(this section was kindly contributed by Natale Calanchi, University of Bologna)
The Southern Tyrrhenian Sea, whose depth exceeds 4,000 meters b.s.l., is characterized by a complicated morphology (an abyssal plain surrounded by various orogenic systems). From a geodynamic point of view, in this area two tectonically contrasting zones coexist (Sartori, 1989): in fact, there are both extensional (e.g. the Vavilov and Marsili basins formed by thinned oceanic crust) and compressive zones (e.g. the Calabrian-Peloritan Arc). Geological and geophysical research, carried out during the past 15 years, confirmed the eastward migration of an extensional stress field from the Oligo-Miocene Sardinian margin to the Pleistocene Marsili basin (Ferrari & Manetti, 1993). The subduction-related volcanic activity showed the same eastward migration going from the Oligo-Miocene Sardinian Arc to the Pliocene Anchise-Ponza Arc and, at last, to the Pleistocene Aeolian Arc (Savelli, 1988).
Fig. 1: Sketch map of the Southern Tyrrhenian Sea showing the location of ocean-type basins and Pliocene to Recent (5-0 Ma) orogenic arcs: AE= Pleistocene Aeolian Arc; AP= Pliocene Anchise-Ponza Arc; IP= Ponza Island; MS= Marsili seamount; SA= Anchise seamount; SV= Vavilov seamount; SM= Magnaghi seamount (from Calanchi et al., 1996).
The orogenic volcanism of the Aeolian Arc initiated in the Pleistocene, showing a prevailing calc-alkaline to shoshonite affinity. In the same period the Perityrrhenian Campanian-Roman volcanism was active: this ultrapotassic volcanism shows important geochemical analogies with volcanism of the Aeolian Arc (Peccerillo, 1985; Ellam et al., 1988).
The Aeolian islands
The Aeolian Arc is a volcanic structure, about 200 km long, located on the internal margin of the Calabrian-Peloritan Arc. The Arc is formed by seven subaerial volcanic edifices (Alicudi, Filicudi, Salina, Lipari, Vulcano, Panarea and Stromboli) and by several volcanic seamounts which roughly surround the Marsili Basin (Beccaluva et al., 1985).
Structural trends and volcanic activity in the area are strongly controlled by the regional stress fields and bring to identify three distinct sectors:
- the western sector (Alicudi and Filicudi islands) dominated by NW-SE-oriented tectonic lineaments (Calanchi et al., 1995);
- the central sector (Salina, Lipari and Vulcano islands) aligned along the important regional transcurrent fault joining the Aeolian Islands to the Malta Escarpment with a NNW-SSE-oriented trend; on Salina and Lipari subordinate E-W-oriented trends are also present (Romagnoli et al., 1989);
- the eastern sector (Panarea and Stromboli islands) characterized by prevailing NE-SW-oriented tectonic lineaments (Gabbianelli et al., 1993).
Fig. 2: Bathymetric map of the Southern Tyrrhenian Sea: the location of submarine and emerged volcanic edifices forming the Aeolian volcanic arc is shown (from Sisifo seamount in the W to Palinuro seamount). (1) = emerged volcanic edifices; (2) = seamount (volcanic and not volcanic) (from Calanchi et al., 1996).
The oldest documented volcanic activity in the arc is represented by the age (1.3 Ma) of a dredged sample coming from the Sisifo seamount (in the submarine western portion of the arc); actually the only volcanoes which can be considered still active are Stromboli, Vulcano and Lipari (although eruptions occurred on Salina and Panarea during the very late Pleistocene, i.e. at <15 ka before present).
During the Late Quaternary Period (from 0.2 Ma to Recent) many fluctuations of the sea level, due to eustatic and/or volcano-tectonic movements, have affected the coeval subaerial portions of the volcanic complexes (Romagnoli, 1990): the presence of numerous marine conglomerate levels (up to 100 meters above sea level) along the coasts of several islands (and also below the present sea level, down to -100 meters) outlines the magnitude of these fluctuations.
The Aeolian volcanism is characterized by orogenic melts with an affinity ranging from calc-alkaline to HK-calc-alkaline, to shoshonite up to ultrapotassic (e.g. lc-tephrites from Vulcano and Stromboli); the early arc-tholeiites are very scarce and come from the submarine portion of the arc.
The peculiarity of the Aeolian volcanism is the space-time superimposition of products with different magmatic affinities along the whole volcanic arc (Beccaluva et al.,1985). For this structure the relationship h-K, typical of volcanic arcs, is not respected and "although among petrologists there is no agreements concerning the role of subduction in generating the heterogeneous mantle source ..... the occurrence of subduction cannot be denied" (Ferrari & Manetti, 1993). Thus, for many authors the origin of the Aeolian volcanism is related to the anomalous and specific model of "passive subduction" of a lithospheric slab (Barberi et al., 1973; Malinverno & Ryan, 1986; Patacca & Scandone, 1989) dipping along a WNW-oriented Benioff plane (representing the remains of the Oligo-Miocene subduction; Scandone, 1979; 1982). The quasi-vertical immersion of the subducted slab beneath the islands would explain the production of melts with different magmatic affinities at different depths (and at specific P-T intervals) and their emplacement in the same point rising along almost coincident volcanic conduits. The anomalously high K content of these products, in comparison with those characteristically occurring in normal island arc suites, may be explained by a mantle enriched in hygromagmatophile elements and affected by partial melting as a source for these melts (Peccerillo, 1985; Ellam et al., 1988).
Subduction versus oblique rifting
Other recent hypotheses about the tectonic setting of Eolian volcanism state that subduction below the archipelago has completely stopped, the tectonics now being dominated by oblique rifting related to right-lateral extensional strike-slip faulting and movement along normal faults and fractures (Mazzuoli et al. 1995). (More soon)