Effects of Hot Plumes and Slab Rheology on the Bending of the Philippine Sea Slab at Shallow Depths Beneath the Kyushu Region, Southwest Japan

Abstract The Philippine Sea (PHS) plate is currently subducting beneath the Kyushu region in southwestern Japan and exhibits a steep dip angle at depths of approximately 70–100 km. It has been suggested that the increase in the dip angle may be attributed to a hot plume, which was purportedly present in the northwestern part of Kyushu approximately 10–6 Ma, flowing southeastward toward southern Kyushu and causing steep shallow bending of the PHS plate. Using numerical simulations, this study quantitatively evaluates the impact of an upwelling hot plume from the northwestern part of Kyushu on the abrupt bending of the PHS plate at shallow depths. We use a time-dependent two-dimensional thermal convection model that incorporates the subduction history of the PHS plate along a vertical plane passing through a hot plume from northwestern Kyushu to the Nankai Trough. Numerical simulations were conducted to investigate the behavior of a slab when colliding with an upwelling hot plume, considering different values of slab viscosity. The results demonstrate that when a hot plume impacts a slab with a viscosity lower than that of a normal strong slab, the bending of the slab is reproduced at a depth of approximately 100 km, which is consistent with the observed PHS slab bending at depths of 70–100 km.