Introduction
The Alps and the Apennines are two mountain ranges in Italy that belong to the central-western Mediterranean region. For a long time, differences and transition between the two ranges, both at least partially facing to the Po Plain, have been discussed considering them as independent geological domains. On the contrary, they represent part of a continuous Alpine orogenic system derived from a complex space-time interaction between the two major European and African plates, and intervening oceanic domains and minor microcontinents. Past and present-day geological features of the Alps/Apennines junction area show structures and sedimentation history associated with complex interfering processes developed during the successive stages of growth of the different segments of the orogenic system related with subduction frames which changed and reversed during time.
The relationships between the Western Alps and the Northern Apennine represent, therefore, a classical and still debated problem in the geology of the Central Mediterranean which finds in the NW parts of the Italian peninsula a topical ground of discussion.
In particular for the junction area, the following general aspects are worth pointing out:
- presence of tectonic units with similar lithostratigraphic features and comparable structural evolution;
- at least in part coeval age of tectonic events and deformations;
- superficial continuities of tectonic units across the Western Alps/Northern Apennine boundary.
These features were partially derived from a paleogeographic heritage of the Alpine and Apenninic realms which were laterally continuous and shared the rifting and the drifting stages of the Ligurian Tethys (Elter 1975; Piccardo 1977; Laubscher and Bernoulli 1977; Stampfli et al. 1998; Manatschal and Bernoulli 1999; Rampone and Piccardo 2000; Lemoine et al. 2001; Piccardo this volume), the ocean which during the Late Mesozoic separated paleoEurope from the southern paleocontinent Adria/Apulia (the “Africa promontory” of Argand 1924). The diachronous closure of this ocean (Dal Piaz 1974; Hunziker and Martinotti, 1984; Laubscher, 1988; Dewey et al. 1989; Polino et al. 1990; Stampfli et al. 1998; Lemoine et al. 2001; Michard et al. 2002; Schmid et al. 1996, 2004; Beltrando et al., 2010 and references; Dal Piaz this volume) during Cretaceous to Eocene and the following Oligocene/Miocene Europe/Adria collision characterized the evolution of the Alps and the early stages of evolution of the Apennines, while the opening of the Provençal basin in the Oligocene and that of the Tyrrhenian sea from the Middle Miocene (in the wake of retreating Adria subduction) represent the key events of the Apenninic evolution (Elter et al. 1975; Laubscher 1971, 1988; 1991; Scandone 1979; Doglioni 1991; Rosenbaum and Lister, 2004; Schettino and Turco, 2006, Argnani, 2009).
The dynamic evolution of the orogenic system(s) and the geometric and kinematic interactions between the two chains are problems still under investigation.
Presently, two opposite contrasting interpretative models are under debate (for a more detailed overview see Molli, 2008 and Vignaroli et al. 2008):
1) According to some authors, the Alps and the Apennines are related to two coeval and opposite-dipping subduction (east-vergent or “alpine” and west-vergent or “apenninic”) active since Late Cretaceous. Major objections to this interpretation concern the structures and evolution of Alpine Corsica and the recognition of the boundary element(s) between the Alps and the Apennines;
2) Alternatively, diachronous east-vergent “alpine” subduction (Late Cretaceous-Middle Eocene) was followed by west-vergent apennine (Late Eocene-onward) subduction. Subduction flip and complex space and time interactions between only partially independent orogens characterize this group of models. The timing, way and causes for reversal of subduction represent major points of discussion.
The present models of the Alps/Apennines relationships derive from more than one century of research which will be briefly outlined hereafter to provide a historical perspective on the treated problem (see Gelati and Pasquarè, 1970; Castellarin, 1994; 2001 for a more complete historical presentation).
In the 19th century the topic was principally faced from a lithologic point of view highlighting the presence of mainly metamorphic rocks as characteristic of the Alps, in contrast to widespread exposures of sedimentary rock-types in the Apennines. In this frame the Sestri/Voltaggio line (east of Genoa) was soon recognized as the possible surface boundary between the two chains. With the recognition of nappe architecture for the Alps (Argand, 1924) some authors suggested also the identity of some structural elements in the two orogens (Pennidic domain of the Alps correlated with metamorphic units exposed in some tectonic windows in the Northern Apennine e.g. Staub, 1933). Subsequently, structural elements such as folds and orogen-scale vergences, together with large scale geometry and general lithostratigraphic features, were quoted as the main distinction between the two chains. In particular it was pointed out that western vergences are characteristic of the Alps, whereas the opposite i.e. eastern vergences can be observed in the Apennines.
The contributions of Elter et al. (1966); Laubscher (1971), Scholle (1970), Boccaletti et al. (1971), Haccard et al. (1972); Sturani (1973), Elter and Pertusati (1973), Dal Piaz (1974), Debelmas (1975), Alvarez et al. (1974) and Grandjacquet and Haccard (1977) were the first to analyze in modern terms the relationships between the western Alps and the Northern Apennine, laying the basis for present research on the subject.
