Reconstruction (continued)

Tyrrhenian Sea

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).

Figure 17 Messinian reconstruction (6 Ma).

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.

Figure 18 Late Pliocene reconstruction (2 Ma).

 

Discussion

The reconstruction of the western Mediterranean since the Oligocene emphasises the role of subduction rollback in convergent plate margins. It shows that a widespread extension took place in the convergent interface separating Africa and Europe, forming marine back-arc basins and new spreading centres. Subduction rollback was probably controlled by the gravitational instability produced during subduction of cold and dense oceanic lithosphere. Back arc extension was likely to occur when the velocity of the slab retreat overcame the absolute motion of the overriding plate (Molnar & Atwater 1978, Dewey 1980, Royden 1993b, Lonergan & White 1997). In the evolution of the western Mediterranean these processes have played a fundamental role since the Oligocene. Subduction rollback was made possible by the consumption of Mesozoic oceanic lithosphere at the subduction zone. This oceanic lithosphere was probably old and cold enough to be gravitationally unstable relative to the surrounding asthenosphere. The subduction zone has therefore progressively retreated from its Oligocene position near the southern margin of Europe to its final configuration in the Calabrian arc, North Africa and the Alboran arc. In the western Mediterranean, subduction rollback occurred during a period of relatively slow convergence between Africa and Europe (Jolivet & Faccenna 2000, Rosenbaum et al. in press). As convergence alone could not compensate the vacant area formed by subduction retreat, back-arc extension occurred in the overriding plate. Thus, a period of relatively slow convergence was actually characterised by large-scale horizontal motions of smaller microplates and allochthonous terranes.

The dispersion of continental terranes, which drifted and rotated during subduction rollback, is clearly seen in our reconstruction. In the Alpine orogen, this process led to the fragmentation of a continuous belt into continental terranes, which in turn, collided with the passive margins of the surrounding continents. We stress that this mechanism may have profound tectonic implications on the way orogens work. Orogens may be subjected to switches from crustal shortening and extension, controlled by the processes of subduction rollback, rifting in the back-arc region and the subsequent accretion of allochthonous terranes onto adjacent passive margins. Thus orogenesis cannot be oversimplified to subduction followed by collision of two continental plates, but includes accretion of numerous continental terranes. Following collisional events, reorganisation of the plate boundaries occurs, associated with termination, jumping or segmentation of subduction zones. A Similar style of orogenesis has been proposed by Nur and Ben Avraham (1982) based on numerous examples of allochthonous terranes throughout the Circum-Pacific (excluding the Andes) and the Alpine Himalayan belts. These authors have suggested that continental slivers and microcontinents could actually migrate great distances before colliding with the continents.

In summary, the style of tectonism suggests that the role of horizontal motions during orogenesis cannot be disparaged. Fragments of continental crust were possibly subjected to large amount of horizontal transportation, block rotations on vertical axes, and episodic alterations from crustal shortening to extension.

Concluding remarks

Extension in the western Mediterranean commenced at 32-30 Ma and was primarily controlled by subduction rollback. The rapid rollback of the subduction hinge was accompanied by a relatively slow convergence between Africa and Europe. Therefore, convergence could not support the rate of subduction rollback, and extension occurred on the overriding plate.
During back-arc extension, marine basins progressively formed from north to south, floored either by thinned continental crust or new oceanic crust. The earliest basins began to form in Late Oligocene in the Gulf of Lion, the Ligurian Sea and Valencia Trough. In Early Miocene, back-arc extension propagated to Provençal, Algerian and Alboran basins, and in the Upper Miocene, extension in the Tyrrhenian Sea commenced.

Rifting led to breakup of continental terranes, which drifted and rotated as long as the subduction zone continued to rollback. Subduction rollback temporally or permanently ceased when continental crust arrived at the subduction zone, impeding subduction processes. The continental terranes have then been accreted to the continents and considerable crustal shortening occurred.