Claudio Faccenna
email: claudio.faccenna@uniroma3.it

Frédéric Gueydan
email: frederic.gueydan@univ-montp2.fr

Dimitrios Sokoutis
email: d.sokoutis@uu.nl

Mélody Philippon
email: melodie.philippon@gm.univ-montp2.fr

Konstantinos Kydonakis
email: konstantinos.kydonakis@univ-rennes1.fr

Christian Gorini
email: christian.gorini@upmc.fr

Jean-Pierre Brun
email: jean-pierre.brun@univ-rennes1.fr

Kevin C Burke

John F Dewey

The two-stage aegean extension, from localized to distributed, a result of slab rollback acceleration

Keywords: Hellenic subduction, slab rollback, trench retreat, Aegean back-arc extension

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The Aegean Tertiary tectonic history corresponds to back-arc extension driven by slab rollback [START_REF] Royden | The tectonic expression slab pull at continental convergent boundaries[END_REF][START_REF] Jolivet | Mediterranean extension and the Africa-Eurasia collision[END_REF][START_REF] Faccenna | Subduction and the depth of convection in the Mediterranean mantle[END_REF], 2014[START_REF] Brun | Exhumation of high-pressure rocks driven by slab rollback[END_REF]. Extension started around 45 Ma ago [START_REF] Brun | 45 m.y. of Aegean crust and mantle flow driven by trench retreat[END_REF] and accommodated up to 600 km of trench retreat [START_REF] Jolivet | Cenozoic geodynamic evolution of the Aegean[END_REF][START_REF] Jolivet | Aegean tectonics: Strain localisation, slab tearing and trench retreat[END_REF]. Extension followed the closure of Vardar and Pindos oceanic domains in Cretaceous-Eocene [START_REF] Dercourt | Atlas Tethys Paleoenvironmental Maps, Beicip-Franlab[END_REF][START_REF] Channell | How many oceans? Meliata, Vardar, and Pindos oceans in Mesozoic Alpine paleogeography[END_REF]Robertson 2004) that led to the stacking of three continental blocks: Rhodopia, Pelagonia and Adria, from top to base (Fig. 1). Tomographic models of the underlying mantle image a single slab [START_REF] Wortel | Subduction and slab detachment in the Mediterranean-Carpathian region[END_REF]Piromallo and Morelli 2003;[START_REF] Widiyantoro | Deformation of the Aegean Slab in the Mantle Transition Zone[END_REF]) indicating that the convergence of continental blocks, now separated by two suture zones, has been accommodated by a single subduction [START_REF] Faccenna | Subduction and the depth of convection in the Mediterranean mantle[END_REF].

The first plate kinematic models of eastern Mediterranean [START_REF] Mckenzie | Active tectonics of the Mediterranean region[END_REF][START_REF] Mckenzie | Active tectonics of the Alpine Himalayan Belt, the Aegean Sea and surrounding regions[END_REF][START_REF] Le Pichon | The Aegean Sea[END_REF] and the present-day displacement field from satellite geodesy [START_REF] Mascle | Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus[END_REF][START_REF] Hollenstein | Crustal motion and deformation in Greece from a decade of GPS measurements, 1993-2003[END_REF]Müller et al. 2013) show that the active pattern of extension combines the effects of the southwestward retreat of the Hellenic trench and the westward displacement of Anatolia along the North Anatolian Fault (NAF). The geological record shows that this interaction between two strongly oblique components of boundary displacement started during Middle Miocene [START_REF] Dewey | Aegean and surrounding regions: complex multiplate and continuum tectonics in a convergent zone[END_REF]Şengör et al. 2005;[START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF], around 10 My before the NAF reached the Aegean [START_REF] Armijo | Westward propagation of the north Anatolian into the northern Aegean: timing and kinematics[END_REF][START_REF] Hubert-Ferrari | Long-term Elasticity in the Continental Lithosphere; Modelling the Aden Ridge Propagation and the Anatolian Extrusion Process[END_REF]Şengör et al. 2005). history during a large part of the Tertiary indicates a process that has not been continuous, neither in time nor in space. This is illustrated by a striking difference in the distribution of Paleogene and Neogene sedimentary basins at Aegean scale (Fig. 2) suggesting that a major change in the dynamics of extension occurred in Middle Miocene, more 30 My after its onset. Paleogene (a) and Neogene (b) basins in the Aegean domain. Acronyms same as in Figure 1.

Fig. 2. Distribution of

The present article describes back-arc extension in the Aegean, in two main stages localized from Middle Eocene to Middle Miocene and distributed since Middle Miocene, and their most significant large-scale features in terms of sedimentation, deformation and metamorphism. Then, it presents a 3D restoration of the extensional displacements in four steps since Middle Eocene. It is shown that the transition in the mode of extension from localized (core complex) to distributed (wide rift), during Middle Miocene, is coeval with an acceleration of trench retreat. Finally, it is argued that this acceleration i) likely resulted from the tearing of the Hellenic slab documented by mantle tomography and ii) was coeval with the onset of Anatolia westward displacement.

Stage 1: Paleogene basins and ductile exhumation of metamorphic rocks

The first stage of extension, from Middle Eocene to Middle Miocene is recorded in the deposition of sedimentary basins and in the exhumation of high-temperature metamorphic core complexes and high-pressure metamorphic belts.

Paleogene basins that mostly contain Middle Eocene and/or Oligocene sediments are located i) on top of the Rhodopia block (Trace Basin: [START_REF] Görür | A fore-arc origin for the Thrace Basin, NW Turkey[END_REF][START_REF] Siyako | Eocene stratigraphic evolution of the Thrace Basin, Turkey[END_REF][START_REF] Kilias | The Thrace basin in the Rhodope province of NE Greece -A tertiary supradetachment basin and its geodynamic implications[END_REF]; Vardar-Thermaikos Basin: Roussos 1994; Carras and Georgala 1998) and ii) on top of Pelagonia (Mesohellenic Trough: [START_REF] Doutsos | Intracontinental wedging and postorogenic collapse in Mesohellenic trough[END_REF][START_REF] Ferrière | Geologic evolution and geodynamic controls of the Tertiary intramontane piggyback Meso-Hellenic basin, Greece[END_REF]) (Fig. 2a). The Thrace Basin that started subsiding in Early Eocene contains sedimentary rocks from Lower-Middle Eocene to Pleistocene, reaching a thickness up to 9000 m. The sedimentary units are dominantly marine-prodelta organicrich shales and turbidites. The transition from Eocene to Lower Oligocene outcrops in the Lemnos Island [START_REF] Maravelis | Organic geochemical characteristics of the late Eocene-early Oligocene submarine fans and shelf deposits on Lemnos Island, NE Greece[END_REF]. The Thrace Basin has recorded only moderate tectonic events. The initial subsidence controlled the deposition of Middle-Eocene to Oligocene sediments. Early Miocene sediments deposited during an event of inversion/folding are unconformably sealed by Middle to Late Miocene deposits. The Mesohellenic Through (MHT) extends over 300 Km with a mean width of 40 km from Albania to Greece along a NNW-SSE trend along the northern side of the Pindos suture zone [START_REF] Doutsos | Intracontinental wedging and postorogenic collapse in Mesohellenic trough[END_REF][START_REF] Ferrière | Geologic evolution and geodynamic controls of the Tertiary intramontane piggyback Meso-Hellenic basin, Greece[END_REF]. Its sedimentary fill also ranges from the Late Eocene to Middle Miocene. A first megasequence is composed, from base to top, of Late Eocene tectonic breccia, mass transport deposits, turbidites, fluvial conglomerates and deltaic plain sediments at the transition with the Oligocene. A second megasequence is characterized by Oligocene carbonates at the base. This platform is rapidly drowned by a rapid subsidence with the deposition of hundred meters of sands and silts organized in deltaic lobes.

