Saw-tooth structures and curved veins related to folds in the south-central Pyrenees (Spain)

Two generation of folds ( F 1 and F 2) and associated structures, developed in the Eocene turbidites of the south-central Pyrenees, are analyzed in this paper. F 1 folds are close, have sub-horizontal axes and southwards vergence. They have an associated cleavage S 1. Competent layers were folded by layer-parallel shortening, tangential longitudinal strain, some possible flexural flow and an obliquely superimposed homogeneous strain due mainly to simple shear. Flexural slip is also an important mechanism in the whole multilayer. F 2 folds are gentle and scarce; they fold the S 1 cleavage. Among the structures associated with F 1 folds, there are sets of veins with curved form in the competent layers. The displacement of each vein gave rise usually to a step in the layer boundary, so that a set of veins produces a structure that is named “saw-tooth structure”. The veins initiated as small faults that made flexural slip difficult and gave rise to a concentration of stress on the steps, leading to an opening of the fractures and a propagation of them along a curved path, as suggested by a simple mechanical model. This propagation agrees with finite element models developed by other authors. ► Curved veins formed associated with folding in turbidites of the Pyrenees. ► The veins gave rise to an exotic structure named “saw-tooth structure”. ► Veins initiated as small faults that made flexural slip difficult. ► Stress concentration led to fracture opening and propagation along a curved path.