Fault zone permeability structure of the Median Tectonic Line, Japan

Christopher A.J. Wibberley1 and Toshihiko Shimamoto

Dept. Geology and Mineralogy, Graduate School of Science, Kyoto University, Japan

1 From Jan 2003: CASP, Dept. of Earth Sciences, University of Cambridge, U.K.

cwibber@ip.media.kyoto-u.ac.jp

Large faults are thought to act as major fluid conduits in the Earth’s crust. The quantification of crustal fluid fluxes is important for modelling and understanding a number of linked hydromechanical and fluid-rock interaction processes. Recent work has shown that different fault rocks can have widely differing permeabilities, so that the fine detail of internal structure in fault zones can strongly influence fluid flow patterns. The small number of studies performed to date give a simple view of symmetrical fracture damage zones flanking a low-permeability fault core, valid in some cases where both fault walls are fracture dilatant materials. A better understanding of the controls on fault zone fluid flow is achieved by linking the protolith and deformation processes to the resulting fault rocks. Fault zone structure and permeability data from the Median Tectonic Line (MTL) in Mie Prefecture, Southwest Japan suggest that such fault permeability models are currently too simplistic for large structurally complex fault zones.

Ryoke Belt granitic 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 (approximately 10 cm wide) 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 the fault rocks studied. Fault permeability models should take into account asymmetry due to contrasting deformation behaviours where different protoliths are juxtaposed (fracture dilatancy in the granitic mylonites vs. shear-enhanced compaction in the metapelitic schists), and large permeability variations within a complex central fault zone ‘core’.

These findings have important implications for hydromechanical processes during seismic slip. Pore pressure evolution during rupture propagation may vary greatly because of this complexity, but pressurization of pore water due to frictional heating is likely if the slip remains within fine-grained clay gouge.