Frictional Sliding Of Pyroxene And Plagioclase Gouges Under Hydrothermal Conditions

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 law has come to the form that we are currently familiar with, known as the rate- and state- dependent constitutive laws. In the constitutive laws, there are two most commonly used evolution equations of the state variable: one of them is referred to as slip law in which the state variable only evolves with the sliding distance, and the other is referred to as slowness law which can describe the time effect of stationary contact.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 can be caused only in the condition of velocity weakening ( a ? b<0) and a larger b ? a value is more favorable for instability to occur .Our group has performed experimental studies on frictional sliding of gabbro, a typical mafic rock in the lower crust. Based on the results, analysis has been made on the possible cause of earthquakes in the lower crust. The major minerals of gabbro, plagioclase and pyroxene are focused in this study under hydrothermal conditions in order to understand their respective contributions on the frictional sliding of gabbro.The experiments were conducted in 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-diameter 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 applied for testing the reproducibility. At temperatures up to 607℃, slip rate steps ranging from 1.22μm/s to 0.122μm/s were applied to obtain the rate dependence of friction. In order to explore the behavior in lower sliding velocity, we conducted several experiments with slip rate steps ranging from 0.224μm/s to 0.0488μm/s.The steady state rate dependence a ? b of plagioclase show only negative values, and for pyroxene gouge, the steady state rate dependence are negative in most of the temperature range except that at ~200℃, both of which are quite different from the results of gabbro gouge. The velocity-strengthening behavior of gabbro is found to be caused by the velocity strengthening of quartz, which is a minor mineral in gabbro and the velocity strengthening counteracts the velocity weakening of plagioclase and pyroxene in the high temperature range.For plagioclase gouge, the coefficient of friction 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. Both the major minerals control the frictional strength of gabbro. The radical differences in velocity dependence between the results under hydrothermal conditions and the oven-dried case show that water has a strong impact on the stability of frictional sliding.