The purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities, in particular with respect to mountain building and other processes of intraplate deformation. Density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields: the product of their magnitude by their depth is shown to characterize their ability to induce deformation. This rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent. For anomalies of lateral extent that is small in comparison with their depth, the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous Newtonian lithosphere. The analytical treatment is based on Fourier transform. Continent‐continent collision thickens not only the crust but the entire lithosphere. The cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away. Thus the continental lithosphere is in a somewhat metastable mechanical state. Adiabatic, i.e. rapid, thickening (or thinning) tends to grow further once initiated. Tectonic phases of strong compression correspond to the climax of such instabilities. The response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions. They yield velocity distributions and stress fields. Instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the Alpine and Appalachian regions. In the presence of lateral variations of the mechanical properties of the lithosphere, the tectonic style is not only function of the local topography and of the nature of its compensation. Deformations in neighbouring provinces are coupled as shown by 3‐dimensional models. For example, thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness. These last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand, and of geometrical constraints related to lateral mechanical heterogeneities on the other.

A competent layer with a nonlinear rheology can, under extension, exhibit pinch-andswell instabilities. Such instabilities can explain small-scale regular deformations of rock. Recently they have also been invoked in relation to the distribution of basins and ranges 40 km for the individual basins and ranges and about 200 km for the Bouguer troughs in Nevada and their associated broad topographies.

Narrow zones of intense shear deformation, i.e. viscous slip zones, are studied analytically with a one-dimensional time-dependent model of two half-spaces of identical or contrasting rheologies and ambient temperatures in relative motion. The rheologies of the half-spaces are strongly temperaturedependent and viscous heating maintains a thin zone of high temperature, low viscosity and large strain rate. The mathematical model is used to describe the structures of slip zones at ridge and plate-boundary transform faults, major continental strike-slip faults and at the top of subducting oceanic crust. No (I priori assumption about slip-zone width or shear-stress magnitude is necessary; the thermal-mechanical structure of the slip zone evolves in time and all its characteristics are self-consistently determined. Slip-zone widths and shear stresses depend on the ambient temperatures, the relative velocity, the rheology and the length of time following the onset of relative motion; for reasonable geologic times, 0.1-10 M yr for example, slip zones are generally several kilometres wide and shear stresses are several hundred bars (tens of MPa). The region of intense shear in a viscous slip zone is an order of magnitude narrower than the width of the accompanying thermal anomaly. The maximum temperature generated by viscous dissipation in a slip zone depends only on the relative motion and the creep properties of the rocks; it is independent of slip-zone age and ambient temperature. Maximum temperatures associated with frictional heating are always less than those required for partial melting. The slip zone on a descending slab is influenced most strongly by the contrast in creep behaviour between the relatively soft oceanic crustal rocks and the hard, overlying mantle rocks; as a result, the slip zone is confined entirely within the oceanic crustal layer. Oceanic crustal rocks deform so readily that frictional heating in a slip zone on a descending slab cannot by itself lead to partial melting and thermal conduction from the hotter overriding mantle must play an essential role in heating the descending crust if Present address: