Rapid propagation of subduction initiation throughout the Iapetus Ocean

The face of our planet changes over geological time due to tectonic reconfigurations permitted by the emergence of new plate boundaries. However, we do not know how, where and how quickly new plate boundaries form, and it is not clear how their development relates to the global plate tectonic system. Supra-subduction zone ophiolites, a common feature of Phanerozoic orogens, record forearc seafloor spreading and nascent arc magmatism during the onset of subduction and tectonic convergence. Here we present high-precision dates for seven such ophiolites spanning >6000 km of the Appalachian–Caledonian Orogen, showing that subduction initiation in the Iapetus Ocean propagated at a uniform rate of 25 cm y-1, from 491 Ma in the east to 467 Ma in the west. These kinematic constraints demonstrate that new convergent plate boundaries can nucleate/appear in one region then propagate through an ocean basin at velocities exceeding plate movement rates. Our results provide valuable new constraints for testing models of subduction initiation, its context and drivers.