Résumé
A characteristic feature of the Earth is that diffuse thermal convection in
the mantle produces localized deformation at the surface: Plate Tectonics.
However, modelling this phenomenon remains a challenge, due to inability to
simulate spontaneous strain localisation in the deep sections of the plates,
where rocks deform by ductile processes and behave as highly viscous fluids.
Analysis of shear zones - the main expression of strain localisation in nature
- reveals that mechanical heterogeneity is key. Here we posit that the
bottleneck for self-consistent generation of strain localisation in
geodynamical models is poor representation of this heterogeneity and its
evolution during deformation, in particular at small scales. To bypass this
obstacle, we introduce a stochastic description of the mechanical properties of
the rocks, which evolves as a function of the local work rate. This approach
not only successfully generates steady-state shear zones from random
rheological heterogeneity fields, but reproduces the full range of observations
in nature and experiments. It allows defining the necessary conditions and a
regime diagram for viscous strain localisation and quantifying the resulting
bulk softening, paving the way for self-consistent modelling of the development
of plate boundaries and initiation of Plate Tectonics on Earth.