Geologic evolution and geodynamic controls of the Tertiary ...

Bull. Soc. géol. Fr., 2004, t. 175, no 4, pp. 361-381

Geologic evolution and geodynamic controls of the Tertiary intramontane piggyback Meso-Hellenic basin, Greece JACKY FERRIÈRE1, JEAN-YVES REYNAUD2, ANDREAS PAVLOPOULOS3, MICHEL BONNEAU4, GEORGES MIGIROS3, FRANK CHANIER1, JEAN-NOËL PROUST5 and SILVIA GARDIN6 Key words. – Sedimentary basin, Greece, Cenozoic, Piggyback, Gravity deposits

Abstract. – The Meso-Hellenic Basin (MHB) is a large, narrow and elongated basin containing up to c. 5 km of Cenozoic sediments, which partially covers the tectonic boundary between the external, western zones (Pindos) and the internal, eastern zones (Pelagonian) of the Hellenide fold-and-thrust belt. New results, based on micropaleontologic, sedimentologic and tectonic field data from the southern half of the MHB, suggest that the MHB originated as a forearc basin during the first stages of a subduction (Pindos basin), and evolved into a true piggyback basin as a result of the collision of thicker crustal units (Gavrovo-Tripolitsa). The late Eocene forearc stage is marked by sharply transgressive, deep sea turbiditic deposition on the subsiding active margin. At this stage, large scale structures of the Pelagonian basement (i.e. the newly defined “Pelagonian Indentor”) control deposition and location of two main subsiding sub-basins located on both sides of the MHB. The Eocene-Oligocene boundary corresponds to a brief tectonic inversion of the basin, at the onset of collision (main compressive event). The true piggyback stage (Oligo-Miocene) is recorded by slope deposition and dominated by gravity processes (from slumped, fine grained turbidites to conglomeratic fan- or Gilbertdeltas). The new elongated geometry of the MHB is controlled by the underthrusted, NNW-SSE trending, thick external zones. During this stage, the locus of subsidence migrates in the same direction (eastward) as underthrusting. This subsidence, favoured by thick dense ophiolitic basement, is attributed to basal tectonic erosion of the upper Pelagonian unit while the tectonic structures of this upper unit control the stepped migration of subsidence. Growing duplexes in the Gavrovo underthrusted unit, which formed local uplifts, were mainly situated on the eastern side of the subsiding areas and associated with normal faulting (late Oligocene–early Miocene). They constituted new loads that could also have been responsible for minor but widespread lithospheric subsidence. The development of the local and regional uplifts explains the basin evolution toward shallow, dominantly conglomeratic deposits and its final emergence at the end of the middle Miocene. This trend toward emersion is emphasized by the late Miocene global sea-level fall. The MHB was subsequently overprinted by neotectonic deformation associated with the development of a continental basin (Ptolemais) and uplift attributed to the evolution of the Olympos structure that developed further east as the underthrusting moved in this direction. These results demonstrate that the Meso-Hellenic Basin evolves as a large scale piggyback Basin and that its sedimentary infill is largely controled by tectonic activity rather than only eustatic sea-level variations.

Evolution géologique et contrôles géodynamiques d’un bassin intramontagneux cénozoïque : le bassin méso-hellénique, Grèce Mots-clés. – Bassin sédimentaire, Grèce, Cénozoïque, Piggyback, Dépôts gravitaires

Résumé. – Le bassin mésohellénique (MHB) est un sillon intramontagneux présentant près de 5 km de séries cénozoïques en son dépôt-centre. Les dépôts post-éocènes scellent partiellement la suture entre les zones externes (Pinde) et les zones internes (Pélagonien) des Hellénides continentales. De nouvelles études sédimentologiques, micropaléontologiques et tectoniques de la partie méridionale du bassin suggèrent que le MHB s'est mis en place en position d'avant-arc lors des premiers stades de la subduction du Pinde (Eocène supérieur), et qu'il a évolué en véritable piggyback lors de la collision des unités du Gavrovo-Tripolitsa (essentiellement à partir de l'Oligocène). Le stade avant-arc du bassin est marqué par une sédimentation turbiditique profonde contrôlée par la subsidence rapide de la marge active et localisée dans des sous-bassins liés à l'héritage structural de la marge pélagonienne (i.e. le « poinçon pélagonien », structure transverse majeure). La limite Eocène-Oligocène est un épisode bref d'inversion tectonique du bassin (phase compressive majeure) correspondant à l'entrée en collision de la marge pélagonienne avec la plate-forme externe. L'Oligocène enregistre ensuite une sédimentation marine relativement peu profonde, dominée par les processus gravitaires (slumps, turbidites fines), et alimentée par l'ouest (la chaîne du Pinde qui commence à émerger). A ce stade, le bassin est contrôlé par les zones externes en sous-charriage, qu'il s'agisse de sa géométrie, désormais allongée NNW-SSE et parallèle au front tectonique ou de sa subsidence qui se met à migrer par à-coups dans la même direction que le sous-charriage de ces zones externes (vers l'est). Cette subsidence, favorisée par la présence d'un substratum ophiolitique dense, est attribuée à de l'érosion tectonique sous l'unité pélagonienne. Elle s'atténue progressivement au Miocène, probablement par 1 2 3 4 5 6

Université de Lille 1, UFR Sciences de la Terre, UMR Pbds 59655 Villeneuve d’Ascq cedex, France. [email protected] Museum National d’Histoire Naturelle, Laboratoire de Géologie, 43 Rue Buffon, 75005 Paris, France. Mineralogy-Geology, Agricultural University of Athens, Iera odos 75, 11855 Athens, Greece. Département de Géotectonique, Université Pierre et Marie Curie, 4 Place Jussieu, BP 129, 75252 Paris cedex 05, France. Géosciences Rennes, Bat. 15, Université de Rennes 1, 35014 Rennes cedex, France Lab. Micropaléontologie, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris cedex 05, France. Manuscrit déposé le 10 octobre 2003 ; accepté après révision le 19 janvier 2004. Bull. Soc. géol. Fr., 2004, no 4

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la croissance de duplex dans les unités sous-charriées, qui seraient bloquées sur la faille égéenne (début de la surrection du massif de l'Olympe). Cet épisode s'enregistre dans le bassin par des failles normales et par le dépôt d'éventails conglomératiques marins de type Gilbert-deltas alimentés par l'est. L'émersion est accélérée par la chute eustatique du Miocène terminal. Au Plio-Quaternaire, le MHB enregistre une néotectonique correspondant à la structuration du relief pélagonien plus loin vers l'est (bassin de Ptolémais, montée de l'Olympe qui se poursuit). Cette étude suggère que l'évolution du MHB est essentiellement imputable à la géodynamique des Hellénides continentales et que les fluctuations eustatiques n'y ont qu'un rôle secondaire.

INTRODUCTION The Meso-Hellenic Basin (MHB), located in northern Greece and Albania (fig. 1), was formerly called “Albanothessalian” by Bourcart [1925], before being named MHB by Brunn [1956] and Aubouin [1959]. It has been said to be “molassic” by these authors as it is filled with detrital sediments unconformably overlying the deformed MesozoicPaleocene basement and some early Tertiary thrusts. The MHB is of importance because of (i) its large size (about 300 km long, 30 km wide), pointing to a relationship

with major orogenic processes, (ii) its lower Cenozoic age, which is a poorly known period of the Internal Hellenic chain; (iii) its nature, as the basin develops as a thrust sheet top or piggyback basin on the main upper tectonic unit of the converging system, by contrast to most basins of that size that form on the lower one [ca. foreland systems: Allen et al., 1986; Sinclair, 1997a, 1997b]. This paper aims to present relevant field observations at various scales from selected areas of the MHB in order to discuss some of the possible mechanisms at the origin of subsidence of this peculiar piggyback basin system. Struc-

FIG. 1. – Simplified geological map of the Meso-Hellenic basin (MHB) in northern continental Greece. 1 to 4: main formations of the MHB, 1: Krania and Rizoma (late Eocene), 2: Eptachorion (latest Eocene?-early Oligocene), 3: Taliaros-Pentalofos (late Oligocene-early Miocene), 4: Tsotyli-Ondrias-Orlias (early to middle Miocene), 5: Ptolemais basin (late Miocene-Pliocene, mp), 6: recent deposits. Abr. Pz: Paleozoic, TJ: Triassic and Jurassic, Ng : Neogen, V: Vourinos massif, S: synclines, A: anticlines (Af: Filippi anticline, At: Theopetra-Theotokos anticline; Fe, Fk, Ft, cf. Fig. 2. AA’: cross-section (fig. 2). Bold lines: major tectonic contacts, with rectangular boxes: late Jurassic thrusts, with white triangles: main Tertiary thrusts. Lines with black triangles: Tertiary back-thrusts or main reverse series. Dashed lines: normal faults. FIG. 1. – Carte géologique simplifiée du basin méso-hellénique (MHB) au nord de la Grèce continentale. 1 à 4 : formations principales, 1 : Krania et Rizoma (Eocène terminal), 2 : Eptachorion (Eocène terminal ?- Oligocène), 3 : Taliaros-Pentalofos (Oligocène supérieur-Miocène inférieur), 4 : TsotyliOndrias-Orlias (Miocène inférieur à moyen), 5 : bassin de Ptolemais (Miocène supérieur-Pliocène, mp), 6 : dépôts récents. Abréviations : Pz : Paléozoïque, TJ : Trias-Jurassique, Ng : Néogène, V : massif du Vourinos, S : synclinaux, A : anticlinaux (Af : anticlinal de Filippi, At : anticlinal de Theopetra-Theotokos), Fe, Fk, Ft cf. fig. 2. AA’ : coupe figure 2. Lignes en trait épais : contacts tectoniques majeurs ; avec barbules rectangulaires : chevauchements du Jurassique supérieur ; avec barbules triangulaires blanches : chevauchements tertiaires majeurs ; avec barbules triangulaires noires : rétro-chevauchements tertiaires ou séries inverses principales. Lignes tiretées : failles normales. Bull. Soc. géol. Fr., 2004, no 4

