Subduction initiation at relic arcs

Although plate tectonics is well established, how a new subduction zone initiates remains controversial. Based on plate reconstruction and recent ocean drilling within the Izu‐Bonin‐Mariana, we advance a new geodynamic model of subduction initiation (SI). We argue that the close juxtaposition of the nascent plate boundary with relic oceanic arcs is a key factor localizing initiation of this new subduction zone. The combination of thermal and compositional density contrasts between the overriding relic arc, and the adjacent old Pacific oceanic plate promoted spontaneous SI. We suggest that thermal rejuvenation of the overriding plate just before 50 Ma caused a reduction in overriding plate strength and an increase in the age contrast (hence buoyancy) between the two plates, leading to SI. The computational models map out a framework in which rejuvenated relic arcs are a favorable tectonic environment for promoting subduction initiation, while transform faults and passive margins are not. Key Points Relic arcs adjacent to a plate interface is a favorable place for spontaneous subduction initiation Both temperature and composition effects at relic arcs contribute as driving mechanism Passive margins tend to be stable because of negative temperature contribution