Generation and driving forces of plate-like motion and asymmetric subduction in dynamical models of an integrated mantle–lithosphere system

Abstract

The dynamical effects of an asymmetric subduction structure on the generation of plate-like motion were investigated using two-dimensional numerical models of the integrated lithosphere–mantle system. To dynamically generate the plate boundary, we introduce a history-dependent rheology in which the yield strength is determined by past fractures. Only the buoyancy due to the internal density contrast consistently drives convective flow, including the motion of the viscous lithosphere, without imposed boundary conditions. We first investigate the effects of plate yield strength, friction at the plate boundary, and plate age on the emergence of plate-like motion with asymmetric subduction. Plate-like motion is generated when maximum plate strength is as high as that estimated by experimental rheology studies. The reason for this is that asymmetric subduction requires a plate-bending force much less than that for symmetric subduction because the plate gently bends when one-sided subduction occurs. In contrast, the strength of the plate boundary has to be very small for emergence of subduction, as several previous studies on the numerical convection and subduction modeling have pointed out. Development of the subducted slab is also controlled by the age of the plate. In the early stages of subduction, older plates increase their velocities faster because of their larger negative buoyancy. After the slab develops, the plate stiffness, that is, both the yield strength and the plate thickness, control plate velocity so that an older plate subducts more sluggishly. We next explore effects of viscosity layering in the underlying mantle, focusing on the mechanism in which the asthenosphere promotes plate motion. The low viscosity under the lithosphere enhances a mantle drag force that drives the plate, not only concentrating the convective flow beneath the plate but also enlarging its aspect ratio. We also examine longevity of the plate-like motion using the convection models with asymmetric subduction. The asymmetrically structured subduction process continues stably at high yield strength, at which episodic resurfacing or the stagnant-lid mode occurs in the previous convection simulation studies. The asymmetric subduction structure therefore has key roles in generating plate-like motion as well as reducing the strength at the plate boundary.