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tural analysis (stress axes, nature, scale and especially timing of deformation) as well as sedimentological studies (1D-2D facies logging and correlation) and biostratigraphical revisions (nannofossils) have been performed to assess or refine the chronostratigraphic pattern and tectonic evolution of the southern half of the MHB, where tectonic features are well expressed and stratigraphic series more complete (i.e. late Eocene is present). BACKGROUND The first studies of the MHB were focused on mapping lithological formations [Brunn, 1956; 1969; Savoyat et al., 1969, 1971a, 1971b, 1972a, 1972b; Mavridis et al. 1979; 1993; Koumantakis et al., 1980; Vidakis et al., 1998]. Other publications provided biostratigraphic refinements [Soliman and Zygojiannis, 1980; Zygojiannis and Muller, 1982], source rock studies from heavy minerals [Zygojiannis and Sidiropoulos, 1981] or olistoliths [Papanikolaou et al., 1988; Wilson, 1993], dynamics of depositional systems [Faugères, 1977a, 1977b; Desprairies, 1979; Ori and Roveri, 1987; Zelilidis et al., 1996, 1997] and large-scale industrial seismic data [Zelilidis and Kontopoulos, 1996; Kontopoulos et al., 1999; Zelilidis et al., 2002]. Tectonic data have also been used to assess the first basin models [Doutsos et al., 1994; Ferrière et al., 1998]. The MHB is a Tertiary basin, 30 km wide and more than 300 km long including its Albanian part (fig. 1). The basin mostly developed east of the main tectonic boundary between external and internal zones of the Hellenides, known as the “Internal Zones Thrust” (fig. 1 and fig. 2), part of a very large thrust system located beneath the MHB. In this area, the internal zones are made up of the Pelagonian continental crust (Triassic to Jurassic metamorphic limestones and Paleozoic gneiss) partly overlapped by upper Jurassic ophiolites thrusted again towards the west onto the Pindos area during the Tertiary events [Brunn, 1956; Aubouin, 1959]. The external zones consist of Pindos flysch nappes just west of the Pelagonian zone below which the thin [oceanic, Bonneau 1982] Pindos crust was underthrusted to the east.

The series cropping out in the Olympos tectonic window have been attributed to the external zones [Godfriaux, 1968; fig. 1 and fig. 2], sometimes to the Parnassos zone [Mercier et al., 1989], but more generally (as in this publication), because of the age of the flysch, to the GavrovoTripolitsa zone [Fleury and Godfriaux 1975]. The MHB is thus located above the main thrust bounding the internal and external zones. The Eocene age of the Olympos series in the window [Fleury and Godfriaux 1975], the ages that can be used to determine the end of deposition of the Pindos series [Lutetian on its eastern part, Lecanu 1976] and the beginning of the tectonic activity along the “Internal zones thrust” recorded in the early MHB deposits (middle and late Eocene), show that the thrust was active below the MHB during its evolution (middle or late Eocene to middle Miocene). Therefore the MHB is a true piggyback basin as defined by Ori and Friend [1984]. However, according to Doutsos et al. [1994], the MHB behaves as a retroarc foreland basin, developing in front of Pindos and ophiolitic thrusts moving towards the east, while the major thrust faults exhibit a clear vergency towards the west. For these authors, the basin architecture is related to the development of these eastward moving thrusts (including blind thrusts). The MHB has been more recently interpreted as a “strike-slip half graben”, mainly based on new interpretations of seismic lines [Zelilidis et al., 2002]. Still, we could not find any clear evidence neither for major eastverging thrust faults, nor for NNW-SSE main strike-slip faults (fig. 2). We here present an alternative hypothesis for the development of the MHB, in which the basin evolves as a piggyback basin controlled by eastward-directed underthrusting, corresponding to the Pindos subduction and the collision of the Gavrovo-Tripolitsa unit since the midlate Eocene . STRATIGRAPHY AND SEDIMENTOLOGY Overview of basin formations The MHB is filled up with about 4500-5000 m of middle Eocene to middle Miocene deposits (fig. 2 and fig. 3). Strata mostly dip towards the east, from subvertical to slightly overturned strata at the western basin boundary, down

FIG. 2. – Cross section of the MHB [modified from Ferrière et al., 1998]. See figure 1 for location. 1 to 4: MHB Formations, same captions as in figure 1. Fk, Fe and Ft: faulted-flexures of Krania (Fk), Eptachorion (Fe) and Theopetra-Theotokos (Ft). F1 and 2: main Tertiary thrusts, Fj: Jurassic obduction. Vertical scale: maximum thickness of the MHB sediments on the cross-section: 4 km. FIG. 2. – Coupe du MHB [modifié d’après Ferrière et al.., 1998]. Voir figue 1 pour la localisation. 1 à 4 : formations du MHB, mêmes légendes que dans la figure 1. Fk, Fe et Ft : flexures faillées de Krania (Fk), Eptachorion (Fe) et Theopetra-Theotokos (Ft). F1 et 2 : chevauchements tertiaires principaux ; Fj : obduction jurassique. Echelle verticale : épaisseur maximum des dépôts sur la coupe : 4 km. Bull. Soc. géol. Fr., 2004, no 4

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slightly westward dipping at the eastern border (fig. 2 and fig. 4). Miocene strata are absent in the west and rest onto the basement in the east. Seismic profiles show a pinch out of deposits at depth [Kontopoulos et al., 1999; Zelilidis et al., 2002]. These data show that deposition is controlled by an overall eastward migration of depocentres and thus of subsidence (fig. 3). The Oligocene to Miocene siliciclastic deposits were first described as six main lithostratigraphic units by Brunn [1956, 1960] from the northern part of the MHB, where Eocene strata are absent. These are: (i) Eptachorion Formation (1100 m, mainly early Oligocene) dominated by silty marls with decimetre thick very fine sandstone beds often resting on thick conglomerates; (ii) Taliaros (or Tsarnos) and (iii) Pentalofos Formations (2500 m, late Oligocene and early Miocene): sandstone beds coarsening upwards to conglomeratic beds; (iv) Tsotyli Formation (600 m, early-mid? Miocene): marls interbedded with sandstones; (v) Ondria and (vi) Orlias For-

mations (350 m or more, early-mid Miocene): sandstones and marls with fossiliferous limestone beds. The chronostratigraphy of MHB formations is still not very precise, mostly because of the scarcity of fossils or due to their reworking in gravity dominated facies. The only available ages are from marls (pelagic foraminifera, nannofossils) or from a few carbonate shelf intervals (benthic foraminifera, invertebrates). Moreover, published ages are significantly divergent, even for the same faunal associations [ca. Zygojiannis and Müller, 1982, vs Kontopoulos et al., 1999]. The lack of precise datations led authors to correlate sedimentary successions on the main base of lithology, but the absence of accurate field data on lateral facies variations induced some mistakes. An attempt of stratigraphic synthesis is given here together with a revision including our additional field and biostratigraphic data in key-areas located in the southern half of the MHB (fig. 5 and fig. 6).

