High-strain zones: a unified model

Where a shear zone is bounded by undeformed wall rocks and there is no volume change, the deformation path within the zone on the bulk scale approximates a simple shear. Where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries, the deformation path within the zone is not a simple shear but still has monoclinic symmetry. Both simple shear and general monoclinic deformation paths should be regarded as special cases in nature, because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries. In this general case, the deformation path is triclinic. We establish a model which includes this general case also taking into consideration volume change. ndependent parameters are required to characterize the rate of deformation of the flow; four are sufficient to characterize the flow kinematics. The model is generally triclinic, and orthorhombic and monoclinic deformations are subgroups. By varying the values of the characterizing parameters, the various types of high-strain zones can be represented. The kinematics of flow within such zones and the accumulated finite deformation geometry are investigated. Many field observations such as, for example, lineations in transcurrent shear zones that vary between vertical and horizontal, cleavage transected folds, and the absence of sheath folds in some shear zones where folds have been significantly rotated, can sometimes be better interpreted in the context of our general model.