article in press · extensional tectonics cenozoic magmatism eastern mediterranean aegean backarc...

15
On the geodynamics of the Aegean rift Samuele Agostini a, , Carlo Doglioni b , Fabrizio Innocenti c,a , Piero Manetti a,d , Sonia Tonarini a a Istituto di Geoscienze e Georisorse-CNR, Pisa, Italy b Dipartimento di Scienze della Terra, Università La Sapienza, Roma, Italy c Dipartimento di Scienze della Terra, Università di Pisa, Italy d Dipartimento di Scienze della Terra, Università di Firenze, Italy abstract article info Article history: Received 10 July 2008 Received in revised form 9 July 2009 Accepted 29 July 2009 Available online xxxx Keywords: Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either a classic backarc basin, or the result of the westward escape of Anatolia, or the effect of a gravitational collapse of an over-thickened lithosphere. Here these models are questioned. We alternatively present a number of geodynamic and magmatic constraints suggesting a simple model for the genesis of the extension as being related to the differential advancement of the upper lithosphere over a heterogeneous lower African plate. The Greek microplate overrides the Ionian oceanic segment of the African plate faster than the Anatolian microplate over the thicker Levantine more continental segment. This setting is evidenced by GPS-velocity gradient in the hangingwall of the HellenicCyprus subduction system and requires a zone of rifting splitting the hangingwall into two microplates. This mechanism is unrelated to the replacement of retreated slab by the asthenosphere as typically occurs in the backarc of west-directed subduction zones. The supposed greater dehydration of the Ionian segment of the slab is providing a larger amount of uids into the low velocity channel at the top of the asthenosphere, allowing a faster decoupling between the Greek microplate and the underlying mantle with respect to the Anatolian microplate. Slab ruptures associated with the differential retreat controlled by the inherited lithospheric heterogeneities in the lower plate and the proposed upwelling of the mantle suggested by global circulation models would explain the occurrence and coexistence of slab-related and slab-unrelated magmatism. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The formation and evolution of backarc extensional basins consti- tutes a crucial point in the comprehension of converging plate margins. Recently, it was proposed that the subduction zones are asymmetric as a function of the subduction polarity (e.g., Doglioni et al., 1999, 2007). The polarity is here dened as the main direction of the subduction with respect to the undulated ow of plate motions (Crespi et al., 2007), overall directed toward the west(Scoppola et al., 2006). In this model of plate tectonics, the westward drift of the lithosphere facilitates backarc spreading only in the hangingwall of the west-directed subduction zones. However, in some cases, as the Aegean and the Andaman regions, backarc basins are located in the hangingwall of NE- directed subduction zones, questioning this global model. Therefore the origin of the Aegean rift seems at odds with a global polarization of tectonics, and its origin might become relevant for testing the global tectonic pattern. Indeed, in spite of the relatively modest extensional rate affecting the AegeanAnatolia region and the consequent limited crustal thinning, the Aegean rift has been interpreted as a backarc spreading (e.g., Le Pichon and Angelier, 1979; Horvath and Berckhemer, 1982). Other models of the Aegean area link the extension with the westward extrusion of the Anatolia plate (e.g. McKenzie, 1972; McClusky et al., 2000) or with a post-orogenic collapse combined with slab retreat (e.g. Gautier et al., 1999; Jolivet, 2001). However, all subduction zones have the trench retreating toward the lower plate; they differentiate whether the upper plate is advancing toward the lower plate faster or slower than the trench (Doglioni et al., 2009). In the rst case a double-verging compressive belt forms, whereas in the second case backarc spreading prevails. More recently, it was inferred that extension in the AegeanWest Anatolia is associated to a differential advancement of the upper plates (Greek and Anatolian microplates) over the Africa plate (Doglioni et al., 2002). Here we describe this model in more detail and in the light of some new geodynamic and geochemical evidences. 2. Geological outlines on the Mediterranean basin The Mediterranean basin consists of different domains formed during the Mesozoic to Present interactions between Eurasia and Africa (Fig. 1). Indeed, the Mediterranean lithosphere is made up of: (i) remnants of the Mesozoic Tethys Ocean subducted from the Cretaceous to the Present as a result of the AfricaEurasia convergence and collision system (central-eastern Mediterranean); or (ii) Tectonophysics xxx (2009) xxxxxx Corresponding author. Istituto di Geoscienze e Georisorse - CNR, Area della Ricerca di Pisa, Via G. Moruzzi, 1 56124 Pisa - Italy. Tel.: +39 050 315 3281. TECTO-124683; No of Pages 15 0040-1951/$ see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.07.025 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto ARTICLE IN PRESS Please cite this article as: Agostini, S., et al., On the geodynamics of the Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Upload: others

Post on 26-Jun-2020

2 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Tectonophysics xxx (2009) xxx–xxx

TECTO-124683; No of Pages 15

Contents lists available at ScienceDirect

Tectonophysics

j ourna l homepage: www.e lsev ie r.com/ locate / tecto

ARTICLE IN PRESS

On the geodynamics of the Aegean rift

Samuele Agostini a,⁎, Carlo Doglioni b, Fabrizio Innocenti c,a, Piero Manetti a,d, Sonia Tonarini a

a Istituto di Geoscienze e Georisorse-CNR, Pisa, Italyb Dipartimento di Scienze della Terra, Università La Sapienza, Roma, Italyc Dipartimento di Scienze della Terra, Università di Pisa, Italyd Dipartimento di Scienze della Terra, Università di Firenze, Italy

⁎ Corresponding author. Istituto di Geoscienze e Geordi Pisa, Via G. Moruzzi, 1 56124 Pisa - Italy. Tel.: +39 0

0040-1951/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tecto.2009.07.025

Please cite this article as: Agostini, S., et al.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 July 2008Received in revised form 9 July 2009Accepted 29 July 2009Available online xxxx

Keywords:Extensional tectonicsCenozoic magmatismEastern MediterraneanAegean backarcGeodynamicsIsotope geochemistryPetrology

The Aegean rift is considered to be either a classic backarc basin, or the result of the westward escape ofAnatolia, or the effect of a gravitational collapse of an over-thickened lithosphere. Here these models arequestioned. We alternatively present a number of geodynamic and magmatic constraints suggesting a simplemodel for the genesis of the extension as being related to the differential advancement of the upperlithosphere over a heterogeneous lower African plate. The Greek microplate overrides the Ionian oceanicsegment of the African plate faster than the Anatolian microplate over the thicker Levantine morecontinental segment. This setting is evidenced by GPS-velocity gradient in the hangingwall of the Hellenic–Cyprus subduction system and requires a zone of rifting splitting the hangingwall into two microplates. Thismechanism is unrelated to the replacement of retreated slab by the asthenosphere as typically occurs in thebackarc of west-directed subduction zones. The supposed greater dehydration of the Ionian segment of theslab is providing a larger amount of fluids into the low velocity channel at the top of the asthenosphere,allowing a faster decoupling between the Greek microplate and the underlying mantle with respect to theAnatolian microplate. Slab ruptures associated with the differential retreat controlled by the inheritedlithospheric heterogeneities in the lower plate and the proposed upwelling of the mantle suggested by globalcirculation models would explain the occurrence and coexistence of slab-related and slab-unrelatedmagmatism.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The formation and evolution of backarc extensional basins consti-tutes a crucial point in the comprehension of converging plate margins.Recently, itwasproposed that the subduction zones are asymmetric as afunction of the subduction polarity (e.g., Doglioni et al., 1999, 2007). Thepolarity is here defined as the main direction of the subduction withrespect to the undulated flow of plate motions (Crespi et al., 2007),overall directed toward the “west” (Scoppola et al., 2006). In this modelof plate tectonics, the westward drift of the lithosphere facilitatesbackarc spreading only in the hangingwall of the west-directedsubduction zones. However, in some cases, as the Aegean and theAndaman regions, backarc basins are located in the hangingwall of NE-directed subduction zones, questioning this global model. Therefore theorigin of the Aegean rift seems at odds with a global polarization oftectonics, and its origin might become relevant for testing the globaltectonic pattern. Indeed, in spite of the relatively modest extensionalrate affecting the Aegean–Anatolia region and the consequent limitedcrustal thinning, the Aegean rift has been interpreted as a backarcspreading (e.g., Le Pichon and Angelier, 1979; Horvath and Berckhemer,

isorse - CNR, Area della Ricerca50 315 3281.

l rights reserved.

, On the geodynamics of the

1982). Other models of the Aegean area link the extension with thewestward extrusion of the Anatolia plate (e.g. McKenzie, 1972;McClusky et al., 2000) or with a post-orogenic collapse combined withslab retreat (e.g. Gautier et al., 1999; Jolivet, 2001). However, allsubduction zones have the trench retreating toward the lower plate;they differentiate whether the upper plate is advancing toward thelower plate faster or slower than the trench (Doglioni et al., 2009). In thefirst case a double-verging compressive belt forms, whereas in thesecond case backarc spreading prevails. More recently, it was inferredthat extension in the Aegean–West Anatolia is associated to adifferential advancement of the upper plates (Greek and Anatolianmicroplates) over the Africa plate (Doglioni et al., 2002). Here wedescribe this model in more detail and in the light of some newgeodynamic and geochemical evidences.

2. Geological outlines on the Mediterranean basin

The Mediterranean basin consists of different domains formedduring the Mesozoic to Present interactions between Eurasia andAfrica (Fig. 1). Indeed, the Mediterranean lithosphere is made up of:(i) remnants of the Mesozoic Tethys Ocean subducted from theCretaceous to the Present as a result of the Africa–Eurasia convergenceand collision system (central-eastern Mediterranean); or (ii)

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 2: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 1. (A) Geodynamic framework of the Mediterranean region (redrawn after Carminati and Doglioni, 2004); (B) magnification of Aegean realm, where different tectonic domains arehighlighted (modified fromBurchfiel, 2008). TheX–X′–X″ andN–S lines represent the trace of the sectionsof Fig. 2. The isobaths inkmof theHellenic slab are fromPapazachos et al. (2000).

2 S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

Cenozoic lithosphere formed in backarc regions of some of thenumerous Eurasia–Africa subduction systems developed in the last60 Ma (western Mediterranean). Currently, four subduction systemsare active in theMediterranean realm (Carminati and Doglioni, 2004),showing different convergence rates and polarity (Fig. 1A).

