Study On The Activity Of Beichuan-Yingxiu Fault In The North-central Section Of Longmen Mountain In The Late Quaternary

LongmenShan is located on the eastern boundary of the Tibetan Plateau,one of the steepest terrain gradient belt in the world.The elevation difference is?4.5kilometers from the Plateau to the western boundary of the Sichuan Basin.It is the steepest terrain gradient belt in the world.The peculiar and complicated topography and geological structure features have led to extensive researches on the eastward extension of the Plateau.There are two endmember hypotheses about the deformation mechanism of the eastern margin of its Plateau:the continental escape and the lower crustal:flow.The LongmenShan fault zone is an active boundary fault belt.The quantitative study of fault behaviours is helpful to verify the models.In the middle and northern sections of the LongmenShan,three fault branches,the Houshan fault(Wenchuan-Maowun fault),the central fault(Beichuan-Yingxiu fault)and the Qianshan fault(Guanxian-Anxian fault),consisted an imbricated thrust belt.The topographical features of the central and northern sections of the Longmen Mountains are obviously inconsistent.The middle part is high and steep,while the north part is much low and gentle.The coseismic surface ruptures of the 2008 Wenchuan Ms8.0earthquake is much similar to the orographic segmentation.The earthquake formed a 240-km-long and 72-km-long surface rupture zone along the central and the Qianshan faults separately.The coseismic displacementsare mainly thrusting in the middlesegment,and right-laterallyslipping in the northern segment.whether or not it indicates the fault behavior of the Longmen Shan fault zone,is equally important to understand the mountain building and the earthquake recurrence.By now,the late Quaternary activity study of the middle and northern sections of the LongmenShan fault zone is far fromsystematic and in-depth.The average vertical slip rate of the three fault branches in the late Quaternary is about 0.2-0.6mm/a and the right-lateral strike-slip rate is about 1mm/a based on the offset Minjiang river terraces.The lateQuaternary average vertical and strike-slip rates of the three faults in the middle section of the LongmenShan are estimated to be?1 mm/a based on offset alluvial fansand gullies.The above work considers that the long-term activity of the entire Beichuan-Yingxiu fault is strike-slip and thrust,but in the Wenchuan earthquake,the southern section is thrust and slippery,and the northern section is strike-slip and thrust.If the understanding of the late Quaternary long-term activity behaviour is correct,then the earthquake misplaced behavior is variability.The correctness of this inference depends on a more systematic study of fault slipping rate or ancient earthquake research,especially in the northern section of the LongmenShan fault zone where data is scarce.The geomorphic index of various watersheds can reflect the structural uplift characteristics of different time scales in the Quaternary,while the river terraces are ideal fault geomorphic landmarks.In this paper,the differences of tectonic activities in the middle and northern sections of the LongmenShan are analyzed from the perspective of geomorphological parameters.The detailed investigation,measurement and dating studies of the river terrace in the northern section of the Beichuan-Yingxiu fault are carried out,and the probability density function method is used to estimate the faults slipping rate and its error.The paper has achieved the following understanding:(1)Through GDEM V2 data and Arcmap 10.1 platform,the geomorphic index of the middle and Northern LongmenShan is studied.The distribution maps of slope,relief and area-elevation integral are obtained.The response of geomorphic index to tectonic uplift is further analyzed.It shows that the tectonic uplift of the middle LongmenShan is stronger than that of the northern LongmenShan.(2)Four fault geomorphic locations in the northern section of Beichuan-Yingxiu fault are selected to carry out high-precision measurement of UAV,and the dislocation of steep slopes of different terraces is obtained by using probability density function method and ground shortening effect analysis.The horizontal displacements of the T1 and T2 terrace scarp,the T2/T3 terrace scarp and the edge of the T3 terraces of the Fenghuang Village River are?12.9m,?17.8m,?30.4m,respectively,and the vertical displacement of the T3 terrace is 4.5±0.5m,and the vertical displacement of T2 terrace is 2.6± 0.6m.According to the age of the trench samples on the T3 terrace,the Holocene strike slip rate of the Fenghuang Village fault section is estimated about 2.3±0.02mm/a,the vertical slip rate is estimated about 0.56±0.07mm/a,and the crustal shortening rate is estimated about 0.65± 0.07 mm/a.