The sources of lithospheric tectonic stresses

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Abstract
The stresses associated with large-scale tectonic deformation have three possible origins: (1) plate-boundary forces counterbalanced by viscous drag beneath the plates; (2) density heterogeneities situated within the plates (say at depths shallower than 200 km); (3) mass heterogeneities in the deep mantle. The first two are shown to be equally important for the understanding of the stress field. No topography (no vertical stress) seems to be associated with lower-mantle mass anomalies. This is most compatible with a two-layer convective mantle where the lower-mantle mass anomalies, mechanically decoupled from the lithosphere, are unable to induce tectonic stresses.