Frictional properties of phyllosilicate‐rich mylonite and conditions for the brittle‐ductile transition

Abstract

AbstractTo understand frictional properties of natural phyllosilicate‐rich mylonite along fault depth, friction experiments on simulated mylonite gouge were conducted at temperatures of 100–600°C, effective normal stresses of 100–300 MPa, and loading rates of 0.04–1.0 µm/s. Experimental results show that at temperatures above 200°C, frictional strength of the mylonite gouge exhibits systematic increase with temperature, in the range of 0.52–0.73. Under 200–300 MPa normal stress conditions, velocity dependence of mylonite gouge shows a transition from initial velocity‐strengthening (Regime 1) to velocity‐weakening behavior (Regime 2) at a temperature between 200 and 300°C and transitions back to velocity‐strengthening behavior (Regime 3) as temperature increases. The latter transition is also found to be promoted by slower loading rates. Friction stability of the mylonite gouge also exhibits a strong pressure sensitivity. While velocity‐strengthening behavior in Regime 3 is absent under 100 MPa normal stress condition, stable frictional behavior is significantly enhanced at higher effective normal stresses, with the transition temperature to velocity strengthening decreasing with effective normal stress. Microstructural analysis shows that the transition of velocity dependence from velocity weakening to velocity strengthening corresponds to transition from cataclastic flow to semibrittle process featured by mylonitic structures (S‐C fabrics). The formation of mylonitic fabrics is due to plastic deformation of phyllosilicates combined with thermally activated grain size reduction of hard clasts (quartz and plagioclase). Our results may help constrain depth range of seismogenesis within phyllosilicate‐rich fault zone and imply that variation of effective normal stress may affect faulting behavior.