FIG. 3. – Lithological formations of the MHB in the study area, from the geological maps of Greece at 1:500,000 and Bornovas and Rondogianni-Tsiambaou, 1983], and 1:50,000 (cf. references), and the synthetic map of Doutsos et al. [1994], modified in the southern half of the MHB, from our field study (see fig.4 for cross sections). Depth contours of the basement below the basin after seismic data of Kontopoulos et al. [1999] completed in the south from field data. Abr: E: early, M: middle, L: late, Eoc: Eocene, Olig: Oligocene, Mio: Miocene, N: Neogene, H: Holocene. FIG. 3. – Formations lithologiques du MHB dans le secteur d’étude, d’après la carte géologique de Grèce au 1:500 000 [Bornovas et Rondogianni-Tsiambaou, 1983] et au 1:50 000 (voir bibliographie), ainsi que d’après la carte de synthèse de Doutsos et al. [1994], modifiée dans la partie méridionale du MHB d’après nos données (voir les coupes fig. 4). Isohypses du toit du substratum d’après les données sismiques de Kontopoulos et al. [1999] complétées dans le sud par des données de terrain. Abréviations : E : inférieur, M : moyen, L : supérieur, Eoc : Eocène, Olig : Oligocène, Mio : Miocène, N : Néogène, H : Holocène. Bull. Soc. géol. Fr., 2004, no 4

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Studied series in the southern basin The southern part of the basin exhibits the most complete sedimentary succession (starting in the late Eocene) exposed within the MHB and shows clear relationships between tectonics and deposition, as demonstrated further in this paper. The Oligo-Miocene Formations have the same characteristics as their stratotypical counterparts in the north, except that Pentalofos and Tsotyli Formations exhibit a higher conglomeratic content. Two areas are detailed here, where upper Eocene deposits are present (fig. 5 and fig. 6). Krania area (western border) The Krania Formation, 1500 m thick, is well developed and preserved only inside a syn-sedimentary syncline (fig. 5B). Toward the syncline edges, various facies laterally pass to each other and finally pinch out below the EoceneOligocene unconformity. The Krania Formation exhibits a set of two sequences of deposits. The deposits rest onto roughly bedded, polygenic clast-supported conglomerate

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beds, interpreted as alluvial fan deposits, onlapping a sole of ophiolitic epiclastites (Krania). In the central part of the basin, the lower sequence is composed of fine-grained fining upwards and homogeneous sandstone beds interpreted as deep water turbidites (Krania, Microlivadon). On the northern side of the syncline, this succession passes laterally to highly bioturbated, fine grained marly sandstones with thin channel bodies of sandstones showing mud pebbles at the base, interpreted as bay-fill deposition (Trikomo), and to roughly bedded conglomerates interpreted as alluvial fans (Parorio). This facies may be highly disrupted by slumps and locally makes part of a large scale olistostrome (Monachiti), set on at the end of deposition of the lower sequence. This olistostrome, mainly composed of Cretaceous limestones collapsed from the northern basin margin, feeds channels and gullies down to the southern basin floor. It is coeval to a paroxysm of slumping in the turbiditic basin. The upper sequence is made up of the same turbiditic sands as the lower one, but exhibits at the base a sharp-based hectometric succession of thicker sandstone

FIG. 4. – Typical cross-sections of the MHB (see fig. 3 for location), compiled from our field work, geological maps of Greece at 1: 50,000 (cf. References) and seismic profiles published by Kontopoulos et al. [1999]. Lithological formations: Kr: Krania Turbidites, Ep: Eptachorion, Ta: Taliaros, Pf: Pentalofos, Ts: Tsotyli . Main faulted-flexures: Fk: Krania, Fe; Eptachorion, Ft: Theopetra-Theotokos. Vertical and horizontal scales are similar (no vertical exaggeration: v.e. = 1). FIG. 4. – Coupes significatives du MHB (voir fig. 3 pour positionnement), compilées d’après nos données de terrain, les cartes géologiques de la Grèce au 1:50 000 (voir bibliographie) et les profils sismiques publiés par Kontopoulos et al. [1999]. Formations lithologiques : Kr : turbidites de Krania, Ep : Eptachorion, Ta : Taliaros, Pf : Pentalofos, Ts : Tsotyli. Flexures faillées principales : Fk : Krania, Fe : Eptachorion, Ft : Theopetra-Theotokos. Les échelles verticales et horizontales sont identiques (pas d’exagération verticale). Bull. Soc. géol. Fr., 2004, no 4

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beds with locally abundant burrows (Skolithos), plant fragments, water escape structures and intraclastic breccias, interpreted as part of a basin floor fan (Monachiti). The Krania basin deposits are topped by the Eocene-Oligocene major unconformity made up of reddish conglomerates and paleosoils truncating the turbidites. Meteora area and eastern border In the famous Meteora area, the MHB Tertiary Formations are thinner, scattered, and show several unconformities. These unconformities are due to the interplay of the NNWSSE Theopetra-Theotokos structure, composed by a structural high (At) bounded to the east by the TheopetraTheotokos Fault (Ft; fig. 5C, see also fig. 12). On the western side of At-Ft, the series are similar to the northern ones with the conglomeratic, Pentalofos Formation (Meteora cliffs) resting on the Eptachorion Formation. By contrast, on the eastern side of Ft, Pentalofos conglomerates are absent, but the succession is more complete at the base and at the top (fig. 5). In the area around Rizoma, the succession starts with a few metres of late Lutetian benthic macroforaminifera-rich

(abundant Nummulites) limestones, corresponding to a carbonate shelf sedimentation (near Lagada these limestones are interfingered with and overlain by more than 100 m of well-rounded conglomerates). The limestones are overlain by a thick upper Eocene succession (more than 200 m) made up of marly distal turbiditic sequences, locally with sandstone beds interpreted as fluvial dominated deltaic mouth bar systems (wood fragments, floating mud pebbles, water escape features, current ripples, Skolithos traces and various burrows). The Eocene succession is covered by Oligocene basal conglomerates or Miocene conglomeratic Tsotyli beds. Mid-Miocene Echinid-rich limestones and marly turbiditic deposits delivering Globigerinidae associations typical of the Ondrias-Orlias Formations, directly deposited on the basement in the southern part of this area, cap the succession. Sedimentary record Depositional setting Our results agree with the previous sedimentological and mineralogical studies, assuming that the Miocene conglom-

FIG. 5. – Schematic stratigraphic sections of the MHB in the central (B) and southern (C) areas, compared to the former published (see text) northern one (A). 1: Pelagonian basement with ophiolites (v), 2: basal conglomerates (unconformity), 3: Conglomerates and sandstones, 4: sandstones (mainly turbiditic), 5: sandstones and shales (mainly turbiditic), 6: shales (partly hemipelagic), 7: major olistoliths, 8: Eocene detrital limestones, 9: Miocene Echinidrich limestones. Nannofossils biozones after: [], Zygojiannis and Müller [1982]; (), Kontopoulos et al. [1994] and Zelilidis et al. [2002]; free thick numbers: this publication. D: major angular unconformities, S: other significant surfaces (main lithological changes), Pz: Paleozoic, TJ: Triassic–Jurassic, UK: upper Cretaceous, Eo: Eocene, Olig: Oligocene, Mio: Miocene, E: early, M: middle, L: late. FIG. 5. – Colonnes synthétiques du MHB dans sa partie centrale (B) et méridionale (C), comparées à la colonne synthétique publiée précédemment pour la partie nord (A, voir le texte). 1 : soubassement pélagonien avec ophiolites (v) ; 2 : conglomérats de base (discordance) ; 3 : conglomérats et grès ; 4 : grès (principalement turbiditiques) ; 5 : grès et shales (principalement turbiditiques) ; 6 : shales (partiellement hémipélagiques) ; 7 : principaux olistolithes ; 8 : calcaires détritiques éocènes ; 9 : calcaires à oursins miocènes. Biozones de nannofossiles d’après : [] Zygojiannis et Müller [1982], () Kontopoulos et al. [1994] et Zelilidis et al. [2002], nombres libres en gras : cette publication. D : principales discordances angulaires ; S : autres surfaces importantes (limites lithologiques). Pz : Paléozoïque, TJ : Trias-Jurassique, UK : Crétacé supérieur, Eo : Eocène, Olig : Oligocène, Mio : Miocène, E : inférieur, M : moyen, L : supérieur. Bull. Soc. géol. Fr., 2004, no 4

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FIG. 6. – Compared ages of MHB deposits from foraminifera (D) or nannoflora biozones. A: Brunn [1956], B: geological maps of MHB areas, IGME, Greece, at 1:50,000 scale (cf References) and Bizon et al. [1968], C: Zygojiannis and Müller [1982], D: Barbieri [1992], E: Doutsos et al. [1994], Zelilidis et al. [1997, 2002] and Kontopoulos et al. [1999], F: this study. Abr. CN: nummulitic limestones, Kr.: Krania, Riz.: Rizoma, Ep.: Eptachorion, Ta.: Taliaros, Pf.: Pentalofos s.l. (Pf and Ta) or s.s. (Pf), Tso.: Tsotyli, l. and u. Met.: lower (Pf) and upper (Ts) Meteora. Single and double lines: different formations; broken thick lines: uncertainties; nannoflora biozones 16 to 25 = NP 16 to NP 25, 17 + minimum age (biozone 17 or younger), 1 to 5 = NN1 to NN5; Foraminifera biozones: P20 and P21 (D). Biozones stratigraphic boundaries and ages from Haq et al. [1987]; new stratigraphical ages from Abreu et al. [1998] on the right. FIG. 6. – Âges comparés des formations du MHB d’après les foraminifères (D) ou la nannoflore calcaire. A : Brunn [1956] ; B : carte géologique de la Grèce au 1:50 000 (voir bibliographie) et Bizon et al. [1968] ; C : Zygojiannis et Müller [1982] ; D : Barbieri [1992] ; E : Doutsos et al. [1994], Zelilidis et al. [1997, 2002] et Kontopoulos et al. [1999] ; F : cette publication. Abréviations : CN : calcaires nummulitiques, KR : Krania, Riz : Rizoma, Ep : Eptachorion, Ta : Taliaros, Pf : Pentalofos s.l. (Pf et Ta) ou s.s. (Pf), Tso : Tsotyli, l. et u. Met : Météores inférieur (Pf) et supérieur (Ts). Lignes simples et doubles : formations ; lignes brisées : indéterminations. Biozones de la nannoflore calcaire 16 à 25 : NP16 à NP25 ; 17+ : âge minimum (biozone 17 ou plus jeune) ; 1 à 5 : NN1 à NN5. Biozones de foraminifères : P20 et P21 (D). Limites stratigraphiques de biozones et âges chronostratigraphiques d’après Haq et al. [1987]. Nouveaux âges d’Abreu et al. [1998] sur la droite.

