Monoclinic model shear zones

Although many ductile shear zones are supposed to have developed by approximately simple shear flow, some must have formed under different conditions. A few types of such ‘non-simple shear zones’ have been proposed in the literature such as transpression-, transtension- and stretching-shear zones. This paper presents a full three-dimensional kinematic model of shear zones with monoclinic flow geometry. Monoclinic shear zone types can be classified according to flow parameters, and according to the geometry and orientation of accumulating finite strain. Modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop in monoclinic shear zones. These include stretching lineations normal to the displacement direction; foliations normal to the shear zone boundary; switching of the position of principal finite strain axes X, Y and Z during progressive deformation; and strain gradients along shear zones that may give rise to obliqueness of fabric elements of the same age in adjacent rock volumes.