## 4 Discussion
### Constraints of Microstructure and Rock Chemical Composition on the Genesis of Pseudotachylyte  
The microstructures indicate that the rocks collected from the Anning River fault zone underwent deformation during different stages, including interseismic plastic flow, coseismic brittle fracturing, pseudotachylyte formation, and even post-seismic relaxation and fracture healing. The deformation of mylonite represents interseismic plastic deformation. Mylonite develops an S-C fabric, with feldspar showing a porphyoclastic texture, while quartz and mica are significantly flattened, elongated, and oriented, forming foliations. The pervasive occurrence of feldspar twins and the undulose extinction of quartz indicate that the rock started to be stress-controlled, and high dislocation densities began to develop within the quartz grains. The fractured structures in feldspar grains, as well as the fine-grained quartz particles developing subgrains, grain rotation, and grain boundary migration, suggest that while quartz started to undergo dislocation creep and other plastic deformations, feldspar deformed mainly by brittle fracturing. The deformation of fractured rocks and pseudotachylytes represents coseismic rupture. The primary deformation characteristic is the presence of fractured feldspar and other minerals, with angular or sub-angular clasts, and fractures filled by fine-grained quartz aggregates. Based on the EBSD analysis, the deformation temperature of the fractured rocks is about 250-350 \({}^{\circ}\)C, which corresponds to the brittle-ductile transition temperature (depth) range of the Anning River fault zone. In sample AF21-4-2, multiple generations of pseudotachylytes are visible. The first generation of veins, formed earlier, is not related to the current coseismic rupture. The second and third generations of veins were both formed during the coseismic rupture phase. After the second-generation vein formation, the viscosity of the melt temporarily restored the fault strength (which may explain why the clasts in the second-generation vein are larger than those in other generations). After the fault was cemented, sliding friction did not stop but continued along the adjacent pre-existing fault surface, forming the third-generation veins. Samples AF19-43G and AF19-44C all show fine-veined pseudotachylytes along foliations. The localization of plastic strain leads to crystal growth, recrystallization, and crystal preferred orientation in the pseudotachylyte matrix. The clasts remaining in the pseudotachylyte may also undergo plastic deformation. These features correspond to the first-generation vein in AF21-4-2, indicating that multiple seismic events occurred in the Anning River fault zone, and each coseismic pseudotachylyte formation underwent later geological deformation and transformation, eventually becoming the \"mother rock\" in the next earthquake event. In samples AF19-43A and AF19-43D, localized devitrification of the pseudotachylyte is evident. Due to the glassy or cryptocrystalline matrix in the pseudotachylyte, the pseudotachylyte is especially prone to recrystallization under fluid (water) influence. Newly formed minerals may partially or completely cover the original microstructure. These observations suggest that pseudotachylytes are difficult to preserve after formation and can be easily modified in later geological processes, which may be one of the reasons why pseudotachylytes are rarely found or reported. In the fractured rock, the feldspar phenocrysts' fractures are healed by quartz veins, indicating post-seismic relaxation. Fine-grained quartz particles exhibit elongation and orientation, showing plastic flow deformation, subgrain development, grain boundary migration, and recrystallization. The chemical composition analysis of pseudotachylyte and its host rocks reveals critical insights into the melting behavior of minerals during coseismic frictional processes.