Deformation Mechanism And Physical-chemical Properties During Seismic Process Along The Xianshuihe Fault Zone

Xianshuihe fault zone is one of the most important large-scale left lateral strike slip faults in the eastern Qinghai Tibet Plateau.It not only controls the eastward extrusion of material beneath the plateau,but also is one of the most fault zones with frequent great earthquake activity in China.After the 2008 Ms8.0 Wenchuan earthquake,the Coulomb stress of Xianshuihe fault zone increased significantly.Especially after the 2013 Lushan Ms7.0 earthquake and the 2017 Jiuzhaigou Ms7.0 earthquake,the Coulomb stress of Xianshuihe fault zone increased sharply,which is very likely to be the fault with the next great earthquake.Therefore,it has become the focus of people's attention.In particular,the deformation mechanism and physical-chemical properties of Xianshuihe fault zone are the key to understand the mechanism of seismic process along the fault zone,and are also the basis and key to solve the 125 most challenging scientific problems published by “Science” in 2021.Taking Huiyuansi near Bamei as the boundary,the Xianshuihe fault zone can be roughly divided into the north and the south segments along the strike.The northern segment can be further divided into three secondary faults,i.e.Luhuo fault,Daofu fault and Qianning fault,which are arranged in left lateral and left step,with relatively single geometry.The geometry and internal structure of the southern segment of Xianshuihe fault are relatively complex,with four right echelon branches——Yalahe fault,Selaha fault,south Mugecuo fault and Zheduotang fault.In this paper,the fault rocks in different sections of Xianshuihe fault zone are taken as the main research object.Through detailed field geological investigation and research,the microstructure of the samples is observed and analyzed by using polarizing microscope and scanning electron microscope,and the chemical composition and age of the samples are analyzed and determined by means of powder X-ray diffraction,Raman spectroscopy,isotope chronology and other testing methods.We have systematically studied the internal structure and material composition of Xianshuihe fault zone,understood the rheological characteristics and deformation behavior of the fault zone,and discussed the weakening mechanism and tectonic evolution of the fault zone.Followings are the main results that we have obtained:(1)Through the study of the internal structure of the fault zone,the types of fault rocks,material composition and deformation characteristics,it is determined that there are obvious differences in deformation behavior between different sections of the northern Xianshuihe fault zone.Black carbonaceous gouge with a thickness of 15?20m is developed in Luhuo fault section,which contains about 30% clay minerals.The gouge is composed of clasts and matrix,with well-developed foliation and "S-C '" fabric.Under the SEM images,the clay minerals are arranged directionally,with local bending or folding,and developed card house and layered pores.The results show that the Luhuo fault has a wide range of pressure solution and experienced creep deformation.However,laser Raman spectroscopy revealed the existence of carbon graphitization.Graphite,as a relatively stable mineral,can be used as evidence of paleoearthquake,indicating that it had experienced rapid earthquake sliding.Therefore,the Luhuo fault may have experienced early creep and later seismic slip.The amorphous carbon in the fault zone may act as a lubricant,leading to fault weakening;the existence of graphitization of carbon indicates that the mechanism of thermal pressurization also plays an important role.The Daofu fault section developed in carbonate rock area has multiple fault cores and surrounding fracture zones.The fault gouge with thickness of about 2 mm developed along the main slip surface.Microstructural analysis revealed that the limestone clasts near the slip surface were cut off,indicating that the fault experienced seismic slip.The discovery of nanoscale spherical particles and embayed flakes of clay mineral on the slip surface is one of the highlights of this paper.Their existence is related to seismic sliding and strong friction weakening.Friction heating and thermal decomposition of carbonate rocks play an important role in the mechanism of seismic slip;in addition,the mechanism of thermal pressurization and fluid lubrication may also be the cause of fault weakening.(2)The southern segment of Xianshuihe fault zone is more complicated in terms of its internal structure,faulting process and deformation behavior.Along the southern segment of Xianshuihe fault zone,the Zheduoshan granite and ductile shear zone with different width are exposed,and the migmatization is also developed locally.The strong cleavage black fault breccia developed near Bamei and the fault gouge at the northwest end of Zheduotang fault are characterized by pressure dissolution,indicating that they have undergone creep deformation.The development of calcite or quartz veinlets indicates the widespread existence of fluid interaction.It can be seen that carbonaceous materials are developed in veinlets,and the source of carbonaceous materials may be related to the original rock or surrounding rock.The local carbonaceous graphitization degree is high,which indicates that the section has undergone rapid slip.The lubrication of amorphous carbon is the dominant mechanism of fault weakening,and the friction heat in the process of seismic sliding leads to the occurrence of carbon graphitization,which also plays a role in promoting seismic sliding.The width of mylonite ranges from several hundred meters to two kilometers.Microstructural analysis reveals that the ductile shear zone is characterized by "S-C" fabric,feldspar "?-type" porphyroblastes,quartz polycrystalline band and "mica fish".The dynamic recrystallization mechanism of quartz and feldspar reveals that the granitic mylonite was deformed at 450? ? 600?.The ages of 13 AFT samples in the Zheduoshan pluton range from 9.2 to 2.2 Ma,and the main uplift rate of the pluton can be calculated to be about 0.36 mm/a to 1.5 mm/a.Combined with the previous thermochronological data,it shows that the uplift age of the pluton tends to be younger from the north to south,which may be caused by the gradual decrease of the horizontal movement rate and the gradual increase of the vertical movement rate of the Xianshuihe fault zone from north to south.(3)Based on the previous geochronological data and the results of this paper,three stages of tectonic evolution of Xianshuihe fault zone since Oligocene are established.I: ca.32 ? 24 Ma,the primary stage of the initiation of left lateral strike slip in Xianshuihe fault zone,the deformation concentrated in the deep crust,showing weak magmatism and anatexis;II: ca.20 ? 13 Ma,the peak deformation period of left lateral strike slip of Xianshuihe fault zone,the deformation spread throughout the whole crust,showing a strong left lateral ductile shear deformation and large-scale magmatism.III: ca.10Ma?,the deformation is concentrated in the upper crust,which is characterized by block rotation and strong uplift,fault weakening and strong seismicity.The tectonic evolution and seismicity of the Xianshuihe fault zone are constrained by the eastward extrusion of the eastern margin of the Tibetan Plateau since the collision of the Indian Eurasian plate.