Reconstruction (continued)

Tectonic setting of south Europe in the Oligocene

During the Oligocene, the area between the Iberian Peninsula and southern France consisted of several terranes, which are now located hundreds of kilometres away. Among these are the internal zone of the Betic-Rif Cordillera, the Balearic Islands, the Kabylies, Corsica, Sardinia, and Calabria (Ricou et al. 1986, Lonergan & White 1997)(Figure 11). Most of these terranes consist of a Hercynian basement and a Mesozoic cover, which largely underwent deformation and metamorphism during Alpine orogenesis. The origin of these terranes is not entirely clear, but they were possibly attached to the Iberian plate before being incorporated in the Alpine orogeny (Stampfli et al. 1998). Since the Middle Miocene, no rotations occurred in Corsica, Sardinia and the Balearic Islands, and their palaeo-positions can be inferred by applying opposite rotations to those obtained from palaeomagnetic studies. Prior to the opening of the western Mediterranean basins, Calabria was located adjacent to Sardinia (Alvarez et al. 1974, Dewey et al. 1989, Minzoni 1991). An alternative hypothesis is that during the Oligocene, Sardinia and Calabria overlapped each other, forming the upper (Sardinia) and the lower (Calabria) units of a metamorphic core complex. This hypothesis, however, requires further research. The largest uncertainty in the Oligocene reconstruction is the position of the Internal Zone of the Rif-Betic cordillera, which is here placed southeast to the Balearic Islands after Lonergan and White (1997). This configuration forms a continuous orogenic belt during the Oligocene, from the Rif-Betic to Calabria, Corsica and the western Alps.

Figure 11 Oligocene reconstruction (30 Ma).

The tectonic setting in the Late Oligocene was characterised by a switch in the vergence of subduction systems and by the occurrence of widespread extension in the Alps (termed ‘the Oligocene Lull’ by Laubscher (1983)) and in the western Mediterranean region. In the Early Oligocene, the Alpine orogen underwent a major orogenic episode, indicated by ~35 Ma ages of high-pressure and ultra-high-pressure rocks exposed in the Internal Crystalline Massifs of the western Alps (Gebauer 1996, Gebauer et al. 1997, Rubatto & Gebauer 1999). The present structural configuration of the Alpine sutures in western Alps and in northeast Corsica suggests that, prior to continental collision, the area had been controlled by a southeast-dipping subduction system (Figure 11). In the Late Oligocene, however, the polarity of the subduction system changed, and a new northwest-dipping subduction system developed in the southern margin of west Europe, producing calc-alkaline volcanism in Provence and Sardinia (Figure 11).

The initiation of a new northwest-dipping subduction system was possibly triggered by continental collision in the Alps at 35 Ma. This collision could block the existing subduction system by the arrival of thick crustal material at the subduction zone. Thus, a new subduction system developed in a more southerly location, where relatively old (Jurassic) oceanic lithosphere was found. Thus, at 30 Ma the subducting oceanic lithosphere was relatively old (>110 Ma) and cold enough to create a gravitational instability, which would cause rollback of the subduction hinge towards the SE. In addition to slab rollback, the motion of Africa relative to Europe has been considerably slow since 30 Ma, and particularly since 25-20 Ma (Jolivet & Faccenna 2000, Rosenbaum et al. in press). Thus, with the absence of sufficient convergence to support subduction rollback, extension commenced in the overriding plate, forming the foundations of the western Mediterranean basins.

Valencia Trough, Gulf of Lion and the Ligurian Sea

Earlier rifting is inferred from syn-rift Late Oligocene sediments deposited on Early Oligocene grabens and half grabens in the margins of Valencia Trough, the Gulf of Lion and the Ligurian Sea (Cherchi & Montadert 1982, Burrus 1989, Bartrina et al. 1992). Rifting probably commenced in the early Late Oligocene (~30 Ma) in the Gulf of Lion (Séranne 1999), and in the latest Oligocene (~25 Ma) in Valencia Trough (Roca et al. 1999). A right lateral strike-slip fault (North Balearic Transfer Zone) separated the Valencia Trough from the Gulf of Lion (Séranne 1999)(Figure 12). Structural observations from the extended margins suggest that horizontal extension was partitioned in different crustal levels, forming rift valleys in the upper crust (e.g., in Sardinia)(Cherchi & Montadert 1982), and low angle extensional detachments in deeper crustal levels (e.g., Corsica and Calabria)(Jolivet et al. 1990, Rosseti et al. 2001). Ductile extensional deformation in Corsica and Calabria has been dated at 32-25 Ma (Brunet et al. 2000, Rosseti et al. 2001), that is, before subduction rollback commenced.

Figure 12 Late Oligocene reconstruction (25 Ma).

As a result of subduction rollback, Extension in the Early Miocene led to the breakup and drifting of continental fragments formerly attached to southern France and Iberia. Thus, during the opening of the Ligurian Sea and the Valencia trough, the Balearic Islands, Corsica, Sardinia and Calabria were subjected to block rotations. Extension in Valencia Trough ceased in early Burdigalian (21-20 Ma) before it was sufficient to form oceanic crust (Bartrina et al. 1992, Watts & Torné1992). However, ongoing southward rollback of the subduction hinge led to the formation of a new rift system between the Balearic Islands and the Kabylies blocks, and further extension resulted in the formation of the Provençal Basin (Séranne 1999)(Figure 13). In the Gulf of Lion, tectonic activity ceased in Aquitanian/early Burdigalian (20-18 Ma) (Cherchi & Montadert 1982, Burrus 1989), possibly due to the collision of Corsica, Sardinia and Calabria with the Apennines (Figure 14). Following collision, Apennine units arrived at the subduction system and impeded rollback, which in turn, led to the cessation of back-arc extension in the Ligurian Sea.

Figure 13 Early Burdigalian reconstruction (21 Ma).