Anatomy of the Ligure-Piemontese Subduction System: Evidence from Late Cretaceous-Middle Eocene Convergent Margin Deposits in the Northern Apennines, Italy

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Anatomy of the Ligure-Piemontese subduction system: Evidence from Late Cretaceous-middle Eocene convergent margin deposits in the Northern Apennines, Italy

Article in International Geology Review · October 2010 DOI: 10.1080/00206810903545493

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TIGR0020-68141938-2839InternationalAnatomy Geology Review,Review Vol. 1, No. 1, December 2009: pp.of 0–0 the Ligure-Piemontese subduction system: evidence from Late Cretaceous–middle Eocene convergent margin deposits in the Northern Apennines, Italy

InternationalM. Marroni et Geology al. Review Michele Marronia,b*, Francesca Meneghinia and Luca Pandolfia,b

aDipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy; bIstituto di Geoscienze e Georisorse, CNR, Pisa, Italy (Accepted 7 December 2009)

In the Northern Apennines, in contrast to the Western Alps and Alpine Corsica, upper structural levels of the Late Cretaceous–middle Eocene subduction complex are still preserved and well exposed. This subduction complex developed in the Ligure- Piemontese basin since the Late Cretaceous time as a consequence of convergence between the Eurasia and Adria plates. Representative successions of this ancient subduction complex are well preserved in the Ligurian units of the Northern Apen- nines, where turbidite and mass-gravity deposits showing pristine stratigraphic features are present. Three main domains, represented by different groups of tectonic units, can be identified, each delineating a different domain of the subduction zone. In this article, we first present a brief history of geological research in the Northern Apennines during the last half of the twentieth century and then a comprehensive picture of the stratigraphy and tectonics of the Ligurian units. A new interpretation of the related tectonostratigraphic units is proposed within the conceptual modern geodynamic framework of convergent margins. Keywords: sedimentation; tectonics; subduction; accretionary prism; ophiolites; turbidites; Late Cretaceous–middle Eocene; Northern Apennines, Italy

Introduction The Apennines and the Western Alps, making up the structural framework of Italy, and the Alpine sector of the island of Corsica, are all complexes classically regarded as

Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 constructed during a long-lived geodynamic history, developed from the Late Cretaceous– middle Eocene closure of the Jurassic Ligure-Piemontese oceanic basin up to the late Eocene–Miocene continental collision. In the Apennine belt, different from the Western Alps and the Alpine Corsica, the superstructure of this ancient subduction complex is still intact. This uniquely qualifies the Apennines belt as a suitable complex to investigate complete sections of the upper part of the Late Cretaceous–middle Eocene subduction complex; the latter developed since the Late Cretaceous, as a consequence of plate convergence in the Ligure-Piemontese basin. These successions are well preserved in the Ligurian units of the Northern Apennines, where turbidite and mass-gravity deposits showing pristine stratigraphic features can be studied. The sedimentary successions of the Ligurian units have been intensively investigated in the past, and some of the concepts of modern

*Corresponding author. Email: [email protected]

sedimentary geology were developed and/or have been elaborated from those studies. For example, field observations that led to the definition of the turbidite concept were performed also in the Ligurian successions (Migliorini 1933). More recently, the concept of olistostromes as a precursor of an advancing nappe (Elter and Trevisan 1973) was defined based on the evidence from the Ligurian units. Therefore, Ligurian units of the Northern Apennines represent a natural laboratory where the shallow-to-medium structural levels of an accretionary prism can be studied and reconstructed. In this article, we present a historical review of the research performed during the last half of the twentieth century as well as an update of the data available for the Ligurian units. Finally, we propose new interpretations of these related deposits based on modern concepts regarding convergent margin tectonics.

Historical picture of the studies on the Ligurian units during the last half of the twentieth century The Northern Apennines and the Western Alps are both collisional belts representing the Alpine sutures between the Europe and Adria plates during the Late Cretaceous to Tertiary convergence (Figure 1). Despite their development in the same geodynamic setting, and an apparent structural continuity visible in the tectonic maps (i.e. Structural Model of Italy, CNR-Progetto Finalizzato Geodinamica 1992), these two collisional belts show striking differences. For instance, although the Western Alps expose the deep structural levels, the Northern Apennines features a tectonic history characterized by a low rate of exhumation. As a consequence, the highest structural levels of the Northern Apennine belt, that is, the Ligurian units, are still well preserved, as testified by the widespread outcrops of very low-grade and unmetamorphosed sedimentary successions, mainly in the Ligurian–Emilian Apennines. The well-exposed sedimentary sequences as well as the great range of deposits represented the objectives of geological studies since the end of the nineteenth century. However, the first modern geological studies, with application of the concepts of alloch- tonous nappes, can be referred to the middle of the twentieth century, when Elter (1960), Elter et al. (1961), and Giannini et al. (1962) described the Northern Apennines as an orogenic belt consisting of an imbricate stack of tectonic units. At the top of this stack, these authors identified a group of tectonic units, i.e. the Ligurian units, belonging to a palaeogeographic domain, i.e. the Ligurian domain, located W of the present-day Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 Apennine belt. Even though the allochtony of the Ligurian units was previously proposed, for instance, by Tillmann (1926), Zaccagna (1932), Rovereto (1939), and Teichmüller (1932), it was only from the 1960s that the idea of the Northern Apennines’ main structure as an imbricate stack of tectonic units was accepted by the whole scientific community. This idea represented a strong impulse for new research in the Northern Apennines. From the 1960s up to the 1970s, numerous articles with significant data about the age, stratigraphy, and tectonics of the sedimentary successions from the Ligurian units, including also the first characterization of the mélanges (the so-called Complessi di base) as sedimentary deposits in most of the successions from the Ligurian unit, were published. The state of the art for the research of that period is well outlined in the special issue of Sedi- mentary Geology edited by Sestini (1970). On the basis of these new data, Elter et al. (1966) and Baldacci et al. (1967) proposed a complete reconstruction of the Cretaceous palaeogeography of the Ligurian domain. In this reconstruction, two different areas were first outlined: the internal basin, characterized by the Late Cretaceous Monte Gottero Sandstone and Val Lavagna Shale, and the external International Geology Review 3 Schistés Lustrées Schistés sement, including ica. The boxed area eanic crust; light grey: Alpine South Alpine ba South indicated. COPH, Dark grey: 2004). post 20 Ma oc ltri Group Piemontese units; SAB, Group Piemontese ltri et al. rsica. The location of the study areas is location of rsica. The fied from Carminati SZ, Sesia zone; TM, Tenda Massif; VBC, Variscan Basement of Cors hern Apennines and Co hern Apennines units; OPH, Sestri Voltaggio Vo units; Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 terranean area (modi terranean odified from Marroni and Pandolfi 2007). from Marroni odified South Alpine sedimentary cover; ian units; IL, internal Ligurian IL, internal ian units; of Corsica; EL, external Ligur of Corsica; EL, external zones; SAS, Ivrea and Canavese orogenic belt. (B) Tectonic sketch map of the Western Alps, Nort Tectonic (B) belt. orogenic Figure 1.Figure the Medi of (A) Geodynamic sketch indicates the of Figure location 7 (m 4 M. Marroni et al.

Figure 2. Proposed reconstruction of the Cretaceous palaeogeography of the Ligurian domain following Elter et al. (1966).

