Brittle/cataclastic deformation and dissolution-precipitation creep in the Goshikinohama fault (Shimanto Belt, SW-Japan): Indication of seismic cycling and possible slow slip?

Microstructural evidence for slow earthquakes is a matter of debate as micromechanical processes are not fully understood and hence resulting deformation microstructures remain unclear. One of the best study areas to investigate the phenomena of seismic and aseismic deformation and also possible paleo-events of slow slip in an exhumed accretionary complex is the Shimanto Belt in SW-Japan, where lithologies, age, and pressure-temperature conditions are well-constrained. Our investigations focus therefore on the Yokonami mélange of the Cretaceous Shimanto Belt. The Goshikinohama fault is a fossil seismogenic fault at the northern margin of the Yokonami mélange. It contains several 20 cm thick cataclastic faults within which thin (less than 1 mm), discrete slip zones occur. Based on vitrinite reflectance the paleo-maximum temperature of the surrounding host rocks is about 250˚C. An exothermic event was identified in the cataclasite of up to 300-360˚C evidenced by paleomagnetic and rock-magnetic analyses [Uchida et al., 2024]. In order to access microstructures related to the seismic cycle as well as to explore whether this proposed thermal event resulted in characteristic changes in deformation mechanism, we conducted observations on the cataclastic shear zone using optical microscopy,electron microscopy,  electron backscatter diffraction (EBSD), energy dispersive X-ray spectroscopy (EDS) and cathodoluminescence (CL).

The studied cataclasite consists of mm- to cm-size fragmented quartz veins in a shale matrix with quartz and feldspar clasts. Quartz displays solution-seam contacts to the shale and various generations of subsequent fracturing and healing are recognized. Characteristic are (i) synkinematic fiber growth microstructures related to a crack-seal mechanism accommodating foliation-parallel stretching of quartz aggregates within the shale matrix as well as numerous generations of blocky veins and (ii) static shattering of quartz grains at the µm-scale and subsequent healing. The static nature of the shattering is interpreted from the lack of any offset or misorientation in the affected quartz grains. In addition, there is some undulous extinction and very minute and local indication of quartz dynamic recrystallization by grain boundary bulging. The shale matrix exhibits a compositional flow banding detected by EDS.

Veining, solution seams and the general clast-in-matrix structure are interpreted to relate to the interplay of brittle fracturing, cataclastic flow and dissolution-precipitation processes. Very few and local evidence for bulging recrystallization fits deformation conditions at the brittle to ductile to viscous transition in accordance with the temperature estimates given before. The origin of shattered quartz is hypothesized to relate to seismic wave-induced shock deformation.Mutual overprinting of brittle/cataclastic deformation and creep deformation as well as synkinematic and static vein growth might be an indication for the formation of these microstructures during the seismic cycle and possible transient creep or slow slip. However, if these processes produced heat to the extent of the proposed exothermic event is a matter of further investigations.

[Ref] Uchida,T., Hashimoto, Y., Yamamoto, Y. and Hatakeyama, T., 2024, Exothermic events in a fossil seismogenic fault acquiring thermoviscous remanent magnetization in an exhumed accretionary complex, Tectonophysics, V. 871,https://doi.org/10.1016/j.tecto.2023.230177.