Dynamic role of tectonic mélange during interseismic process of plate boundary mega earthquakes

Tectonic mélange has a key role in subduction zones because its thick pile forms the plate boundary itself; therefore, the plate boundary process is nearly identical to the mélange-forming process. We examined three tectonic mélanges in the Shimanto Belt in southwest Japan to decipher their progressive deformation process with subduction, especially within the seismogenic zone. Here we report detailed observation of sandstone blocks and the strain history of the shale matrix of a sediment-dominated tectonic mélange. The necessity of tectonic mélange accompanying fossil seismogenic décollement is unveiled. Several deformation processes continue by turn until the depth of the down-dip limit of the seismogenic zone is reached. The results support the space–time partition of deformation in terms of seismic behavior and suggest a possible candidate for a geological consequence of recently observed slow earthquakes.

► We described tectonic mélanges showing progressive deformation within the seismogenic zone.
► The formation process of the tectonic mélange is essentially the deformation of sandstones.
► Inferred brittle deformation within the seismogenic zone suggests the mélange formation during interseismic period.
► Sandstone deformation is capable of releasing seismic waves.