Heterogeneous strain behaviour in competent layers during folding in transpressive regimes

The understanding of folding processes in transpression is crucial in structural geology. However, the study of folding in multilayers where adjacent strata exhibit high competence contrast is impossible if the observations are only restricted to geometrical features. Nevertheless, using detailed three dimensional finite strain analyses it is possible to constrain the solutions. Strain data from the deformed northern Portugal Ordovician quartzites show that folding mechanisms in transpression are highly variable. For the more deformed materials found, not only in the short limbs, but also in the thinner layers of the long limbs, the strain ellipsoids orientations can be explained by a combination of flexural shear (related to the folding process) and sinistral wrench component parallel to the bedding plane (a pervasive regime in the southern branch of the Ibero-Armorican Arc). In the thicker layers of the long limbs, orthogonal flexure seems to be the dominant mechanism. Concerning the strain ellipsoid shape, the constrictional forms predominate mainly in the long limbs. Strain partitioning in transpressive regimes is important at the mesoscopic fold scale, especially in asymmetric folds. In the more deformed short limb, the data indicate that transpression tends to be always an active mechanism, while in the long limb the regional wrench component it's often less important.