Stratigraphic Evolution in the Ligurian Alps Between Variscan Heritages and the Alpine Tethys Opening: a Review

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Earth-Science Reviews 125 (2013) 43–68

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Stratigraphic evolution in the Ligurian Alps between Variscan heritages and the Alpine Tethys opening: A review

A. Decarlis a,⁎, G. Dallagiovanna b, A. Lualdi b, M. Maino b, S. Seno b a IPGS – EOST, Université de Strasbourg, CNRS UMR 7516; 1, rue Blessig F-67084, Strasbourg Cedex, France b Università degli Studi di Pavia, Dipartimento di Scienze della Terra e dell'Ambiente, Via Ferrata 1, 27100 Pavia, Italy article info abstract

Article history: The Ligurian Alps are the southernmost segment of the Alpine orogenic belt. Their formation is the result of a Received 11 September 2012 multistage process that comprises two orogenic cycles: the Variscan and the Alpine one. Since the Late Accepted 4 July 2013 Paleozoic onwards, superposed sedimentary successions deposited in this area, as a response to the changing Available online 12 July 2013 tectonic setting in the frame of the Western Mediterranean evolution. The transition between the two orogenies was characterized by the formation of a net of sedimentary basins controlled by diffused extension, which is not Keywords: correlated to a clear geodynamic context yet. As the fault network is severely affected by Alpine reworking, the Ligurian Alps Prepiedmont multiple pre-Alpine tectonic stages experienced by the Ligurian sector can be unravelled only by the analysis of Briançonnais the well exposed stratigraphic succession, which offers very distinctive feature for each tectonic phase. Geodynamics From the present day situation of a nappe-pile orogen backward to the originary basinal setting, the aim of Central Mediterranean this work is to reconstruct the paleogeographic evolution of the Ligurian Alps between Permian and Jurassic, through a detailed stratigraphic analysis. Several sedimentary packages belonging to different tectonic units are analysed, grouped into specific domains and correlated. Five major steps in the evolution of the Ligurian Alps have been pointed out; they have been contextualized within the latest paleogeographic reconstructions of the Alpine sector, contributing to detail the role of the study area in the plate tectonic dynamics. The sedimentary record is referred to successive geodynamic stages, from the Pangea break-up and Variscan belt dismantling, through a Triassic diffused extension, to the Alpine Tethys rifting and finally to the spreading that generated the Piedmont–Ligurian oceanic branch. The stratigraphic reconstruction of the Ligurian sector also indicates the lacking of an ocean interposed between the European continent and the Alpine collisional wedge, thus representing the southward termination of the Valais basin. On the whole, the paleogeographic reconstructions provided in this work highlight that the Ligurian Alps were a domain in which for over a hundred million years orogenesis, rifting and oceanisation strongly affected the integrity of the upper crust. The heterogeneity of the crust that suffered multiple mantle uplifts, partial melting and extensive faulting over a long period is testified by field evidences and several effusive events. The last and most important rifting and the following Alpine Tethys oceanisation developed around a narrow, elongated area of crustal weakness generated by multiple geodynamic events. © 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction and geological setting ...... 44 2. Alpine structure and evolution ...... 45 3. Paleogeographic reconstruction ...... 48 4. Stratigraphy ...... 48 4.1. Permian ...... 48 4.2. Uppermost Permian–Lower Triassic ...... 52 4.3. Middle Triassic ...... 52 4.3.1. Costa Losera Formation (CLO; ) ...... 52 4.3.2. San Pietro dei Monti Dolomite (SPMD; ) ...... 52 4.4. Upper Triassic ...... 54

⁎ Corresponding author. Tel.: +33 368850427; fax: +33 368850402. E-mail address: [email protected] (A. Decarlis).

4.4.1. Capo Noli Formation (CNF; ) ...... 54 4.4.2. Rocca Prione Formation (RPF; ) ...... 54 4.4.3. Monte Arena Dolomite (MAD; ) ...... 54 4.4.4. Veravo Limestone (VL; ) ...... 54 4.5. Jurassic ...... 54 4.5.1. Siderolitico (SID) ...... 55 4.5.2. Rocca Livernà Limestone (RLL; ) ...... 55 4.5.3. Monte Galero Breccia (MGB; ) ...... 57 4.5.4. Montaldo calcschists (MCS; ) ...... 57 4.5.5. Nasino sandstone (NS; ) ...... 57 4.5.6. Arnasco Radiolarite (AR; ) ...... 57 4.5.7. Rio di Nava Limestone (RNL; ) ...... 57 4.5.8. Val Tanarello Limestone (VTL; ) ...... 57 5. Syn-sedimentary tectonics ...... 58 5.1. Permian ...... 58 5.2. Triassic ...... 58 5.3. Jurassic ...... 59 5.4. Paleomargin orientation ...... 59 6. Major events recorded in the tectonic evolution ...... 59 6.1. Events during the Permian period ...... 59 6.2. Middle Triassic events ...... 61 6.3. Upper Triassic–Lower Jurassic events ...... 61 6.3.1. First rifting phase ...... 61 6.3.2. Second rifting phase ...... 61 6.3.3. Third rifting phase and post-rift ...... 63 6.3.4. Paleogeography remarks ...... 65 7. Conclusion ...... 65 References ...... 66

1. Introduction and geological setting In the southern western Alps, at least four different domains are preserved: The renewed interest that has arisen around passive rifted margins leads us to reconsider the stratigraphy and geodynamics of (1) The Eastern Provençe domain representing the more external this peculiar and misunderstood sector of the Italian Alpine belt. areas with respect to the chain, mainly outcropping in France. The Western Italian Alps comprise one of the most extensively stud- It has been interpreted to derive from the proximal European ied areas in geological sciences, but the Ligurian Alps, representing a Tethyan passive rifted margin. At present it is well-preserved large part of the western belt, have been left out from recent debates due to the scarce involvement in the later Alpine build up. and are rarely cited in papers on pre-collisional geodynamic recon- (2) The Briançonnais domain is a debated key-area for the Tethyan structions. In our opinion this is due to the scarcity of international paleogeography and rift dynamics. It is formed by a basement literature on this Alpine sector, and to the widespread metamorphism and by a volcanic and sedimentary cover whose details are and tectonic displacement affecting almost all of the sedimentary presented in this paper. It is characterized by a long-lasting covers, discouraging field research. Nevertheless, once the complex depositional gap in the sedimentary succession from the puzzle of tectonic units has been restored, the Ligurian Alps represent Late-Middle Triassic to the Early–Middle Jurassic that records one of the best places to observe a complete section of a Mesozoic a peculiar isostatic history, contrasting with that of the Europe- passive margin. Furthermore, the preserved geological record per- an passive margin (Dauphinois), with the Adriatic Margin tains to two distinct orogenic events: the late Variscan and the Alpine, (Austroalpine and Southalpine) and with oceanic Tethyan documenting a complete Wilson cycle. basin realms (Piedmont–Ligurian). The latter, in fact, preserve The European margin of the Alpine Tethys is now partly preserved a quasi-complete succession with less sedimentary gaps. The in the Western Alps as a nappe-pile accreted during the Eocene Briançonnais has classically been interpreted as an “anoma- convergence. The distribution of tectonic slices in the present-day lous” domain placed between the proximal and the distal belt reflects their original pre-collisional geographic pertinence; part of the European passive margin of the Tethys ocean, suf- thus the integrated study of their geometric position (tectonic analy- fering a substantial uplift that finally led to the formation of sis) inside the chain structure with the stratigraphy (facies analysis) two conjugate shoulders in the Early to Middle Jurassic rifting allows to distinguish the various “Alpine domains” (Fig. 1). These stage (e.g. reconstruction in Lemoine et al., 1986). Briançonnais are considered areas with homogeneous stratigraphic features each in these reconstructions represented a wider shoulder than one with their own paleogeographic behaviour leading to the inter- the Southern margin one, more narrow and localized (the pretation of the various steps of the evolution of the Alpine area, Canavese Domain in Stampfli et al., 1991). Stampfli (1993) from the Mesozoic rifting stage to the Cenozoic collision. It is impor- proposed that the Briançonnais was formerly a part of the Te- tant to note that the “domain creation” has been originally modelled thys margin belonging to the Iberian Plate. It became detached in a functional way to the Mesozoic-onward history summarizing the from it and from the Europe by the formation of another key-events for the Alps s.s. However, some of these subdivisions still oceanic branch with Atlantic affinity, the Valais Ocean, from remain valid also for pre-Mesozoic times, suggesting how the scenar- the Cretaceous onwards (for more details, see below). Finally io in which the Alpine cycle begins is connected with the latest part of it shifted to the actual position by the later eastward drifting the preceding orogenesis: the uplift and dismantling of the Variscan of the Iberian plate. It results in a duplication of the European chain. passive margin. An early model has been proposed by Lavier A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 45

tic ve el H

Valais is o c in ti h e p lv u e is H na a - on D ra nc lt ria U -B n ub ia S r e u n ig i Study area L lp t- a s n o i r ? a o e t n in s l n m lp a o d a u c e o A n c i tr e n s d v ) P l t u o a n a i n A r n r l r a P a o t . te B n n . r m L x e e o E E t d n e n I i m i p d p

e e l r i

P a ( P

e h t s

e u v o

a ? ? S

? a C Europe Adria 30 km 500 km

Fig. 1. Hypotetichal paleogeographic scheme for the Western and Central Alps during The Upper Jurassic. Modiﬁed from Vanossi (1991).

and Manatschal (2006), interpreting the Briançonnais as a separating the Briançonnais and the Dauphinois domains (see discus- “hanging wall block” (H block) formed between Europe and sion below). In addition, the Ligurian sector is also lacking a pre- Adria during Jurassic rifting. The results from their numerical Eocene Subbriançonnais section. This last is the domain classically modelling predict the behaviour and peculiar significance of interpreted to be the transition between the Briançonnais block and this domain in the dynamics of rifting. In the Ligurian Alps the Valais, and it is characterized by peculiar sedimentary succes- the Briançonnais is formerly divided into an internal, an inter- sions. The Subbriançonnais domain thins out toward the south and mediate and an external area, which experienced different crops out in the Ligurian Alps only as an enigmatic Eocene Flysch sedimentary gaps. This subdivision also corresponds to their (Flysch Noir) or as a tectonic slice lying below the Helminthoid Flysch paleogeographic position (Fig. 2C). nappe (Flysch di Bajardo, Eocene Sup-Oligocene?; Vanossi, 1991). (3) The Prepiedmont domain, close to the eastern part of the Briançonnais, that formed the Alpine Tethys distal margin It 2. Alpine structure and evolution is formed by basement and by a complete sedimentary succession from Middle Triassic to Cretaceous. It represents the tran- The Ligurian Alps are the NW–SE oriented southernmost segment sition to the Ophiolite bearing units (classically interpreted as of the Alpine orogenic belt (Fig. 2). They are made up of a nappe the remnants of the oceanic realm s.s.) and it fully differenti- pile constituted by, from top to bottom, the Piedmont–Ligurian, ates from the Briançonnais from the Late Triassic onwards. Prepiedmont and Briançonnais tectonic units, all stacked onto the Since that time, deepening upward carbonate sedimentation Dauphinois domain (Fig. 3; Vanossi et al., 1986). recorded increasing tectonic subsidence due to the concentra- The present-day architecture of the Ligurian Alps is the result of tion of extensional activity close to central rift axis. the relative movements of three main plates (Africa, Adria and (4) The Piedmont–Ligurian domain is formed by the post-Middle Europe) and several interposed oceanic basins, which led to the Jurassic ophiolite bearing nappes and “Helminthoid Flysch formation of the Alpine belt. nappes”. The former represent the remnants of the “Oceanic Since the Cretaceous, as a consequence of the SE–NW directed con- floor” and of its sedimentary covers that escaped subduction. vergence between the European and Adria plates (Schmid and Kissling, The latter are thick portions of the Cretaceous carbonate flysch 2000; Handy et al., 2010), the Piedmont–Ligurian Ocean was consumed deposited in the Tethys Ocean during the Late Cretaceous and in a south-east to south directed subduction zone (Stampfliand that outcrops over wide areas, ever detached from their ocean- Marchant, 1997). Large sectors of the Piedmont–Ligurian oceanic do- ic crust substratum. main and the European continental margin (Prepiedmont basements and Internal Briançonnais) were gradually involved in the subduction The main object of this paper is to review the stratigraphic and channel between Adria and Europe plates (e.g. Schmid et al., 1996; geodynamic evolution of the Briançonnais and Prepiedmont domains Stampfli and Marchant, 1997; Beltrando et al., 2007b). Geochronological in the Ligurian Alps: that may be also useful to integrate the recent and metamorphic data from Western Alps indicate that high-pressure rifting models and paleogeography in the western Mediterranean metamorphism propagated from the internal (SE) orogenic zones (the area. It is worth considering that one of the most debated argument oceanic domain) towards the outer (NW) ones (the European base- in the pre-collisional evolution of the Alpine domains is whether ment) until ca. 35 Ma (Schmid et al., 1996; Rosenbaum and Lister, the Valais realm exists as a true oceanic branch and if it continues 2005; Berger and Bousquet, 2008; Bousquet et al., 2008 cum ref.). This toward the south (Fig. 1; for a complete review see de Graciansky et forelandward shift of the subduction zone during progressive accretion al., 2011). This domain has been emphasized in the main paleogeo- of the overriding plate is also recorded in the Ligurian Alps: the oceanic graphic reconstructions as a true oceanic branch of Cretaceous age units (now represented by the Voltri Massif and the Montenotte unit, lying at the back of the Briançonnais domain and directly connected Vanossi et al., 1986) experienced eclogite–blueschist facies metamor- to the opening of Bay of Biscay (e.g. Handy et al., 2010 cum ref.). phism between 49 and 40 Ma (Federico et al., 2005). As indicated by In the study area, there is no evidence about an oceanic through the Lutetian age (~48–40 Ma) of the syn-orogenic sediments (Cabella 46 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68

