Keywords: Tectonics, Southern Provence, La Nerthe range, L'Etoile range, Paleogene, Neogene

    Comments on the paper entitled : " New structural constraints on the southern Provence thrust belt (France) : evidences for an Eocene shortening event linked to the Corsica-Sardinia subduction " . by

Observations sur la note intitulée : « Nouvelles contraintes structurales sur la ceinture de plis de la Provence méridionale (France) : arguments en faveur d'un raccourcissement Eocène lié à la subduction corso-sarde ». Mots-clés.-Tectonique, Provence méridionale, chaîne de la Nerthe, chaîne de l'Etoile, Paléogène, Néogène.

Résumé. -Nous discutons dans cette note l'interprétation tectonique d 'Andreani et al. des chaînes de l'Etoile et de La Nerthe qui constituent la partie occidentale du chevauchement Sud-provençal défini par [START_REF] Aubouin | Le chevauchement sud-provençal : de l'Etoile à la Sainte-Baume[END_REF]. En référence à des travaux antérieurs, nous soulignons l'indépendance structurale des deux chaînes. La partie occidentale de la chaîne de l'Etoile est chevauchée par la Nerthe méridionale le long du couloir aptien de Graffiane, apparaissant ici comme un secteur tectonique-clé. Contrastant avec le concept autochtoniste de la Nerthe développé par Andreani et al., nous montrons que les données structurales et paléogéographiques sont en faveur de l'existence d'un important chevauchement à vergence nord dans la partie méridionale de la Nerthe, mis en place pendant la phase pyrénéo provençale majeure (Bartonien). Ce chevauchement fut suivi d'un événement compressif daté du Ludien (Priabonien) qui a replissé les structures de la partie méridionale du massif de la Nerthe. Contrairement à l'interprétation d 'Andreani et al., les données de terrain conduisent à considérer le système de failles WE qui affectent la Nerthe et l'Etoile, comme s'étant formé dans un régime distensif

Introduction

The authors provide an overview of the tectonic history of the southern Provence, by reinterpreting facts previously established, using a microtectonic analysis of three structural regional units : La Nerthe range, the Western Etoile range and the Sainte-Victoire range. Since the works from M. [START_REF] Bertrand M | La grande nappe de recouvrement de la Basse-Provence[END_REF], the southern Provence thrust belt is regarded as formed by plurikilometers long, WE-trending, north or south-verging overthrusts related to the Pyrenean-Provence compressional event of late Middle Eocene age [the so-called Bartonian phase : [START_REF] Vasseur | Note préliminaire sur la constitution géologique du bassin tertiaire d'Aix-en-Provence[END_REF][START_REF] Aubouin | Essai sur la structure de la Provence[END_REF]. Overthrusts are interpreted as due to a major shallow décollement of the Mesozoic cover above the Triassic evaporites [START_REF] Aubouin | Essai sur la structure de la Provence[END_REF]. However, the hercynian basement is involved in the thrusts as shown by the Sicié overthrust [START_REF] Bertrand | Sur un témoin d'un nouveau pli couché près de Toulon, phyllades superposés au Trias[END_REF][START_REF] Arthaud | -Les structures pyrénéennes du Languedoc et du Golfe du Lion (Sud de la France)[END_REF]. According to [START_REF] Tempier | -Modèle nouveau de mise en place des structures provençales[END_REF] and [START_REF] Lacombe | Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny[END_REF] the basement would also be involved in La Nerthe and Sainte-Victoire overthrusts. As referred by paleostress analyses, the southern Provence overthrusts would result from a nearly N-S compression during the major Bartonian phase [START_REF] Lacombe | Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny[END_REF]. But, the shortening amount of the sedimentary cover was debated : 60 km [START_REF] Guieu | Structures, paléostructures, paléogéographie et genèse des bauxites en Provence[END_REF] vs 25 km [START_REF] Tempier | -Modèle nouveau de mise en place des structures provençales[END_REF]. New studies in La Nerthe and L'Etoile ranges are of particular interest in the debate relative to the tectonic of the southern Provence belt, due to their vicinity to the Eocene-Oligocene Arc basin and to the Oligocene Marseilles and Saint-Pierre basins. Moreover, the conclusions of the last synthetic tectonic work on L'Etoile et La Nerthe ranges [START_REF] Guieu | Notice tectonique de la feuille Martigues -Marseille à 1/50[END_REF] are challenged by the Andreani et al. paper. Indeed, these authors provide new interpretations about the structure of La Nerthe and L'Etoile overthrusts and on the age and the direction of the compressional constraints.

