Alternative geodynamic models for the Apennines: a discussion

As discussed, the coupled Oligocene to Present extensional-compressional wave in the Apennines-Provençal-Tyrrhenian system and the associated magmatism is related by a large number of authors (e.g., Scandone, 1979; Dercourt et al., 1986; Malinverno and Ryan, 1986; Beccaluva et al., 1989; Doglioni et al., 1997; Carminati et al., 1998; Gueguen et al., 1998; Faccenna et al., 2001) to the radial retreat of a NW-directed subduction of oceanic lithosphere beneath the European southern paleo-margin. However the same geological and igneous petrological observations were ascribed alternatively to different hypotheses such as: a) mantle upwelling (called “asthenolith” or “mantle megadome” in the literature) impinging the south-eastern European lithosphere around Late Oligocene (e.g., Wezel, 1982; Burrus, 1984; Doblas and Oyarzun, 1990; Lavecchia and Creati, 2006; Centamore and Rossi, 2008), migrating in W-E direction from the Alboran to the Aegean basin from Oligocene to the Present (Morelli, 1998); b) lateral expulsion of crustal wedges (e.g., Carmignani et al., 1994, Assorgia et al., 1995; Pasci, 1997; Mantovani et al., 2002); c) delamination of a lower crustal and lithospheric mantle slab, whose tear migrates through time to the NE (e.g., Channell and Mareschal, 1989). In some cases the geochemical characteristics of the Apennines Plio-Quaternary magmatism have been considered unrelated to any type of subduction processes (e.g., Scalera, 2006), but rather generated by an alternatively hot (e.g., Bell et al., 2006) or “wet” (with “normal” temperature; Lavecchia and Creati, 2006) uprising mantle. In particular, Bell et al. (2006) propose the existence of a large mantle plume, with a diameter of at least 1000 km from Sardinia to the NW Alps and peninsular Italy. The existence of a large range of chemical and isotopic compositions among the Plio-Quaternary Italian igneous rocks is simply related to a chemically and isotopically heterogeneous alleged mantle plume. Lavecchia and Creati (2006) state that “the Apennines and the Maghrebides are not related to subduction, but represent a recent example of «plume-induced orogenesis»”. In their view, the seismically active Benioff-Wadati plane beneath the Calabro-Peloritani arc would represent the strongly deformed (originally SE-dipping) Alpine slab (Lavecchia and Creati, 2006). Also Locardi and Nicolich (2005) proposed the existence of an eastward migrating deep mantle plume (called asthenolith) to explain the opening of the Ligurian-Provençal and Tyrrenian basins, their reduced lithospheric thickness, the rotation and collision of several microplates in the Western Mediterranean and the igneous activity. All these papers are essentially based on the original model proposed by Wezel (1982). In other cases a mix model, requiring the existence of both subduction processes and a mantle plume has been proposed in literature. Gasperini et al. (2002) and Cadoux et al. (2007) explained the geochemistry of the peninsular Italy rocks with the involvement of a mantle plume uprising from deep mantle just in a hole of the subducting Apennines slab.

Also the igneous rocks of Sicily or Sicily Channel have been related to the existence of active or fossil deep mantle plumes. On the basis of detailed global seismic tomographic considerations, Montelli et al. (2004) suggested the presence of a deep mantle plume beneath Mt. Etna, similarly to what proposed by Hoernle et al. (1995) at a much larger scale, including the entire Mediterranean Sea and neighbouring continental areas. On the basis of major and trace element and isotopic constraints, Civetta et al. (1998) proposed that the magmatism of Pantelleria, as well as that of Mt. Etna, is related to the activity of a mantle plume. Rotolo et al. (2006) instead proposed the presence of a fossil plume head beneath the Sicily Channel, Mt. Etna, the Hyblean Plateau, Ustica Island and the westernmost island of the Aeolian Archipelago (Alicudi). Their model is based on the relatively homogeneous EAR-like (EAR = European Astenospheric Reservoir; Granet et al., 1995) major and trace element and Sr-Nd-Pb isotopic characteristics of the magmatic rocks.

We believe that there is no necessity to relate the igneous activity in these areas to the existence of active or fossil upraise of solid mantle from very deep. These are often unsupported by geophysical, thermal, sedimentological and geological data. Also geochemistry (commonly considered the strongest evidence for the existence of such type of processes) can be explained in different ways, without invoking such large-scale events (Lustrino and Wilson, 2007, and references therein). More detailed considerations on alternative hypotheses proposed to reconcile the chemical-isotopic composition of Plio-Quaternary igneous rocks of Italy with geodynamics are fully approached in Lustrino et al. (2011).