Abstract

Progressive opening of the Rheic Ocean led to the drifting away of one or several ribbon terranes, generally ascribed to Avalonia, and inaugurated a passive margin stage on the newly formed margin of NW Gondwana. In Iberia, which remained on the Gondwanan side of the ocean, the rift to drift transition is recorded in the Ossa Morena Zone in latest Furongian times and migrated towards more internal parts of the margin during the Lower Ordovician. The passive margin stage is characterized by development of open marine platform sedimentation locally punctuated by eruption/intrusion of mainly basaltic, alkaline volcanic rocks, during transient periods of tectonic extension. A progression from outer (Ossa Morena Zone), through intermediate (Central Iberian and West Asturian-Leonese Zone, to inner (Cantabrian Zone) shelf environments can be generally established, although with significant variations related to local tectonic development. The end of the passive margin stage is marked by the formation of syn-orogenic basins, which roughly migrate in the same direction, i.e. from external to internal parts of the margin, as a response to the propagation towards the foreland of the Variscan orogenic wedge.