Figure 10. Computer animation
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.
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.
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.
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.
During the Early-Middle Miocene, intense tectonic activity took place in North Africa due to the opening of the Provençal, Algerian and Alboran basins, and the subsequent emplacement of the Kabylies block and the Internal Rif onto the African margin. Extension in the Provençal Basin commenced after rifting in Valencia Trough had failed. In early Burdigalian (~21 Ma), continental breakup occurred between the Balearic Islands and the Kabylies blocks, and a new basin developed. Extension was probably governed by a rapid southward rollback and subsequently led to sea floor spreading and formation of a new oceanic crust in the Algerian-Provençal Basin (Rehault et al. 1985)(Figure 13 & Figure 14).
Evidence for extensional fabrics associated with the opening of the Algerian-Provençal Basin are found in the Kabylies metamorphic core complexes, which are now accreted to the African margin. These complexes are characterised by the occurrence of low-angle extensional detachments, which juxtaposed upper crustal rocks on top of high-grade metamorphic rocks (Caby & Hammor 1992, Tricart et al. 1994, Saadallah & Caby 1996). The occurrence of core complexes implies that a considerable amount of crustal thinning was made possible by non-coaxial shearing along extensional detachments. Based on 40Ar/39Ar dating, it has been suggested that extensional deformation occurred at 25-16 Ma (Monié et al. 1984, 1988, 1992).
The Kabylies blocks drifted southward in response to the southward rollback of the subduction zone until they collided and accreted to the African margin (Cohen 1980, Tricart et al. 1994)(Figure 15). Collision occurred when the Mesozoic oceanic lithosphere, which had previously separated the Kabylies from the African margin, was totally consumed by subduction. The collision occurred between 18-15 Ma, based on the cessation of extensional tectonics in the Algerian-Provençal Basin and the Kabylies core complexes, and the commencement of thrusting in the External Maghrebides (Frizon de Lamotte et al. 2000). The accretion of the Kabylies block and the final consumption of old oceanic lithosphere in this region permanently terminated southward subduction rollback. South-directed thrust systems propagated southward after accretion (Frizon de Lamotte et al. 2000), but subduction processes were impeded and eventually halted with the presence of the relatively buoyant continental crust at the subduction zone. The Middle Miocene cessation of subduction processes in North Africa resulted in the segmentation of the western Mediterranean subduction system, with an eastward dipping subduction in the Alboran region, and a westward dipping subduction in the Tyrrhenian region (Lonergan & White 1997)(Figure 15).
The tectonic evolution of the Alboran Sea is a matter of controversy, and several different models have been proposed. This reconstruction follows Royden (1993a) and Lonergan and White (1997), who suggested a slab rollback model for the opening of the Alboran Sea. Alternatively, it has been suggested that extension in the Alboran Sea developed as a result or an extensional collapse of a thickened continental crust and its underlying lithospheric mantle (e.g. Platt & Vissers 1989, Housman 1996). These models are not entirely supported by field observations from the Rif-Betic cordillera, which imply episodic alterations from crustal shortening to extension (Azañón et al. 1997, Balanya et al. 1997, Martínez-Martínez & Azañón, 2002). Neither are such models supported by the block rotations inferred from palaeomagnetic data.
According to our model, the origin of the Internal Zone of the Rif-Betic cordillera is similar to the origin of Corsica, Sardinia, Calabria and the Kabylies (Figure 11). Thus, the Rif-Betic in its Oligocene position formed a continuous orogenic belt together with the Kabylies, Calabria, Corsica and the western Alps that underwent high-pressure metamorphism during Alpine orogeny (Figure 11). As mentioned before, these terranes (excluding the western Alps) were part of an overriding continental slab above a northwest dipping subduction zone, which started to retreat oceanward during the Oligocene. Once rollback of the subduction hinge commenced, rocks of the Rif-Betic Internal Zone were subjected to an extensional regime leading to the formation of metamorphic core complexes and exhumation of high-pressure metamorphic rocks below extensional detachments (Platt et al. 1983, Azañón et al. 1997, Balanya et al. 1997).
Westward rollback of the subduction hinge in the westernmost Mediterranean was probably triggered by an original curvature of the subduction zone in its western terminus. Rollback, therefore, led to southwestward migration of the subduction hinge accompanied by southwestward drifting of the extended continental fragments of the overriding plate. Southerly migration, however, could not proceed after the subduction zone reached the passive margin of Africa (see previous section). Rollback continued only in areas where the existence of oceanic lithosphere still permitted oceanward retreat of the subducting lithosphere (Figure 15). Thus, in the Middle Miocene (16-15 Ma), the western east-dipping segment of the subduction zone rolled back in the direction of the oceanic lithosphere.
The formation of the Alboran Sea occurred during the westward migration of the subduction hinge. Rapid rollback was compensated by wholesale extension in the overriding continental crust, which was thinned to ~15 km between 23-10 Ma (Lonergan & White 1997). Contemporaneously, fragments of continental crust were thrust onto the passive margin of Africa and Iberia (the External Zone), forming rotation patterns consistent with oblique thrusting derived by the westward rollback of the subduction zone. Final accretion of the Rif-Betic Cordillera occurred at ~10 Ma, when the subduction zone rolled back as far as Gibraltar (Figure 16). Subduction rollback then ceased, together with the cessation back-arc extension in the Alboran Sea (Lonergan & White 1997).
The Tyrrhenian Sea is the youngest basin in the western Mediterranean, forming since the Tortonian (~9 Ma). It was opened, according to this reconstruction, as a result of a southeastward rollback of subduction systems near the margins of the Adriatic plate (Malinverno & Ryan 1986).
We suggest that the collision of Corsica and Sardinia with the Apennines at ~18 Ma led to a relative quiescence in back-arc extension between 18-10 Ma (Figure 14-Figure 16). During this period, continental crust of Apennine units incorporated in the subduction zone, and impeded further eastward subduction rollback. Thus, considerable crustal shortening occurred in the Apennines accompanied by thrust systems that propagated eastward.
The opening of the Tyrrhenian Sea occurred in two stages: an early (9-5 Ma) opening of the northern Tyrrhenian Sea (Figure 17), and a late (5-0 Ma) opening of the southern Tyrrhenian Sea (Mantovani et al. 1996)(Figure 18). It was accompanied by coeval crustal shortening in the Apennines (Malinverno & Ryan 1986) and counterclockwise rotations of nappe stacks. The reason for the opening of the northern Tyrrhenian is not entirely clear. It may be associated with subduction of oceanic crust located between the Apennine belt and the Adriatic foreland, which promoted lithospheric gravitational instability during Late Miocene, and further eastward subduction rollback. It is possible that deep-sea sediments of the Lagonegro and Molise formations are remnants of these intra-Adriatic basins (Sengör 1993). At this stage, the subduction zone was oriented ~N-S, that is, roughly parallel to the direction of convergence. Therefore, the rate of convergence at the trench was very low, and consumption of oceanic lithosphere was mostly driven by the negative buoyancy of the subducting slab (Faccenna et al. 2001).
During the latest Miocene or the Early Pliocene (5 Ma) extension ceased in the northern Tyrrhenian Sea and migrated southward to the southern Tyrrhenian Sea (Figure 18). This stage was characterised by considerable extension that culminated during the Pliocene-Pleistocene, when new oceanic crust formed. Contemporaneously, crustal shortening occurred in the Southern Apennines and Sicily accompanied by counterclockwise block rotations in the former and clockwise rotations in the latter. These processes have been controlled by rapid rollback of oceanic Ionian lithosphere beneath the Calabrian arc.