2D visco-elasto-plastic subduction models in the presence of a high density and viscosity continental block: a case study for the slab geometry beneath the Kii Peninsula, southwest Japan

Abstract The Philippine Sea (PHS) plate embracing the Shikoku Basin subducts with a relatively young age varying between ~ 15 and 26 Ma. It exhibits a diversity of subduction styles along the Nankai Trough, from shallow beneath Shikoku and Chugoku regions to steep subduction beneath the Kii Peninsula. Although the onset of subduction along the entire Nankai Trough is synchronous at 15 Ma, present-day subduction geometries differ greatly between Shikoku and the Kii Peninsula. Seismic tomography beneath the Kii Peninsula reveals several high-velocity zones associated with the presence of a high density block. Some studies suggest that the location of this high density block coincides with the acidic rock group represented by the Kumano pluton, which is probably responsible for the high dip angle subduction of the Philippine Sea (PHS) slab beneath the Kii Peninsula. This study analyzes the effect of a high density and viscosity block placed in the vicinity of the Nankai Trough beneath the Kii Peninsula on the subduction dynamics of the PHS slab. We develop a series of 2D subduction numerical models using a visco-elasto-plastic rheology with spontaneous slab bending where we incorporate a high density and viscosity block in the proximity of the convergent margin. Our numerical results are compared with seismic tomography, present-day slab geometry, and seismicity distribution along an across-arc profile passing through the Kii Peninsula. The results show a good relationship between the slab geometry dynamics and high density and viscosity materials localized near the subduction zone. The high density and also possible high viscosity block identified in seismic tomography may hamper subduction of the PHS plate with a low dip angle beneath the Kii Peninsula, resulting in steep subduction.