FLEXURAL ACCOMMODATION OF OROCLINAL BUCKLING AT THE CORE OF PANGEA: A STRUCTURAL STUDY OF THE CANTABRIAN OROCLINE, NW IBERIAN MASSIF

The Variscan orogen provides the European record of the Upper Paleozoic continental collisions that ultimately became the core 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 lithospheric-scale vertical-axis folds of orogens are accommodated, are poorly understood. Structural continuity between the northerly Cantabrian and the southerly Central Iberian oroclines suggests that they formed contemporaneously and 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-degree Cantabrian orocline. In the Western flank of the Narcea Antiform, the Narcea Slates are characterized by a penetrative steep to vertical slaty cleavage (S1) and subparallel 2-km wide reverse shear zones with a penetrative fabric (S2) that are affected by asymmetric meso- to outcrop-scale vertical-axis folds with a dominant vergence toward the oroclinal hinge; i.e. fold sense is dominantly dextral in the southern limb of the Cantabrian orocline and dominantly sinistral in its northern limb. Vertical-axis folds affecting the Narcea Slates are of the appropriate scale and geometry to be parasitic structures developed in response to flexural shear within the limbs of the Cantabrian orocline. A model of formation of the Iberian coupled oroclines by buckling in response to a principle compressive stress oriented at a high angle to orogenic trend is therefore supported, providing new insight into the complexities associated with the final stages of Pangean amalgamation.