Subduction is the main driver of plate tectonics on Earth. However, the mechanisms of subduction zone initiation remain highly controversial.
While subduction zone initiation (SZI) is particularly important in maintaining plate tectonics, the process of initiating new subduction zones remains poorly understood. This is partly due to the fundamental differences between the dynamics of individual subduction zone initiation events, but also to incomplete or missing and geographical discontinuos geological evidence, as well as the long timescales and the numerous physical processes involved in forming new convergent plate boundaries.
Evidence of both subduction initiation leading to the onset of modern style ocean-plate tectonics (before 800 Ma), and more geologically recent (<800 Ma) initiation of individual subduction zones is rare. SZI is different from early Earth subduction initiation (SI) processes. Although early SI dynamics are still debatable (some sort of subduction may have existed since the beginning of the planet’s evolution), it is much more certain that the initiation of new subduction zones has actually occurred several times over the last hundred million years. Reconstructing the exact timing and location of SZI events, however, remains challenging.
The onset of subduction zones is a highly time and space dependent physical process occurring in a variety of tectonic settings, possibly a function of continental arrangements. SZI is also geodynamically diverse due to a complex interaction between internal and external plate forces, plate structure, material flow and buoyancy. Also, SZI occurs while subduction is occurring elsewhere on the planet so it is possibly influenced by other subduction zones. Newly formed subduction zones are sometimes difficult to distinguish from pre-existing ones, and the sources and directions of the forces triggering them are often unclear.