Radial profiles of temperature and viscosity in the Earth's mantle inferred from the geoid and lateral seismic structure

In the framework of dynamical modelling of the geoid, we have estimated bas ic features of the radial profile of temperature in the mantle. The applied parameterization of the geotherm directly characterizes thermal boundary l ayers and values of the thermal gradient in the upper and lower mantle. In the inverse modelling scheme these parameters are related to the observable s (geoid and seismic structure of the mantle) through the viscosity profile which is parameterized as an exponential function of pressure and temperat ure. We have tested similar to 10(4) model geotherms. For each of them we h ave found proper rheological parameters by fitting the geoid with the aid o f a genetic algorithm. The geotherms which best fit the geoid show a signif icant increase of temperature (similar to 600-800 degrees C) close to the 6 60-km discontinuity. The value of the thermal gradient in the mid-mantle is found to be sub-adiabatic. Both a narrow thermal core-mantle boundary laye r and a broad region with a superadiabatic regime can produce a satisfactor y fit of the geoid. The corresponding viscosity profiles show similarities to previously presented models, in particular in the viscosity maximum occu rring in the deep lower mantle. The best-fitting model predicts the values of activation volume V* and energy E* which are in a good agreement with th e data from mineral physics, except for V* in the lower mantle which is fou nd somewhat lower than the estimate based on melting temperature analysis. An interesting feature of the viscosity profiles is a local decrease of vis cosity somewhere between 500 and 1000 km depth which results from the steep increase of temperature in the vicinity of the 660-km discontinuity. (C) 1 998 Elsevier Science B.V. All rights reserved.