[P-PS07] Planetary Sciences

[PPS07-05] Two-dimensional numerical experiments on mantle convection with stress-history-dependent rheology: Toward self-consistent reproduction of plate tectonics

*Hiroki Taito¹, Masanori Kameyama¹ (1.Graduate School of Science and Engineering , Ehime University)

Keywords:stress-history-dependent rheology, mantle convection, plate tectonics, cold Thermal Boundary Layer (cTBL)

In this study we developed a numerical model of thermal convection of highly viscous incompressible fluid, aiming at reproducing plate tectonics in the framework of mantle convection. A two-dimensional basally-heated convection is considered under the Boussinesq approximation in a half of cylindrical annulus whose inner and outer radii are close to the Earth's mantle. The viscosity of fluid is assumed to nonlinearly depend on "degree of damage'' as well as temperature and pressure. The heart of the present rheology model lies in the hysteresis betweenthe "intact'' and "damaged'' branches at low and high stress, respectively (Ogawa,2003; Miyagoshi et al., 2020). The hysteresis induces the dependence on stress-history in viscosity for a particular range of applied stress, which enables us to distinguish the stiff, mechanically strong plate interior and soft, highly deformable boundaries on the lithosphere. In a series of calculations by systematically varying the temperature-dependence in viscosity, we obtained a regime of convection where the nature of highly viscous lithosphere of cold fluid is quite similar to that of the Earth's plates; the lithosphere is divided into three or four pieces of surface plates each of which rigidly moves, and the surface heat flow of plates begins decreasing at divergent-plate-margin (ridge) in accordance with the half-space cooling model. A careful analysis on the mechanical states in the cold thermal boundary layer (cTBL) shows that the occurrence of the "plate-like'' (PL) regime is closely related with the hysteresis in viscosity; the PL regime takes place only when the stress level σ_cTBL in cTBL, estimated by the product of its thickness and temperature contrast can induce the stress-history dependence in viscosity. By comparing the convecting flow structures in our experiments with the earlier ones using 2-D Cartesian geometry (Ogawa,2003), we also found that the cTBL has a larger thickness and a smaller temperature contrast for the cases in spherical geometry than in the Cartesian one. This significantly reduces the effects of spherical geometry on stress levels σ_cTBL in cTBL, which in turn allows the conditions relevant for the PL regime almost unaffected by the model geometry. Our findings not only highlight the ultimate importance of stress-history-dependent rheology in the self-consistent reproduction of tectonics plates, but also offer hints for future attempts toward integrated models of mantle convection and plate tectonics in three-dimensional spherical geometry.