Deformation and geochronology of a ductile shear zone on the south side of Foping Dome in the South Qinling

The Foping area in the Qinling orogenic belt is characterized by the concentrated development of granulite-migmatite-gneiss domes, making it a crucial region for studying the rheological mechanisms of the middle to lower continental crust and the Mesozoic tectonic evolution of the Qinling Mountains. The Yangtianba-Shimudi ductile shear zone, located at the southern margin of the Foping dome, preserves abundant information on mid- to deep-level structural deformation during the orogenic process. Multi-scale structural analysis and kinematic vorticity analysis indicate that this shear zone is dominated by pure shear with a right-lateral ductile shear deformation. In the felsic mylonites, quartz primarily exhibits prism <a> and prism <c> slip systems, suggesting deformation occurred under amphibolite facies conditions at approximately 550-650 °C. The characteristics of the metamorphic mineral assemblages and the garnet-biotite-plagioclase thermobarometry results indicate a clockwise P-T path, with peak metamorphic conditions of 568-611 °C/5.2-5.3 kbar and 630-654 °C/7.1-7.9 kbar. The isothermal decompression stage M2 recorded conditions of 590-616 °C/3.5-4.5 kbar. Zircon U-Pb dating of the leucosomes in the migmatites within the shear zone yielded an age of 180.8±3.8 Ma, representing the lower limit of the ductile shear deformation. Integrated with regional geological data, the metamorphic and deformational evolution of the study area can be reconstructed as follows: Prior to ~210 Ma, the central segment of the South Qinling tectonic belt was dominated by collisional orogenesis, leading to crustal thickening and the development of progressive metamorphism (M1) in the Foping area. During 210-200 Ma, the Foping region transitioned into a tectonic regime shifting from collisional orogenesis to post-collisional extension. This transitional phase was characterized by a bidirectional stress regime combining horizontal shortening and vertical collapse, which triggered ductile shear deformation (D1) in the Yangtianba-Shimudi area and initiated the isothermal decompression metamorphic event (M2). By 180 Ma, the region had fully entered the post-collisional extensional stage, during which a decompression-induced partial melting event developed in the northern part of the study area. Subsequently, during the exhumation of the shear zone, the mylonitic foliation and metamorphic foliation (S1) were overprinted by late-stage folding and crenulation cleavage, resulting in foliation transposition (D2).