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.

Overriding plate controls on subduction evolution

“…Sustained asymmetric subduction following subduction initiation has been a long‐standing issue in time‐dependent plate tectonic models [ Bercovici , ]. Sustained asymmetric subduction is the predominant style of subduction on Earth and is thought to play a key role in generating plate‐like motions [ Zhong and Gurnis , ; Nakakuki et al , ]. Anisotropic Frictional Rheology in the upper layer of the subducting plate and interplate shear zone was not only crucial for asymmetric subduction for the models in this study; in addition, its use enabled the plate boundary interface to evolve with time as a function of the plate boundary stresses.…”

“…Another common feature of modern subduction zones on Earth is subduction asymmetry; in other words, one plate tends to subduct beneath the other rather than both plates descending vertically into the mantle [ Zhong and Gurnis , ; Nakakuki et al , ; Crameri et al , ]. Subduction is thought to initiate via slip along a preexisting fault zone [ Gurnis et al , ].…”

Dynamical mechanisms controlling formation and avalanche of a stagnant slab

“…1. The model is constructed based on the long-time 96 integrated model in Nakakuki et al (2008). Major changes from the previous model 97 include: (1) the 410-km and 660-km phase transitions, (2) enlargement of the model size 98 (10000 km 2000 km), (3) the 6-km thick layer with history-dependent yielding (Tagawa 99 et al, 2007b), and (4) viscosity and yield-stress reduction due to hydration in the wedge mantle area (see Section 2.3 for details).…”

“…We construct 2-D dynamical models of the subduction system that interacts with the mantle phase transitions at 410-km and 660-km depths. In our model, plate motion and plate-boundary migration are generated dynamically without velocity conditions imposed on the plate as our previous studies (Tagawa et al, 2007b;Nakakuki et al, 2008). We introduce the viscosity reduction in the transition-zone and/or the lower-mantle slab because of the grain-size reduction that accompanies the phase transitions at 410-km (Rubie, 1984;Karato, 1989;Riedel and Karato, 1997;Shimojuku et al, 2004;Yamazaki et al, 2005) and 660-km (Ito and Sato, 1991;Karato et al, 1995;Kubo et al, 2000) depths.…”

The Structure of Mantle Flows and Stress Fields in a Two-Dimensional Convection Model With Non-Newtonian Viscosity

“…В ходе суперконтинентального цикла сдвиговые напряжения в верхней части плит достигают 40 МПа. Более сложная реология была использована в работе [Nakakuki et al, 2008], где авторами проведено 2D дина-мическое моделирование асимметричной субдукционной структуры в комплексной системе литосфе-ра-мантия. Для генерирования границы плит авторы вводят зависящую от истории реологию с преде-лом прочности, который определяется предшествующими разломами.…”