Sublithospheric loading and plate-boundary forces

Sublithospheric loading arises from anomalous densities in the mantle, such as cool subducting slabs and regions affected by hot spots. Such loading gives rise to isostatic flexure and to tectonic stress in the strong upper lithosphere. Simple models of sublithospheric loading have been studied by finite-element analysis. The maximum loading stress produced by a simple load increases with its width towards the theoretical density-moment function value, but is found to be almost independent of depth for a narrow load (in contrast to a wide load). A layer of low viscosity above the load reduces the stress, depending on its thickness and viscosity. If the lithospheric stress arising from loading is intersected by a zone or plane of weakness, then plate-boundary tractions develop on the adjacent plates resulting from the redistribution of stress. It is shown, by modelling, that ridge push, slab pull and trench suction can be explained in this way.