Effect of temperature on frictional behavior of smectite and illite: Implication for the updip limit of seismogenic zone along subduction thrust

Abstract:

Along plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures of ~150C is one of the key mineralogical changes thought to control the updip limit of seismicity. Saffer and Marone (2003) reported illite shale exhibited only velocity-strengthening behavior, whereas illite is widely expected to be velocity-weakening behavior. The limitation of their experiments were temperature, in which the frictional experiments were csrried out at room temperature, although the updip limit of seismogenic zone is thermally controlled that occurs at temperature around 150C. Therefore, in this study, we determined the effect of temperature of frictional properties of smectite and illite and discuss whether the smectite-illite transition accounts for the updip limit of seismogenic zone along subduction thrust. In the frictional experiments, we determined the velocity dependence of sliding friction, which is a key parameter for stable or unstable sliding. After steady-state sliding, the loading velocity of 3 μm/s was abruptly changed to 33 μm/s in each frictional experiments to determine the velocity dependence of these clay minerals. The velocity dependence of both smectite and illite at room temperature shows always positive at normal stress higher than 40 MPa, which is similar to the results of Saffer and Marone (2003). However, at temperature of 200C, illite shows negative values of (a-b), suggesting that illite exhibits unstable velocity-weakening behavior. den Hartog et al. (2012) also showed the velocity-weakening for illite gouge by high temperature ring shear experiments, but the transition from velocity strengthening to weakening occurs at temperature around 250C. These results explain that smectite is potentially aseismic for stable sliding at the subduction thrust, whereas illite becomes seismic due to a negative velocity dependence and unstable sliding at high temperatures. Thus, the smectite-illite transition has a potential to explain the updip limit of seismogenic zone along subduction thrust.