Development of Deformation Bands and Deformation Induced Weathering in a Forearc Coal-Bearing Paleogene Fold Belt, Northern Japan

Abstract

We conducted microstructural and microchemical analyses of deformation bands in a forearc fold belt consisting of the Eocene Urahoro Group located in northern Japan. In the study area, there was one flexure (or monocline) developed where deformation bands pervasively occurred in arkosic sandstone intercalated with mudstone and coal layers. Deformation bands formed at the maximum burial depth of c. 1.5–2.5 km; this was inferred from both the thickness of the overlying strata and vitrinite reflectance values (%RO) of the coal layers (c. 0.5). These bands were inferred to have originated as phyllosilicate bands, which developed into cataclastic bands with increasing strain on sandstones with up to c. 10% volume of phyllosilicate. In the cataclastic bands, the detrital grains in the host parts were crushed into sizes less than one-half to one-fifth of the original ones, and the long axis of the fractured grains tended to align parallel to the deformation bands. It was found that the deformation bands became a site of intense weathering at later stages, where not only detrital biotite grains were altered to vermiculite and kaolinite, but also authigenic clay minerals such as smectite grew in pore spaces created by the fracturing of detrital grains.