## 5 Discussion  
### Phase D\({}_{1}\) Deformation: Timing and Dynamical Context  
Muscovite \({}^{40}\)Ar-\({}^{39}\)Ar dating was conducted on intensely deformed albite-quartz-mica schist subjected to brittle-ductile shearing. The results yield a deformation age of approximately \(\sim\)79 Ma. Electron backscatter diffraction (EBSD) analyses of quartz from albite-muscovite-quartz schist and mylonitized dactic within the S\({}_{1}\) foliation of the D\({}_{1}\) deformation phase have been conducted by previous researchers. The results reveal a deformation temperature \(\leq\) 380 \({}^{\circ}\)C, characterized by subgrain rotation recrystallization, and deformation-metamorphism temperatures of 420-350 \({}^{\circ}\)C, estimated using the chlorine geothermometer [25]. These data suggest that the obtained age approximately corresponds to the peak stage of brittle-ductile shear deformation. Meanwhile, the overlying Late Jurassic to Late Cretaceous strata deposited above the Yeba Group significantly increased the burial depth, providing suitable pressure-temperature conditions for D\({}_{1}\)-phase brittle-ductile deformation. The kinematic features of high-angle, westward-plunging mineral stretching lineations indicate the westward extrusion of the Yeba Group as a whole (Figure 3j). Consequently, under a nearly N-S-oriented principal stress field, an extrusion-related structure formed within a transpressional tectonic setting. Kinematic analyses reveal the coexistence of sinistral and dextral shear-rotated porphyroclasts within the Yeba Group. In the absence of cross-cutting relationships between these shear fabrics, we propose that the coexisting synthetic and antithetic porphyroclasts formed synchronously under a single phase of brittle-ductile shearing within a compressional stress regime [26]. Field observations and regional correlations indicate that phase D\({}_{1}\) deformation is well developed in the Lower Jurassic Yeba Formation and the basal DuodigouFormation but notably absent in the overlying Linbuzong Formation and younger units [27]. These observations suggest that the D\({}_{1}\) deformation may have occurred during the subduction stage of the Neo-Tethys Ocean, likely after the Early Cretaceous. Additionally, stratigraphic studies of the Cretaceous sequences in the Southern Gangdese region indicate that the area experienced a back-arc compressional setting after approximately \(\sim\)90 Ma. It is further inferred that large-scale folding and detachment-related structural deformation occurred between \(\sim\)85 and 69 Ma [15]. The aforementioned studies suggest that this deformation phase was likely initiated post-Early Cretaceous and was governed by Neo-Tethys Ocean subduction. The brittle-ductile shearing fabrics developed under a compressional regime rather than an extensional setting. During the Late Cretaceous (\(\sim\)90-79 Ma), the Neo-Tethys Ocean subducted northward beneath the Lhasa terrane [28; 29], leading to the development of a large-scale Gangdese magmatic arc along the southern margin of the Lhasa terrane. Under the influence of this northward subduction, the Cretaceous Gangdese back-arc basin experienced N-S-directed compressional stress. Based on the \({}^{40}\)Ar-\({}^{39}\)Ar muscovite age of \(\sim\)79 Ma obtained from phyllite in this study, we infer that, around \(\sim\)79 Ma, a ductile shear zone developed within the Yeba Group under a transpressional regime, resulting in the D1-phase ductile deformation event (Figure 7a). Figure 7: Tectonic evolution model of the Yeba Volcanic Arc. **(a)** Ductile shear zones developed in the Yeba Rock Group under a transpressional regime during the Late Cretaceous; **(b)** fold deformation formed in the Yéba Rock Group and Duodigou Formation under N-S compressional settings during the Late Cretaceous; **(c)** ductile shear zones in the Yeba Rock Group developed due to combined effects of GCT activity and the exhumation of the Gangdese baltholit during the Oligocene-Miocene.