An Experimental Study On Frictional Sliding Of Granitic Mylonite Under Hydrothennal Conditions

Laboratory experiments on frictional sliding are considered to be possible modelbehaviors of fault motions in the earth’s crust. To date, quasi-static experimental resultsabout crystalline rocks such as granite and gabbro have been used to discussearthquake nucleation on natural fault surfaces. However, these results may besigificantly different to the sliding behavior of rocks at deep portion of a fault zone.To our knowledge, mylonite, as a typical fault rock at deep portion of fault zones,has not been examined on its sliding stability as related to earthquake nucleation. Inorder to investigate the mechanical properties of deep portion of fault zones andearthquake nucleation, we chose a mylonite sample collected from the Red River faultzone to conduct experiments of frictional sliding under hydrothermal conditions, withthe rate and state dependent constitutive relation as a theoretical framework.Experiments were performed in a triaxial testing system with argon gas asconfining medium with effective normal stress of200MPa, pore pressure of30MPaand temperature varied from100to600°C. For temperatures over400°C, porepressure of30MPa corresponds to supercritical water conditions. During theexperiments, axial loading rate steps were applied to obtain the rate dependence offriction in a velocity range from0.04μm/s to1μm/s. Constant normal stress control isapplied to the fault plane. In order to understand the experimental data of mylonite,supplemental experiments were performed on quartz at400℃.Through simple data analysis, first we obtained the steady-state rate dependencea-b and steady-state coefficient of friction. Further, other parameters in theconstitutive relation (a, b/a and dc) were also obtained through data fitting to the theconstitutive relation Combining with previous mechanical results and observations onmicrofabrics of deformed myloniite, we obtain a preliminary understanding aboutfrictional sliding of mylonite, as follows： (1) The friction coefficient of mylonite increases with an increase in temperaturebelow300℃but decreases with an increase in temperature at higher temperatures.This trend is basically consistent with the results of granite in the literature.(2) Thevelocity dependence of mylonite shows velocity weakening at200℃and400℃, butvelocity strengthening occurred at other temperatures. The velocity weakening at200℃is manifested by sustaining quasi-static oscillation; At400℃, sliding mode isstable sliding at low rate, that at medium speed exhibits oscillation, and a step changefrom intermediate loading rate to high loading rate triggers stick-slip motions.(3) Themineral compositions of the mylonite sample and Westerly granite are similar, and itseems that, quartz, as one of the principal minerals of the two rocks, controls thefrictional sliding behavior of them to some extent. However, the frictional slidingbehavior of the mylonite sample may not be the same as previous data on Westerlygranite.(4) The transition from velocity weakening to velocity strengthening for thegranitic mylonite sample occurs at about430℃. According to this, the bottom ofearthquake nucleation can be deeper than the previous estimate under conditionswhere frictional sliding is possible around the transition temperature(depth).