The structure of 2D mantle convection and stress fields: Effects of viscosity distribution |
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| Authors: | A A Baranov A M Bobrov |
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| Institution: | 1.Schmidt Institute of Physics of the Earth,Russian Academy of Sciences,Moscow,Russia;2.International Institute of Earthquake Prediction Theory and Mathematical Geophysics,Moscow,Russia |
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| Abstract: | Spatial fields of temperature, velocity, overlithostatic pressure, and horizontal stresses in the Earth’s mantle are studied
in two-dimensional (2D) numerical Cartesian models of mantle convection with variable viscosity. The calculations are carried
out for three different patterns of the viscosity distribution in the mantle: (a) an isoviscous model, (b) a four-layer viscosity
model, and (c) a temperature- and pressure-dependent viscosity model. The pattern of flows, the stresses, and the surface
heat flow are strongly controlled by the viscosity distribution. This is connected with the formation of a cold highly viscous
layer on the surface, which is analogous to the oceanic lithosphere and impedes the heat transfer. For the Rayleigh number
Ra = 107, the Nusselt number, which characterizes the heat transfer, is Nu = 34, 28, and 15 in models with constant, four-layered,
and p, T-dependent viscosity, respectively. In all three models, the values of overlithostatic pressure and horizontal stresses σ
xx
in a vast central region of the mantle, which occupies the bulk of the entire volume of the computation domain, are approximately
similar, varying within ±5 MPa (±50 bar). This follows from the fact that the dimensionless mantle viscosity averaged over
volume is almost similar in all these models. In the case of temperature- and pressure-dependent viscosity, the overlithostatic
pressure and stress σ
xx
fields exhibit much stronger concentration towards the horizontal boundaries of the computation domain compared to the isoviscous
model. This effect occurs because the upwellings and downwellings in a highly viscous region experience strong variations
in both amplitude and direction of flow velocity near the horizontal boundaries. In the models considered with the parameters
used, the stresses in the upper and lower mantle are approximately identical, that is, there is no denser concentration of
stresses in the upper or lower mantle. In contrast to the overlithostatic pressure field, the fields of horizontal stresses
σ
xx
in all models do not exhibit deep roots of highly viscous downwelling flows. |
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