Competence contrast between pelitic schist and silicified pelitic schist in the Iwakuni—Yanai area of the Ryoke Belt, southwest Japan.

Abstract

Pelitic metamorphic rocks of the Ryoke Belt are distributed in the Rokuroshi area, southern part of Iwakuni district. Dark-brown biotite schist was locally silicified and decolorized to form milky-white “silicified domain”. Quartz veins were developed in both of the biotite schist and silicified domain. The veins in biotite schist are generally parallel to the schistosity and form boudinage due to ductile flow of the host rock. The veins in silicified domain are oblique to the schistosity with medium to high angles and have not undergone ductile deformation except for some schistosity-parallel veins. The mode of occurrence of these veins indicates that the silicified domain is much more competent than the biotite schist. Ductile deformation after the silicification was accommodated by viscous flow of biotite schist. The silicification probably results from dissolution-precipitation processes which may have raised pore pressure to cause hydraulic fracturing.