Late Eocene subduction initiation of the Indian Ocean in the North Sulawesi Arc, Indonesia, induced by abrupt Australian plate acceleration

Abstract

Subduction of oceanic lithosphere is the main driving force of plate tectonics on Earth, yet, how subduction began (i.e. subduction initiation; SI) remains enigmatic, due to limited SI examples. Here, we report new whole-rock geochemical and zircon U-Pb-Hf isotopic data of arc magmatism from the North Sulawesi Arc, Indonesia, to constrain the petrogenesis, magmatic tempo, and tectonic setting, as well as the timing and trigger of SI. Zircon U-Pb dating yielded crystallization ages of ~29.7–16.7 Ma for parental magmas, which are calc-alkaline I-type in nature and show typical subduction-related signatures. Geochemical data and high positive zircon ε Hf (t) values indicate magma generation by partial melting of preexisting mafic basement rocks. Our new results, together with literature data, allow the identification of an Indian SI in the Late Eocene, as evidenced by the last vanishing of BAB-type mafic basement rocks at ~37 Ma and the first appearance of calc-alkaline felsic arc magmatism at ~30 Ma in the North Sulawesi Arc, as well as ~36–32 Ma ophiolite and ~34–29 Ma metamorphic sole in the nearby East Sulawesi Ophiolite. Such a temporal duration (maximum of ~7 Ma) from SI to typical arc magmatism is identical to that (~8–7 Ma) of the Izu-Bonin-Mariana system. Given the contemporaneousness of a sudden increase (from ~4–5 to ~8–9 cm/y) in northward drifting rate of the Australian plate, we propose that Late Eocene Indian SI was probably linked to the sharp Australian plate acceleration. Such plate reorganization may have played a significant role in driving SI in Earth history. • Calc-alkaline I-type felsic rocks in the North Sulawesi Arc developed at ~30–16 Ma. • Arc magmatism formed by melting of preexisting mafic rocks during Indian subduction. • Indian SI occurred in the Late Eocene, driven by Australian plate acceleration.