The competition between folding and faulting in the upper crust based on the maximum strength theorem

It is proposed to complement the numerous geometrical constructions of fault-related folds relevant to fold-and-thrust belts by the introduction of mechanical equilibrium and of the rock limited strength to discriminate between various deformation scenarios. The theory used to support this statement is the maximum strength theorem that is related to the kinematic approach of limit analysis known in soil mechanics. The classical geometrical construction of the fault-propagation fold (FPF) is proposed for illustration of our claim. The FPF is composed of a kink fold with migrating axial surfaces ahead of the region where the ramp propagates. These surfaces are assigned frictional properties and their friction angle is found to be small compared with the usual bulk friction angle to ensure the full development of the FPF, a first scenario. For larger values of the axial surface friction angle, this development during overall shortening is arrested by the onset of fault breaking through the front limb, a second scenario. The amount of shortening at the transition from folding to break-through faulting is established.