Deformation Process and Mechanism of the Frontal Megathrust at the Nankai Subduction Zone

The frontal megathrust of the Nankai Trough subduction zone is recognized as a seismogenic fault based on a record of frictional heating, but the underlying micromechanical processes that act on the fault surface are poorly known. Here we present a layer of fault gouge similar to 2 mm thick within a core drilled across the megathrust, in which smectite-rich siltstone has been transformed into a preferentially oriented illite aggregate. The nearly complete breakdown of smectite is consistent with fast frictional heating on this fault; however, the microtextures of the gouge and its surroundings are asserted one produced experimentally by slow slip. We suggest that slow slip with small shear strain has overprinted the textures produced by the previous faster and larger slip. This interpretation based on microtectonic evidence suggests a slow slip around the frontal megathrust took place during slow down, afterslip, or interseismic as observed now going in subduction zone. We suggest that the illite-dominated gouge is conditionally stable, likely to shift from rapid to slow slip at different times.