basin, characterized by Late Cretaceous–early Tertiary carbonate turbidites, reported as Helminthoid Flysch. The internal and external basins were separated by a ridge, known as Ruga del Bracco, consisting of deformed and uplifted ophiolites (Figure 2). In the same time span, the analysis of the relationships between tectonics and sedimentation produced some notable definitions as chaotic complex (Abbate et al. 1970) or precursory olistostromes (Elter and Trevisan 1973). The first term indicates a sedimentary and/or tectonic mixture of rock bodies of different size, age, and lithology set in a shaly matrix, generally associated with normal bedded deposits. The second one is referred to submarine landslides derived from the front of an advancing nappe and sedimented within the foredeep deposits. With the diffusion of the plate tectonic theory, the geological evolution of the Northern Apennines was reinterpreted. The first article placing the Ligurian domain in the frame of plate tectonic theory was provided by Boccaletti et al. (1971) and represented a true mile- stone for the history of the geological studies in the Northern Apennines. This is the first article where a subduction is proposed to explain the stratigraphic and tectonic features of the Ligurian units from the Northern Apennines. In the reconstruction proposed by Boccaletti et al. (1971), the internal and external Ligure-Piemontese basins, both characterized by a Jurassic oceanic crust, are regarded as domains belonging to different geodynamic Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 settings, as subsequently suggested also by Baldacci et al. (1972). The internal domain was involved in a first deformation stage (Late Cretaceous to Eocene), connected with an east-dipping ‘alpine’ subduction, whereas the external domain was deformed successively (late Oligocene–Miocene) in a west-dipping ‘apennine’ subduction (Figure 3). This reconstruction implied a reversal of the subduction during late Eocene time. Reconstructions with a subduction reversal have also been proposed subsequently by Haccard et al. (1972), Elter and Pertusati (1973), Grandjacquet and Haccard (1977), and Boccaletti et al. (1980), mainly differing in the age and the location of the ‘alpine’ and ‘apennine’ subductions. A model with a single, west-dipping ‘apennine’ subduction was subsequently proposed by Ohnenstetter et al. (1976) and Abbate et al. (1980). Whereas Ohnenstetter et al. (1976) proposed a single subduction below the Adria plate, Abbate et al. (1980) depicted a model where the subduction plane migrated progressively westward by multiple convergence zones (Figure 4). In this model, the Late Cretaceous–early Tertiary turbidites from both internal Ligurian (IL) and external Ligurian (EL) domains were interpreted as trench deposits related to the progressive migration of the deformation across the oceanic areas. International Geology Review 5

Subligurian Unit Tuscany Macigno basin 17 15 14 16 20 7 19 718 9 1013 11 13 12 848 2 3

B 25 M.Y. (Upper Oligocene–Lower Miocene)

Western Ligurian Units Subligurian Basin ‘Scaglia’ basin 18 17 15 16 Eastern Ligurian Basin 19 14 3 20 7 11 10 12 84sea level

A 40 M.Y. (EOCENE)

Figure 3. Proposed reconstruction (from Late Cretaceous to early Miocene) of the internal and external Ligure-Piemontese basins by Boccaletti et al. (1971). Schematic sequences of sections across northern Corsica illustrating a model for the tertiary development of the Alps–Apennines structures associated with trenches (no vertical scale). Black zone, oceanic crust; crosses, continental crust; large vertical hatched zone, upper mantle; white, asthenosphere; light stipple, ‘miogeosynclinal’ covers; heavy stipple, ‘eugeosynclinal’ covers; dense vertical hatched zone: subligurian unit. 1, Apuane Palaeozoic units; 2, Massa zone; 3, Apuane autochthonous cover; 4, Tuscany nappe; 5, M. Cervarola unit; 6, Umbro-Marchigiana zone; 7, Briançonnais and Sub-Briançonnais zone; 8, Canetolo zone; 9, M. Caio unit; 10, Basal Complex of Ligurian flysches; 11, M. Cassio unit; 12, M. Sporno unit; 13, Oligo-Miocene Ranzano Sequence; 14, M. Gottero unit; 15, M. Antola unit; 16, Bracco ophiolitic unit; 17, Massiccio di Voltri ophiolitic unit; 18, Schistes lustrés unit; 19, Balagne Ligurian unit; 20, Corsica autochthonous cover; 21, Po Valley post-orogenetic formations. Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Figure 4. Model proposed by Abbate et al. (1980) for the Alps–Apennines geodynamic system during the Late Cretaceous (from Abbate et al. 1980). 6 M. Marroni et al.

The debate around these deeply different reconstructions promoted a new strong impulse for detailed studies on the Ligurian units during the 1970s. Most of these studies focused on the Jurassic ophiolites, detected at the base or as slide-blocks into the sedimentary basins in both the internal and external domains. Fundamental is the paper by Decandia and Elter (1969) that first interpreted the ophiolites of the Northern Apennines as a fossil Jurassic oceanic crust, located at the base of the sedimentary successions of the Ligurian domains. The features of the ophiolites were analysed in numerous articles (Abbate et al. 1970; Bezzi and Piccardo 1970; Bortolotti and Passerini 1970; Decandia and Elter 1972; Gianelli and Principi 1974; Galbiati 1976; Piccardo 1976; Abbate et al. 1980; Beccaluva et al. 1980; Cortesogno et al. 1987 and many others) that proposed a detailed picture of the stratigraphy and the petrology of the ophiolites as well as their comparison with the crust of the present-day oceanic basins. Another fundamental contribution to the geological investigations of the Northern Apen- nines sedimentary succession is presented in the article by Mutti and Ricci Lucchi (1972), who applied the facies association concept to the analyses of the turbidites from the Ligurian units. On the same lines, the articles by Aiello et al. (1977), Martini et al. (1978), Sagri (1979, 1980), Casnedi (1982), Nilsen and Abbate (1983–1984), and many others described the sedimentological features of the turbidite deposits of both the IL and EL units. Another crucial contribution was provided by Pertusati and Horremberger (1975), who first applied the techniques of structural geology to the successions of IL units. The first articles interpreting the Ligurian units as remnants of an accretionary prism are those by Treves (1984) and Principi and Treves (1984), where, for the first time, the sedimentation and the deformation of the sedimentary deposits are discussed according to the data from modern subduction zones. The proposed model considers the deformed deposits with different ages as sediments accreted at different time in an accretionary prism developed in a westward subduction zone (Figure 5). Several contributions followed Treves (1984) and Principi and Treves (1984), in which further analyses of the Ligurian successions and the reconstruction of the structural history of the IL units provided clear evidence that the deformation is coherent with an involvement of sediments in an accretionary prism setting (Van Wamel et al. 1985; Van Zupthen et al. 1985; Meccheri et al. 1986; Marroni et al. 1988; Marroni 1991; Marroni et al. 2004; Hooger- duijn Strating 1994; Leoni et al. 1996; Marroni and Pandolfi 1996; Ducci et al. 1997). In addition, calcareous nannofossil biostratigraphic analyses have provided a refining of the age for most of the formations of the Ligurian sedimentary successions (Rio and Villa 1983, 1987; Rio et al. 1983; Marroni and Perilli 1988, 1990; Villa 1991; Cobianchi Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 and Villa 1992; Marroni et al. 1992; Cobianchi et al. 1994; Gardin et al. 1994). More recently, the finding by different authors of a tight association between ophiolites and fragments of the lower and upper continental crust in sedimentary mélanges allowed refinement of the configuration of the EL and IL domains. Although the IL units derived from an oceanic basin, the EL domain can be divided into two areas: the western- most domain characterized by an ocean–continent transition crust consisting of a subcontinental mantle topped by continental extensional allochthonous, and the easternmost domain characterized by a thinned, continental crust (Marroni and Tribuzio; 1996; Molli 1996; Montanini 1997; Marroni et al. 1998, 2001; Montanini and Tribuzio 2001; Piccardo et al. 1990, 2002, 2004). The improvement of the geological knowledge of the Ligurian units stimulated new geodynamic models for the closure of the oceanic basin in the Late Cretaceous–early Tertiary time span. Most of these models, as for instance those of Bettelli et al. (1989), Elter and Marroni (1991), Castellarin (1992), Marroni and Treves (1998), Vescovi et al. (1999), and Laubscher (1988, 1991), proposed a deformation of the Ligurian units in an International Geology Review 7 Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Figure 5. Model proposed by Principi and Treves (1984) for the Alps–Apennines geodynamic system during the Late Cretaceous–early Miocene time span (modified from Principi and Treves 1984).

oblique subduction setting, where strike-slip faults perpendicular and/or parallel to the belt strike were active from the intraoceanic subduction up to continental collision. In the last 10 years, further contributions on both structural (Pini 1999; Corsi et al. 2001; Ellero et al. 2001; Bettelli and Vannucchi 2003; Marroni et al. 2004; Levi et al. 2006; Meneghini et al. 2007; Dellisanti et al. 2008) and stratigraphic features (Marroni and Pandolfi 2001; Zuffa et al. 2002; Argnani et al. 2004; Bracciali et al. 2007) have been published. Contemporaneously, several models of pre- (Marroni et al. 2001; Piccardo et al. 2004; Principi et al. 2004; Marroni and Pandolfi 2007) and syn-convergence evolution (Daniele and Plesi 2000; Marroni et al. 2002; Del Castello et al. 2005; Molli et al. 2006; Nirta et al. 2007; Molli 2008; Vignaroli et al. 2008) have been proposed. The various syn- convergence models mainly debate the Late Cretaceous–early Tertiary dipping of the 8 M. Marroni et al.

subduction by proposing different solutions. However, the problem is still open and represents the target of future research.

