Subduction initiation at passive continental margins: A review based on numerical studies

The Wilson cycle predicted that the passive continental margin finally collapses and forms a new subduction zone, because the transition between oceanic and continental lithosphere provides as a natural boundary with great density and rheology contrasts. However, this process is hardly constrained, due to the lack of obvious Cenozoic examples in the natural Earth. The rather stable passive margin of Atlantic Ocean, without any clear sign for subduction initiation (SI), also challenges this hypothesis. In this study, we have reviewed the main ideas and models for the SI at passive margin, which are classified into two categories, i.e. driven by either local forces or external forces. The local forces, mainly due to the local gravitational instability, are generally insufficient to break the lithosphere and lead to SI on the present Earth, although they are possible driving mechanisms for the SI in early Earth with higher potential temperature of the mantle. The external forces may come from the push of mid-ocean ridge and/or mantle plume, the lateral drag of neighboring sinking slab, as well as the basal drag of large-scale mantle convection. They are feasible to drive SI for young oceanic basins with thin and weak lithosphere on the present Earth. But the old and stable Atlantic type margin with high rheological strength is difficult for SI, which requires pre-existing weakness and/or significant weakening mechanism. Previous numerical studies have generally focused on the geodynamic conditions of SI. However, the SI for either present or early Earth is still not well constrained, especially in the aspects of geological responses and possible petrological records, which thus requires further systematic comparative studies between numerical models and geological records.