Brandon Shuck and his colleagues suggest that SZI is not always either horizontally driven or vertically driven, but can indeed be both:
Geodynamic models suggest that if horizontal forces dominate, the upper plate experiences compression and uplift followed by extension and subsidence, whereas vertically forced subduction involves only extension. Geologic evidence of past subduction initiation events has been interpreted in terms of these alternatives; however, it is unclear whether they are mutually exclusive or represent different stages of early subduction. Here, we present seismic images of the Puysegur plate boundary south of New Zealand that reveal space–time relations of stress during subduction initiation. Our data show evidence for a stress transition (compression followed by extension) that spread from north to south as the trench nucleated and propagated along the plate boundary. Both the magnitude and duration of compression diminish from north (8 Myr) to south (5 Myr). This indicates that transition to self-sustaining subduction accelerates after nucleation of a downgoing slab increases driving forces and decreases fault strength near the propagating tipline of the nascent trench. Instead of horizontally forced versus vertically forced initiation, we propose a four-dimensional evolution, where horizontal forces initially dominate at the site of nucleation, but with time, vertical forces accelerate, propagate along strike and facilitate the development of self-sustaining subduction.
Shuck et al. (2022, Nature Geoscience)