The Ligurian units of the Northern Apennines: the state of art Geological background The Ligurian units crop out with good continuity in the Ligurian–Emilian Apennines. Therefore, the data reported in this article refer to this area, although extensive outcrops of Ligurian units occur also in Southern Tuscany (Nirta et al. 2007 and quoted references). The Ligurian units consist of an assemblage of tectonic slices interpreted as tectonic fragments of a Jurassic oceanic area, i.e. the Ligure-Piemontese basin, and its transition to the continental margin. During Jurassic time, this basin was located between the Adria plate, to the SE, and the Europe plate, which included at this time also the Corsica–Sardinia microplates, to the NW (Figure 6). According to the scheme proposed by Elter et al. (1966), the Ligurian units can be divided into two main groups, the IL and EL units, on the basis of their structural and stratigraphic features. The IL units include a Jurassic ophiolite sequence, still preserved at the base of a sedimentary succession characterized by mainly siliciclastic turbidites of Late Cretaceous–early Palaeocene age. In contrast, the EL units are characterized by the widespread occurrence of the Late Cretaceous carbonate Helminthoid Flysch. Despite the ubiquitous occurrence of Late Cretaceous Helminthoid Flysch, the EL units have been further subdivided by Marroni et al. (2001), according to the lithostratigraphic features of their basal complexes. The first EL group (‘western successions’) includes all the successions characterized by the occurrence of sedimentary mélanges with both oceanic and continental slide-blocks, whereas the second group (‘eastern successions’) displays successions showing a Triassic–Jurassic sedimentary base derived only from the thinned Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Figure 6. Geodynamic sketch of the Tethyan domain during the Jurassic time (modified after Cavazza et al. 2004). International Geology Review 9

Adria continental margin. According to these features, the first group is considered to be placed ‘oceanward’, near the Ligure-Piemontese oceanic domain, whereas the second group can be located ‘continentward’ at the distal edge of the Adria continental margin (Marroni and Pandolfi 2007 and references therein). In summary, three groups can be distinguished into the Ligurian units, hereafter referred to and described as IL, ‘oceanward’ western EL (WEL) and ‘continentward’ eastern EL (EEL) units (Figure 7). Each group of units is representative of a specific palaeoge- ographical domain and, hence, records a different sedimentary history.

The internal Ligurian units The IL units are arranged in a stack of tectonic units cropping out in the Ligurian Apennines, from the Sestri-Voltaggio line to the Ottone-Levanto-Carrara line (Figure 7). Along the N–S trending Sestri-Voltaggio line, these units are juxtaposed against the Voltri Group, represented by Jurassic ophiolite sequences and related sedimentary cover, metamorphosed up to eclogite facies. The Sestri-Voltaggio line has been regarded as an east-dipping, low-angle normal fault of early Tertiary age successively reworked in the Oligo–Miocene time (Hoogerduijn Strating 1994). Very recently, Capponi et al. (2009) and Federico et al. (2009) have provided evidence for a dextral transpression along the Sestri-Voltaggio line during the Oligo–Miocene, interpreted as related to the indenter of Adria into the Western Alps arc. Despite the different interpretations, a pressure gap of about 8 kbars can be detected across the Sestri-Voltaggio line between the Voltri Group and the IL units. Elsewhere in the Ligurian Apennines, the IL units are thrust over the EL ones along the Ottone-Levanto-Carrara line, a steep tectonic surface, interpreted as a pre- Oligocene east-verging thrust by Elter and Pertusati (1973). However, Elter and Marroni (1991) and Marroni and Treves (1998) proposed a different interpretation of this line as a sinistral strike-slip fault, developed during the northwestward indenter of the Adria plate. The IL units, as well as the adjacent alpine units, are unconformably overlain by post- orogenic succession of the Tertiary Piedmont Basin whose older deposits are mainly represented by the late Eocene Monte Piano Marls and by the early Oligocene, continental/marine conglomerates (Mutti et al. 1995). The group of the IL units includes several units known as, from SE to NW, Colli- Tavarone (including Lizza-Serò), Bracco-Val Graveglia, Gottero, Due Ponti, Vermallo, Portello, Cravasco/Voltaggio and Monte Figogna units. Despite the complexity derived Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 from a great number of local names, all these units are quite homogeneous from a stratigraphic point of view. In fact, all these units contain part of a general succession that consists of a Jurassic ophiolite sequence capped by the Middle Jurassic/early Palaeocene sedimentary cover. A stratigraphic log of the IL succession (Figure 8) can be fully reconstructed by the integration of data available from the different tectonic units. The ophiolites are characterized by a reduced sequence, not thicker than 1 km, consisting of a basement made up of mantle lherzolites, intruded by gabbros and covered by a volcano–sedimentary complex, where sedimentary breccias, basaltic flows and radiolarites are complexly intermixed (Abbate et al. 1980 and quoted references). This stratigraphy has been interpreted as representative of an ophiolite sequence developed into a slow-spreading ridge (Treves and Harper 1994 and quoted references). The ophiolite sequence is capped by pelagic/hemipelagic deposits represented by Radiolarite Formation (Callovian–Tithonian), Calpionella Limestone (Berriasian-Valanginian) and Palombini Shale (Valanginian–Santonian). The Radiolarite Formation mainly derived from the reworking of pelagic siliceous ooze by 10 M. Marroni et al. MS, Epimesoalpine successions; Epimesoalpine MS, n of Figure 9 is indicated as A–A’. tion of the geological sectio ion. Inset: EL, external Ligurian units; E units; Ligurian external EL, Inset: ion. rpretative cross-sect and loca Umbrian and units. The Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 tocene deposits; TU, tocene deposits; Tuscan Figure 7. Apennines with inte Tectonic sketch map of the Northern IL, internal Ligurianunits; Plio-Pleis PP, International Geology Review 11

Figure 8. Reconstruction of the succession of the internal Ligurian units (redrawn from Marroni

Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 and Pandolfi 2007). Details of the ophiolite sequences are shown in the boxed areas.

turbidites and oceanic bottom currents, whereas the Calpionella Limestone and the Palom- bini Shale derived from distal carbonatic and mixed siliciclastic-carbonatic turbidites, mainly reworking a source area located in the uppermost part of the Europe/Corsica continental margin (Pandolfi 1997; Bracciali et al. 2007). The Palombini Shale grades upward to siliciclastic turbidites, ranging from Campanian to early Palaeocene (the Manganesiferi Shale, the Monte Verzi Marl and the Zonati Shale, which form the Val Lavagna Shale Group, the Ronco Formation, the Canale Formation, and the Monte Gottero Sandstone), interpreted as a complex turbidite fan system fed by the Europe–Cor- sica continental margin (Nilsen and Abbate 1983–1984). The Manganesiferi Shale (early Campanian) is composed of siliciclastic, coarsening-upward basin plain turbidites and grades up to mixed siliciclastic–carbonatic turbidites known as Monte Verzi Marl (early to late Campanian). This formation is represented by siliciclastic basin plain turbidites inter- bedded with carbonatic megaturbidites. The upper part of the turbidite system is an overall 12 M. Marroni et al.

