The interaction between oblique and layer-parallel shear in high-strain zones: Observations and experiments

Abstract

The Scandinavian Caledonides consist of a rigid basement, a weak décollement zone, and a series of crystalline nappes. The basement is cut by oblique, west-dipping extensional shear zones, associated with west-verging folds in the overlying décollement zone. The oblique shear zones may, theoretically, have formed after, prior to, or during post-contractional, top-to-the-west shear movements. Physical modelling of this geometrical/mechanical situation shows that each of the three deformation histories results in a characteristic structure. If the oblique, extensional shear zones postdate the layer-parallel (décollement) shear, a normal-type, oblique shear zone structure occurs. If the layer-parallel shear occurs after the development of oblique shear zones, the resulting structure is characterized by a low-strain region above the oblique shear zone. Some folds typically develop above this “protected” region. However, simultaneous oblique and layer-parallel shear provides abundant asymmetric folds with gently dipping axial planes and sub-horizontal, shear zone-parallel fold axes in the region above the oblique shear zone. A neutral surface may be encountered above, and parallel to the oblique shear zone, separating a lower, thin zone of layer-parallel extension from the main, contractional region. The differences between the three types can be used to interpret field observations of this type of structures. The large-scale, post-contractional structures in the Scandinavian Caledonides are consistent with layer-parallel shear related to back-movement of nappes, synchronous with oblique shear zones in the basement. Similar geometries are found as meso- and micro-scale structures in ductile high-strain zones, where oblique shear bands or shear zones affect more competent layers within the tectonites. Also, these structures compare well with those produced experimentally by simultaneous layer-parallel and oblique shear, and are consistent with the general assumption that oblique shear bands form contemporaneous with the general shear in shear zones.