B Ceva 4877 Cuneo

9 8

5 Savona 4876 4

2 4875 1 6 3 Tenda 7

4874 A

CH France 4873 Italy

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4872

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C margin Distal H e lm in th gin ianco o l mar astelb i oxima rnasco-C d Pr io-C. A f lvecch ly Caste s taldo c Mon h rto a C. Tube Orme llare Ma tta a ato-M. M.So Armett Pampar N E IGURIA a L s armo MONT te M.C PIED r ONT n l EPIEDM - terna PR P In ro v e n iddle c M its a IS ent un l ONNA basem nal RIANC ntiated Exter B undiffere N

Fig. 2. A) Location of the study area; B) sketch of the Ligurian Alps showing the tectonic units locations. Legend of colours in panel c; stars indicate the location of stratigraphic sections of Fig. 3: (1) Ormea, (2) Caprauna–Armetta, (3) Castelvecchio Cerisola, (4) Mallare, (5) Pamparato–Murialdo, (6) Case Tuberto, (7) Arnasco–Castelbianco, (8) Monte Sotta, (9) Montalto. C) Scheme of main tectonic units of Ligurian Alps and of their juxtaposition in the Late Jurassic. After Vanossi (1974b, 1991). et al., 1991; Dallagiovanna, 1995), the blueschists Ligurian Internal Arnasco Castelbianco, Monte Sotta; Fig. 1)escapedthesubduction, Briançonnais units (i.e. Pamparato–Murialdo, Mallare and Internal recording a low-grade metamorphism (anchimetamorphism to Ormea units; Figs.1,3) were involved in the subduction later than the greenschist facies). oceanic rocks. In contrast, the External Briançonnais (Monte Carmo and The onset of metamorphism corresponds to the NW-ward External Ormea units; Fig. 1) and the Prepiedmont units (Case Tuberto, obduction of part of the oceanic accretionary wedge (the Piedmont– A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 47 A B Prepiedmont L. Briançonnais Duplex Units Tertiary (Internal Units) Piedmont Ligurian Apennines Helminthoid Piedmont-Ligurian Basin Units Flysch Units Units Ligurian Sea Po Plain 0 0 Briançonnais (External Units) ? European ? -20 Crust Penninic -20 (Dauphinois) Front ADRIA -40 -40 Km a) Present-day 0 20 km 40 Km

L. Briançonnas Duplex Prepiedmont Piedmont-Ligurian Units Units (Internal Units) Tertiary Ligurian Units Piedmont of the Apennines Elminthoid Basin Flysch Units 0 0 Briançonnais (External Units)

-20 European -20 Crust Penninic (Dauphinois) Front ADRIA -40 -40 Km b) ~Early Oligocene 0 20 km 40 Km

L. Briançonnas Duplex Prepiedmont Helminthoid (Internal Units) cover Flysch Units Units 0 0

Dauphinois Briançonnais -20 (External Sector) -20 Prepiedmont basement Units ADRIA

-40 Piedmont-Ligurian -40 Km c) Lutetian-Bartonian oceanic crust Km

European continent European margin Alpine Thethis Piedmont-Ligurian Briançonnais Prepiedmont domain Dauphinois External sector Helminthoid Flysch 0 Internal sector 0

-20 oceanic crust -20

-40 d) Early Cretaceous 0 20 km 40 Km -40 Km B Legend: SL Po Plain Tertiary piedmont Basin (TPB) TPB DM Apennines units VMVG A Cuneo Genova External massifs Argentera Brianconnais and Prepiedmont Br-Pp units (Br-Pp) Ligurian Sea Internal massifs (DM: Dora Maira) HF Schistes lustres complex (SL) and Dauphinois ophiolitic rocks (VM: Voltri Massif)

N 0 50 Km A Helminthoid Flysch units (HF)

Fig. 3. Retrodeformed crustal cross-sections along the central traverse of the Ligurian Alps. A) Present day structure; B–D) main stages of the Alpine evolution of the Ligurian belt. From Bonini et al. (2010). 48 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68

Ligurian Helminthoid Flysch; Kerckhove, 1969; Merle and Brun, Alpine blueschist–greenschist metamorphism and a following counter- 1984) and with the deposition of syn-orogenic sediments into basin clockwise (CCW) rotation relative to Europe (values of 47 ± 13° in the developed onto the European foreland (Dauphinois domain). These Briançon–Guillestre area, Thomas et al., 1999; 68 ± 15° and 117 ± 19° deposits record a diachronous marine transgression that migrated to- in the Ubaye and Liguria regions, respectively, Collombet et al., 2002). ward NW during the Middle to Late Eocene (Ford et al., 1999). In the Paleomagnetic data from the Early Oligocene–Miocene sediments of whole of the Western Alps, the early thrust system propagation (D1 the TPB, unconformably resting upon the Ligurian Alps nappes, indicate phase) were mainly toward northwest (Malavieille et al., 1984; an ~50° CCW rotation dated at the Early–Middle Miocene times Dumont et al., 2011) but has been re-arranged by rotation during (Maffione et al., 2008). the Neogene bending of the arc (Collombet et al., 2002), resulting in From these data two different interpretations have been sug- the present-day radially directed structures. gested: (1) the regional ~50° CCW rotation of the TPB and Western At around 35 Ma the motion of the Adria plate changed from Alps, inducing the bending of the arc, occurred synchronously with NW-ward to WNW-ward (Schmid and Kissling, 2000; Handy et al., the Corsica–Sardinia drifting (Maffione et al., 2008); in this view the 2010). The Western Alps become a zone of frontal collision between significantly higher value from the Liguria samples can be attributed the Ivrea body (as frontal portion of Adria) and the European margin to local shear zones. (2) A second hypothesis considers a diachronous (Ford et al., 2006). In Liguria this Late Eocene–Early Oligocene phase rotation of the Western Alpine arc with respect the TPB. The Miocene is recorded by the Piedmont–Ligurian and Briançonnais units, which 50° CCW rotation of the TPB-Ligurian Alps occurred after a previous achieved retrogressive metamorphism during the last ductile defor- (Oligocene) bending of the Western Alps associated with Adria rota- mational phases (D2–D3; Maino et al., 2012a). tion and indentation (Dumont et al., 2011). In this case, subtracting The Late Oligocene–Miocene evolution of the Ligurian Alps is associ- the Miocene rotation, the remaining 70° CCW rotation from the Ligu- ated with the development of the Liguro-Provençal rifting. This rift ria samples, is in agreement with values from the other sector of the opened west of the Ligurian Alps at ~30 Ma (e.g. Séranne, 1999). During Western Alps. Unfortunately, a precise timing for the Briançonnais the Early–Middle Miocene, the oceanisation of the Liguro-Provençal zone rotation is lacking, thus preventing a definitive choice between basin and the consequent drifting of the Corsica–Sardinia block caused the two hypothesis. In the Section 4 (syn-sedimentary tectonics) we a 50° anticlockwise rotation of the Ligurian Alps (Vanossi et al., 1994; discuss the two possible restorations resulting from the application Maffione et al., 2008). This triggered transtensional and transpressional of either 50° or 115° CCW rotational values. brittle deformation (D4 and D5) including extensional faulting, long-wavelength open folds and northeast-ward thrusts (Maino et al., 4. Stratigraphy 2013). 4.1. Permian 3. Paleogeographic reconstruction The Late Paleozoic volcano-sedimentary successions of the Liguri- Each tectonic unit constituting the Ligurian belt displays a coher- an Alps rest unconformably upon basement rocks deformed and ent crustal segment of either continental or oceanic crust before Al- metamorphosed during the Variscan orogenesis (Cortesogno et al., pine orogeny and are separated from the other nappes by tectonic 1993; Gaggero et al., 2004; Maino et al., 2012b). These successions contacts developed during the Alpine deformation. In spite of the are well preserved in the Briançonnais units while they are poorly polyphase deformation, the Ligurian Alps units show a relatively or not represented in the Prepiedmont units where the Mesozoic well-preserved original stratigraphy especially in the Prepiedmont rocks are tectonically detached (Figs. 4, 5). The volcano-sedimentary and External Briançonnais units, which were generally affected by successions in the Briançonnais are highly variable in terms of both low-grade metamorphism (Vanossi et al., 1986). The distinctive Me- thickness and facies. A simplified model for the sedimentation and sozoic stratigraphy, and the abundance of easily recognizable distribution of the Permian deposits and volcanics is represented in lithostratigraphic markers, facilitates detailed analysis of the complex Fig. 6. The Early Permian Briançonnais successions show continental structural history. The restoration of the thrust sequences, folding and metasediments (from coarse-grained clastics of fluvial origin to fine faulting phases through a coherent kinematic model allows to deter- decantation lacustrine deposits) alternated with various calcalkaline mine the time–space trajectories of each units and thus their volcanic flows. pre-Alpine relative position (e.g. Tricart, 1984; Butler, 1986; The emplacement of large amount of K-alkaline volcanics in the Lemoine et al., 1986; Butler et al., 2006; Bonini et al., 2010). The re- Late Permian closed the Paleozoic cycle in the Ligurian Alps. The pas- construction of a regional paleogeography derives from the compari- sage from the first cycle characterized by Lower Permian calcalkaline son of the stratigraphic and tectonic evolution of the Ligurian sector volcanics to the second with Upper Permian K-alkaline supports the with other sections of the Western Alps (i.e. the Briançon area, existence of a Middle Permian gap. This last may be interpreted either France; see de Graciansky et al., 2011 cum ref.). as a stage of non-deposition/volcanism or as derived from a stage of This exercise has been approached in the Ligurian Alps since the vigorous erosion. '70s and definitively applied and detailed in the last 10 years The first sedimentary event recorded in the Ligurian Briançonnais suc- (e.g. Vanossi, 1971a; Vanossi et al., 1986; Seno et al., 2005a,b; cessions is represented by the deposition of the Lisio Formation (Li, Bonini et al., 2010; Maino et al., 2013). In the present work we Late-Carboniferous–Asselian?), composed of arkosic metarenites and adopt the reconstruction (Fig. 3) presented in the more recent papers metaconglomerates. It unconformably covers the orthogneiss Variscan (Seno et al., 2005a; Bonini et al., 2010; Maino et al., 2013), where sim- basement of the Internal Briançonnais units with a maximum thickness ilarities and differences of the Alpine tectonic evolution between the of 200 m. The post-Variscan volcanism begins with the emplacement Ligurian sector and the neighbouring Western Alps are highlighted of up to 100 m thick ignimbritic rhyolites/rhyodacites (Case Lisetto and thoroughly discussed. Metarhyolite, CLMr) onto Li or directly upon the metamorphic basement. One of the major difficulty encountered in the paleogeography They have been dated at 285.6 ±2.6 Ma (Sakmarian; Dallagiovanna et al., reconstruction, is the assessment of the rotational movement experi- 2009). These rhyolites are followed by thick (up to some hundreds of me- enced by the Ligurian Alps during the late orogenic stages. Paleomag- tres) continental metasediments (Ollano Fm.; OL). Some ignimbritic and netic studies from the Western Alpine arc and the Tertiary Piedmont pyroclastic agglomerates (the Osiglia Porphyres, OP, dated as 278 ± basin (TPB) present contradictory results (Collombet et al., 2002; 3.4 Ma, Artinskian; Dallagiovanna et al., 2009) are intercalated with Maffione et al., 2008). Data from the Briançonnais sedimentary cover these conglomeratic–sandy–pelitic fluvial–lacustrine deposits. The conti- document a magnetic overprint, subsequent to Eocene–Early Oligocene nental suite evolves towards fine-grained sediments now preserved as Briançonnais Prepiemont External Middle Internal