The Southern Rhodope Core Complex (SRCC) (Fig. 3) [START_REF] Dinter | Late Cenozoic extension in northeastern Greece: Strymon Valley detachment system and Rhodope metamorphic core complex[END_REF][START_REF] Sokoutis | A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension[END_REF][START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF], located to the North of the Vardar suture zone, started to develop in Middle Eocene, around 45 Ma, and was controlled by the SW dipping Kerdylion detachment. The detachment hanging-wall is made of the metamorphic units of the Chalkidiki peninsula whose tectonic-metamorphic evolution is dominantly Mesozoic and which correspond to the western and most external part of Rhodopia (Kydonakis et al. 2015b and in press). The detachment footwall (i.e. the core) consists mostly a Hercynian basement made of orthogneisses, paragneisses and marbles dated as Permian at their base (i.e. a Pelagonian-type assemblage) affected by a high-temperature metamorphism that at many places reached partial melting in Barrovian-type metamorphic conditions [START_REF] Dimitriadis | Sillimanite grade metamorphism in the Thassos Island, Rhodope Massif, Greece, and its regional significance[END_REF]). To the North, the Nestos Thrust [START_REF] Burg | Rhodope: From Mesozoic convergence to Cenozoic extension. Review of petro-structural data in the geochronological frame[END_REF][START_REF] Nagel | The exposed base of a collapsing wedge: The Nestos Shear Zone (Rhodope Metamorphic Province, Greece)[END_REF]) separates these core units from the northern part of Rhodopia. Therefore, at regional-scale, the SRCC corresponds to a tectonic window in which Pelagonia is exposed. Consequently, we interpret the Nestos thrust that separates northern Rhodopia from Pelagonia as the Vardar Suture Zone. Prior to the 50 Ma migmatisation [START_REF] Wawrzenitz | Exhumation and doming of the Thasos metamorphic core complex (S Rhodope, Greece): structural and geochronological constraints[END_REF], the Pelagonian core units were sheared and duplicated by SW directed thrusting [START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF]. From Middle Eocene to Middle Miocene, the same units, as well as Oligocene and Lower Miocene granite intrusions [START_REF] Kyriakopoulos | Rb-Sr geochronological, petrological and structural study of the Kavala plutonic complex (N. Greece)[END_REF][START_REF] Kyriakopoulos | Thermochronological evolution of Symvolon and Pangeon pluton and their country rocks, Kavala area, N. Greece: an apatite fission track analysis[END_REF][START_REF] Kolocotroni | The origin and emplacement of the Vrondou granite, Serres, NE Greece[END_REF][START_REF] Dinter | U-Pb and Ar/Ar geochronology of the Symvolon granodiorite: Implications for the thermal and structural evolution of the Rhodope metamorphic core complex, northeastern Greece[END_REF], recorded a second shearing event again toward the SW but in extension, during the exhumation of the core complex [START_REF] Dinter | Late Cenozoic extension in northeastern Greece: Strymon Valley detachment system and Rhodope metamorphic core complex[END_REF][START_REF] Sokoutis | A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension[END_REF]; that extensional phase was primarily controlled by the SW-dipping Kerdylion Detachment [START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF]. Duchêne et al. 2006 in Jolivet andBrun 2010) and whose detachment dip northeastward to the West and Northward to the East. Red arrows: syn-metamorphic senses of shear. Senses of shear associated to core complex extension after [START_REF] Sokoutis | A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension[END_REF] and [START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF] for the SRCC and Gautier et al (1993) and [START_REF] Gautier | Crustal-scale geometry and kinematics of late-orogenic extension in the central Aegean (Cyclades and Evvia Island)[END_REF] for the CCCC.

Everywhere within the exhumed units, the gneisses are mylonitic but approaching the Kerdylion Detachment they become ultramylonitic with thick cataclasites at the hangingwall contact. Paleomagnetic data [START_REF] Dimitriadis | Dextral rotation and tectonomagmatic evolution of the southern Rhodope and adjacent regions (Greece)[END_REF] indicate that the core complex exhumed during a 30° dextral rotation of the hanging-wall (Chalkidiki Peninsula) what is in agreement with the northwestward-closing triangular shape of the core complex map contours at regional scale [START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF]. Since Middle Miocene, the exhumed core units have been segmented by two sets of normal faults trending NE-SW and NW-SE that controlled the deposition of Neogene basins [START_REF] Lalechos | Correlations and observations in molassic sediments in onshore and offshore areas of northern Greece[END_REF]Snel et al. 2006, Brun and[START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF]. In its largest width, to the Southeast, the bulk extensional displacement, including the brittle segmentation of the core complex, reached around 120 km. The Central Cyclades Core Complex (CCCC) [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF]) is located the South of the Vardar Suture Zone and outcrops in the central islands of the Cyclades (Fig. 3): Naxos-Paros and Mykonos-Delos-Rhenia. This core complex developed entirely within the Adria crustal block. The core units display a high-temperature metamorphism reaching partial melting that was superposed to a previous stage of high-pressure metamorphism. In Paros a fast heating from 350°C to 700°C occurred between 35 and 20 Ma [START_REF] Bargnesi | Miocene core complex development and coeval supradetachment basin evolution of Paros, Greece, insights from (U-Th)/He thermochronometry[END_REF]. In Naxos, where the peak of high-pressure metamorphism is dated around 45-50 Ma (Duchene et al 2006;Martin et al 2006), partial melting commenced prior to 20.7 Ma [START_REF] Keay | The timing of partial melting, Barrovian metamorphism and granite intrusion in the Naxos metamorphic core complex, Cyclades, Aegean Sea, Greece[END_REF]. A characteristic PTt trajectory of Naxos migmatites is shown in Figure 3 (modified after [START_REF] Duchêne | Pressure-temperature-time evolution of metamorphic rocks from Naxos (Cyclades, Greece): constraints from Thermobarometry and Rb/Sr dating[END_REF]. Core units show a stretching lineation dominantly oriented N-S with an associated top N sense of shear in Naxos-Paros (Gautier et al. 1989[START_REF] Gautier | Structure and kinematics of Upper Cenozoic extensional detachment on Naxos and Paros (Cyclades Islands, Greece)[END_REF][START_REF] Urai | Alpine deformation on Naxos (Greece)[END_REF]Buik 1991) and NE-SW-oriented with a top NE sense of shear in Mykonos-Delos-Rhenia (Leconte et al. 2010) in agreement with a N-dipping but wavy detachment in Naxos-Paros and a NE-dipping detachment in Mykonos-Delos-Rhenia. These differences in structural and kinematic patterns are often interpreted as two separated core complexes with independent and nonconnected detachments (e.g. Jolivet et al. 2010;[START_REF] Denèle | Granite intrusion in a metamorphic core complex: the example of the Mykonos laccolith (Cyclades, Greece)[END_REF]. Consequently, this would imply distinct dynamics of development. The two groups of islands are separated by a regional-scale discontinuity trending NE-SW [START_REF] Gautier | Crustal-scale geometry and kinematics of late-orogenic extension in the central Aegean (Cyclades and Evvia Island)[END_REF]; later called -Mid-Cyclades Lineament‖ by Walcott and White 1998). [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF] showed that the restoration of post-Middle Miocene faulting using available paleomagnetic data brings the two trends of stretching lineations into parallelism and the two groups of islands in geometrical continuity, revealing that hidden below the scattering of islands was initially a single core complex (i.e. the CCCC). The restoration also showed that the Mid-Cyclades Lineament can be interpreted as a dextral strike-slip fault, with an offset in the order of 50 km, that was called by [START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF] Myrthes-Ikaria fault (MIF in Fig. 3) as it transforms the post-Middle Miocene opening of the Myrthes and Ikaria basins located at its SW and NE tips, respectively. In this frame, the wavy shape of the Naxos-Paros detachment appears to result from an E-W component of shortening (as already suggested by [START_REF] Avigad | Ductile and brittle shortening, extension-parallel folds and maintenance of crustal thickness in the central Aegean (Cyclades, Greece)[END_REF] at the scale of the central Cyclades and by [START_REF] Urai | Alpine deformation on Naxos (Greece)[END_REF]Buik (1991) from the observation of outcrop-scale folds in Naxos).

