Influence of viscous layers on the growth of normal faults: insights from experimental and numerical models

The influence in space and time of viscous layers on the deformation pattern of brittle layers is investigated using wet clay/silicone putty analogue models in extension. Brittle and brittle–viscous experiments at various extension velocities are compared. Numerical models are also performed to confirm the results and to control the boundary conditions. Our results show that: (i) the presence of a basal viscous layer localizes the deformation by creating faults with very large throw. This kind of deformation distribution constrains the location of small faults, with scattered orientations, in the vicinity of the larger, in particular in relay zones. (ii) A lower strength of the viscous layer (i.e. a low extension velocity) enhances this localization of the deformation. (iii) The displacement–length relationship and the spatial distribution of small-scale faults are strongly influenced by both the rheology of the model and the amount of extension. This study shows that they are important parameters, especially when characterizing the whole fault network evolution and the relationship between large and small faults.