1-Ormea 2-Caprauna 3-Castelvecchio 4-Mallare 5-Pamparato 6-Case Tuberto 7-Arnasco 8-Monte 9-Montaldo Armetta Cerisola Murialdo Castelbianco Sotta VTL

PNQ VTL North South

Late BVR PNQ VTL VTL RLL BVR VL CLO MP2 Jurassic AR RNL VTL CVP MAD Middle SID SID PNQ RPF MGB SPMD MP1 SPMD CLO SPMD

CVP Jurassic

BVR Middle Late CLO VM RLL MCS .Dcri ta./ErhSineRves15(03 43 (2013) 125 Reviews Earth-Science / al. et Decarlis A. CVP CLO

Triassic PNQ EZ MP1 RLL PNQ PNQ RLL

Early Middle BVR Early VL VL VL BVR BVR MAD

Late MP2 MP1 MP1 MAD MP2 MAD

Middle SPMD

CLO Tr iassic

PNQ

MP1 Early Middle Late

EZ – VM 68 Legend: Permian 300 m Sedimentary gapDetritic limestone Rhyolite

Early Ol

Conglomerate Limestone Andesite 200

EZ Breccia Dolostone Undiff. Basement OP VM 100 Stratigraphic boundary Sandstone Radiolarite correlations OL Stratigraphic gaps Li Pelite Cherty limestone 0 correlations Pre-Permian

Fig. 4. Composite stratigraphic columns for the main stratigraphic units. The stratigraphic logs are chosen as representative for the most relevant tectonic units in Fig. 2A and C. Section location in Fig. 2B. Litostratigraphic units abbreviation in text. 49 50

External Briançonnais succession Internal Briançonnais succession Prepiedmont succession

Val Tanarello Limestone Late

Val Tanarello Limestone Nasino sandstone Thermal cooling Monte Galero breccia Arnasco radiolarite group Oceanization Rio di Nava Limestone Middle Margin collapse Jurassic

Rocca Livernà limestone Early

“External Briançonnais gap” Enhanced subsidence

(Tethys rifting s.s.) “Internal Briançonnais gap” 43 (2013) 125 Reviews Earth-Science / al. et Decarlis A. Tethys rifting+Middle Trias event) Veravo limestone

Monte Arena dolostone rifting

San Pietro dei Monti dolostone Back arc -Meliata rift Back arc -Meliata rift San Pietro dei Monti dolostone or crustal cooling or crustal cooling platforms platforms group Triassic group Costa Losera Fm. Costa Losera Fm. Costa Losera Fm. Ponte di Nava Quartzites Ponte di Nava Quartzites Ponte di Nava Quartzites M. Pianosa Formation Early Middle Late

Variscan late molasse group

M. Pianosa Formation – Late 68

Melogno Ph. 2

Melogno Ph. 1 Melogno Ph. 1

Middle Melogno Ph. 1

Permian Viola Sch.

Permian Viola Sch. Lisio Fm. Volcano-Metasedimentary succession Ollano Fm. Early

Lisio Fm.

Continental basement Pre- Permian

Fig. 5. Stratigraphic scheme of the Late Paleozoic–Mesozoic successions of the Ligurian Alps. The three most relevant sectors of the study area are reported, showing with their characteristic depositional gaps. Formations have been joined in groups and their geodynamic signiﬁcance is reported in the lateral columns. A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 51

E. Brianconnais

I. Brianconnais

Middle Permian unconformity

C° 0 E. Brianconnais 80 Man C° tl 00 e 11

I. Brianconnais F) Late Permian (Wuckiapingian)

E. Brianconnais

C° 800 Ma C° I. Brianconnais ntle 00 11

E) Early Permian (Kungurian)

C° 0 80 C° E. Brianconnais Ma 00 ntle 11

I. Brianconnais D) Early Permian (Artinskian-Kungurian) Cover r ec y c l in

E. Brianconnais

C° 0 80 C° Ma 00 I. Brianconnais ntle 11

C) Early Permian (Artinskian)

C° Ma 0 E. Brianconnais ntle 80

C° 00 11 I. Brianconnais B) Early Permian (Sakmarian)

Melogno Fm. 258.5 ± 2.8 Ma Ollano Fm. Melogno Fm. 272.7 ± 2.2 Ma Case Lisetto rhyolites 285.6z ± 2.6 Ma Eze Fm. Lisio Fm. M C° ant 0 le 80 Borda granodirites Murialdo-Viola Fm. ° 0 C 10 Osiglia Fm. 278 Ma 1 A) Early Permian? (Asselian?)

Fig. 6. Simplified paleoenvironmental–depositional models for the Paleozoic succession of the Ligurian Alps. 52 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 metamorphic schists (Murialdo Formation and Viola; MV) interbedded Quartzite and Case Valmarenca Pelite, occasionally interfingered with andesitic lavas and pyroclastites up to 250 m thick (Eze Formation). with carbonate shoreline and lagoon deposits. These events prelude the huge volume of calcalkaline rhyolitic to dacitic ignimbrites of the Melogno Porphyroids—Lithozones A, B and 4.3. Middle Triassic C (MP1). Nearly missing in the inner zones, they largely crop out in the central and external Briançonnais sectors with thickness Directly overlying the CVP or the PNQ, a ubiquitous carbonate exceeding 1000 m. These volcanites are dated at 272.7 ± 2.2 Ma platform developed on the siliciclastic succession. In the internal (Kungurian, Dallagiovanna et al., 2009). All the different Early Perm- Briançonnais units the Middle Triassic deposits have been eroded at ian volcanic and sedimentary products were emplaced in narrow least to their basal terms (Anisian). basins showing a great chemical or lithological variability (Cabella et During the Middle Triassic a shallow subsiding carbonate ramp al., 1988; Cortesogno et al., 1993). The Late Paleozoic igneous activity covered both the Briançonnais and the Prepiedmont units. It devel- ends with polychrome K-rhyolitic fine to medium-grained ignimbrites oped through three main stages (cycles in Baud and Megard-Galli, of the Melogno Porphyroids—Lithozone D (MP2; up to 150 m) dated at 1977; Megard-Galli and Baud, 1977; Lualdi, 1985): (1) growth of a 258.5 ± 2.8 Ma (Wuckiapingian, Dallagiovanna et al., 2009). Different- shallow carbonate platform with occasional detrital inputs, (2) em- ly from the Early Permian magmatic products, these volcanites are placement of a tidal flat succession, typically arranged in multiple widespread in all the Prepiedmont–Briançonnais units without signifi- shallowing-upward cycles, and (3) establishment of a dolomitic cant lateral variation of thickness. The K-rhyolites were coeval with monotonous succession of a hypersaline coastal plain. A lower the emplacement of rhyolite dikes cutting the Variscan base- lithostratigraphic unit of Anisian age, the Costa Losera Formation ment (260.2 ± 3.1: Maino et al., 2012b). The late Variscan volcano- (Lualdi and Bianchi, 1990; Decarlis and Lualdi, 2009), has been distin- sedimentary complex is unconformably followed by the uppermost guished from an upper one, the San Pietro dei Monti Dolomites, Permian–Lower Triassic conglomerates of the Verrucano Formation established by Vanossi (1969). (BVR). 4.3.1. Costa Losera Formation (CLO; Lualdi and Bianchi, 1990) The CLO Formation is formed by a succession about 40 m 4.2. Uppermost Permian–Lower Triassic (Caprauna Armetta, Castelvecchio Cerisola, Case Tuberto units) to 150 m thick (Ormea unit) of greyish, well-bedded and fine-grained The late Variscan Ligurian detritic succession, composed of polygen- limestones. Sericite, muscovite, chlorite and quartz grains constitute ic well-rounded conglomerates and interbedded green and violet the “marbres phylliteux” facies of the French authors, while intensive- schists (“Briançonnais Verrucano” Auct.: Uppermost Permian?–Lower ly burrowed layers form the “calcaires vermiculés” facies. Nodular Triassic?), ends with the deposition of the Ponte di Nava Quartzite chert, interpreted as the diagenetic replacement of sulphates (Lualdi (PNQ; Vanossi, 1969). This formation is composed of coarse-grained and Porro, 1988), is concentrated in well-defined and widespread grey quartzarenites and conglomerates with fining-upwards cycles; in levels. A few tephra layers intercalate in the succession and form a its lower part a coarser facies with rough bedding and lacking sedimen- distinctive horizon at the regional scale (Caby and Galli, 1964). tary structures dominates, while towards the top a finer lithofacies can The lower part of this unit is dated as Late Olenekian (?)–Lower be recognized. This last is composed of thinner beds of medium-to-fine Anisian by the rare occurrence of Meandrospira pusilla Ho (cf. Baud et quartzarenites interbedded with greenish pelites with chlorite, musco- al., 1971)andDadocrinus gracilis Buch associated with Glomospira sp., vite and sericite; low-angle cross-bedding and oscillation ripple marks Glomospirella sp., Isocrinus sp., Pentacrinus dubius Gold. In the middle can be found. The total thickness of Verrucano and Ponte di Nava and upper part, CLO contains Agathammina judicariensis Premoli Silva, Quartzite varies from about 90 m (Internal Briançonnais, Mallare Frondicularia sp., Calcitornella sp., ?Planiinvoluta sp., Tolypammina and Pamparato–Murialdo units) to 250 m (External-Intermediate gregaria Wendt, Tetractinella trigonella Schloth., Decurtella decurtata Gi- Briançonnais; see Figs. 4, 5). At the top, the Quartzite passes into differ- rard, Coenotyris vulgaris Schloth., Neritaria sp., Loxonema sp., Dadocrinus ent lithostratigraphic units according to the original paleogeographical gracilis Buch., Encrinus liliformis Mill., Isocrinus sp., Encrinus granulosus setting: in the Briançonnais domain it is transitionally followed by the Muenst., Encrinus pentacrinus Bronn, and Spirigera isseli Rovereto. In ankeritic green shales of the Case Valmarenca Pelite (CVP; about 15 m particular, the association Endothyranella wirzi koehn–zaninetti, thick), while in the Prepiedmont units thin-bedded, fine micaceous Trochammina almtalensis Koehn-Zaninetti, Glomospira densa (pàntic), calcarenites and calcsiltites have been unformally recognized. Primary and Meandrospira dinarica Kochansky-Devidé & Pàntic date the upper ankerite (Vanossi, 1974a; Lualdi and Porro, 1988) and thin calcareous part of the formation to the Upper Pelsonian–Illyrian (Upper Anisian; beds suggest a deposition along an arid coastal plain close to a carbon- Lualdi and Bianchi, 1990). The CLO could be compared to the St.- ate platform system. Triphon Formation of the classic Briançonnais domain of the Préalpes Vanossi (1969) suggested that the vacuolar structure of PNQ could Medianes (Megard-Galli and Baud, 1977) for its stratigraphic position be related to the dissolution of original gypsum cement. Furthermore, and depositional character (Decarlis and Lualdi, 2009). in the topmost Lower Triassic beds of the Briançonnais succession, a The depositional setting can be related to a system of a carbonate boundary layer with altered carbonate clasts has been recognized; and/or mixed siliciclastic tidal shelf (Fig. 7: 1) locally passing to they can be interpreted as remnants of a nearby shoreline. The pale- oxygen-depleted coastal lagoons (Fig. 7: 3); fine clastic inputs ontological content of PNQ does not allow precise dating (quartzites (Fig. 7: 2) testify to the erosion of exposed paleozoic terrains, are mostly azoic) and their attribution to the Lower Triassic is based remnants of the Variscan chain. on the stratigraphic position of similar clastics in the Alpine realm (Decarlis and Lualdi, 2009). 4.3.2. San Pietro dei Monti Dolomite (SPMD; Vanossi, 1969) The extremely rare occurrence of Estheria minuta (Schloth.) or The SPMD forms a thick carbonate succession (about 200 m) Myacites fassaensis (Bittn.) found in the Ponte di Nava Quartzite composed of massive to well-bedded grey dolostones and lime- (Mazzuoli and Issel, 1884; Zaccagna, 1887), and its stratigraphic position, stones. At the base, greenish tephra that indicates Early Ladinian vol- date it to the Lower Triassic. canic activity can be found locally (Caby and Galli, 1964; Cortesogno The Variscan Ligurian detritic succession suggests a wide spec- et al., 1982). It follows a succession of cyclically arranged carbonates trum of depositional environments in the marginal-marine domain, (“calcaires rubanées”, Megard-Galli and Baud, 1977) of about 150 m thick- from the alluvial fan to fan-delta of Briançonnais Verrucano, to coastal ness. They are formed by shallowing-upward cycles organized as follows: plain and shallow siliciclastic shelf deposits of the Ponte di Nava a subtidal part, with crinoidal mudstones/wackestones (average 50 cm), A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 53

