The Research And Application Of Magnetic Fabric Response Of Rock Under The Coupling Of Temperature And Stress

Magnetic fabric as a rapid,economical and nondestructive measurement method has been widely used in structural geology and other fields.However,the study in the past on the magnetic fabric of rocks mostly stayed on the qualitative description of the relationship between stress and deformation of rock and magnetic fabric,or used to solve practical problems such as regional structure,deformation and evolution.Under different deformation conditions,the response characteristics of the magnetic fabric in the process of rock fracturing is still one of the leading frontiers in the research of magnetic fabric theory.In addition,the Taigu and Hancheng fault zones located in the central and southern of Shanxi province is the two important boundary faults of the Fenwei graben system,where has a dense distribution of more coal,iron ore,gypsum ore mining engineering sites.Because of the lack of effective strain indicators,the systematical study on regional tectonic stress is less,which has affected the precise construction of in-situ stress field in mining engineering regions.Based on the advanced rock mechanics and magnetism test conditions andequipment in the State Key Laboratory of Earth Dynamics,Institute of Geology,China Earthquake Administration and Paleomagnetism and Geochronology Laboratory,Institute of Geology and Geophysics,Chinese Academy of Sciences,The igneous rocks,as the research object,were performed the experiments of rock magnetic fabric response to deformations under room temperature loading,atmospheric pressure heating,constant load heating and the coupling of temperature and pressure.At the same time,referred to the research method of rock deformation and failure process in the mechanism of electromagnetic radiation,the rock acoustic emission experiment was performed to discuss the microscopic mechanism of magnetic fabric variations.Finally,based on the theory of the rock magnetic fabric,combined with the stress tensor inversion of earthquake focal mechanism solutions and regional geological data obtained in the process of field mapping,we investigated the direction,distribution and evolution of regional tectonic stress field around the Taigu and Hancheng fault zones.The main conclusions of this study are as follows:1)In the uniaxial compression process at room temperature,as the increase of loading,the mean magnetic susceptibility and the degree of magnetic susceptibility anisotropy(AMS)increase slowly;the shape factor increases obviously;the magnetic foliation develops gradually;the magnetic susceptibility ellipsoid changes from the "pencil type" to "flat-ball type";the maximum and intermediate axis of magnetic susceptibility ellipsoid rotates rapidly around the minimum axis,and in the lower hemisphere equal area projection the maximumaxis moves closer to the center of projection,and the intermediate axis end away from the center of projection roughly.2)Under the atmospheric pressure heating condition and with temperature increasing,the mean magnetic susceptibility shows an upward trend below the temperature 240℃,but once the temperature reaching at 240℃,the mean magnetic susceptibility overall declines.At the same time,with the increase of temperature,the degree of AMS,shape factor,axis of magnetic susceptibility ellipsoid and their projections in the lower hemisphere equal area projection show no regularity change trend.This indicates that the generation and transformation of minerals is often accompanied in the process of rock heating,resulting in increase or decrease of magnetic susceptibility,and the heating and heat corrosion in the process of heating to a certain extent weakens the anisotropy of magnetic susceptibility of rocks.The effect of temperature on the magnetic fabric of rocks is influenced by the type of magnetic minerals,content,particle size,and the preferred orientation of minerals.3)Under the coupling of temperature and pressure,mechanical properties of rock mass deteriorate significantly,the internal structure change is more intense,and the response of magnetic fabric deformation and failure of rock is more significant.Deformation,change of magnetic fabric of rocks is mainly controlled by the load caused by the load.At the same time,the diversity of types of magnetic minerals,temperature changes produced by thermo-chemical reaction and thermal alteration factors such as the rock magnetic responsecharacteristics more complicated.4)The rock magnetism and magnetic fabric characteristics of the Shaojialing,Zhubeizhuang and Shangyukou three sites located at the NE section of Hancheng fault reveals that the NE section of Hancheng fault zone is mainly controlled by the NW-SE extension with the effect of a relatively weak NE-SW to the horizontal compressive stress.Certain differences in stress exist in different region.A relatively weak stress was observed at the sites of Shangyukou and Zhubeizhuang,while Shaojialing site shows strong extrusion stress.5)The magnetic fabric of Taigu fault zone revealed that the tensile stress of the fault zone is controlled by NW-SE,with relatively weak NE-SW trending horizontal compressional stress,which is consistent with the geological evidence,the earthquake focal mechanisms occurred nearby the fault zone,and also the stress tensor inversion based on the focal mechanism solutions.Different from previous experiments which used synthetic samples,in this paper the igneous rock samples were used as the research object.Our results enrich the understanding of the magnetic fabric theory and provide a reference for further research on the quantitative relationship between the rock magnetic fabric and stress.The application of the magnetic fabric method on the above two tectonic fault zones reveals the direction,distribution and evolution of the regional stress field,and the results contribute a scientific evaluation of the faultactivities and the in-siut stress field for the engineering sites and their surrounding areas.