Please cite this article as: Agostini, S., et al., On the geodynamics of the

The westernMediterranean mainly consists of basins developed inthe last 30–40 Ma. The progressive southeast-ward retreat of theApennines–Maghrebides subduction system led to the developmentof the Provençal, Valencia, Alboran, Algerian and Tyrrhenian basins(Boccaletti et al., 1974; Carminati and Doglioni, 2004). These basins

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 3: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

3S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

can be regarded as typical backarc basins and are characterized bymarked crustal thinning and subsequent formation of new oceaniccrust. Instead, the Eastern Mediterranean is basically a relic of theNeothethys (e.g., Garfunkel, 2004) whose original passive margins arestill preserved to the east and to the south in the Levantine andHerodotus basins (Fig. 1A). To the north, the Eastern Mediterranean ischaracterized by the northeast-directed subduction of the Africanplate under the Crete and Cyprus trenches. It is noteworthy that,whereas thinner oceanic lithosphere is currently subducting underCrete, a thicker continental margin is subducting into the Cyprustrench (e.g. Robertson, 1998). Unlike the Western Mediterranean, theonly rift of the Eastern Mediterranean formed in a “backarc” setting isthe Aegean Sea. This basin formed by thinning of a variety of tectonicunits, which were mainly emplaced during the Upper Cretaceous–Paleocene convergence–collision processes (e.g. Boccaletti et al.,1974; Robertson et al., 1991). These units are continental massifssuch as the Serbo-Macedonian-Rhodope, the Pelagonian–Attic-Cycla-dic, and the external Hellenides that are connected by ophiolitic beltssuch as the Vardar and Sub-Pelagonian-Pindos (Fig. 1B). It isnoteworthy that: 1) the Hellenic subduction system was activesince at least the Late Cretaceous and the “backarc” rift developedlater; and 2) despite the long-lasting formation of the Aegean basin(≈40 Ma), the extension rate is relatively low, so that no oceanic crustwas generated. This area is currently undergoing a widespreadregional extension, which can be dated back to the Eocene–EarlyMiocene (e.g., Seyitoğlu and Scott, 1996; Gautier et al, 1999; Jolivet,2001; Doglioni et al., 2002). Western Anatolia has a structuralarchitecture very similar to the Aegean region. Indeed, the Sakaryamassif is considered to be analogous to the Rhodope massif (e.g.Şengör and Yilmaz, 1981); the Izmir–Ankara zone is similar to theVardar ophiolite belt; the Menderes massif can be correlated to thePelagonian massif, whereas the Lycian nappes could represent theeastward projection of the Pindos ophiolites (Fig. 1B) (e.g. Robertsonet al., 1991; Stampfli, 2000).

3. Geophysical constraints on the slab geometry

The subduction of the African plate under the Crete trench ismarked by seismic and volcanic belts, which display a well-definedBenioff plane extending down to about 160–180 km (USGS Earth-quake Catalogue, http://neic.usgs.gov/neis/epic/, Konstantinou andMelis, 2008, Fig. 1B). Earthquake focal mechanisms indicate down-dipextension in the slab (Papazachos et al., 2005). The angle of thesubduction, calculated according to the earthquake hypocentersdistribution is about 16°, and tends to flatten beneath NE–SW cross-sections, that is the direction of the subduction parallel to the relativeplate motions) (Papazachos et al., 2005). The slab dip increases (up to30–40°) moving southward where the subduction is oblique or inlateral ramp. Shear wave splitting analysis of SKS phases reveals theoccurrence of an anisotropic fabric of the lithospheric mantle in the

Fig. 2. (A) S–N cross-section of upper mantle structure inferred from S-wave speed perexaggeration). Blue portion represents zone with positive anomalies, and is interpreted asvelocity zone; NAF = North Anatolian Fault). (B) 3D reconstruction of the Hellenic slab bas

Please cite this article as: Agostini, S., et al., On the geodynamics of the

Central-Northern Aegean, suggesting a pure shear extensionalmechanism involving the whole lithosphere (Kreemer et al., 2004).The direction of this anisotropy (Hatzfeld et al., 2001) is NE-trendingin the Central and Northern Aegean andWestern Anatolia. However itis worth noting that, closer to the convergent margin, its direction isrotating, from NNW in the Peloponnesus to roughly ENE close toRhodes (see Figs. 2 and 6 of Kreemer et al., 2004). Furthermore, lowS-wave velocities in the mantle beneath North-Central Aegean foundby Bourova et al. (2005), suggest the occurrence of a low-viscositychannel at relatively shallow depths (around 100 km), compatiblewith a flat slab beneath the overriding Aegean rift (Fig. 2), matchingthe slab shape drawn according to hypocenter locations (e.g.,Christova and Nikolova, 1993). The tomography of the area shows asteep high velocity body, which is usually referred as the evidence forthe Hellenic slab (Fig. 3A). However, the seismicity has a verydifferent trend, pointing for a less inclined, shallower and shorter slab(Fig. 3B). The misalignment between tomography and slab-relatedseismicity highlights a fundamental problem, i.e., question on the realnature of the high velocity body.

In the entire Aegean–Western Anatolian region, no earthquakesdeeper than 180 km occur (http://neic.usgs.gov/neis/epic/), and onlyshallow to intermediate earthquakes are present in the Central andNorthern Aegean. These events have maximum depths of 55–60 km,well inside the lithosphere of the upper plate. The slab depicted byseismicity does not correspond to the slab imaged by tomography. Weprefer to rely on thegeometryof the slab detected by the seismicity (e.g.,Christova and Nikolova, 1993; Papazachos et al., 2005) rather than toconsider the tomography (e.g., Piromallo and Morelli, 2003), which isbased on a mantle velocity model. The absence of deep seismicitycontradicts the presence of a deep slab since the few cm/yr convergencerate and the oceanic nature of the down-going slab should rather favor adeep seismicity.

Most of the authors interpret faster bodies detected by tomogra-phy as colder bodies, i.e., subducted slabs (e.g., Wortel and Spakman,2000; Piromallo and Morelli, 2003). However seismic waves speedvariations may depend by some other factors, such as chemicalheterogeneities in the mantle, or phase changes (e.g. Trampert et al.,2004). Recently, as an alternative hypothesis, Doglioni et al. (2009)proposed a mechanism able to generate the ghost of a slab along E- orNE-directed subduction zones; deeper mantle portions, more rigidand with faster seismic velocities with respect to the shallowermantle, may be upraised in response to the slab suction. This wouldgenerate volumes of faster mantle much wider than the usual slabthickness and it could explain the absence of seismicity below 200 kmin the Aegean realm because there the slab would be missing.

According to this view, we interpret the faster body detected bytomography not as a real subducting lithosphere, but as a volumeof uplifted mantle sucked up by the slab that is moving southwest-ward relative to the mantle, a direction opposed to the dip of thesubduction.

turbations along the Aegean region (redrawn from Bourova et al., 2005; no verticalthe subducted slab, whereas red portions represent negative anomalies. (LVZ = Lowed on seismicity (after Christova and Nikolova, 1993). Location of the sections in Fig. 1.

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 4: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 3. Tomographic section (A) and seismicity (B) across the Hellenic subduction-Aegean rift (after Piromallo and Morelli, 2003; Papazachos et al., 2000, respectively). White circlesin (A) are earthquake hypocenters (M≥5), black box represents the area of section (B). Note the different scales between (A) and (B) (1000 and 200 km depth, respectively). There isa relevant discrepancy between the high velocity body in the tomographic section, usually interpreted as the deep prolongation of the slab, and the seismicity. The earthquakesdepict a less inclined and much shorter slab, as also described by shear waves (Fig. 2). Since the accuracy of seismicity is finer than tomography, we infer a possible alternative originfor the high velocity body: the slab is moving SW-ward relative to the mantle and is sucking up the underlying faster mantle. The slab is suffering down-dip extension, compatiblewith the model.

Fig. 4. GPS-velocity field (Eurasia fixed). Redrawn after Reilinger et al. (2006). Circlesrepresent 1σ velocity uncertainties; circles without arrows represent GPS-stationswhose velocity is negligible (lower than error). Note that the pole of rotation of the sitearrows might be apparent since the sites are located on different tectonic settings andmicroplates.

4 S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

4. Kinematic constraints on plate motions

The active tectonics of the Aegean region has been the subject ofmany analyses, both concerning its structural setting and itsgeophysical characteristics (e.g. Jackson, 1994; Gautier et al., 1999).Given the occurrence of normal faults and graben systems throughoutthe Aegean–Western Anatolian region (e.g. McKenzie, 1972; Yilmazet al., 2000; Kreemer et al., 2004) and the focal mechanisms of crustalearthquakes, all these papers agree on the fact that the region iscurrently undergoing extensional tectonics.

A significant improvement in the description of this extension wasrecently provided by geodetic studies, which allow the reconstructionof detailed plate kinematics based on GPS-data (e.g., Le Pichon et al.,1995; Reilinger et al., 2006). The horizontal velocities measured in theEastern Mediterranean area evidence the occurrence of a counter-clockwise rotation of a broad region relative to Eurasia, from theArabian plate, to the Turkey, and to the Aegean, at rates in the range of20–30 mm/yr (Reilinger et al., 2006). In this reference frame, fixed toEurasia, these authors note that the values in the velocity fieldincrease toward the Crete trench (Fig. 1A) system, stressing theoccurrence of extension in theWestern Anatolia–Aegean system, withan average NW–SE trend (Fig. 4). However all sites used in thisreconstruction pertain to different and disrupted tectonic units, oreven independent deforming plates (Arabian, Anatolian, Greek) anddo not constrain the rotation of a coherent plate. It is noteworthy thatall the plates around the Eastern Mediterranean (i.e. Eurasia, Arabiaand Africa) are moving northeast-ward in the ITRF (InternationalTerrestrial Reference Frame), with respect to the hypothetic center ofthe Earth (e.g., Heflin et al., 2008). In a hotspot reference framerelative to the mantle, all those plates rather move in the oppositedirection (west-ward or southwest-ward) (Gripp and Gordon, 2002).

Given that the most prominent factor in shaping the EasternMediterranean area is the subduction of Africa plate underneath the

Please cite this article as: Agostini, S., et al., On the geodynamics of the

Crete and Cyprus trenches (Fig. 1A), Doglioni et al. (2002) suggestedthat it could be useful to analyze platemovements keeping the Africanplate fixed, rather than Eurasia. In this reference frame, the GPS siteslocated in the Aegean area exhibit faster southwest-ward velocitieswith respect to the sites in northern Greece, and adjacent areas of theBalkan Peninula, Western Anatolia, and Cyprus. This means that theregion surrounding northern Greece and Western Anatolia are actingas separate microplates and override Africa with different velocities,being the Aegean area the diffuse transfer zone of separation betweenthe two (Fig. 5).

