3.4.1. Background

The lithosphere of cratonic Iberia was consolidated during the Variscan orogeny. During the Mesozoic break-up of Pangea, the Iberian microcontinent became step-wise isolated in response to opening of the oceanic Alpine-Tethys (Middle Jurassic), North Atlantic (Early Cretaceous) and Bay of Biscay (Mid-Cretaceous) basins (Ziegler, 1988; Stampfli et al., 2002; Stampfli and Borel, 2004). With the Late Cretaceous onset of Africa-Europe convergence (Rosenbaum et al., 2002), subduction of the Alpine-Tethys beneath die Iberian microcontinent commenced during the Campanian (± 80 Ma), as indicated by the age of the oldest high-pressure metamorphic rocks (Zeck, 1999; Faccenna et al., 2001b). At the same time closure of the Pyrenean rift started, involving northward subduction of continental Iberian lithosphere beneath Europe and southward subduction of the oceanic Bay of Biscay beneath Iberia (e.g. Ziegler, 1988). During Campanian to Late Eocene times Iberia was at least partly coupled to the African plate with the African-Eurasian plate boundary coinciding with the Cantabrian-Pyrenean orogen. Evolution of the latter (Munoz, 1992; Vergéz and Garcia-Senez, 2001) was accompanied by a gentle clockwise rotation of Iberia (Rosenbaum et al., 2002), involving initially tensional and later strike-slip reactivation of fault systems along its Atlantic margin. The Campanian-Maastrichtian alkali basalt magmatism of southern Portugal (Tavares Martins, 1999) can be related to the activation of the NE Atlantic mantle plume (Hoernle et al., 1995).

During the Late Eocene-Early Miocene Iberia acted as an independent micro-plate with Africa-Eurasia convergence being increasingly compensated by activity along the Betic-Balearic arc-trench system. Upon Early Miocene consolidation of the Cantabrian-Pyrenean Orogen, Iberia became welded to the Eurasian plate with Africa-Europe convergence being fully taken up at the Betic-Balearic arc-trench system. At the same time the convergence direction of Africa with Europe changed from NNE during the Late Oligocene-Burdigalian to NNW during the Langhian-early Tortonian and from late Tortonian onward to NW (De Vicente et al., 2004).

During the Late Cretaceous the eastern parts of the Iberian microcontinent were covered by an extensive carbonate platform, reflecting that they were located near sea level (Stampfli et al., 2001). During the Late Eocene and Oligocene evolution of the Cantabrian-Pyrenean and the Betic-Balearic orogens, the Iberian microcontinent was subjected to repeated pulses of intraplate compression, causing inversion of the Mesozoic extensional Catalan Coast Ranges and the Iberian Chain basins (Salas et al., 2001; Guimerà et al., 2004) and Oligocene-Early Miocene upthrusting of the Central System basement block that had not been affected by Mesozoic extension (De Vicente et al., 1996; Gomez-Ortiz et al., 2005). This resulted in the development of major topography, tectonic silling of the flexural Ebro foreland basin and subsidence of the Tagus Basin (Andeweg, 2002).

Roll back of the northwest-dipping Alpine-Tethys slab started during the Late Oligocene, causing development of the Gulf of Lions-Valencia Trough rift system by back-arc extension that culminated in late Aquitanian (21.5 Ma) separation of Corsica-Sardinia from Iberia and the opening of the oceanic Liguria-Provençal Basin. At the same time the Kabylia-Alboran block was separated from the Balearic promontory, resulting in the opening of the oceanic Algerian Basin, In the evolving Betic-Balearic orogen, crustal shortening persisted until late Mid-Miocene times (Rocca, 2001). During the Late Miocene, the Valencia Trough was tensionally mildly reactivated. Compressional stresses build up again during the Plio-Pleistocene, as evidence by a subsidence acceleration of the Liguria-Provençal-Valencia Basin (Rocca, 2001).

Seismic tomography (Fig. 55) indicates that the Alpine-Tethys slab is still attached to the African lithosphere in the area of the Rif fold belt but that opening of a slab window had commenced in the Maghrebian domain and in time progressed westward (Spakman and Wortel, 2004), as evidenced by late Middle Miocene to Pliocene slab detachment-related magmatic activity (Wilson and Bianchini, 1999). The Tortonian and younger alkaline magmatism (10-0.1 Ma) along the Iberian Mediterranean margin (e.g. Calatrava, Colombretes, Olot-Gerona-La Selva) appears to be plume-related (Wilson and Bianchini, 1999; Rocca, 2001). Tomographically imaged upper asthenospheric low velocity anomalies beneath NE Iberia and southern France can be interpreted as a plume head that has spread out above the 410 km discontinuity (Fig. 54). As these anomalies appear to extend under large parts of Iberia (Sibuet et al., 2004; Spakman and Wortel, 2004), they may be related to the NE Atlantic plume that rises up from the deep mantle (Hoernle et al., 1995).

In the Pyrenean and Betic collision zones subduction processes apparently ceased at End-Oligocene and Mid-Miocene times, respectively. Subsequent convergence of Africa with Iberia and Europe at rates of 3.3 - 5 mm yr-1 (Jiménez-Munt et al., 2003), combined with Atlantic ridge push forces, is held responsible for the seismicity and neotectonic deformation of Iberia, with the latter exerting a strong control on its topography (Cloetingh et al., 2005b).