Microstructures, Quartz Fabrics, Flow Vorticity Analyses and Zircon U-Pb Ages of the Tuerhongshate Ductile Shear Zone, Eastern Irtysh Tectonic Belt, Southern Chinese Altai Orogen, and Tectonic Implications

Abstract

The Irtysh tectonic belt lies on the southern margin of the Chinese Altai Orogen. Several secondary shear zones with NW-SE strikes have developed in this tectonic belt, and the deformation processes are of great significance to understanding the tectonic regime of the Altai Orogen in the Late Paleozoic. The Tuerhongshate ductile shear zone is located in the eastern Irtysh tectonic belt with obvious deformed structures. The felsic rocks are strongly mylonitized, exhibiting S-C fabrics, asymmetric rotational porphyroclasts, and bookshelf structures of the plagioclases, indicating a sinistral shear sense. The deformation mechanisms, lattice preferred orientations (LPOs) of quartz, and opening angles of quartz c-axis suggest that the deformation temperatures range from 400 to 500 °C, consistent with higher-greenschist to lower-amphibolite facies conditions. The calculated kinematic vorticity values (W_k) of the studied samples range from 0.53 to 0.89 and indicate general shear to simple shear, based on rotational rigid porphyroclast method and oblique grain-shaped/quartz c-axis fabric method. The U-Pb ages of magmatic zircons in felsic mylonites indicate that the sinistral shear occurred after 296.7 ± 3.0 Ma (Early Permian) in the Tuerhongshate shear zone and persisted for approximately 13 Ma. Combined with the tectonic setting and the observed sinistral strike-slip shear indicators in the mylonite zone, these features demonstrate that the Irtysh tectonic belt was in a post-orogenic and strike-slip environment following the closure of the Irtysh Ocean.

Keywords

Irtysh tectonic belt / sinistral shear / microstructures / quartz c-axis fabrics / flow vorticity / post-orogenic / rheology / tectonics

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