erate-rich formations (Pentalofos, Tsotyli) correspond to remnants of deltaic bodies [Desprairies, 1979] and, especially in the Meteora area, to Gilbert-type, piedmont fan deltas [Ori and Roveri,1987]. Some other conglomerates have been attributed by Zelilidis et al. [1997] and Kontopoulos et al. [1999], to alluvial fans (as those locally observed at the base of Eptachorion Formation) or shelf delta deposits (southern parts of the Pentalofos and Tsotyli Formations). The finest-grained deposits can be related to prodeltaic domains, gradually passing to the sandstones (Eptachorion to Taliaros) or interfingered with them (Rizoma). Even in the most marly, hemipelagic deposits (i.e. upper Eptachorion near Alatopetra), graded or rippled sandy laminae still occur, pointing to the permanence of turbiditic flows at the basin floor. Most of the non-conglomeratic formations have also been interpreted by Kontopoulos et al. [1999] as inner or outer fans (Krania, Eptachorion, northern Pentalofos and Tsotyli Formations) but this is partly confusing as fans may occur either on a shelfal epicontinental setting, or at the floor of a deep basin. Our observations show that most of the southern MHB deposits are related to deposition along a steep basin profile characterised by many slope instabilities (slumps, olistolites,

debris flows, mass flows, turbiditic flows) in most of reported lithologies. The maximum impact of slope failure is recorded in Eocene sandstones of Krania, one of the deepest – or the deepest – settings (common occurrence of Zoophycos traces) recorded in the MHB evolution. However, because depositional systems cannot be traced from tributaries to basin, doubt remains about the permanence of a true shelf-devoid, steep basin margin profile throughout the whole MHB evolution. For instance, Eocene Rizoma sandstones exhibit rather features of very rapid deposition (floating wood clasts, flames etc..) but no typical sliding. Sediment sources The clast petrography and mineralogy of the MHB deposits designate the proximal borders of the basin as the sources for sediment supply. The Eocene Krania deposits have a high ophiolitic content. Ophiolites constitute the bulk of the basin basement and also most of the mountains that bordered the former Krania basin to the west. To the east, by contrast, upper Eocene Rizoma deposits are not resting on ophiolites but on Mesozoic marbles and Paleozoic gneisses (due to the interplay of the basement “Pelagonian indentor”, Bull. Soc. géol. Fr., 2004, no 4

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see below). This lithological contrast is reflected in the clast composition of the deposits above. Paleocurrent analysis based on clay mineralogy and clasts imbrication, show that most of the basin fill was sourced in the west before the late Oligocene and in the east after that time [i.e. Pentalofos and Tsotyli; Desprairies, 1979]. However, Miocene transport paths may locally be much more complex, especially because of the northward deepening of the MHB (see fig. 3). Syntectonic deposition Some unconformities at the formation boundaries are the result of major basin deformation, as for example between deposition of Krania and Eptachorion Formations (fig. 5 and fig. 10). This deformation brings about emersion, as evidenced by the occurrence of reddish conglomerates and paleosoils above the unconformity in the west of Krania. Other unconformities only express the depocentre migration to the east, as evidenced by the contact of Tsotyli Formation onto the pelagonian basement (fig. 5, see also fig. 11). At the boundary between the lower and the upper part of Krania Formation, the tectonic imprint is marked by the presence of olistolithic events associated with paroxysmal slope failures (fig. 5 and fig. 10). This Eocene tectonic activity is also underlined by the unconformity between these two parts at the northern border of the Krania basin (fig. 10). The first analysis of the large scale formation geometries pointed to synsedimentary polyphased deformation along folded and/or faulted structures, described as “faulted-flexures” [Ferrière et al., 1998]. This is the case for: (i) the northern Krania basin border in the late Eocene, which controls the development of a large turbiditic synsedimentary fan and is the source of olistolithic deposition in the basin (fig. 5B and fig. 10); (ii) the TheopetraTheotokos structure bounding the late Eocene Rizoma Formation; this axis was a topographic high in the Oligocene (conglomerates only on the eastern side), still active and controlling the fan delta conglomerates of MeteoraPentalofos and Tsotyli in the Miocene (Ft; fig. 5, see also fig. 11); (iii) the western side of the MHB (locally faulted Fe flexure; fig. 4, see also fig. 9) at the beginning of Oligocene, where Eptachorion Formation also exhibits a rapid decrease of the angle of dip of turbiditic strata, locally sharply based by scattered reefal limestone build-ups, that suggests a very sudden sink of depositional areas [Ferrière et al., 1998]. Subsidence The subsidence evolution pattern is particularly well expressed in the southern MHB. As described above, subsidence overall migrates to the east. However, this is not a continuous evolution. In the late Eocene, sedimentation (Krania and Rizoma sub-basins) takes place on both sides of the MHB. During the Oligocene and early Miocene, deposition is mainly restricted to the west of the TheopetraTheotokos axis, infilling the basin in an overall west to east accretion trend but without significant subsidence migration. In the early Miocene, subsidence abruptly shifts to the east of the MHB (from Pentalofos to Tsotyli Formations; fig. 5C and fig.8). Bull. Soc. géol. Fr., 2004, no 4

Concerning the amount of subsidence and its behaviour in 3D, the lack of available boreholes, drillholes, and seismic profiles, as well as local erosion within the series and the fact that turbiditic slope depositional systems are poor bathymetric indicators, bring about uncertainties. Despite these uncertainties, some curves based on extrapolated vertical sedimentary records, from outcrops, map-derived cross-sections and published seismic profiles [Kontopoulos et al., 1999] are proposed (fig. 7). Because of the overall eastward migration of the depocentres, the onset of subsidence differs from one area to another. However, some geometrical changes, as at the Eocene-Oligocene boundary (Krania, fig. 7), or, even, main facies changes, as between Eptachorion marls and Taliaros-Pentalofos conglomerates (Alatopetra, fig.7), can be observed on the corresponding curves. The two subsidence curves proposed by Kontopoulos et al. [1999] evidence an uplift stage during Pentalofos sedimentation, from 21 to 16 Ma, but they are

FIG. 7. – Subsidence curves concerning the central part of the MHB (1: Alatopetra, 2: Grevena and 3: Krania series), from our field and seismic published data. Abbreviations: Ep: Eptachorion Formation, P: Pentalofon Formation, Ts: Tsotyli Formation. Approximations on the subsidence calculations are related to some age (see fig. 6) or formation thicknesses uncertainties, and mainly to paleobathymetric data, particularly for deep water facies (i.e. turbidites). Backstripping has been computed with SUBSILOG [Dubois et al.., 2000], using the standard parameters defined by Sclater and Christie [1980]. Grey area (c.35-33 Ma) corresponds to the main compressional episode. FIG. 7. – Courbes de subsidence pour la partie centrale du MHB (1 : Alatopetra, 2 : Grevena, 3 : Krania), d’après nos données et les profils sismiques publiés. Abréviations : Ep : formation d’Eptachorion, P : formation de Pentalofon, Ts : formation de Tsotyli. Des approximations sur le calcul de la subsidence sont liées aux incertitudes sur les âges (voir fig. 6), sur l’épaisseur des séries ou sur les estimations paléobathymétriques, particulièrement pour les faciès profonds. La restauration verticale des séries a été réalisée à l’aide du logiciel SUBSILOG [Dubois et al., 2000], utilisant les paramètres standards de Sclater et Christie [1980]. Les zones en grisé (35-33 Ma) correspondent à l’épisode compressif principal.

