We conducted triaxial friction experiments on opal gouge at a confining pressure of 150 MPa, a pore water pressure of 50 MPa, temperatures (T) of 25–200°C, and axial displacement rates (V_axial) changed stepwise among 0.1, 1 and 10 μm/s, in order to investigate its temperature-dependent frictional properties and their controlling factors. Because opal is known as a main component in chert on the Pacific plate subducting at the Japan Trench, its frictional properties would be applicable to those of chert subducting there.

The results showed frictional strength increasing with increasing T, rate-weakening behavior at T ≧100°C, where frictional strength decreases or increases when V_axial is increased or decreased, and stick slips at T ≧150°C. When fitted by the rate- and state-dependent friction constitutive law, steady-state friction coefficient μ_ss increases with increasing T or decreasing V_axial, while a – b (rate dependence of μ_ss implying stable (aseismic) faulting if positive, i.e., rate strengthening, and potentially unstable (seismic) faulting if negative, i.e., rate weakening) decreases with increasing T or decreasing V_axial, and becomes ≈0 at 50°C and negative at T ≧100°C, being larger negative at higher T or lower V_axial in the latter case.

These results indicate that frictional properties of opal gouge are dependent on not only T but also V_axial, suggesting that a thermally activated process is responsible for the observed frictional properties. Gouge layer at higher T contains a smaller fraction of submicron-size particles, which have likely been dissolved away at higher T. In addition, gouge layers at T ≧100°C do not show clear cataclastic microstructures, but are rather dense. These microstructures suggest the activation of thermally activated pressure solution during the experiments, which promoted gouge densification to increase frictional strength with increasing T or decreasing V_axial, thereby decreasing a – b to <0 at T ≧100°C. Our results also show that a – b ≈ 0 at 50°C, suggesting that the transition from aseismic faulting to seismic faulting occurs at ≈50°C along the megathrust at the Japan Trench subduction zone if the megathrust is located in chert on the subducting Pacific plate.

Stress build-up processes within fault zones have been discussed as phenomena that occur during the seismic cycle. Geological studies of this nature are few but bear importance in bridging geophysical and microstructural observations. Methods for investigating stress records in rocks are performed through the application of palaeo-piezometre. This however requires that the rocks are deformed through the dislocation creep mechanism.

Our studies are conducted on the exhumed mylonitic samples from the Ryoke belt along the Median Tectonic Line. The samples contain f-type quartz microstructures and are reported to preserve records of high differential stress deformations. Although deformed under similar temperature conditions close to the brittle-ductile transition, these samples display heterogeneous strain distributions [1], thus suitable to examine stress accumulation processes. However, the samples display trends of the weakening of CPO fabric intensity with increasing strain for the composing quartz crystals. Generally, this phenomenon is accredited to the onset of grain boundary sliding from the dislocation creep mechanism, thus inhibiting the application of the palaeo-piezometre.

Our samples contradicted numerous studies have shown that the CPO fabric intensity increases with strain (e.g. Heilbronner & Tullis, 2006; Muto et al., 2011). Thus, we aim to investigate the effect of strain on the evolution of the CPO fabric intensity and clarify the mechanism involved. The recrystallised fraction of dynamically recrystallised grains is adopted as a proxy for strain evaluated using a newly developed routine to process the EBSD data. Achieving the target enables us to determine sample conditions that are suitable for the application of the palaeo-piezometre, which result will be conducted for the study of stress-strain relationship along the Median Tectonic Line.

[1] Katori et al., 2021. JSG. [2] Heilbronner & Tullis., 2006. JGR. [3] Muto et al., 2011. JGR.