Subduction of oceanic lithosphere and delamination of continental lithosphere constitute the two predominant mechanisms by which the Earth’s surface is recycled into the mantle. Continental plate delamination typically occurs in collisional orogens by the separation of the lithospheric mantle from the overlying lighter crust, aided by weak layers within continental lithosphere. By contrast, oceanic lithosphere is generally considered to be sufficiently rigid to inhibit delamination. Here we show from seismic imaging and numerical simulations that delamination of oceanic lithosphere is occurring offshore Southwest Iberia. Specifically, seismic tomography reveals a high-velocity anomaly that we interpret as a delaminating block of old oceanic lithosphere, a process that we reproduce with numerical simulations. We propose that this process was triggered by plate convergence and assisted by a thick serpentinized layer that allows the lithospheric mantle to decouple from the overlying crust. We suggest that such oceanic delamination may facilitate subduction initiation, a long-unsolved problem in the theory of plate tectonics, and may be responsible for some of the highest-magnitude earthquakes in Europe, including the M8.5–8.7 Great Lisbon Earthquake of 1755 and the M7.9 San Vincente Earthquake of 1969.