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 5: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 5. ITRF2000 velocity field computed with respect to HELW (Egypt, African plate), redrawn after Doglioni et al. (2002). Notice the faster southwest-ward motion of the Greekmicroplate relative to Africa with respect to the motion of the Anatolian microplate. The differential velocity between the two upper plates of the subduction system impliesextension in the Aegean realm.

5S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

Moreover, in the hotspot reference frame (Gripp and Gordon,2002), the Hellenic slab, which is attached to the African plate, ismoving westward relative to the mantle (see Figs. 22 and 23 inDoglioni et al., 2007). Therefore it is moving in a direction opposed tothe subduction, suggesting that it actually moves out of the mantle.The subduction is active and continues to retreat relative to Africabecause the upper plate (Greek lithosphere) is overriding Africasouthwest-ward faster (Doglioni et al., 2007). In this interpretation,unlike thewest-directed subduction zones that generate a corner flowin the mantle wedge (e.g., Turcotte and Schubert, 1982), the Hellenictype setting in which the slab is supposed to upraise, it should rathergenerate a suction flow from the underlying mantle (Doglioni et al.,2009). Such an interpretation could explain: i) why the tomographicimages show relatively fast velocities along the ideal prolongation ofthe slab (e.g., a volume of an uplifted underlying mantle sucked up bythe slab); ii) the anomalous thickness of the fast velocity body imagedby tomography (several hundred kilometers versus the real 70–100 km thickness of the subducted lithosphere).

5. Why the Aegean cannot be considered a typical backarc

In the Central Mediterranean area, two subduction zones have a stillactive volcanic arc, the Aeolian and the Aegean, and both arecharacterized by the formation of a backarc basin, the Tyrrhenian Seain the hangingwall of aWest-directed subduction, and the Aegean Sea inthe hangingwall of a NE-directed subduction (Fig. 1A). The southernTyrrhenian Sea, alongwith the Provençal and Algerian basins are flooredby post 20 Ma oceanic crust, formed in the hangingwall of the eastwardretreating Apennines–Maghrebides slab. In contrast, the Aegean basinhas some peculiar physiographical and morphological characteristics.Thefirst peculiarity is that the stretching is quite limitedwhen comparedto the duration of the subduction (Late Cretaceous,≈60–90 Ma). In fact,thecrust is still continental and thicker than20 km(Makris, 1978;Makriset al., 2001), whereas the backarc basins usually experience fastoceanization contemporaneous with the subduction. For example, theProvençal and Tyrrhenian basins are eastward migrating and rejuvenat-ing backarc basins formed contemporaneously in the hangingwall of theOligocene to Present Apennines subduction zone (Carminati andDoglioni, 2004). TheAegean rift developedmostly after the emplacementof Hellenic–Taurides nappes (e.g., Jolivet et al., 1994; Jolivet, 2001).

Please cite this article as: Agostini, S., et al., On the geodynamics of the

In addition, the Hellenic subduction shows a number of charactersthat fall into theE-orNE-directedclass of subductionzones that include:the shallowdip of the forelandmonocline at the base of the accretionaryprism in theMediterraneanRidge (0–2°) (Lenci andDoglioni, 2007); theshallow depth of the seismicity (mostly shallower than 200 km)coupled with the shallow dip of the slab (see above; Christova andNikolova, 1993); the down-dip extension of the intra-slab seismicity(Papazachos et al., 2000, 2005). Moreover in the Aegean area, there arescattered high-pressure rocks in the thrust sheets of the Hellenides thatare disrupted by the normal faults associated with the rift. This type ofrocks requires deep seated thrusts cross-cutting the whole crust andpossibly the upper mantle, a tectonic setting that typically occurs onlyalong the E- or NE-directed subduction zones (Doglioni et al., 1999). Inthe opposite W-directed subduction setting, these rocks may occur asscattered relics, but they are inferred tobe remnantsof pre-existingE- orNE-directed subduction zones (Doglioni et al., 1999). Moreover,extension does not take place only in the backarc area, but is alsopresent in the forearc (e.g., between Crete and the active Aegean arc,Fig. 1B) and in the island of Crete (Papanikolaou and Vassilakis, 2008;van Hinsbergen and Meulenkamp, 2006).

At least three different models have been proposed in theliterature to explain the Aegean–Western Anatolia extension:

(i) Some authors consider the Aegean basin to be a typical backarcbasin (Fig. 6A) and link extension to slab retreat and steepening(e.g., Berckhemer, 1977; Le Pichon and Angelier, 1979). In thisview, the engine driving the extension is both the “margin pushforce” and the “slab pull force”. Thismodel requires a progressivesteepening of the African subducted slab and has recently gaineda lot of popularity because of tomographic images showing asteep slab sinking down all through the upper mantle to themesosphere (e.g., Wortel and Spakman 2000; Piromallo andMorelli, 2003).

(ii) Since the early contribution ofMcKenzie (1972), anothermodelexplains extension in the Aegean–West Anatolia as a result ofthe Africa–Eurasia collision and the west-ward extrusion ofAnatolia driven by the Arabia indenter on Eurasia, and thelateral escape bounded to thenorth by theNorthAnatolian Fault(NAF, Fig. 6B). In this view, the extrusion spreads out toward theless constrained Ionian margin, and the subduction is seen as aconsequence of the Anatolian escape.

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 6: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 6. Sketch map of the three main different models proposed so far to explainextension in the Aegean region: (A) Backarc matched by trench migration, induced byslab rollback and steepening. Extensional stressfield detected in land (black arrows) andsea (white arrows) reported in Angelier et al. (1982) are shown. However all slabs havethe subduction hinge retreating relative to the lower plate, the slab is still shallow andwith low dip. (B) Anatolia extrusion and Aegean spreading induced by Arabia–Eurasiacollision. Black arrows represent GPS-velocity pattern with respect to a fixed Eurasia(after McClusky et al., 2000). In this model we would rather expect the velocity of sitesdecreasing (e.g., the white arrows) from the energetic source (the Arabia indenter),which is not. (C) Gravitational collapse of an over-thickened lithosphere (redrawn afterJolivet, 2001). In thismodel the spreading should be active in spite of a practically absenttopographic gradient between the center of the Aegean and the foreland.

6 S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

(iii) The Aegean extension is also ascribed to post-orogenic collapsethat is a gravity spreading of a continental lithosphere over-thickened during the “Alpine” collision (Fig. 6C) (Seyitoğlu andScott, 1996; Gautier et al, 1999; Jolivet, 2001).

The increase in geological and geophysical data in the regionpoints out few weaknesses in these models. In particular, recentprecise geodetic measurements of plate velocities (e.g., McCluskyet al., 2000; Doglioni et al., 2002) show that velocities of Anatolianplate increase from east to west (Fig. 4). This velocity pattern is notcompatible with model (ii) because the velocity field should insteaddecrease moving away from the energy source, that is the Arabianindenter (Fig. 6B) and being negligible the inertial forces ingeodynamics (Turcotte and Schubert, 1982). Moreover, the westwardAnatolian escape would rather close the Aegean Sea. Model (iii)suffers the absence of a quantitative balance of the acting forces. We

Please cite this article as: Agostini, S., et al., On the geodynamics of the

consider that the topographic gradient, which has a slope of less than1° (Doglioni et al., 2002), could be not enough to activate gravitationalsliding of a brittle crust, even combined with a slab retreat. Model (i)is strictly linked with the occurrence of slab rollback and itsprogressive steepening. The presence of a steep slab under theAegean is inferred only on the basis of model-dependent tomographicimages, but it is not confirmed by the seismicity (Fig. 3B), whichreveals a low angle slab down to 180 km depth and an aseismic “slab”at greater depths (see for example Figs. 7 and 9 of Papazachos et al.,2005). GPS-data relative to a fixed Africa (see above) indicate that theGreek microplate is overriding Africa at the Crete trench towards theSW faster than the Anatolia microplate that overrides Africa at theCyprus trench (Fig. 7). Thus, extension in the region can simply derivefrom this velocity pattern, which is responsible for the onset of adiffuse extensional margin between the two plates (Doglioni et al.,2002). Different velocities between the Greek and Anatolian micro-plates (Fig. 7) could be due to the fact that the Aegean subductionsystem is coupled to relatively fast southwest-ward migration of theCrete trench (e.g., about 4 cm/yr), whereas the Anatolian subductionis coupled to the slower slab retreat of the Cyprus trench (1 cm/yr;Doglioni et al., 2002). In other words, the hangingwall has adifferential velocity, which has to be accommodated by riftingbetween the Greek and Anatolian microplates (Fig. 8).

This simple intra-upper plate velocity gradient is a differentmechanism from the one that can be envisaged in the Tyrrhenian Sea,where the backarc area is the loci where the asthenosphere replacesthe lithosphere subducting and retreating beneath the Apennines. Itis noteworthy that, in a cross-section, the Hellenic system iscomposed of three plates (Africa, Greek and Anatolian), whereasthe Apennine subduction system involves only two plates, Africa (orAdria in the north), in the footwall and Europe in the hangingwall(Fig. 9). These different kinematic constraints and mechanisms arealso supported by heat flow data (Hurtig et al., 1991) that are muchhigher in the Tyrrhenian Sea (N200–250 mW) with respect to theAegean sea (b100–120 mW).

When compared to the W-directed Apennines subduction, thehigher topography of Greece, in spite of the Aegean rift, better fits withthe E–NE-directed subduction zones, as does the gravity signature ofthe Hellenic subduction system (Harabaglia and Doglioni, 1998).

The subduction of the Ionian oceanic (?) lithosphere (de Voogd etal., 1992; Panza et al., 2003, 2007) should determine dehydration ofthe slab. The fluids are partly metasomatizing the mantle wedge andgenerating the magmatic arc. However, the large flux of lithosphereinto the subduction should generate large amount of fluids, whichremain entrapped in the asthenosphere (e.g. Peccerillo et al., 2008).These fluids are able to decrease the viscosity in the low velocity layer(Manea and Gurnis, 2007), hence triggering a faster decouplingbetween lithosphere and underlying mantle. The western portion ofthe subducting lithosphere is oceanic (the crust is 11–16 km thick,with a 4–6 km thick sedimentary cover, Makris and Stobbe, 1984; deVoogd et al., 1992), whereas its eastern portion is represented by acontinental margin. Therefore the larger amount of fluids released bythe Hellenic subduction with respect to the Anatolian subductioncould explain the faster southwest-ward motion of the Greekmicroplate with respect to the Cyprus–Anatolia segment of the Africaplate subduction. This hypothesis is supported by the occurrence of alow-viscosity channel under the Aegean lithosphere, as evidenced bythe low values of P-wave velocities (Papazachos et al., 2005) andabsolute S-waves velocities (Bourova et al., 2005) (Fig. 2).

