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.

Tectonics and topography for a lithosphere containing density heterogeneities

An alternative mechanism of flood basalt formation

Low-pressure metamorphic facies (i.e. high T/P ratios) are widespread in a wide range of tectonic settings. Explanations offered for the occurrence of these facies include extensional and/or magmatic models. However, these fail to explain that the low-P facies metamorphism is commonly coeval with a phase of pervasive crustal thickening, with T/P ratios increasing during, or slightly lagging behind, the thickening. We propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion (thinning) of the mantle lithosphere.

A model for low‐pressure facies metamorphism during crustal thickening

Convective patterns under the Indo-Atlantic « box »

The mechanical state of the lithosphere in the Altiplano-Puna segment of the Andes

Convection experiments are carried out in a tank with two isothermal heat sinks: The top plate and one of the sidewalls. Heating is supplied either by an isothermal bottom plate or by generation within the fluid. This situation is similar to that of the earth's subcontinental mantle in the presence of a neighboring subducting oceanic lithosphere. There, the mantle material loses heat not only to the base of the continental lithosphere but also to the cold dipping oceanic slab. The thermal structure of the convective fluid is observed by a variety of techniques, including differential interferometry and strioscopy. Two parameters characterize the observations: the usual ‘vertical’ Rayleigh number and a newly defined ‘lateral’ Rayleigh number. The lateral cooling induces a large roll with axis parallel to the cold wall. The variation of its width relative to the values of the two Rayleigh numbers has been determined. An application to the earth's upper mantle would predict rolls 5 times wider than high. The circulation in the large roll remains two-dimensional for Rayleigh numbers up to about 3 × 104. Beyond this value, boundary layer instabilities are observed within the persisting large roll. Their period has been determined. The interferometric method is shown to be very useful for visualizing other time-dependent processes such as the growth of the induced large rolls. This growth is rapid enough to allow us to propose the existence of such large rolls in the earth and argue that their action could have led to continental break-up.

C. Froidevaux

Papers

Laboratory convection experiments: Effect of lateral cooling and generation of instabilities in the horizontal boundary layers