High-pressure metamorphism (In blue in Fig. 4) affected Adria and partly Pelagonia during the closure of the Vardar and Pindos oceanic domains. Their exhumation was accommodated by extension, dominantly during -Stage 1‖ and partly during -Stage 2‖. The Cycladic Blueschist Unit (CBU) is constituted by: i) at the base, a Hercynian basement dominantly made of granite orthogneisses, ii) a sedimentary cover where alternate marbles and schist sequences whose depositional ages range from Visean to Eocene and iii) at the top, an ophiolitic mélange made of serpentine schists with Triassic to Cretaceous metagabbro and metabasalt knockers that could represent either relicts of the Pindos Ocean itself or part of the Adria subcontinental lithospheric mantle that was partially molten during the stretching and rifting stage of the Pindos ocean. The rather similar ranges of pressure peaks in the oceanic-type rocks (0.8-2.2 GPa) and in the sedimentary cover (0.6-1.8 GPa) indicate that the basement, its sedimentary cover and the oceanic-type rocks were subducted at comparable depths. The compilation of geochronological data obtained by various methods in 10 different Cycladic islands (Fig. 5 in [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF] shows: i) that the ages of high-pressure metamorphism range between 58 and 40 Ma and ii) that the blueschist-greenschist transition Fig. 4. High-pressure metamorphism in the Adria and Pelagonia blocks. PTt diagrams illustrate the exhumation history of the Cycladic Blueschist Unit in Tinos (Numbers in circles: Time in My) (After Parra et al. 2002 in Jolivet andBrun 2010) and of the " Phyllite-Quartzite Nappe » in Peloponnese (Numbers in circles: Time in My) (After Jolivet et al. 2010a). Black arrows: sense of shear related to subduction. White arrows: sense of shear related to exhumation. Senses of shear associated to subduction (black arrows) after [START_REF] Huet | Thrust or detachment? Exhumation processes in the Aegean: Insight from a field study on Ios (Cyclades, Greece)[END_REF] and [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF]. Senses of shear associated to exhumation (white arrows) after [START_REF] Gautier | Crustal-scale geometry and kinematics of late-orogenic extension in the central Aegean (Cyclades and Evvia Island)[END_REF] for the Cyclades and Jolivet et al (2010a) for the Peloponnese and Crete. occurred in a narrow age range of 3 My between 35 and 32 Ma. This indicates that the CBU was exhuming in Late Eocene-Early Oligocene as a whole coherent unit, in agreement with the preservation, in many islands, of the superposition cover on basement with or without ophiolitic mélange at the top. In addition, nummulitic turbidites (Lutetian) are involved in thrust deformation and have recorded a metamorphic pressure of 1-1.2 GPa [START_REF] Shaked | Alpine high-pressure metamorphism at the Almyropotamos window (southern Evia, Greece)[END_REF][START_REF] Ring | Samos Island, Part I: metamorphosed and nonmetamorphosed nappes, and sedimentary basins[END_REF][START_REF] Rosenbaum | Structure and metamorphism of Amorgos: a field excursion[END_REF]. Together with the Middle Eocene age of the youngest pressure peaks, this shows that subduction-related thrusting was active until Middle Eocene. As the blueschist-greenschist transition occurred between 35 and 32 Ma, the delay between the end of subduction and the onset of exhumation has been short, more likely less than 5 My. The PTt trajectory of Tinos blueschists (Fig. 4) [START_REF] Parra | Relation between the intensity of deformation and rétrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chlorite-mica local equilibria[END_REF][START_REF] Jolivet | Cenozoic geodynamic evolution of the Aegean[END_REF] shows that an isobaric heating occurred during exhumation between 37 and 33 Ma. As Tinos is close to the CCCC (Fig. 3) this is more likely related with the heating observed in Paros in the same range of ages [START_REF] Bargnesi | Miocene core complex development and coeval supradetachment basin evolution of Paros, Greece, insights from (U-Th)/He thermochronometry[END_REF]. Three main events of pervasive ductile deformation that can be characterized at regional scale by stretching lineations and associated senses of shear characterize the CBU [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF]): ( 1) prior to at least 40 Ma, a subduction-related layer-parallel shear top to SW and top to S to the NW and to the SE of the MIF, respectively (Black arrows in Fig. 4), ( 2) from 40 to 20 Ma, an exhumationrelated layer-parallel shear of weak to moderate intensity, prior to the onset of the North Cycladic detachment, top to N or NE to the NW and to the SE of the MIF, respectively (White arrows in Fig. 4), (3) from 20 to 13 Ma and observed in the islands adjacent to the North Cycladic detachment, a top to NE shear in general of strong intensity affects the blueschists as well as high temperature rocks of the CCCC (Red arrows in Fig. 3). The high-pressure metamorphic unit of Peloponnese and Crete is characterized by an exhumation history younger than the CBU one (Fig. 4). It was still undergoing burial when the CBU was already exhuming. Exhumation occurred in Late Oligocene-Lower Miocene without significant heating during exhumation. In this most external part of Adria, high-pressure low-temperature (HP-LT) metamorphism is only recorded in the -Phyllite-Quartzite Nappe -(PQN) (see comprehensive review by Jolivet et al. 2010b and references therein). The HP-LT PQN is sandwiched between two thick thrust units composed of Triassic to Eocene formations, namely the Gavrovo-Tripolitza Nappe (GTN) on top and the -Plattenkalk Nappe‖ below. As pointed out by Stöckhert et al. (1999), the average resistance of the PQN was much smaller than the overlying and underlying units and consequently localized a large part of the deformation during burial and exhumation. From a mechanical point of view, the PQN is therefore a décollement (i.e. dominated by layer-parallel shear) and relating its deformation to a detachment (shear zone cutting down section), as done in most published works, is rather misleading. This is confirmed by the presence of the GTN over the whole Peloponnese-Crete region and the common outcropping of the PQN in windows of the GTN. A detachment, significant at regional-scale, should have exposed the PQN over a large domain without the GTN on top. In Peloponnese, stretching lineations related to thrusting and exhumation trend EW but with a sense of shear top to W for thrusting (black arrows in Fig. 4) and top to E for exhumation (white arrows in Fig. 4). In Crete, stretching lineations related to exhumation trend NS (white arrows in Fig. 4). The orthogonality of stretching directions related to exhumation and the convergence of associated senses of shear imply that Peloponnese and Crete have undergone a nearly 90° relative rotation after the end of ductile deformation.

