At least three scales of convection in a mantle with strongly temperature-dependent viscosity

Abstract

We studied the convective patterns developing at high Rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature-dependent viscosity. Within this sluggish lid regime, three different scales of convection develop. The largest convective scale is cellular, with cold downwelling sheets of viscous fluid encasing hotter, less viscous, parts of the tank. Within each of those cells develop several (typically 3–7) hot 3D upwelling plumes. Upon impinging under the cold thermal boundary layer, each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank. Applying those results to the Earth’s mantle, we suggest that the large-scale features of mantle convection and the co-existence of several scales of convection, i.e. slabs and plumes, are produced by thermal convection in a mantle with high Rayleigh number and a strongly temperature-dependent viscosity material. This generates a convective pattern in two large-scale cells : the Pacific and the Indo-Atlantic “boxes”. Our experiments further suggest that what has been named the two hot superplumes, i.e. the two seismically slow regions encased within the subduction rings, are in fact each constituted of several hot instabilities. Moreover, plumes impacts under the lithosphere should be surrounded by cold rings of small extent. The asthenosphere appears therefore as the graveyard of both small-scale convection and hot plumes generated in the system.