In the present paper we consequently will focus our comments on these two ranges and we will discuss the value of the new interpretations by taking into account the stratigraphical and tectonic works carried out in this region during last decades.

Distinction between La Nerthe range and L'Etoile range.

Both La Nerthe and L'Etoile ranges correspond to the western part of the major EW-trending « Southern-Provence thrust » as defined by [START_REF] Aubouin | Le chevauchement sud-provençal : de l'Etoile à la Sainte-Baume[END_REF]. La Nerthe range lies along the southern margin of the Etang-de-Berre Late Cretaceous syncline, while L'Etoile range is a north-verging thrust emplaced over the Late Cretaceous-Eocene Arc basin. According to Andreani et al. (p. 550,fig. 3) the EW-trending La Nerthe range connects progressively eastward to the NE-SW-trending L'Etoile range. In this viewpoint, no structural boundary lies between the two tectonic units. However, in our opinion, the sensu stricto La Nerthe range (also labelled the Martigues range) strictly relates to the EW-trending branch of the overall structure described by Andreani et al. The boundary between La Nerthe range and L'Etoile range lies in the Graffiane aptian corridor (fig. 1) that represents a key-tectonic area previously described in details by [START_REF] Guieu | Sur les relations tectoniques entre les massifs de la Nerthe et de l'Etoile dans le secteur du vallon de Graffiane au Nord-Ouest de Marseille[END_REF].

The main results of this study have to be recalled (fig. 2): i) along the Graffiane corridor, the western part of l'Etoile range is underthrusted beneath the southern part of La Nerthe (Chapats) unit ; ii) the contact between the two units is laid out by Late Aptian, Albian, Cenomanian and Senonian para-autochthonous thrust slices (not referred by Andreani et al.) occuring in front of the Chapats unit overthrust ; iii) subsequently to the emplacing of La Nerthe overthrust, the Graffiane corridor played as a strong flexural axis, allowing the western part of L'Etoile range to raise. Therefore, the Graffiane corridor appears as an important tectonic boundary which allows a fundamental distinction between La Nerthe and L'Etoile ranges, due to significative differences in structure between the two overthrusts. Guieu et Philip, 1972 , modifié). Coupes a, b, c, localisation Fig. 1 ; coupe d : reconstitution du couloir tectonique de Graffiane avant le soulèvement flexural de la partie occidentale du massif de l'Etoile. U-J : Jurassique supérieur-VAL : Valanginien-HAUT : Hauterivien-URG : Barrémien (Urgonien)-L-APT : Aptien inférieur-U-APT : Aptien supérieur-CEN : Cenomanien-L-TUR: Turonien inférieur-U-TUR: Turonien supérieur -L-SEN: Senonien inférieur (Coniacian-Santonian). A. Chevauchement de la Nerthe méridionale (unité des Chapats)-B. Unité de Graffiane-C. Partie occidentale du massif de l'Étoile (synclinal crétacé supérieur de Méjean-La Redonne).

Fig. 2-Coupes en série du couloir tectonique de Graffiane (d'après

Structure of La Nerthe range

Andreani et al. carried out a microtectonic study of the nearly WE-trending faults that affect La Nerthe range, in an attempt to infer the kinematic movements that may have occurred along these faults ; from north to south : Valtrede fault, Laurons-Ensuès fault and la Folie-Valapaux faulted zone [the so-called « la Couronne unit », [START_REF] Guieu | Etude tectonique de la région de Marseille[END_REF]] (fig. 1). Andreani et al. provided a structural interpretation of the range according to a roughly north-south trending section (p. 551, fig. 4 A-A'), but without re-mapping the given area. The Andreani et al.'s study of La Folie-Valapaux faulted zone deserves particular comments. Firstly, the northern fault that limits this zone is interpreted by these authors as a right-lateral, northward gently-dipping fault, bounding the Chapats unit from the Middle-Late Cretaceous core of La Folie-Valapaux zone. Unfortunately, the authors did not discuss the interpretation proposed by [START_REF] Guieu | L'évolution tectonique de la chaîne de la Nerthe, au Nord-Ouest de Marseille[END_REF][START_REF] Guieu | Notice tectonique de la feuille Martigues -Marseille à 1/50[END_REF] and [START_REF] Assas | Mécanismes de mise en place d'une nappe de charriage épiglyptique[END_REF] who interpreted this fault as the north-verging thrust emplacing the Chapats unit over la Folie-Valapaux zone (fig. 3). It is noteworthy that the Guieu's interpretation was an alternative explanation to the subsurface structure documented by the CFP-La Folie borehole [START_REF] Dufaure | -Description sommaire d'un sondage dans la chaîne de Martigues (Bouches-du-Rhône)[END_REF] ; location fig. 1]. Second, a question arises about the structural significance of the southern fault that bounds La Folie-Valapaux zone. Following the authors, this fault should be a south-dipping reverse fault rooted in the Jurassic Arc syncline basement (p. 551, fig. 4A-A'). We strongly disagree with such an interpretation. As shown by numerous authors [START_REF] Lutaud | La tectogenèse et l'évolution structurale de la Provence[END_REF][START_REF] Aubouin | Essai sur la structure de la Provence[END_REF][START_REF] Guieu | Etude tectonique de la région de Marseille[END_REF], this fault corresponds to a non-rooted, major flat or southward gently-dipping thrust contact, that results in the overthrusting of the Middle Cretaceous series of La Folie-Valapaux zone by the lower Cretaceous thrust unit of the southern La Nerthe range [the so-called « Sausset unit », Guieu 1968 ; fig. 1]. In front of this thrust, there are Turonian and Senonian crushed thrust-slices [START_REF]Les formations calcaires à rudistes du Crétacé supérieur provençal et rhodanien[END_REF] that are misinterpreted by Andreani et al. as related to the autochthonous cover of La Folie-Valapaux Middle-Cretaceous serie. It is worth noticing that the complex Cretaceous series of La Folie-Valapaux faulted zone is not only sealed by the Early Miocene marine sediments as indicated by Andreani et al. (p. 551) but also by early Rupelian gently-folded limestones which are overlying an overturned Late-Middle Cretaceous thrust slice in La Beaumaderie area (fig. 4). Guieu, 1977, modifié). Localisation Fig. 1

(coupe XX'). A. Unité des Chapats-B. Zone faillée de La Folie-Valapaux-C. Unité de Sausset. Φ2 : chevauchement de la Nerthe méridionale

In other words, the Andreani et al. new interpretation corresponds to an autochthonous concept of La Nerthe range that would represent, according to the authors, the folded and faulted southborder of the Arc syncline, without significant northward or southward displacement of the different compartments. Unfortunately, the eastward motion of these compartments along the above mentioned faults, inferred by the authors as right-lateral faults, is not quantified and no discussion was presented about the relationship of these faults with the Chapats overthrusting on the Graffiane Upper Aptian corridor (fig. 2).

The Andreani et al.'s autochthonous concept of La Nerthe sweepingly contrasts with the allochthonous concept from [START_REF] Guieu | L'évolution tectonique de la chaîne de la Nerthe, au Nord-Ouest de Marseille[END_REF] for whom the southern La Nerthe overthrusts (Sausset unit+Chapats units) constitute an unique north-verging thrust system, subsequently folded and then cut by normal faults. In this view, La Folie-Valapaux faulted zone is considered to be a tectonic window (fig. 3). This interpretation also takes into account the existence of the parautochthonous Upper Cretaceous thrust slices, isopic to the L'Etoile range Upper Cretaceous Méjean-La Redonne series (location fig. 1). The Andréani et al.' s study does not take into account these structural and palaeogeographic data and minimizes the role of the Sausset thrust sheet.

Age and kinematics of La Nerthe WE-trending faults.

The microstructural study allows Andreani et al. to interpret La Nerthe range as a right-lateral transpressive flower structure. According to them, the structural data collected along the WEtrending faults of the range argue for a NW-SE shortening direction attributed by Andreani et al. (p. 552) to the Eocene (Bartonian) major tectonic event. A similar shortening direction would also exist in the western part of L'Etoile range and in the northern part of the Sainte-Victoire range. Moreover, the right-lateral strike-slip faults of La Nerthe range would be coeval with the NW-verging thrusting event of the western part of L'Etoile range. However, there is no convincing evidence of a late Middle-Eocene (Bartonian) right-lateral motion along the WE-trending faults of La Nerthe range.