In summary, the exhumation of core complexes (high-temperature metamorphism) and blueschists (high-pressure metamorphism) results from significantly different mechanisms of development, primarily controlled by temperature-dependent rheology of the crustal units. Therefore, their location in the Aegean, as well as their relative timing of development, has an important mechanical significance: The SRCC started to develop in Middle-Late Eocene in North Aegean when the CBU started to exhume in central Aegean, The CCCC developed in central Aegean in lower Miocene almost synchronous with the onset of HP-LT PQN exhumation in Peloponnese and Crete.

The sense of shear and detachment dip in core complexes and sense of shear in highpressure rocks, is top to SW in North Aegean (SRCC), to NE in central Aegean (CBU and CCCC) and to E and N in South Aegean (HP-LT PQN). The part of exhumation synchronous with ductile deformation ended in Middle Miocene in all types of metamorphic rocks, either high-temperature (SRCC and CCCC) or high pressure (CBU and HP-LT PQN) and whatever age of onset.

Remark: The high-pressure to ultrahigh-pressure metamorphic units of the Southwest Rhodope (Kydonakis et al. 2015b) and the North Rhodope [START_REF] Mposkos | Diamond, former coesite and supersilicic garnet in metasedimentary rocks from the Greek Rhodope: a new ultrahigh-pressure metamorphic province established[END_REF]Kostopoulos 2001, Liati 2005) developed during the Cretaceous, and were exhumed before Eocene, prior to the onset of back-arc extension in the Aegean [START_REF] Kydonakis | Upper Cretaceous exhumation of the western Rhodope Metamorphic Province (Chalkidiki Peninsula, northern Greece)[END_REF], Kydonakis et al. 2015band 2015c) controlled by the Hellenic slab rollback. Therefore, they are outside the scope of the present paper.

Stage 2: Neogene basins, segmentation of metamorphic units and dextral transtensional faulting

The Neogene basins (Fig. 2b) whose deposition started in Middle Miocene constitute one of the most striking geological features of the Aegean domain, both onshore and offshore. They emplaced on all types of rock units (Paleogene basins, high-temperature or highpressure metamorphic units, plutonic massives and volcanic buildups) of Rhodopia, Pelagonia and Adria and over around 1000 km from Crete to Rhodope. The earlier deposits are Langhian-Serravalian in some basins of the North Aegean (e.g. Prinos; [START_REF] Chiotis | A Middle Miocene thermal event in northern Greece confirmed by coalification measurements[END_REF][START_REF] Beniest | Structure and seismic stratigraphy of deep Tertiary basins in the northern Aegean Sea[END_REF]. Where structural data are available, field measurements or seismics, tectonic setting of most basins is extensional or transtensional (e.g. [START_REF] Mercier | Change in the state of stress in the overriding plate of a subduction zone: the Aegean Arc from the Pliocene to the present[END_REF][START_REF] Mercier | Extensional tectonic regimes in the Aegean basins during the Cenozoic[END_REF][START_REF] Lyberis | Tectonic evolution of the North Aegean trough[END_REF][START_REF] Mascle | Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus[END_REF][START_REF] Koukouvelas | fault structure and related basins of the North Aegean:Sea and its surroundings[END_REF][START_REF] Sakellariou | Strike slip tectonics and transtensional deformation in the Aegean region and the Hellenic arc: preliminary results[END_REF]). In the islands of the North Aegean and Eastern Cyclades where it can be observed, the Late Miocene corresponds to continental lacustrine and alluvial series (Lesbos and Chios: Jones 1971; Samos: [START_REF] Weidmann | Neogene stratigraphy of the eastern basin, Samos Island, Greece[END_REF][START_REF] Beniest | Structure and seismic stratigraphy of deep Tertiary basins in the northern Aegean Sea[END_REF]. In Samos, the sediments are dated from 10 to 6 Ma by the intercalation of volcanic rocks [START_REF] Weidmann | Neogene stratigraphy of the eastern basin, Samos Island, Greece[END_REF]) and mammals [START_REF] Koufos | The Late Miocene Mammal Faunas of the Mytilinii Basin, Samos Island, Greece: New Collection. 16[END_REF], with a very rapid subsidence until 8 Ma [START_REF] Deschamps | Caractérisation des séries lacustres du Miocène Supérieur de l'Ile de Samos. Un exemple de paléolac alcalin et salin[END_REF]. During the Messinian, a sea of brackish-water character developed between the Paratethys to the North and the Mediterranean to the South. At that time the Aegean Sea was principally fed by surrounding rivers and/or by Paratethyan waters. The sedimentary sequence deposited is principally composed of limestone containing brackish-water Paratethyan fauna, with some intercalations with Mediterranean species indicating the re-establishment of a non-permanent Atlantic-Mediterranean connection and, thus, replenishment of marine waters [START_REF] Bache | A two-step process for the reflooding of the Mediterranean after the Messinian salinity crisis[END_REF]. The Pliocene-Quaternary is characterized by a definitive restoration of the marine conditions in the Aegean.

Low-temperature thermochronology ages, obtained by various methods (apatite and zircon fission-track and U-Th/He on apatite and zircon) in high-temperature and highpressure metamorphic units, which were exhumed during the first stage of extension, are dominantly Serravalian-Tortonian, over the whole Aegean (Fig. 5). This indicates that metamorphic rocks of the SRCC, the CBU-CCCC and Peloponnese-Crete, whose onsets of exhumation were different, were reaching the surface in Middle-Late Miocene. In the core complexes and high-pressure metamorphic rocks of the Aegean these ages of exhumation are commonly considered to provide an age for the end of normal sense displacement along a detachment. But the synchronism, at the whole Aegean scale, between the first sediment deposition in Neogene basins and the final stages of metamorphic rock exhumation put this type of interpretation in question, as most Neogene basins cannot be put in relation with any major crustal-scale detachment. In the Rhodope, Serravalian-Tortonian sediments are deposited on the hanging-wall of normal faults that cut through the metamorphic rocks of the SRCC [START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF]. Simultaneously, the footwall of these faults was uplifted, reaching altitudes up to 1000-2000m. These normal faults trend either NW-SE, almost parallel to the core complex detachment, or NE-SW, perpendicular to the detachment. Whatever the fault trend, rocks from their footwall give the same range of thermochronology ages (Fig. 5), showing that these ages are not related to the functioning of the core complex

Fig. 5. Frequency distribution histograms of low-temperature thermochronology ages (apatite and zircon fission tracks (AFT and ZFT) and UTh/He on apatite and zircon) in

