Formation mechanism of the Permo-Triassic unconformity and its tectonic-sedimentary response in the East Junggar orogen, NW China

The East Junggar orogen is a major constituent of the southwestern Central Asian Orogenic Belt (CAOB). The Permian-Triassic strata exposed there is the first terrestrial sedimentary cover overlaying the Palaeozoic volcanic basement, and are well developed in the Kalamaili Mountain in the East Junggar. An obvious angular unconformity developed between the Permian and Triassic in the East Junggar, which is also widely seen in the Tian Shan and the Junggar Basin. Based on field observations, petrology, sedimentology, and detrital zircon U-Pb geochronology, we evaluate different mechanisms for the Permian-Triassic angular unconformity, clarify the characteristics of the tectonic movement during Permian-Triassic time and its control on the sedimentation in the East Junggar region, and identify the mountain-basin coupling processes.

The sedimentary characteristics of the deposits show an abrupt change in depositional environments from a fluvial delta-lacustrine depositional system to an alluvial fan across the unconformity. The yellow conglomerate developed in the Middle Permian Pingdiquan Formation contains rounded and imbricated clasts, which indicate a relatively strong hydrodynamic environment within a fluvial delta. The red-brownish conglomerates in the Lower Triassic Cangfanggou Formation have poor sorting and are interpreted as typical alluvial fan deposits. Meanwhile, the heavy minerals statistics from sandstones show a higher abundance of stable heavy minerals in the Middle Permian Pingdiquan Formation than in the Lower Triassic Cangfanggou Formation, which indicates a provenance change from distal to proximal.

The detrital zircon U-Pb ages from 9 clastic rock samples from the south flank of the Kalamaili Mountain show that in the Early Permian, a prominent age peak of ~330 Ma occurred. These sediments may have been derived from neighboring source areas, such as the Early Carboniferous volcanic rocks in the Kalameli tectonic belt. The Middle Permian samples show multiple age peaks of ~260 Ma, ~320 Ma, ~360 Ma, and ~510 -420 Ma, indicating that the source area gradually became enriched from other sediment sources. The source might include the Early Carboniferous volcanic rocks, Late Carboniferous granites, northern Early Paleozoic Yemaquan Island arc (~450 Ma), and/or the Early Paleozoic Karameli ophiolite (~490 Ma). The occurrence of these units marks the uplift of the East Junggar orogen range and significant deep terrane exhumation. The age peaks of the red conglomerate in the Lower Triassic are more diverse ~830-260Ma, which suggests that this region is still in a state of uplift and denudation, providing continuous supply to the basin. Combined with sedimentary characteristics and published data, our new data suggest that the East Junggar has also gone through a tectonic inversion in the Late Permian after large-scale orogenesis in the Late Carboniferous and post-collisional extension in Early-Middle Permian. Together, these events caused the Permian-Triassic angular unconformity and different sedimentary environments. In addition, results are consistent with the published conclusion that an uplift occurred in the Northern-Central Tianshan and the Bogda mountain in the southeastern Junggar in the Late Permian. These results provide vital data for reconstructing the Pangea supercontinent and the Hercynian movement of the Central Asian Orogenic Belt.