Conclusion and open problems
The present-day morphostructural domains of the Western Alps/Northern Apennine junction area result from a kinematically complex interaction between interfering orogenic systems active since the Oligocene and related to the opposite-dipping east-southeast “alpine” and west-northwest “apenninic” subductions.
Within parts of the present morphostructural domains, however, reworked and reactivated structures of a formerly continuous Late Cretaceous/mid-Eocene intraoceanic and continental subduction-related “alpine” wedge are preserved and incorporated within the younger “apenninic” system.
From north to south in the junction area between Western Alps and Northern Apennine the following domain can be recognized:
- Domain of Cottian-Maritime Alps represents the southwesternmost segment of the Western Alps it mantains the complete alpine signature from surface to deep crustal levels. Although the deep structure of the southernmost part of this domain is still uncompletely known due to the lack of seismic data of other alpine segments (ECORS CROP and NFP-20 in particular Roure et al., 1990; Schmid and Kissling, 2000; Lardeaux et al., 2006) major differences in the overall architecture can be observed if compared with other western Alps transects (e.g. Schmid et al., 2004). This can be primarily the result of the Apenninic kinematic interference;
- Domain of Ligurian Alps/Tertiary Piemonte Basin/Ligurian units p.p. This crustal segment records the complex interactions in space and time between two evolving and interfering Alps/Apennines system. The domain includes a segment of the axial Alpine system, the Ligurian Alps, recording a peculiar syn to late-to collisional evolution characterized by a very fast exhumation occured as soon after the inception of continental subduction recorded in the Brianconnais/European crust. This process occured in a span time between 50 Ma - age of the early involvement of continental subduction in Corsica southernmost prolongation of the alpine system (Doglioni, 1991; Molli, 2008, Molli and Malavieille, 2010; Doglioni et al. this volume) - and the 35 Ma age of involvement of thinned continental crust of the Dora Maira unit in the CMA (Ford et al., 2009; Beltrando et al., 2010; Dal Piaz this volume).
The fast exhumation was possibly developed and enhanced within a transpressive zone which may have laterally constricted the subduction channel (Federico et al. 2007, Crispini et al., 2009) and thus accelerated extrusion similarly to what occured some My later for the Dora Maira (Ford et al., 2006). As alternative or concurrent mechanism for the fast exhumation, a northward propagation of the slab detachment could be envisaged (Molli, 2008; Molli and Malavieille, 2009; Molli and Malavieille, 2010).
Figure 10. Conceptual 3D reconstruction of the Western Alps-Northern Apennine junction area at the latest Oligocene-earliest Miocene.
At that time, two opposite dipping subduction system were active after Eocene subduction reversal affecting the southern (Corsica-Liguria) segment of the Alpine system. In deep blue, axial Alps and their southern prolongation in Corsica; in deep green, uppermost units of the alpine nappe stack, including the Chenaillet unit (Ch), the Helminthoid Flysch units of Swiss Prealps, Embrunais-Ubaye and Ligurian Alps, the Antola unit, the Balagne (Ba), Nebbio and Macinaggio units in Corsica; in light green, Ligurian and sub-Ligurian accretionary wedge of Apennines.
Thrust-sheet-top basins (B, Barrême; D, Dévoluy) and foreland basins on European crust according to Ford and Lickorish (2004); basins on Adria crust and atop the axial belt (E, Epiligurian basins; TPB, Tertiary Piedmont Basin) according to Garzanti and Malusà (2008). Amount of shortening and lateral slip during Eocene – early Miocene as summarized by Malusà et al. (2009) and reference therein. Southern face of the 3D model inspired by Doglioni et al. (1998) (CROP M-12A, CROP-03, CROP M-16). Major faults: AR, Aosta-Ranzola; BF, Briançonnais; FPF, Frontal Pennine; IF, Insubric; IHF, Internal Houiller; OS, Ostriconi; RF, Rio Freddo; SF, Simplon; ST, Stura; SV, Sestri-Voltaggio.