siliciclastic sequence showing a thickening and coarsening upward trend, represented by the Zonati Shale (late Campanian–early Maastrichtian) and the Monte Gottero Sandstone (early Maastrichtian–early Palaeocene). The Zonati Shale, which can be correlated with the Ronco Formation and the Canale Formation, is made by thin-bedded turbidites, interpreted as basin plain deposits, and grades upward to the Monte Gottero Sandstone, composed of coarse-grained siliciclastic turbidites and interpreted as the proximal portion of the deep-water fan. According to Abbate and Sagri (1982), Nilsen and Abbate (1983– 1984) and Pandolfi (1997), the turbidite sequences recognizable in the IL succession are characterized by turbiditic facies indicative of a connection between continental margin and deep-sea deposits. The arenites from Val Lavagna Shale Group, Gottero Sandstone, and Bocco Shale are arkoses and subarkoses characterized by an almost complete siliciclastic framework and by a metamorphiclastic composition of the fine-grained lithic fragments (Valloni and Zuffa 1984; van de Kamp and Leake 1995; Pandolfi 1997). The arenite framework is dominated by the presence of mono- and polycrystalline quartz, plagioclase, and K-feldspar. Lithic fragments of volcanic nature are common and include porphyritic rhyolite and dacite fragments. Intrusive coarse-grained fragments such as granitoids are also common. Metamorphic rock fragments include low-grade schists and micaschists. Carbonate extrabasinal fragments are scarce and they are represented by oolitic and peloidic grainstones and mudstones. According to these data, the source area of the sedimentary cover of the IL units can be placed on the upper part of a continental crust belonging to the Corsica–Europe continental margin (Valloni and Zuffa 1984). The youngest formation of the IL units is represented by the early Palaeocene coarse- grained deposits known as the Bocco Shale (see also Giaiette Shale or Colli/Tavarone formations), characterized by the occurrence of thin-bedded turbidites, where ophiolite-bearing slide, debris flow, and high-density turbidity current-derived deposits are imbedded. Facies analysis and provenance studies indicate, for the last deposits, a formation by small and scarcely evolved flows that reworked a typical oceanic lithosphere and its sedimentary cover. These sedimentary processes can be interpreted as the downcurrent evolution of submarine landslides developed along a steep slope. The thin-bedded turbidites are instead indicative of a different facies association derived from more evolved low-density turbidity currents. The composition and the stratigraphic features of the thin-bedded turbidites indicate a source area different from that of the slide and debris flow deposits. Particu- larly, the source was the same area that supplied the Gottero Sandstone and the Val Lavagna Shale, i.e. the uppermost part of a continental crust. It is worth to noting that, in Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 all the investigated area, the Bocco Shale lies on top of the sedimentary succession, from the Gottero Sandstone to the Palombini Shale, by means of a well-preserved unconform- ity. Marroni and Pandolfi (2001) have interpreted this formation as lower slope and trench deposits related to the frontal tectonic erosion of the accretionary prism. On the whole, the described sedimentological features of this succession, in particular the transition from pelagic to turbidite deposits, have been interpreted as reflecting the trenchward motion of an area belonging to the Ligure-Piemontese oceanic lithosphere. This interpretation is coherent with the pre-Oligocene (Di Biase et al. 1997) polyphase deformation history described and summarized for the IL units by several authors (Meneghini et al. 2007 and quoted references) and interpreted as achieved during progressive underthrusting (D1a), underplating (D1b and D1c), and later exhumation (D2a and D2b) in an accretionary prism. The folding phase related to the main underplating event (D1b) is predated by extensive dewatering and rapid fluid escape (D1a event) and followed by the development of shear zones (D1c). The D1b folds show similar geometry and westward vergence, associated with a slaty cleavage developed under P/T conditions International Geology Review 13

ranging from to low-grade blueschists in the lowermost units to very low grade in the uppermost one (Leoni et al. 1995; Ellero et al. 2001). The D1c subphase, characterized by west-verging thrusts, is particularly meaningful for the understanding of the dynamics of the Ligure-Piemontese accretionary prism because it testifies active shortening of the IL units during and after their accretion. In addition, the D1c thrusting event represents the transition from the accretion-related deformation to extensional tectonics, characterized by parallel folds (D2a) and low-to-high-angle normal faults (D2b). The extensional tectonics is interpreted as the consequence of the thickening of the Ligure-Piemontese accretionary prism, produced by either the continuous underplating at its base or the shortening of the previously underplated units. Finally, the extensional tectonics resulted in the exhumation of the IL units up to the surfaces during the early Oligocene, when its sedimentary succession represented one of the source areas of the conglomerates deposited in the Tertiary Piedmont Basin (Di Biase et al. 1997).

The western external Ligurian units The WEL unit group includes Bettola, Caio, Orocco, Ottone, Monte delle Tane, and Groppallo units. As for the IL units, all WEL units show part of a more general succession comprising sedimentary mélanges at the base and two, Late Cretaceous and Tertiary, flysch deposits. All successions are detached from their original base. The tectonic setting of the WEL units is quite complex. They are generally juxtaposed between the IL units, through the Ottone-Levanto-Carrara line, and the EEL units, onto which they are overthrust in the Emilian Apennines. However, in some areas, the EEL unit group is overthrust by the WEL units as a result of the Oligo–Miocene east-verging deformations. Concerning the stratigraphy, the most typical deposit of the western successions is represented by the sedimentary mélanges cropping out extensively in the Ottone, Monte delle Tane, and Groppallo units and known as Casanova, Monte Ragola, and Pietra Parcellara complexes, respectively. Among them, only the Casanova complex is stratigraphically overlain by the Helminthoid Flysch, whereas the Monte Ragola and Pietra Parcellara complexes are bounded by thrusts, and their original stratigraphic relationships with the Helminthoid Flysch can be only suggested. Although distinguished by their tectonic position, all basal complexes show the same stratigraphic and sedimentological characteristics. They are all detached from their stratigraphic base and consist of huge slide-blocks enclosed in a matrix made of clast-supported Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 breccias and coarse-grained, turbidite-derived rudites and arenites. The most significative differences among these mélanges arise from the lithologies represented as slide-blocks or clasts. The best exposed and preserved sedimentary mélange is represented by the Casanova complex (Passerini 1965; Naylor 1982; Elter et al. 1991), an up to 1500 m-thick succession that consists of monomict-to-polymict pebbly-mudstones, polymict pebbly sandstones and huge slide-blocks showing a transition to well-bedded, coarse-grained arenites and rudites, known as Casanova Sandstone (Figure 9). The slide-blocks, generally monolithologic, and the clasts in the breccias are composed by various lithologies. The most representative are probably the mantle ultramafics, consisting of spinel-lherzolites with common pyroxenite bands, deformed and partly recrystallized in the plagioclase stability field, with the formation of tectonite-mylonite fabrics (Piccardo et al. 2004). The ultramafic slide-blocks are interpreted as slices of subcontinental mantle, emplaced at low structural levels during the early stages of rifting of the Jurassic Ligure-Piemontese basin (Piccardo et al. 2002). The rare slide-blocks of 14 M. Marroni et al. bbly-mudstone of the bbly-mudstone relationships among internal among relationships eviations used are according to the Flysch; ORO, Orocco Flysch (modified Flysch; Apennine) showing the Apennine) showing Sandstone; CCVb, matrix supported pe Sandstone; CCVb, matrix supported unit is overturned. The abbr unit Monte Monte Veri complex; OTO, Ottone igurian–Emilian Northern igurian–Emilian Northern e; CCVa, Casanova e succession of the Ottone the Ottone of succession e tion: APA, Palombini Shal Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 , slide-blocks of serpentinites; MVE, , slide-blocks of serpentinites; ∑ ts and Subligurian units. Th Subligurian ts and along the uppermost part of the Aveto Valley (L ect (CARG). Explana ect (CARG). , slide-blocks of basalt; b 2005). et al. from Elter Casanova complex; complex; Casanova Ligurian unit, western external Ligurian uni western external unit, Ligurian mapping proj Italian 1:50,000 Figure 9. Geological cross-section International Geology Review 15