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match areas of drastic facies changes and follow tectonic structures (as at the Krania sub-basin northern limit, fig. 5B). For instance, the major geographic gap between Pentalofos and Tsotyli Formations (fig. 3 and fig. 5, see also fig. 11), follows the Theopetra-Theotokos structure, although no major facies change occurs at the boundary between these two conglomeratic formations in this area. By contrast, the main lithologic changes from Eptachorion marls to Pentalofos conglomerates are not associated to major changes in basin limits. Thus, an eustatic sea-level fall [Haq et al., 1987; Abreu and Haddad 1998] could be partly responsible for this evolution. However, tectonic movements also exist, notably uplifts evidenced by the development of Gilbert-deltas, as in the Meteora area. Zelilidis et al., [2002] argues that all 'the stratigraphic occurrence of lowstand facies compares closely with published eustatic sea-level curves’.. We consider that this apparent correlation remains more than questionnable, because (i) there is no accurate biostratigraphic control and (ii) the required tectonic calendar is not considered, although the authors admit the existence of syntectonic sedimentation and major tectonic contacts. TECTONICS General view

FIG. 8. – Paleogeographic synthesis of possible basin and sub-basins extension at different stages of MHB evolution. The limits here minimise the depositional areas (e.g. we could not exclude a possible connection of the sea between Krania and Rizoma in the upper Eocene, especially along the tectonic structures bounding the Pelagonian Indentor). Abbreviations: see figure 3. FIG. 8. – Reconstitution paléogéographique des bassins et sous-bassins et de leur extension aux différentes étapes de l’évolution du MHB. Les limites minimisent les aires de dépôt (par exemple on ne peut exclure une connexion possible entre les sous-bassins de Krania et Rizoma à l’Eocène supérieur, particulièrement le long des structures tectoniques du poinçon pélagonien). Abréviations : voir figure 3.

only representative of the axis of the present MHB (areas of maximum residual thicknesses). The general pattern of the migration of MHB depocentres and associated subsidence is synthesized by the map in figure 8 (see also schematic curves fig. 13). Paleogeography The paleogeographical sketch (fig. 8) is based on subsidence but also lateral and vertical facies variations. The frequent absence of shoreline deposits and possible erosion brings about uncertainties concerning the true extension of the basin limits through time. Therefore, the proposed limits minimise the marine depositional area extension: i.e., the Rizoma and Krania sub-basins probably respectively extended to the north and to the east along the tectonic structures of the Pelagonian Indentor, but they were partly eroded at the Eocene-Oligocene boundary. These limits

In cross-section, the MHB is an asymmetrical syncline, the western flank of which is steeper than the eastern one. A few vertical or nearly overturned strata are even observed at its western boundary (Fk on D or Fe on E fig. 4, fig. 9 and fig. 10) or at the eastern border of the Theopetra-Theotokos structure (TTS, see below) (B on fig. 4 and fig. 11). To the south, the main syncline splits into two narrow synclines separated by this structural high (TTS), expressed as an anticline (At) faulted (Ft) on its eastern border (fig. 9 and fig. 11). Relationships observed between sedimentary and tectonic features (olistoliths; progressive variations of strata dips, as shown inside Eptachorion Formation near Alatopetra; angular unconformities) show that the syncline folding is mostly coeval to basin infilling, lasting even well after marine retreat, as demonstrated by the folding of the last middle Miocene deposits south of Rizoma (fig. 11 and fig. 12). Normal faults of metric to hectometric offset, mostly directed sub-parallel to the basin axis, are common in all formations (fig.4), and important ones follow the basin borders (Vourinos and Koziakas, fig. 3 and 4). Some large scale normal faults were identified from seismic lines within the basin [Kontopoulos et al.., 1999]. Some of these normal faults are certainly of Plio-Quaternary ages [Aubouin, 1956]. Most of large scale complex fault zones have experienced several motions, including normal faulting prior to some compressional deformation. They appear now as subvertical or even slightly reversed faults (e.g. Ft at the eastern border of the TTS, fig. 4, fig. 11 and fig. 12). Large faults transverse to the basin axis, as reported on the 1: 500,000 maps [Bornovas and Rondogianni-Tsiambaou, 1983] are also common. Some of them linked to deep transverse structures of the MHB basement appear to have had a control on the Eocene sub-basins organisation (i.e. southern and northern borders of Krania sub-basin). Bull. Soc. géol. Fr., 2004, no 4

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FIG. 9. – Synthetic structural map of the Greek part of the MHB and internal zones of Hellenides. Note the coincidence between the Pelagonian Indentor (double thin lines bounded by double dashed lines for the northern flexure), the Theopetra-Theotokos structure (TTS=At+Ft) and the Rizoma elongated subbasin (1B) on the west; and the structural saddle of Kozani (S.4) and Krania sub-basin on the north-west. Abbreviations: S.1 to S.4: synclines; Af: Filippi anticline, At: Theopetra-Theotokos anticline or structural high ; Fk, Fe and Ft: faulted-flexures of Krania, Eptachorion and Theopetra. Enclosed map: 1 to 7: sites of tectonic data reported from stereograms. a (1 to 3): compression observed in late Eocene formations (1: Monachiti, 2: Krania and 3: Rizoma areas); b (4 to 7): extension observed in the different formations (4: Eptachorion, 5: Krania-Grevena, 6: West Vourinos and 7: Meteora-Theotokos areas); aD (compression) and bD (extension) from Doutsos et al. [1994]. FIG. 9. – Carte structurale synthétique de la partie grecque du MHB et des zones internes des Hellénides. Noter la coincidence entre, d’une part, le poinçon pélagonien (ligne double limitée par ligne pointillée double pour la flexure nord), la structure de Theopetra-Theotokos (TTS=At+Ft) et le sous-bassin allongé de Rizoma (1B) à l’ouest, et, d’autre part, l’ensellement de Kozani (S4) et le sous-bassin de Krania au nord-ouest. Abréviations : S1 à S4 : synclinaux, Af : anticlinal de Filippi, At : anticlinal de Theopetra-Theotokos (ou haut structural), Fk, Fe et Ft : flexures faillées de Krania, Eptachorion et Theopetra. Carte en insert : 1 à 7 : sites de mesure des données reportées dans les stéréogrammes. a (1 à 3) : compression observée dans les formations de l’Eocène supérieur (1 : Monachiti, 2 : Krania et 3 : Rizoma) ; b : (4 à 7) : extension observée dans différentes formations (4 : Eptachorion, 5 : KraniaGrevena, 6 : Vourinos-ouest et 7 : Météores-Theotokos) ; aD (compression) et bD (extension) d’après Doutsos et al. [1994].

While a lot of microtectonic data concerning the MHB have been published by Doutsos et al. [1994], the chronology and causality of these deformations are still not well established. Our results show that micro- and meso-structures are different within late Eocene and Oligo-Miocene formations (fig. 9). Oligo-Miocene formations Metric to decametric mostly longitudinal conjugate normal faults dominate, part of them being post-depositional. A few are coeval to deposition (as in the north of Theotokos in the Oligocene). The main direction of σ3 axis is overall perpendicular to the main basin direction (fig. 9). Hectometric open folds are also present in the vicinity of major polyphased tectonic structures, mainly affecting Miocene Bull. Soc. géol. Fr., 2004, no 4

strata, such as for example between Theotokos and Asproklissia in Tsotyli Formation (fig. 4C) or to the south of Rizoma in Ondria Formation (FF’; fig. 12). Upper Eocene formations In addition to the features described above, well-expressed compressional structures are present, thus, related to an Eocene-Oligocene boundary folding episode. These compressional structures are mainly metric to decametric reverse faults and small folds associated or not with slight cleavage. In Krania and Mylia (SW of Krania) series, these deformations are pervasive, coupled with decametric thrusts towards the east (fig. 4D). The main stress axis (σ1) deduced from field data (fig. 9) is generally about NE-SW, subperpendicular to the MHB elongation. The Monachiti

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FIG. 10. – Geologic map and cross-section of the northern border of Krania sub-basin (Monachiti transverse structure); see figure 3 for location. 1: ophiolitic basement (v: peridotites, dots: lavas), 2: upper Cretaceous limestones, 3: upper Eocene, lower series within (3a) and beyond (3b-3c) the northern subbasin limits, 3a: turbiditic shales and sandstones (flysch) on basal breccias and conglomerates, 3b: fine grained sandy bay deposits, 3c: well rounded conglomerates; 4: large olistostromes with a breccia matrix (main blocks: black triangles), 5: pluridecametric, massive or little brecciated olistoliths; 6: upper series (late Eocene); turbidites (flysch) above thick sandstones (dashed lines); 7: early Oligocene (latest Eocene?) conglomerates and sandstones. S (S1, S1b, S2): surfaces indicating major tectonic events; D: angular unconformities (see also fig. 5). Vertical and horizontal scales are similar (no vertical exaggeration, v.e.=1). FIG. 10. – Carte géologique et coupe de la bordure nord du sous-bassin de Krania (structure transverse de Monachiti) ; voir figure 3 pour localisation. 1 : soubassement ophiolitique (v : péridotites, pointillés : laves) ; 2 : calcaires du Crétacé supérieur ; 3 : Eocène supérieur, séries inférieures au sein du sous-bassin (3a) et à l’extérieur de celui-ci (3b-3c) : 3a : shales et grès turbiditiques (flysch) reposant sur brèches et conglomérats de base, 3b : dépôts fins de baie, 3c : poudingues ; 4 : grands olistostromes à matrice bréchique (blocs principaux : triangles) ; 5 : olistolites décamétriques, massifs ou faiblement bréchifiés ; 6 : conglomérats et grès des séries supérieures (Eocène terminal ?). S (S1, S1b, S2) : surfaces indiquant des événements tectoniques majeurs ; D : discordances angulaires (voir également fig. 5). Les échelles verticales et horizontales sont identiques (pas d’exagération verticale).