6. Aegean–Western Anatolian magmatism

6.1. Age distribution

Since the Paleogene and throughout the Neogene, the Aegean–Anatolian region has been characterized by several episodes of

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 7: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 7. Geodynamic setting of EasternMediterranean region, evidencing the occurrence of two separate upper plates (Greek and Anatolianmicroplates), and their kinematics relativeto Africa. White arrow indicates the diffuse extensional margin between the two upper plates. A–A′ and B–B′ are locations of the cross-sections shown in Fig. 8. NAF, North AnatolianFault.

7S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

magmatic activity (Figs. 10 and 11; Table 1). The older products arethe Upper Eocene to Upper Oligocene calc-alkaline volcanics ofRhodope, Thrace and North–West Anatolia (e.g., Innocenti et al., 1984;Yanev et al., 1998; Altunkaynak and Genç, 2007), which aresometimes associated with intrusive bodies.

Since the Miocene, orogenic products are found throughoutWestern Anatolia and the Aegean, with a rough trend of youngingsouthward (Fig. 11; Table 1). This geographic trend indicates that theslab has been retreating consistently (i.e., the subduction hinge wasmoving towards the African lower plate). The orogenic volcanic belt,which definitely constitutes the most abundant and widespreadactivity in the region, starts at the base of Early Miocene both in thenorthern Aegean and northwestern Anatolia (e.g. Aldanmaz et al.,2000; Innocenti et al., 2005 and references therein) and is matched inWestern Anatolia by quasi-contemporaneous emplacement of plu-tonic bodies (e.g. Bingöl et al., 1982; Delaloye and Bingöl, 2000). To the

Fig. 8. Simplified kinematic setting in which the lower plate (L) is fixed. The upper plateis subdivided into two subplates (U1 and U2–U3). The subduction hinge has twovelocities (H1NH2) corresponding to the faster advancement of microplate U1 withrespect to microplate U2–U3. The model is tentatively applied to the Hellenic–Cyprussubduction zone. The velocity gradient between U1 (Greek microplate) and U2–U3

(Anatolia microplate) generates the rifting such as in the Aegean Sea because U1 isoverriding Africa faster than U2–U3. The different velocity of hinge migration (H1 andH2) determines different subduction rates, whereas the shortening in the accretionaryprism might be similar because the difference between the hinge migration and theconvergence (H1–U1 and H2–U3) is the same in both profiles.

Please cite this article as: Agostini, S., et al., On the geodynamics of the

south in the Central Aegean region, analogouswidespread high-K calc-alkaline volcanism took place a few million years later, between theEarly and Middle Miocene. Significantly younger are less abundanthigh-K calc-alkaline volcanics found in and south of the Cycladic andMenderes massifs (Figs. 10 and 1A), which are mostly Upper Miocene.Along with the calc-alkaline volcanism, mainly anatectic S-typegranites were emplaced inside the Cycladic and Menderes Massifsduring the Middle to Late Miocene (e.g., Hetzel et al., 1995; Pe-Piperand Piper, 2002). Further south occurs the Pliocene–Quaternary SouthAegean volcanic arc (Fig. 10), which can be dated back to EarlyPliocene and it is still active (e.g., Francalanci et al., 2005 and referencestherein).

It is noteworthy that from the northern to the southern part of thisregion, activity starts at different times but is characterized bymagmas sharing the same geochemical characteristics. They are allhigh-K calc-alkaline and range from basaltic andesite to rhyolite incomposition, with basalts being virtually absent, and andesitic lavasand rhyolitic tuffs being the most abundant products. In addition,volcanic activity follows a similar geochemical evolution all over theregion (Fig. 11; Table 1). The older high-K calc-alkaline activity isstrictly followed by the emission of shoshonitic products, with ageneral K2O-increasing trend mirrored by abundance decrease oferupted products. In some places, this trend evolves to sporadicemission of younger ultra-K products that sometimes have lamproiticaffinity (Innocenti et al., 2005).

In addition, at the beginning of LateMiocene the Central part of theAegean–Anatolian region is the locus of a narrow belt of high-Mgandesitic products (Pe-Piper and Piper, 2002; Agostini et al., 2005).From Late Miocene onwards, alkali basaltic lavas were emplacedthroughout the Aegean, Thrace and Western Anatolia. These basaltsmay display either a K- or Na-alkaline affinity and are characterized bya roughly southward younging trend. They are commonly scattered,low-volume occurrences, except for the Kula volcanic field that coversan area of about 350–400 km2 (Tokçaer et al., 2005).

6.2. Geochemical and isotope characters of the volcanic products

A number of recent studies (e.g. Francalanci et al., 1990; Aldanmazet al., 2000; Alıcı et al., 2002; Innocenti et al., 2005; Agostini et al.,2007) were aimed at identifying the sources involved in the genesis ofthe volcanism in the region, as well as their evolution and theinteractions between the different sources. The rocks belonging to the

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 8: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 9. Aegean vs. Apennine subduction systems. Considering fixed Africa, along the Apennines subduction, there are only two plates (Africa and Europe), and the backarc forms fromby the replacement of the retreated slab by the asthenosphere. Along the Crete–Cyprus subduction zone, the system is composed by three plates (African, Greek and Anatolian). TheAegean Sea rift forms in spite of a lithospheric thickening and the long-lasting (N60 Ma) subduction system. Nevertheless the rift is still in a continental stage and the crust is N20 kmthick. The Greek microplate is overriding Africa faster than Anatolian microplate, implying extension between Greece and Anatolia and the formation of the diffuse region of rifting.Differences in thickness and composition of the subducting African lithosphere may explain the faster subduction of the Ionian oceanic lithosphere with respect to the Levantine Sea.

8 S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

calc-alkaline, shoshonitic and ultra-K associations have the typicalgeochemical and isotope features of supra-subduction rocks, sincethey are enriched in Fluid Mobile Elements (FME) with respect toHigh Field Strength Elements (HFSE), as shown by their high Ba/Nband Rb/Zr ratios. They are also characterized by high 87Sr/86Sr(≈0.707–0.710), and low 143Nd/144Nd (≈0.5122–0.5125) ratios.

The Late Miocene to Pleistocene alkali basalts can be subdividedinto two main groups because some of them are mildly SiO2-undersaturated and have potassic affinity whilst some others arestrongly undersaturated and sodic alkaline. Interestingly, the K-basalts have limited variability in major elements but have very highvariability in trace element abundances, FME/HFSE ratios and Sr andNd isotope ratios (87Sr/86Sr≈0.704–0.708; 143Nd/144Nd≈0.5124–0.5129). In contrast, the sodic alkaline rocks show greater majorelement variation (especially those from Kula, Fig. 10B), ranging frombasanites to phono-tephrites but have quite constant FME/HFSE, Srand Nd isotope ratios (87Sr/86Sr≈0.7032, 143Nd/144Nd≈0.5129). Thegeochemical and Sr–Nd isotope features of calc-alkaline, shoshoniticand ultra-Kmagmas inWestern Anatolia point out that these magmaswere generated from the same mantle domain, which has beeninterpreted to be a mantle wedge strongly modified by a subductioncomponent (Innocenti et al., 2005). Differing enrichments ofsubduction-related components, with a minor role played by theassimilation of crustal materials, may explain the geochemicalvariability observed in these rocks.

Fig. 10.Map showing the distribution ofmagmatism in Aegean–Western Anatolian region. Thwhereas age data, as well as potassic (K) or (Na) affinity for alkali basaltic rocks are reporteActive Volcanic Arc. Stars mark the occurrence of U-K magmas.

Please cite this article as: Agostini, S., et al., On the geodynamics of the

The Na-alkaline basalts show the typical geochemical character-istics of intraplate (OIB-type) magmas and were sourced in the sub-slab asthenosphere. By contrast, the K-alkaline basalts derive fromsome kind of interaction between sub-slab melts and the mantlewedge. From Pliocene to present, arc volcanism related to Aegeansubduction is taking place in the South Aegean (e.g. Keller, 1982; Pe-Piper and Piper, 2002). The mantle source of this calc-alkalinevolcanism is considered to be the asthenospheric mantle wedge, asthe case for the older Central Aegean–Western Anatolian activity.With respect to the Miocene calc-alkaline belt, the Pliocene–Holocenesouth Aegean arc exhibits a greater variation in Sr, Nd and Pb isotopes,as well as FME/HFSE ratios, in the less evolved rocks, which has beenrelated to different amount of a metasomatizing component added tothe mantle source (Francalanci et al., 2005).

In Fig. 12, Sr isotope ratios of Aegean–Anatolian volcanics areplotted against their age. Here we subdivided the region into threezones, from south to north. Using the 87Sr/86Sr ratio as an indicator ofsubduction component, we found that in all of the three zones there isa change from strongly subduction modified (87Sr/86Sr≈0.708) tounmodified mantle sources. It is noteworthy that, through time, rockssharing the same mantle source (i.e., calc-alkaline, shoshonitic andultra-K) mark a progressive lowering of the subduction signal. This isparticularly evident for the rocks of CentralWestern Anatolia, where amore coherent dataset is present (Fig. 12B). In addition to the mantle-sourced magmas, anatectic granites and rhyolites characterized by

e age data and petrogenetic affinity for the subduction-related rocks is evidenced in (A),d in (B). C-A, calc-alkaline; Sho, shoshonitic; U-K, ultra potassic; SAAVA, South Aegean

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 9: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

9S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

Please cite this article as: Agostini, S., et al., On the geodynamics of the Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 10: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 11. Scheme representing time distribution and petrogenetic affinity of magmatism in the different regions of the Aegean realm. Black-grey arrows, subduction-related products;Blue, potassic alkali basalts; Red, sodic alkali basalts; Green, crustal related magmas. C-A, calc-alkaline; HK C-A, high potassium calc-alkaline; Sho, shoshonitic; U-K, ultra potassic;Rhy, rhyolites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

10 S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

high 87Sr/86Sr ratios (0.709 to 0.715) were emplaced, especially in theareas experiencing a major amount of crustal thinning or extension(Attic-Cycladic and Menderes massifs) (e.g. Altherr et al., 1982;Innocenti et al., 1982; Altherr et al., 1988; Hetzel et al., 1995).