gabbros, locally characterized by the occurrence of ductile shear zones, consist of trocto- lite- to olivine-bearing gabbro derived from low-pressure fractional crystallization of mid- ocean ridge (MOR)-type melts (Montanini et al. 2008). Slide-blocks of basalts, sometimes with preserved stratigraphical relationships with late Callovian to early Oxfordian radiolarites (Conti et al. 1985), frequently occur as pillow lava and massive bodies, but basaltic dikes are also widespread in lherzolites, gabbros, and massive basalts. They are normal to transitional MOR basalts, generated by a few per cent of fractional melting of a slightly depleted astenospheric mantle in the spinel stability field (Vannucci et al. 1993; Montanini et al. 2008). Slide-blocks of sedimentary rocks also occur, corresponding to the Palombini Shale (Valanginian–Late Cretaceous), the Calpionella Limestone (Berriasian-Valanginian), and the Radiolarite (Late Jurassic) Formation. All these lithologies belong to the same succession, which is the typical sedimentary cover of the Jurassic ophiolites from the Ligure- Piemontese basin (Decandia and Elter 1972). Continent-derived rocks are also recognized, consisting mainly of slide-blocks of granitoids of late Palaeozoic age (310–280 Ma; Ferrara and Tonarini 1985), commonly affected by a cataclastic deformation younger than Middle Triassic (Marroni et al. 1998). Where primary relationships with basalts are well preserved, the granitoids are intruded by basalt dikes, although basaltic flows stratigraphically covering the granitoids and their brittle structures are also found (Molli 1996). Other continent-derived rocks, found as clasts in polymict breccias, mainly consist of micaschists, orthogneisses, and garnet-bearing paragneisses. One of the most striking feature recognized in the sedimentary rock-derived slide- blocks is the presence of a pre-brecciation deformation recognized inside the clasts of Calpionella Limestone and Chert. The deformation is represented by folded veins in the Calpionella Limestone clasts and by a well-developed foliation (confined inside the clasts) in the siliceous shales associated with the Chert-derived clasts. The matrix of these slide-blocks is mainly represented by the Casanova Sandstone, regarded as an early Campanian complex deep-sea turbidite system characterized by the association of coarse-grained and poorly evoluted high-density turbidites with thick to very thick fine-grained low-density turbidites. The Casanova Sandstone shows a lithoarenitic composition where the rock fragments are mainly represented by serpentinites, basalts, cherts, Calpionella Limestone, and Palombini Shale (Di Giulio and Geddo 1990). Granitoids, low-grade metamorphic rocks, and arkosic arenites fragments are also present. According to Di Giulio and Geddo (1990), a petrofacies characterized by an arkosic continental block-derived composition (sensu Dickinson 1985) can also be recognized in the Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 arenites of the Casanova Sandstone, although it is only found in the fine-grained low-density turbidites. The Casanova Sandstone, generally 300–600 m thick, shows a gradual transition to the late Campanian Ottone Flysch. Moreover, in the lower part of the Ottone Flysch, several intercalations of debris flow deposits (known as Monte Veri complex), as those recognized in the Casanova complex, are widespread (Elter et al. 1991). The Monte Ragola complex (Marroni and Tribuzio 1996) exhibits a 600 m thick succession assigned to Santonian–early Campanian by nannofossil assemblages (Elter et al. 1997). Differently from the Casanova complex, the Monte Ragola complex is characterized by the prevalence of mantle ultramafics, the scarce presence of basalts, and the absence of gabbro bodies, but the main feature is the occurrence of slide-blocks of mafic and acid granulites. According to Marroni and Tribuzio (1996) and Montanini (1997), the mafic granulites can be interpreted as remnants of an igneous complex, derived from crystallization at moderate pressure of tholeiite-derived and crustally contaminated liquids. The relics of igneous textures, and mineral and whole-rock mineral variations indicate that the granulites intruded at deep structural levels into extending continental 16 M. Marroni et al.

lithosphere and re-equilibrated in subsolidus conditions at 0.6–0.9 GPa and at 810–920°C under granulite facies in the Late Carboniferous–Early Permian time (Meli et al. 1996). The mafic granulites show a subsequent metamorphic history younger than the Middle Triassic (Meli et al. 1996), commonly accompanied by a deformation changing from plas- tic to brittle, during their exhumation along an intermediate P/T gradient from granulite to amphibolite and greenschist facies conditions to upper crustal levels, probably in association with the subcontinental mantle (Marroni et al. 1998). In addition, slide-blocks of garnet- bearing acid granulites showing primary contacts with the mafic granulites have been distinguished. They are interpreted as granulite facies metasediments showing a post-late Palae- ozoic retrograde metamorphic history from granulite to amphibolite and greenschist facies associated with mylonite and cataclasite development (Marroni et al. 1998). The Pietra Parcellara complex (Elter et al. 1997), probably of Santonian–early Campa- nian age, crops out in the eastern areas of the Northern Apennines. The slide-blocks recognized in the Pietra Parcellara complex show features similar to that recognized in the Casanova and Monte Ragola complexes even if the continent-derived rocks, that is, the granulites, the granitoids and the associated metamorphic rocks, are absent. In summary, all the sedimentary mélanges recognized in the western successions seem to be derived from a source area corresponding to an ocean–continent transition at the margin of the Adria plate (Marroni and Pandolfi 2007 and quoted references). According to the reconstruction proposed by Marroni et al. (2001), this area was characterized by a basement of subcontinental mantle and lower continental crust, covered by extensional allochtons of upper continental crust and intruded by basalts (Figure 10). Even if characterized by some differences, mainly consisting of the occurrence of the granulites and the other continent- derived rocks, the overall characteristics, as, for instance, the widespread occurrence of subcontinental mantle ultramafics, suggest that the mélanges probably belonged to the same geodynamic setting, with the differences only reflecting heterogeneities in the source area. By contrast, other WEL units display successions represented mainly by Helminthoid Flysch with no or small remnants of sedimentary mélanges at their base. The Helminthoid Flysch consists of calcareous turbidites characterized by rhythmic alternation of calcareous-marl, marly-limestone, and marl layers showing medium-to-very thick beds with fine- to-medium arenitic base. One of the main features of these layers consists of an a:p ratio <<1 which, in some layers, can reach values >20. This feature, together with the presence of incomplete Bouma sequences, the lack of erosive structures, the parallel plane geometry of Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Figure 10. Simplified model of the ocean–continent transition at the Adria continental margin based on data from the external Ligurian units during the Middle–Late Jurassic during the radiolarite sedimentation (from Marroni et al. 1998). International Geology Review 17

the strata, and the carbonate-free hemipelagic background sediments, indicates a deposition by low-density turbidity currents in a deep-sea environment located below the local CaCO3 compensation level. The arenites show an arkosic composition characterized by monominer- alic fragments of quartz, feldspar, and rock fragments derived from granitoides and low- grade metamorphites. Close to the stratigraphic transition with the Casanova complex, few strata with lithoarenitic composition, comparable to that of the underlying ophiolitic sandstone from the Casanova complex, can be recognized. This evidence is together with the absence of carbonate in hemipelagic background sediments, is indicative of an abyssal plain environment located below the local CaCO3 compensation level (Scholle 1971). The Helminthoid Flysch reveals the same stratigraphic setting in both the WEL and EEL units. The most complete succession in the WEL units (Figure 11) can be identified in the Caio unit (Vescovi et al. 1999), where a late Campanian–Maastrichtian Helminthoid Flysch is typically characterized, at the base, by thin intercalations of mafic and ultramafic fragment- bearing debris flows. The Monte Caio Flysch is overlain by Palaeocene–middle Eocene carbonate flysch, that is, the Marne Rosate Formation. The Bettola unit (Marroni et al. 2001) displays a well-preserved succession where a late Cretaceous Helminthoid Flysch, i.e. the Bettola Flysch, is overlain by a Palaeocene–middle Eocene carbonate flysch, i.e. the Val Luretta Flysch. The successions of the Caio and Bettola units show close similarities and are reported together in the geological sketch map of Figure 7. Even if consisting only of a Late Cretaceous Helminthoid Flysch devoid of its basal complex, the Orocco unit (Elter and Mar- roni 1991) is included in this group by its tectonic setting, similar to that of the Caio unit. In the WEL units, as well as in the EEL ones, polyphase deformation history associated with metamorphism typical of diagenesis zone can be observed. These deformations, whose structures are sealed by uppermost middle Eocene deposits of the Epiligurian basin, affected also the lowermost middle Eocene deposits of the EL successions; thus, the age of all of these deformations can be confined to the middle Eocene (Ligurian phase of Elter 1975). In all the EL units, this polyphase deformation history includes a D1 phase consisting of Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Figure 11. Reconstruction of the western and eastern successions of the external Ligurian units (redrawn from Marroni et al. 2001). 18 M. Marroni et al.

west-verging isoclinal to close parallel folds (Levi et al. 2006) associated with a cataclastic shear zone (Marroni et al. 1999), which generally represent the present-day boundary of the recognized tectonic units. The relationships between the WEL and the EEL units are acquired during this phase. The subsequent D2 phase is, in turn, represented by east- verging close to open large-scale folds with about NW–SE axis and subhorizontal axial planes (Meccheri et al. 1982; Cerrina Feroni et al. 1989; Costa et al. 1991, 1995; Cerrina Feroni et al. 1994; Corsi et al. 2001; Marroni et al. 2002; Levi et al. 2006). As a whole, the structures of the D2 phase, which include not only overturned folds with subhorizontal axial planes, but also shear surfaces cutting down in the stratigraphic sequence and passive rotation of the linear structural elements, are coherent with the gravitational spreading and tectonic transport of both WEL and EEL units towards the easternmost areas.

