The dynamics of multiscale, multiphysics faults: Part I - The long-term behaviour of the lithosphere

Here we used a physics-based geomechanics approach to show that the long-term strength of the lithosphere is controlled by multiple steady states that arise as a function of significant material weakening at and above a critical value of local dissipation. Ultimately, the long-term strength of the lithosphere is governed by whether it can reach this critical value to form plate boundaries and intraplate lithosphere scale fault zones. We identify two different regimes: (1) post-yield steady state creep strength, (2) post-yield localization strength. Accordingly, in regime (1) the plate tectonic loading is insufficiently large to breach the critical local dissipation needed to reduce the overall long-term strength of the lithosphere. This regime is described in classical thermal-mechanical geodynamic models based on material values derived from laboratory experiments. The approach leads to the “jelly-sandwich” model of long term lithosphere strength. The constitutive models predict that the strongest portion of the lithosphere is the upper mantle followed by the upper crust; both can support mountain ranges for long geological times and lead to large effective elastic flexural rigidities. For regime (2) we identify two possible responses. One in which the plate strength is reduced by chemical and thermal feedback processes. And the other one where crystal plastic deformation causes micron scale Fick diffusion controlled micro-shears, that in turn release fluid to feed millimetre-metre scale hydraulic subshears (Darcy diffusion controlled) to finally drive kilometre scale Fourier diffusion controlled plate boundary shears. Under these special conditions the mantle can become extremely weak such as in the “crème brûlée” model. We conclude that both models have their validity. They reveal two different end members of dynamic thermodynamic feedbacks at different critical dynamic loadings and chemical composition. •New continuum formulation of multi-physics and multi-scale couplings in terms ofchemical, hydraulic and thermal reaction diffusion equations•Multiple steady states characterized through characteristic parameter defined by the ratio of the diffusion over the reaction time scales of the processes.•Localization phenomena are observed if the critical characteristic non-dimensionalparameter is large enough multi-scale.•Scale of localization governed by the characteristic diffusion length scale, Fourier, Darcy and Fick diffusion respectively.•Nested localiza