Deformation Mechanism Of Fault Rocks During Co-seismic Slip

Pseudotachylytes and cataclastic rocks recorded transient creep and co-seismic slip in the brittle-plastic transition zone of fault.Studying the genetic mechanism of pseudotachylytes could provide the direct geological evidence for revealing the seismic slip and co-seismic process.Earthquakes occured frequently in the Anning River fault zone,where brittle fractures coexist with ductile shear deformations at the exhumed granitic rocks along the fault zone.Pseudotachylytes is found in the ductile-deformed fault rocks.This paper focus on the deformation of pseudotachylytes andfault rocks（cataclasites and mylonites）collected in the Anning River fault zone.The microstructures were analyzed using an optical microscope（OM）,scanning electron microscopy（SEM）and electron backscatter diffraction（EBSD）;the chemical composition was analyzed using electron microprobe and scanning electron microscopy with energy dispersive spectrometry（EDS）;high-velocity frictional experiments were carried out to understand the process and conditions of the formation of pseudotachylytes from granite faults during co-seismic.The granitc gneiss and mylonites,which represent the plastic deformation of the interseismic period,have a porphyritic structure,augen-shaped plagioclase with crushed structure inside and pressure-dissolution characteristics on both sides.Fine-grained quartz and biotite bands constitute the foliation.The cataclasties which represents co-seismic fractures,developed cracks in fragmented plagioclase and the cracks were filled with recrystallized quartz veins;Using EBSD to analyze the quartz fabric in the cataclasties,the results show that the dominant sliding system of quartz is the basal<a>sliding,which indicates the deformation temperature is 250-350°C,and corresponding to the relative depth is 15-20 km.There are three types of pseudotachylytes veins developed in the cataclasties.This paper focus on the straight-veined pseudotachylytes which is typical molten rocksin the exhumed granitic rocks along the fault zone.The chemical compostion analysis shows that the pseudotachylytes in the Anning River fault zone is generally rich in iron and calcium,and the content of Si O₂ is similar to the host rocks,but the melt composition has a tendency to evolve in two directions to low silicon and high silicon.The content of Na₂O、K₂O、Al₂O₃、Ca O、Fe O and the content of Si O₂ is negatively correlated,and decreases monotonically with the increase of Si O₂.The chemical composition results show that the pseudotachylytes is the product of the melting of the host rocks.The results of high-velocity frictional experiments show the friction strength evolution of both dry samples and samples with fluid pore pressure undergoes three stages:initial weakening,strengthening and secondary weakening.The chemical composition analysis of the experimental samples shows that the characteristics of the melt composition of the dry sample and its evolution trend with Si O₂ are similar to those of the pseudotachylytes in the Anning River fault zone.Therefore,it is speculated that the formation conditions of pseudotachylytes in the Anning River fault zone are closer to the dry environment.Based on the deformation characteristics of mylonites,cataclasties and pseudotachylytes in the Anning River fault zone,combined with high-velocity frictional experiments,the deformation of fault rocks in the Anning River fault zone at different stages of co-seismic,post-seismic relaxation and inter-seismic periods was preliminarily constructed.The model provides a reference for understanding the deformation process of the source rock of the earthquake fault.