Carboniferous to Triassic polyphase tectonic transition in the western North China craton: A retroarc basin response to termination of the middle segment of the Paleo-Asian Ocean

Abstract Knowledge of the closure of the archipelago-type southern Paleo-Asian Ocean has been limited by uncertainty regarding whether the Alxa tectonic belt was isolated from the North China craton by an ocean around the Helanshan tectonic belt. Newly released seismic reflection profiles across the Alxa-Ordos border showed that the Alxa tectonic belt and Ordos Basin were located in a paleogeographically linked basin throughout the Paleozoic, indicating that the Tianshan and Solonker sutures were contiguous. The ca. 320–285 Ma successions in the Helanshan tectonic belt and surroundings, fed by a northeasterly Inner Mongolia continental arc source, display a retrogradational stacking pattern along with diabase sill emplacement. This inferred retroarc extensional setting was coeval with asthenospheric upwelling and thinning of the continental arc driven by retreating subduction of the Paleo-Asian Ocean. The subsequent ca. 280–260 Ma sedimentary hiatus coincided with magmatic flare-up, crustal thickening, and retroarc foreland formation in the Inner Mongolia continental arc, indicating a geodynamic transition from retroarc extension to compression in the mid-Permian. Then craton-scale drainage reorganization occurred during the Early Triassic (ca. 250–230 Ma), manifested by the discharge of the Middle Triassic fluvial system southeastward to the central Ordos Basin; this was steered by southeastward expansion of the Alxa tectonic belt during the final termination of the Paleo-Asian Ocean. Postcollision lithospheric adjustment since ca. 230 Ma resulted in vigorous alkaline magmatism and supracrustal stretching in the Inner Mongolia continental arc as well as mafic magmatism and rifting in the study area. Hence, Carboniferous to Triassic polyphase tectonic transitions in the western North China craton were retroarc basin expressions related to complicated subduction-closure processes in the southern Paleo-Asian Ocean.