Deformation Mechanism Of The Brittle-plastic Transition Zone In Different Periods Of The Earthquake Cycle

The transition from microscopic brittle deformation to microcrystalline plastic deformation is called brittle-plastic transition.The brittle-plastic transition zone is considered as a key position for determining the limit of lower continental crust seismicity.In addition to being controlled by temperature,the depth and deformation mechanism of the brittle-plastic transition zone,during the different deformation stages of the seismic cycle,changes in the strain rate and fluid pressure will also cause it to change.By studying the deformation mechanism of the fault zone tectonite,could reveal the deformation characteristics of the fault brittle-plastic transition zone in different phases of the seismic cycle,and effectively understand the multi-seismic tectonic deformation superposition phenomenon.In this paper,microstructural observation and analysis were carried out on the tectonite samples collected from the Red River fault.The tectonite samples included mylonites,pseudo-genite and cataclasite rocks.Using polarized and scanning electron microscopy,the morphology,micro-deformation characteristics,mineral composition,mineral water-rock reaction,pressure dissolution,out dissolution,fissure filling and deformation characteristics of the main minerals in the sample were systematically observed;the mineral components were examined using the EDS.Quantitative analysis was carried out,and some of the samples were full scanned.Electron backscatter diffraction(EBSD)experimental data was used to study and analyze the structure of major minerals in the sample.Based on these analyses,the deformation model of fault brittle-plastic transition zone in the period of interseismic,co-seismic and post-seismic relaxation was established.Plastic deformation is the main deformation mechanism of the fine-grained mylonite.The main rock-forming minerals in the samples,such as plagioclase,quartz and biotite were elongated and oriented,forming significant stretch lineation.The EBSD analysis shows that quartz is mainly basal <a> slip and has the characteristics of medium-high temperature deformation.In the augen-mylonite,The plagioclase phenocrysts undergoes intragranular fractures,and the edge undergoes a water-rock reaction to produce low-strength minerals such as quartz and mica.Quartz undergoes significant dynamic recrystallization and forms a strongly deformed quartz band,forming a shear foliation with the elongated mica.EBSD analysis shows that quartz is mainly basal <a> slip and has low temperature deformation characteristics.The deformation characteristics of these two kinds of mylonite samples represent the plastic flow in the deep fault zone and the plastic deformation of the brittle-plastic transition zone in the inter-seismic period.The pseudo-basaltic glass veins and mylonite boundaries developed in mylonites are distinct.The quartz in mylonites undergoes low-temperature plastic deformation and consists of a large number of needle-like chloritoids and vitrics in the vitreous,representing brittle plasticity.Friction melting formed during the co-seismic rupture process of the transition zone.The cataclastic rock is characterized by the joint development of brittle deformation and plastic deformation.Quartz has low-temperature plastic deformation characteristics.The feldspar is fragmented,and the fracture is filled with static recrystallized quartz or calcite veins.The calcite vein undergoes plastic deformation,which represents the post-seismic relaxation deformation.Based on the analysis of the deformation mechanism of the Red River fault tectonite and combined with previous studies,the evolution process of the deformation mechanism of fault brittle-plastic transition zone in the seismic cycle is summarized as an oscillation model over time.In the interseismic period,plastic shear deformation occurs in the rocks in the deep faults,forming tectonite represented by fine-grained mylonites and augen-mylonites;as the stress accumulates,when the stress reaches the fault strength,the rock undergoes coseismic brittleness.Cracked,false basalt glass is the product of coseismic deformation;in the subsequent relaxation stage,high stress and high strain rate lead to brittle fracture of high-strength minerals such as feldspar in rocks,and plasticity of low-strength minerals such as quartz and mica Deformation,under the influence of fluids,fractures are healed by quartz or calcite filling,and as the stress accumulates,the minerals such as quartz and calcite in the fracture are transformed from static recrystallization to dynamic recrystallization.With the healing of fractures and the accumulation of stress,the fault strength gradually increases,accumulates energy for the next earthquake ingestion,and the fault deformation and earthquakes form a periodic cycle.