Early Cretaceous terminal convergence between the North China-Mongolia and Siberia continents: Imprints from the Chagan Obo shear zone in central Inner Mongolia, North China

Abstract

The ENE-striking Chagan Obo shear zone in central Inner Mongolia is critical to constrain the termination of convergence between the North China-Mongolia and Siberia continents. We recognized three ENE-striking zones with strong deformation in the Permian biotite monzogranite within the shear zone and investigated the structural and kinematic characteristics. The strong deformation zones developed dense ENE-striking and NNW-dipping foliations. The widespread shear sense indicators such as S-C foliations, mica fish, σ-type, structures, and typical tight-to-isoclinal asymmetric folds show top-to-the-SSE thrusting through NNW–SSE compression. Two 40Ar-39Ar ages of muscovites from the muscovite quartz schists in this shear zone show 144.1 ± 1.2 Ma and 142.1 ± 0.9 Ma, respectively, providing the peak compression to be Early Cretaceous (144–142 Ma). Combined with the regional syn-/post-collisional magmatic rocks (166–155 Ma) related to the closure of the Mongol-Okhotsk Ocean, this shear zone's ages (144–142 Ma) suggest that the continental convergence between the North China-Mongolia and Siberia continents lasted from Late Jurassic to Early Cretaceous. The Early Cretaceous shoshonitic volcanism (137–133 Ma) and rift basins in this region indicate the onset of orogenic collapse in the eastern Central Asian Orogenic Belt.