Effects of the Lithospheric Strength on the Generation, Longevity, and Structure of the Subduction

Rheology of the lithosphere is the most important property to determine tectonics of the terrestrial planets. Because subduction of the plate is a fundamental character of the plate tectonics, we here focus on influences of the strength of the plate interior and boundary on the generation of the subduction. We develop self-consistent dynamical models of the plate incorporated in the mantle convection system, which reproduce the growth of the subducted plate from the initiation to a deep slab. We use 2-D viscous fluid in a rectangular box, and no external forces are applied to the surface boundary and the plate. Rheology depending on the hysteresis of the stress is utilized to produce a thrust fault at the plate boundary (Honda et al., 2000). The rheology includes Arrhenius-type temperature and pressure dependence and maximum yield strength as large as those reported by the experimental studies. We also introduce heterogeneity of the surface region, i.e., "oceanic" and "continental" region, which is treated as a horizontal heterogeneity of the yield stress and/or the composition at the surface. In our models, Earth-like one-sided successfully reproduced. Systematic survey for the strength of the plate (maximum strength of the yielding, a friction coefficient of brittle fracture of the plate interior, and a friction coefficient at the plate boundary fault) shows that generation of the subduction is sensitive to the friction the plate boundary but not to the maximum strength of the plate interior. The subducted slab is able to be formed when the friction coefficient of the fault zone at the plate boundary is small enough (< 0.01), and then the subduction successfully occurs even at the highest value in the model (600 MPa). Our models with long-time integration (100 to 200 Myr) show that the subduction of the plate stably continues and maintains high temperature in the wedge mantle above the subducted plate for long duration. Our results show that fine adjustment for the strength of the lithosphere as previous studies reported is not necessary to generate Earth-like subduction when there exist mechanisms to weaken plate boundary. We have also examined effects of the rheology in the plate and slab on the structure of the subducted slab. We consider difference of the tensional and compressional strength of the lithosphere and/or weakening induced by grain size reduction by the phase transition. When both the effects are introduced into the model with a thin overriding plate, the slab migrates ocean-ward, and that causes shallow dip angle of the slab.