Rhodope (data from [START_REF] Wuthrich | Low temperature thermochronology of the North Aegean Rhodope Massif[END_REF], Cyclades (Compilation of data: [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF] and Peloponnese-Western Crete (data from [START_REF] Brix | Thermobarometric data from a fossil zircon partial annealing zone in high pressure-low temperature rocks of eastern and central Crete, Greece[END_REF][START_REF] Marsellos | Rapid Exhumation of High-Pressure Metamorphic Rocks in Kythera-Peloponnese (Greece) Revealed by Apatite (U-Th)/He Thermochronology[END_REF]. Numbers correspond to the number of ages within a bin. detachment but to a superposed event of faulting that segmented the core complex. The Cyclades archipelago resulted from the segmentation by normal faults within the CBU and the CCCC (see restoration in [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF]. As quoted in section 2.2, the Myrthes-Ikaria fault (MIF), which trends NE-SW, separated the Cyclades in two main domains (Fig. 6) [START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF]. To the NW of the MIF, the islands correspond to the residual reliefs remaining above the sea level from the uplift of normal fault footwalls. To the SE border of MIF, the high altitude of Paros-Naxos (up to 600 m) is likely related to upright folding of the core complex under a component of EW shortening [START_REF] Avigad | Ductile and brittle shortening, extension-parallel folds and maintenance of crustal thickness in the central Aegean (Cyclades, Greece)[END_REF][START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF]. Two reasons could explain the lack of Middle-Late Miocene basin remnants in the Cyclades: i) the islands represent the upper part of normal fault footwalls and ii) erosion could have removed possibly inverted basins. However, Samos to the East of the Cyclades shows Tortonian sediments that were deposited in transtension and locally reworked in compression around 9 Ma (Ring and Ochrusk 2007). The statistical distribution of thermochronology ages (Fig. 5) obtained in 10 Cyclades islands, in either the CBU or the CCCC, is Middle-Late Miocene with a strong peak in Tortonian (See data compilation in Fig. 12 of [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF]. [START_REF] Carbotte | New integrated Data Management System for Ridge 2000 and MARGINS Research[END_REF]). The Myrthes-Ikaria fault that connects the Myrthes and Ikaria basins [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF][START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF] is the offshore extend of the onshore Izmir-Balikeshir Transfer Zone (Sozbilir et al. 2010), located between the Izmir-Ankara suture zone and the Menderes Massif (pink).

Fig. 6. Major strike-slip faults and sedimentary basins in the Aegean Sea, as displayed by bathymetry (GMRT bathymetry data from

Dextral strike-slip faulting is an important part of the active Aegean fault pattern, in particular in the North Aegean, recognized through dynamic analysis of fault systems [START_REF] Mercier | Change in the state of stress in the overriding plate of a subduction zone: the Aegean Arc from the Pliocene to the present[END_REF][START_REF] Mercier | Extensional tectonic regimes in the Aegean basins during the Cenozoic[END_REF][START_REF] Lyberis | Tectonic evolution of the North Aegean trough[END_REF][START_REF] Lyberis | Tectonic evolution of the North Aegean area during the Plio-Pleistocene[END_REF], structural mapping [START_REF] Armijo | Westward propagation of the north Anatolian into the northern Aegean: timing and kinematics[END_REF][START_REF] Papanikolaou | Postcollisional tectonics and magmatism in the Mediterranean region and Asia[END_REF], seismicity and earthquake focal mechanisms [START_REF] Hatzfeld | The Mediterranean Basins: Tertiary Extension within the Alpine Orogen[END_REF][START_REF] Taymaz | Active tectonics of the north and central Aegean Sea[END_REF]Goldworthy et al. 2002). Among these faults, the 1200 km-long North Anatolian Fault (NAF), which today accommodates the westward displacement of Anatolia, focus considerable and justified attention related to seismic risk from both societal consequences and scientific points of view.

Fig. 7. Present-day displacements (after Nyst and Thatcher 2004) superposed to the map of Neogene basins (Yellow) showing that a major change occurred in the course of post-Middle Miocene deformation when the North Anatolian Fault (NAF) localized and connected to the North Aegean Through (NAT).

In the frame of this paper, our concern is the role, in time and space, that the NAF played in the Aegean dynamic evolution since Middle Miocene, called here stage 2 extension. Thirty-six years ago, [START_REF] Dewey | Aegean and surrounding regions: complex multiplate and continuum tectonics in a convergent zone[END_REF] recognized that the NAF had been preceded by a larger zone of displacement initiated in Late Miocene, connected to the Bitlis suture in eastern Anatolia, as the -Proto-Anatolian Transform‖. More recently, Şengör et al. (2005) on the basis of quantitative arguments, derived from the study of Neogene basins in North Anatolia, showed that the NAF resulted from strain localization within the 100 km wide North Anatolian Shear Zone (NASZ). Localization in Western Turkey (Dardanelle strait) is estimated around 5 Ma [START_REF] Armijo | Westward propagation of the north Anatolian into the northern Aegean: timing and kinematics[END_REF]. To the West of the Dardanelle Strait, the NAF takes a NE-SW direction and joins the SSW-NNE-trending North Aegean Through (NAT) [START_REF] Lyberis | Tectonic evolution of the North Aegean trough[END_REF][START_REF] Koukouvelas | fault structure and related basins of the North Aegean:Sea and its surroundings[END_REF] that is the major bathymetric depression of North Aegean [START_REF] Papanikolaou | Morphotectonic structure of the western part of the North Aegean Basin based on swath bathymetry[END_REF]. The NAT likely originated in Paleogene as a transtensional structure to laterally accommodate the dextral rotation of the Chalkidiki block with reference to North Rhodope during the exhumation of the Southern Rhodope Core Complex. In the junction area between the NAT and the NAF, positive flower structures [START_REF] Roussos | Structure of the Central North Aegean Trough: an active strike-slip deformation zone[END_REF] suggest a reactivation of pre-existing faults of the NAT at the propagating tip of the NAF. The dextral strike-slip displacements that characterize the NAF become transtensional in the NAT with a series of North-dipping normal faults oriented WNW-ESE that branch onto the sharp and linear eastern border of the NAT (Fig. 6). The superposition of the GPS displacement field on top of the map of Neogene basins (Fig. 7) shows that the NAT separates two domains where displacements are southward and low rate (< 0.5 cm/y) to the North and southwestward and medium rates (> 2.1 cm/y) to the South. Synsedimentation rollover geometry of Neogene basins of Southern Rhodope indicates a NE-SW direction of stretching in Pliocene [START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF]. Kinematic analysis of fault systems in North Aegean [START_REF] Lyberis | Tectonic evolution of the North Aegean trough[END_REF][START_REF] Lyberis | Tectonic evolution of the North Aegean area during the Plio-Pleistocene[END_REF][START_REF] Mercier | Change in the state of stress in the overriding plate of a subduction zone: the Aegean Arc from the Pliocene to the present[END_REF][START_REF] Mercier | Extensional tectonic regimes in the Aegean basins during the Cenozoic[END_REF]) revealed that a change in the direction of stretching from NE-SW to N-S (Fig. 8) occurred in Lower Pleistocene. [START_REF] Lyberis | Tectonic evolution of the North Aegean trough[END_REF]; Bathymetry data GMRT from [START_REF] Carbotte | New integrated Data Management System for Ridge 2000 and MARGINS Research[END_REF].

