Tectonic stresses in the lithosphere

Various types of observables (earthquake focal mechanisms, in situ measurements and geological deformations) give information about the large scale lithospheric stress field. The latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates. This approach ignores the role of mass heterogeneities within the lithosphere. Here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude. The presence of internal sources makes the problem three‐dimensional. We show however that it can be reduced to a two‐dimensional plane stress formulation, whereby the edge forces are expressed by the ‘non hydrostatic stresses’ and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress. For the oceanic lithosphere we derive a compression that increases with age. The comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag. For the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans.