Study On The Tectonic Geomorphology Feature And The Strain Differential Transfer Mechanism Of The Longmenshan

The collision between the Indian plate and the Eurasian plate not only formed the current plateau,but also led to the rapid uplift of the Longmenshan on the eastern edge of the Tibetan Plateau.The Longmenshan fault zone has strong activity.In less than five years,the 2008 Ms 8.0 Wenchuan earthquake and the 2013 Ms 7.0 Lushan earthquake occurred,which attracted wide attention from the geoscience community.The geology and geomorphology of this area are remarkable,and the water systems are well developed.It has become a typical area for studying the tectonic-geomorphology-water systems evolution and surface processes.In this research,the tectonic geomorphic differences of the Longmenshan river basin area are systematically analyzed.On this basis,the strain differential transfer mechanism and tectonic geomorphological response model of the Longmenshan are established.This study mainly draws the following conclusions:(1)Using multiple geomorphic parameters(elevation,average slope,relief,hypsometric integral,mathematical fitting function of river longitudinal profile,hydraulic erosion model),combined with predecessor data,this study shows that the tectonic geomorphology of the Longmenshan watershed area has obvious differences,and the main gains are as follows:(1)In terms of deformation,the middle and southern segments of Longmenshan are dominated by thrust,whereas the northern segment is dominated by strike-slip.(2)In terms of uplift strength,the middle and southern segments,and Minshan have a strong uplift(the tectonic uplift strength of the middle segment and Minshan is stronger than that of the southern segment),whereas the northern segment has a weak uplift.(3)In terms of the topography,the middle and southern segments and Minshan areas have higher and steeper geomorphology(the middle segment is the steepest,and the southern segment is slower and wider than the middle segment),and the main influencing factor of geomorphological evolution is tectonic uplift,which is in the development stage of Ohmori mode;whereas the northern segment is relatively low,and the main influencing factor of geomorphological evolution is rainfall,which is now in the recession of the Ohmori model.(4)The overall tectonic uplift of the Minjiang River(This article refers specifically to the Minjiang River basin on the west side of the Sichuan Basin)is weak,and it is now in the maturity stage of the Strahler model.With the trace erosion of the Minjiang River,it develops into the old stage.Based on the Burbank's fault connection growth model,the activity of the middle segment fault zone is analyzed,which indicates that most of the strain in middle segment may be transferred to the Beichuan-Yingxiu fault,whereas the Maoxian-Wenchuan fault is less active in the late Quaternary.(2)Based on the obvious differences in uplift strength,topography and deformation characteristics of the Longmenshan watershed,the strain differential transfer mechanism between the Longmenshan segments is established,by analyzing the principal stress direction,seismic interpretation profile,sandbox simulation results,shallow detachment distribution data,seismic data and the data of liquefaction caused by the Wenchuan earthquake:(1)The main influencing factor of strain differential transmission between the northern and southern-middle segments may be the difference in the direction of strain transfer(the middle-southern segments is perpendicular to the fault zone,the north segment is parallel to the fault zone)and the difference in the total amount of strain(due to the blocking effect of the Minshan,the total strain in the northern segment is smaller than that in the middle-southern segments).(2)The main factor for the differential transfer of strain between the middle and southern segments may be the thickness of the Triassic gypsum rock layer(shallow detachment);the thicker detachment in the frontal area of the southern segment leads to its strain to be easier to transfer to the basin which is reducing the activity of the faults in the southern segment.(3)Based on the strain differential transfer mechanism in the middle and south segments of the Longmenshan,the formation mechanism of the ‘seismic gap' in the southern segment and the tectonic geomorphological response mode of the strain differential transfer in the Longmenshan are studied,and the main gains are as follows:(1)Based on the seismic capacity analysis of the Shuangshi-Dachuan fault and range front blind thrust(RFBT)in ‘seismic gap',the seismic capacity of the ‘seismic gap' is likely to be less than Ms 7.0.(2)The rock mass of the ‘seismic gap' is weaker than that of the south side,which leads to its strain to be easier to transfer to the basin,which may be the best cause of fault-related fold development in the middle of the Longquanshan.(3)The Longmenshan strain differential transfer mechanism: the differential strain transfer is mainly controlled by four factors including the strain transfer direction,the total amount of strain,the strength of rock physics and the thickness of the shallow detachment in the leading edge area.