structure, as well as other transverse structures (E-W to NESW) might have had some strike-slip activity, especially at the Eocene-Oligocene boundary, but no clear evidence of such significant strike-slip motion could be evidenced from field analysis. Longitudinal basin-scale strike-slip faults (NNW-SSE) have been proposed by Zelilidis et al. [2002],

based on some microtectonic data [Doutsos et al, 1994] and interpretations of a few seismic lines. Still, these authors do not provide any precision about the importance of the displacements implied nor about their chronology. Moreover, such large longitudinal strike-slip faults could not be observed from field analysis: for example, the contact between Bull. Soc. géol. Fr., 2004, no 4

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FIG. 11. – Geologic map of the southern area of the MHB (Meteora). 1: Paleozoic gneisses and schists (Pz), 2: Triassic-Jurassic marbles (TJ, on the east) and upper Cretaceous(UK) limestones of the Theopetra anticline (At), 3: ophiolites, 4: Middle Eocene clastic limestones overlain by upper Eocene finegrained turbidites (unifites) or deltaic sandstones (Rizoma sub-basin), 5: Oligocene conglomerates, sandstones and marls (Eptachorion Formation), 6: Lower Meteora conglomerates (Pentalofos Formation), 7: Upper Meteora conglomerates (Tsotyli Formation). 8 and 9: Miocene Ondria Formation: Echinidrich limestones (8), sandstones and Globigerinidae marls (9), 10: Quaternary and recent deposits. See figure 12 for cross-sections F to H. FIG. 11. – Carte géologique de la partie méridionale du MHB (Météores). 1 : gneiss et schistes paléozoïques (Pz) ; 2 : marbres du Trias-Jurassique (TJ, à l’Est) et calcaires du Crétacé supérieur (UK) de l’anticlinal de Theopetra (At) ; 3 : ophiolites ; 4 : calcaires clastiques de l’Eocène moyen, surmontés par des turbidites fines ou des grès deltaïques de l’Eocène supérieur (sous-bassin de Rizoma) ; 5 : conglomérats, grès et marnes de l’Oligocène (formation d’Eptachorion) ; 6 : conglomérats inférieurs des Météores (formation de Pentalofos) ; 7 : conglomérats supérieurs des Météores (formation de Tsotyli) ; 8 et 9 : formation miocène d’Ondria : calcaires à oursins (8), grès et marnes à globigérines (9) ; 10 : dépôts quaternaires. Voir figure 12 pour les coupes F à H.

Eptachorion and Pentalofos Formations supposed to be a major strike-slip fault [Zelilidis et al., 2002], is clearly stratigraphic at all localities where it could be observed. Tectonic structures of the western basin border The western border of the elongated MHB is basically an Oligocene flexure, the eastern flank of which is collapsed, and may locally be defined by reverse faults (Fe, fig. 4 and fig. 9). North of the Krania sub-basin, the basal Oligocene deposits onlap the Filippi anticline, an elongated window of Pindos Eocene flysch below the overthrusted ophiolites (fig. 4E and Af fig. 9). Because of the characteristics of the Oligocene strata on the eastern side of the Filippi anticline (quick deepening of the facies, progressive changes of the dippings), the growth of this fold is thought to have lasted during Oligocene. South of Krania, Oligocene strata dip less, but still rest either onto ophiolites, Pindos flysch or Koziakas series (bringing about a pre-Oligocene and postLutetian age for the Internal thrust zone and the E-W Kastaniotikos transverse flexure; fig. 9). The upper Eocene Krania sediments form a complex perisyncline termination, which was mainly active during the late Eocene (see above and fig. 10). It is bounded to the south by transverse (approximately E-W) faults, and to the west by a subvertical flexure, locally reverse and faulted (Fk, fig. 10). East-verging tectonic structures, mostly decametric reverse faults, are numerous in the Krania subBull. Soc. géol. Fr., 2004, no 4

basin fill, especially near Mylia (SW of Krania) or Microlivadon (fig. 4D and fig. 10). They are related to the main compressive phase (at the end of Eocene). To the north, the Krania sub-basin is bounded by a large WSWENE flexure, the Monachiti-Trikomo structure (MTS), active at least during the late Eocene because: (i) sediments of this age are different on both sides of the MTS; (ii) the middle Krania sandstones (S1b, fig. 5B and fig. 10) rest unconformably on the northern side of the MTS, source of the hectometric olistoliths and olistolithic channel-fills of the Krania sub-basin; (iii) Oligocene strata of the MHB truncate the uppermost, subvertical upper Eocene turbiditic sandstone strata of the Krania sub-basin onto the southern flank of the MTS. As the Oligocene transgression proceeded, the northern side of the MTS was probably still high, as indicated by the presence, exclusively on its southern side, of Oligocene conglomeratic alluvial fans, pointing to southward paleoflows (fig. 10). The geometry of the northern boundary of the Krania sub-basin suggests that the MTS could have developed as a WSW-ENE dextral strike-slip fault, as already suggested by Papanikolaou et al. [1988] and admitted by Zelilidis et al., [2002]. This remains to be demonstrated as there are no equivalent structural patterns on both sides of this structure (fig. 10) and if horizontal slickensides exist, they are rare. The compressive stresses observed in the Krania sub-basin (σ1 approximately SW-NE, fig. 9) are able to trigger a

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FIG. 12. – Cross-sections of the MHB southern area (see fig. 11 for location). 1: Paleozoic gneisses and schists (Pz), 2: Triassic-Jurassic marbles (TJ), 3: ophiolites, 4: Cretaceous limestones, 5: Upper Lutetian-Bartonian p.p. clastic limestones (above Theopetra Cretaceous rocks and at the base of Rizoma sub-basin), 6: Upper Eocene fine-grained turbidites (unifites) and deltaic sandstones (Rizoma sub-basin), 7: Oligocene conglomerates, sandstones and marls (Eptachorion Formation), 8: Lower Meteora conglomerates (Pentalofos Formation), 9: Upper Meteora conglomerates (Tsotyli Formation). 10 and 11: Miocene Ondria Formation with Echinid-rich limestones (10), sandstones and Globigerinidae marls (11). TTS: Theopetra-Theotokos structure. Vertical and horizontal scales are similar (no vertical exaggeration: v.e.=1). FIG. 12. – Coupes de la partie méridionale du MHB (voir fig. 11 pour la localisation). 1 : gneiss et schistes paléozoïques (Pz) ; 2 : marbres du Trias-Jurassique (TJ) ; 3 : ophiolites ; 4 : calcaires du Crétacé ; 5 : calcaires clastiques du Lutétien supérieur-Bartonien p.p. (surmontant le Crétacé de Théopetra et à la base des séries du sous-bassin de Rizoma) ; 6 : turbidites fines et grès deltaïques de l’Eocène supérieur (sous-bassin de Rizoma) ; 7 : conglomérats, grès et marnes de l’Oligocène (formation d’Eptachorion) ; 8 : conglomérats inférieurs des Météores (formation de Pentalofos) ; 9 : conglomérats supérieurs des Météores (formation de Tsotyli) ; 10 et 11 : formation miocène d’Ondria avec calcaires à oursins (10), grès et marnes à globigérines (11). TTS : structure de Theopetra-Theotokos. Les échelles verticales et horizontales sont identiques (pas d’exagération verticale).

strike-slip displacement along this WSW-ENE structure. However, the direction of the main observed decametric fold (near Trikomo), subparallel to the MTS, argue in favour of a large WSW-ENE directed flexure (fig. 10) responsible for the N-S steep depositional profile (from bay-fill to more southern basin floor-fan, via canyon-fills and large slumps). The Theopetra-Theotokos tectonic structure (median MHB) Deposits on both sides of this newly defined TheopetraTheotokos tectonic structure (TTS fig. 9, fig. 11 and fig. 12) are mostly of different ages (fig. 1, fig. 5 and fig. 9). The TTS, well expressed in the southern MHB area, corresponds to a NNW-SSE anticline (At), the eastern flank of which is bounded by a main fault (Ft). This fault is a major basement boundary as ophiolites appear mainly west of the fault. The TTS has controlled deposition in the southern basin from Eocene to Miocene times (fig. 5C and fig. 12). (i) De-

position of the upper Eocene Rizoma deltaic and prodeltaic systems was restricted to the east of this structure. If they ever existed to the west, their total erosion suggests they had to be very thin compared to the 200-300 m thick deposits in the Rizoma sub-basin (fig. 11 and fig. 12). (ii) During the Oligocene (Eptachorion Formation), the structure still existed as a morphological step, as demonstrated by the presence of conglomeratic alluvial-fans along the footwall of its eastern side and correlative fine-grained turbidites deposits onlaping the western flank of the structure (see H in fig. 12). (iii) In the Miocene, the lower Meteora conglomerates (Pentalofos Formation) developed only on the western side of this structure. The tilting towards the west of the substrate of Pentalofos Formation is responsible for the sudden reversal of sediment sources from west to east and, for the internal architecture of these conglomerate fan deltas (fig. 12). In the same way, due to the interplay of the TTS, Tsotyli series are restricted to the east of this structure (fig. 11). The TTS area is also affected by a NE-SW compression at the Eocene/Oligocene boundary, characterised by reverse Bull. Soc. géol. Fr., 2004, no 4

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faulting towards the NE or ENE in the upper Eocene Rizoma sediments, unconformably covered by nearly horizontal Oligocene strata (fig. 12). Many reverse faults (dipping 60° to the west) recorded in the Cretaceous limestones are probably linked to the same compressive event, as these faults are mainly located near the main fault (Ft).

sin remnant on an active marine margin. The pattern of subsidence, in that case, should be constrainable by mechanical parameters of the subducting crust (thickness, temperature, angle of subducting slab). However, the boundaries of Krania and Rizoma basins are still controlled by structures of the upper unit at this time (flexures at the edges of the Pelagonian Indentor) (stages A, B fig. 14A and fig. 14B).

