INFERRING THE VISCOSITY AND THE 3-D DENSITY STRUCTURE OF THE MANTLE FROM GEOID, TOPOGRAPHY AND PLATE VELOCITIES

In a dynamic Earth, mantle mass heterogeneities induce gravity anomalies, surface velocities and surface topography. These lateral density heterogeneities can be estimated on the basis of seismic tomographic models. Recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates. The relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle. Here we use this model, combined with a generalized least-squares method, in order to infer the viscosity profile of the Earth from the surface observations, and to get some new insight into the 3-D density structure of the mantle. The computed radial viscosity profile presents a continuous increase of more than two orders of magnitude. The asthenosphere has a viscosity close to 2 x 10(20) Pa s. No sharp discontinuity is requested at the upper-lower mantle interface. The largest viscosity 7 x 10(22) Pa s is reached in the middle of the lower mantle. At greater depth, approaching the core-mantle boundary, the viscosity decreases by one order of magnitude. The model suggests that the well-known degree-2 and order-2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs. It also slightly increases the degree-2 and order-0 in the lower mantle and decreases it in the upper mantle. In other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes.