The Variscan orogeny marks the amalgamation of continental plates to form the supercontinent Pangaea in the Upper Palaeozoic. From a comparison between the existing plate tectonic models, it is clear that the number of Variscan oceans and their relative importance to the evolution of the Variscan orogen is still a matter of much debate. Despite these differences, the majority of the authors accept the view that the late Palaeozoic Variscan orogeny involved the closure of the Rheic Ocean, bordering Gondwanan France and Iberia.
The Iberian Massif represents a well-exposed cross section of continental crust affected by Variscan continent-continent collision during Late Devonian to Carboniferous. It is mainly composed of folded, thrusted and metamorphosed rocks of late Precambrian and Palaeozoic age and large batholiths of granitoids emplaced during and after collisional tectonics. According to Ribeiro et al. (1990) and Quesada (1991), the evolution of the Iberian Variscan Fold Belt can be described in terms of accretion of a number of tectonostratigraphic terranes with contrasting geological histories, through a long and complex process involving subduction and eventual obduction, followed by oblique convergence and transcurrent faulting.
Ribeiro et al. (1990), Quesada (1990, 1991) and Shelley and Bossière (2000) distinguished the following accreted terranes within the Iberian Massif:
(1) The Iberian autochthon terrane, comprising the Cantabrian Zone (CZ), the West Asturian Leonese Zone (WALZ) and the Central Iberian Zone (CIZ) and consisting of a Proterozoic basement with Gondwanan affinities, covered by lower-middle Palaeozoic shelf sediments (Fig. 1). In the northern part of the CIZ, the Palaeozoic succession is underlain by the Ollo de Sapo acid plutonics emplaced in the Early Ordovician (Gebauer et al., 1993).
Figure 1. Major geotectonic zones of the Iberian Massif
Major geotectonic zones of the Iberian Massif (modified from Julivert et al., 1974 and Farias et al., 1987). The location of Fig. 2 is indicated.
The Northwest overthrust terranes, represented by the allochthonous nappes of the Galicia Tràs-os-Montes Zone (GTMZ) (Fig. 1). These include klippe with ophiolitic catazonal complexes (Cabo Ortegal, Ordenes, Bragança and Morais), overthrusted by sheets of Precambrian basement rocks and Palaeozoic clastic cover sediments.
The Ossa Morena terrane (OMZ), containing oceanic and volcanic arc material of Cadomian age, covered by Palaeozoic rocks with a more distal character than those of the CIZ (Fig. 1).
The Pulo de Lobo oceanic terrane composed of Ordovician-Silurian oceanic lithosphere and low to middle Devonian turbidites which are thought to represent an accretionary prism. Both the Pulo do Lobo and the Ossa-Morena terranes are overstepped by Late Devonian and/or Lower Carboniferous flysch sediments.
The exotic South Portuguese terrane (SPZ) formed by pre- to syn-orogenic, shelf type deposits of Upper Devonian and Carboniferous age, on top of an unexposed basement of unknown origin (Fig. 1). The SPZ is separated from the Iberian terrane by the Pulo de Lobo oceanic unit (Silva et al., 1990). This suture zone is believed to represent the remnants of the Rheic Ocean (Capdevila and Mougenot, 1988)
As a result of the presence of large, northward and southward overthrusts, the Variscan fold belt has a broad fan-like aspect (Matte, 1986). This bilateral symmetry is accompanied by an outward migration of deformation, metamorphism and granite emplacement through time. A consistent decrease in the age of tectono-metamorphic events from the internal to the external zones of the orogen has been reported by several authors (e.g. Matte and Burg, 1981; Ribeiro, 1981; Matte, 1986).
In the internal zones, collisional tectonics was contemporaneous with and immediately followed by rapid exhumation of crystalline complexes, voluminous granite intrusions and high temperature / low pressure metamorphism (e.g. Burg et al., 1994; Schulmann et al., 2002).