Beside paleomagnetic data (e.g. Collombet et al., 2002; Maffione et al., 2008), the interconnection between the Southalpine part of the Adriatic foredeep, filled by the Gonfolite clastic wedge, and the Apenninic part filled by the Macigno-Modino clastic wedge (Garzanti and Malusà, 2008) is a strong argument supporting the Neogene rotation of the whole Tertiary Piedmont – Ligurian Alps block.
The axial domain of the LA were later on rapidly buried under shallow to deep-water sediments as result of the inception of the opposite west-ward Apenninic subduction starting to the south (Doglioni, 1994; Molli, 2008; Malusà et al., 2008; Molli and Malavieille, 2010). A partial reshaping of the deep structure with the presence of the Ligurian Moho testified the “Apenninic” reworking of this crustal sector;
- Domain of the NA shows accretionary wedge units (the Ligurian/sub Ligurian nappe stack) overlying continental derived basement (Alpi Apuane) but mainly detached cover units Adria-derived. The nappe stack in the westernmost part of the belt was strongly excised during Mid-Late Miocene sin-contractional exhumation of the deeper wedge units related with east-ward retreating of Apenninic subduction and later on (from Pliocene to present ) further thinned by crustal-scale extension (Molli, 2008 and references).
The deep configuration of the major domains forming the Western Alps/Northern Apennine junction area can be therefore considered as the result of the complex interference of different orogenic processes such as the late collisional indentation of Europe and Adria, the development of structures related with opposite dipping subduction after a subduction flip and the kinematic development of the northern Apennines in a frame of slab retreat with the related opening of the backarc Liguro-Provençal basin and then the Tyrrhenian sea in the wake of Apenninic subduction.
The different structures formed during this complex geodynamic evolution are still far to be completely well understood and between others two major still open problems are hereafter highlight:
i) The problem of the rotation pole/pole(s)
One kinematic problem related to the Western Alps/Northern Apennine junction area concerns the back rotation of the LA and TPB basin after the preliminary retrodeformation of structures developed since the end of rotation c.16 Ma onward.
The kinematic balancing has to address the question of location of a single and fixed pole or the successive poles and the way in which they migrated in space and time. Moreover, the whole rotation model vs. differential block rotation or domino-like accomodation are different kinematic solutions with obvious major implications for the retro-deformation of the structures.
To unravel this problem an accurate 3D-balancing (Schumacher and Laubscher, 1996; Piana and Polino,1995; Mosca et al., 2009) of the major structures has to be achieved by combined structural, sedimentological and paleomagnetic data altogether with the new acquisition of high resolution 3D-seismic profiles.
ii) The role of the Pyreneean-Provençal deformation and its heritage
The geology of the junction area and the relationhips between Alps and Apennines have been described in this paper as a kinematic problem of the structural heritage of a former single and continuous orogenic system (the pre-Late Eocene Alps) partially reworked during the development of a younger “Apenninic” orogenic system (from the Late Eocene onward). However, another important element plays a possibly relevant, although almost unknown role, that is the eastern prolongation of the North Pyrenean fault and the development of the Pyrenean orogen itself (Malavieille, 1983; Lacombe and Jolivet, 2005). These structures were active and formed during Eocene times in response to the displacement and collision between the Iberia and the European plate (Lagabrielle and Boudinier, 2008 and ref.). The Pyreneen orogen and related structures found their prolongation on land with the Languedoc-Provençe belt, undersea in the Gulf of Lyon, and further east north of Corsica where one might expect an interference with the Late Eocene Alpine system (Fig.10). Complex structures are to be expected in such an area now in part reworked in western Liguria, a point still demanding focussed analyses.
In conclusion, although highly debated for more that one century the subject of the relationships between the Alps and the Apennines is a still uncompletely solved problem. The Western Alps/Northern Apennine junction area, in particular, hides and preserves most of the solutions of this 3-D kinematic problem therefore it might be considered a sort of type area for studying the geological processes occurring at interfering orogens, thus challenging present and future research.