Study On The Structural Geomorphology Of The Southern Section Of The Longmenshan Tectonic Belt

Longmenshan Thrust Belt (LTB) locates between the Tibet plateau and Sichuan Basin. Since Mesozoic period, there were at least two shortening events that made the LBT a typical thrust belt. The 2008 Ms8 Wenchuan earthquake indicates that the LBT is an active seismic zone. After Wenchuan earthquake, some studies showed that the Coulomb stress increased in the South LTB, which means the seismic hazard level in the S LBT increased. In 2013, a Ms7.0 earthquake shocked Lushan County seems confirmed this point. The Lushan earthquake happened in the foreland area of LTB and some studies show that area is still high in earthquake hazard. But the studies of this area are rare and show that Yanjing-Wulong Fault were active in late Pleistocene and Shuangshi-Dachuan Fault occurred an earthquake in Holocene and produced surface rupture.Fluvial terrace is a great object for active tectonic study. By means of measuring the deformation styles(offset, title, warp and et al.), we can informed the active tectonic characters of the study area. In the South LTB, Qingyijiang River cuts across Longmenshan in its upper stream and generate four terraces which provide a good opportunity for this study. Through measure the terrace altitude and height above the river precisely, and in combine with geomorphic index extracted from DEM, this conclusions are generated:(1) Most of the fluvial terraces cross the upper stream of Qingyijiang River are preserved in the small basins of Longdong, Wulong, Muping and Lingguan and only four terraces can be distinguished. T1 is aggradational terrace and the sedimentary structure is clear, the gravel are sub-rounded with sandy lens. T2-T4 are covered by slope deposit and soil and the fluvial deposit were exhausted. Upper Wulong, the terrace above river bed is 3-5m,10m,30m and 80m respectively, between Wulong and Xiaoguanzi the terrace above river bed are 10m,20m,60m and>100m, down Xiaoguanzi, arc 7m,20m,40m and>60m.(2) Hypsometry index and river bifurcation ratio indicate that the geomorphology is less evolved in the north-west of Gengda-Longdong Fault and is most evolved in the south-east of Xiaoguanzi Fault. In between the geomorphology changes gradually.(3)The topography between Zhonggang Fault (Yongfu Fault) and Xiaoguanzi Fault is contrast to adjacent area in the respect of altitude, relief and slope which implys that the hinterland of South LTB. Along Yongfu Fault, relief and slope are of high value while altitude is decrease. In topographic swath profiles, the area between Longdong Fault and Wulong Fault is high altitude while relief and slope value are low. But the relief, slope and topography value are high from Wulong Fault and Xiaoguanzi Fault expect where is affected by river.(4)In between Wulong Fault and Xiaoguanzi Fault, the topography is warping, especially in swarm profile p2. And this feature is in accordance with slope and relief. Change of SL/K index is similar to them. R2-R4 SL/K index is greater than upper and down stream in between Wulong Fault and Xiaoguanzi Fault, which means a steep river channel. There are no correlation of SL/K and lithology, means that the change of channel steepness is due to tectonics.This implies that a blind tectonics between Wulong Fault and Xiaoguanzi Fault lead to the warp of fluvial terrace, steepness of river channel and high altitude. In the southeast of Xiaoguanzi Fault, the terraces are not deformed, may means the Xiaoguanzi Fault has some impact on the blind tectonics. So we should pay attention to the blind tectonics and Xiaoguanzi Fault.