THE EPISODES OF OCEANIC OPENING AND THEIR SPACE-TIME RELATIONS WITH POST-OROGENIC EXTENSION AND CALC-ALKALINE VOLCANISM OF THE WEST-MEDITERRANEAN-TYRRHENIAN REGION

SE-directed flat-slab subduction of European plate and eventual collision with Africa, ended at Oligocene start, produced the submerged orogen which saw the West-Mediterranean-Tyrrhenian oceanic opening sequence which is generally considered above WNW-directed steep-slab subduction of Africa plate. Post-orogenic continental extension and calc-alkaline (CA) volcanism of silicic to intermediate nature initiated in the Oligocene (c. 33/32 Ma ago). Extension reached the stage of oceanic spreading between 20 and 16 Ma (Burdigalian) in two basins of the west Mediterranean. The onshore rims of the deep basins of Sardinia-Provence (European plate) and north Algeria (the western part of submerged orogen) have been affected by volcanism showing Burdigalian age and CA, silicic to mafic composition.

In the eastern part of submerged orogen, the supra-subduction Tyrrhenian basin opening, on the contrary, has occurred when volcanism was present only in the bathyal plains of Vavilov (c. 7 Ma) and Marsili (c. 1.8 Ma). Marsili deep-seated spreading preceded the voluminous CA volcanism extending from the Aeolian islands and Marsili axial seamount to the “Roman Magmatic Province” (< 1 Ma).

The contrast between lack and occurrence of peri-bathyal CA volcanism, at various times of oceanic spreading, would indicate different tectonic setting. The commencement of WNW-directed steep subduction under the Tyrrhenian orogen may have occurred at c. 16/15 Ma (the waning of Burdigalian oceanic spreading). If so, Oligo-Burdigalian continental extension, oceanic spreading and CA volcanism took place in the absence of subduction, recalling the Basin and Range continental extension and CA volcanism following the Laramide orogeny. The west-Mediterranean- Tyrrhenian region may have had important geological connections with the Alps. The tectonic mode of orogenic accretion, initiated in the Alps in Cretaceous time, has been temporarily supplanted by extension accompanied by CA magmatism in the Oligo-early Miocene. The mid-Miocene resumption of orogenic accretion and waning of magmatism in the Alps may have been coeval with the start of west Mediterranean oceanic opening while crustal thinning continued in the Tyrrhenian.

The space-time study indicates that calc-alkaline volcanism is not linked only to subduction.