A regime diagram for subduction styles from 3-D numerical models of free subduction

Previous models of subduction (both analogue and numerical) have observed a number of distinct styles of subduction, each with particular subduction motions (partitioned between slab rollback and forward plate advance) and associated slab morphologies. We use 3-D numerical models to investigate subduction dynamics by varying the strength of slabs as well as the buoyancy, and propose a new classification based on these parameters. The slab strength is specified both through the ratio of viscosities between the subducting plate and upper mantle (ηplate/ηum) as well as the plate thickness, hplate. Only a very restrictive range of plates (“strong” plates with smaller buoyancy) tend to favor modes of subduction which are exclusively advancing. Plates which have greater negative buoyancy will eventually transition into a retreating style. We find that the flexural strength and the buoyancy determine the subduction style (as distinguished by a characteristic slab morphology), and control several subduction characteristics including the partitioning between slab rollback and plate advance, the trench curvature, and the slab's radius of curvature. Plates that are 80–100 km thick with ηplate/ηum ~ 100–300 are classified here as “weak” and are the only plates to exhibit slab geometry with several recumbent folds atop the more viscous lower mantle. This regime of weak plates with their associated slab morphologies (predominant folding) is argued to be most similar to slabs on Earth based on the presence of folded slab piles in Earth's upper mantle (as interpreted from seismic tomography).