The eastern external Ligurian units The second group of EL units is represented by the Media Val Taro, Cassio, Farini, Antola, and Sporno units, showing thick, well-preserved basal sequences where, in contrast to the WEL units, the mafic and ultramafic slide-blocks are totally lacking. Although the Antola unit is found at the top of the IL units, the Media Val Taro, Cassio, Farini, and Sporno units are generally located at the top of the WEL units, even if this occurrence is locally modified by both middle Eocene and Oligo–Miocene east-verging deformations. Whereas the Farini and Sporno units (Cerrina Feroni et al. 1994) are characterized by successions that include Palaeocene–middle Eocene carbonate flysch, all other EEL units show well-preserved basal complexes below the flysch deposits. The Media Val Taro unit (Vescovi et al. 1999) consists of a basal complex represented by the Palombini Shale, the S. Siro varicoloured Shale, and the Ostia Sandstone formations. The Cassio unit shows the most representative succession of the EEL domain, with a complete transition from the basal complex, to Late Cretaceous Helminthoid Flysch, up to early Tertiary, predominantly shaly, deposits. The basal complex includes the Palombini Shale (Early Cretaceous), arenites correlated to the Ostia Sandstone (Case Baruzzo Sandstone of Vescovi et al. 1999 and Scabiazza Sandstone), and varicoloured, hemipelagic shales (Cenomanian–late Campanian). The varicoloured, hemipelagic shales are characterized by intercalations of conglomerates, known as Salti del Diavolo conglomerate. Pebbles feature intrusive, volcanic, low- to medium-grade metamorphic, siliceous and carbonate sedimentary rocks (cfr. Baldacci et al. Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 1972). Intrusive rocks are represented by medium- to coarse-grained syeno- and monzogran- ites. Volcanic rocks are characterized by dacites and rhyolites. Pebbles of metamorphic rocks are common. Low- to medium-grade metamorphic rocks such as phillites, schists, muscovite-biotite- and garnet-bearing micaschists and gneisses are recognized, as well as cordierite-bearing gneisses. Pebbles of carbonate (mainly limestones and dolostones not older than Barremian-Hauterivian age) and siliceous rocks can be also recognized, with the carbonate pebbles mainly made of mudstones, radiolarian-bearing wackestones and packstones, peloidic grainstones, and Calpionella-bearing mudstones. Recrystallized dolostones and arenites made up of carbonate platform fragments are also observed. Siliceous rocks are red and green radiolarites, silicified radiolarian-bearing mudstones and siltstones, radiolarian-bearing packstones, and cherty-limestones. Arenites from Ostia Sandstone, Varicoloured Shale, and Salti del Diavolo conglomerate are sublitharenites characterized by a mixed siliciclastic-carbonate framework composition and by a mixed composition of the fine-grained lithic fragments (Bracciali et al. 2007 and quoted references). The extrabasinal siliciclastic-arenite framework is characterized by International Geology Review 19

mono- and polycrystalline quartz, plagioclase, and K-feldspar grains. Coarse-grained lithic fragments of granitoids are common. Metamorphic rock fragments include low-grade schists, micaschists, and minor fragments of medium-grade cordierite- and garnet-bearing gneisses. Siliceous rock fragments are represented by radiolarian-bearing cherts, cherty- limestones, siliceous mudstones, and siliceous siltstones. Carbonate mudstones, radiolarian- bearing wackestones, and medium- to coarse-grained dolostones are the most common extrabasinal carbonate rock fragments. Calpionella-bearing mudstones, oolitic grainstones, and radiolarian-bearing packstones are also found. According to Bracciali et al. (2007), the geochemical data relative to the siliciclastic fraction of these deposits indicate an ultramafic source standing out also from the mafic- felsic components contributing as well to the sediment. This is in agreement with the occurrence of millimetre-sized Cr-spinel fragments in the corresponding arenitic fraction (Mezzadri 1964 and Wildi 1985), typically derived from mantle coarse-grained ultramafic rocks and indicative of a source area characterized by the presence of mantle rocks cropping out below the continental crust. The source area was located in correspondence with the Adria continental margin where the metamorphic and magmatic rocks belonging to the basement were exposed (Elter et al. 1966). In this picture, the presence of mantle rock exposed at the sea floor can be also hypothesized. The Varicoloured Shale grades upward to the late Campanian-Maastrichtian Monte Cassio Flysch, showing arenites with a hybrid composition (Fontana et al. 1994), characterized by the presence of an extrabasinal arkosic to lithic-arkosic assemblage with fragments of granitoids, low-grade metamorphic rocks, dolostones, Early Cretaceous micritic limestones, and cherts, associated with coeval intrabasinal debris made up of planktonic and benthonic bioclasts and glaucony. At the top of the succession, the Palaeocene Viano Shale (Laccarino and Rio 1972), consisting of red and grey shales alternating with carbonate turbidites, occurs. On the whole, the entire succession from the Cassio unit is characterized by deposits supplied by a continental margin where ophiolites are lacking, even if the presence of mantle rock exposed at the sea floor can be hypothesized. Locally, an assemblage of tectonic slices has been recognized at the base of the Cassio unit succession (Vercesi and Cobianchi 1998). Even if dismembered in multiple tectonic slices, the original succession can be fully reconstructed. It consists of Middle Triassic dolomitic limestones with stromatolitic structures, capped by sedimentary breccias with Diplopora-bearing dolomitic clasts (Late Triassic). On top of the sedimentary breccias lie cherty-limestones (Lias) and marls (Dogger-Malm), showing a transition to cherts Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 (Malm), Aptychus-bearing calcareous red marls (Malm), and Calpionella-bearing pelagic limestones (Early Cretaceous). The pelagic limestones, similar to the Maiolica Formation of the Southern Alps and Tuscany, are characterized by intercalations of sedimentary breccias with dolomitic and cherty clasts. This sequence can be interpreted as the remnants of the continental substratum of the succession from the Monte Cassio unit and can be probably extended to the other WEL successions (Marroni et al. 2001) (Figure 11). Similar to the Cassio unit, the Antola unit displays an up to 3000 m-thick succession that can be roughly subdivided into three main parts: the lower basal complex (Montoggio Shale and Gorreto Sandstone), the middle carbonatic turbidite deposits (Monte Antola Flysch), and the upper mixed siliciclastic-carbonatic turbidite deposits (Bruggi–Selvapi- ana Formation and Pagliaro Shale) (Catanzariti et al. 2007 with quoted references). The part of the basal complex represented by the Montoggio Shale has been further subdivided. The lowermost portion is made up of black manganiferous, carbonate-free, hemipelagic shales, interlayered with fine-grained turbiditic sandstones, showing an arenite/pelite ratio >1 and a thickness of at least 100 m. The uppermost portion consists of 200–300 m of 20 M. Marroni et al.

varicoloured hemipelagic shales, followed by the turbiditic succession of the Gorreto Sand- stone, characterized by thin-bedded turbidites of mixed siliciclastic/carbonatic composition. The basal complex grades upward to the Monte Antola Flysch, consisting of calcareous turbidites and megaturbidites, showing a siliciclastic-arenite composition (Rowan 1990; Fon- tana et al. 1994), interlayered with rare siliciclastic beds and thin hemipelagic, carbonate- free shales, indicating sedimentation below the calcite compensation depth (Scholle 1971). The Monte Antola Flysch is overlain by the carbonatic megaturbidite sequence of the Bruggi–Selvapiana Formation, partly corresponding to the Bruggi and Selvapiana members of Abbate and Sagri (1967). This formation is characterized by the alternation of turbiditic, marly megabeds with thin-bedded siliciclastic turbidites and shales. The top of the Antola unit succession, the Pagliaro Shale (Bellinzona and Boni 1971), mainly consists of thick shale beds alternating with siliciclastic, thin-bedded turbidites and minor calcareous turbidites (corresponding to the Cabella Member of Abbate and Sagri 1967) (Figure 12). Also, the Late Cretaceous succession of the Solignano unit, characterized by varicoloured shales and a Maastrichtian Helminthoid Flysch, known as Solignano Flysch, is here regarded as belonging to the eastern domain, according to the modal analyses of the arenites of the Solignano Flysch, which reveal a hybrid composition characterized by dolostone fragments without any evidence of ophiolite fragments (Fontana et al. 1994). Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Figure 12. Stratigraphic log of the Antola unit according to Catanzariti et al. (2007). International Geology Review 21