In the 80’s, papers by Treves (1984) and Principi and Treves (1985), expanding on former propositions of Scholle (1970) and Scandone (1979), presented the interpretation of the Apennine as an accretionary wedge and suggested the presence of a wide east-west-striking, distributed zone of deformation north of the Voltri Massif. For the authors this domain accomodates the displacement between the opposite-dipping Alpine (i.e. eastward) and Apenninic (i.e. westward) subduction zones, drawing a direct comparison with the present convergent setting of New Zealand.
Laubscher (1988, 1991), Polino et al. (1992); Schumacher and Laubscher (1996); Polino et al. (1994); Piana (2000); Mosca et al. (2009) underlined the nature of the problem for the boundary between the Alps and the Apennines in its 3D-kinematics.
Doglioni (1991), Doglioni et al., (1998) and Carminati et al., (2004) make an overall comparison foregrounding the general distinct characters of the two chains in terms of both geological and geophysical expression and the different positions of basal décollement, very deep in the Alps and more shallow in the Apennines. These general features are related to the deep geometry of underlying subduction following or opposing the “eastward” undulated mantle flow (Doglioni et al., 1999). Within this general model they underlined the problem in terms of the time interference between the southern prolongation of the Alps and the Apennines, with incorporation of segments of the former in the inner side of the latter, although no detailed analyses or definition of the boundary problems were ever attempted.
The surface boundary between the Alps and Apennines has been traditionally placed along the Sestri Voltaggio Zone or the Sestri Voltaggio Line (Cortesogno et al., 1979; Cortesogno and Haccard, 1984; Hoogerdujn Strating, 1991). The Sestri Voltaggio Zone is a km-wide north-south oriented structural domain which includes different metamorphic tectonic units in contact with unmetamorphic units (historically ascribed to the Northern Apennine). Several interpretations for this structural domain have been proposed. The Sestri Voltaggio line was interpreted as a transform fault kinematic boundary between Alps and Apennine (Scholle, 1970; Elter and Pertusati, 1973; Sturani, 1973; Ten Haaf, 1975), as a thrust fault, which juxtaposes rocks from different crustal levels (Cortesogno and Haccard 1979, Cortesogno and Haccard, 1984; Capponi, 1991; Castellarin, 1994), as a dextral strike-slip fault (Giglia et al., 1996, Crispini 1996, Crispini and Capponi 2001) or transpressional oblique-slip fault (Spagnolo et al. 2007, Crispini et al. 2009) and extensional detachment (Hoogerdujin Strating, 1991; 1994; Vignaroli et al., 2008; 2009). Miletto and Polino (1992) considered the Sestri/Voltaggio zone a major backthrust zone of Alpine units above Apenninic ones. Crispini (1994); Crispini and Capponi (2001); Capponi and Crispini (2002); Crispini et al. (2009) describe the details of the tectonics of the zone and suggest an evolving structural significance from early nappe contacts to the more recent Oligo-Miocene age shallow crustal reactivations.
Sturani (1973) and Elter and Pertusati (1973) were the first to point out that whatever was the original significance of the Sestri-Voltaggio Zone, the sealing of the basal Tertiary Piemonte Basin (TPB) on the internal structures of the Sestri Voltaggio zone, the Ligurian Alps and parts of the Ligurian units implies a subordinate role of the Sestri-Voltaggio zone in the Alps/Apennines kinematics. They also indicated in the Villalvernia-Varzi-Ottone-Levanto line (Elter and Pertusati, 1973) the kinematic boundary between metamorphic and unmetamorphic units originally part of the pre-Late Eocene Alpine orogen (dominated by western vergences) and other Ligurian units with “only Apenninic eastward” history. They firstly introduced the idea of a complex interference between two evolving orogenic systems.
Relations between the Western Alps and the Northern Apennines are deeply related with another classical issue in Alpine geology, the origin of the tight curvature of the Western Alps, a crucial point for any kinematic restoration and paleogeographic reconstruction on the inner side of the Alpine belt (e.g. Gougel, 1963; Laubscher, 1971; Debelmas, 1986; Giglia et al., 1996; Schmid and Kissling, 2000). The main question, faced in different contributions (e.g. Argand, 1916; Gougel, 1963; Laubscher, 1971; Debelmas, 1972; Elter and Pertusati, 1973; Debelmas, 1986; Malavieille et al., 1984; Chouckroune et al., 1986; Ricou and Siddans, 1986; Laubscher, 1988; Lacassin, 1989; Vialon et al., 1989; Platt et al., 1989; Hoogerdujin Strating et al., 1991; Vanossi et al., 1994; Giglia et al., 1996; Schmid and Kissling, 2000; Ford et al., 2006) is whether the shape of the Western Alps arc simply derives from a torsion superimposed on a more linear chain, or it reflects an inherited pre-collisional physiography later tightned during the Alpine orogeny.