Abstract: As frictional sliding of rocks is closely related to faulting and earthquake activities,it is essential to properly describe its constitutive relations.Dieterich proposed a preliminary rate-dependent constitutive law based on his experimental studies.After the refinement by Ruina,the constitutive relation has come to the form that we are currently familiar with,known as the rate-and state-dependent constitutive laws.The parameter a-b in the friction laws has been shown to be important in controlling the stability of frictional sliding.Analysis of small perturbation around a steady state for a spring-slider system(linear analysis)shows that the system is stable no mater how much the stiffness is in the condition of velocity strengthening(a-b>0);and sliding instability occurs only in the condition of velocity weakening(a-b<0).Under the framework of rate-and state-dependent friction laws,recent experimental work has been performed on gabbro gouge under both dry and hydrothermal conditions.This work is to study the different contributions of major mineral constituents of gabbro to the overall sliding behaviour of gabbro gouge under hydrothermal conditions.The experiments were conducted on pyroxene and plagioclase gouges separated from a gabbro rock sample with a triaxial system using gas as the pressure medium.A mineral powder layer of 1 mm thick was placed along an inclined saw-cut in a 20-mm-dia～meter cylinder sample to simulate a fault with gouge.The experiments were mainly conducted under pore pressure of 30MPa and effective normal stress of 200MPa and a series of experiments of plagioclase under effective confining pressure of 100MPa were also conducted for testing the reproducibility.At temperatures up to 607℃,standard slip rate steps switching between 1.22μm/s and 0.122μm/s were applied to obtain the rate dependence of friction.Slower rate steps switching between 0.224μm/s and 0.0488μm/s were also applied to explore possible change of sliding behaviour in slow slip rates.The steady state rate dependence a-b of plagioclase shows only negative values,and the steady state rate dependence of pyroxene gouge is negative in most of the temperature range except that at～200℃.Both of these results are quite different from the results of gabbro gouge documented in a previous study.Accordingly,the velocity-strengthening behavior of gabbro gouge above 510℃ may be caused by the presence of minor minerals such as hornblende,mica,and others in gabbro.Values of friction coefficient were picked for comparison.For plagioclase gouge,the friction coefficient shows an increasing trend with increasing temperature in the low temperature range and a decreasing trend with increasing temperature in the higher temperature range.The maximum is attained at～300℃.For pyroxene gouge,the coefficient has no systematic variation due to temperature elevation and varies around an average of 0.74.The results under hydrothermal conditions in this study are radically different from the oven-dried case in a previous study,and this indicates that water has a strong influence on the stability of frictional sliding.

Key words: plagioclase, pyroxene, gabbro, friction coefficient, velocity dependence, velocity weakening

摘要： 在热水条件下对辉长岩的2种主要组成矿物斜长石和辉石进行了摩擦滑动实验研究。结果表明:1)斜长石在整个实验温度范围内(<607℃)显示速度弱化,而辉石除了在200℃附近表现出微弱的速度强化外,在其他温度范围也都表现为速度弱化;2)斜长石在低温时强度随温度的升高而略有增加,高温时强度则随温度的升高而降低。辉石的摩擦强度随着温度的增加没有系统性的变化,在0.74上下波动;3)在热水条件下辉长岩的摩擦性质并不是这2种主要矿物的叠加,有可能是辉长岩中的次要矿物的存在导致了辉长岩的速度强化;4)本实验所得热水条件下的实验结果与干燥条件下的结果有很大差异,再一次表明水的存在极大地影响了岩石的摩擦稳定性。

关键词: 斜长石, 辉石, 辉长岩, 摩擦强度, 速率依赖性