A thick pipe-like heat-transfer mechanism in the mantle: nonlinear coupling between 3-D convection and variable thermal conductivity.

We have investigated the dynamics and heat-transfer of 3-D mantle convection with a realistic temperature-ana pressure-dependent conductivity in a constant viscosity medium. Variable thermal conductivity is found to stabilize greatly the interior flow and the boundary-layer dynamics, as compared to constant conductivity solutions for Rayleigh number up to 5 x 10(5). Due to the nonlinear interaction between variable conductivity and convective flow, a great deal of heat can be transported coherently through the mantle-within thick pipe-like structures with a radius of around 500km. These pipe-like solutions can be understood from the intermediate asymptotic nature of the nonlinear-heat diffusion equation. We suggest that these thick plume-like structures in the lower mantle imaged recently by tomography may be a manifestation of this nonlinearity in the conductivity of the temperature equation.