The transition from single layer to foliation boudinage: A dynamic modelling approach

Foliation boudinage is a deflection of foliation in the vicinity of a central discontinuity in foliated rocks, mostly filled with vein material. It shows evidence for brittle deformation and void-opening during ductile flow. We used a two-dimensional visco-elastic spring model based on a discrete element approach to study the dynamic development of foliation boudinage and the behaviour of anisotropic visco-elastic material deformed under pure shear conditions. The anisotropies are set by defining rheological heterogeneities in the models with (1) a single layer in a weaker matrix; (2) multi-layers with different elastic properties and (3) random-distributed “micas”, rows of horizontally aligned elements with the same elastic properties. The models show the nucleation and propagation of fractures during ductile deformation and flow perturbation and localization in the vicinity of fractures. Voids localize along extension fractures, at intersections of conjugate shear fractures and in small pull-apart structures along shear fractures, which look identical to boudin necks in isolated stiff layers. Results show that strain localization is strong in models with lower elastic constants and anisotropy whereas it is distributed in the models with higher elastic constants and anisotropy in which asymmetric and complex geometries develop. The models successfully show the development of and transitions between a single layer, multi-layer and foliation boudinage that are common structures in high strain zones.