Mechanics of the transition from localized to distributed fracturing in layered brittle–ductile systems

Abstract

The mechanical coupling between brittle and ductile layers in the continental lithosphere produces rheological contrasts, which are supposed to trigger localized or distributed mode of faulting. A plane-strain 2D finite-element model is used to highlight the mechanical role of the brittle–ductile coupling in defining the patterns of fracturing. The coupling is performed through the shortening of a Von Mises elasto-viscoplastic layer rimmed by two ductile layers behaving as Newtonian incompressible fluids. By varying the viscosity of the ductile layers or the amount of softening in the brittle layer, the fracturing mode evolves from localized to distributed. The mechanics of brittle–ductile coupling is explained by the limitation of the fault displacement rate imposed by both brittle and ductile rheologies. On these bases, an analytical approach is presented in order to estimate the maximum velocity along each fault permitted by both brittle and ductile media. This velocity is then compared to the velocity required by the boundary shortening rate. If the velocity in the fault is not large enough, the development of new faults is necessary. From this analysis, we define four fracturing modes in a brittle–ductile media: the localized mode with the onset of a few large faults, the distributed mode with very dense fault patterns, and finally, the ductile-control mode and the brittle-control mode, where the number of faults increases with an increase in the ductile viscosity and a decrease in the brittle softening respectively.