Boron and lithium isotope studies contributed to clarify the role ofthe subduction-derived fluids in modifying the mantle sourcesinvolved in themagma genesis and their temporal evolution (Tonariniet al., 2005; Agostini et al., 2008). A progressive lowering of δ11B, aswell as δ7Li, throughout the time is observed and is matched by aprogressive decrease of B/Nb ratio (Fig. 13). This fact points out thatthe shift from C-A to U-K rocks was due to the addition of a decreasingamount of metasomatizing fluids in the mantle wedge, from aprogressively dehydrating slab. In particular, the ultra-K rocks weresourced in a mantle wedge left as residuum due to the previousextractions of calc-alkaline magmas, and slightly metasomatized bythe last fluids coming from an almost dehydrated slab. The high δ11Bvariability of K-alkaline basalts, along with their limited variation inmajor elements and Li isotopes, and wider variations of FME/HFSEratios, Sr–Nd isotopes, point out that these basalts did not result frommixing between intraplate-type and orogenic melts, rather they wereformed by interactions between sub-slab asthenosphericmagmas andresidual slab fluids (Tonarini et al., 2005; Agostini et al., 2008).

6.3. Magmatic constraints

Some fundamental constraints on the geodynamic evolution of theregion can be gained by the time distribution, the geochemical andpetrological features of the magmatism. These constraints can beschematically listed as follows:

(i) The northern, central and southern parts of the Aegean–western Anatolian region are characterized by similar evolu-tion of magmatism, with a time shift from north to south(Fig. 11), implying that the same geodynamic setting progradesfrom north to south.

(ii) Especially in the central Aegean and western Anatolia, weobserve an Early–Middle Miocene evolution from calc-alkalineto ultra-K magmas. The geochemical and isotope features ofthese rocks (see above) implies that they were generated after

Please cite this article as: Agostini, S., et al., On the geodynamics of the

partial melting endured in the samemantle domain, suggestingthe occurrence of a non-convective mantle wedge.

(iii) The progressive decline of slab released fluids coupled to anextreme B–Li negative signature point out that the slab wasdehydrated up to almost being dewatered completely. Theoccurrence of such a process again may be indicative of astagnant slab (i.e. very slowly sinking, dehydrated as aconsequence of progressive thermal perturbation).

(iv) Li isotopes are easily re-equilibrated to common mantle valuesin the mantle wedge, so that most subduction-related magmasworldwide show no Li isotope variability. The persistence ofhigh Li isotope differences, as well as the B–Li isotopecorrelation, in Western Anatolia orogenic rocks are thusindicative of a limited interaction between slab fluids andoverlaying mantle, that is a very tiny, low-volume, mantlewedge (Agostini et al., 2008).

(v) The youngest Na-alkaline magmas were sourced in the sub-slab asthenosphere and suffered no interaction with slab fluidsor any slab component. The genesis of such magmas is usuallylinked to asthenosphere partial melting after extensionaldynamics and mantle upwelling. In addition, to allow suchmagmas to reach the surface without any contamination byslab material, the occurrence of a slab window is invoked.

(vi) The Late Miocene K-alkaline basalts resulted from interactionbetween sub-slab magmas (i.e. the intraplate-type magmasrepresented by the later Na-basalts) with residual slab fluids.This points out that, during the earlier stages of slab windowopening, limited interactions between intraplate magmas andslab fluids occurred.

7. Summary and conclusions

The Western Anatolia–Aegean region since the Late Oligocene–EarlyMiocenewas, and still is, affected by extensional tectonics takingplace in a backarc position. The Aegean area is one of the fewexamples of a backarc basin in the hangingwall of a NE-directedsubduction zone. This would contradict the model of a globalpolarization of tectonics, which claims that backarc basins form onlyin the hangingwall of W-directed slabs (e.g., Doglioni et al., 1999).

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 11: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Table 1Geochronology and Sr isotopes data of volcanics from Aegean region.

Zone Locality Age (Ma) Affinity* 87Sr/86Sr Ref.

Rhodope-ThraceRhodope Bezvodno 37–35.5 HK Calc-alk 22Rhodope Borovitsa Caldera 34–32 HK Calc-alk 0.7079–0.7083 29, 23Rhodope Borovitsa post-

Caldera29.5–27.5 Sho 0.7079–0.7083 22, 23

Thrace N of Xanthi 30.7–24.6 HK Calc-alk 0.7063–0.7077 18Thrace Fere-Dadia 30.6–23.6 HK Calc-alk 0.7075–0.7081 18Thrace N of

Alexandroupoulis33.1–30.0 HK Calc-alk 18

Thrace Karatepe-Mahmutkoy

11.7–8.4 Na Bas 0.7032–0.7035 20, 3

North Aegean–Northwest AnatoliaN Aegean Samothraki 22.3 HK Calc-alk 8N Aegean Limnos 22.7–17.8 HK Calc-alk 14N Aegean Agios Efstratios 23.2–18.0 HK Calc-alk 14Biga P. Balıkceşme-

Kırazlı37.3–27.6 HK Calc-alk 0.7049–0.7058 4

Biga P. Assos-Ayvacık 20.3–19.5 HK Calc-alk 0.7083–0.7088 2Biga P. Assos-Gulpınar 21.5–16.8 Sho 7Biga P. Ezine-Ayvacık 9.9–8.3 Na-Basalts 0.7031–0.7033 2, 7Bergama Bigadiç 23.0–17.8 HK Calc-alk 0.7070–0.7091 11, 16Emet Emet 20.3 HK Calc-alk 28

Central Aegean–Central West AnatoliaC Aegean Lesvos 18.1–16.2 HK Calc-alk 0.7095 7, 24, 25C Aegean Lesvos 15.5 Sho 0.7082 7, 12C Aegean Skyros 15.0 HK Calc-alk 14C Aegean Psara 17.7–15.0 HK Calc-alk 15C Aegean Chios 17.0–14.3 HK Calc-alk 6C Aegean Kalogeri 6.0 Na Bas 0.7032 17, 1C Aegean Psathoura 0.7 Na Bas 0.7043 15, 1Bergama Bergama-Nebiler 20.4–18.5 HK Calc-alk 0.7056–0.7085 2, 19, 16Bergama Ayvalık 20.3 HK Calc-alk 9Bergama Dikili 15.7 Sho 7Izmir Urla-Karaburun P. 21.3–18.2 HK Calc-Alk 0.7085–0.7079 7, 19Izmir Urla-Karaburun P. 17.3–16.6 Sho 7Izmir Urla-Karaburun P. 11.9–11.3 K Bas 0.7047–0.7075 7, 19Izmir Foça 16.6–16.0 HK Calc-Alk 5Izmir Foça 15.0–14.0 Sho 5Izmir Foça 7.0 K Bas 0.7065 19, 21Izmir Izmir-Menemen 18.2–17.0 0.7078–0.7086 7, 16, 19Izmir Akhisar 16.7 U-K 0.7080 19Uşak-Kula Uşak-Gediz 18.9–17.6 HK Calc-alk 0.7079–0.7083 19, 28Uşak-Kula Uşak-Gediz 15.9–14.6 Sho 0.7074–0.7078 19, 28Uşak-Kula Uşak-Gediz 16.3–14.2 U-K 0.7072–0.7079 19Uşak-Kula Uşak-Gediz 8.3 K Bas 0.7042 19Uşak-Kula Kula 1.3–0.0 Na Bas 0.7031–0.7033 7, 19, 30

South Aegean–South West AnatoliaS Aegean Samos 10.2–8.7 HK Calc-alk 24S Aegean Samos 8.5–8.1 Sho 0.7059–0.7065 26, 27Aydin Söke 7.0 HK Calc-alk 0.7050–0.7095 10, 16Bodrum Bodrum 10.6–10.5 Sho 0.7063–0.7071 15, 27Bodrum Bodrum 8.1–7.9 K Bas 0.7051–0.7058 26, 27S Aegean Patmos 7.2–6.2 Sho 0.7067–0.7076 13, 31S Aegean Patmos 4.4 K Bas 0.7049–0.7051 31

References1, Agostini et al., 2007; 2, Aldanmaz et al., 2000; 3 Aldanmaz et al., 2006; 4, Altunkaynakand Genç, 2007; 5, Altunkaynak et al., 2004; 6, Bellon et al., 1979; 7, Borsi et al., 1972;8, Eleftheriadis et al., 1994; 9, Ercan et al., 1979; 10, Ercan et al., 1985; 11, Erkül et al.,2005; 12, Francalanci et al., 1990; 13, Fytikas et al., 1976; 14, Fytikas et al., 1979;15, Fytikas et al., 1984; 16, Güleç, 1991; 17, Innocenti et al., 1982; 18, Innocenti et al.,1984; 19, Innocenti et al., 2005; 20, Kaymakcı et al., 2007; 21, Kissel et al., 1987; 22, Lilovet al., 1987; 23 Marchev et al., 2004; 24, Pe-Piper and Piper, 2002; 25, Pe-Piper et al.,2003; 26, Robert and Cantagrel, 1977; 27, Robert et al., 1992; 28, Seyitoğlu et al., 1997;29, Singer and Marchev, 2000; 30 Westaway et al., 2004; 31, Wyers and Barton, 1987.* HK Calc-alk, high-K calc-alkaline; Sho, shoshonitic; U-K, ultra potassic; K Bas, potassicalkali basalts; Na Bas, sodic alkali basalts.

Fig. 12. 87Sr/86Sr vs. Age diagram for Northern, Central and Southern WesternAnatolian–Aegean rocks. Colors indicate provenance of studied samples (Blue, BigaPeninsula; Green, Dikili-Bergama; Violet, Izmir; Red, Kula-Uşak; Yellow, Samos-Aydın;Black, Patmos-Bodrum), whereas symbols represent petrogenetic affinity (diamonds,calc-alkaline, shoshonitic and ultra-K; squares, potassic alkali basalts; circles, sodicalkali basalts).(For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

11S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

However, the Aegean rift has a number of different characteristicswith respect to typical backarcs. Indeed, it is diachronous with respectto the onset of the subduction that dates back at least to the Late

Please cite this article as: Agostini, S., et al., On the geodynamics of the

Cretaceous. The Aegean rift is characterized by a low degree ofstretching, so that no oceanic crust was generated through 40 Ma ofrifting. Other structural features of the Aegean region that contrastwith W-directed subduction systems and backarc basins are: i) theshallow dip of the foreland monocline at the base of the accretionaryprism; ii) the shallow depth of the seismicity with an absence ofearthquakes deeper than 180 km; and iii) a low subduction angle(≈16°) in the first 200 km of the subducted lithosphere which tendsto flat northward beneath the rift (Fig. 2).

