[S-CG62] Structure, evolution, and deformation of island arcs associated with subduction processes

[SCG62-P01] Numerical simulations of spontaneous subduction initiation in the oceanic lithospheres: Rheological effects of rheology of hydrated fracture zones

*Misuzu Ishikawa¹, Ken-ichi Hirauchi¹, Masaki Yoshida² (1.Department of Geosciences, Faculty of Science, Shizuoka University, 2.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

Keywords:fracture zone, spontaneous subduction initiation, numerical simulation, diffusion creep, antigorite, fault strength

Intra-oceanic spontaneous subduction might initiate at fracture zones or transform faults between the oceanic plates, where rheological and/or density contrasts exist. Recent petrological studies using peridotite samples recovered along the oceanic transform fault reveal that seawater infiltration into the fracture zones results in the formation of hydrous minerals (e.g., amphibole, chlorite, and serpentine) and the operation of diffusion creep of dynamically recrystallized olivines. In this study, we conducted two-dimensional visco–elasto–plastic numerical simulations of spontaneous subduction initiation to understand the rheological effects of oceanic fracture zones on subduction initiation. In the present numerical models, we assume that seawater infiltration results in the hydration of mantle peridotites characterized by serpentinization (antigorite) and structural hydroxyl concentration in olivine along with the existence of pore fluid pressure. Our results show that at a given age offset between the overriding and subducting oceanic plates, incorporating a power-law creep law of antigorite and a diffusion creep law of olivine into the entire fracture zones leads to the formation of a low-viscosity (10¹⁹ Pa s) zone, which acts to facilitate oceanic plate subduction. However, we confirm that when considering the dehydration depths of antigorite, which vary from 13 to 28 km depending on the thermal state of the fracture zones, remarkably low effective friction coefficients are needed to realize a self-sustaining subduction zone.