Inner Inner

Outer 35 Outer

sea level sea 32 level 36

C 33 ontin Co 34 enta 37 ntine l ntal ** **

* L) Toarcian-Bathonian * I) Late Pliensbachian * *

Inner Inner

Outer Outer

sea l sea l evel 30 evel 26 29 31 28 Co ntine Contin ntal en tal ** * * G) Sinemurian * H) Early Pliensbachian * * *

Continental s.s Supratidal

Inner 20 Intertidal 21 Subtidal Outer 26 sea l evel 25 sea le 23 vel

27 24 C Con ontin 22 tine ent ntal al Tid al flat

Su btid al - T F) Hettangian fact Rhaetian ory ** E) **

Supratidal Chemical erosion Karst activity Supratidal Intertidal 10

16 17 Subtidal Intertidal - sea level + 18 11 Subtidal Vadose sea level Ph 19 12 reatic 13 14 Contine 15 ntal

Tid C al flat ontin ental Sa lty gr oun D) Norian Sub dwa tidal ter Tida C) Carnian l flat Sub tidal

Supratidal

Intertidal SABKHA 4 Subtidal

Intertidal

5 8 sea level 1 Subtidal 6 6 sea level 7 9 6 2 6 8 3 Tidal 6 flat Contin e 9 ntal Lagoo n fla t Tra nsitio nal Sub tida Ba Lag lT-M rrier oon facto Subtid Fla ry S al t B) Ladinian lope - Cfac A) Anisian tory

Fig. 7. Schematic paleoenvironmental models from Middle Triassic to Middle Jurassic compiled for sedimentary facies of the Ligurian Alps. Models show the organization of facies along an idealized coast sector (not to scale) during the development of different platforms. * indicates lack of field evidence (mainly due to tectonics). See text for facies numbers. 54 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 an intertidal part with flaser lenticular wackestones and oolitic This formation is locally affected by such pervasive tectonics that grainstones (30 cm) and whitish dolomitic mudstones with gypsum it was interpreted as a decollement cataclasite since the early 1960s pseudomorphs, topped by thin reddish pelites (30 cm; supratidal). by some authors (Debelmas and Lemoine, 1963; Vanossi, 1971a). These cycles are interrupted by several major emersion events and Later, certain primary depositional features (i.e. thin argillitic inter- others related to evaporitic diagenesis, evidenced by metre-thick beds) were identified, testifying to the sedimentary nature of this autoclastic breccias and gypsum–anhydrite pseudomorphs. unit (Jeanbourquin and Lualdi, 1994). Owing to its stratigraphic posi- The upper part of the SPMD is formed by massive-bedded grey do- tion, the formation has been ascribed to Carnian s.l.(Boni et al., 1971). lomites composed of dolomitic wackestones with Diplopora annulata The RPF can be related to a hypersaline coastal flat with sulphates Schafht., D. uniserialis Pia. and Costatoria goldfussi (Alberti). leading to an alluvial plain with lacustrine facies (Fig. 7:12–13). This unit has been ascribed to the Ladinian (Vanossi, 1969; Decarlis and Lualdi, 2009) by the occurrence of the above-cited 4.4.3. Monte Arena Dolomite (MAD; Boni et al., 1971) fauna and the abundance of Encrinus liliformis Mill. near the base. The base of the unit is formed by a monogenic dolomitic breccia with The suggested depositional environment is a tidal flat with wide laminated clasts filling large pockets (Monte Nero and Monte Croce (hundred km2) supra-intertidal evaporitic areas (Fig. 7: 4 coastal Breccia; thickness 3–10 m; Vanossi, 1971b). It follows whitish, ponds, 5 shallow subtidal). In the Monte Carmo unit, a barrier–island coarsely-crystallized dolostones arranged in thick beds. Algal stromato- complex formed by the emergence of sandy/oolitic bars (Fig. 7:6) lites are frequent, as well as emersion surfaces that become abundant in rims the inner platform. Intra-bar areas were dominated by faunas the uppermost part of the unit. Thickness can be generally estimated at in response to open or restricted water circulation (Fig. 7: 7 crinoidal about 300 m with local variation (about 70 m in Case Tuberto). The oc- flat, 8 dasycladacean flat). The outer platform passed to a currence of Worthenia contabulata (Costa) dates the MAD to the Norian. distally-steepened ramp dominated by slumps and by detrital input Glomospira spp. and Glomospirella spp. are commonly found. from the islands (Figs. 7, 9). Deposition was cyclic with peritidal rhythms being largely algal-mat dominated (Fig. 7: 18). Towards the top, long-lasting expo- 4.4. Upper Triassic sure with caliches (Fig. 7: 16) alternated with subtidal units (Fig. 7: 17). As a response to a progressive climate humidification, the top- – The Upper Triassic represents a key moment in the Ligurian platform most metres (7 9) of the succession are composed of dark micritic evolution: during the Late Ladinian (or perhaps the Carnian p.p.)thede- dolostones with ostracods enriched in a clayey matrix of residual “ ” position in most of the Ligurian Briançonnais ended due to the emer- origin ( Unità dolomitica di transizione , Lualdi, 1983; Fig. 7: 19). gence of the platform. Chemical/physical weathering just probably begun from this time onwards and till the Liassic (and possibly the 4.4.4. Veravo Limestone (VL; Boni et al., 1971) Dogger), possibly in a generalized geodynamic-related uplift during This formation contains peritidal cycles composed of grey limestones which widespread paleokarst are created “by karstic events during a sin- capped by dark dolostones altered to yellow. A Retiophyllia clathrata gle long-lasting karst period” (“Siderolitico”, Decarlis and Lualdi, 2008). (Emm) boundstone divides a lower argillitic succession from an upper The erosion of the Ligurian Briançonnais platform acted progressively pure carbonate one. Dark argillites containing Rhaetavicula contorta in the different sectors, increasing from the external paleogeographic Portlok and Frondicularia woodwardi Howchin are interbedded with zones (i.e. geodynamic sensu) to the internal ones. The amount of erosion subtidal bioclastic limestones. Some peculiar high-energy lithofacies in the inner sectors affected the whole of the Middle and Lower Triassic have been found as clasts in the Jurassic Monte Galero breccias deposits (Vanossi et al., 1986), where successions were probably origi- (Dallagiovanna et al., 1984). They testify to the existence of oolitic bar ‘ ’ nally reduced, accentuating the sedimentary gap. In contrast, the complexes ( winnowed platform edge sands , sensu Wilson, 1975)and Prepiedmont units show the persistence of sedimentation, although foram-rich ramps. The total thickness of VL varies from a few metres – – – diastemic, during Carnian times. Four formations have been recognized: (6 10 m in Case Tuberto) to 30 50 m (Arnasco Castelbianco units). the Capo Noli Formation, the Rocca Prione Fm (Vanossi, 1971a) cropping The upper lithozone consists of burrowed mudstones, packstones and out mainly in the Case Tuberto unit, the Monte Arena Dolomite, and the grainstones topped by algal-laminated inter-supratidal dolostones with Veravo Limestone (Vanossi, 1971b). mudcrack surfaces. The faunal assemblage consists of: Rhaetina gregaria (Suess), Atreta intusstriata (Emm.), Cardita munita (Stopp.), C. cloacina (Quenst.), fam. Pleurotomariacea and Cerithiacea, Isocrinus sp., 4.4.1. Capo Noli Formation (CNF; Lualdi, 1991) Plegiocidaris sp., Triasina hantkeni (Majzon), Involutina sinuosa fi This unit is characterized by two main lithofacies: the rst pragsoides (Oberh.), Glomospirella friedli Kristan-Tollmann, Trochammina (lithofacies a) is composed of thinly-bedded dolomitic mudstones almtalensis Kohen-Zaninetti, Frondicularia woodwardi howchin, with anhydrite/gypsum pseudomorphs interlayered with black dolo- Agathammina austroalpina Kristan-Tollmann & Tollmann, Planiinvoluta ? – mitic schists (Ormea, Monte Carmo units: 8 10 m); lithofacies b is mesotriassica Baud, Zaninetti & Brönnimann, Trocholina sp., and made up of brownish dolomites interbedded with waxy dolomitic Ammodiscus sp. On the basis of the faunal association, the formation has – marls (Case Tuberto unit: 15 30 m). The CNF can be considered as been ascribed to the uppermost part of the Triassic (Rhaetian; Vanossi, “ the Ligurian counterpart of the Complexe schisto-dolomitique basal 1971b). ” “ des Ourdeis and Complexe bréchique inférieur et supérieur de The sedimentation took place on a tide-controlled shallow carbon- ” Clot-la-Cime (Briançonnais s.s.; Megard-Galli, 1972). An assemblage ate platform (Fig. 7: 21 intertidal, 22 restricted circulation “Rhaetavicula of Lamelliconus procerus (Liebus) and L. multispirus (Oberh.) indicates beds”, 23 subtidal) upon which widespread patch-reef developed a Carnian age (Lualdi, 1991). (mounds and carpets; Fig. 7: 24, 25). Black argillites with brachiopods A hypersaline coastal plain is the suggested sedimentation and selective faunas were deposited in the inter-supratidal ponds environment (Fig. 7: 14 intertidal, 15 subtidal). (Fig. 7:20).

4.4.2. Rocca Prione Formation (RPF; Boni et al., 1971) 4.5. Jurassic The RPF is composed of vuggy dolomitic breccias interbedded with dark-to-reddish limestones and yellow-reddish claystones. A major facies change observed in the Jurassic sediments marks a Thickness ranges between 0 and 80 m. In the Monte Sotta unit a renewed geodynamic trend. In the Prepiedmont domain, the drowning few metres of thinly-laminated, micaceous, dark siltites have been of the future distal margin produced large amounts of megabreccias found (“Schistes à Equisetum mytharum”; Bloch, 1958; Fig. 7: 10). (Monte Galero Breccia) lying on outer shelf carbonates (Rocca Livernà A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 55

A B

C D

E F

Fig. 8. (A, B) Small-scale extensional faults conﬁned to a single bed and (C, D) soft-sediment deformation (crumpled beds) in the Ladinian San Pietro dei Monti Dolomite, testifying to the widespread seismic activity that matches the development of Middle Triassic platforms. (E) The “Siderolitico” in its two typical facies: residual soil, and collapse breccia (Late Triassic?–Early Jurassic). (F) Synsedimentary extensional tectonic activity during the deposition of the top of the “Siderolitico” (Lias).

