DYNAMICAL CONSEQUENCES OF MID-MANTLE VISCOSITY STRATIFICATION ON MANTLE FLOWS WITH AN ENDOTHERMIC PHASE-TRANSITION

Recent geophysical evidences from seismology and geoid inversions have pointed out the existence of some layering between 900 and 1000 km de pth and the possibilities of a low viscosity zone between 660 and 1000 km depth. Using a finite-difference and spectral code, we have conduc ted 3-D calculations of thermal convection with the endothermic phase transition at 670 km depth and a family of viscosity profiles with emp hasis on the presence of two low viscosity zones, one in the upper man tle between 100 and 250 km depth and the second one between 670 and 10 00 km depth. Below 1000 lan the viscosity increases with depth. The su rface Rayleigh number used was in the range of 10(7). The dominating e ffect is that due to the second low viscosity zone below the phase cha nge. Layered convection is induced by the presence of this second asth enosphere, which causes horizontal flow to develop in there. Alternati vely, a viscosity increase at 670 lan depth would shift convection to the single-layer regime. The potential sharp variations of the viscosi ty structure between 670 and 1000 km depth can greatly influence the g lobal flow dynamics of the mantle.