The Pelagonian Indentor (eastern MHB) The Pelagonian Indentor (PI), displaying a main control on the MHB evolution, is here described for the first time (fig. 9). It corresponds to an elongated Pelagonian block transverse to the MHB. It is bounded to the east by the Aegean fault (between Olympos and Thermaikos Gulf), to the west by the TTS and late Eocene Rizoma outcrops, to the south by Larissa and Trikkala subsiding areas, and to the north by the NE-SW saddle of Kozani-Krania [Kozani straight of Brunn, 1956; Kozani saddle of Aubouin, 1959], where some remnants of the Vermion nappe and Vourinos ophiolites are preserved (fig. 1 and fig. 9). The narrowest section of the MHB occurs in front of the PI, in the Meteora sector. The upper Eocene sub-basins of Krania and Rizoma develop and/or are preserved at its margins (Krania sub-basin in the axis of the Kozani saddle, Rizoma sub-basin along the TTS and the PI; fig. 9). The deformation around the PI extends far into the external zones, as series preserved in Pindos and Gavrovo zones right to the west of the saddle of Kozani-Krania are younger (Eocene flysch) than external series (mainly Mesozoic sediments) preserved to the south of the Kastaniotikos transverse structure in front of the PI (fig. 9). Several hypothesis may explain the high elevation of the PI regarding to surrounding areas: i) the location beneath the PI of a thick crustal body subducted from the west of the Pelagonian margin, before (or during) the Lutetian; ii) a coeval tectonic phase that would have been purely transverse, as the Cenozoic ones (before Lutetian p.p.) described in these parts of the internal zones [Ferrière, 1982]. BASIN INTERPRETATION Our new data show that the piggyback MHB evolution is mainly controlled by geodynamic processes and that eustatic controls are of relatively minor importance. The main trends (depth variations, existence and nature of tectonic episodes, amount of subsidence) are interpreted to result from the behaviour of the underthrusted unit (i.e: thickness of the crust) while the detailed sedimentological organisation and architecture depend on the tectonic structures of the upper unit. The eustatic sea-level changes have a weaker control than tectonics on the basin evolution (fig. 13, fig. 14 and fig. 15). Opening: from subduction to collision Late Lutetian-late Eocene: Pindos subduction Sedimentation in Krania and Rizoma sub-basins takes place during the onset of convergence and related Pindos subduction, responsible for the development of an accretionary prism below the ophiolitic-pelagonian upper unit (A, B, fig. 15). Because it originated at the back of a rising accretionary prism, the deep sub-basin of Krania, and the sub-basin of Rizoma, may be considered as a forearc baBull. Soc. géol. Fr., 2004, no 4

Eocene-Oligocene transition: Gavrovo-Tripolitsa collision The Eocene-Oligocene transition corresponds to a major angular unconformity, especially near large tectonic structures (e.g. Monachiti, fig. 10 and Theopetra, Ft in fig. 11). This is the period of maximum compressional deformation (eastward verging reverse faults in Krania-Mylia and TheopetraAvra areas, folding and slight cleavage). These features are consistent with the continuing rise of the Pindos accretionary prism, opening the main flysch windows below the ophiolitic nappes (fig. 4 E), and leading to the emersion of the Krania sub-basin (stage C fig. 14A), as evidenced by reddish conglomerates at the unconformity. We consider that these events are the results of the arrival in the subduction zone of the Gavrovo-Tripolitsa platform unit (C fig. 15). The collision seems obvious, even though the Gavrovo-Tripolitsa crust may have been slightly thinner than a normal continental crust, as suggested by its continuous subsidence during the Mesozoic and early Paleogene [Aubouin, 1959]. From this hypothesis it appears that the time required for the subduction of the whole Pindos basin is about 10 Ma (45-43 Ma to 35-33 Ma). The width of the Pindos zone, 300 to 600 km in the northern Hellenides, is deduced from i) mapping of the isopic zones (fig. 1), ii) the amount of tectonic shortening estimated from outcrops and reconstructed cross-sections, and iii) the estimated extension rate of this continental [Thiebault, 1982] or oceanic basin crust [Bonneau, 1982]. These approximate values give an average Pindos subduction rate of about 3 to 6 cm/year, that is consistent for such basins. Eptachorion stage (Oligocene p.p.): underthrusting of the Gavrovo-Tripolitsa unit The fine-grained Eptachorion Formation is characterized by a very strong tectonic subsidence, marked by synsedimentary normal faults and the onlap of relatively deep turbiditic facies against sub-aerial to shallow marine basal conglomerates or reef limestones due to a sharp steepening of depositional profiles. At the same time, the former Pindos accretionary prism and anticlines keep rising, as demonstrated by the syntectonic fans recorded in the lower part of turbiditic deposits. Such a strong subsidence in the course of the major underthrusting of a thick crust has to be explained. As the MHB evolves into a major NNW-SSE marine trough, the process is necessarily at the scale of the whole margin. The presence of thick dense ophiolitic bodies west of the Pelagonian unit, forming the basement of the MHB, as in Albania [Robertson and Shallo, 2000] could partly control the location of the main subsiding areas. However, the NNW-SSE direction of the new Oligo-Miocene elongated MHB trough, parallel to the Hellenic front thrust and to the external zones, argue for a main control by the external underthrusted units. Delamination at the base (basal tectonic erosion) of the Pelagonian upper unit beneath the basin (fig. 15D) due to the passing through of the

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FIG. 13. – Synthesis of MHB stages of evolution. Left: tectonic context, right: eustatic curves [from Haq et al., 1987 and Abreu and Haddad, 1998], bottom: subsidence through time from 1a as early Lutetian. Tp and Tt: local transpression or transtension. See text for discussion. FIG. 13. – Schéma synthétique résumant l’évolution du MHB. A gauche : contexte tectonique ; à droite : courbes eustatiques [d’après Haq et al., 1987 et Abreu et Haddad, 1998] ; en bas : évolution de la subsidence au cours du temps depuis le stade 1a : début du Lutétien. Tp et Tt : transpression ou transtension locale. Voir le texte pour discussion.

Gavrovo-Tripolitsa thick platform is here proposed to explain some subsidence processes (C, D fig. 15).

the most recent charts edited for the European basins [Abreu and Haddad, 1998] (fig. 13).

Closure: from collision to underplating

This basin stage, starting with Pentalofos Formation, is mainly controlled by uplift of the domain east of the MHB which becomes the main drainage area (E fig. 15). The high sedimentation rate (maybe the first time in the basin evolution that sedimentation may take subsidence over) associated with conglomeratic deposition of Pentalofos leads to rapid overfilling of the basin west of the TTS (stage E fig. 14B). The subsidence jump to the east (Tsotyli Formation) across this Theopetra-Theotokos structure is interpreted as the eastward progression of the process of delamination beneath the basin, coupled with an increasing uplift rate (because of underplating) in the hinterland. These eastward migration of subsidence and uplifted areas give probably rise to major normal faults well-expressed in the MHB. The basin closure is linked to the general uplift of the MHB area possibly emphasized by the sea-level fall known in the mid-late Miocene [Abreu and Haddad, 1998]. Flexuration (south-east of Rizoma) and folding (east of Theotokos) last during and/or after basin closure, due to a renewed tectonic activity at the eastern side of the basin before the Plio-Quaternary normal faulting well-known in this part of the Hellenides [Aubouin, 1959; Mercier et al., 1989].

Taliaros-Pentalofos-Tsotyli stage (latest Oligocene-middle Miocene) The transition between the Eptachorion (turbidites and marls) and Taliaros (deltas) Formations locally gradational (as observed near Alatopetra), reflects an overall progradation trend during the Oligo-Miocene times. Whereas conglomerates of Pentalofos overlie Taliaros deltas in the northern MHB, they directly truncate Eptachorion turbidites in the south. The onset of conglomeratic fan-deltas, that dominate deposition during the late Oligoceneearly Miocene times is partly due to the tectonic activity. This is demonstrated by the reactivation of existing tectonic structures at the time of Pentalofos Formation deposition (movements along the Theopetra-Theotokos Structure) and deposition of Gilbert-deltas (as the Pentalofos conglomerates in the Meteora area), that implies progradation on a steep basin slope (stage E fig. 14A, B). The additional imprint of the Rupelian-Chattian eustatic sea-level fall [Haq et al., 1987] could explain this sharp Eptachorion-Pentalofos boundary, but this event seems less prominent on some of

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FIG. 14A. – Main stages of MHB evolution from Pindos subduction (upper Eocene) to collision (from Oligocene), Krania-Vourinos section (central MHB). See text for explanations. FIG. 14A. – Etapes principales de l’évolution du MHB depuis la subduction du Pinde (Eocène supérieur) jusqu’à la collision (depuis l’Oligocène), coupe dans la partie centrale du bassin (Krania-Vourinos). Voir explications dans le texte.