First, the subsequent, nearly WE anticline structure of La Folie-Valapaux faulted zone (fig. 3) implies a strong folding, then an erosion, of la Nerthe range, after the emplacing of the southern overthrust and prior to the deposit of early Rupelian limestones (fig. 4). Fault-slip data were mainly collected by Andreani et al. in the Chapats unit, along La Folie-Valapaux faulted zone. According to them, many N080° to N110°-trending fault planes display right-lateral subhorizontal motion. Besides, we have measured N050° horizontal striae in the Late Cretaceous series of La Folie-Valapaux zone near la Baumaderie area. In our opinion, the shortening event resulting in folding and perhaps strike-slip movements along La Folie-Valapaux faulted zone has occurred belatedly during a stratigraphic interval bracketted by the Bartonian emplacing of the southern overthrusting structures and by the first step of early Rupelian deposits. Therefore, this significative morpho-tectonic evolution of La Nerthe range, has probably occurred during Ludian (Priabonian) times.

Second, we consider the Ponteau, Laurons-Ensuès and Valtrede, WE-trending faults systems to have been formed subsequently to the deposit of early Rupelian series [START_REF] Nury | L'Oligocène de Provence méridionale[END_REF] which transgressively overlie the northern Lower-Cretaceous autochthonous compartment of the range (Saint-Pierre syncline and neighbouring areas). Indeed, by contrast with the Andreani et al. interpretation (p. 551, fig. 4 A-A'), it is worth to precise that the early Rupelian limestones are tilted or reversed along the Ponteau WE-trending fault limiting to the south the Saint-Pierre Oligocene syncline. Moreover, the Valtrede WE-trending fault system acted as synsedimentary normal faults during the deposit of late Oligocene syn-rift coarse breccias and olistoliths filling the northern part of the Saint-Pierre syncline [START_REF] Nury | L'Oligocène de Provence méridionale[END_REF]. The fault-planes do not display any mark of horizontal striae. As a result of these data, it seems irrelevant to consider the shortening direction inferred by Andreani et al. from microtectonic data as having resulted in the formation of the WE-trending faults of northern La Nerthe range. These faults are linked to the NS-distensive regime that started from early Rupelian and that was accentuated during late Chattian, leading to the formation of syn-rift continental and shallow marine deposits along the southern border (« Carry basin») of the range [START_REF] Oudet | Land and sea study of the northeastern golfe du Lion rifted margin : the Oligocene-Miocene of southern Provence (Nerthe area, SE France)[END_REF]. It is noticed that the post-rift Burdigalian horizontal deposits unconformably overlie the early Rupelian SSW dipping strata in La Beaumaderie area (fig. 4 ) and seal the various faults and tectonic compartments of La Nerthe range (fig. 1).

Thirdly, the horizontal striae recorded by Andreani et al. along the Valtrede (2 sites) and Les Laurons (1 site) WE-trending faults could be interpreted as a strike-slip motion due to the reactivation of southern thrusting structures during middle Oligocene times as it is revealed by tilting and slightly folding that affects the early Rupelian limestones and the thrust slices of « La Couronne » compartment in La Baumaderie area (fig. 4). Such a shortening event, already suggested by [START_REF] Nury | Sur l'évolution tertiaire de la chaîne de la Nerthe (Bouches-du-Rhône)[END_REF] and [START_REF] Guieu | L'évolution tectonique de la chaîne de la Nerthe, au Nord-Ouest de Marseille[END_REF], is consistent with the roughly north-verging overthrusts and reverse south-dipping faults, affecting early Rupelian limestones, which have been documented in the southern border of the Marseilles basin [START_REF] Guieu | Etude tectonique de la région de Marseille[END_REF][START_REF] Weydert | Données nouvelles sur la stratigraphie et la tectonique de la zone méridionale du bassin de Marseille[END_REF]. Moreover, NS and N 140 shortening directions have been recorded in the early Rupelian limestones filling the Marseilles basin [START_REF] Nury | Étude tectono-sédimentaire du fossé oligocène de Marseille (Bouches-du-Rhône). Évolution postérieure[END_REF]. Whereas the authors have attributed these different directions to alpine shortening stages [START_REF] Nury | Étude tectono-sédimentaire du fossé oligocène de Marseille (Bouches-du-Rhône). Évolution postérieure[END_REF][START_REF] Hippolyte | Relations entre tectonique extensive et sédimentation continentale: exemple des bassins oligocènes de Marseille et de Basse-Provence[END_REF] a middle-Oligocene transpressive strike-slip reactivation of late Middle Eocene (Bartonian) major structures cannot however be excluded.

