How do new subduction zones start?
In plain words
Plate tectonics needs plates to sink back into the mantle at subduction zones, but a strong intact plate resists being bent and broken to start sinking. Whether new subduction zones begin by themselves or are pushed by neighboring plates is debated.
Precise statement
Determine the conditions under which subduction initiates: spontaneously (gravitational foundering of old dense lithosphere along a weak zone) or induced (by plate convergence), and the roles of mantle plumes and inherited weak zones. An answer is a criterion in terms of lithospheric age, weak-zone strength and applied force, from thermomechanical models with lab-constrained rheology, tested against Cenozoic initiation events such as Izu-Bonin-Mariana near 52 Ma (review: Stern and Gerya, Tectonophysics 2018, https://doi.org/10.1016/j.tecto.2017.10.014).
What would settle it
Dated magmatic and stratigraphic records of forearcs at young subduction zones compared with thermomechanical models using only experimentally constrained rheology.
Status in the literature
Ocean drilling of the Izu-Bonin-Mariana forearc in 2014 recovered the volcanic sequence expected from spontaneous initiation; whether induced initiation dominates elsewhere remains debated.