Fig. 8. Change in the principal direction of stretching between Pliocene to Lower (?) Pleistocene (a) and Lower (?) Pleistocene to Present (b) related to the localization of the North Anatolian Fault in the North Aegean (Principal directions of stretching after

This suggests that the present-day displacement pattern became fully installed rather recently, in Lower-Middle Pleistocene. A similar change in the direction of stretching from NE-SW to N-S is also observed in the Southern Hellenides during the Pliocene (See Fig. 3 in [START_REF] Papanikolaou | Disruption of the Hellenic arc: Late Miocene extensional detachment faults and steep Pliocene-Quaternary normal faults-Or what happened at Corinth?[END_REF]. Figure 6 shows that the Myrthes-Ikaria fault (MIF) is the offshore extend of the onshore Ismir-Balikeshir transfer zone (IBTZ) (Sozbilir et al. 2010;[START_REF] Ersoy | Petrogenesis of the Miocene volcanism along the Izmir-Balıkesir Transfer Zone in western Anatolia, Turkey: implications for origin and evolution of potassic volcanism in postcollisional areas[END_REF][START_REF] Uzel | Structural evidence for strikeslip deformation in the İzmir-Balikesir transfer zone and consequences for late Cenozoic evolution of western Anatolia (Turkey)[END_REF]) (Fig. 6). Lower(?)-Upper Miocene sedimentary-volcanic basins were deposited in this transtensional corridor, located at the northwestern border of the Menderes Massif [START_REF] Ersoy | Petrogenesis of the Miocene volcanism along the Izmir-Balıkesir Transfer Zone in western Anatolia, Turkey: implications for origin and evolution of potassic volcanism in postcollisional areas[END_REF]. Simultaneously, grabens developed in the Menderes, accommodating a NE-SW direction of stretching. Without entering here into the complexities of the paleomagnetic record of block rotations in IBTZ and Menderes [START_REF] Uzel | Paleomagnetic evidence for an inverse rotation history of Western Anatolia during the exhumation of Menderes core complex[END_REF], it must be mentioned that dextral strike-slip shear in IBTZ accommodated a CCW rotation of the southern Menderes of around 30° [START_REF] Pourteau | High-pressure metasediments in central Turkey: Constraints on the Neotethyan closure history[END_REF]van Hinsbergen et al. 2010), likely comparable to the 33° CCW rotation of the Naxos-Paros block in the Cyclades [START_REF] Morris | First palaeomagnetic results from the Cycladic Massif, Greece, and their implications for Miocene extension directions and tectonic models in the Aegean[END_REF] accommodated by dextral strike-slip offsets along the MIF [START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF][START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF]. Over 450 km, from Myrthes Basin to Balikeshir, this dextral strike-slip fault zone was active since Middle Miocene -i.e. around 10 My before the arrival of the NAF in the North Aegean. We lack of direct markers to identify when displacements ceased on this fault. We may hypothesize that it should be around 5 Ma when the NAF fully localized, reaching the western part of Marmara Sea This would be in agreement with the last exhumation ages recorded by low-temperature thermochronology (Fig. 5). The Myrthes-Balikeshir fault zone accommodated the difference in amount of stretching between the two domains that it separates. Therefore it is interesting to note that, in terms of displacement, the bulk 50 km offset of dextral strike-slip along the fault that was estimated in the central Cyclades [START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF]) is in rather good agreement with the 50 km of NE-SW trending extension estimated in the southern part of the Menderes Massif [START_REF] Van Hinsbergen | A key extensional metamorphic complex reviewed and restored: The Menderes Massif of western Turkey[END_REF].

In summary, the mode of extension during this second stage of back-arc extension is in strong contrast with the one that characterizes the first stage. Extension passed in Middle Miocene from the core complex mode to the wide rift mode [START_REF] Buck | Modes of continental lithospheric extension[END_REF][START_REF] Brun | Narrow rifts versus wide rifts: Inferences for the mechanics of rifting from laboratory experiments[END_REF], as demonstrated by the deposition of extensional or transtensional Neogene basins across the whole Aegean offshore and onshore. The interruption of ductile exhumation in Middle Miocene, in all types of metamorphic rocks (HT as well as HP) whatever their age of onset, as well as the segmentation of the metamorphic units and the deposition of Neogene basins on top of them suggest that the transition between the two modes of extension was not progressive and likely occurred in a rather short delay.

Restoration of 45 My of back-arc extension

Restorations of deformation in the shallow upper crust have demonstrated the difficulties and limitations inherent to this type of exercise. At lithosphere scale, like attempted here, restoration is face to more difficulties and depends even more than at smaller scales on the interpretation that is made, prior to restoration, of i) the tectonic setting in time and space of the domain to be restored and ii) the understanding of mechanical processes involved. In other words, such a restoration cannot be expected to put in evidence something that has not been identified and understood prior to restoration, whatever the particular techniques used for restoration. However, in spite of these limitations and in a case like the Aegean whose 3D evolution is rather complex, it is the best way: i) to test the coherence of the proposed interpretations, ii) to discuss their 3D implications, and iii) to make them more easily accessible and opened to critical assessment by anyone.

The 3D reconstruction of the 45 My history of back-arc extension presented in this section was carried out in two steps, in map view and then in cross-sections. It was performed manually using standard graphical techniques because a computer procedure appropriate for restoration at lithosphere scale does not exist yet. The two-step restoration, in map view and in cross-section, was performed at 5, 15 and 45 Ma for which enough geological and geophysical data were available to satisfactorily constrain a model. An intermediate model at 30 Ma was obtained by interpolation.

Restoration in map view was carried out using the data from references quoted in the two previous sections plus: i) Geological map of Greece at various scales (IGME), ii) offshore maps (synthesis by [START_REF] Mascle | Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus[END_REF], ii) paleomagnetic data (see compilations in: i) from NW Peloponnese to North Rhodope (Section A) and ii) from Southeast Peloponnese to the West of Marmara Sea (section B) (Fig. 10). The present-day geometry of the slab in section A shows a fold that passes laterally to a slab tear in section B. In section A, extension is mostly located to the North of the Vardar suture zone (Vardar-Thermaikos Gulf and SRCC). At 50 Ma, the Pelagonian crust that has been entirely subducted starts to delaminate from the underlying mantle allowing the asthenosphere to flow below the crust. As a consequence, fast and strong crustal heating, up to partial melting, strongly weakened the crust. At 45 Ma, after the subduction of the narrow Pindos oceanic domain, the Adria crust was dominantly subducted and core complex extension (SRCC) started at the back of the thrust wedge, controlled by the Kerdylion Detachment. At 15 Ma, core complex extension gave place to wide rift mode of extension with deposition of Neogene basins, the larger one being the Vardar-Thermaikos Gulf basin. Along this section, located to the West of the Corinth Gulf, trench retreat reached around 300 Km. Most of the extension was located in the North Aegean and, conversely, the Adria crust (Southern Hellenides) was only weakly extended. The section B shows the same sequence of tectonic events but with two major differences: i) trench retreat reached around 500 Km and extension affected the full Adria crustal block and ii) to the North, instead of a core complex, extension gave birth to the Thrace Basin on top of Rhodopia. The series of sections illustrates that the exhumation of high-pressure metamorphic rocks of Adria and partly Pelagonia occurred entirely in extension. Between 45 and 15 Ma, the whole initial thrust pile collapsed southward and both Pelagonia and Adria underwent a layer-parallel top to North sense of shear bringing them in a lower crustal depth. Since 15 Ma, the southern part of the extending thrust wedge that was located to the South of the slab tear, underwent a strong distributed extension that achieved the exhumation of metamorphic rocks up to surface. Kissel and[START_REF] Kissel | The Tertiary geodynamic evolution of the Aegean arc: a paleomagnetic reconstruction[END_REF][START_REF] Van Hinsbergen | Revision of the timing, magnitude and distribution of Neogene rotations in the western Aegean region[END_REF] at Aegean scale, ii) [START_REF] Dimitriadis | Dextral rotation and tectonomagmatic evolution of the southern Rhodope and adjacent regions (Greece)[END_REF] for northern Greece, Morris and Anderson 1996 for the Cyclades and [START_REF] Kissel | Preliminary paleomagnetic evidence of a post-Eocene clockwise rotation of the Western Taurides thrust belt, east of the Isparta reentrant (Southern Turkey)[END_REF] for Western Anatolia), iii) Principal directions of stretching and shear sense recorded in HT and HP metamorphic rocks during subduction (Black arrows) and extensional exhumation (White arrows) (Figs. 3 and4). Colors of the restored maps are the same than those of the present-day map (Fig. 1). The darker blue band in Adria corresponds to the external fold and thrust belt of the Pindos Nappe. All maps show the position of Black Sea and the location of the rotation pole of Scutary-Pec (Albania) [START_REF] Kissel | Paleomagnetism of external southern Dinarides and northern Albanides: implications for the Cenozoic activity of the Scutari-Pec shear zone[END_REF] around which the Southern Hellenides and western Cyclades rotated clockwise by 50° since Oligocene [START_REF] Kissel | The Tertiary geodynamic evolution of the Aegean arc: a paleomagnetic reconstruction[END_REF][START_REF] Van Hinsbergen | Revision of the timing, magnitude and distribution of Neogene rotations in the western Aegean region[END_REF]. The present restoration integrates more detailed restoration models dedicated to the Cyclades [START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF] and to Northern continental Greece (Kydonakis et al. 2015a).

