A Study On Rate Dependence Of Friction Of Gabbro Under High Temperature And Pressure

According to different deformation mechanisms, the lithosphere is considered to consist of brittle, brittle-ductile transition and ductile regions, and the appearance of this kind of layered structure is repetitive with increasing of depth and variations in rock types. Because existence of faults is ubiquitous in the brittle region, its strength is determined by the strength of fault Most of global earthquakes occur in the shallow brittle region. Stick slip (unstable sliding) is a possible mechanism of earthquakes, hence studying the stability of factional sliding is crucial to understanding the mechanics of earthquake nucleatioa Previous researches on frictional constitutive relation by Dieterich and Ruina lead to a rate and state dependent friction law, where the steady state rate dependence of friction stress may be positive (velocity strengthening) or negative (velocity weakening) according to the conditions applied. Velocity weakening is a necessary condition for stick slip and earthquake nucleatioa Early experimental studies on frictional stability as related to temperature, pressure and fluid condition failed to capture the intrinsic nature of stability of frictional sliding, for stability also depends on the stiffness of the specific testing rig. But rate dependence of friction is independent of the stiffness as applied by the specific testing rig, and so it can better reveal the intrinsic property related to stability of frictional sliding. Many detailed studies have focused on rate dependence of friction of granite, the typical rock in the upper crust, whereas similar studies on gabbro (a typical rock in the middle-lower crust) are quite limited To our knowledge, no study on rate dependence of friction of gabbro is documented so far. This work is to study frictional properties of gabbro under high temperature and pressure, especially focuses on rate dependence of frictioa1. Experimental resultsThe experiments of are conducted to study frictional rate dependence on dry gabbro under conditions of temperature ranging from 23 ℃ to 645℃, normal stress of 200MPa and 300MPa and sliding velocity of 1×10-4mm/s and 1×10~3mm/s in a biaxial apparatus using argon gas as the pressure medium. The cylindrical sample (20 mm diameter x 40 mm length) with a sawcut plane inclined 35 to the sample axis contains a 1-mm-thick layer of gabbro gouge.At both 200MPa and 300MPa normal stresses, the strength of friction tends to decrease with the increase of temperature. At 200MPa normal stress and temperature up to 650℃, the strength becomes large with the increase of sliding velocity (velocity strengthening) and the rate dependence peaks at 427℃. At 300MPa normal stress, the rate dependence varies with the temperature. At temperature between 103℃ and 264℃, friction shows velocity strengthening, and the rate dependence decreases with temperature. For temperature between 360℃ to 475℃, rate dependence is positive and increases rapidly with the raise of temperature, and at 475℃ the rate dependence exhibts a maximum in all temperature conditions, and the rate dependence reduces with the increase of temperature. At temperature of 274℃ to 360℃, the strength of friction decreases or does not change with the increase of velocity (velocity weakening). For temperatures above 475℃, the rate dependence decreases with the increase of temperature, and at 528℃ and 612℃ it approaches to 0.Thin section observation revealed that the angle between Rl (Riedel shear) shear plane and the fault plane is related with the strength of friction and tends to reduce with the decrease ofstrength of frictioa With the increase of temperature, both boundary shear and Y shear become dominant, and Riedel shears become secondary.2. Application of experimental results and discussionIn this study, friction coefficient of gabbro has also been measured at temperature ranging from 23 ℃ to 645℃. From temperature up to 420℃, the friction coefficient is almost constant, with values very close to that expected by Byerlee's law. At temperatures...