Influence Study from the Activity of the Northern Margin Fault Zone of Qinling Mountains on Geological Hazard
The northern margin fault zone of the Qinling Mountain is located between the Qinling Mountain orogenic belt and Weihe basin.This fault zone is a boundary fault in terms of tectonic,and also an important activity fault in terms of seismology.Xi’an,which is the capital for thirteen dynasties,is located there and also a major strategic starting point for China’s "One Belt And One Road" countries.The activity of the fault zone is a direct threat to the security of the region.The mode of movement,the active period and the movement mode of the zone remain controversial.In this paper we used the method of geotectonics research,combined advanced research techniques,to: 1)depict the fault structure characteristics;2)get the late quaternary activity times;3)reveal the regional movement model of the northern margin fault zone of the Qinling Mountain.Moreover,we attempt to provide a scientific basis for regional stability.On the basis of field geological investigation,and the outcrop and typical section of observation and analysis,using optical stimulated luminescence dating(OSL),thermohtminescence dating techniques(TL),electron spin resonance(ESR),magnetic fabric,regional fracture physical mapping analysis,and 3D crustal movement data(GPS),combined block method means,we discuss the structural feature of the fault zone,adjacent regions kinematics model and fracture effect of disasters.According to systematic research and analysis,this paper has the following main achievements:(1)According to the detailed field geological survey,the northern margin fault zone of the Qinling Mountain is mainly oriented towards the near east-west direction,which trends to be north,and there is a partial inversion phenomenon.It can be divided into three parts: western,middle and eastern.According to the comparison of the typical section of the field and the characteristics of scratches,it is found that the faults in the northern margin of the Qinling Mountains is in the early stage of the early adverse shock.Scratch based on fault near the conjugate joints and statistics of tectonic stress inversion,found near ns compressional,nearly east-west extrusion and nearly south-north extension,it is consistent with the formation of three types of faults.(2)The loess magnetic fabric study of Kou Town fault in the northern Weihe basin suggests that the fault is a synsedimentary active fault.The hanging wall is active.Loess magnetic fabric in footwall has only one direction which represents the palaeowind direction.Magnetic fabric features reflect three tectonic stress in three periods that nearly north,north-west and south-west direction.It show that regional stress changes exist multiple phase change.(3)According to the age data of this age determination and collection,it was found that there were seven active periods in the faults in the northern margin of the Qinling Mountains faulting the late Pleistocene.The time interval between activities has been shortened since the Holocene.It show that the activity of the faults in the northern margin of the Qinling Mountains fault is strengthened,and it is coupled with the Holocene uplift rate in East Qinling Mountains.There are seven stages of fault activity,which prove that there should be diversity in the regional movement pattern.(4)Based on the field geological survey and the mapping of the regional fault system,it is found that there are five major motion systems in the study area,and the east-west left strike-slip movement system is an important model leading to the expansion of the Weihe basin.The northwest-south-eastern strike-slip system may be related to the compaction and dispersion of the Ordos basin,and the right-hand strike-slip system of the Kouzhen-Guanshan fault may represent the relative eastward movement or even rotation of the Ordos block caused by the Liupan Mountain orogeny.The North and South compressional movement system of the East Qinling Mountains leads to the strong bending of the faults in the northern margin of the Qinling Mountains,and maintains the relative uplift state of the Huashanand Xiaoshan Mountains.The dynamic wedge extrusion system of the Middle Qinling Mountains is consistent with the present GPS data.It represents the current state of compression and convergence in the Weihe basin.(5)Combined with GPS vector data and fault system,it is found that the present tectonic stress field in the central and eastern part of the southern edge of Ordos is a whole squeezing stress environment,and the wedge block extrusion escape with squeezing.The escape of wedge blocks complicates the movement tendancy which was reflected by GPS vector.The relative motion pattern of the present block in the southern margin of Ordos accords with the "three-head collision" model,Three blocks came together and formed high extrusion stress region which is the Sichuan block,the Ordos block and western Henan block.The "triple-head collision" of the three blocks of fault uplift in western Henan Province forms a high-stress zone,and a "head-escape zone" forms a strike-slip zone and a relatively low-stress zone.(6)Geological hazard distribution and region based zoning map,found geological disasters of landslides,landslide and debris flow,disaster prone areas of high shape mainly ribbon and dendritic ribbon,there are nearly east-west,North East and North West.There are coupling strip shape distribution of faults in high prone areas with the system,dendritic and water distribution,mainly reflected in the topography of faults,landform control water.The whole fracture was found of direct or indirect control of geological disasters.Based on the distribution characteristics of geological disaster and geological hazards easy-happening areas map,we found out that disasters is dominated by landslides,collapse and debris flow in the northern margin fault zone of the Qinling Mountains and adjacent area,and by ground fissure in the Xi’an.The main shape of disaster distribution characteristics and geological hazards easy-happening areas is ribbon and arborization.The ribbon is mainly at E-W,NE,and NW direction.It coupling with faults system in study area.The arborization related to tectonic landform and is mainly embodied that fault structure.(7)The formation of ground fissure of Xi’an are resulted from various factors such as groundwater exploitation,the plastic deformation of the Loess and the tectonic activity by analysis of the ground fissure distribution,profile characteristics and GPS vector data in Xi’an.Using the current movement pattern,we judge that “three-head collision” is the main tectonic formation.The southeast extension amount of each fissure about 0.2-0.3mm/a,it can’t be avoided from the angle of disaster prevention and reduction.It is suggested that the urban planning should preventing collision with the ground fissure and the building should far from settlement area of the ground fissure.