Abstract
The evolution of the ‘mantle – moving deformable continents’ system has been studied by numerical experiments. The continents move self-consistently with the mantle flows of thermo-compositional convection. Our model (two-dimensional mantle convection, non-Newtonian rheology, the presence of deformable continents) demonstrates the main features of global geodynamics: convergence and divergence of continents; appearance and disappearance of subduction zones; backrolling of subduction zones; restructuring of mantle flows; stretching, breakup and divergence of continents; opening and closing of oceans; oceanic crust recirculation in the mantle, and overriding of hot mantle plumes by continents. In our study, the continental crust is modeled by active markers which transfer additional viscosity and buoyancy, while the continental lithosphere is marked only by increased viscosity with neutral buoyancy. The oceanic crust, in its turn, is modeled by active markers that have only an additional buoyancy. The principal result of our modeling is a consistency between the numerical calculations and the bimodal dynamics of the real Earth: the oceanic crust, despite its positive buoyancy near the surface, submerges in subduction zones and sinks deep into the mantle. (Some part of the oceanic crust remains attached to the continental margins for a long time.) In contrast to the oceanic crust, the continental crust does not sink in subduction zones. The continental lithosphere, despite its neutral buoyancy, also remains on the surface due to its viscosity and coupling with the continental crust. It should be noted that when a continent overrides a subduction zone, the subduction zone disappears, and the flows in the mantle are locally reorganized. The effect of basalt-eclogite transition in the oceanic crust on the mantle flow pattern and on the motion of continents has been studied. Our numerical experiments show that the inclusion of this effect in the model considerably alters the pattern of mantle flows and leads to distinct changes in the evolution of continents. Moreover, a new effect arises – bulging of heavy material (eclogitized former oceanic crust) at the core-mantle boundary, wherefrom it arises with the mantle plumes on the surface of the Earth.
Highlights
The effect of continents on mantle convection has been studied for several decades
Continents moved by mantle convection themselves change the convection and create a supercontinental cycle, including the convergence of all continents into a supercontinent and its subsequent disintegration accompanied by radical restructuring of all the mantle flows
In [Bobrov, Trubitsyn, 2008], the movements and mixing of the submerged oceanic lithosphere material in the mantle and its subsequent rise to the surface were determined in the successive stages of the calculated supercontinental cycle
Summary
The effect of continents on mantle convection has been studied for several decades. Already early works [Trubitsyn et al, 1999; Gurnis, 1988] showed that the presence of continents in the model considerably changes the mantle convection pattern. Lowviscosity tracers, in our opinion, too combine into continents and the supercontinent with their subsequent dispersion; these aggregations are roundshaped Despite these simplifications, a fundamental conclusion in the qualitative form was obtained about the reasons of the difference in the continental and oceanic hemispheres of the Earth. We consider a two-dimensional Cartesian model and numerically investigate the evolution of mantle flows, temperatures, viscosity fields, and the displacement of the material during the motion of two continental plates on the surface of the convective mantle. This property of the drop in the viscosity of the material when the critical value of the maximum shear stress is reached (non-Newtonian viscosity, quasi-plastic behavior) plays a very important role Due to such rheology, bending of thick rigid oceanic plates (in the region of high stresses) and their subduction becomes possible.
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