Lulu Zhang and colleagues provide us with a view on subduction zone initiation where, for once, the lighter of two plates is going down:
The oceanic lithosphere is characterized by the existence of transform faults. These faults are large linear features where the age, thickness, and density of the rocks at each side are different. When a fault is in a compressive tectonic setting, it makes a natural candidate for SZI. Due to the difference in density and thickness, it is generally expected that the heavier plate goes down and the lighter one stays on the surface. However now, there seem to be several places in the western Pacific where the opposite is happening: younger and more buoyant plates are starting to sink underneath denser and older plates. These rarely studied scenarios are suggesting that SZI is indeed a complex process heavily impacted also by the mechanical properties of the rocks. Rheology might, in some cases, even become more important for SZI than the density contrast of the converging plates. This work therefore investigates the specific, and apparently anomalous, conditions under which a younger, less dense plate can start to sink under an older and heavier one.
Zhang et al. (2021, G-cubed)
Comparison between the physical forward model and a conceptual geologic evolution of the Gagua Ridge.