Continental Tip With Thinned Lithosphere and Thickened Crust Is a Favorable Location for Subduction Initiation

The collapse of passive margins and subsequent subduction initiation (SI) are key steps in the Wilson Cycle. However, the occurrence and mechanism of SI at passive margins have long been questioned due to the lack of Cenozoic observations. In this study, we propose that the continental tip, where arc/plateau blocks with thinned lithosphere and thickened crust accrete beside the passive margin, is a favorable location for SI. Our three-dimensional numerical model and force analysis demonstrate that the continental tip, with its reduced frictional resistance and increased gravitational instability, is better suited for SI than other parts of the passive margin. Upon completion of SI, the retreat of self-sustained subduction can create a transform plate boundary by tearing the passive margin. Our model explains why SI can occur at continental tips such as the Scotia and the Caribbean, as well as locations with similar tectonic settings, while most parts of the Atlantic margin remain stable over long geological periods. Plain Language Summary The formation of new subduction at tectonically-inactive continental margin is an important step in Earth's tectonic evolution. However, it is unclear whether and how subduction initiation (SI) can occur at tectonically-inactive continental margin due to the lack of examples in recent 66 million years. We suggest that the continental tip region consisting of buoyant blocks such as volcanic arc and oceanic highland is a favorable place for SI. Compared with the adjacent oceanic lithosphere at tectonically-inactive continental margins, the thinner lithosphere and thicker crust of the volcanic arc or oceanic highland can reduce friction resistance and increase lateral buoyancy contrast, both promoting SI at the continental tip. Our three-dimensional numerical model also indicates that SI is easier to occur at the continental tip region than at other locations of the tectonically-inactive continental margin. When the SI at the continental tip is completed, the rapid retreat of the subduction zone can cause the rupture of the tectonically-inactive continental margin, which changes to a transform plate boundary accordingly. Our model results not only shed new light on the SI process occurring at continental tips such as the Scotia and the Caribbean but also provide important clues to identify future SI locations.