Aegean Crustal Thickness Inferred from Gravity Inversion. Geodynamical Implications

Earth and Planetary Science Letters 228 (2004) 267–280 www.elsevier.com/locate/epsl

Aegean crustal thickness inferred from gravity inversion. Geodynamical implications

Ce´line Tirela,*, Fre´de´ric Gueydana, Christel Tiberib, Jean-Pierre Bruna

aGe´osciences Rennes UMR 6118 CNRS, Universite´ de Rennes 1, Rennes, France bLaboratoire de Tectonique UMR 7072 CNRS, Universite´ Pierre et Marie Curie, Paris, France Received 19 April 2004; received in revised form 8 October 2004; accepted 18 October 2004 Available online 21 November 2004 Editor: E. Bard

Abstract

Since Oligo–Miocene times, the Aegean domain has undergone regional extension due to the southward retreat of the Hellenic subduction zone. Boundary conditions have been more recently modified by the westward extrusion of Anatolia. A new map of the Aegean crustal thickness inferred from gravity inversion is proposed here to better constrain the variations in space and time of crustal thinning that has accumulated since Oligo–Miocene times. Moho topography is obtained by inversion of satellite marine gravity data. Data are first corrected for terrain anomalies and deep mantle effects (African subducting slab). They are then filtered between 50 and 300 km to avoid short wavelength intracrustal effects. Results are consistent with previous 2D geophysical studies (seismic refraction, receiver functions) and show that an overall regional isostatic compensation of the crust holds for the Aegean area, with a mean crustal thickness of 25 km. Three different provinces (North Aegean, Cyclades and Cretan Sea) can be identified. Thinner crust is observed both in the North Aegean region (NE–SW trending of thinning, with crustal thickness lower than 24 km) and in the Cretan Sea (crustal thickness of 22–23 km). Between these two regions, the Cyclades are marked by a rather flat Moho at 25 km. A two-stage model of the Aegean extension could well explain the observed crustal thickness variation. From Oligocene to middle Miocene, gravitational collapse of the Hellenides, due to the southward retreat of the African slab, reduced the Aegean continental crust from 50 km (by reference to continental Greece and Anatolia) to a mean value of 25 km at the scale of the whole Aegean. From upper Miocene to present, the westward extrusion of Anatolia modified the extension and the associated crustal thinning in the North Aegean domain. During this second episode, crustal thinning related to the southward retreat of the African slab tends to localize in the Cretan Sea. The Cyclades likely behave as a rigid block translated toward the south–west without significant deformation, in agreement with the GPS velocity field and the lack of major earthquakes. D 2004 Elsevier B.V. All rights reserved.

Keywords: Aegean Sea; crustal thickness; gravity inversion; geodynamic

* Corresponding author. E-mail address: [email protected] (C. Tirel).

1. Introduction recent basins is not available in the whole Aegean domain. Following continental collision and crustal thick- In the present paper, we propose a new map of the ening, the Aegean domain has undergone two Aegean crustal thickness obtained from inversion of successive stages of extension since Oligocene times. marine gravimetric data. To assess that the gravity From Oligocene to middle Miocene, extension was signal only reflects the crustal thickness, a series of first marked by the development of core complexes in corrections were applied: water load and terrain the Cyclades, Menderes and Rhodope, with a domi- corrections, effects of the subducting African slab nantly N–S direction of stretching [1–4]. During this and bandpass filtering of short wavelengths related to period, extension likely corresponds to a gravity crustal heterogeneities smaller than 50 km. The results collapse of the previously thickened and thermally are then compared to available reflexion and refrac- softened lithosphere, controlled by the southward tion seismic profiles and receiver functions [26–30] retreat of the south Hellenic subduction zone [5–9]. that provide local estimates of crustal thickness. The Since the late Miocene, the effects of the westward variations of crustal thickness are finally discussed in displacement of Turkey were superimposed on the terms of a two-stage deformation history of the previous kinematic pattern [4,10–14]. Particularly, the Aegean since Oligo–Miocene times. Cyclades underwent considerable stretching during the first stage and became rather inactive during the second one [10,15–20]. Deformation tends to localize 2. Gravity inversion within a restricted number of active faulting areas on the edges of the Aegean domain (continental Greece 2.1. Data processing and Western Turkey, North Aegean Through, Cretan Sea) and the volcanic arc. Active normal faulting The complete Bouguer anomaly (CBA) of the resulting from N–S stretching is especially well Aegean area is used in this study to image the Moho represented in the Gulf of Corinth and Evia rifts variations. A complete Bouguer anomaly (CBA) is [13,15,21–23]. In the Peloponnesus and Crete, the first compiled from the satellite derived free-air present active extension is parallel to the subduction anomaly (FAA; Fig. 1b, [31,32]). The anomaly is then arc [24]. This is in good agreement with locations of corrected from possible deep and crustal sources to major earthquakes and the present day displacement only retain the crustal thickness information. pattern demonstrated by GPS measurements Sandwell and Smith [31] and Smith and Sandwell [10,19,20]. The present day kinematics and strain [32] high-density satellite 2 min grid gravimetry and patterns depict the superposition of a dominantly topography data set provide the free-air gravity dextral shearing along the North Anatolian Fault and anomaly (FAA) and bathymetry maps for the Aegean the North Aegean Through (Fig. 1a) to the north, and (Fig. 1b and a, respectively). The CBA (Fig. 2a) is a N–S stretching due to the subduction retreat to the then computed by removing the effects of the water south. load and terrain correction. These two corrections are In the North Aegean, the location of Plio-Quater- computed with a 3D grid composed of elementary nary sedimentary basins (Fig. 1d, [25]) is relatively prisms of 22 min basal area and a thickness set to well correlated with regions that have undergone the bathymetry (Fig. 1a). The gravity signal of this 3D important thinning during the second stage of structure is computed with water and crustal densities deformation. However, a complete map of these of 1000 and 2670 kg m3, respectively. It is removed

