Activity Of The Eastern Liupanshan Piedmont Fault Zone Since Late Quaternary

The eastern piedmont fault zone of Liupanshan is an important part of the north-south tectonic belt. The geological structure and GPS velocity profile has a lot of similarities with Longmenshan fault zone. The occurrance of Wenchuan ML8.0 earthquake suggested that we can not neither ignore the weak velocity gradient changes on both sides of the fault zone, nor underestimate this type of fault of seismic hazard. Therefore, the further study of the activity of the eastern Liupanshan piedmont fault zone can provide important base data for regional seismic hazard assessment. In this paper, the most important work is:(1) Establish the landscape space and time sequence of LiuPanShan area,(2) Find new evidence of recent fault activities and calculate the parameters of fault activity,(3) Build structure uplift rate difference model, bring in the structural uplift rate parameters(?UT), and the geomorphology parameter Channel steepness index( ksn), analyse of regional uplift activity,(4) Analyse the vertical movement velocity of Liupanshan tectonic belt and the vertical movement and deformation characteristics of Liupanshan piedmont fault zone between the north and the south section, based on the regional precise leveling data and different extents geological profile With these parameters, we assess the eastern Liupanshan piedmont fault zone tectonic uplift activity and tectonic deformation, and we discussed the phenomenon of the weak velocity gradient changes on both sides of the eastern Liupanshan piedmont fault zone.Following is the mains results of this article,(1) There are two secondary faults Caohecun fault(f1) and Baojiabu fault(f2).They are between the Haiyuan fault and the eastern Liupanshan piedmont fault. Their activities mainly are left-lateral strike-slip properties. They may have inherited the tail end of the Haiyuan fault activity habits.(2)The length of eastern Liupanshan piedmont fault is approximately 90 km,the main trend is SN. The properties of the fault is mainly the thrust and left-lateral strike-slip. The research gets the geometry and the subsection of the eastern Liupanshan piedmont fault with remote sensing image interpretation, DEM analysis, field investigation. The trend of the north segment(F1) is 320°-330°. The properties is left-lateral strike-slip and high-angle thrusting. The trend of the middle part(F2) is 350°-SN, and main properties are high-angle thrusting. The trend of the south segment(F3) is 330°-340°.There are thrust structural system in the bedrock and high-angle thrust appeared in the Quaternary sediments.(3)In the middle part(F2), we got the average vertical active rates about 0.55mm/a since the late period of late Pleistocene. In the north segment(F1), we got a Horizontal slip rate about 0.92-1.16mm/a since the late period of late Pleistocene, and about 0.31-0.41mm/a vertical active rates since the Holocene.(4)We collect the previous study achievements of the Yellow River terraces of the east and west Liupanshan, the Weihe River terrace and the Jing River terrace, meanwhile, we incorporate the formation age according to the paleomagnetic and 14 C methods, the measurements of tug-of-war and so on. Based on these data, we build a tectonic differential uplifting rates(?UT)model to reflect the regional activity and the intensity of tectonic uplift based on the parameters of terraces in both the east and west sides of Liupanshan. First of all, we compared the tectonic differential uplifting rates of the Jing River with the Lanzhou segment of Yellow River terraces, the results show that there are four times of ?UT, which are 76.87 mm/ka, 42.88 mm/ka, 421.06 mm/ka and 1050mm/ka. The average of the four rates is 397.70mm/ka. Then, we compared the tectonic differential uplifting rates of the Jing Rive with the Weihe River terraces of Longxi segment, the results show that there are three times of ?UT, which are 177.37 mm/ka, 171.83 mm/ka and 168.27mm/ka, and the average rate of them is 172.49mm/ka. Two sets of date in each time have been shown positive. The result implies that the activity of the west Liupanshan is stronger than the east, meanwhile, the north section of Liupanshan tectonic belt is more active than the south section and increasing gradually.(5)We extracted the Channel steepness index(ksn) of Liupanshan using the equilibrium channel longitudinal profiles model. The result shows that the channel steepness index of the north, middle and south section is respectively 65.11,73.53 and 71.76. The change of the Channel steepness index(ksn) mainly influenced by regional uplift rate differences and other factors, such as climate, Lithostratigraphic, rainfall and so on. The change of the Channel steepness index indicates the distribution feature of the different region Structure uplift rate. The uplift rate of northern section is low and the regional activities is weak. The uplift rate of middle part is fastest and the region activities is strong. The south part is between these two parts.(6)We obtained fault activity parameters, tectonic differential uplifting rates(?UT), channel steepness index(ksn), precise leveling measurement, cross fault leveling, different segment fault-crossing geological profile and other datas by geological survey and analysis. These parameters reflect that the activity of the west Liupanshan is stronger than the east,the tectonic deformation and activity of the north and middle segment is becoming stronger. The current GPS velocity profile shows that the eastern piedmont fault zone of Liupanshan has weak velocity gradient but have obvious phenomenon of stress accumulation and may have obvious stress transformation. Regional horizontal motion mode is the vertical movement and different extents of Liupanshan both sides of the fault has different vertical movement intensity.