Limestone). They were followed by radiolarites (Arnasco Radiolarite) paleokarstification product of the host rock (partly in the vadose and basinal limestones (Menosio Limestone). zone). The resulting karst network locally penetrates the underlying In contrast, the Ligurian Briançonnais units were uplifted and Middle Triassic rocks for a few hundred metres (details in Decarlis and recorded a prolonged sedimentary gap giving karsts and residual Lualdi, 2008). Residual soils testify to a generalized non-deposition in red soils (Siderolitico), mainly developed during the Early Jurassic the Ligurian Briançonnais during apparently 50 Ma, with large-scale but possibly active since the Late Triassic. From the Bathonian, shelf emersion episodes (Fig. 7: 11). carbonates were widespread (Rio di Nava Limestone), followed by Due to the lack of paleontological data, the age of this unit is still a pelagic nodular limestones (Val Tanarello Limestone; cf. “Marbres de matter of debate. It is reasonable to assume that it was generated by Guillestre” of the French Authors). several karstic events during a single long-lasting karst period (sensu Bosak et al., 1989). The suggested age for most of the chemical 4.5.1. Siderolitico (SID) vs physical degradation (facies 1) is Late Triassic-to-Liassic (up to This informal unit is composed of two main facies: (1) red–green Upper Bajocian?; Decarlis and Lualdi, 2008). pelites and schists with chloritoid and haematite (Fig. 8E–F), and (2) Fe-oxide cemented breccias, black or green marls, filling erosional 4.5.2. Rocca Livernà Limestone (RLL; Boni et al., 1971) pockets and host-rock cavities. The first facies is considered to be a This unit isformed byathicksuccession(up to 600 m in Arnasco– physico-chemically degraded paleosol, while the second facies is a Castelbianco unit, few to 20 m in Case Tuberto) of well-bedded grey 56 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68

A B

C D

E F

Fig. 9. Development of the Alpine Tethys passive margin in the Ligurian Alps: (A) The beginning of the tectonic collapse that involved the Prepiedmont passive margin is testified to by small-scale normal faults affecting the Rhaetian platform roof (Veravo Limestones). (B) Early and Middle Lias hardgrounds testifying to episodes of tectonic quiescence and sedimentation on pelagic highs during the early phases of rifting. (C) Open-shelf sedimentation on the highly subsident margin during the Lias (Rocca Livernà cherty limestone). (D) Large amounts of carbonate breccia generated by the collapse of the Tethyan margin during late Lias–Early Dogger. Accumulation of breccia is fed by faults mainly affecting the Triassic successions (M. Galero Breccia). (E) Heterogenic breccia developed when the dismantling of the Triassic succession in the source area was almost complete and the Paleozoic units begin to be eroded. (F) The base of the Case Morteo Rhyodacite flow reworking the underlying breccia. limestones with banded cherts (Fig. 9C). Its lower boundary is It follows the Monte Pesalto Limestone member, a thick succession formed by an encrusted and bored condensed horizon containing of monotonous dark grey limestones with chert nodules and brightly ammonites belonging both to the Johnstoni (Caloceras johnstoni coloured lists. In its middle part, rhythmically arranged couplets of (Sow), C. torus (d'Orb.) and the Bucklandi zones (Conybeari subzone: mudstone/fine calcarenite mark frequent turbiditic inputs. In the up- Metophioceras conybeari (Sow); Cantaluppi and Lualdi, 1983; Lualdi, permost part of the member, a few pluri-metric breccia bodies indicate 1986, 2005). Most of the formation is composed of sponge spicule the first fault-fed mass-flows. An assemblage composed of Arietites mudstones/wackstones with occasional Nannobelus acutus (Mill) bucklandi (Sow.), Vermiceras spiratissimum (Quenst.), Metophioceras rostra. There are several clastic inputs at first, and then breccia sp., Epammonites sp., Lima costata (L.) and Involutina liassica (Jones) bodies are interbedded; they progressively increase towards the has been found in the lower part of this subunit. top. Recently (Decarlis and Lualdi, 2011) three members have been The Rio Morteo Limestone member completes the succession with identified. grey calcarenitic limestones (locally cross- and parallel-laminated) con- The Pizzo Ceresa Limestone member is composed of pure grey taining crinoids and bivalves. They are separated from the overlying micrites without cherts, and bioclastic calcarenites containing Lobothyris spiculitic limestones by thin Fe–Mg encrusted surfaces. cf. ovatissimaeformis Quenst., Calcirhynchia cf. rectemarginata (Emm.), On the whole, the base and the top of the formation are diasthemic, and Chlamys cf. valoniensis (Defr.). showing multiple condensed and omission surfaces, while the middle A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 57 member represents continuous sedimentation in a strongly subsiding (with well-bedded siltites at the top) can be recognized along 60 m basin. The age of the RLL can be precisely defined only at the base, as of total thickness. No fossils have been found within the NS; its strati- Early Hettangian. A late Lower Hettangian to Lower Sinemurian omis- graphic position indicates a Dogger s.l. age (possibly Early Dogger). sion (unconformity 1; Fig. 7: 26 open shelf, 27 hardground; Fig. 9B) The NS is interpreted as a fluxoturbidite complex deposited in the and then an Upper Sinemurian (unconformity 2; Fig. 7: 28 open shelf, progressively subsiding Piedmont basin and generated by reworked 29 hardground) omission are recorded by two distinct hardgrounds. deposits from the uplifted Briançonnais area. Sedimentation generally took place in an open-shelf environment (lower and middle member p.p.), with local pelagic highs. Some fine 4.5.6. Arnasco Radiolarite (AR; Boni et al., 1971) debris fluxes began to shade on the cherty carbonates of the outer The AR has been divided into three distinctive parts (Dallagiovanna platform (Fig. 7: 31, 30–32). Later on, sporadic mass flows were and Di Giulio,1984) from the bottom: grey-yellowish marly shale deposited on the lower part of a fault-dislocated ramp (Fig. 7:33 interbedded with coarse sands, red cherts and argillites and green and coarse carbonate breccia, 34 fine carbonate debris). The upper mem- red cherty argillites with radiolarians and spiculae. In this latter, small ber testifies to an evolution towards a crinoidal ramp occasionally carbonate clasts containing Calpionella sp., early Globigerinids, aptychi subjected to extrabasinal fluxes of fine oxidized terrigenous clastics. and Saccocoma sp.havebeenfound.Thisassemblageindicatesa The uppermost condensed and mineralized horizon of the Rio post-Kimmeridgian–Tithonian age for the uppermost layers. On the Morteo Limestone member has been related to the Pliensbachian– whole, the AR represents a condensed sedimentary sequence from Toarcian (Decarlis and Lualdi, 2011) on the basis of regional correlation. Dogger (Bathonian?) to Tithonian (average thickness: 10–50 m). Thus an Early Hettangian–Toarcian? age is proposed for this formation. The depositional environment was initially a deep shelf-to-basin ramp, still occasionally invaded by fine turbidites (1). It evolved to a 4.5.3. Monte Galero Breccia (MGB; Boni et al., 1971) true basinal plain (2) where settling of radiolarian mud prevailed, and This formation consists of a thick breccia succession (50 to 400 m) ended with an outer shelf complex (3) with argillites and less-evolved with coarse heterometric elements (from huge blocks to centimetric clastic flows (conglomerates and micaceous sandstones). The presence clasts) linked by carbonate cement (Fig. 9D–E). These are megabreccia of local interbedded rudites, with reddish limestones, granitic deposits, with hardly definable bedding and poor internal organization blocks and hardgrounds with Calpionella alpina Lorenz, indicates the (Dallagiovanna and Lualdi, 1986). Recently (Decarlis and Lualdi, 2011) occurrence of a still-unknown dismantling pelagic high. two main sedimentary bodies have been recognized in the Arnasco Castelbianco unit, both showing a similar internal compositional 4.5.7. Rio di Nava Limestone (RNL; Boni et al., 1971) evolution from clasts belonging to Mesozoic carbonate rocks (found at The RNL is composed of a thick succession (from 40 to 100 m in the the base of each body) to Paleozoic basement towards the top. Ormea unit) of well-bedded dark limestones with marly schists in the It is worth recording the presence of rhyodacite lavas and pyroclas- lower part; assemblages contain Alzonella cuvilleri Bernier & Neumann, tics forming thick layers that interfinger and are reworked within the Orbitammina elliptica f. A and f. B d'Arch, Mesoendothyra croatica gusic, top of breccias (Cortesogno et al., 1981; Fig. 7:37;Fig. 9F). The age of Valvulina lugeoni (Sepft), Bactroptyxis bacillus d'Orb., Fibuloptyxis sp., this unit, deduced from the stratigraphic position, is Late Liassic–Early and Melanioptyxis sp. A wide variability characterizes the emplacement Dogger. The MGB can be related to gravity-driven mass-flows of this unit, in which coral bafflestones (Montlivaltiidae, Thecoseris superimposed upon the RLL open shelf (Fig. 7: 36, 35). The selective schardti (Koby), Cladocoropsis mirabilis (Felix)) and sand-bar oolitic composition of clasts reflects the erosion of Ligurian successions from grainstones alternate with low-energy mudstones (Lualdi, 1994; their younger rocks to the older ones, caused by the activity of subma- Bertok, 2006). The transgressive base of this unit is evidenced by the rineand/orsubaerialtranstensionalfaultsystems(Vanossi et al., 1986). widespread unconformity and by the first clastic layers containing both Siderolitico and Triassic to Jurassic carbonate clasts (3–30 cm). 4.5.4. Montaldo calcschists (MCS; Dallagiovanna and Vanossi, 1987) These beds are followed by plurimetric plasticlast conglomerates indi- This informal unit has been recognized by Dallagiovanna and cating gravity-driven debris flows triggered by tectonic accommodation Vanossi (1987), Dallagiovanna (1993) and Vanossi (1991). It was of the basin. Bertok (2006) recorded seismites in the lower part of the originally created as a unique formation with three members but RNL. after a stratigraphic revision it can be more likely considered as a On the basis of the faunal contents, the age of the RNL can be fixed group linking three formational units. as Bathonian–?Callovian. The depositional environment was related The basal part of the succession is mainly composed of recrystallised to a diverse carbonate platform with both open circulation and limestones in thin beds, rarely interbedded with graphite-bearing restricted areas locally limited by coral patches and sand-bars. phyllades. The latter contain fine-grained quartz grains slightly decreas- ing toward the top of the succession and being replaced by an increas- 4.5.8. Val Tanarello Limestone (VTL; Boni et al., 1971) ing amount of mica. The successive calcschists are formed by The VTL consists of well-bedded light grey limestones for a total brown-yellowish metapelites with rare limestone beds. These beds thickness up to 100 m. Four lithofacies can be recognized, from the disappear toward the top, where they are replaced by decimetric bottom: calcarenite beds. The most peculiar character of this formation is the occurrence of interstratified olistolites of serpentinites and subordinately (1) a transgressive base (locally unconformable) with fine crinoidal of prasinites, of metre to hundreds metres size (Vanossi, 1991). The top calcarenites or whitish coarse quartzarenites can be observed of the formation is composed of recrystallized limestones, fine in the Ormea unit only; in the Deviglia Klippen (Dallagiovanna, gravity-driven carbonate deposits and lenses of carbonate breccias. 1993) the base of the VTL is generally represented by conglomer- The total thickness of the original formation is variable from 300 to ates and/or breccias, up to 10 m thick, with quartz clasts derived 400 m. The above cited authors dated MCS as dubitatively ranging from the underlying Variscan Ligurian molasse, and it rests upon from Lias to Cretaceous by stratigraphic position and by comparison PNQ or BVR. with analogous sections in the Prepiedmont domain. (2) Waxy light grey to pink limestones, with belemnites and Globigerina oxfordiana Grig. 4.5.5. Nasino sandstone (NS; Decarlis and Lualdi, 2011) (3) Grey detrital limestones passing to light-red nodular limestones This unit includes polygenic sandstones and subordinate micro- (“Marbres de Guillestre” of the French authors). conglomerates. They are composed of quartz, mica and carbonate grains (4) Light grey limestones in thicker beds with Lytoceras welded by a shaly micrite or carbonate cement. A fining-upwards trend subfimbriatum d'Orb, L. crebrisulcatum Uhl., L. strangulatum 58 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68

