## 5 Discussion
### Strain Localization in the Flexure and Implications in Tectonics  
We have not found any occurrence of deformation bands in the study area other than the boreholes (B4, 5 and 6) and outcrops where samples KT1, 2 and 3 were collected. Therefore, these deformation bands only occurred in the sandstones of the Shakubetsu Formation, which includes mudstones and coal layers other than sandstones, not in sandstones from the uppermost unit of the Shitakara Formation, which solely consists of sandstones. It should be noted that these deformation bands are oriented nearly parallel or slightly oblique to the bedding of strata, and they are also considered to be CSBs or SECBs formed in the flexure under the setting of a strong contraction. Therefore, these deformation bands could have formed by overall layer-parallel shortening, which also yielded a component of layer-parallel shearing (samples B6\(8\)2 m, KT1 and 3) and the conjugate shear plane (sample KT2) during the initial stage of flexure formation, as schematically illustrated in Figure 15. It has also been shown by [7] that bedding-parallel deformation bands formed during the initial stage of folding in the Jurassic Navajo Sandstone, San Rafael monoclinic, Utah, USA, when the dip of forelimb was around 25\({}^{\circ}\). It has been shown that deformation bands were localized around a regional thrust, and conjugate sets of SECBs formed at an early stage of buckling in the stress concentration area at the thrust tip [4; 56]. In the present case, fault gouges in borehole sample B4_5.1 m could be interpreted to constitute the fault core, which was surrounded by a damage zone characterized by the pervasive development of deformation bands in the borehole core sample B6_8.2 m and outcrop samples KT1, 2 and 3. This fact suggested that the present deformation bands also developed in relation to thrusting, although the sense of shear in the gouge from borehole core sample B4_5.1 m could not be constrained. It has been recently described that the layer parallel deformation bands could have formed as R-bands in combination with the formation of Y- and P-bands in the initial stage of folding in a monoclinic consisting of the Upper Triassic to Lower Jurassic sandstones in the western United States [9]. The composite planar structures could be compared with those found in B4_5.1 m (Figure 4) and B6_8.2 m (Figure 5c) core samples. If this is correct, the present gouge and deformation bands could have also formed in the early stage of flexure formation. Accordingly, the composite planar structures in gouge in core sample B4_5.1 m (Figure 4) and layer parallel deformation bands (Figure 5a,b) and oblique foliation (Figure 5c) in core sample B6_8.2 m could be correlated with R- and Y- or P-bands (Figure 15, [9]). It could be inferred that this flexure, which perhaps accompanies a blind thrust (Figure 15), developed in the Shakubetsu Formation because this formation containing mudstones and coal layers other than sandstones could be much weaker than the sandstone unit of the Shitakara Formation. Therefore, the weakest part in the studied Urahoro Group could have been selected for the development of flexure accompanied by the formation of deformation bands and gouge. Finally, we would like to emphasize the following points. As far as we know, this study first reported that deformation bands developed in arkosic sandstones from forearc fold-and-thrust belts in the Japanese Islands. Although deformation bands have been reported from the accretionary prism in the Nankai Trough region of Shikoku [57] and the Miura-Boso Peninsula, central Japan [58], the host materials are argillaceous or volcanicistic sediments, and thus these deformation bands are not comparable to the present ones formed in arkosic sandstones. Furthermore, microstructures of fractured grains constituting the deformation bands have not been reported in these studies.