ИСТИНА

supercontinent into spherical mantle model with phase transitions, the temperature- and pressure-dependent rheology, while assuming that the mantle is heated from the base and from within. Before implementation of the supercontinent, the overlithostatic horizontal stresses in the main part of the upper mantle are in the range of ±15 MPa, while the horizontal stresses along subduction zones and around mantle plumes are significantly larger (±25 MPa and more). The supercontinent covered one third of the Earth`s surface and it is modeled as an undeformable, highly viscous immobile lid with respect to the ambient mantle and it is abruptly imposed on well-developed mantle convection. The area of supercontinent is limited by a spherical angle (θ ≤ 66.4◦). After implementation, the mantle flow is rearranged and a group of upwelling mantle plumes is formed under the supercontinent and their hot heads increase in size due to the heat-insulating effect of the supercontinent, while quasi-linear subduction zones increase in the oceanic regions. As a result, the average temperature of the area under the supercontinent rises over time and becomes higher than the average temperature of the suboceanic area, where cold descending mantle flows intensify. Аt the depth of (300 ÷ 400) km under the supercontinent the temperature rises on average by 60 K. Formed under the supercontinent, mantle plumes dramatically change the stress pattern in the supercontinental area producing tensional stresses in the supercontinent and overlithostatic compressive horizontal stresses in the subcontinent mantle. Tensile overlithostatic horizontal stresses inside the supercontinent are about (25 ÷ 50) MPa, whereas beneath the supercontinent the overlithostatic compressive horizontal stresses in the subcontinent mantle of about (20 ÷ 60) МPа are detected. Only for the model with weak zone around the supercontinent stresses can reach 100 MPa.