d'Orb., L. honnoratianum d'Orb., Costidiscus recticostatus d'Orb., Alpine deformation (D1 and D2) the WNW-striking faults were main- Neolissoceras grasi d'Orb., Desmoceras (Pusozia) lechicum Uhl., ly reactivated as fore- or back-thrusts, while the NNE-striking faults Haploceras sp., Holcostephanus astieri d'Orb., Duvalia lata Blainv., acted preferentially as lateral ramp or transfer faults (Bonini et al., D. emerici Rasp., Calpionella alpina Lorenz, Saccocoma sp. and 2010). The Alpine imprint prevents to directly define the original Globochaete alpina Lomb. kinematics of these faults but the general tectonic regime can be inferred from the geometry of the volcano-sedimentary bodies distrib- The age of the VTL is still a matter of debate in terms of both its uted within the Briançonnais and Prepiedmont units. Here, the boundaries. Concerning the base, a Callovian–Oxfordian hiatus was remnants of the Early Permian volcano-sedimentary sequence within suggested by Lanteaume (1968), while Lecanu et al. (1978) proposed fault-bounded basins suggest the development of transtensional a sedimentary gap limited to the Lower Callovian only. The top of the pull-apart structures (Cabella et al., 1988; Cortesogno et al., 1998), as formation is capped by a regional disconformity mostly associated occurred also in the adjacent Variscan sectors (e.g. Bruguier et al., with a mineralized Aptian–?Cenomanian hardground (Caron et al., 2003; Schaltegger and Brack, 2007; Ronchi et al., 2008; Cassinis et al., 1971; Lualdi et al., 1989). According to French and Italian authors, 2012; Casini et al., 2012; Gretter et al., 2013). the uppermost VTL beds are heterochronous (Faure-Muret and After an ~10–15 Ma gap in the magmatic and sedimentary activi- Fallot, 1955; Vanossi, 1963; Fallot in Lanteaume, 1968; Rioult and ty, in the Late Permian, alkaline ignimbrites were outpoured in the Royant, 1975; Lualdi et al., 1989), ranging from the Kimmeridgian whole of the Briançonnais and Prepiedmont domains (Cabella et al., to the Barremian. 1988; Cortesogno et al., 1993). In our opinion, the VTL ranges from Upper Callovian/Lower Oxfordian Differently from the Early Permian deposits, these volcanites were (for the presence of Hibolites Mayer; lithofacies (2)) to the Early emplaced in wide basins approximately corresponding to the actual Cretaceous p.p. (ammonite fauna in Faure-Muret and Fallot, 1955; Alpine domains (or sub-domains). Within each preserved basin, the lithofacies (4)). Late Permian volcanites are relatively homogeneous, whereas they The depositional environment can be related to an open platform show abrupt thickness increase from 40 m to 100 m and 150 m gradually inundated by pelagic seas associated with the nodular beds. moving from the Internal to External Briançonnais, respectively Toward the top of the Formation, a regressive trend marks the return (Cortesogno et al., 1993). This distribution suggests that the basins to an open shelf-ramp setting preceding the Aptian–?Cenomanian were separated by faults, presently represented by WNW–ESE or sedimentary gap. NW–SE directed Alpine thrusts. These lineaments have been interpreted as the original faults that facilitated the raise of hot mate- 5. Syn-sedimentary tectonics rials from the mantle, which was the source of the Late Permian alkaline volcanites (Dallagiovanna et al., 2009). Pre-Alpine structures have been reconstructed in different kinematic models through the comparison of the stratigraphic succes- 5.2. Triassic sions and the restoration of the Alpine thrust sequences (e.g. Tricart, 1984; Butler, 1986; Lemoine et al., 1986; Butler et al., 2006; Bonini The uppermost Permian–Early Triassic Verrucano sediments were et al., 2010). Large part of the pre-alpine faults experienced polyphase deposited with increasing thickness from the inner (80 m, Internal reactivation during the Alpine phases. During the Eocene early ductile Briançonnais) toward the outer zone (250 m, External Briançonnais). deformation (D1 and D2) the pre-Alpine faults were reactivated as These deposits do not preserve evidences of large-scale syn- thrust, or lateral ramp/transfer faults depending on their original ori- sedimentary faulting but it is arguable that the last faulting phase entation with respect the new imposed stress (Bonini et al., 2010). (Late Permian) produced suitable gradients for the erosion and trans- Later, extensional and transtensional tectonics operated by the port of Verrucano clastic components from the inner to the outer sec- reactivation of the early compressional structures during the tors (Cabella et al., 1988). The thickness variation of these deposits Oligo-Miocene late-Alpine stages (D4–D5, Maino et al., 2013). Obvi- follows the same general trend that controlled the emplacement of ously, the primary features of the pre-Alpine faults are now partially the Permian ignimbrites. On the whole, these topographic conditions or completely obliterated by the Cenozoic deformation. Therefore, were protracted in time, later controlling the sedimentary variations the kinematic of these faults may be only indirectly inferred, mainly (facies and thickness) of the Mesozoic cover (Cortesogno et al., by stratigraphic analysis. Nevertheless, pre-Alpine synsedimentary 1998). During the Middle Triassic, the shallow subsiding carbonate tectonic structures can be directly observed in the less deformed ramp covered both the Briançonnais and the Prepiedmont domains. areas; in the Ligurian Alps the most preserved stratigraphic succes- Subsidence was concentrated in the outer (External Briançonnais) sions belong to the External Briançonnais or the Prepiedmont do- and innermost (Prepiedmont) zones, whereas the Internal Briançonnais mains, which experienced low-grade metamorphism during the constituted a relative high, as indicated by the extremely reduced (or Alpine orogenic phases. lacking) Anisian–Ladinian limestones or dolomites (Vanossi et al., 1986). Mesoscopic evidence of synsedimentary Middle Triassic tecton- 5.1. Permian ics is concentrated in two main stratigraphic ranges: at the transition between CLO and SPMD (Anisian–Ladinian?), and the lower part of The early pre-Alpine tectonic structures recognized in the Ligurian SPMD (Lower Ladinian?). The former contains metric offsets of normal Alps have been detected in the Permian volcano-sedimentary succes- faults in peritidal carbonates and plurimetric clusters of poor-sorted sions deposited during the late Variscan stages (Cortesogno et al., breccias, with up to decimetric-sized, roughly-shaped clasts. Breccias 1993, 1998; Dallagiovanna et al., 2009). Detailed mapping of the concentrate towards the eastern areas, suggesting widespread tectonics Early Permian lithostratigraphic formations in well exposed localities near the transition to the Internal Briançonnais uplifted zone, where the (Ormea–Viozene and Ollano–Calizzano in the External and Internal Middle Triassic successions are strongly reduced. During the Early Briançonnais domains, respectively; see Cabella et al., 1988; Seno et Ladinian, a renewed tectonic phase is indicated by seismic-induced al., 2010 for details) shows sudden lateral thickness variation, in the structures affecting the San Pietro dei Monti Dolomite: fluidification, order of up several hundred of metres. The reconstruction of each vol- crumpled bedding (Fig. 8c–d; Bogacz et al., 1968) and shock-induced, canic and sedimentary body highlights the presence of small, strongly small-scale normal faulting (Fig. 8a–b). These structures can be associ- subsiding continental basins bordered by synsedimentary faults ated directly with seismic activity because other mechanisms are (Cabella et al., 1988; Cortesogno et al., 1998). These faults show two ruled out by the tidal flat depositional environment (providing stable prominent directions: WNW–ESE and NNE–SSW. During the ductile sedimentation with stable tectonics). Extensional tectonics continued A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 59 throughout the Ladinian, as testified by the occurrence of a experienced Cenozoic rotation as, conversely, the Penninic units of trough-shaped physiography that pre-dates the Capo Noli Formation the Alps. The same NE–SW alignment may be inferred also from the (Carnian). Evidence of this can be found in the M. Carmo–Rialto Unit Mesozoic faults from Corsica and Sardinia if they are restored to (Decarlis and Lualdi, 2009), where a distally steepened carbonate their pre-Miocene position (e. g. Gattacceca et al., 2007). In the ramp separates an external Briançonnais shallow platform from an Penninic domains of the Western Alps, the restoration of the Mesozo- internal Briançonnais one. The recognition of these km-length ic structures to their pre-Oligocene rotation position is more compli- paleoescarpments suggest the presence of fault bounded basin cated. As discussed in Section 3, in Liguria two possible CW rotational limited by WNW–ESE normal faults. A Triassic extensional activity values can be applied: 117° (Collombet et al., 2002) or 50° (Maffione of these faults can also be inferred by the highly differentiated sedi- et al., 2008). In the first case, the pre-Alpine orientation of the major mentation within the basins. From these field data a general exten- Permian–Jurassic normal faults of Liguria turns out to being NE–SW sional setting controlling the subsidence during the Middle Triassic striking, which is the same main trend of the fault-network of the times can be deduced. At regional scale the pre-existing Permian neighbouring European margin of the SE France and of Corsica– km-length faults dividing the post-Variscan basins and sub-basins Sardinia. On the contrary, applying a CW 50° rotational value, the were probably still active, as confirmed by the development of the same faults become NNW–SSE-trending, resulting in a different structural high represented by the Internal Briançonnais. In the orientation of the Ligurian fault-system with respect the other part Late Triassic the Ligurian Briançonnais platform stopped subsiding; of the continental margin. Unfortunately, as a definitively Oligo- it was uplifted and subjected to subaerial chemical and physical Miocene rotational value of the Ligurian Alps is not available, a pre- degradation until the Dogger (Decarlis and Lualdi, 2008). The distri- cise paleo-orientation of the Ligurian margin cannot be determined. bution of shallow-water Carnian formations suggests that, after a generalized sea-level fall recorded in different Alpine domains 6. Major events recorded in the tectonic evolution (Jadoul et al., 1992; Loriga Broglio and Neri, 1995), they were deposited in the most depressed areas, as the heritage of the Late 6.1. Events during the Permian period Ladinian tectonic structures. Conversely, in the adjoining Prepiedmont domain, the Norian sub- In the Alpine area, the Late Paleozoic Variscan crust was affected sidence strengthened, marked by the deposition of the thick Monte by extension, wrenching, major volcanic activity and basin develop- Arena Dolomite (Hauptdolomit). The increased subsidence in the ment. In Liguria, as well as in other segments of the belt (e.g. Pro- more internal sector is a common characteristic of the whole vence, Sardinia, South-Alpine), the development of intramontane Peritethyan margin during the Norian. Then it followed a period of lacustrine basins and the emplacement of conspicuous volumes of tectonic quiescence during the deposition of the Rhaetian platform calc-alkaline volcanic rocks was associated with a strike-slip regime (i.e. Veravo Limestone). (Cortesogno et al., 1998; Ziegler and Stampfli, 2001). The volcanism between ~286 and 272 Ma accompanied the collapse and subsidence 5.3. Jurassic of the lithospheric blocks controlled by WNW–ESE and NNE–SSW trending normal and strike-slip faults. The upwelling of magma asso- The comparison of the Jurassic stratigraphic sequences between ciated with high thermal flow was induced by the major crustal faults the Briançonnais and Piedmont domains suggests the non-linearity tapping magmatic reservoirs at depth (Cortesogno et al., 1998). The of the Paleo-European continental margin (Dallagiovanna and fault network of the Ligurian post-Variscan succession reflects the Lualdi, 1986; Galbiati, 1986; Vanossi et al., 1986). In the well Early Permian development of pull-apart basins, which are a common preserved Arnasco–Castelbianco unit (Prepiedmont domain), m- to features in the whole southern Variscan realm (Bruguier et al., 2003; hm-length faults WNW–ESE-striking (e.g. Zuccarello fault) bordering Schaltegger and Brack, 2007; Ronchi et al., 2008; Cassinis et al., 2012). enormous volumes of breccias (MGB; Dallagiovanna and Lualdi, This regional transtensive setting can be related to a generalized lith- 1986) are evidences of the Jurassic syn-sedimentary extensional tec- ospheric thinning associated with large-scale crustal wrenching tonics. These breccias are confined by WNW–ESE-striking normal (Dallagiovanna et al., 2009; Fig. 10a). The sudden termination of the faults implying the paleo-escarpments located on the continental calc-alkaline magmatism in most of the southern Variscides and margin. Although many of these faults experience Alpine contraction- the consequent Mid-Permian magmatic gap, has been related to the al reactivation, the mainly extensional kinematic of these faults can main phase of a general strike-slip tectonic event, which led the be indirectly derived from the stratigraphic setting. Seismites have intra Pangaea reorganization (Muttoni et al., 2003; Gutiérrez-Alonso been moreover found near the base of the Middle Jurassic Rio et al., 2008). In Liguria this gap is marked by the renewal of volcanic di Nava Limestone (Bertok, 2006), suggesting a sedimentation activity, represented by the alkaline ignimbrites, at ~258 Ma controlled by extensional tectonics. The WNW–ESE faults are locally (Lithozone D, MP2, Dallagiovanna et al., 2009). Differently from the truncated by NNE–SSW faults, which can be probably related as orig- previous calc-alkaline volcanics, the high-K rhyolites erupted inal transfer faults. throughout all the Briançonnais and Prepiedmont domains, with variable thickness controlled by WNW-trending graben structures. This 5.4. Paleomargin orientation extensional regime lasted until the Middle Triassic times, inducing erosion focused upon the relative highs (horsts) and sedimentation The syn-sedimentary Permian–Jurassic major faults detected in within the grabens (Fig. 10b). Diffused thermal perturbation is also the Ligurian area, show a constant strike towards WNW–ESE. These recorded in the post-Variscan succession by U–Pb analyses: frequent normal faults are connected by secondary NNE–SSW-striking faults. ages, scattered between 207 ± 8 and 261 ± 11 Ma, from altered zir- The trend of the former corresponds to the elongation of the major con rims, suggest multiple resetting of the U–Pb system, probably due tilted blocks and graben structures, while the second ones may corre- to the Late Permian and Triassic thermal anomalies (Dallagiovanna et spond to the paleo-transfer faults. This fault network depicts the al., 2009). For the same period, conspicuous magmatic activity is also structure and the shape of the Ligurian continental paleomargin documented in Provence, Corsica, Sardinia, the Penninic and overlooking the Mesozoic Alpine Thetys. Austroalpine zones, and the Southern Alps. It was represented mainly The synrift faults developed in the adjoining European crust of the by effusive products (rhyolites, andesites, basalts) and dykes, SE France (between the Massif Central and the External Alpine base- highly variable in both thickness and geochemical characterization ments of Pelvoux, Belledonne and Argentera) are mainly NE–SW (both calc-alkaline and alkaline; e.g. Schaltegger and Brack, 2007; directed (Lemoine et al., 1989, 2000). These structures did not Beltrando et al., 2012). 60 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68

