geomorphology.In the study of tectonic geomorphology of the thrust and fold belts,the piedmont river system is an important research object.On the one hand,the geological and geomorphic landscapes of the thrust and fold belts are shaped by the tectonic,climate,fluvial sedimentation and erosion process.On the other hand,the processes of river vertical and lateral erosion,material migration and settlement,as well as channel migration and usurpation are all significantly affected by the tectonic activities of folds and climate change.Therefore,how to quantitatively assess the contribution of tectonic and climate to the geological and geomorphological evolution of the thrust and fold belts,and how the tectonic,climate and surface processes interact and respond in the thrust and fold belts are still difficult and frontier scientific problems in the study of tectonic geomorphology.However,constraining the influence and contribution of tectonic activities is one of the core of understanding the interaction and response of tectonic,climate and surface processes.The South Junggar Thrust Belt（SJT）was developed at the northern foot of the Tianshan,and the 3-4 rows of folds developed from south to north absorbed the deformation of SJT.The geomorphic evolution of the South Junggar Thrust Belt was influenced by tectonics and climate.Its rich tectonic,stratigraphic,geomorphologic and seismic records provide favorable conditions for the study of fault and fold deformation history at different time scales,and it is also a natural laboratory for the study of the interaction of tectonics,climate and surface processes in continental thrust and fold belts.However,although a large number of studies have been carried out on the late Cenozoic,especially for the late Quaternary,deformation of SJT,most studies intended to determine the activity characteristics of SJT through the deformation of single anticline or fault in the thrust and fold belts,lacking systematic studies on the spatio-temporal distribution of the late Quaternary deformation and rate of SJT.There is great controversy about the Late Quaternary activity of the passive back-thrust fault（SJT1）of the northern flank of the first row anticlines.The shallow detachment layer（SJT2）under the second anticline is a blind thrust fault.Its complex relationship between slips and surface displacements of blind thrust fault makes it a difficult challenge to constrain the activity characteristics of SJT2.In addition,there is still a lack of detailed quantitative research on the interaction and response of the tectonic activities of the thrust fault and fault-fold,climatic changes,as well as sedimentation and erosion processes in the South Junggar Thrust Belt.This paper,based on the fault-related fold theory,systematically studied the late Quaternary activity characteristics of SJT and branch tectonic units（the first row South-Manas anticline and the second row Tugulu and Manas anticline）along the Manas River section,by means of field observations,mapping,seismic reflection profile interpretation,UAV remote sensing image and topographic survey,and luminescence dating.First,we constrained the activity and strain distribution of SJT along the Manas River section.Then combining with the fluvial geomorphic evolution processes of the Manas River,we discussed how tectonic activity,climate change,as well as fluvial sedimentation and erosion processes jointly act on the geological and geomorphic landscape of the Manas River basin,and their mutual response relations.The main findings and conclusions are as follows.（1）Luminescence dating of fluvial cobble is a reliable and effective dating technique,providing a new chronology method in the study of active tectonics and palaeo-earthquake in mid-west of our country.Compared with the conventional silt-sand luminescence dating,this method not only extends the range of materials applicable to the luminescence dating,but also has obvious advantages in evaluation of bleaching degree and the calculation of dose rates.In addition,on the basis of the conventional age-depth（A-D）plateau of rock surface OSL dating,the introduction of age-temperature（A-T）plateau in post-infrared infrared stimulated luminescence（MET-p IRIR）protocol enables us to simultaneously evaluate the bleaching and fading of the luminescence signals of the rock slices,and judge the reliability of the age results of the rock slices.In particular,the A-T plateau can indicate the bleaching information of the OSL signal through single rock slice,which is of great significance for the cobble with limited bleaching depth.（2）The Southern Junggar Thrust（SJT）is a complex tectonic wedge system with multiple detachment layers.Its slip is absorbed by the passive back-thrust fault（SJT1）of the northern flank of the first row anticlines,the shallow detachment layer SJT2under the second row of Horgos-Manas-Tuguru anticline and the regional deep detachment layer SJT3.In the Manas River basin,the growth strata of the southern flank of the Tugulu anticline and the underlying shallow detachment layer SJT2basically conform to the curved-hinge kink-band migration deformation model.Its shortening rate was 2.0^+0.6/_-0.4mm/a between～360-120ka B.P.,and increased significantly to 5.0^+0.8/_-0.7mm/a at～120ka B.P.,and from～25.5ka B.P.until now it decreased to 2.1^+1.0/_-0.7mm/a.（3）The shortening rates of the passive back-thrust fault（SJT1）of the northern flank of the South-Manas anticline and the shallow detachment layer SJT2 under the Manas anticline were respectively 1.2±0.2mm/a and 2.5±0.3mm/a since～26.5ka B.P.Therefore,along the Manas River section,the slip of SJT to the basin mainly spread to SJT1 and SJT2 since late Quaternary.The total slip of SJT over the past～25ka B.P.was～74-103m,in which the passive back-thrust fault（SJT1）of the northern flank of the South-Manas anticlinal has absorbed～31-43%,and the shallow detachment layer SJT2under the Tugulu-Manas anticlinal has absorbed～57-69%.（4）The Manas river incised rapidly along fixed channel at a basically constant rate（up to～10-16mm/a）since～25ka B.P.,while the lateral erosion rate can reach 10²-10³mm/a.The vertical incision and lateral erosion processes of Manas river are mainly controlled by climate.The variation of water-sand ratio（Qw/Qs）caused by climate change affects the vertical and lateral jiggling of Manas River.（5）The geomorphic evolution of the Southern Junggar Thrust Belt is influenced by both tectonic and climate.The geomorphic landscapes of the Manas river basin were shaped by the interaction of the tectonic activities of the faults and folds belt,regional climatic changes,as well as fluvial sedimentation and erosion processes.The vertical incision and lateral erosion of the Manas River have obvious responses to tectonic activities and climatic changes.Combined with the results of SJT late Quaternary tectonic activity characteristics along the Manas River section,we believe that the Manas River may have undergone the following fluvial geomorphologic evolution process:the fold uplift was accelerated and the off-lap relationship displayed by growth strata section at～120ka B.P.In the syncline area to the south of anticline,the sedimentation rate simultaneously increased significantly probably because the acceleration of fold uplift may have formed a greater obstacle to the Manas River;Although both the rate of fold uplift and the rate of sedimentation in the synclinal area increased significantly at～120-40ka B.P.,their ratios may basically remain stable.Thus,growth strata intersected at one point forming a steady on-lap point on section;At～25-30ka B.P.,the climate transitioned from interglacial to glacial,and fold uplift slowed down.The T6 terrace of Manas River began to deposit and cross the entire anticline,displaying the over-lap relationship with the whole previous sedimentary strata.Meanwhile,the Manas River jiggled to both sides at a very high lateral erosion rate and formed a wide terrace planation surface.