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Two-layer model of upper-mantle circulation

1971; American Geophysical Union; Volume: 76; Issue: 20 Linguagem: Inglês

10.1029/jb076i020p04744

2156-2202

Walter M. Elsasser,

Geomagnetism and Paleomagnetism Studies

In the model whose implications are investigated here, the lithosphere is assumed to be 100 km thick and the asthenosphere 300 km thick. A linearized viscosity is assumed, its mean value for the lithosphere being 1023 poises and for the asthenosphere 1019 poises. The mesosphere, lying below 400 km depth, is assumed to be more viscous again than the asthenosphere, but no specific values are required. The viscosity contrast of 104 between lithosphere and asthenosphere is the dominant feature of this model. The model reproduces a surprisingly large number of observed features in a qualitative manner and without the need for subsidiary assumptions of an essential kind. The existence of a mathematical theorem due to Helmholtz is recalled: it says that for small Reynolds number the actual flow is such as to minimize viscous dissipation. This theorem suggests that only extremely simple flow patterns can be used in describing convection in the mantle. In particular, the shallowness of upper-mantle convection is in part due to the shallowness of the asthenosphere, in part to the mechanical advantage obtained by the flow's hugging a free boundary during nearly half a closed circuit. It is next pointed out that the asthenosphere appears as the uniquely determined locus of the chemical differentiation of sial from primary mantle material. The shortness of vertical distance from asthenosphere to the lithosphere facilitates the progressive incorporation of sialic material into the continental plates, especially into their roots, as seems to be observed.