Amalgamation of Mariana-type forearc lithosphere during accretionary orogenesis: Insights from the North Qilian orogenic belt, northeastern Tibet

This study presents a spatiotemporal analysis of the Proto-Tethyan North Qilian ophiolites and related accretionary tectonics. Integrated geochemical and geochronological data suggest that southern ophiolite-arc complexes of the North Qilian orogenic belt mainly represent relics of Mariana-type forearc oceanic lithosphere, accompanied by minor abyssal peridotites and late-stage island-arc complexes. The tectonic reconstruction further reveals the presence of three overprinted arc systems along the southern part of the North Qilian orogenic belt: the Cambrian–early Ordovician Mariana-type, the Neoproterozoic–early Ordovician Japan-type, and the Ordovician Japan-type. The Mariana-type arc system was formed by northward intra-oceanic subduction during the ca. 530–480 Ma interval, whereas the Japan-type arc systems developed through two phases of southward ocean-continent subduction (ca. 550–480 Ma and ca. 480–450 Ma). These ca. 530–480 Ma dual-subduction systems with opposite polarities culminated in the arc-arc collision during the early Ordovician. This process not only contributes to the tectonic emplacement of the forearc lithosphere along active continental margin, but also transformed long-lived continental margin subduction from a continuous tectonic regime to discrete tectonic episodes. Amalgamation of the Mariana-type arc system during accretionary orogenesis therefore modified the dynamic processes along the Proto-Tethyan convergent margin and exerted a profound influence on the architecture of the North Qilian orogenic belt.