Ludian strata, was mentioned by [START_REF] Philip | Carte Géologique de la France à 1/50 000[END_REF] in the Saint-Martin-de-Londres basin in front of the Pic Saint-Loup north-verging thrust. This local compressive event (a reactivation of the Pic Saint-Loup overthrust ?) preceded the whole NW-SE-distensive regime which prevailed in this region during late Oligocene (Chattian). Likewise, [START_REF] Roca | -Oligocene to Middle Miocene evolution of the central Catalan margin (northwestern Mediterranean)[END_REF] dealing with the tectonic evolution of the central Catalan margin have mentioned Eocene-middle Oligocene compressional regime and a late Oligocene extensional one in this region.

Conclusion

La Nerthe range appears as a multistage tectonic puzzle, marked by the existence of a major north-verging overthrust probably occurring during the late Middle-Eocene shortening event, followed by an early Rupelian transgression and finally affected by a complex episode of faulting prior to the Burdigalian transgression. The Andreani et al.'s microtectonic data do not significantly alter this tectogenetic model but question the age and origin of horizontal strike-slip motions recorded by these authors along WE-trending fault-systems in both La Nerthe and L'Etoile ranges and referred by these authors as linked to the major Eocene (Bartonian) compressional event. However, stratigraphical and tectonic regional data suggest a shortening event during Ludian (Priabonian) times, resulting mainly in a strong folding of the southern La Nerthe range, then a Middle Oligocene strike-slip reactivation of the WE-trending system faults of La Nerthe range, coeval to the formation of north-verging overthrusts and reverse south-dipping faults, affecting early Oligocene sedimentary units of the Marseilles basin.

Fig. 1 -

 1 Fig. 1-Geological and structural map of La Nerthe range and Western L'Etoile range (after Guieu 1977, modified). A : Chapats unit-B : La Couronne unit-C : Sausset unit-S : La Folie borehole-a,b,c : location of sections on Fig. 2 -XX' location of the section Fig. 3. Fig. 1-Carte géologique et structurale du massif de La Nerthe et de la partie occidentale du massif de L'Étoile (d'après Guieu, 1977, modifié). A. Unité des Chapats-B. Unité de la Couronne-C. Unité de Sausset-S. Forage de La Folie-a,b, c : localisation des coupes sur la Fig. 2-XX' Localisation de la coupe Fig. 3.

Fig. 2 -

 2 Fig. 2-Cross-sections of the Graffiane corridor (after Guieu and Philip, 1972, modified). Sections a,b, c : location on Fig. 1 ; section d : reconstruction of the Graffiane corridor before the flexural raising of the Western Etoile range. U-J : Upper Jurassic-VAL : Valanginian-HAUT : Hauterivian-URG : Barremian (Urgonian)-L-APT : Lower Aptian-U-APT : Upper Aptian-CEN : Cenomanian-L-TUR: Lower Turonian-U-TUR: Upper Turonian-L-SEN: Lower Senonian (Coniacian-Santonian). A : Southern La Nerthe overthrust (Chapats unit)-B : Graffiane unit -C : Western Etoile range (Méjean-La Redonne Late Cretaceous syncline).

Fig. 3 -

 3 Fig. 3-Cross-section of La Nerthe range (after Guieu, 1977, modified). Location on Fig. 1 (section XX'). A. Chapats unit-B. La Folie-Valapaux faulted zone-C. Sausset unit. Φ2 : Southern La Nerthe overthrust. Fig. 3 Coupe du massif de La Nerthe (d'après Guieu, 1977, modifié). Localisation Fig. 1 (coupe XX'). A. Unité des Chapats-B. Zone faillée de La Folie-Valapaux-C. Unité de Sausset. Φ2 : chevauchement de la Nerthe méridionale

Fig. 4

 4 Fig. 4 Structure of the Southern La Nerthe range at La Baumaderie (location Fig. 1). The red arrow points out the angular unconformity between the underlying Lower Senonian folded marls of La Folie-Valapaux Faulted Zone and the overlying SSW dipping Lower Oligocene (Rupelian) lacustrine limestones. An erosional surface (yellow line) bounds the unconformably transgressive Middle-Burdigalian bioclastic limestones from the substrate. Fig. 4.-Structure de la partie méridionale du massif de la Nerthe à la Baumaderie (localisation Fig. 1). Noter la discontinuité stratigraphique (flèche rouge) entre les calcaires lacustres de l'Oligocéne inférieur (Rupélien) et les marnes du Sénonien inférieur de la Zone faillée de la Folie-Valapaux. Une surface érosive (ligne jaune) délimite les calcaires bioclastiques transgressifs et discordants du Burdigalien Moyen de leur substratum.

Acknowledgements. -François Roure and an anonymous reviewer are warmly acknowledged for improving an earlier version of the manuscript.