Calcite fabrics in a natural shear environment, the Helvetic nappes of western Switzerland

Abstract The aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the Upper Jurassic limestones of the Helvetic nappes between the Mont Blanc and Aar massifs, over a distance along strike of about 180 km. The c-axis preferred orientation pattern, which could be inferred, is regionally very constant. The c -axis point maxima are everywhere subperpendicular to the regional strike of the chain. There is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains: the c -axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain-shape fabric. This fabric geometry indicates either continuous shear deformation, or, alternatively, a two stage deformation history. The obliquity of the grain-shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process, or it may represent a later finite deformation, which has led to the formation of a regionally developed second cleavage. There is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern. This suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction. The observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms, a combination of slip on r and twinning on e . However, the e -twinning observed in the specimens investigated is younger than the development of the crystallographic fabric. A new method for stress determination from e -twinning is proposed.