Response Of Piedmont Landforms To Tectonic Activity And Climatic Transitions Since The Late Pleistocene

Evolution of the piedmont landform not only reflects the deformation characteristics of tectonic activities,but also an important record carrier of the climate fluctuations.How to identify and evaluate the contribution of climate and tectonic in the evolution process of geomorphology is the key to quantitatively study the tectonic components of the piedmont tectonic geomorphology.The Tianshan orogenic belt is in the hinterland of Central Asia.It can be divided into SW Tianshan,the Western Tianshan and the Eastern Tianshan by geographical distribution from west to east.Since the uplift of Tianshan orogen was revived by the extrution from Qinghai-Tibet Plateau,there has been a significant difference in deformation pattern between the Eastern and the Southwestern Tianshan since the late Pleistocene.A multi-stage alluvial fan surface has been developed in the southern margin of the Kepingtage SW Tianshan and in the piedmont of the Balikun Mountain NE Tianshan.The evolution of these tectonic alluvial fans is an invaluable for us to quantitative the neotectonic deformation features of the Tianshan Mountains since the late Pleistocene.This thesis is based on the typical tectonic geomorphology of the southern margin of the Kepingtage SW Tianshan and the piedmonts of the Balikun Mountain NE Tianshan.With the geology mapping,s UAV structural geomorphology mapping combined with the10 Be exposure dating methods,paleo-seismology and sedimentary dating method and the paleoclimate and tectonic record data.Based on the comparison of the origin,the tectonic deformation characteristics,the 10 Be exposed age and paleoclimatic records,a fine tectonic activity and the age of the paleo-seismology events obtained by the paleo-earthquake trench,We have determined the recurrence behavior and the deformation rate of different sections of the Kepingtage Fault.And we obtained the activity and deformation rates of thrust faults and folds at the piedmont of the Balikun Mountain and the central of the Balikun Basin NE Tianshan.The main understandings of this paper are as follows:First,10 Be exposure ages of 24 alluvial fans both in the two study areas are concentrated in ~10ka,~30ka,~50ka,~90ka and ~150ka of five communities,and the age of the oldest age is 163.91±15.41 ka from the top of Kusutai Fold.Secondly,the alluvial fans form during ~20ka to ~160ka,the exposure age is roughly matched with the northern hemisphere climate cycle interval driven by the precession of 19~23ka,indicating that the abandonment of these old alluvial fans is mainly controlled by climate fluctuations.Moreover,the abandoned age of alluvial fans is also obviously synchronized with the humid period in the climate records such as deserts,lakes and river terraces,suggesting that climate fluctuations.The fan formed since ~20ka is the most direct response to the tectonic uplifting activity.And the age of tectonic event is closer to the10 Be exposure age of the new alluvial fan.Third,based on the GIS processing of 30 m resolution ASTER GDEM topographic data,it can be seen from the distribution of HI index of different alluvial fans in piedmont.The high-value HI area in the eastern of Barkol Basin is consistent with the distribution of the height tectonic uplift rate,which are located in the area of? ? Kuisu-Yamizitai.The distribution of HI index value in the Kepingtage piedmont alluvial fans is consistent with the fault activity rates.In the front edge of the alluvial fan,high-value areas of HI may be related to the thrust rupture activity that migrates southward in front of the mountain.Fourth,at least two events with the recurrence cycle of ~3kyr of the fault is recorded in the Xikeer in the western section,and the vertical slip rate is0.33~0.53mm/a with the shortening rate is about 1.1~1.7mm/a since(5.1+0.9-1.9)ka.At the Sanchakou,at least 3 ancient earthquake events with the average recurrence period is ~2kyr have been recorded on faulted alluvial fan.The vertical slip rate is0.41~0.65mm/a,and the shortening rate is 1.1~1.8mm/a since(5.8+1.5-1.2)ka of the fault in the eastern section.Based on the results of paleo-seismology and chronology analysis from the trench,the maximum value of the left-lateral strike-slip rate of(4.04~4.72±0.5)mm/a since(60.60±5.96)ka in the western segment of the BSF Fault.And in the eastern section the shortening rate is 3~4mm/a with a dip angle of 45° in? ? Yemizitai-Kuisu since ~40ka.Similarly,the recurrence interval of earthquake of HNF Fault in the Dewaili is(6.6±1.9)kyr and shortening rate 0.76~0.96mm/a.Fifth,based on the quantitative analysis of faulty alluvial fans in response to tectonic and climatic effects,we found that under the division of the NNW Piqiang Fault,the slip rate in Kepingtage Eastern section is close to West since ~6ka.And,under the regional compression stress,the tectonic activities are superimposed to form a horizontal shortening rate of 4.5~5mm/a from the Kuisu to Dewaili at the middle part of the Balikun Mountains.The horizontal slip component transmitted by the left-lateral strike-slip fault system on both sides of the Balikun Mountain is3.1~4.6mm/a,while the horizontal slip component gradually decreasing to the east and west sides of the Balikun Mountain may be absorbed in sequence by the thrust structure developed along the left-lateral fault.Finally,for the lack of high-resolution of DEM topographic data in the two study areas,the extracted basins can't reflect the structural effects at the ~10m magnitude.And the paleo-earthquake records in the two study areas lacked more detailed chronological control since ~20ka,so it is difficult to effectively distinguish with the climate fluctuation events.Therefore,if we can further supplement the relevant parameter information of paleo-earthquake records and tectonic geomorphic deformation features in the study area,it will provide more abundant and reliable evidence support for the discussion of the more complex evolution model of the tectonic geomorphology with coupling effects both from tectonic and climate since~20ka.