The role of bedding in the formation of fault-fold structures, Portalegre-Esperança transpressional shear zone, SW Iberia

Abstract

Fold-fault structures within a major transpressional shear zone of the SW Iberian Massif were investigated by combining geological mapping, cross-section analysis and microtectonic studies. A significant example of contractional deformation is displayed in the Portalegre-Esperanc¸a Shear Zone (PESZ) where a heterogeneous Ordovician stratigraphic sequence, showing a strong competence contrast between quartzites, slates and quartzo-feldspathic rocks favoured strain localization and fault nucleation that controlled fold formation. The presence of politic layers within the thick-bedded quartzites had probably provided weakness zones that were more favourable for the strain localization than the previous foliation present in the quartzites. The quartzites and the quartzo-feldspathic rocks (granites and volcaniclastic rocks) accommodated heterogeneous high strain developing different degrees of mylonitization. The quartzites with protomylonitic textures are dominant and represent coarse-grained siliciclastic sediments that suffered metamorphism and partial dynamic recrystallization. Ultramylonites occur within discrete high-strain shear zones. It is probable that the strain localization in the PESZ involved both the effect of having layers of different competence and layers or stratigraphic contacts with rocks that experienced grain size reduction dominated by cataclasis and dislocation creep. Folds within quartzites with sub-horizontal to gently plunging hinges vary from closed to open in thick-bedded quartzites and from tight to closed in thin-bedded quartzites. Observed changes in structural style of deformed quartzites, slates and quartzo-feldspathic rocks are interpreted to result from the constraints imposed by the mechanical properties of the different lithologies. As the folds tightened during shortening, the alternating zones of contrasting competence favoured the disruption of the bedding. The deformation history in the PESZ was dominated by thrusting and strike-slip faulting along incompetent layers that locally developed staircase geometry and transected the lower Ordovician stratigraphic sequence. The kinematic model proposed for the PESZ is consistent with the models of transpressional shear zones stretched along strike with the development of coeval strike-slip and low angle oblique-slip faults associated to active buckle folds with axes parallel to the principal extension direction.