FLUID FLUX AND PERMEABILITY ALONG THE ROOF THRUST OF THE MÉLANGE ZONE IN A GEOLOGICAL TIME SCALE: AN EXAMPLE FROM THE FOSSIL SEISMOGENIC THRUST OF MUGI MÉLANGE, THE CRETACEOUS SHIMANTO BELT, SW JAPAN

The aim of this study is to estimate fluid flux and permeability along the seismogenic subduciton interface. The silica reduction from host rocks to deformed rocks due to rock-fluid interaction within the fault-fluid system was examined qualitatively. The study area is the northern-most part of the Mugi mélange, the Cretaceous Shimanto Belt, Shikoku, SW Japan. There is the boundary fault zone of the roof thrust of the Mugi mélange with a pseudotachylyte. The P-T condition of the fault ranging from 200 ºC/200MPa to 210 ºC/220MPa which might be corresponding to the depth around the onset of the seismogenic zone along subduction interface.

Rocks are classified into three types on the basis of deformation texture as mélange, breccia and gouge. Bulk chemical analysis was conducted by X-ray fluorescence (XRF) on those rocks. The average mass and volume loss from melange host rocks to fault rocks estimated from isocon method indicate about 5 % and 8 % respectively. In particular, Si reduction was observed clearly because the amount of Si is larger than the others.

From the Si reduction and other some geological constraints such as P-T condition of fluid, length of fault, duration between deposition to exhumation ages, amount of fluid and fluid flux were roughly examined, those are about thousands times of fluid volume to the rock volume and 10 ^-8 m/s of fluid flux. From the fluid flux and P-T condition of fluid, permeability of fault is estimated, which represent about 10^-16 order in breccia and 10^-17 order in gouge. Whereas the permeability of breccia is similar to the value from laboratory experiments, that of gouge is quite larger than that from laboratory experiments. This might indicate that the permeability of gouge is not constant like laboratory experiments but can be changed dynamically at the time of faulting or for some duration after faulting.