In the kinematic theory of lithospheric plate tectonics, the position and parameters of the plates are predetermined in the initial and boundary conditions. However, in the self consistent dynamical theory, the properties of the oceanic plates (just as the structure of the mantle convection) should automatically result from the solution of differential equations for energy, mass, and momentum transfer in viscous fluid. Here, the viscosity of the mantle material as a function of temperature, pressure, shear stress, and chemical com position should be taken from the data of laboratory experiments. The aim of this study is to reproduce the generation of the ensemble of the lithospheric plates and to trace their behavior inside the mantle by numer ically solving the convection equations with minimum a priori data. The models demonstrate how the rigid lithosphere can break up into the separate plates that dive into the mantle, how the sizes and the number of the plates change during the evolution of the convection, and how the ridges and subduction zones may migrate in this case. The models also demonstrate how the plates may bend and break up when passing the depth boundary of 660 km and how the plates and plumes may affect the structure of the convection. In con trast to the models of convection without lithospheric plates or regional models, the structure of the mantle flows is for the first time calculated in the entire mantle with quite a few plates. This model shows that the mantle material is transported to the mid oceanic ridges by asthenospheric flows induced by the subducting plates rather than by the main vertical ascending flows rising from the lower mantle.

No abstract

We compute the transfer of oceanic lithosphere material from the surface of the model to the inner convective mantle at successive stages of the supercontinental cycle, in the time interval from the beginning of convergence of the continents to their complete dispersal. The sequence of stages of a supercontinental cycle (Wilson cycle) is calculated with a two-dimensional numerical model of assembling and dispersing continents driven by mantle flows; in turn, the flows themselves are forming under thermal and mechanical influence of continents. We obtain that during the time of the order of 300 Myr the complete stirring of oceanic lithosphere through whole mantle does not occur. This agrees with current ideas on the circulation of oceanic crust material. Former oceanic crust material appears again at the Earth's surface in the areas of mantle upstreams. The numerical simulation demonstrates that the supercontinental cycle is a factor which intensifies stirring of the material, especially in the region beneath the supercontinent. The reasons are a recurring formation of plumes in that region as well as a global restructuring of mantle flow pattern due to the process of joining and separation of continents. The computations of viscous shear stresses are also carried out in the mantle as a function of spatial coordinates and time. With a simplified model of uniform mantle viscosity, the numerical experiment shows that the typical maximal shear stresses in the major portion of the mantle measure about 5 MPa (50 bar). The typical maximal shear stresses located in the uppermost part of mantle downgoing streams (in a zone that measures roughly 200 × 200 km) are approximately 8 times greater and equal to 40 MPa (400 bar). K e y w o r d s : supercontinental cycle, mass transfer, shear stresses, numerical experiment

No abstract

No abstract