Mechanics of non-critical fold–thrust belts based on finite element models

The mechanics of fold–thrust belts and accretionary wedges is investigated using a two dimensional, plane strain, elastic–plastic (cohesive Mohr–Coulomb) mechanical model solved with the Finite Element Method. Results show that when a layer with an initially non-critical geometry is compressed from the rear, it does not form a wedge that is at failure throughout, as assumed in critical wedge theory. Rather, the wedge consists of narrow plastic shear zones that propagate sequentially outward with time, loading rocks ahead while unloading rocks behind. Not only are stress states within the wedge not everywhere at failure but principal stress orientations vary strongly in time and space, particularly across shear zones, near the basal detachment and in the hanging wall of active structures, where local surface extension may be observed. The reason the investigated wedges are not stressed to compressive failure throughout is related to strength reduction associated with strain localisation that enables material outside shear zones to unload and return to an elastic stress state. This mechanism is intrinsic to elastic–plastic materials and occurs regardless of any material degradation such as loss of cohesion. Even though the stress state of the investigated wedges is generally non-critical, the overall geometry may still be consistent with cohesionless critical wedge theory, since the local surface slope is created when a particular part of the wedge is at a limit state. Prowedge tapers display non self-similar growth through time but eventually evolve to the minimum critical taper. Retrowedges on the other hand, may get caught within this initial transient state and thus may have tapers anywhere between the minimum and maximum critical taper. However, if the basal detachment is such that lateral propagation is not kinematically inhibited, retrowedges are shown to also eventually evolve towards minimum critical tapers, resulting in a symmetrical doubly-vergent orogen. Because all of these wedges have non-critical stress states, both pro- and retrowedges are predicted to display stable non-equilibrium taper angles in the presence of external perturbations, for example related to erosion or sediment deposition.

Research Highlights
► Elastic–plastic wedges in FEM mechanical models are not always stressed to failure (critical).
► Strain localisation leads to elastic unloading outside shear zones.
► Stick–slip events along the base also unload the wedge.
► Retrowedge are noncritical if they are kinematically constrained on the base.
► Otherwise, retrowedges may resemble prowedges in terms of slope and width.