Recognition, age determination and tectonic significance of the Kunlun River ductile shear zone in the East Kunlun Mountains

Along the Kunlun River of Qinghai Province in the East Kunlun Mountains, a large-scale ductile shear zone is developed. The rocks that involved in the ductile shear deformation and experienced extensive mylonitization include the metamorphosed basic to intermediate volcanic rocks, clastic rocks and marble of the Middle-Upper Proterozoic Wanbaogou Group, the metamorphosed clastic rocks, basalt and marble of the Ordovician Nachitai Group, and the Silurian intermediate to acid intrusive rocks. The structural marks observed in the shear zone, such as the σ-type rotated porphyroclasts, S-C fabrics and the asymmetrical folds of mylonite foliation, indicate in general a dextral strike-slip shear superimposed by a later sinistral shear. Zircon LA-ICP-MS U-Pb ages of the deformed granodiorite and the undeformed muscovite granite in the shear zone are 432.3±1.2Ma and 423.3±1.5Ma, respectively, suggesting that the shear zone formed between ca. 432Ma and 423Ma. Combined with the regional tectonic evolution, it is suggested that the Kunlun River shear zone represents the tectonic response to the collisional orogeny in the Middle-Late Silurian between the North and South Kunlun blocks after the closing of the Proto-Tethys Ocean. The later sinistral shearing was controlled by the strike-slipping of the South Kunlun fault zone in the early Yanshanian period.