A Fluvial Geomorphology Research About The Late Quaternary Deformation Of The Southern Longmen Shan And Its Foreland Basin

As the eastern boundary of the Qinghai-Tibet Plateau and the contact zone between the Bayanhar block and the Sichuan basin,Longmen Shan experienced an intensive uplift since the Miocene period.Hence it became the steepest mountain belt in the world.In the eastern edge of the plateau,the altitude reduces from 5.0 km to 0.5 km in a distance of 50 km.Longmen Shan has become an important place to study the uplift mechanism of the Qinghai-Tibet Plateau eastward movement and the eastern margin of the Qinghai-Tibet Plateau.There are two kinds of end-unit models for the deformation mechanism on the eastern margin of the Qinghai-Tibet Plateau:one is the continental escaping model,which is dominated by the crustal shortening,and the deformation of the eastern margin of the Qinghai-Tibet Plateau is mainly concentrated on the important active boundary faults.Another is the lower crustal flow pattern,indicating that the eastern part of the Qinghai-Tibet Plateau is controled by a holistic uplift deformation.The rationality of these two models needs to be measured by the construction of quantitative data.In the structural style,there are obvious north-south segmentation characteristics of Longmen Shan.The tectonic assemblage of the middle and northern part of the Longmen Shan is simple,mainly composed of three fractures,such as the back range fault?Wenchuan-Maowen fault?,the central fault?Beichuan-Yingxiu fault?and the front fault?Guanxian-Anxian fault?.But the thrusting structure in the southern part of Longmen Shan is composed of two parts:the thrust belt and the foreland fold belt.Besides,the northwest side of southern Longmen Shan is still connected with the Jintang arc-shaped tectonic belt,which makes the tectonic pattern more complex.The Wenchuan Ms 8.0 earthquake formed a 270 km long rupture zone along the middle part of the Longmen Shan,forming a few meters high fault scarp along the central rupture and the front mountain fault,providing direct evidence of the latest activity of the Longmen Shan.But the Lushan Ms 7.0 earthquake in the southern part of Longmen Shan in 2013 did not produce significant surface rupture,and it caused troubles for the study of seismogenic tectonics.How the Lushan earthquake is controlled by the thrust structure in the southern part of the Longmen Mountains,although widely concerned,is still difficult to discuss considering the absence of active tectonic proofs.At present,the study of the late Quaternary activity in the southern section of the Longmenshan fault zone is limited to the Dachuan-Shuangshi and Yanjing-Wulong faults.According to the deformation of the Qingyi River terrace,the average vertical velocity of the Late Quaternary in the Yanjing-Wulong fault is about 0.6¹.2 mm/yr,and the Dachuan-Shuangshi fault has no obvious vertical dislocation.The sum of the thrusts of these two faults is significantly smaller than the total vertical displacement rate of about 2³ mm/yr in the middle of the Longmenshan fault zone.In view of the roughly equal terrain of the southern part of Longmenshan and the middle section of the Longmenshan fault,it is the most basic structural problem in the region whether the thrust deformation in the southern section of the Longmenshan fault zone is likely to be distributed to other branch faults or foreland fold belts.Therefore,the quantitative study of the Late Quaternary tectonic activity in the southern part of Longmenshan and its precursors will not only help to understand the orogeny in this area,but also provide basic data for assessing the future earthquake hazard in the area.The Qingyi River is a large river flowing through the Longmen Shan mountain belt and its foreland area,which developed a multi-level basin-wide terraces,and in the foreland area formed two ancient alluvial fans named Mingshan-Qionglai Fan?MQF?and Danleng-Simeng Fan?DSF?.In this paper,the detailed investigation,measurement,contrast and dating studies of these river topography are carried out,and the deformation of the Late Quaternary tectonics in the southern part of Longmenshan and the foreland region is studied quantitatively by the deformation of river geomorphology.Besides,the deformation mechanism and development trend of southern Longmen Shan are further discussed.The main application methods in this paper include:river sediment element analysis,heavy mineral analysis,¹⁴C,OSL,ESR and other chronological tests,geomorphic high-precision DEM analysis and interpretation,network RTK high-precision field measurement provided by the Sichuan Surveying and Mapping Company?the accuracy of mobile measurement is±5 cm horizontally and±8 cm in elevation?.The main progress of the paper is as follows:?1?On the basis of morphological survey,alluvial contrast and chronological test,the geologic facies sequence of the Quaternary rivers of Qingyi River was established.Through the DEM to restore the Mingqiong and Dansi ancient alluvial fan surface morphology,and based on heavy mineral analysis,the Mingqiong and Dansi fans were determined to have derived from the ancient Qingyi River system.During the early and middle Pleistocene?537±64 ka³05±49 ka?,the Qingyi