The Aegean rift is here interpreted as an atypical backarc basin,whose opening is not related to progressive slab rollback, steepening,and asthenospheric replacement as it can be inferred for example forthe Tyrrhenian or the Western Pacific basins. Geodetic data, if

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 12: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 13. δ11B and δ7Li vs. B/Nb and age for a selected suite of Western Anatolia and Aegean rocks. Bars represent 2σ errors on age determinations (CA, calc-alkaline; Sho, shoshonitic;U-K, ultra potassic; K bas, potassic alkali basalts, Na bas, sodic alkali basalts).

12 S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

computed relative to fixed Africa, indicate that the Greekmicroplate isoverriding Africa southwest-ward faster than the Anatolia microplate(Figs. 7 and 8). This simple intra-upper plate velocity gradient is ableto explain extension in the region and the onset of a diffuseextensional setting between the Greek and Anatolian microplates.The differential velocity of the Greek microplate with respect toAnatolia is accommodated by the faster southwest-ward migration ofthe subduction hinge along the Crete trench with respect to theCyprus trench. The differential velocity could be explained by thelarger dehydration of the Ionian oceanic slab, subducting all along theHellenic trench, with respect to the eastern Levantine continentalsegment of the slab, subducting for example along the Cyprus trench.The west-ward Anatolia escape is an unsatisfactory physical inter-pretation because the velocity increases from east to west, indicatingthat the source of the energy cannot be the Arabia indenter alone.Moreover, the Hellenic slab (African plate) is moving west-wardrelative to the mantle (i.e., in the hotspot reference frame) that iftaken alone suggests that it is not sliding and sinking down into themantle, but is rather moving “out” of the mantle in a directionopposed to that of the subduction. Thus, the subduction is activebecause the upper plate moves southwest-ward faster than the lowerAfrica plate, pushing down the slab, and forcing the subduction hingeto migrate in the same direction. The intra-slab down-dip extension iscompatible with a subduction zone attached to a surface plate (i.e. theAfrican plate) that is moving in the direction opposed to thesubduction (Doglioni et al., 2007, 2009). This mechanism is feasiblein the frame of the “westward” drift of the lithosphere relative to themantle. The thickness of the continental upper plate (80–100 km), theshallow dip of the slab, and the source of themagmatism from a depthof about 100 km constrain the mantle wedge to be only continental

Please cite this article as: Agostini, S., et al., On the geodynamics of the

lithospheric mantle or have only a thin underlying asthenosphericlayer. However, the differential SW–SSW retreat of the subductionhinge (faster along the Greek segment with respect to the Anatolianone) should produce tear zones and windows in the slab. Such slabruptures could allow sub-slab asthenospheric upwelling.

Our analysis of the complex magmatic activity in the regionprovided fundamental constraints to develop and test this model:(i) the orogenic activity exhibits a geochemical evolution from calc-alkaline, to shoshonitic, to ultra-K, pointing out the occurrence ofprogressive dehydration of the slab (extremely low B isotope values)and a very tiny mantle wedge (variable and low values of Li isotopes)(Figs. 13 and 14A); (ii) the later occurrence of alkali basalts sourced inthe sub-slab testify for the occurrence of upwelling and melting ofmantle asthenosphere. Moreover, the time shifting from K-alkalibasalts, with residual slab imprinting, to OIB-type Na-basalts pointsout for a progressive influx of sub-slab mantle whose rising isconsidered due to the stretching of the subducted slab, with theformation of ruptures, or vertical slab windows, allowing suchmagmas to reach the surface (Fig. 14B).

The Hellenides and Taurides (Fig. 1) have been shown as polygenicorogens where a few microplates gradually docked, sandwiching theintervening oceanic regions (e.g., Pindos, Sub-Pelagonian, Vardar,Izmir–Ankara zone), supporting a very heterogeneous paleogeogra-phy and several lithospheric anisotropies as visible in Fig. 1B. Forexample, the lower K content in the Pliocene–Holocene South Aegeanactive volcanic arc could be related to a change in composition of thesubducting slab such as from a stretched passive continental marginduring the Miocene, giving rise to high-K calc-alkaline to shoshoniticmagmas, to an oceanic slab during the Plio-Pleistocene (low- tomedium-K calc-alkaline magmas). The differential slab retreat was

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 13: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

Fig. 14. Cartoon showing the geodynamic evolution of Africa–Aegean converging system, along A–A′ section of Fig. 7. During the Early to Middle Miocene (A), the progressive slabdehydration metasomatized the mantle wedge (MW), leading to the formation of calc-alkaline, shoshonitic and ultra potassic magmas. The southwest-ward migration of thesubduction hinge during the Late Miocene to Present produced a wide extensional tectonics affecting both the upper plate, themantle wedge (MW) and the subducted slab, and wasmatched by asthenosphere upwelling (B). In this tectonic setting, sub-slab OIB-type magmas formed and reached the surface after small (potassic alkali basalts) or no interaction(sodic alkali basalts) with the overlaying mantle.

13S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

enhanced by the lower plate lithospheric variability, and kinemati-cally required slab ruptures, which can explain the occurrence ofalkaline Na-rich final magmatic products.

Acknowledgements

Randy G. Keller and an anonymous referee are acknowledged fortheir constructive and helpful comments. We thank M. Fytikas(Aristotle University of Thessaloniki, Greece), N. Kolios (IGME, Athens,Greece), and M.Y. Savaşçın (Dokuz Eylül University, Izmir, Turkey) forstimulating discussions during fieldwork. We are also indebted withG. Panza (Univeristy of Trieste, Italy) for stirring discussions onEastern Mediterranean geophysical context.

F. Innocenti devoted his last days for revising the earlier version ofthis paper. In his enthusiastic conception of science this paper wouldnot be the last one. For the co-authors, it has been a privilege to workwith him for several decades until his very last days.

References

Agostini, S., Doglioni, C., Innocenti, F., Manetti, P., Savaşçın, M.Y., Tonarini, S., 2005.Tertiary high-Mg volcanic rocks from western Anatolia and their geodynamicsignificance for the evolution of the Aegean area. In: Fytikas, M., Vougioukalakis, G.(Eds.), The South Aegean Volcanic Arc: Present Knowledge and Future Perspectives:Elsevier Book Special Series, Developments on Volcanology, vol. 7, pp. 345–362.

Agostini, S., Doglioni, C., Innocenti, F., Manetti, P., Tonarini, S., Savaşçın, M.Y., 2007. Thetransition from orogenic to intraplate Neogene magmatism in Western Anatoliaand Aegean area. In: Beccaluva, L., Bianchini, G., Wilson, M. (Eds.), CenozoicVolcanism in the Mediterranean Area: Geol. Soc. Am. Sp. Paper, vol. 418, pp. 1–15.

Agostini, S., Ryan, J.G., Tonarini, S., Innocenti, F., 2008. Drying and dying of a subductedslab: coupled Li and B isotope variations in Western Anatolia Cenozoic volcanism.Earth Planet. Sci. Lett 272, 139–147.

Please cite this article as: Agostini, S., et al., On the geodynamics of the

Aldanmaz, E., Pearce, J.A., Thirlwall, M.F., Mitchell, J.G., 2000. Petrogenetic evolution oflate Cenozoic, post-collision volcanism in western Anatolia, Turkey. J. Volcanol.Geotherm. Res. 102, 67–95.

Aldanmaz, E., Koprubaşı, N., Gürer, O.F., Kaymakcı, N., Gourgaud, A., 2006. Geochemicalconstrants on the Cenozoic, OIB-type alkaline volcanic rocks of NW Turkey:implications for mantle sources and melting processes. Lithos 86, 50–76.

Alıcı, P., Temel, A., Gourgaud, A., 2002. Pb–Nd–Sr isotope and trace elementgeochemistry of Quaternary extension related alkaline volcanism: a case study ofKula region (western Anatolia, Turkey). J. Volcanol. Geotherm. Res. 115, 487–510.

Altherr, R., Keuzer, H., Wendt, I., Lenz, H., Wagner, G.A., Keller, J., Harre, W., Hohndorf,A., 1982. A Late Oligocene/Early Miocene high temperature belt in the Attic-Cycladic crystalline complex (SE Pelagonian, Greece). Geol. Jahrb. 23, 97–164.

Altherr, R., Henjes-Kunst, F., Mathews, A., Friedrichsen, H., Hansen, B.T., 1988. O–Srisotopic variations in Miocene granitoids from the Aegean: evidence for an originby combined assimilation and fractional crystallization. Contrib. Mineral. Petrol.100, 528–541.

Altunkaynak, S., Genç, S.C., 2007. Petrogenesis and time-progressive evolution of theCenozoic continental volcanism in the Biga Peninsula, NWAnatolia (Turkey). Lithos102, 316–340.

Altunkaynak, S., Rogers, N., Kelley, S., 2004. Petrogenetic evolution of the bimodalCenozoic volcanism in Western Anatolia (Turkey): the Foca volcanic center. 32ndIntl. Geol. Congress, Florence, Italy, 20–28 August 2004, Abstracts, pp. 1294–1295.

Angelier, J., Lyberis, N., LePichon,X., Barrier, E., Huchon, P., 1982. The tectonic developmentof the Hellenic Arc and the Sea of Crete: a synthesis. Tectonophys 86, 159–196.

Bellon, H., Grissolet, G., Sorel, D., 1979. Age de l'activité volcanique néogène de l'île deChios (Mer Egée, Grèce). C. R. Acad. Sci. Paris 288D, 1255–1258.

Berckhemer, H., 1977. Some aspects of the evolution of marginal seas deduced fromobservations in the Aegean region, northeastern Greece. In: Biju-Duval, B.,Montadert, L. (Eds.), Int. Symp. Structural History of the Mediterranean Basins,Split, Yugoslavia, 25–29 October 1976. Technip, Paris, pp. 303–313.

Bingöl, E., Delaloye, M., Ataman, G., 1982. Granitic intrusions in Western Anatolia: acontribution of the geodynamic study of this area. Eclogae Geol. Helv. 75, 437–446.

Boccaletti, M., Manetti, P., Peccerillo, A., 1974. The Balkanids as an instance of back-arcthrust belt: possible relation with the Hellenids. Geol. Soc. Am. Bull. 85, 1077–1084.

Borsi, S., Ferrara, G., Innocenti, F., Mazzuoli, R., 1972. Geochronology and petrology ofrecent volcanics in the Eastern Aegean Sea (West Anatolia and Lesvos Island). Bull.Volcanol. 36, 473–496.

Bourova, E., Kassaras, I., Pedersen, A.H., Yanovskaya, T., Hatzfeld, D., Kiratzi, A., 2005.Constraints on absolute S velocities beneath the Aegean Sea from surface waveanalysis. Geophys. J. Int. 160, 1006–1019.

