Research On Characteristics Of Neotectonics In The Middle Section Of Longmenshan Structural Belt

The Longmenshan structural belt is located in the eastern margin of Tibetan Plateau and is still active that make itself the steepest topography of the border mountains around Tibet Plateau. The Neotectonics in the area is very strong which causes frequent geological hazard. However, there are still some geological questions such as the structural deformation and response of structural landform in late Cenozoic, initial time of strong uplift and model of orogenics.Focusing on the Neotectonics in middle section of Longmenshan structural belt and adopting multi-discipline and technology, This thesis conducts deep research on the tectonic landform, co-seismic surface deformation, gravel fabric characteristics of conglomerate layers, growth analysis of stalactite and seismic reflection profile, etc. the main results achieved on the characteristic of neotectonics in middle Longmenshan are as follows:(1) Distribution of active faults in the middle section of Longmenshan Mountain: the main active faults in the research area comprises of Wenchuan-Maoxian fault(F1), Yingxiu-Beichuan fault(F2), Guanxian-Jiangyou fault(F3) and Pengxian fault(F4). Their average trend is NE30°-50°with NW dip direction. The faults are imbricate arrangement with a steep surface dip and gradually meet together into the floor thrust fault in the depth of15km. The faults go along deep-cutting valleys, strap and large relief fluctuation.(2)Nature and strength of neotectonics in the middle section of Longmenshan structural belt: the strong neotectonics in the research area is revealed by the thrusting and dextral slipping of pre-existing faults. the co-seismic surface deformation of Wenchuan earthquake reflect that the motion of Yingxiu-Beichuan fault is thrust and dextral slip and motion of Guanxian-Jiangyou fault is thrust. The deep-cutting rate of river in Longmenshan is0.46mm/a.(3) Primary time of rapid uplift of middle section of longmenshan:ESR dating and gravel fabric characteristics of conglomerate layers demonstrate that Pengguan basement complex began to be eroded into surface in late Miocene and early Pliocene(5.4Ma of ESR date). The regional topography was large enough and accelerated to uplift.(4) Activity and period of earthquake in longmenshan area: the frequency of seismicity is lower and activity of seismicity is not strong. The destructive earthquakes(Ms≥4.7) mainly distribute along the faults. the temporal and spatial distribution is heterogeneous. The depth of earthquake focus is5-25km and display two depth strap of10and15km. the research of stalactite reveals that the seismicity in Longmenshan has an periodic characteristic and the period of large earthquake is2500a.(5) Uplift pattern of Longmenshan:the large elevation of Longmenshan is1000m higher than the averge elevation of Tibetan Plateau. Based on the result of stalactite and surface rupture of Wenchuan earthquake, the abrupt uplift rate caused by Wenchuan earthquake is0.8-2mm/a and the geodesic survey reveals the slow uplift of Longmenshan is0.3-0.4mm/a. it is clear that the uplift pattern of Longmenshan is the combination of abrupt uplift by earthquake and slow deformation and the dominant uplift is abrupt uplift by earthquake.(6)Evolution of middle Longmenshan structural belt:Longmenshan began to uplift and deposited continental sediments. The depositing center was located in the eastern section of Wenchuan-Maoxian fault. During Jurassic and Paleogene, Longmenshan structural belt continued to thrust into Sichuan basin. The depositing center migrate into around Guanxian-Jiangyou fault. During middle and late Jurassic, strong thrust into Sichuan basin made Longquanshan Pre-forebulge of Longmenshan. In Cenozoic, longmenshan continued to thrust into Sichuan basin. Chengdu basin became the depositing center and Longquanshan was the forebulge of Sichuan basin, in late Miocene and early pliocene, the middle section of Longmenshan began rapid uplift and SE thrusting. In Middle Pleistocene, there was also a strong thrust into Sichuan basin in middle Longmenshan. In late Pleistocene, the thrust began to grow weak.(7)Dynamical mechanism in the middle section of Longmenshan structural belt: As the general vertical uplift of Tibetan Plateau since Pliocene, Bayanhar block began SE-direction escape along the pre-existing faults and the escape motion is hindered back in Longmenshan area by Sichuan basin which leads to strong thrust of Longmenshan faults system, overlay of strata, uplift of elevation and vertical acceleration. in this way, vertical deformation and acceleration accommodates the horizontal shortening of crust.