River formed the Qiong alluvial fan,which distributed between the Longmen Mountain and the Xiongpo anticline.During the middle and late Pleistocene?305±49 ka³00±60 ka?,the ancient Qingyi River rechanneled,as the Mingqiong alluvial fan abandoned,Qingyi River formed the early Dansi alluvial fan.At the same time,the T₆ terrace developed in the upper reaches of Qingyi River.During the middle and late Pleistocene?300±60 ka²68±54 ka?,the Danish alluvial fan was further developed and expanded,and the T₅ terrace was developed in the upper reaches of the Qingyi River.At the end of the Pleistocene?185±19 ka?,the Dansi alluvial fan has been completely abandoned.The Qingyi River formed the latest valley landscape and followed by the development of T₄^T₁ terraces,the abandoned age of terraces were 185±19 ka¹63±26ka,93±10 ka⁷3.08±9.73 ka,51.36±7.35 Ka⁴0.36±4.98 ka,7.63±0.58⁵.91±0.03 ka.?2?Based on the elevation measurement,contrast of alluvial sediments,and chronology test,longtitudinal profile of T₁^T₆ terraces along Qingyi River is established.And the characteristics of the Late Quaternary tectonic activity in the southern part of Longmenshan and its foreland region are indicated,according to the deformation of the river terrace in the whole river basin.It is found that the fault zone in the southern Longmenshan section is dominated by thrusting,and the foreland area is mainly composed of fold and deformation,while the Jintang arc-shaped fault zone has no obvious activity.The average vertical slip rates of the Gonda fault,the Longdong fault,the Yanjing-Wulong fault and the Xiaoguanzi fault in the late Quaternary are 0.21⁰.30 mm/yr,0.12⁰.21 mm/yr and 0.10⁰.12 mm/yr,respectively.And the Quaternary activity of Dachuan-Shuangshi fault is not obvious.The crustal shortening rate of the thrusting structure in the southern Longmen Shan section is 0.48⁰.77 mm/yr,and the crustal shortening rate of the folding structure in the foreland area is 2.38³.78 mm/yr.The total crustal shortening rate is 2.86⁴.55 mm/yr.It indicates that,the foreland area of southern Longmen Shan absorbs most of the crustal shortening,therefore its future earthquake risk can not be ignored.?3?The fault ramp model of the Lushan earthquake in 2013 is proposed.According to the longitudinal profile of the Qingyi River terrace,it is found that in the Lushan earthquake area,the late Quaternary deformation is characterized by a new anticlinal fold.Considering the deep geophysical profile,uplifting of the fold may be dominated by a flat-ramp-flat structure,and the late Quaternary shortening rate is estimated to be 0.45¹.14 mm/yr.The deformation characteristics of the anticline are consistent with that of the co-seismic ground deformation in Lushan earthquake.According to the characteristics of earthquake relocation and surface deformation,it is presumed that the fault slope constitutes a latched structure,so the earthquake rupture occurs along the fault slope.The upper flat continue to extend to the foreland region,forming a decollement structure which controls the foreland fold belt.?4?In the south of the Xiongpo anticline,a new east-dipping Quaternary thrust fault is found,which named Danling Fault.And the western boundary of the Dansi alluvial fan is controlled by the southern section of the fault.In the north section,it formed a fold uplift.According to the quantitative analysis of the displacement of the alluvial fan,the late Quaternary slipping velocity of the Datang fault on the north side of Xiongpo anticline is determined to be 0.12⁰.15 mm/yr.As a conclusion,the existence of two north-south faults in the foreland area of the southern Longmen Shan indicates the easternward extension of the Daliang Shan area on the eastern side of the Sichuan-Yunnan block,which makes the foreland area of the southern Longmen Shan obtain a superposition extrusions in both the NW-SE and the E-W directions.The main innovations of the paper are as follows:?1?For the first time,the chemical weathering indexes of alluvial sediments are used for quantitative comparison of river geomorphology,and the terraces in the whole river basin is measured by SCGNSS network RTK with centimeter accuracy.The CIA,A-CN-K and other chemical weathering indicators derived from bulk and trace elements can be used as a quantitative basis for comparison of river terrace.The scatter measurement accuracy of the SCGNSS network RTK system provided by Sichuan Survey and Mapping Bureau is±5 cm horizontally and±8 cm in elevation,which allows the high precision study of the terrace deformation in the whole river basin and the single fault slipping analysis locally.?2?Based on the longitudinal profile of the Qingyi River terrace,the late Quaternary deformation of the Lushan earthquake area is characterized by a new anticline uplifting,which coincides with the coseismic deformation characteristics of the Lushan earthquake in 2013.But the new uplifting structure is different from the deformation pattern of the Paleogene strata in the area.Therefore it may be a newly borned anticline which is controlled by a fault bend fold.?3?A new N-S striking reverse fault with late Quaternary activity was detected distributing between the Xiongpo anticline and the Sansuchang Anticline,which was named Danling fault.