VARISCAN TRANSPRESSIONAL INVERSION OF A RIFTED CONTINENTAL MARGIN: OSSA-MORENA ZONE (SW IBERIA)

The Variscan orogen has its most complete outcrop in the Iberian Massif, where the southwestern half of the Ibero-Armorican Arc is exposed. Northern areas display an intensely imbricated pile of east-verging far-traveled nappes whereas southern areas show a predominantly left-lateral transpressional structure. In fact the Ibero-Armorican Arc has been interpreted to be the result of the impingement of a promontory in NW Gondwana (Ibero-Aquitanian indentor) against the margin of Laurussia. In this scenario, deformation regimes during collision varied from nearly orthogonal imbrication in frontal parts of the indentor to mostly transpressional, strike-slip escape of units at both lateral margins (sinistral in SW Iberia and dextral in the French Armorican Massif).

In the SW lateral margin of the indentor presently exposed in SW Iberia, the onset of collision forced the left-lateral escape of marginal units, accommodated by slip along major preexisting features and by subduction of the remnant oceanic domain beneath the escaping blocks. The most prominent unit typifying the evolution in SW Iberia is the Ossa-Morena Zone which escaped some hundred kilometres towards the SE along the so-called Badajoz-Córdoba shear zone (a preexisting Cadomian suture). This displacement was accommodated by N-directed oblique subduction of the previously adjacent ocean under the Ossa-Morena Zone. Between both active margins (left-lateral subduction at the suture zone, in the S, and also sinistral transcurrent displacement along the Badajoz-Córdoba shear zone, in the N) the deformation of the Ossa-Morena Zone during Variscan collision was typically transpressional.

The present overall geometry of the zone defines a hectokilometre-scale strike-slip (sinistral) duplex. Basement involved thick-skinned structures were formed, but in some horses a detachment of the Paleozoic cover took place leading to thin-skinned nappe formation, decoupled from the underlying basement. Significantly, the largest stratigraphic differences among the various horses occur within a Cambrian-Ordovician rift sequence, which is interpreted as evidence for inversion of a preexisting horst and graben structure acquired during Early Paleozoic rifting, that culminated in generation of a new oceanic tract, probably a part of the Rheic ocean.