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FIG. 14B. – Main stages of MHB evolution from Pindos subduction (upper Eocene) to collision (from Oligocene), Koziakas-Meteora-Rizoma section (southern MHB). Vertical and horizontal scales are similar. See text for explanations. FIG. 14B. – Etapes principales de l’évolution du MHB depuis la subduction du Pinde (Eocène supérieur) jusqu’à la collision (depuis l’Oligocène), coupe méridionale (Koziakas-Météores-Rizoma). Les échelles verticales et horizontales sont identiques (pas d’exagération verticale). Voir explications dans le texte. Bull. Soc. géol. Fr., 2004, no 4

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FIG. 15. – Tertiary geodynamic evolution from Gavrovo-Pindos external zones to eastern Olympos – Vardar/Thermaikos areas and MHB evolution. See text for detailed discussion on mechanisms. Note that basal tectonic erosion could be partly responsible for subsidence from stages C to D. FIG. 15. – Evolution géodynamique des Hellénides au Cénozoïque, depuis les zones externes du Gavrovo-Pinde à l’ouest jusqu’aux zones internes du Vardar-Thermaïque à l’est, en relation avec l’évolution du MHB. Noter que l’érosion tectonique pourrait être responsable de la subsidence dans les étapes C à D. Bull. Soc. géol. Fr., 2004, no 4

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Olympos rise (after Miocene times) The uplift of the domain east of the MHB is thought to be linked to the underplating of basal Pelagonian material and deep tectonic ramps and duplex caused by locking to the east of the underthrusted units, probably against the Aegean fault and block (F fig. 15). The rise of the Olympos area (50 km east of the MHB) would reflect the piling up of such duplexes. Metamorphic rocks cropping out in the Olympos windows have been analysed, mainly from fission tracks. The results about the Olympos units, excluding the Ambelakia series (they are not demonstrated to belong either to the internal or external zones), published by Schermer et al. [1989], Kilias et al. [1991] and Schermer [1993], do not disagree with the general trend of the tectonic calendar proposed from our MHB study (fig. 15) : i) 42-36 Ma: blueschists; ii) 36-25 Ma: open folds; iii) 23-16 Ma: low-angle normal faults; iv) open folds and high-angle normal faults. Our model states the progression of underthrusting as far as a backstop exists (for instance the Aegean faultblock). It is thus expected that this migration may have a similar impact on basin generation further east from the MHB. The development of the Ptolemaïs basin, by its shape (elongated NNW-SSE trough), age (mainly upper MiocenePliocene), depositional environments (continental) and location on the western side of a main (Olympos) uplift, supports this idea. SUMMARY AND CONCLUSIONS Numerous large-scale complex, polyphased structures affect the southern MHB. These features show that deposition is largely syntectonic, as expected for a piggyback basin. This is supported by the basin fill nature and stratigraphic organisation. The following results may be highlighted. 1) Important tectonic structures influenced the basin evolution. These are either transverse structures, which control upper Eocene depositional areas (i.e. the northern border of the Krania sub-basin), or major structures parallel to the NNW-SSE basin axis (here named faulted-flexures), active during the main tectonic phases, especially at the Eocene-Oligocene boundary. These latter structures, as the Theopetra-Theotokos structure, reactivated during the Miocene, controls the Meteora system. These different structures are mainly linked to major heterogeneities within the Pelagonian basement, the main one is described here as the Pelagonian Indentor. 2) Several phases of basin evolution are distinguished. During the late Eocene (lasting ca. 10 Ma), scattered, possibly deep depocentres evolve as Pindos (thin, probably oceanic, crust) subduction proceeds. A brief episode of major compression follows (lasting 2-3 Ma), as the GavrovoTripolitsa (thicker crust) collides the Pelagonian basement of the MHB. This brings about reverse faulting and emersion of the western Pelagonian margin. During the underthrusting of Gavrovo zone beneath the Pelagonian unit, from Oligocene to mid-Miocene (ca. 20 Ma), the MHB evolves, parallel (NNW-SSE) to the main thrusts and external zones, which are thus considered as the main control factor on the basin shape. Tectonic erosion at the base of the Pelagonian unit, migrating in the same direction as underthrusting, may control subsidence at that time. The

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overall coarsening of deposits from Oligocene (Eptachorion Formation) to Miocene (Pentalofos Formation) is related to the continuing uplift of the basin borders, mainly on the eastern side, due to the formation of tectonic ramps in the underthrusted complex. Numerous normal faults appear as subsidence and adjacent uplifts work and migrate to the east. 3) Our results are consistent with those expected from a piggyback basin (this latter point being demonstrated by the presence of external Hellenides in the Olympos window). They do not support the hypothesis of a retroarc type basin [Doutsos et al., 1993], because the observed reverse faults towards the east at the western basin boundary are nearly vertical or of limited extension, and mainly active during a very short period of time (Eocene/Oligocene boundary). In the same way, our observations do not support the hypothesis that the MHB could be a “strike-slip half graben” [Zelilidis et al.] as no any major NNW-SSE strike-slip faults have been recognised, especially in the areas where such faults are drawn by these authors. Some of our results may have a general significance for large-scale piggyback basins. a) Multi-scale facies and stratal architecture are mainly controlled by tectonics. – Many observed sedimentary features (as olistoliths and slumps) within the MHB are the consequence of tectonic events (from elementary seisms to main uplifts inducing gravity slides). Because tectonics is active, depositional systems are not graded to their equilibrium profiles. For instance, there is a general lack of shelfal deposits but presence of Gilbert deltas. This does not rule out the possible impact of eustatic changes on local stratal patterns. A sea-level rise could have emphasized the Oligocene transgression within the MHB area, and a sea-level fall during mid-Oligocene times could partly explain the transition from Eptachorion marls to Pentalofos conglomerates. However, main tectonic controls (compressional or extensional events, vertical motions, folding) are clearly demonstrated for each main change within the MHB; thus eustatic controls are considered to have weaker effects than tectonic ones. b) Both units, above and below the main thrust, successively control subsidence within the piggyback basin. – Before collision, dense ophiolitic basement and large structural heterogeneities of the Pelagonian crust (upper unit) controlled the basin subsidence (location of Krania and Rizoma sub-basins linked to the presence of a major Pelagonian uplifted structure, here named the “Pelagonian Indentor”). The development of subsidence during the collision process, is more difficult to explain. Because the Oligo-Miocene MHB becomes a large basin parallel to the thrust front and to the external zones, it seems to be mainly controlled by the NNW-SSE elongated units underthrusted below the Pelagonian crust. Basal tectonic erosion of the upper unit and tectonic load (duplexes) on the lower one have been considered to explain the development and migration of this Oligo-Miocene subsidence. c) Subsidence migrates. – As in foreland basins, subsidence migrates as thrusting progresses. Foreland basins are developing in front of the main thrusts and subsidence migrates in the same direction as thrusting. On the contrary, large piggyback basins, as the MHB, develop on top of the main thrusting unit and subsidence migrates in the opposite Bull. Soc. géol. Fr., 2004, no 4

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direction of the thrusting motion. This emphasizes the major role of the underthrusting unit (“subducting” slab) on the development of this type of basin. However, this general behaviour is still to be demonstrated for piggyback basins from other case studies. d) Basin closure is mainly related to underplating. – Just before the final basin closure (calcareous, marly Ondrias-Orlias Formations), coarse grained deposition (Pentalofos and pro parte Tsotyli formations) is related to the development of tectonic ramps beneath and beside the basin (mainly on the eastern side of the MHB) which leads to the steepening of proximal depositional profiles. The basin remained below sea-level most of the time, due to the interplay of a strong subsidence, but the overall regressive trend linked to underplating, maybe emphasized by a sealevel fall, leads to the final emersion before or during late Miocene.

Finally, the MHB seems a good example for reevaluating the relative importance of piggyback basins in convergent settings. The Hellenic chain records a continuum of tectonics versus sedimentation processes from an early subduction to an established collision, and only two well expressed basin systems are recorded: the first one is equivalent to a deep forearc, and the second one to a true piggyback (the latter being more important in its volume and stratigraphic record). Other piggyback basins of very large extent could have been mistaken for foreland basins, especially in areas where blind thrust may have developed.

Acknowledgements. – This work has been supported by EC Projects (PLATON) and by CNRS (UMR Pbds 8110) and Agricultural University of Athens fundings. The authors would like to thank the different reviewers, especially P. Vergely, for their helpful remarks.

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