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The Palaeozoic ocean that bordered the northern coast of Gondwana from the Ordovician until its closure in the Late Devonianearly Carboniferous is known as the Rheic Ocean (e.g. Nance et al. 2010, and references therein). It separated Gondwana from Laurussia (Laurentia-Baltica-Avalonia) following the closure of the Iapetus Ocean (e.g. Nance & Linnemann 2008). The closure of the Rheic Ocean produced a suture that extends over 10000 km from Mexico to Turkey, gave rise to the vast Variscan-Alleghanian-Ouachita orogen (e.g. Nance et al. 2010, and references therein) and assembled the supercontinent Pangaea.One of the most complete sections of the Rheic passive margin sequence of Gondwana is exposed in the Iberian Massif (western Iberia, Fig. 1) where Palaeozoic rocks lie within a complex arcuate segment of the Variscan orogenic belt (Weil et al. 2001(Weil et al. , 2013Catalan 2011Catalan , 2012Shaw et al. 2012). The Rheic Ocean was bordered on its southern side by an extensive passive margin (Pastor-Galan et al. 2013a,b) that originated after the rift-drift of Avalonia from northern Gondwana at the Cambrian-Ordovician boundary (Murphy et al. 2006(Murphy et al. , 2008. The passive margin, now preserved in Iberia, was more than 200 km wide (Murphy et al. 2008) and previous interpretations are consistent with palaeogeographical reconstructions that place NW Iberia adjacent to North Africa along the southern flank of the Rheic Ocean during the Palaeozoic (e.g.

The Variscan orogen provides the European record of the late Paleozoic continental collisions that culminated with formation of the supercontinent Pangea. An S-shaped pair of isoclinal coupled oroclines characterizes the Variscan orogen of the Iberian Massif. Though oroclines are common features of the world's orogenic belts, the mechanisms that drive oroclinal formation, and the manner in which these continental-scale vertical-axis folds of orogens are accommodated are poorly understood. The northerly Cantabrian and the southerly Central Iberian oroclines are structurally continuous and pericontemporaneous, suggesting that they formed in the same fashion. Exposures of the Ediacaran Narcea Slates within the so-called Narcea antiform trace a 150-km-long arcuate belt around the 180° Cantabrian orocline. In the western flank of the Narcea antiform, the Narcea Slates are characterized by a penetrative steep to vertical, rough to slaty cleavage (S1) and subparallel 2-km-wide reverse shear zones with a penetrative fabric (S2) that are postdated by asymmetric meso-to outcrop-scale verticalaxis folds (plunge >65°) with a dominant vergence toward the oroclinal hinge; i.e., fold geometry is dominantly dextral (Z-shaped) in the southern limb of the Cantabrian orocline and dominantly sinistral (S-shaped) in its northern limb. Axial planes are consistently steeply dipping, but they are typically oriented at a high angle to S1/S2 and are therefore variable in strike about the orocline hinge. Vertical-axis folds affecting the Narcea Slates are of the appropriate scale and geometry to be interpreted as parasitic structures developed in response to a component of flexural shear within the limbs of the forming Cantabrian orocline. A model of formation of the Iberian coupled oroclines by buckling accommodating significant orogen-parallel shortening along an initially linear Iberian Variscan belt is therefore supported, providing new insight into the complexities associated with the final stages of Pangean amalgamation.