TRANSTENSIVE Phase ALPINE TETHYS PASSIVE-MARGIN Phase Early Permian Late Triassic - Norian A E 23°26’N

TA IA L E M 23°26’N C IA PALAEOTETHYS CL MAA LI MA NEOTETHYS

S DO PIN

0° EB IB EB IB PP N E O T E TH Y S R IF T ING

0°

BACK-ARC to MELIATA RIFTING or CRUSTAL COOLING Phase Early Jurassic Late Permian F B 23°26’N

g in t f i r A T s IA y L h t E e M g T n e fti C ri n i A A LI T p MELIA l MA NEOTETHYS A

PALAEOTETHYS S PINDO N EB IB PP 2233°°2266’’N 0° EB IB Central Atlantic

NEOTETHYS

Middle Jurassic Middle Triassic C G

TA LIA ME

A C T LIA IA A L M E M NEOTETHYS 23°26’N PALAEOTETHYS

OS PIND

EB IB PP EBIB PP 23°26’N

NEOTETHYS

0°

Late Triassic - Carnian Late Jurassic D H

TA IA L ’N E 23°26 M C A LI MA VARDAR NEOTETHYS ALPINE TETHYS

S DO PIN 23°26’N

EB IB PP S PINDO NEOTETHYS

0° A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 61

In the external basements of the Helvetic domain, the Permian– Permian thermal event; our paleogeographic reconstruction follow Triassic magmatism is lacking, suggesting a different paleogeographic the Ziegler and Stampfli (2001) setting in which the Alpine area position of the internal zones, which are considered to have a south- was part of a wider zone that evolved with a diffused extension also ern origin, in between the basements of Provence–Corsica–Sardinia testified by the opening of different oceanic branches (Meliata, Maliac and the Southern Alps (Bertrand et al., 2005). and Pindos?) which persisted until the upper Triassic. This setting is The structural and magmatic features of the future northern rift consistent with the observation from Cassinis et al. (2008), which margin suggest a diffused and prolonged thermal anomaly affecting evidenced that the coeval strike-slip to extensional tectonics of the the continental crust through constant upwelling of magma from Southern Alps is linked to arc/back-arc conditions of an eastern the mantle, as also confirmed by the emplacement of gabbros within subduction zone that bounded the Meliata Basin. Of course we have the South-Alpine lower crust. The diffused crustal weakness became no way to control the large scale “engine” of our extension and subsi- established within a crust strongly wrenched by the Permian tecton- dence from the very local point of view of the Ligurian Alps and then ics (Fig. 12A). the thermal relaxation hypothesis still remains a highly valuable alternative. 6.2. Middle Triassic events 6.3. Upper Triassic–Lower Jurassic events The Anisian–Ladinian deposits of the Briançonnais and Prepiedmont domains provide a good example of a carbonate platform controlled by From the upper Triassic onward three distinct phases of rifting active extensional tectonics (Megard-Galli and Baud, 1977). At this time connected to the opening of Tethys have been reported in the Alpine the Internal Briançonnais represented a narrow horst between two realm and can be recognized in the Ligurian area. shallow subsiding areas (External Briançonnais and Prepiedmont; Fig. 12B). The magmatic activity continued through all the Middle 6.3.1. First rifting phase Triassic, as shown by scarce calc-alkaline rhyolites, andesites, and felsic At the beginning of the Late Triassic the study area may be consid- and transitional basalts (Caby and Galli, 1964; Cortesogno et al., 1982; ered as a subsident part of Laurasia affected by a diffused extension. Lemoine et al., 1986). The Latest Carnian and Norian marked a change in the geodynamic The tectonic and magmatic events that occurred between the Late pattern. In the Austro- and South-Alpine domains, the main tectonic Permian and the Middle Triassic are interpreted either as reflecting phase that led to the final break-up of the Alpine Tethys was preceded the initiation of Tethyan rifting, or linked to an eastern Tethyan by a long “transtensive phase” (Favre and Stampfli, 1992): strong and segment: the Meliata ocean (Stampfli and Borel, 2002; Fig. 10C). Par- continuous subsidence allowed the deposition of some thousand ticularly, in the Southern Alps, calcalkaline magmatism occurred metres of shallow-water carbonates (Hauptdolomit). This 1st phase under a transtensional to locally transpressional regime (Castellarin probably acted from Late Carnian times (cf. Favre et al., 1991) and and Rossi, 1980; Doglioni, 1984; Cassinis et al., 2008). These condi- led to a tensional deformation affecting the future rifting area tions have been related to the subduction of Paleotethys beneath (Fig. 10D–E; Fig. 12C). It was responsible for the Late Triassic breccias Laurasia. From this viewpoint, the Southern Alps could represent found in the Briançonnais s.s. and Austroalpine domains (Stampfly the western continental termination of the Meliata oceanic back-arc and Marchant, 1997) and it probably also favoured the opening basin (Ziegler and Stampfli, 2001). Differently from the Southern of Early Jurassic rifting-associated rim basins developing on a Alps, in Liguria, as well as Sardinia, Corsica and the Western Alps, a pre-weakened lithosphere. In the Southern Alps a change in the igne- purely extensional regime controlled the sedimentation of the ous rock composition towards tholeiites from the late Carnian is post-Variscan detritic succession (Verrucano) and the carbonate reported by Cassinis et al. (2008) and suggested as symptomatic of platform. the onset of the Alpine Tethys rifting. The Triassic extension has also been alternatively interpreted as In Liguria, the 1st phase of rifting is evidenced by the deposition of an effect of the generalized and widespread thermal relaxation of the Norian Monte Arena Dolomite in the Prepiedmont domain, that the crust that followed Permian thermal event (Schuster and Stüwe, indicates renewed subsidence only along Alpine Tethys future distal 2008). In this model, the “Permian event” was terminated by the margin areas (Fig. 10E). Evidence of extensional activity can be opening of the Meliata ocean and was followed by a long lasting found in the neptunian dykes and synsedimentary faults affecting subsidence (sag stage) matching a slow lithospheric cooling. the Norian dolomites as well as monogenic breccias and sedimentary Concerning the Ligurian Briançonnais, from the Early Triassic to structures that are related to local seismic activity. The Late Carnian– the Carnian, widespread extensional tectonics generated depressed Norian phase probably transitionally overlaps the previous exten- troughs. As enhanced by the Monte Carmo Unit (Decarlis and sional event (which did not switch off, since the Meliata, Maliac and Lualdi, 2009), the basin axes cross through the main paleogeograph- Pindos oceans survived during the Alpine Tethys rifting), generating ical domains (Briançonnais–Prepiedmont, which have been defined the widespread subsidence that created enough accommodation for with regard to the Alpine Tethys axis; see Fig. 11). In the proposed the Hauptdolomit s.l. From the late Carnian the large part of the paleogeographical reconstruction they intersect the subsequent Briançonnais domain began to be uplifted (Fig. 12). (Norian–Middle Jurassic) Alpine Tethys rifting axis (Fig. 10C). Because of the tectonic deformation this angle is not directly measurable, but it 6.3.2. Second rifting phase has been deduced from facies distribution (Fig. 11). The successive Alpine Tethys rifting phase s.s. (Fig. 10F; Fig. 12D) This prolonged phase of Mesozoic extension developed into a has been extensively studied in the Southern Alps (Manatschal et complex and not yet completely understood geodynamic context: al., 2007, cum ref.) and found to be formed by two distinct tectonic it is not completely accelerated if it was generated as a response pulses. During Early Hettangian–Sinemurian times the first pulse to the southeastern subduction of Paleotethys (leading to a wide brought widespread crustal stretching that caused the formation of back-arc zone) or by a generalized crustal relaxation following the a net of subparallel rim basins developing along the future oceanic

Fig. 10. Regional-scale paleogeographical reconstruction from the Early Permian to Late Jurassic focused on the major geodynamic settings in the Western Mediterranean area. Colours indicate suggested sedimentation environment: brown is for emerged land, light blue for shallow water platform and blue for deeper marine. The red star correspond to the Ligurian Alps location. The insets detail the study area interpretation, the yellow line surrounds data-constrained areas. Continuous red line indicates the fault-pattern, red dotted lines the supposed fault pattern. Modified from Stampfli and Hochard (2009), Stampfli et al. (2002) and Ziegler and Stampfli (2001). 62 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68

Bria nçon nais its un Prep xt. iedm E ont M C . So . Tub tta erto M. C arm A o . Ca stelb.

Lias s.l.

Bria nçon nais its un Prep xt. iedm E ont M C . So . Tub tta erto M. C arm A o . Cas telb.

Norian

Bria nçon nais its un Prep xt. iedm E ont M C . So . Tub tta erto M . Ca rmo A. Cas telb.

? Carnian

Bria nçon nais its un Prep xt. iedm E ont M C . So . Tub tta erto M . Ca rmo A. Cas telb.

