serie NOVA TERRA nº 49

70 structures, such that the unit provides unique information about early stages in the tectonic evolution of the upper units which is usually lost elsewhere as a result of higher grade metamorphic recrystallization. Second, large numbers of ma fi c dykes intrude the low-grade metasedi- ments and show various relationships with the regional deformation events. The relative timing of ma fi c dyke emplacement can be establ- ished by fi eld relationships,and thenewU – Pb age of dykeemplacement can be used to date the structural evolution of this uppermost section. The age data and cross-cutting relationships clearly indicate that this part of the upper units was deformed in Middle Cambrian times. This structural evolution therefore provides important informa- tion about the dynamics of the peri-Gondwanan arc represented in the upper allochthonous terrane of NW Iberia, including the structures and kinematics developed during the transition between the contractional and extensional regimes that preceded the Cambro- Ordovician opening of the Rheic Ocean at the periphery of the West African Craton. 2. Structure of the upper units at the Ares – Sada region: the coastal section In contrast to the rest of the upper units, which are characterized by a pervasive regional S 2 foliation developed under medium- to high- grade conditions that prevents analysis of earlier deformation events, their uppermost low-grade unit can be studied as a typical slate belt by applying classical macroscopic and microscopic geometrical analysis and superpositional criteria. On this basis, three main deformation events can be distinguished, each associated with different structures and kinematics. Hence, the uppermost unit provides a unique oppor- tunity to identify the earliest deformation event in the upper units of the Fig. 1. (a) Structural map, and (b) cross section of the Órdenes Complex showing location of area depicted in Fig. 2 . AC, A Coruña granodiorite; AS, A Silva granodiorite; C, Corredoiras granodiorite; CD, Corredoiras detachment; F, Fornás unit; FD, Fornás detachment; MC, Monte Castelo gabbro; PCD, Ponte Carreira detachment; S, Sobrado unit; XD, Xesteda detachment. 353 F. Díaz García et al. / Gondwana Research 17 (2010) 352 – 362