Internal structure and permeability of major strike-slip fault zones: the Median Tectonic Line in Mie Prefecture, Southwest Japan

Abstract

Fault zone structure and permeability data from the Median Tectonic Line (MTL) in Mie Prefecture, Southwest Japan suggest that fault permeability models are currently too simplistic for such large structurally complex fault zones. Ryoke Belt mylonites are cut by mineralised brittle structures up to 300 m North of the MTL that show evidence of fluid circulation. The Sambagawa schist on the south side of the MTL is deformed into foliated quartz/phyllosilicate gouge across a 15-m-wide zone. The complex fault contact area has foliated cataclasite up to 4 m wide, and is cut by a narrow central planar slip zone that probably represents the most recent seismogenic principal displacement zone. Laboratory-determined permeability data show wide variation with fault rock microstructure (e.g. gouge microclast size), controlled by structural position in the fault zone and slip zone intersections. Central slip zone gouges have the lowest permeabilities of all of the fault rocks studied. Fault permeability models should take into account asymmetry where widely contrasting protolith lithologies exist and large permeability variations within a complex central fault zone ‘core’. Pore pressure evolution during rupture propagation may vary greatly because of this complexity, but thermal pressurisation is feasible above certain pressures if the slip remains within fine-grained gouge. The different deformation behaviours of contrasting protolith lithologies control the fault zone fabrics and hence final permeability structure.