The effect of energy feedbacks on continental strength

The strong shall be weak The strength of the Earth's crust controls the evolution of continents, from mountain formation to the distribution of earthquakes. How strong are the continents? Numerical calculations have been used to resolve the energy, momentum and continuum equations that determine continental deformation and strength. The continents turn out to be weaker than expected due to energy feedback processes. This explains various enigmatic observations related to continental behaviour, from the lack of seismicity in the mantle below continents, to the development of major weaknesses just where continents should be strongest. The classical strength profile of continents 1 , 2 is derived from a quasi-static view of their rheological response to stress—one that does not consider dynamic interactions between brittle and ductile layers. Such interactions result in complexities of failure in the brittle–ductile transition and the need to couple energy to understand strain localization. Here we investigate continental deformation by solving the fully coupled energy, momentum and continuum equations. We show that this approach produces unexpected feedback processes, leading to a significantly weaker dynamic strength evolution. In our model, stress localization focused on the brittle–ductile transition leads to the spontaneous development of mid-crustal detachment faults immediately above the strongest crustal layer. We also find that an additional decoupling layer forms between the lower crust and mantle. Our results explain the development of decoupling layers that are observed to accommodate hundreds of kilometres of horizontal motions during continental deformation.