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 14: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

14 S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

Burchfiel, B.C., 2008. The Aegean: a natural laboratory for tectonics. IOP Conf. Ser.: EarthEnviron. Sci, 2, p. 012001. doi:10.1088/1755-1307/2/1/012001. 4 pp.

Carminati, E., Doglioni, C., 2004. Europe–Mediterranean tectonics. Encyclopedia ofGeology. Elsevier, pp. 135–146.

Christova, C., Nikolova, S.B., 1993. The Aegean region: deep structures and seismologicalproperties. Geophys. J. Int. 115, 635–653.

Crespi, M., Cuffaro, M., Doglioni, C., Giannone, F., Riguzzi, F., 2007. Space geodesyvalidation of the global lithospheric flow. Geophys. J. Int. 168, 491–506.doi:10.1111/j.1365-246X.2006.03226.x.

de Voogd, B., Truffert, C., Chamot-Rooke, N., Huchon, P., Lallemant, S., Le Pichon, X.,1992. Two-ship deep seismic soundings in the basins of the Eastern MediterraneanSea (Pasiphae cruise). Geophys. J. Int. 109, 536–552.

Delaloye, M., Bingöl, E., 2000. Granitoids from western and northwestern Anatolia:geochemistry and modelling of geodynamic evolution. Int. Geol. Rev. 42, 241–268.

Doglioni, C., Harabaglia, P., Merlini, S., Mongelli, F., Peccerillo, A., Piromallo, C., 1999.Orogens and slabs vs their direction of subduction. Earth-Sci. Rev. 45, 167–208.

Doglioni, C., Agostini, S., Crespi, M., Innocenti, F., Manetti, P., Riguzzi, F., Savaşçin, M.Y.,2002. On the extension in western Anatolia and the Aegean sea. J. Virtual Explor. 8,169–184.

Doglioni, C., Carminati, E., Cuffaro, M., Scrocca, D., 2007. Subduction kinematics anddynamic constraints. Earth-Sci. Rev. 83, 125–175.

Doglioni, C., Tonarini, S., Innocenti, F., 2009. Mantle wedge asymmetries along oppositesubduction zones. Lithos. doi:10.1016/j.lithos.2009.01.012.

Eleftheriadis, G., Pe-Piper, G., Christofides, G., Soldatos, T., Esson, J., 1994. K–Ar dating ofthe Samothraki volcanic rocks, Thrace, north-eastern Aegean (Greece). Bull. Geol.Soc. Greece 30, 205–212.

Ercan, T., Dincel, A., Gunay, E., 1979. Uşak volkanitlerinin petrolojisi ve plakatektoniğiAcisindan ege bolgesindeki yeri (Petrology of the Uşak volcanics andtheir place in the Aegean region according to plate tectonics). Türk. Jeol. KurumuBül. (Bull. Geol. Soc. Turkey) 22 (2), 185–198.

Ercan, T., Satır, M., Türkecan, A., Akyürek, B., Çevikbaş, A., Günay, E., Ateş, M., Can, B.,1985. Batı Anadolu Senozoyik volkanitlerine ait yeni kimyasal, izotopik veradyometrik verilerin yorumu. Türk. Jeol. Kurumu Bül. (Bull. Geol. Soc. Turkey)28, 121–136.

Erkül, F., Helvacı, C., Sözbılır, H., 2005. Stratigraphy and geochronologyof theEarlyMiocenevolcanic units in the Bigadiç borate basin, western Turkey. Turk. J. Earth Sci. 14, 1–27.

Francalanci, L., Civetta, L., Innocenti, F., Manetti, P., 1990. Tertiary–Quaternary alkalinemagmatism of the Aegean–Western Anatolia area: a petrological study in the lightof new geochemical and isotopic data. IESCA Proc. 2, 385–396.

Francalanci, L., Vougioukalakis, G.E., Perini, G., Manetti, P., 2005. A West–East traversealong themagmatism of the South Aegean volcanic arc in the light of volcanological,chemical and isotope data. In: Fytikas, M., Vougioulakakis, G. (Eds.), The SouthAegean Volcanic Arc: Present Knowledge and Future Perspectives: Elsevier BookSpecial Series, Developments on Volcanology, vol. 7, pp. 65–111.

Fytikas, M., Giuliani, O., Innocenti, F., Marinelli, G., Mazzuoli, R., 1976. Geochronologicaldata on recent magmatism of the Aegean Sea. Tectonophys 31, T29–T34.

Fytikas, M., Giuliani, O., Innocenti, F., Manetti, P., Mazzuoli, R., Peccerillo, A., Villari, L.,1979. Neogene volcanism of the northern and central Aegean region. Ann. Géol.Pays Hell. 30 (1), 106–129.

Fytikas, M., Innocenti, F., Manetti, P., Mazzuoli, R., Peccerillo, A., Villari, L., 1984. Tertiaryto Quaternary Evolution of Volcanism in the Aegean region. In: Dixon, J.E.,Robertson, A.H.F. (Eds.), The Geological evolution of the Eastern Mediterranean:Geological Society Special Publication, vol. 17, pp. 687–699.

Garfunkel, Z., 2004. Origin of the Eastern Mediterranean basin: a reevaluation.Tectonophysics 391, 11–34.

Gautier, P., Brun, J.P., Moriceau, R., Sokoutis, D., Martinod, J., Jolivet, L., 1999. Timing,kinematics and cause of Aegean extension: a scenario based on a comparison withsimple analogue experiments. Tectonophysics 315, 31–72.

Gripp, A.E., Gordon, R.G., 2002. Young tracks of hotspots and current plate velocities.Geophys. J. Int. 150, 321–361.

Güleç, N., 1991. Crust–mantle interaction in western Turkey: implications from Sr and Ndisotope geochemistry of Tertiary and Quaternary volcanics. Geol. Mag. 128, 417–435.

Harabaglia, P., Doglioni, C., 1998. Topography and gravity across subduction zones.Geophys. Res. Lett. 25 (5), 703–706.

Hatzfeld, D., Karagianni, E., Kassaras, I., Kiratzi, A., Louvari, E., Lyon-Caen, H., Makropoulos,K., Papadimitriou, P., Bock, G., Priestley, K., 2001. Shear-wave anisotropy in the uppermantle beneath the Aegean related to internal deformation. J. Geophys. Res. 106 (12),30737–30753.

Heflin, M. et al., 2008, http://sideshow.jpl.nasa.gov/mbh/series.html.Hetzel, R., Passchier, C.W., Ring, U., Dora, O.Ö., 1995. Bivergent extension in orogenic

belts: the Menderes Massif (southwestern Turkey). Geology 23, 455–458.Horvath, F., Berckhemer, H., 1982. Mediterranean backarc basins. In: Berckhemer, H.,

Hsü, K.J. (Eds.), Alpine Mediterranean Geodynamics: Am. Geophys. Un., Geody-namics Series, vol. 7, pp. 141–173.

Hurtig, E., Cermak, V., Haenel, R., Zui, V., 1991. Geothermal atlas of Europe. HermannHaack Verlagsgeselleschaft mbH, Geographisch-Kartographische Anstalt Gotha.GeoForschungs Zentrum Potsdam. 74 pp., 110 plates. Publication n.1.

Innocenti, F., Kolios, N., Manetti, P., Rita, F., Villari, L., 1982. Acid and basic late Neogenevolcanism in central Aegean Sea: its nature and geotectonic significance. Bull.Volcanol. 45, 87–97.

Innocenti, F., Kolios, N., Manetti, P., Mazzuoli, R., Peccerillo, A., Villari, L., 1984. Evolutionand geodynamic significance of the Tertiary orogenic volcanism in northeasternGreece. Bull. Volcanol. 47, 25–37.

Innocenti, F., Agostini, S., Di Vincenzo, G., Doglioni, C., Manetti, P., Savaşçin, M.Y.,Tonarini, S., 2005. Neogene and Quaternary volcanism inWestern Anatolia: magmasources and geodynamic evolution. Mar. Geol. 221, 397–421.

Please cite this article as: Agostini, S., et al., On the geodynamics of the

Jackson, J., 1994. Active tectonics of the Aegean region. Annu. Rev. Earth Planet. Sci. 22,239–271.

Jolivet, L., 2001. A comparison of geodetic and finite strain pattern in the Aegean,geodynamic implications. Earth Planet. Sci. Lett. 187, 95–104.

Jolivet, L., Brun, J.-P., Gautier, P., Lallement, S., Patriat, M., 1994. 3D kinematics ofextension in the Aegean region from the earlyMiocene to the Present, insights fromthe ductile crust. Bull. Soc. Géol. Fr. 165, 195–209.

Kaymakcı, N., Aldanmaz, E., Langereis, C., Spell, T.L., Gurer, O.F., Zanetti, K.A., 2007. LateMiocene transcurrent tectonics in NW Turkey: evidence from palaeomagnetismand 40Ar–39Ar dating of alkaline volcanic rocks. Geol. Mag. 144 (2), 379–392.

Keller, J., 1982. Mediterranean Island arcs. In: Thorpe, R.S. (Ed.), Andesites: OrogenicAndesites and Related Rocks. Wiley, pp. 307–325.

Kissel, C., Laj, C., Poisson, A., 1987. A paleomagnetic overview of the Tertiarygeodynamical evolution of the Hellenic Arc. Terra Cognita 7, 108.

Konstantinou, K.I., Melis, N.S., 2008. High-Frequency Shear-Wave Propagation acrossthe Hellenic Subduction Zone. Bull. Seismol. Soc. Am. 98 (2), 797–803. doi:10.1785/0120060238.

Kreemer, C., Chamot-Rooke, N., Le Pichon, X., 2004. Constraints on the evolution andvertical coherency of the formation in the Northern Aegean from a comparison ofgeodetic, geologic and seismologic data. Earth Planet. Sci. Lett. 225, 329–346.

Le Pichon, X., Angelier, J., 1979. The Hellenic arc and trench system: a key to theneotectonic evolution of the eastern Mediterranean area. Tectonophysics 60, 1–42.

Le Pichon, X., Chamot-Rooke, N., Lallemant, S., Noomen, R., Veis, G., 1995. Geodeticdeterminationof the kinematics of centralGreecewith respect to Europe: implicationsfor eastern Mediterranean tectonics. J. Geophys. Res. 100, 12675–12690.

Lenci, F., Doglioni, C., 2007. On some geometric prism asymmetries. In: Lacombe, O., Lavé, J.,Roure, F., Verges, J. (Eds.), Thrust belts and Foreland Basins: From Fold Kinematicsto Hydrocarbon Systems. In: Frontiers in Earth Sciences. Springer, pp. 41–60.

