ИСТИНА

Numerical modelling are carried out of the fields of the main normal stresses values in the mantle, the maximal shear stresses, and also their spatial orientation in each point. Calculations are fulfilled for sequence of stages of a supercontinental cycle (Wilson cycle) on numerical two-dimensional model of assembling and dispersing continents, driven by mantle flows; in turn, the flows themselves are forming under thermal and mechanical influence of continents. Without such self-consistent interaction, Wilson cycle would be impossible, and all structure of currents and stress fields would be another, qualitatively differing from taking place in the real Earth. Our computer modeling demonstrates a number of realistic features of the process: closing and then opening ocean; appearance of the marginal seas and inclined subduction zones, etc. Mantle sources of radiogenic heat are also included in the model (in the considered case they contribute one third of full thermal flow at the surface). Results of model calculations have shown, that areas of the maximal shear stresses are located in the upper parts of descending mantle flows, in particular, in the upper parts of inclined subduction zones. The model gives that the sizes of such area where shear stresses are equal or exceed 35 MPa (350 bar), are approximately 200 x 200 km. These magnitudes are about 10 times more the stress values in the bulk of the manle model (3 - 5 MPa). At the same time the upgoing mantle flows are, with respect to downgoing ones, more slow and wide, having significantly less stress magnitudes. Further, the area before a leading edge of moving continent is outlined: the pair consisting of a descending and an ascending mantle streams, close located to each other (similar to Andian subduction zone of South America and closely located East-Pacific raising) which exists during rather long time, forming the mantle area of essentially (approximately in 2.5 times in comparison with average value) increased stresses. Later, in process of advance of continent, the ascending mantle flow deviates aside continent, and the rest of a descending flow becomes more flat, comes under continent and gradually disappears; stresses in it also gradually decrease. The picture is qualitatively similar to a situation at Pacific margin of North America. This work was supported by the Russian Foundation for Basic Research (project 05-05-65190).