Re-understanding on the tectonic implication and evolutional model of the "North Tianshan Ocean" from the eastern Tianshan area, Xinjiang, NW China

Several Paleozoic ophiolitic mélanges with various ages and types are distributed between the Junggar-Tuha and Yili-Central Tianshan blocks. All of these mélanges are commonly regarded as the relicts of the North Tianshan Ocean with the time span from Cambrian to Late Carboniferous or even Permian. Based on the latest results of geological investation in the eastern Tianshan area, the tectonic implication and evolution of the North Tianshan Ocean are revised by the study on the provenance and paleobiogeography of the Silurian-Devonian active continental margins on both sides of the recently identified Kanggurtag-Dacaotan ophiolitic mélange zone. Our contributions reveal that the Junggar-Tuha block and Yili-Central Tianshan block belonged to distinct provenance systems and paleobiogeographic units during Silurian to Early Devonian, indicating that the North Tianshan Ocean contained tectonic paleogeographic separation significance. In contrast, in the Middle Devonian, the coral faunas belonging to the same paleobiogeographic realm on both sides of the North Tianshan Ocean indicate that this ocean had evolved into a remnant ocean basin. The unconformity of the latest Devonian-early Carboniferous terrestrial molasse and volcanic rocks upon the underlying rocks and the interpenetration of the provenance of both sides suggest that the North Tianshan Ocean in the eastern Tianshan area had completely closed since Late Devonian. The convergence of Junggar-Tuha and Central Tianshan blocks probably occurred in the early Late Devonian (~370 Ma). During Carboniferous-Early Permian, the composite Junggar-Tuha-Central Tianshan block formed by the earlier welding had probably been subjected to the northward roll-back subduction of the South Tianshan Ocean located to the south, leading to the creation of back-arc basin with immature oceanic crusts along the Kanggur-Yamansu belt. This limited ocean lasted until its final closure in Ealry Permian (~290 Ma). The North Tianshan and Kanggur Oceans are two distinct ocean basin systems with different ages, types and evolution processes. © 2022 Geological Society of China. All rights reserved.