FRICTIONAL SLIDING OF PYROXENE AND PLAGIOCLASE GOUGES UNDER HYDROTHERMAL CONDITIONS
LUO Li HE Chang-rong(State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration,Beijing 100029,China)
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-b0);and sliding instability occurs only in the condition of velocity weakening(a-b0).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.
【Fund】： 国家自然科学基金(40574080);; 地震行业科研专项(200708014)共同资助
【CateGory Index】： P589.1
【CateGory Index】： P589.1