Structural Geology and Chronology of Shear Zones along the Shangdan Suture in Qinling Orogenic Belt, China: Implications for Late Mesozoic Intra-Continental Deformation of East Asia

The Shangdan suture zone (SDZ) in the Qinling orogenic belt (QOB) is a key to understanding the East Asia tectonic evolution. The SDZ gives information about convergent processes between the North China Block (NCB) and South China Block (SCB). In the Late Mesozoic, several shear zones evolved along the SDZ boundary that helps us comprehend the collisional deformation between the NCB and SCB, which was neglected in previous studies. These shear zones play an essential role in the tectonic evolution of the East Asia continents. This study focuses on the deformation and geochronology of two shear zones distributed along the SDZ, identified in the Shaliangzi and Maanqiao areas. The shear sense indicators and kinematic vorticity numbers (0.54–0.90) suggest these shear zones have sinistral shear and sub-simple shear deformation kinematics. The quartz’s dynamic recrystallization and c-axis fabric analysis in the Maanqiao shear zone (MSZ) revealed that the MSZ experienced deformation under green-schist facies conditions at ∼400–500 °C. The Shaliangzi shear zone deformed under amphibolite facies at ∼500–700 °C. The 40Ar/39Ar (muscovite-biotite) dating of samples provided a plateau age of 121–123 Ma. Together with previously published data, our results concluded that QOB was dominated by compressional tectonics during the Late Early Cretaceous. Moreover, we suggested that the Siberian Block moved back to the south and Lhasa-Qiantang-Indochina Block to the north, which promoted intra-continental compressional tectonics.