The Study On The Activity Of Xigeda Fault In Sichuan During The Late Quaternary

The Xigeda fault is located on the southwestern edge of the South-North seismic belt in China.Inside the Sichuan-Yunnan block,it is a large fault dominated by left-lateral strike-slip.At present,the arguments about the characteristics of the activity and the genetic mechanism since the Quaternary of the fault are still insufficient.Therefore,this article will take the Xigeda fault as the research object,explaining the characteristics,the genetic mechanism of the Xigeda fault since the Late Quaternary,and the relationship with the activities of the Sichuan-Yunnan block,by using technical methods such as structural geomorphology,plate kinematics and geophysics.This article has investigated a large number of predecessors’ research materials and regional background data,and has carried out sufficient indoor and outdoor work.According to the geological and geomorphological conditions,combined with the analysis of time samples,trenches,electrical exploration,modern tectonic stress field,GPS velocity field,etc.,this paper has gained the following points:(1)The strike of the Xigeda fault is generally in the NNW direction,and the overall trend of the section is east.The fault-breaking late Pleistocene-Holocene stratum can be seen in many places along the line.The fault is a late Pleistocene-Holocene active fault,and the overall performance is dominated by left-lateral strike-slip and has the characteristics of extrusion thrust.With the Xinjiu as the boundary,the Xigeda fault is divided into the southern and northern segments(including the eastern branch and the western branch).The southern segment of the Xigeda fault(Xinjiu-Jiangbian segment)has an average horizontal slip rate of about 0.4 mm/a and a vertical slip rate of about 0.3 mm/a since the late Pleistocene.For the eastern branch of the northern segment of the Xigeda fault,the analysis of the trench in Haita Village indicates that the fault has displaced the Holocene lacustrine sedimentary layer,and the latest earthquake event time is 240±30a BP.For the western branch of the northern segment of the Xigeda fault,the fault mud TL dating of the fault outcrop is 85600±7000 a.(2)Based on the analysis of the spatial differences in the relevant geomorphology,fault space distribution and activity of the Xigeda fault,this paper segmented the Xigeda fault: There is obvious bifurcation in the fault near Xinjiu,and the fault extends east and west branches to the north.With the new nine as the boundary,the Xigeda fault can be divided into the northern section(including the eastern branch and the western branch)and the south section.The western branche is a late Pleistocene active fault;the eastern branch is a Holocene active fault;the southern section is a Holocene active fault.(3)According to the regional stress field of the Xigeda fault and the GPS velocity field of the Sichuan-Yunnan block,the genetic mechanism of the Late Quaternary activity of the Xigeda fault is analyzed,and the following conclusions are obtained:(1)For the modern tectonic stress field of the Xigeda fault,the horizontal principal compressive stress is NNW-SSE direction,and the horizontal principal tensile stress is NEE-SWW direction.This tectonic stress field causes the Xigeda fault to be dominated by left-lateral strike-slip,supplemented by thrust.(2)Due to the squeeze from the west to the east caused by the plateau uplift and the blocking effect of the South China plate,the Sichuan-Yunnan block is characterized by the movement of the SE direction and the clockwise rotation.The Xigeda fault shows the activity characteristics of left-lateral strike-slip and squeeze thrust in the background of this block activity.