TECTONIC EPISODICITY AND CONVECTIVE FEEDBACK MECHANISMS

The evolution of the Earth is characterized by irreversible processes: radioactive decay of the major heat-producing elements, thermal conve ction and chemical segregation. The prevailing heating from within and the temperature dependence of the viscosity are essential for thermal convection. In the present paper, the chemical and thermal evolution of the mantle and the generation of the continent material are represe nted by a two-dimensional and finite-difference Boussinesq convection model. We have incorporated the above-mentioned principal features in this model, a geochemical paper by Hofmann (1988, Earth Planet. Sci. L ett., 90: 297-314) constituting our starting point for the distributio n of the radionuclides. The concentration of the radionuclides and the viscosity are functions of the location and time developing according to our system of differential equations. Although the real Earth is a much more complex system, we have dared to make a comparison with obs erved geophysical and geological data; we obtain a depleted upper mant le and acceptable values for the heat flow on the surface of the Earth as well as for the distribution of temperature, viscosity and of the velocity of creep in the mantle. The ups and downs of the convective v igour of the model roughly resemble the supercontinental cycles, the w orld-wide distribution of mineral dates in time, the sea-level variati ons and the variations of a number of geochemical parameters. (C) 1997 Elsevier Science B.V.