| Field geological surveys reveal that carbon or even graphite is often enriched within natural fault zones.As carbon is a high conductive material,its types,contents and distribution are crucial parameters influencing electrical conductivity of fault zones.Extensive researches have been carried out to study the types and contents of carbon in faults zones,and fruitful results have been achieved.However,there has b e e n n o systematic studies on thestructures of carbon distribution in the fault zones so far.In this paper,to study the effects of fault motion on carbon distribution structure andthe evolution of fault electrical characteristics to carbon contents and distribution,we conducted friction experiments and electrical conductivity measurements on mixtures of graphite-quartz gouges with different contents of graphite(3,5,6 and 7wt%).The experiments were performed at a normal stress of 2 MPa and slip rates ranging from 500 ?m/s to 1 m/s,under room temperature and humidity.Similar tests were also performed on natural fault gouges collected from Hongkou Bajiaomiao outcrop of the Longmenshan fault zone,at a normal stress of 2 MPa and a slip rate of1 m/s,hoping to reveal the possible changes of carbon materials in fault movement.The electrical conductivity results indicated that the initial samples of the simulated graphite-quartz gouge have low conductivity,andthis characteristic was not closely related to graphite contentsunder our experimental conditions.With the start of the friction slip and the increasing of sliding distance,the conductivity of the samples in the direction parallel andperpendicular to the slip surface show significantly different features.In the direction parallel to the slip surface,the conductivity of the simulated samples increases rapidly towards a steady state,within about tens of centimeters in displacement.The electrical conductivity in this case is also closely related to the contents of graphite and the shear strain.Furthermore,the visible increase in electrical conductivity of the samples can only be observed when the graphite content is greater than 5wt% and the shear displacement exceeds tens of c e n t i m e t e r s.Such increases is independent of therate of shear slip when graphite still survived(without oxidation during frictional heating).In contrast,in the direction perpendicularto the slip surface,the sample conductivity almost does not change with slip velocity and distance at all.The mechanical data show that the steady-state friction coefficients of the simulated samples are negatively correlated with the graphite contents at slip rates ranging from 500 ?m/s to 1 m/s.SEM microstructures analysis show that the anisotropic of electrical conductivity caused by frictional slip directly reflects the changes of distribution structure of graphite.The electrical conductivity of mixtures of graphite-quartz gouges are mainly controlled by the conductive structure of graphite.The amount of graphite in rocks and its connection status at the grain boundary are the key to improving the conductivity of rock.The electrical conductivity of natural samples measured at atmosphere(in the presence of oxygen)suggested that the carbon in samples might undergo oxidation reaction due to the significant temperature rises caused by high-speed frictional sliding.In the future,we will conduct high-speed friction experiments on nature samples under anaerobic environment,trying to get a more comprehensive view on the influence of fault motion on the various changes of carbon materials.This study deepens our understanding on the electrical characteristics of shallow fault zones,and provides an important constraint on the interpretation of filed magnetotelluric data.Furthermore,our results are of great importance in understanding the mechanical properties of carbon-bearing fault zones and the distribution characteristics of weak minerals phases and itsevolutionprocess with displacement in the shear deformation. |
PDF Full Text Request