Change of direction of overthrust shear in the Helvetic nappes of western Switzerland

The lowest of the Helvetic nappes in western Switzerland, the Morcles and the Doldenhorn nappes, are large-scale recumbent folds in equivalent tectonic positions in the Alpine chain. A comparison of the orientations of stretching lineations in both nappes shows that the lineations in the two stratigraphically normal sequences trend N-S, whereas those in the two stratigraphically inverted sequences trend NW-SE. A similar geometrical relationship has been found from an analysis of the calcite c-axis directions of these limestones: the two normal sequences show NW-SE directed c-axes, whereas the two inverted sequences show W-E directed c-axes. These observations lead to the following conclusions: (1) Regionally consistent shear movements occurred over a strike distance of more than 60 km, and these gave rise to similar deformation patterns in both nappes. (2) A progressive rotation of these shear movements from a northerly direction in the earlier-formed limbs to northwesterly in the later-formed inverted limbs can be deduced. The finite-strain stretching lineations lag behind the crystallographic fabric axes formed in the later part of the deformation history.