ИСТИНА

The spatial fields of overlithostatic pressure, the vertical and horizontal stresses in the mantle and in a moving continent are studied in numerical models of mantle convection. The continent moves consistently with time-dependent forces, which act from the viscous mantle. By introducing the variable viscosity, we gain the possibility for taking into account the oceanic lithosphere that surrounds a continent, and the difference between the viscosity of the upper and the lower mantle. The calculations are carried out for three different patterns of the viscosity distribution in the mantle: an isoviscous model, a four-layer viscosity model, and a temperature- and pressure dependent viscosity model. At the considered stages of motion and in different parts, the continent is characterized by the following typical values of stresses: the overlithostatic pressure ranges from –5 to +15 MPa; the horizontal overlithostatic tensile stress amounts up to –4 MPa; and the compressive stress in case of the overriding of the subduction zone attains +35 MPa. The typical overlithostatic horizontal stresses in the main part of the mantle are ±(7–9) MPa; in the highly viscous regions and, particularly, in the subduction zones near continental margins they are about five times larger. We find significant differences between the fields of the horizontal stresses, vertical stresses, and the pressure. The pressure field reveals both vertical and horizontal features of slabs and plumes, clearly showing their long thermal conduits with more broad heads. The horizontal stresses are sensitive to the subhorizontal features of the flows, whereas the vertical stresses mainly reveal the vertical substructures of the flows. These fields exhibit strong concentrations in the areas of descending slabs, where the values are much higher (50 MPa). This agrees with current views on the oceanic slabs as the most important factor of the mantle convection.