## 4 Results
### Microstructural Analysis  
#### 4.2.1 Microstructure  
In the B4 core sample, a c. 20 cm thick gouge (see [45; 46] for definition) developed at the depth interval between 5.0 and 5.2 m. In this gouge sample (B4_5.1 m, Table 2), the original sandstone layers were pervasively fractured, and anastomosing cracks developed along which detrital chlorite and illite grains were aligned (Figure 4). The anastomosing cracks were mostly oriented in two different directions, which could be correlated with composite planar structures R-Y-P (e.g., [47]). However, since it is difficult to identify which of these cracks can be correlated with the R, Y or P plane, the sense of shear along this gouge zone cannot be clearly inferred. The detrital chlorite grains with a very elongated shape aligned along the anastomosing cracks which were possibly formed by fracturing along cleavages from the original large detrital chlorite grains. Adjacent to this gouge zone, a weakly sheared sandstone occurred. This sandstone contained many detrital biotite grains, some of which were fractured and peeled off from the host biotite grains along cleavages. The same microstructures occurred in detrital biotite grains in the host parts of the deformation bands in borehole core sample B6 which will be described below. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample No.**} & **Core or** & \multirow{2}{*}{**Formation**} & \multirow{2}{*}{**Lithology**} & \multirow{2}{*}{**Analysis**} \\  & **Outcrop** & & & \\  & **Sample** & & & \\ \hline B4\_5.1 m & core & Shakubetsu F. & gouge & micro. \\ B4\_5.3 m & core & Shakubetsu F. & coal & vitri. \\ B6\_8.2 m & core & Shakubetsu F. & v.f.s. to f.s. with mudst. and coal & micro. image a. EPMA \\ B6\_8.6 m & core & Shakubetsu F. & coal & vitri. \\ KT1 & outcrop & Shakubetsu F. & v.f.s. to f.s. & micro. SEM-EDS \\ KT2 & outcrop & Shakubetsu F. & f.s to m.s. with mudst. and coal & micro. SEM-EDS, EPMA, image a. \\ KT3 & outcrop & Shakubetsu F. & v.f.s. & micro. SEM-EDS, EPMA \\ KT4 & outcrop & Shitakara F. & f.s. to m.s. & image a. \\ KT5 & outcrop & Shakubetsu F. & v.f.s. with mudst. and coal layers & vitri. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summaries of analyzed samples. Figure 4: Microstructures in gouge from borehole core sample B4 (crossed polarized light). Dashed lines indicate composite planar structures, along which elongated chlorite detrital grains were aligned. Inset diagram shows the cross-section of borehole core sample B4, the locality for which is indicated in Figure 3b. The dot in the inset diagram shows the locality of the thin section. The up direction of the borehole is shown by arrows. Qz, quartz; Chl, chlorite. The deformation bands developed in borehole core sample B6 (B6_8.2 m) are typical deformation bands (Figure 5), where very-fine fractured detrital grains constitute thin bands up to 1 mm (mostly a few hundreds of microns) wide. These deformation bands are surrounded by undeformed or weakly deformed very-fine- to fine-grained sandstones consisting of detrital grains with diameters varying between 50-150 mm (Host, Figure 5a). Another interesting feature is that very thin coal layers (less than 1 mm) were separated by many extension fractures, which are reminiscent of a coal cleat [48]. Further, these coal layers peel off parallel to the bedding and were incorporated into the deformation bands as very-thin coal seams (Figure 5b). The coal seams were flowing in the deformation bands, showing an irregularly curved shape, which suggested that fractured detrital grains flowed in the deformation bands (i.e., granular-particulate flow, e.g., [44]). Also, there were many fractured very-fine-grained detrital biotite grains embedded in the deformation bands, which were aligned slightly oblique to the deformation band boundary forming a foliation (Figure 5c). In addition, there are kinked and extended detrital biotite grains in host parts, which were caused by sliding along the cleavage plane (cleavage fracturing, Figure 5d).