Lilov, P., Yanev, Y., Marchev, P., 1987. K/Ar dating of the Eastern Rhodopes Paleogenemagmatism. Geol. Balc. 17 (4), 49–58.

Makris, J., 1978. The crust and upper mantle of the Aegean region from deep seismicsoundings. Tectonophysics 46, 269–284.

Makris, J., Stobbe, C., 1984. Physical properties and state of the crust and upper mantleof the eastern Mediterranean Sea deduced from geophysical data. Mar. Geol. 55,345–361.

Makris, J., Papoulia, J., Papanikolaou, D., Stavrakakis, G., 2001. Thinned continental crustbelow northern Evoikos Gulf, central Greece, detected from deep seismicsoundings. Tectonophysics 341/1-4, 225–236.

Manea, V., Gurnis, N., 2007. Subduction zone evolution and low viscosity wedges andchannels. Earth Planet. Sci. Lett. 264, 22–45.

Marchev, P., Raicheva, R., Downes, H., Vaselli, O., Chiaradia, M., Moritz, R., 2004.Compositional diversity of Eocene–Oligocene basaltic magmatism in the EasternRhodopes, SE Bulgaria: implication. Tectonophysics 393, 301–328.

McClusky, S., Balassanian, S., Barka, A., Demir, C., Ergintav, S., Georgiev, I., Gurkan, O.,Hamburger, M., Hurst, K., Kastens, K., Kekelidze, G., King, R., Kotzev, V., Lenk, O.,Mahmoud, S., Mishin, A., Nadariya, M., Ouzounis, A., Paradissis, D., Peter, Y.,Prilepin, M., Reilinger, R., Sanli, I., Seeger, H., Tealeb, A., Toksoz, M.N., Veis, G., 2000.Global Positioning System constraints on plate kinematic and dynamics in theeastern Mediterranean and Caucasus. J. Geophys. Res. 105 (B3), 5695–5719.

McKenzie, D.P., 1972. Active tectonics of the Mediterranean region. Geophys. J. R.Astron. Soc. 30, 109–185.

Papanikolaou., D., Vassilakis, E., 2008. Middle Miocene E–W tectonic horst structure ofCrete through extensional detachment faults. IOP Conf. Ser.: Earth Environ. Sci. 2012003 (6pp). doi:10.1088/1755-1307/2/1/012003.

Papazachos, B.C., Karakostas, V.G., Papazachos, C.B., Scordilis, E.M., 2000. The geometryof the Wadati-Benioff zone and lithospheric kinematics in the Hellenic arc.Tectonophysics 319, 275–300.

Papazachos, B.C., Dimitriadis, S.T., Panagiotopoulos, D.G., Papazachos, C.B., Papadimi-triou, E.E., 2005. Deep structure and active tectonics of the southern Aegeanvolcanic arc. In: Fytikas, M., Vougioulakakis, G. (Eds.), The South Aegean VolcanicArc: Present Knowledge and Future Perspectives: Elsevier Book Special Series,Developments on Volcanology, vol. 7, pp. 47–64.

Panza, G.F., Pontevivo, A., Chimera, G., Raykova, R., Aoudia, A., 2003. The Lithosphere–Asthenosphere: Italy and surroundings. Episodes 26 (3), 169–174.

Panza, G.F., Peccerillo, A., Aoudia, A., Farina, B., 2007. Geophysical and petrologicalmodelling of the structure and composition of the crust and upper mantle incomplex geodynamic settings: the Tyrrhenian Sea and surroundings. Earth ScienceReviews 80, 1–46.

Pe-Piper, G., Piper, D.J.W., 2002. The igneous rocks of Greece: the anatomy of an orogen.Gebrüder Borntraeger, Berlin. 573 pp.

Pe-Piper, G., Matarangas, Reynolds, P.H., Chatterjee, K., 2003. Shoshonites from AgiosNectarios, Lesbos, Greece: origin by mixing of felsic and mafic magma. Eur. J.Mineral. 15, 117–125.

Peccerillo, A., Panza, G.F., Aoudia, A., Frezzotti, M.L., 2008. Relationships betweenmagmatism and lithosphere–asthenosphere structure in the western Mediterra-nean and implication for geodynamics. Rendiconti Lincei 19, 291–301. doi:10.1007/s12210-008-0020-x.

Piromallo, C., Morelli, A.D., 2003. P wave tomography of the mantle under the Alpine–Mediterranean area. J. Geophys. Res. 108 (B2), 2065. doi:10.1029/2002JB001757.

Reilinger, R., McClusky, S., Vernant, P., Lawrence, S., Ergintav, S., Cakmak, R., Ozener, H.,Kadirov, F., Guliev, I., Stepanyan, R., Nadariya, M., Hahubia, G., Mahmoud, S., Sakr, K.,ArRajehi, A., Paradissis, D., Al-Aydrus, A., Prilepin, M., Guseva, T., Evren, E., Dmitrotsa,A., Filikov, S.V., Gomez, F., Al-Ghazzi, R., Karam, G., 2006. GPS constraints oncontinental deformation in the Africa–Arabia–Eurasia continental collision zone andimplications for the dynamics of plate interactions. J. Geophys. Res. 111, B05411.doi:10.1029/2005JB004051.

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025

Page 15: ARTICLE IN PRESS · Extensional tectonics Cenozoic magmatism Eastern Mediterranean Aegean backarc Geodynamics Isotope geochemistry Petrology The Aegean rift is considered to be either

15S. Agostini et al. / Tectonophysics xxx (2009) xxx–xxx

ARTICLE IN PRESS

Robert, U., Cantagrel, J.M., 1977. Le volcanisme basaltique dans le Sud-Est de la MerEgée. Données géochronologiques et relations avec la téctonique: VI Coll. Geol.Aegean Reg., Athens, vol. 3, pp. 961–967.

Robert, U., Foden, J., Varne, R., 1992. The Dodecanese Province, SE Aegean: a model fortectonic control on potassic magmatism. Lithos 28, 241–260.

Robertson, A.H.G., 1998. Mesozoic–Tertiary tectonic evolution of the easternmostMediterranean area: integration ofmarine and land evidence. Proc. ODP Sci. Results160, 723–782.

Robertson, A.H.F., Clift, P.D., Degnan, P.J., Jones, G., 1991. Palaeogeographic andpalaeotectonic evolution of the eastern Mediterranean Neotethys. In: Channell, J.E.T.,Winterer, E.L., Jansa, L.F. (Eds.), Palaeogeography and Paleoceanography of Tethys:Palaeogeog., Palaeoclim., Palaeoecol., vol. 87, pp. 289–343.

Scoppola, B., Boccaletti, D., Bevis, M., Carminati, E., Doglioni, C., 2006. The westwarddrift of the lithosphere: a rotational drag? Bull. Geol. Soc. Am. 118, 199–209.

Şengör, A.M.C., Yilmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach.Tectonophysics 75, 181–241.

Seyitoğlu, G., Scott, B.C., 1996. The cause of N–S extensional tectonics in westernTurkey: tectonic escape vs back-arc spreading vs orogenic collapse. J. Geodyn. 22,145–153.

Seyitoğlu, G., Anderson, D., Nowell, G., Scott, B., 1997. The evolution fromMiocene potassicto Quaternary sodic magmatism in western Turkey: implications for enrichmentprecesses in the lithospheric mantle. J. Volcanol. Geotherm. Res. 76, 127–147.

Singer, B., Marchev, P., 2000. Temporal evolution of arc magmatism and hydrothermalactivity, including epithermal veins, Borovitsa caldera, Southern Bulgaria. Econ.Geol. 95, 1155–1164.

Stampfli, G.M., 2000. Tethyan oceans. In: Bozkurt, E., Winchester, J.A., Piper, J.D.A.(Eds.), Tectonics and Magmatism in Turkey and the Surrounding Area: GeologicalSociety Special Publications, vol. 173, pp. 1–23.

Please cite this article as: Agostini, S., et al., On the geodynamics of the

Tokçaer, M., Agostini, S., Savascın, M.Y., 2005. Geotectonic setting, origin andemplacement model of the youngest Kula volcanics in western Anatolia. Turk. J.Earth Sci. 14, 145–166.

Tonarini, S., Agostini, S., Innocenti, F., Manetti, P., 2005. δ11B as tracer of slabdehydration and mantle evolution in western Anatolia Cenozoic magmatism. TerraNova 17, 259–264.

Trampert, J., Deschamps, F., Resovski, J., Yuen, D., 2004. Probabilistic tomography mapschemical heterogeneities throughout the lower mantle. Sciences 306, 853–856.

Turcotte, D.L., Schubert, G., 1982. Geodynamics. Wiley, New York. 450 pp.USGS Earthquake Cathalogue, http://neic.usgs.gov/neis/epic.van Hinsbergen, D.J.J., Meulenkamp, J.E., 2006. Neogene supradetachment basin

development on Crete (Greece) during exhumation of the South Aegean corecomplex. Basin Res. 18, 103–124.

Westaway, R., Pringle, M., Yurtmen, S., Demir, T., Bridgland, D., Rowbotham, G., Maddy,D., 2004. Pliocene and Quaternary regional uplift in western Turkey: the GedizRiver terrace staircase and the volcanism at Kula. Tectonophysics 391, 121–169.

Wortel, M.J.R., Spakman, W., 2000. Subduction and slab detachment in theMediterranean–Carpathian region. Science 290, 1910–1917.

Wyers, G.P., Barton, M., 1987. Geochemistry of a transitional ne-trachybasalt—Q-trachyte lava series from Patmos (Dodecanesos), Greece: further evidence forfractionation, mixing and assimilation. Contrib. Mineral. Petrol. 93, 297–311.

Yanev, Y., Innocenti, F., Manetti, P., Serri, G., 1998. Upper Eocene–Oligocene collision-related volcanism in Eastern Rhodopes (Bulgaria)–Western Thrace (Greece):petrogenetic affinity and geodynamic significance. Acta Vulcanol. 10 (2), 279–291.

Yilmaz, Y., Genc, S.C., Gurer, F., Bozcu, M., Yilmaz, K., Karacik, Z., Altunkaynak, S., Elmas,A., 2000.When did the western Anatolian grabens begin to develop? In: Bozkurt, E.,Winchester, J.A., Piper, J.D.A. (Eds.), Tectonics and Magmatism in Turkey and theSurrounding Area, vol. 173. Geological Society Special Publications, pp. 353–384.

Aegean rift, Tectonophysics (2009), doi:10.1016/j.tecto.2009.07.025