The series of maps (Fig. 9) shows: i) the change in geometry and location of the Vardar and Pindos suture zones, ii) the approximate position of the trench, iii) the development of major strike-slip faults, in particular the Myrthes-Ikaria Fault (MIF) between 15 and 5 Ma and the North Anatolia Fault (NAF) since 5 Ma and iv) the location of Paleogene basins (in orange). The present-day strong obliquity between stretching directions related to ductile exhumation in Peloponnese and Crete is almost entirely restored into a single NE-trending direction at 5Ma. At 15 Ma, the stretching directions of Peloponnese, Crete and Cyclades are all parallel and trending NNE with Crete located below the Cyclades. In northern Greece, the progressive exhumation of the Southern Rhodope Core Complex occurred during the deposition of the Thrace Basin (TB). The lateral transition between these two major geological features of the North Aegean was likely accommodated by sinistral transcurrent displacements along the North Aegean Trench (NAT), from 45 to 5 Ma. At 30 Ma, all the Paleogene basins (Mesohellenic Trough (MHT), North Aegean Trough (NAT), Xanthi Basin (XB) and Thrace Basin (TB)) are close to each other, forming a band slightly oblique to the trend of the suture zones of Vardar and Pindos. Our restoration is in a rather good agreement with the one of [START_REF] Royden | Slab segmentation and late Cenozoic disruption of the Hellenic arc[END_REF]their Fig. 15), up to lower Oligocene, but at strong variance with the one of van Hinsbergen and Schmid (2012; their Fig. 12) that requires a component of EW stretching across the whole Aegean accommodated by N-S or NE-SW trending extensional detachments, giving in the southern Aegean an arc-parallel extension up to 650 Km between 15 Ma and present (i.e. more EW displacement than the bulk amount of trench retreat). In addition, the van Hinsbergen and Schmid's model, contrary to our model, totally ignores the kinematics of HP-metamorphic rocks exhumation, as their model implied stretching directions trending perpendicular or strongly oblique to the stretching directions recorded in rocks.

Restoration in cross-section was done using the following input data: i) restored maps (previous section), ii) peak pressures recorded in high pressure metamorphic rocks (see (2011). The slab tear of section AA' passes to a fold in section BB' (See slab tear contours in red on map). At 45 Ma, the restoration shows the geometry of the thrust wedge resulting from the pilling up of Rhodopia (Brown), Pelagonia (Purple) and Adria (Blue) continental blocks separated by the suture zones of the Vardar (Green) and the Pindos (Orange). Extension that is a direct function of the amount of trench retreat is much larger in section AA' than in section BB' where it is mostly located to the North of the Vardar suture zone. review in Jolivet andBrun 2010 and[START_REF] Philippon | Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)[END_REF], iii) present-day crustal thickness [START_REF] Tirel | Aegean crustal thickness inferred from gravity inversion. Geodynamical implications[END_REF]) and iv) present-day geometry of the top slab surface from Swave tomography [START_REF] Salaün | Structure et déformation du manteau supérieur de la région Egée-Anatolie par tomographie en ondes de Rayleigh[END_REF]. Two parallel sections oriented NE-SW have been restored: i) from NW Peloponnese to North Rhodope (Section A) and ii) from Southeast Peloponnese to the West of Marmara Sea (section B) (Fig. 10). The present-day geometry of the slab in section A shows a fold that passes laterally to a slab tear in section B. In section A, extension is mostly located to the North of the Vardar suture zone (Vardar-Thermaikos Gulf and SRCC). At 50 Ma, the Pelagonian crust that has been entirely subducted starts to delaminate from the underlying mantle allowing the asthenosphere to flow below the crust. As a consequence, fast and strong crustal heating, up to partial melting, strongly weakened the crust. At 45 Ma, after the subduction of the narrow Pindos oceanic domain, the Adria crust was dominantly subducted and core complex extension (SRCC) started at the back of the thrust wedge, controlled by the Kerdylion Detachment. At 15 Ma, core complex extension gave place to wide rift mode of extension with deposition of Neogene basins, the larger one being the Vardar-Thermaikos Gulf basin. Along this section, located to the West of the Corinth Gulf, trench retreat reached around 300 Km. Most of the extension was located in the North Aegean and, conversely, the Adria crust (Southern Hellenides) was only weakly extended. The section B shows the same sequence of tectonic events but with two major differences: i) trench retreat reached around 500 Km and extension affected the full Adria crustal block and ii) to the North, instead of a core complex, extension gave birth to the Thrace Basin on top of Rhodopia. The series of sections illustrates that the exhumation of high-pressure metamorphic rocks of Adria and partly Pelagonia occurred entirely in extension. Between 45 and 15 Ma, the whole initial thrust pile collapsed southward and both Pelagonia and Adria underwent a layer-parallel top to North sense of shear bringing them in a lower crustal depth. Since 15 Ma, the southern part of the extending thrust wedge that was located to the South of the slab tear, underwent a strong distributed extension that achieved the exhumation of metamorphic rocks up to surface.

Discussion: Dynamics of back-arc extension in the Aegean

Back-arc extension in the Aegean occurred in two main stages, first between Middle Eocene and Middle Miocene and second since Middle Miocene. The first stage, prior to Middle Miocene, is illustrated by the deposition of Paleogene basins (Fig. 2) and the simultaneous exhumation of high-temperature and high-pressure metamorphic rocks in a core complex northward and a blueschist belt southward. This occurred in two steps: i) In Middle Eocene (45 Ma) started the exhumation of the SRCC in the Rhodope (Fig. 3) and the CBU in the Cyclades (Fig. 4) and in Lower Miocene (around 23 Ma) the exhumation of the CCCC in the Cyclades (Fig. 3) and the PQN blueschists in Peloponnese and Crete (Fig. 4). During the second stage, since Middle Miocene, the HP and HT metamorphic units exhumed during the first stage were segmented dominantly in transtension, synchronous with Neogene basin deposition.

