Fluidization of granular material in a subduction thrust at seismogenic depths

Subduction thrusts exhumed from seismogenic depths may record the dynamic slip processes during subduction zone earthquakes. The thrust faults in the Shimanto accretionary complex of southwest Japan imbricate the mélange derived from sedimentary rocks and basalt, which represent a duplex underplating associated with the downward step of the subduction thrust at seismogenic depths (4–6 km in depth, temperature = 130–150 °C). One well-exposed duplex-fault zone consists mainly of foliated cataclasites and ultracataclasites derived from basalt. The fault zone structure suggests slip localization along the 2–20 cm thick ultracataclasite layer at the boundary between different lithologies. The common features observed along the ultracataclasite layer are the injection of granular material into the mélange without the sorting of fine-grained particles along with a small detection probability of the fragmented counterparts in the injected materials, which suggests a rapid injection of fluidized granular material during faulting. The ultracataclasite layer is marked by fluid-related processes, as indicated by the alteration and precipitation of amorphous silica gel along the ultracataclasite and the concentration of veins near the ultracataclasite. The fluidization during localized slip under fluid-infiltrated conditions could be due to thermal pressurization, which causes the fluid pressure to approach lithostatic values. The injection of fluidized granular material may result from hydraulic fracturing in response to an increase in the fluid pressure associated with thermal pressurization. The plate boundary subduction thrust could be lubricated when the granular material fluidizes due to thermal pressurization during earthquakes.