STRESSES INDUCED IN CONTINENTAL LITHOSPHERES BY AXISYMMETRIC SPHERICAL CONVECTION

Abstract

We employ an axisymmetric spherical shell model of mantle convection to examine the magnitude of deviatoric tensile stresses generated in a stationary continental plate resulting from the subduction of oceanic plate material below an active continental margin. The model includes depth-dependent physical properties, uniform internal heating, compressibility and mineral phase-change boundaries at depths of 400 and 660 km. Below 100 km, mantle viscosity is assumed constant. Above 100 km, plate-like behavior may be approximated in axisymmetric spherical geometry by imposing (i) a factor of 10 viscosity contrast between the upper 100 km and the underlying mantle and (ii) a small variation of surface velocity within each plate such that plate mass is conserved and significant vertical mass flux at the base of the plates is therefore confined to the near vicinity of the plate boundaries. We find surface stresses generated by counterflow under our model continental margins is insufficient to actively rift a supercontinent in all but one case. Earth-like curvature appears to be a major factor in reducing surface stresses relative to those found previously in constant viscosity models in plane layer geometry. A simple internal loading model allows us to estimate this effect as a 20 per cent average reduction in stress generation. This suggests that continental rifting requires the pre-existence of localized zones of weakness.