Location of the Ligurian succession in the Late Cretaceous–early tertiary palaeogeography of the Ligure-Piemontese basin According to the described geological features, some considerations about the location of the successions preserved in the Ligurian units, in the framework of the Late Cretaceous– early Tertiary palaeogeography, can be attempted. The IL units are characterized by an oceanic basement topped by a thick sedimentary cover, spanning from Late Jurassic to early Palaeocene. In this sedimentary cover, a transition from pelagic (cherts, Calpionella Limestone, Palombini Shale Formation) to turbidite (Val Lavagna Shale Group and Monte Gottero Sandstone Formation) and lower slope deposits (Bocco Shale) has been outlined by several authors (Marroni et al. 1992 and quoted references). This transition has been interpreted as achieved during the trenchward motion of an oceanic lithosphere (Treves 1984; Marroni and Pandolfi 2001). Matching with this reconstruction are the contrasting sedimentary features shown by the pelagic versus trench plus lower slope deposits (Marroni and Perilli 1990). In fact, while the pelagic deposits show a very low sedimentation rate, as demonstrated by their long time span of sedimentation (about 80 Ma) and reduced thickness (about 1000 m), the trench and lower slope deposits are characterized by a short time span of sedimentation (about 20 Ma) but a relevant thickness (about 3000 m). The interpretation of the transition from pelagic to trench and lower slope deposits as achieved during a trenchward motion is coherent with the subsequent structural and metamorphic evolution, typical of units subjected to underplating into an accretionary prism (Meneghini et al. 2007 and quoted references). Thus, the IL units can be regarded as a fragment of the accretionary prism developed during subduction of the Ligure-Piemontese oceanic basin in the Late Cretaceous–early Tertiary. Regarding the palaeogeographic location of the IL successions, clear evidence is provided by the Monte Gottero Sandstone, whose arenite composition indicates that the source area is represented by the crystalline basement of the Europe/ Corsica plate (Valloni and Zuffa 1984). Thus, the IL domain can be interpreted as located in an area characterized by oceanic lithosphere bounded by the Corsica continental margin in its northwestern side, whereas the opposite side is represented by the accretionary prism, from which the ophiolite-bearing deposits of the Bocco Shale were derived (Figure 13). A different picture can be drawn for the EL units, mostly characterized by a succession that includes late Campanian–middle Eocene carbonate turbidites. It is crucial to highlight that the turbidites are represented by a monotonous sequence of beds without evidence of

Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 sin-sedimentary deformations as slumps, intraformational breccias, or unconformities, as also stated by Marroni and Treves (1998). This is particularly evident for the Monte Antola unit succession where Catanzariti et al. (2007) have recognized a continuous sedimentation from late Campanian to late Palaeocene. Also, the Caio unit displays a succession continuous from late Campanian to middle Eocene (Rio et al. 1983). Thus, the basin from which the EL successions were derived is lacking any evidence of deformation from late Campanian until the above described lowermost middle Eocene deformation event. As previously discussed, the slide-blocks preserved in the basal sedimentary mélanges from WEL, and the arenite composition from the Cenomanian to Campanian turbidite from EEL units, indicate that the EL domain was located along the western edge of the Adria plate, in correspondence with the ocean–continent transition (WEL units) and the adjacent thinned continental margin (EEL units). Thus, whereas the IL units experi- enced the Late Cretaceous–early Tertiary subduction-related sedimentary and tectonic events, the EL domain escaped deformation until the middle Eocene, i.e. until the inception of the continental collision. This feature can be explained only by placing the basin, 22 M. Marroni et al.

Figure 13. Interpretive section of the subduction zone in the Ligure-Piemontese oceanic basin during the Palaeocene. In the blow-up, the details of the frontal tectonic erosion of the accretionary wedge slope are shown. The arrows point out the provenance of the two groups of facies deposits of the Bocco Shale (hollow arrow: deposits supplied from the Europe plate; solid arrow: deposits supplied from accretionary wedge by frontal tectonic erosion). Modified from Marroni and Pandolfi (2001).

from which the EL units derived, in a suprasubduction setting, located between the Adria Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 thinned continental margin and the rear of the accretionary prism. However, both the WEL and EEL units are characterized by the sedimentary evidence of a Santonian–early Campanian tectonic pulse, as testified by the occurrence of the basal sedimentary mélange of the WEL units, which suggests a tectonic-controlled deposition, according to the huge size of the slide-blocks and the evidence of brittle deformation of the slide-blocks before their inclusion in the mélange. The occurrence of Santonian– Campanian tectonics is also testified by radiometric data on the continental crust slide- blocks. The partial annealing of fission tracks in zircons from quartzo-feldspathic granulites indicates that the temperature must have exceeded 200°C at about 80 Ma during a thermal event of Late Cretaceous age (Balestrieri et al. 1997). This is consistent with the metamorphic overprint under subgreenschist facies conditions responsible for the development of chlorite and sericite in the quartzo-feldspathic granulites. In addition, the mafic granulites provided an 40Ar/39Ar age around 80 Ma (Meli et al. 1996), which can be related to the development of a metamorphic overprint in the pumpellyite-actinolite facies (Marroni and Tribuzio 1996; Montanini 1997). This tectonic pulse is also recognized in the EEL units International Geology Review 23

where unconformities within the Cenomanian to Santonian succession have been identified (Vescovi et al. 1999). Thus, this tectonic pulse seems to be recognizable in the whole EL domain. The geodynamic significance of this tectonic pulse is puzzling. The only interpretation in the frame of plate tectonics is that of Naylor (1982), who proposed a faulted distal passive margin as the source area of the Casanova complex sedimentary mélanges. However, this interpretation is unrealistic, because it cannot explain the occurrence of normal faults cutting the ocean–continent transition after 80 Ma from the oceanic spreading inception. Some authors (Principi and Treves 1984; Treves 1984) interpreted the sedimentary mélanges from EEL units as lower slope deposits supplied from an accretionary prism. In this picture, the carbonate turbidites, i.e. the Helminthoid Flysch, are interpreted as trench deposits, but their residence time in the basin of about 30 Ma, without evidence of deformation, does not fit very well with the proposed scenario. Subsequently, Bertotti et al. (1986) interpreted the mélanges as a sedimentary result of back-thrusting in the rear of an accretionary prism connected to an east-dipping subduction. In addition, Elter and Marroni (1991) and Marroni and Treves (1998) proposed an interpretation of these deposits as related to a strike-slip tectonics connected with a major reorganization of the plates in the Tethys area during the Campanian. However, none of these interpretations are completely satisfying, and the geodynamic meaning of this tectonic pulse remains unclear. Despite this unsolved point, the IL and EL units show features that clearly indicate their belonging to two different palaeotectonic domains. The IL units represent the remnants of an accretionary prism, whereas the EL ones can be considered as derived from a supra-subduction basin that, even if affected by tectonics in the Santonian–Campanian boundary, did not suffer any subduction-related deformation and remained undeformed from the late Campanian until the middle Eocene. It is important to outline that the EL domain is deformed only when all the oceanic lithosphere of the Liguria-Piemontese basin was subducted. In addition, no record of the possible transition between the IL and EL domains has been identified so far. Similarly, even if both are characterized by the occurrence of Helminthoid Flysch, the transition between the ophiolite-bearing and ophiolite- free deposits of the WEL and EEL units is not detected in the present-day unit pile of the Northern Apennines. The evidence of these main gaps have been considered by Marroni and Treves (1998) to support their interpretation of the IL, WEL, and EEL units as terranes separated by strike-slip faults active from the Late Cretaceous up to the Oligocene in a regime of transpression. Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Geodynamic history of the Ligure-Piemontese basin: a proposal The discussed features of the Ligurian successions allow us to propose a possible scenario for the Late Cretaceous to middle Eocene geodynamic history of the Ligurian Apennines. This reconstruction is depicted step by step in Figure 14. Important key elements from the neighbouring domains, as the Alpine Corsica, have been taken into account in the proposed reconstruction. Our reconstructed evolution starts in Pre-Santonian time, when the Ligure-Piemontese basin was dominated by pelagic sedimentation. The architecture of the basin in this time span was inherited from the rifting history leading to the opening of the Ligure-Piemon- tese oceanic basin (Marroni and Pandolfi 2009 and quoted references). The Early to Middle Jurassic asymmetric stage of the rifting depicted in the reconstruction by Marroni et al. (1998) resulted in profoundly different architecture and lithology of the paired continental margins. The Adria plate was characterized by a wide ocean–continent transition floored by 24 M. Marroni et al. Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010

