Near-surface thermal/chemical boundary layer convection at infinite Prandtl number: Two-dimensional numerical experiments

Chemical differentiation and convective removal of internal heat make the Earth's lithosphere a thermal and a chemical boundary layer. Thin layers of chemically light material form near the Earth's surface and become embedded within the cold thermal boundary layer associated with interior heat removal. The likelihood of near-surface thermal and chemical boundary layer interactions influencing the Earth's thermo-tectonic evolution prompts the models presented herein. A simplified system, consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer, is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar, well-studied, thermal boundary layer systems. A major cause of differences between the two systems resides in the ability of the deformable near-surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below. In thermal equilibrium, regions of chemical boundary layer accumulation locally enforce an effectively near-constant heat-flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material. For cases in which chemical accumulations translate laterally above the unstable layer, the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non-equilibrium type, i.e. the thermal condition at the top of the convectively unstable layer is time-, as well as space-, variable. A second major cause of differences is that, for the thermal/chemical system, chemically induced theologic variations can offset, or compete with, those due to temperature. More specifically, the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an, on average, strong upper thermal boundary layer. Both of these factors have low-order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow. Some implications of this, in regard to elucidating how near-surface chemical boundary layer deformation, e.g. continental tectonics, might interact with, and influence, mantle convection, are discussed.