## 3 Results
### Microstructures of Pseudotachylyte
#### 3.1.3 Multi-Stage Pseudotachylyte Veins in Granite Gneiss
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Vein Type** & **Host Rock** & **Morphology** & **Key Characteristics** & **Representative Samples** \\ \hline \multirow{7}{*}{Straight vein-type} & & & Dark to black color with clastic texture & \\  & & & Angular/sub-angular plagioclase and & \\  & & & quartz clastics & \\  & & & Evidence of brittle fracturing & \\  & & & forming cataclastic & \\  & & and cataclastic & appearance with & Localized S+C structures from & AF19-44A, AF21-4, \\  & & & clear boundaries & initial myelitization & AF19-43A, AF19-43D \\  & & & Flattened and elongated minerals with & \\  & & & folding under SEM & \\  & & & Formed during coseismic rupture with & \\  & & & high-temperature melting & \\ \hline \multirow{7}{*}{Fine vein-type} & & & Clear but discontinuous boundaries & \\  & & & Intermittent or tapering, curved shapes & \\  & & & Alternatives with quartz bands in the & \\  & & & directional arrangement & \\ \cline{1-1}  & & & exhibits plastic deformation features & \\ \cline{1-1}  & & & flow types of quartz grains larger deformed & \\ \cline{1-1}  & & & & grains and the aggregates & \\ \cline{1-1}  & & & Shows medium-temperature dislocation & \\ \hline \multirow{7}{*}{Multi-stage veins} & & & Three stages are visible with clear boundaries & \\ \cline{1-1}  & & & First stage: irregular shape, high & \\ \cline{1-1}  & & & matrix content & \\ \cline{1-1}  & & & Second stage: lighter color, higher clastic & \\ \cline{1-1}  & & & proportion & \\ \cline{1-1}  & & distinct cutting relationships & Third stage: similar to straight vein-type, & \\ \cline{1-1}  & & & higher matrix content & \\ \cline{1-1}  & & & De-glassified cryptocrystalline mineral & \\ \cline{1-1}  & & & aggregates in matrix & \\ \cline{1-1}  & & & Evidence of multiple coseismic slip events & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of pseudotachylyte vein types. system of quartz deformation can be identified, thus allowing for the determination of the deformation temperature. In this study, the host rock of pseudotachyllyte consists of granite gneiss and granitic mylonite. To determine the depth (temperature) conditions of pseudotachyltyte formation, the most typical straight vein-type pseudotachyltyte samples, AF21-4-2 and AF21-4-3, were selected for EBSD analysis of quartz in the host rock (parent rock of the melt), fractured granite gneiss (Figure 11). For sample AF21-4-2, EBSD analysis of fine-grained quartz in the host rock identified 1249 distinct grains. The pole figure of the c-axis [001] shows a clear orientation, consistent with low-temperature basal dislocation slip creep. For sample AF21-4-3, a surface scan was performed on quartz in the host rock, and noise reduction was carried out using Channel 5 software. A total of 501 distinct quartz grains were identified, and a pole figure was generated (Figure 11B). Similarly, the pole figure of the c-axis [001] of quartz in this sample shows clear orientation, and the extreme density pattern indicates low-temperature basal <a> dislocation slip creep characteristics. EBSD analyses of quartz in the granitic host rock reveal a pronounced c-axis alignment characteristic of low-temperature basal <a> dislocation creep, indicating deformation temperatures of roughly 250-350 \({}^{\circ}\)C in the fractured granite gneiss surrounding straight vein-type pseudotachyltyte. By correlating these temperatures with typical geothermal gradients, the host-rock deformation--and hence melt injection--depth is constrained to the brittle-ductile transition zone at approximately 15-20 km. Importantly, this melt is not derived from high-temperature magmatic intrusion but is instead generated in situ by coseismic, high-velocity frictional heating along fault planes. Thus, EBSD-derived quartz slip system characterization provides a robust thermobarometric framework for estimating both host rock deformation conditions and the depth of pseudotachyltyte formation.