SPECTRAL AND GEOPHYSICAL CONSEQUENCES OF 3-D SPHERICAL MANTLE CONVECTION WITH AN ENDOTHERMIC PHASE-CHANGE AT THE 670 KM DISCONTINUITY

Numerical computations of 3-D compressible convection have been conducted in a spherical Earth's mantle with an endothermic phase change at the 670 km discontinuity. The results validate the trends of layering of mantle convection induced by the phase change. Indeed, partial layering and two-layer convective circulation prevail for the same value of Clapeyron slope as was observed with an axisymmetrical approach. These 3-D results show that as far as the purely 3-D effects of convection are not concerned, the 2-D geometry allows relevant conclusions; in particular, on the cylindrical shape of the avalanches of upper-mantle material into the lower mantle which were observed at the poles of the axisymmetrical numerical simulations. However, the 3-D simulations allow direct computations of the geophysical consequences of mantle flows as thermal anomalies, topographical deflections of the bottom, upper and internal surfaces and the resulting geoid anomalies. Two-layered or intermittently layered convection induces geophysical anomalies that are in qualitative agreement with the real geophysical date. Whereas surface topographies computed from one-layer convection models or whole-mantle circulation models are too large compared with observations, it is possible to generate a satisfactory geoid and topography when a realistic phase change is considered at the 670 km discontinuity. This property is verified both for a layered and an intermittently layered structure of convection in the Earth's mantle.