Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling

Abstract We describe a natural shear zone with triclinic symmetry, present a general model for triclinic shear zones based on natural examples, and investigate the kinematics and strain geometry within such zones. In the Roper Lake shear zone in the Canadian Appalachians, the orientation of a stretching lineation is oriented approximately down-dip near the shear zone boundary and becomes gradually shallower towards the centre. The structures in the central portion of the shear zone exhibit approximately monoclinic symmetry where the poles to both the S- and C-surfaces, the stretching lineation on the S-surfaces and the striations on the C-surfaces all plot in a great circle girdle. However, the lineations from the marginal portion do not plot in the same girdle, and the bulk symmetry of the shear zone is triclinic. Theoretical modelling shows that the observed strain geometry can be interpreted by an oblique transpression with a larger ratio of simple shear to pure shear in the centre of the shear zone than in the margin. The latter suggests a higher degree of localization of the zone boundary-parallel movement component relative to the boundary-normal compression component. We emphasize that, as the imposed boundary displacements for most natural shear zones lie between dip-slip and strike-slip, their movement pictures are generally triclinic; monoclinic shear zones are special end members. Structural data that exhibit monoclinic symmetry do not necessarily mean that they resulted from a monoclinic movement picture; the present modelling demonstrates that a triclinic movement picture with a high ratio of boundary-parallel movement to boundary-normal movement can result in apparent monoclinic structural geometry. The results of the modelling also show that the simple statement made for simple shear zones that stretching lineations will align with, and therefore indicate, the shear direction cannot be extrapolated to three-dimensional transpressional (or transtensional) shear zones.