The restoration of displacements that is well constrained in continental Greece and central Aegean by numerous paleomagnetic data, kinematic indicators in metamorphic rocks and geochronological data shows that an acceleration of trench retreat started in Middle Miocene (Fig. 11). The rate of trench retreat that was rather low, around 0.6 cm.y -1 , during the first stage of extension increased to around 1.7 cm.y -1 between Middle Miocene and Pliocene to reach 3.2 cm.y -1 during the last 5 Ma. This is in agreement with the -dramatic acceleration‖ of back-arc extension deduced by Van Hinsbergen et al. (2010) from their study of the Menderes Massif in western Anatolia. Fig. 11. Rate of trench retreat along section AA' (Fig. 10) showing an acceleration by a factor 5 since Middle Miocene, from 0.6 to 3.2 cm y -1 , that correlates i) with a change in the mode of extension from localized/stage 1 (exhumation of HT and HP metamorphic rocks) to distributed/stage 2 (segmentation of exhumed metamorphic units and deposition of Neogene basins over the whole Aegean domain) and ii) with the strong contrast in shape and distribution of Paleogene and Neogene basins (Fig. 2). This acceleration of trench retreat (i.e. extensional boundary displacement), first by a factor 2 after Middle Miocene and then by a factor 5 after Pliocene, was likely responsible for the observed change in the mode of extension, from localized to distributed (i.e. from core complex to wide rift ;Buck 1990;[START_REF] Brun | Narrow rifts versus wide rifts: Inferences for the mechanics of rifting from laboratory experiments[END_REF][START_REF] Tirel | Extension of thickened and hot lithospheres: Inferences from laboratory modeling[END_REF][START_REF] Tirel | Dynamics and structural development of metamorphic core complexes[END_REF][START_REF] Gueydan | Continental rifting as a function of lithosphere mantle strength[END_REF]Kydonakis et al. 2015a). Mechanical modeling of the extension of a two-layer brittle-ductile system shows that an increase in strain rate increases the strength of the ductile layer and consequently the coupling between the brittle and ductile layers, giving a transition from localized to distributed extension [START_REF] Brun | Narrow rifts versus wide rifts: Inferences for the mechanics of rifting from laboratory experiments[END_REF][START_REF] Schueller | Brittle-ductile coupling: Role of ductile viscosity on brittle fracturing[END_REF][START_REF] Schueller | Mechanics of the transition from localized to distributed fracturing in layered brittle-ductile systems[END_REF]. In addition to this major change in the style of extension, it is interesting to note that the difference in P/T ratios between the high-pressure metamorphic rocks of Cyclades and Crete agrees well with an increase in the velocity of trench retreat [START_REF] Gueydan | P/T ratio in high-pressure rocks as a function of dip and velocity of continental subduction[END_REF]. The acceleration of trench retreat is probably related to a lateral tearing of the Hellenic slab below western Anatolia [START_REF] Brun | 45 m.y. of Aegean crust and mantle flow driven by trench retreat[END_REF]Van Hinsbergen et al. 2010) identified by P-wave tomographic models of the upper mantle (Piromallo and Morelli 2003;[START_REF] Biryol | Segmented African lithosphere beneath the Anatolian region inferred from teleseismic P-wave tomography[END_REF]. The improved resolution of mantle structure provided by Swave tomographic modeling (Salaün et al. 2012) allowed a mapping of the slab tear with three main trends (Salaün 2011) (Fig. 10): WNW-ESE, below the central part of North Aegean Sea, and NNW-SSE and E-W almost parallel to the two mean coastline trends of Southwest Anatolia. Section B (Fig. 10) shows the geometry of the top slab below the North Aegean. Whereas the exact timing of slab tearing is difficult to constrain, the sudden change in the mode of extension that is associated with the acceleration of slab retreat strongly supports that slab tearing should have started to develop earlier, possibly in Early Miocene, to become fully efficient from 15 Ma onward. The transtensional deformation pattern that results from the interaction between Hellenic trench retreat and Anatolia westward displacement and that is still active in the Aegean took place in Middle Miocene, showing that the westward displacement Anatolia was coeval with the acceleration of trench retreat. Whereas the North Anatolian fault plays a major role in the present-day kinematic pattern, the Myrthes-Ikaria-IBTF was the first large dextral strike-slip fault zone to develop. Its location close to the Izmir-Ankara suture zone and parallel to it suggests that the suture zone was acting as weak zone able to localize displacements at the onset of Anatolia westward displacement, as illustrated in the laboratory experiments of [START_REF] Philippon | The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene[END_REF]. However, this interaction between two plate boundary displacements raises a still opened fundamental issue: What is the dynamic relationship between slab tearing and Anatolia displacement? Which one controlled the development of the other?

Fig. 1 .

 1 Fig. 1. Simplified geological map of the Aegean domain in Tertiary showing the three main continental blocks, Rhodopia, Pelagonia and Adria, separated by the Vardar and Pindos suture zones. The cross-section shows the present-day crustal-scale structure of the Aegean domain and the geometry of the Hellenic subduction. NAF: North Anatolian Fault. NAT: North Aegean Through. MIF: Myrthes-Ikaria Fault.

Fig. 3 .

 3 Fig. 3. The two core complexes (High-temperature metamorphism) of the Aegean domain. SRCC: Southern Rhodope Core Complex, which exhumed between 45 Ma and 18 Ma and whose detachment dip southwestward. CCCC: Central Cyclades Core Complex, whose exhumation history is shown by a PTt diagram from Naxos (Numbers in circles: Time in My) (AfterDuchêne et al. 2006 in Jolivet and Brun 2010) and whose detachment dip northeastward to the West and Northward to the East. Red arrows: syn-metamorphic senses of shear. Senses of shear associated to core complex extension after[START_REF] Sokoutis | A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension[END_REF] and[START_REF] Brun | Kinematics of the Southern Rhodope Core Complex (north Greece)[END_REF] for theSRCC and Gautier et al (1993) and[START_REF] Gautier | Crustal-scale geometry and kinematics of late-orogenic extension in the central Aegean (Cyclades and Evvia Island)[END_REF] for the CCCC.

Fig. 9 .

 9 Fig. 9. Restoration of Aegean extension in map view at 5, 15, 30 and 45 Ma. Colours of continental blocks and suture zones same as in Figure 1. Orange: Paleogene basins (MHT: Mid Hellenic Trough; XB: Xanthi Basin; TB: Thrace Basin). Yellow: Neogene basins; only shown in the present-day map (e). Red and white arrows (in b to e): principal

Fig. 10 .

 10 Fig. 10. Restoration of Aegean extension along two NE-SW trending lithosphere-scale cross-sections AA' and BB' (see location on map). Shape of the Hellenic slab (in sections)and geometry of the slab tear (map view) from the S-wave tomographic model ofSalaün (2011). The slab tear of section AA' passes to a fold in section BB' (See slab tear contours in red on map). At 45 Ma, the restoration shows the geometry of the thrust wedge resulting from the pilling up of Rhodopia (Brown), Pelagonia (Purple) and Adria (Blue) continental blocks separated by the suture zones of the Vardar (Green) and the Pindos (Orange). Extension that is a direct function of the amount of trench retreat is much larger in section AA' than in section BB' where it is mostly located to the North of the Vardar suture zone.

Acknowledgements

J-P Brun thanks Ali Polat for inviting the paper. Research was financially supported by ANR EGEO (ANR-06-BLAN-0156) lead by L. Jolivet (J-PB, FG and MP), the European Union FP7 Marie Curie ITN "TOPOMOD" contract 264517, lead by C. Faccenna (J-PB, CF, FG, KK and DS) and by the Netherlands Research Centre for Integrated Solid Earth Science (DS). We thank the guest editor C. Şengör, D. Van Hinsbergen and an anonymous reviewer for their comments and suggestions of improvement.