Figure 14. Possible scenario for the Late Cretaceous to middle Eocene geodynamic history of the Ligurian Apennines. (A) Campanian–Maastrichtian; (B) early Eocene; (C) late Eocene; (D) early Oligocene. Explanation: IL, internal Ligurian domain; WEL, western external (oceanward) Ligurian domain; EEL, eastern external (continentward) Ligurian domain; SBL, Subligurian domain; ANT, Antola unit; T, Tuscan domain; TPB, Tertiary Piedmont Basin; ELS, Epiligurian Basin. International Geology Review 25

exhumed subcontinental mantle rocks, intruded by gabbros and covered by extensional allochthons of upper continental crust, in turn topped by basaltic flows and pelagic sediments. This ocean–continent transition was adjacent to a wide area characterized by thinned continental crust. According to Marroni et al. (2001), the WEL and EEL basins were separated by a ridge consisting of a huge extensional allochton made up of upper continental crust, which can be compared with the AlKaPeCa microcontinent identified in the southernmost areas of the western Tethys by Michard et al. (2002). In contrast, the transition from continental to oceanic crust at the Europe/Corsica margin was sharp and characterized by escarpments induced by high-angle normal faults (Marroni and Pandolfi 2007). The architecture inherited form the rifting history remained unmodified until the inception of convergence in the Ligure-Piemontese basin, but, as shown below, it played a fundamental role during the convergence processes. Convergence possibly started in the Santonian–Campanian boundary, as suggested by the onset of sedimentation of tectonic- controlled deposits as turbidites and mélanges (Marroni et al. 1992). This interpretation fits very well with the 84 ± 5 Ma of the eclogite metamorphism detected in metaophiolites from Corsica by Sm/Nd analyses (Lahondere and Guerrot 1997) and that represents the oldest reliable age for the high pressure (HP), subduction-related metamorphism. Conver- gence led to the onset of intraoceanic subduction that located in close proximity of the ocean–continent transition towards the Adria plate. Some considerations on the dipping of the subduction are crucial to complete this scenario, even if this is still a matter of strong debate. Evidence of top-to-the-west shear sense during the underplating of the IL units have been identified during structural analyses by several authors (Hoogerduijn Strating E.H. 1994; Marroni and Pandolfi 1996; Marroni et al. 2004). However, the strongest constraints are provided in Corsica Island by the Tenda Massif, which represents a fragment of the Europe/Corsica continental margin subjected to HP metamorphism at the middle–late Eocene boundary and associated with top-to-the-west shear zones (Molli et al. 2006). It is important to outline that HP metamorphism detected in the Adria continental margin developed only since the late Oligocene– early Miocene, i.e. more than 20 Ma after the HP metamorphism detected in the Europe/ Corsica continental margin (Brunet et al. 2000). Thus, all the structural and radiometric data seem to be coherent with a Late Cretaceous scenario dominated by an east-dipping (‘alpine’) subduction where the role of lower plate was played by the Europe/Corsica margin. This reconstruction is coherent with the data reported by Peccerillo et al. (2001), Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 which provide evidence for an earliest metasomatic modification of the present-day Tus- can mantle as a result of the ‘alpine’ subduction. Therefore, during the Late Cretaceous–Palae- ocene, the IL basin was progressively shortened and consumed by subduction of oceanic lithosphere, whereas the EL basin was characterized by the continuous, monotonous sedimentation of the carbonate turbidites. In this frame, the EL basin can be coherently placed in a suprasubduction basin, located between the accretionary prism and the Adria continental margin. However, a tectonic pulse, testified by the sedimentation of sedimentary mélanges, affected also the margin of this basin at the Santonian–Campanian boundary, possibly representing the inception of oceanic subduction. In our reconstruction, the origin of the sedimentary mélanges owing to transpressive tectonics is adopted, according to the model proposed by Elter and Marroni (1991), Pandolfi (1997) and Marroni and Treves (1998). A few other considerations help a more detailed timing of the subduction evolution, because they allow estimation of the time of oceanic lithosphere total consumption by subduction and that of the involvement of continental crust into the system. The youngest preserved 26 M. Marroni et al.

sedimentary cover of the oceanic lithosphere identified along the whole alpine–apennine belt is represented by the IL units, where the Gottero unit shows a top of the sedimentary cover of the ophiolite sequence of early Palaeocene age (Marroni and Pandolfi 1996). Thus, the Gottero unit probably represented the last portion of the oceanic lithosphere of the Ligure-Piemontese basin subjected to underthrusting in the subduction zone. On the other hand, the continental units affected by HP metamorphism from Corsica Island provide valuable insights also for the early–middle Eocene time span. As previously reported, the Tenda Massif was affected by HP metamorphism with an 40Ar/39Ar age of 34.9 ± 0.4 Ma (middle–late Eocene boundary, Brunet et al. 2000). In addition, an 40Ar/39Ar age of 46.7 ± 0.6 Ma (early Eocene boundary, Brunet et al. 2000) has been found on the continental slices with eclogite metamorphism enclosed in the metaophiolites of Corsica. Therefore, this east-dipping subduction, probably intraoceanic until the Palaeocene– Eocene boundary, involved subsequently the continental crust of the Europe/Corsica plate. Thus, in the early Eocene, the underthrusting of the Europe/Corsica continental crust below the alpine accretionary prism, represented by the ophiolites showing HP metamorphism today preserved in Corsica Island and in the Northern Apennines, can be envisaged. In the same time span, the suprasubduction basin corresponding to the EL domain was still undeformed. The lack of subduction-related magmatism during the Late Cretaceous–middle Eocene time span has been explained by Marroni and Treves (1998) as owing to shallow dip angle and slow rate of subduction. In the middle Eocene, a major tectonic event occurred in the Ligure-Piemontese basin. The east-dipping subduction stopped, probably as a consequence of the involvement of the thick continental crust of Europe/Corsica plate in the subduction zone. The architecture of the Europe/Corsica margin inherited from the rifting process, i.e. a sharp transition from oceanic crust to thick continental crust, is coherent with this interpretation. As a consequence of the stop in the ‘alpine’ subduction, the convergence migrated to the EL basin. In our reconstruction, the first result of the convergence in the EL basin is a large westward displacement of the EEL units, as testified by the present-day tectonic setting of the Antola unit over the IL units, i.e. the former alpine accretionary prism. The west-verging tectonics was followed by east-verging deformations connected with the underthrusting of the EL units below the alpine accretionary prism. The resulting structure was a triangular, crocodile-like zone, where the EL units were thrust either below or over the alpine accretionary prism, whose upper structural levels are today represented by the IL units. A similar structure is also today recognized along the seismic profiles of the Downloaded By: [University of Stellenbosch] At: 12:13 26 April 2010 western (Biella et al. 1997) and central (Lüschen et al. 2004) Alps. According to that proposed in the model by Carminati et al. (2004), our model envisages, at Eocene time, also the break-off of the alpine slab and the inception of the west- dipping ‘apennine’ subduction. The subduction along the northernmost areas of the Ligure- Piemontese basin was probably driven by the west-dipping subduction active in the southernmost areas, where a true, wide oceanic area is hypothesized to have been consumed by several geodynamic reconstructions (e.g. Figure 8 of Molli 2008 and quoted references). In addition, the subduction in the EL basin was favoured by the characteristics of its crustal structure inherited by rifting history, i.e. a wide area, very easily subducted because it was characterized by subcontinental mantle covered by thinned continental crust. Thus, in the late Eocene, two different paired structures with different tectonic origins were coupled: (i) the alpine accretionary prism (IL units), where in its western margin the contractional deformation was substituted by the extensional tectonics in the early Oli- gocene and (ii) the proto-Apennine belt (EL units), derived from the subduction of the thinned Adria continental margin. The structures resulting from this series of events are all International Geology Review 27

sealed by the deposits of Epiligurian (middle Eocene to Tortonian) in the E and the Tertiary Piemontese (late Eocene to Tortonian) basins in the W. The proposed model fits well with a sharp occurrence of calcalkaline magmatism in the Adria plate in the early Oligocene (Mattioli et al. 2002), whose remnants are today preserved in the Subligurian units (Elter et al. 1997). This picture is also confirmed by the interpretation of the early Oligocene Aveto Formation as the first, true foredeep deposit related to eastward migration of the compressive front related to west-dipping subduction (Elter et al. 1999; Catanzariti et al. 2003). The subsequent tectonic history of the Ligurian units is achieved during their thrusting onto the Subligurian and Tuscan units in the Oligocene–Miocene time span and the subsequent extensional tectonics, related to the eastward progressive migration of the compressive front, continuous from middle Eocene up to Quaternary time. During this evolution, the structural setting achieved during the Late Cretaceous–middle Eocene by the Ligurian units was only weakly modified and the upper structural levels were still well preserved as a consequence of the very low exhumation rate of the Northern Apennines belt.

Acknowledgements This research was supported by MIUR (Project PRIN), by CNR (Istituto di Geoscienze e Georisorse, Unità Operativa di Pisa), by funds ATENEO grant by Pisa University and by Galileo Programme 2008/2009. All the authors are indebted to Professor Piero Elter, who introduced all the authors to the mystery of the Northern Apennines geology.

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