? Ladinian

? A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 63 axis. It was followed by a gap in tectonic activity that preceded the collapse during ongoing rifting. Mass and debris flows, and second pulse (Late Pliensbachian–Toarcian) in which the stretching later turbidites, are testified to by megabreccias and chaotic concentrated into a narrower zone above the Alpine Tethys axis deposition at the feet of faulted slopes. Transtensional dynamics (cw. “focused rift” in Berra et al., 2009). Evidence of the latter can intervened to constantly provide new material to be eroded be found in the westernmost area of the Southalpine domain away and different depositional areas were established. The (Bertotti et al., 1993; Ferrando et al., 2004). Similar two-phase rifting acidic volcanic flows, interfingered between the uppermost development has been evidenced in the Austroalpine by Froitzheim breccia layers and the finer clastics that preceded pelitic- and Manatschal (1996): Hettangian–Sinemurian and Toarcian– radiolarite deposition (Case Morteo rhyodacite), were a prelude Dogger; a similar setting has been suggested in the Dauphinois and to the basic magmatism involved in the opening of the future Helvetic domain, where rifting generated two distinct subsidence Penninic Ocean. They were symptomatic of the lithospheric thin- pulses (e.g. Chevalier et al., 2003) In the Briançonnais domain of the ning, which lead small amount of magma to rise up along the French Alps, Dumont (1988) and Chevalier et al. (2003) pointed out pre-existing normal fault pattern (Fig. 12). that the strongest extension with the development of wide normal faults, block tilting and erosion (Claudel and Dumont, 1999) indicat- 6.3.3. Third rifting phase and post-rift ing important crustal stretching took place from the Upper The third phase corresponds to the evolution of an extremely Hettangian (Schlotheimia zone) to the Dogger; in the Western Alps stretched margin lithosphere that finally led to the exhumation of this time-span can be separated into two parts which clearly indicate subcontinental mantle in the distal domains (Fig. 12E). In the the timing of the extension: (1) Upper Hettangian–Sinemurian, and Austroalpine domain, the distal Adriatic margin preserves a complete (2) Upper Liassic–Dogger. Along deep faults, wide crustal sectors succession of a “supra-detachment basin” that developed just above drowned quickly, putting pelagic mudstones directly onto peritidal thinned continental crust and exhumed mantle (Samedan basin, carbonates. Locally they are divided by fossiliferous hardgrounds to Masini et al., 2011). Breccias made up by dolomites, basement prove the persistence of offshore shallow shelves with reduced depo- rocks, serpentinite and prasinite clasts have been found in the sition already detached from the coast in Early Sinemurian times Prepiedmont Rochebrune unit of the French Alps (“Formation de (Bucklandii zone). Prafauchier”, Dumont, 1983). The age of this formation is not well In the Ligurian Alps, the rifting sequence is very well preserved in constrained but it is capped by Upper Jurassic limestones and it can the Prepiedmont domain units (Fig. 12D), and acted in two main sub- be dubitatively dated as Dogger s.l. For what concerns the Ligurian stages as follows: Alps, locally, in the most internal Prepiedmont area (i.e. Montaldo), the Middle Jurassic? detritism is finer and mostly made of carbonate (1) The first of these events in the Ligurian Alps can be recognized rocks, but associated with occasional metre-to-hundred metre lenses in the sedimentary succession of the Prepiedmont ramp, where of serpentinite and prasinite. Originary, they were interpreted as the carbonate sequence of the Arnasco–Castelbianco unit re- slices and blocks originary formed into an “intraoceanic fracture cords several tectonic events at the Hettangian–Sinemurian zones or olistolites” (Dallagiovanna and Vanossi, 1987). The boundary like those in other sectors of the Western Alps be- improved knowledge of the rifting Alpine kinematics now allows us longing to the European continental margin (see Elmi, 1983; to try to correlate this deposits with the Prafauchier Breccia of Lemoine, 1984; Dumont, 1988): the main first one was the ac- Dumont (1983); they probably deposited during the latest phase of tivation of normal and/or transcurrent faults joined to tilted rifting leading to mantle denudation and they are symptomatic of blocks and horst/grabens, while a second was a rapid deepen- the final evolution of a hyperextension of the crust (Mohn et al., ing linked to fast subsidence. In the Ligurian Alps, the Arnasco– 2010, cum bibl). Castelbianco unit, the post-Hettangian fast subsidence allowed After this “paroxysmal” phase of rifting, there followed an uplift the deposition of about 600 m of monotonous spongolitic pulse of the Tethyan rift shoulder and Ligurian Briançonnais domain. limestones. As already indicated, the thickness of the Calcari Similar behaviour has been found in all of the Alpine domains were di Rocca Livernà is variable from some tens of metres to the clastic infill have been interpreted as a result of the progressive 600 m, depending on the position with respect to the eroding erosion of the shoulder (Kalin and Trümpy, 1977; Eberli, 1988). continental margin; unfortunately, no clear ancient tectonic The uplifted areas separate conjugate rim basins (i.e. Adriatic Lombar- lineaments can now be found owing to the allochthony of the dian basin to the south and European Helvetic–Dauphinois–Subalpine unit. In some outcropping parts of the Arnasco–Castelbianco basins to the north) from the subsiding Penninic basin. In the Ligurian tectonic unit, strong erosion occurred in (late?) Hettangian Alps, a (Toarcian–Bajocian?) small system ofturbiditicsandstones times, as the whole Rhaetian sequence is lacking and the (Nasino sandstone) was generated by the dismantling of the top of the Calcari di Rocca Livernà now lie directly on the Norian M. nearest uplifted margin. The Briançonnais units were at this time Arena Dolomites. After the Sinemurian there followed a time emerged and subject to rapid erosion with the previously deposited of increased subsidence, but there is no evidence of relevant marine succession. tectonic events in which the homogeneous basinal succession Rapid cooling followed the beginning of “oceanisation” in the Li- of RLL2 was deposited (Pliensbachian). gurian sectors from the most external area (Fig. 10G); so the External (2) The second substage took place when, during the Early–Middle Briançonnais was the first to be flooded and receive the first Jurassic, some rift-related faults produced conspicuous amounts post-Bajocian marine sediments: the Rio di Nava Limestones. The In- of breccia that flowed down along the margin and formed the ternal Briançonnais remained exposed in a subaerial environment till Monte Galero Breccia. The Brèche inférieur of Mid Liassic–Early to the Upper Jurassic and separated the external units from the Alpine Dogger age in the Breccia Nappe of the Préalpes Médianes Tethys basin, dominated by pelagic deposition (Arnasco Radiolarite (Chessex, 1959; Steffen et al., 1993) may be considered as equiv- on the margin). With the progressive opening of the ocean, the alent to the Monte Galero Breccia, and underline the margin thermal cooling also affected the Internal Briançonnais which slowly

Fig. 11. Large-scale paleoenvironmental reconstruction of the study area based on both the facies genetic model of Fig. 6 and on the displacement of the tectonic units. It is worth considering that while a quasi-continuous extension is recorded from Ladinian to Jurassic, it acted along different directions, probably generating incident (here orthogonal) basins: an EW Meliata-related basin and a successive NE–SW Piedmont–Ligurian basin. For graphic simplicity, the amount of extension in both directions is not considered in this ﬁgure. Tectonic units (that separates each other later during the Cenozoic compression) are then supposed to maintain a conformable position each other during the considered time span. Up- and down-arrows indicate areas respectively interpreted as uplifting or subsident. For present-day locations of tectonic units, see Fig. 2. 64

MIDDLE JURASSIC LATE-EARLY JURASSIC UPPER TRIASSIC MIDDLE TRIASSIC LATE PERMIAN nprdfrom Inspired 12. Fig. Basement-undifferentiated Middle /Upper Middle Triassic Basement-undifferentiated Undifferentiated mantle Basement-undifferentiated Middle Triassic Middle Basement-undifferentiated Undifferentiated mantle Undifferentiated mantle Basement-undifferentiated Middle/Upper Triassic Middle/Upper Basement-undifferentiated Basement-undifferentiated Middle Triassic Middle Basement-undifferentiated Undifferentiated mantle Variscan molasse Basement-undifferentiated Briançonnais s.l. Briançonnais s.l. Briançonnais s.l. Briançonnais s.l. nepeaiescin costeTty rmLt ema oMid-Jurassic. to Permian Late from Tethys the across sections Interpretative Briançonnais .Dcri ta./ErhSineRves15(03 43 (2013) 125 Reviews Earth-Science / al. et Decarlis A. Stamp Early Permian Shearzone relics ﬂ n atae (1990) Marthaler and i (weakened crust) Prepiedmont Prepiedmont Widespread Anisian-Ladinianvolcanics ashes Prepiedmont Prepiedmont Prepiedmont Variscan molasse Variscan molasse Variscan molasse Variscan molasse o.wt lmnsfrom elements with mod. , Piedmont-Ligurian Adriatic margin Adriatic margin Adriatic margin Adriatic margin Thinning faults Synrift deposits Synrift Exhumation faults deposits Synrift – Upper Triassic 68 Adriatic margin one l (2010) al. et Mohn C B A E D

Exhumation Thinning Stretching Stretching only ? . Alpine Tethys rifting Back-arc to Meliata rifting ? A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 65 subsided and formed a unique pelagic realm with the External preserves its peculiar characters also during the first stages of Briançonnais, testified to by the continued deposition of the Val post-rift (during thermal subsidence stage) when the internal part Tanarello Limestone. of it still remain uplifted until the Upper Jurassic. Following the Mohn et al. (2012) model (see also Masini et al., 6.3.4. Paleogeography remarks 2013), the area acting as upper plate shoulder (e.g. Stampfli et al., The above mentioned three steps evolution of rifting in the Liguri- 1991; “necking zone” in Mohn et al., 2010), has to be placed in the an Alps fits with recent numerical modelling proposed by Lavier and easternmost part of the Southern Subalpine basin (a peculiar south- Manatschal (2006) for the evolution of hyperextended, magma-poor ern area of the Dauphinois s.l. domain also known as the Eastern rifted margins. The first stage, called “stretching phase”, in which Provençal domain). Here the pre-rift succession shows different brittle normal faults initially accommodate the extension is closely characters from the Ligurian Briançonnais but a stratigraphic omis- followed by a “thinning phase”, in which localized detachment faults sion of Liassic deposits occurs on a wide area (Dardeau, 1984). occur. This stage also correspond to the moment in which both sides of the so called “H block” of the model (corresponding to the 7. Conclusion Briançonnais s.s. domain) are sheared off. The main difference with previous classical models is that the This paper represents the sum of extensive fieldwork on one of the Briançonnais s.s. does not represent only a broad rifting shoulder most polydeformed and polymetamorphosed sectors of the Alpine (e.g. Stampfli et al., 1991) generated by the simple shear mechanism chain. The stratigraphic and tectonic revisions of this complex tecton- of rifting that induced an astenosphere upwelling just below the ic framework from the Permian onwards have been deciphered and upper plate. Instead, it actually represents the residual part of a interpreted in the light of the most recent developments in Alpine block that has been detached from the European plate just during geodynamics to offer a point of view on the phases that preceded the second phase of rifting and then maintained its peculiar isostatic the collision of the Alpine chain. Our work shows five evolutionary history during later times. steps, deduced from the geological record of the Ligurian Alps and The third stage lead to the exumation of middle crust and upper from comparison with similar situations in other sectors of the Alpine serpentinized mantle through concave downward faults. Following chain: Manatschal and Muntener (2009), exhumation may occur in different areas of the rifting zone, allowing the formation of multiple basins (1) The Early–Middle Permian tectonics, considered as the product floored or partially floored by upper mantle rocks (i.e. the “Proto of a megashear zone evolution or as a response to a changed Liguria–Piemonte and Valais” domains; Mohn et al., 2010). geodynamic context, probably acting from the Upper Carbonif- One of the most controversial points about Alpine paleogeography erous, affected the Ligurian Alps, as we can see particularly in is the presence and significance of a minor oceanic through (i.e. the the Briançonnais domain, with the rejuvenation or birth of Valais domain; Trümpy, 1980), interplaced between the Briançonnais new faults bordering graben/half-graben or pull-apart struc- and the Dauphinois domains (see de Graciansky et al., 2011). In “clas- tures during the emplacement of the different volcanic bodies sic” and more recent paleogeographic reconstructions it is considered (see Section 2.1). as a Northwestern oceanic branch parallel to the Ligurian–Piedmont (2) During Late Permian–Early Triassic times, the Ligurian sectors Ocean (e.g. Stampfli, 1993; Handy et al., 2010). It is supposed to be underwent extensional tectonics only. The first phase Cretaceous in age (Frisch, 1979) and closely connected with the corresponded to the development of graben/half-graben struc- opening of the Bay of Biscay (Stampfli, 1993). de Graciansky et al. tures in which the emplacement of the Upper Permian (2011) underline the difficulty to correlate these two sectors with K-rhyolites occurred. Afterwards, a second phase aided in pro- no evidence of ocean floor (i.e. in Provence). Some other criticism ducing suitable gradients for erosion of clastic components of arise around the age of the Valais ophiolites: Scharer et al. (2000), the Verrucano Formation (VBR). Beltrando et al. (2007a) and Masson et al. (2008) founded Paleozoic (3) From the Middle Triassic onwards, the Ligurian Alps area was ages for the Versoyen magmatics. These last has been interpreted as affected by diffuse extension possibly generated in a a Permian intrusion connected to the extensional collapse of the back-arc/rifting setting caused by northwestward subduction of Variscan chain. In addition, the structural position of some outcrops Paleotethys which led to the opening of multiple oceanic troughs that were previously related to the Valais zone (Balma–Chiavenna), (i.e. Pindos, Maliac and Meliata; Ziegler and Stampfli, 2001) have been deeply revised and they were more likely attributed to or in a generalized crustal cooling setting that followed the the Piedmont–Ligurian Ocean (Beltrando et al., 2007a). Numerical previously-described Permian thermal event (Schuster and modelling (Lavier and Manatschal, 2006) suggests a contemporane- Stüwe, 2008). At these times, the studied area was located in a ous spreading for the Tethys and Valais throughs, later supported sector approaching the convergence of the axes of basins, by field observation and radiometric ages from the Tasna nappe opening like a fan toward the East (Fig. 11). The climax of (Manatschal et al., 2006). Dumont et al. (2011, 2012) interpreted this activity occurred in the Ladinian; directions of extension the Valais domain as a V-shaped basin with a sharp westward termi- are multiple and complex because of this peculiar paleogeo- nation in the Vocontian basin; the same authors suggest that the suc- graphical position. Nevertheless, our data show that the ex- cessive Valais subduction may had took place also in absence of a true tensional basin axes at this time were markedly different from oceanic crust by the interposition of thinned crust north of the the Alpine Tethys ones that overprint the whole pre-Jurassic Brianconnais domain. geological record. The Ligurian Briançonnais represents the southern termination of (4) The Late Carnian to Norian was a key point in the evolution of the Briançonnais domain that is actually preserved in the western Al- the Mediterranean area: in the Ligurian Alps it represented the pine belt; actually no evidence is known to testify the presence of the transition between the Middle Triassic stage and the Alpine Valais through interplaced between Briançonnais and Dauphinois in Tethys rifting s.s. Field evidence is in favour of an overlap of the study area. We support the idea that this domain may close the two extensional mechanisms rather than a complete southward in a “V shaped” basin, character envisaged in numerical switch-off of the first. It is our opinion that this may explain modelling and that shows a marked compatibility with similar basins the great subsidence affecting several Alpine domains during generated in rifting areas, like the Woodlark basin or the Porcupine the Norian. Basin (e.g. Reston et al., 2004). The Ligurian Briançonnais represents (5) The Alpine Tethys rifting developed from the Norian and an isolated block all along the second and third phase of rifting. It reached its climax in the Early Jurassic, when subsidence 66 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68

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