## 3 Results
### Microstructures of Pseudotachylyte  
#### 3.1.1 Straight Vein-Type Pseudotachylyte in Granite Gneiss and Cataclasite  
The straight vein-type pseudotachylyte appears in the cataclasite, with a straight, black pseudotachylyte vein clearly delineated from the host rock. (1) Sample AF19-44A is a pseudotachylyte vein developed in granite gneiss, with a distinct boundary between the vein and the host rock. The vein has a banded appearance, with a width of approximately 0.5 cm (Figure 4A). This is the dominant type of pseudotachylyte distribution in the study area. The pseudotachylyte vein is dark to black with a clastic texture both in field samples and under Optical Microscopy and SEM. The angular clasts consist of plagioclase and quartz. The plagioclase clasts in the host rock are angular or sub-angular, while the quartz clasts are sub-angular or sub-rounded (Figure 4B-D). In the host rock, brittle fracturing to form cataclasite (Figure 4A, red) and localized S-C structures from initial mylonitization (Figure 4A, yellow) are observed. Under scanning electron microscopy, plagioclase, quartz, and dark minerals are seen to be flattened and elongated in a directional manner, with folding (Figure 4E-G). The microstructure of this sample suggests that the granite gneiss, with fractured rocks forming cataclasite, produced the pseudotachylyte vein during the coseismic rupture process. It is hypothesized that the mylonitized and cataclasitic granite gneiss surrounding the pseudotachylyte vein serves as its protolith. Due to the high-temperature melting and coseismic deformation, the contact area between the pseudotachylyte and the host rock underwent plastic deformation. \begin{table}
\begin{tabular}{l l l l} \hline \multicolumn{1}{c}{**Analytical Method**} & \multicolumn{1}{c}{**Research Objective**} & \multicolumn{1}{c}{**Technical Features**} & \multicolumn{1}{c}{**Co-Application Necessity**} \\ \hline EBSD & \begin{tabular}{l} Quartz crystallographic \\ orientation determination \\ Deformation \\ mechanism inversion \\ \end{tabular} & \begin{tabular}{l} Nanoscale crystallographic \\ orientation imaging \\ CPO pattern reconstruction \\ \end{tabular} & \begin{tabular}{l} The EBSD crystallographic \\ orientation data and XRD \\ phase analysis jointly \\ constrain the deformation \\ history and the formation \\ depth of the melt \\ \end{tabular} \\ \hline XRF & \begin{tabular}{l} Whole-rock major \\ element analysis \\ \end{tabular} & \begin{tabular}{l} Bulk chemical \\ composition determination \\ \end{tabular} & \begin{tabular}{l} XRF bulk chemistry \\ coordinates with EPMA \\ \end{tabular} \\ \hline EPMA & \begin{tabular}{l} Mineral micro-domain \\ chemical variation analysis \\ \end{tabular} & \
\begin{tabular}{l} Micron-scale element \\ quantification (0.1–5 μm) \\ \end{tabular} &
\begin{tabular}{l} micro-chemistry to reveal \\ element migration patterns \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 1: Cont. Figure 4: Microstructures of a straight vein of pseudotachylyte in AF19-44A. (**A**) AF19-44A under plane-polarized light (PPL). Red solid lines demarcate brittle fractures; yellow dashed lines indicate domains of plastic deformation. (**B**) AF19-44A under cross-polarized light (XPL). (**C-G**) Scanning electron micrographs (SEM) of AF19-44A. (2) In sample AF21-4-3, the boundary of the black pseudotachylyte vein is straight and distinct (Figure 5A-C), representing the deformation of the coseismic rupture stage. The host rock is an equigranular granite gneiss, where numerous brittle fractures are observed (Figure 5A, red) and localized plastic deformation is present around the vein (Figure 5A, yellow). The host rock exhibits localized fracturing to form cataclasite, with the fractured clastic angular clasts being angular or sub-angular plagioclase and sub-angular or sub-rounded quartz grains (Figure 5D,E). This indicates that the host rock of the pseudotachylyte is cataclasite fractured from granite gneiss.