Strain partitioning in banded and/or anisotropic rocks: Implications for inferring tectonic regimes

Among the many concepts that have contributed to the development of structural geology in the last half-century, two are particularly important. These are: (i) the link between the symmetry of the structure, the vorticity of the associated deformation process and the tectonic regime, and (ii) the widespread partitioning of strain. However, a clear understanding of these concepts and the links between them are not always apparent in structural analyses carried out to determine the prevailing tectonic regime of an area through the use of structural symmetries and kinematic indicators. This contribution, based on field studies from Cap de Creus, experimental data and theoretical concepts, highlights some of the problems encountered when attempting to deduce the type of strain and associated regional tectonic regime from field structures. The relation between the symmetry of a structure and its associated kinematics is not unique. For example, it is demonstrated that in mechanically anisotropic materials, symmetry variations do not necessarily reflect marked vorticity variations, but rather the angular variations between the kinematic framework and the anisotropy planes. Because of widespread strain partitioning, local and general kinematics do not generally match. This mismatch is especially accentuated in rheologically heterogeneous rocks. Nevertheless, correct interpretations are possible by performing continuous multi-scale structural analyses in which the impact of strain partitioning and of the presence of material anisotropies and heterogeneities are considered.