Frictional Properties of Phyllosilicate-rich Mylonite and Conditions for the Brittle-Ductile Transition

Abstract:

Phyllosilicate-rich mylonite (60wt. % phyllosilicates which are mainly chlorite and muscovite) is collected from a ductile thrust fault zone along Gengda-Wenmao fault of Longmenshan fault zone. Frictional experiments on artificial mylonite gouge were conducted under elevated temperature in the range of 100-600oC and effective normal stress of 100MPa, 200MPa and 300MPa to represent the deep portion conditions of the fault zone. In order to obtain velocity dependence of friction, loading rates were stepped up and down in the range of 0.04μm/s -1.0μm/s.

In our experiments, the frictional coefficient of mylonite exhibits systematic increase with increasing temperature. Under 200MPa and 300MPa effective normal stress condition, velocity dependence of mylonite gouge shows a transition from initial velocity-strengthening behavior (Regime 1) to velocity-weakening behavior (Regime 2) at about 300oC and then transitions back to velocity-strengthening behavior (Regime 3) as temperature increases. The velocity dependence of mylonite also shows strong pressure sensitivity. When the effective normal stress is increased to 300MPa, the stable frictional behavior is significantly enhanced with larger (a-b) compared to that under the lower pressure condition. Microstructure in Regime 3 is characterized by pervasive mylonitic foliations, which is attributed to plastic deformation of phyllosilicates combined with thermal activated particle size reduction of hard clasts (quartz and plagioclase). At 300MPa effective normal stress and 400oC, loading rate exerts an obvious influence on the transition of velocity dependence, from velocity strengthening behavior to velocity weakening behavior as loading rate increases from 0.04μm/s to 1.0μm/s.

In the framework of rate and state friction constitutive law, the effects of frictional properties of mylonite on faulting mechanics are discussed. From our experimental results of mylonite, unstable slip events may nucleate in mylonite gouge at 100MPa effective normal stress and temperatures of 300-600oC. At higher effective normal stresses of 200MPa and 300MPa, the temperature range for the nucleation of unstable slip events narrows, corresponding to 300-500oC and 300-400oC, respectively.