Flow perturbations: a tool to study and characterize heterogeneous deformation

The classification of the myriad of small-scale structures that are used in tectonic analysis is presently based on their geometry, which makes it difficult to discover transitions and groups amongst them. An alternative would be to classify structures according to the flow type by which they form, but this is difficult. Although most structures form by heterogeneous flow, modelling studies are often focussed on bulk homogeneous flow, since the mathematical treatment of heterogeneous flow is cumbersome. Also, heterogeneous deformation patterns seem to occur in as many types as there are small-scale structures. This paper introduces the use of the geometry of flow perturbations to improve our understanding of heterogeneous flow, and to allow classification of structures based on kinematics. A flow perturbation is the deviation of a heterogeneous flow pattern from the background homogeneous flow. Perturbation patterns can be visualised as open or closed loops of vectors, which occur in pairs, rows or groups of four. They usually cross-cut rheological interfaces, and can be subdivided into those that cross interfaces twice or four times. Most geological structures belong to the first group, while initiating buckle folds belong to the second group.