Résumé
A fundamental characteristic of plate tectonics is the contrast between strong plate interiors and weak plate boundaries. The weakness has both a structural aspect, as deformation zones can be reactivated during different tectonic episodes, and a dynamic aspect, as there is no major compositional difference between the lithosphere at active and inactive regions. These characteristics point to a structural origin of the weakness of plate boundaries, as deformation affect structures but structures do not heal rapidly. This presentation summarizes recent advances made in estimating the rheology of polymineralic aggregates under conditions appropriate for various levels of the lithosphere. We construct a new generation of strength profiles that take into consideration the differences in microstructure that are observed in continental shear zones: reduced grain size, metamorphism, and weak phase interconnection/layering. For each lithosphere stratigraphy, two strength profiles can be constructed, one for the undeformed material, the other for a high-strain material. The low-strain profile assumes a reference strain rate, constant with depth, an untextured mixture of plagioclase and biotite in the upper and middle crust (biotite being unstable in the lower crust, and an olivine-pyroxene untextured mixture in the mantle. As deformation proceeds, the fabric evolves until the phases become interconnected, and olivine grain size is reduced in the upper mantle. A high-strain strength profile can be derived for constant strain rate for reference. However, strain rate is likely higher in the high-strain state, but the degree of this enhancement is poorly constrained. We can follow two hypotheses. In the first, stress at each depth is the same as in the initial state. The strength profile is unmodified, but we can solve at each depth for a new strain rate. Alternatively, we can impose that the stress integrated over a lithospheric column is the same and solve for a depth-independent strain rate after the structural changes are in place. In both approaches, the enhancement of strain rate corresponds to a measure of localization achieved by the structural change.