Kinematics and Geochronology of Late Paleozoic–Early Mesozoic Ductile Deformation in the Alxa Block, NW China: New Constraints on the Evolution of the Central Asian Orogenic Belt

Abstract

Strike-slip faults are widely developed throughout the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic collages in the world, and may have played a key role in its evolution. Recent studies have shown that a large number of Late Paleozoic–Early Mesozoic ductile shear zones developed along the southern CAOB. This study reports the discovery of a NW–SE striking, approximately 500 km long and up to 2 km wide regional ductile shear zone in the southern Alxa Block, the Southern Alxa Ductile Shear Zone (SADSZ), which is located in the central part of the southern CAOB. The nearly vertical mylonitic foliation and subhorizontal stretching lineation indicate that the SADSZ is a ductile strike-slip shear zone, and various kinematic indicators indicate dextral shearing. The zircon U-Pb ages and the 40Ar/39Ar plateau ages of the muscovite and biotite indicate that the dextral ductile shearing was active during Middle Permian to Middle Triassic (ca. 269–240 Ma). The least horizontal displacement of the SADSZ is constrained between ca. 40 and 50 km. The aeromagnetic data shows that the SADSZ is in structural continuity with the coeval shear zones in the central and northern Alxa Block, and these connected shear zones form a ductile strike-slip duplex in the central part of the southern CAOB. The ductile strike-slip duplex in the Alxa Block, including the SADSZ, connected the dextral ductile shear zones in the western and eastern parts of the southern CAOB to form a 3000 km long E-W trending dextral shear zone, which developed along the southern CAOB during Late Paleozoic to Early Mesozoic. This large-scale dextral shear zone was caused by the eastward migration of the orogenic collages and blocks of the CAOB and indicates a transition from convergence to transcurrent setting of the southern CAOB during Late Paleozoic to Early Mesozoic.

An Evolving Lithospheric‐Scale Wrench Fault System Along the Eastern End of the Altyn Tagh Fault: Kinematics and Quaternary Activity of the Heishan Fault System, Western China

“…They determined the timing of the shearing to be 260∼240 Ma using zircon U‐Pb dating and 40 Ar/ 39 Ar dating of muscovite and biotite from the fault zone tectonites. This is consistent with previously reported late Paleozoic‐early Mesozoic regional dextral shear along the northern margin of the Hexi Corridor, including the Heishan, possibly driven by eastward migration of orogenic terranes of the southern Central Asian Orogenic Belt (B. Zhang, Zhang, Zhao et al., 2021; J. Zhang et al., 2022 and references therein). Regional‐scale dextral shear is also supported by aeromagnetic trends (J. Zhang et al., 2022; Y. Zhang, Zhang, Zhao, et al., 2021; Zhao et al., 2022).…”

“…This is consistent with previously reported late Paleozoic‐early Mesozoic regional dextral shear along the northern margin of the Hexi Corridor, including the Heishan, possibly driven by eastward migration of orogenic terranes of the southern Central Asian Orogenic Belt (B. Zhang, Zhang, Zhao et al., 2021; J. Zhang et al., 2022 and references therein). Regional‐scale dextral shear is also supported by aeromagnetic trends (J. Zhang et al., 2022; Y. Zhang, Zhang, Zhao, et al., 2021; Zhao et al., 2022). Thus, we suggest that the sinistral shear along the main HF documented in this study (e.g., Figures 9d, 9e and 10, 11, and 13) represents reactivation and tectonic inversion of an older dextral shear zone.…”

“…In the Kuantanshan, Helishan, and Longshoushan (Figure 1), B. Zhang, Zhang, Zhao, et al. (2021) and Zhao et al. (2022) also reported nearly E‐W‐trending ductile shear zones with mylonitic foliation and sub‐horizontal stretching lineations (J. Zhang et al., 2007, 2022).…”

“…Regional-scale dextral shear is also supported by aeromagnetic trends (J. Zhang et al, 2022;Y. Zhang, Zhang, Zhao, et al, 2021;Zhao et al, 2022).…”

“…Prior research concerning the tectonic geomorphology and landscape evolution of these ranges has suggested that they were uplifted during 2∼0.6 Ma (Hetzel et al., 2004; Pan et al., 2021; W. J. Zheng, Zhang, Ge, et al., 2013). However, apatite fission track analysis of exhumed bedrock suggests that the Kuantanshan and Heishan were rapidly uplifted and eroded initially during the Eocene (50∼35 Ma) perhaps as a distant response to the early stages of the Indo‐Asia collision (Figure 2, An et al., 2020; H. Zhang, Zhang, Li, et al., 2021).…”