Fig. 1. (a) Topographic and bathymetric map of the study area (isolines every 200 m). N.A.T.—North Aegean Trough; E.T.—Edremit Trough; L.P.T.—Lesbos–Psara Trough; Ma.—Mandouthi; An.—Andros; Ti.—Tinos; Am.—Amorgos. (b) Free air anomaly (FAA) deduced from satellite altimetry (isolines every 20 mGal). (c) Seismotectonic map modified after Le Pichon and Angelier [7], Gautier and Brun [57], Jolivet et al. [2], Kahle et al. (extension strain rates) [19], Engdahl et al. [18] (0–70 km deep seismicity from [18] is marked by black circles). (d) Location of the principal sites cited in the text, together with the location of the main sedimentary basins modified after Mascle and Martin [25]. S.F.— Sikinos–Folegandros; I.–C.—Iraklion–Central; I.–S.—Ikaria–Samos; W.–S.—West–Santorini; B.—basins. C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280 269 270 C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280

Fig. 2. (a) Complete Bouguer anomaly (CBA; isolines every 20 mGal). (b) Gravity anomaly due to the subducting African slab (isolines every 10 mGal). (c) Complete Bouguer anomaly without the subducted African slab effect (CBAS; isolines every 20 mGal), and (d) the same filtered with a bandpass 50–300 km (FCBAS; isolines every 10 mGal). This late filtered map is used to compute the Moho depth variation in Fig. 3a. C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280 271 from the FAA to obtain the CBA (Fig. 2a). Note that a frequency signal, prior to the inversion, we filter the low value for crustal density is used to model the CBAS between 50 and 300 km with a bandpass presence of sedimentary rocks near the water–crust taper through the Fourier domain using the Generic interface. Mapping Tools (GMT) software [38].Alower The comparison between FAA and CBA (Figs. 1b minimum value of the bandpass filter (30 km instead and 2a) shows that the large positive signal observed of 50 km for example) does not remove all the short in the south Aegean is amplified and more localized to wavelength variations. Setting the maximum of the reach a value of 160 mGal in the Cretan Sea. The bandpass filter to a larger value will decrease the increase of CBA (up to 80 mGal) in the North Aegean amplitude of the CBAS signal. Indeed, a test with a Trough emphasizes the strong influence of the water bandpass filter of 50–1000 km shows a signal load in zones of high bathymetry. amplitude decrease of about 10 mGal and the same The effect of deep low-frequency sources, partic- shape of anomaly signal than a 50–300 km filter. ularly the African subducting slab in this region The choice of a bandpass filter between 50 and 300 [33,34], must be taken into account. It is beyond the km was thus found sufficient to remove intracrustal scope of this article to discuss in detail the shape of density variations and to preserve the amplitude of the subducting slab. We are particularly aware that the CBAS signal. The filtered complete Bouguer this shape will control the gravity signature induced anomaly without slab effect (FCBAS; Fig. 2d) only by the slab. However, the estimate of the slab gravity reflects the crustal thickness variations. The Cretan effect made by Tiberi et al. [34] presents the Sea and the North Aegean Trough correspond to two advantage of taking a non-ad hoc shape for the maxima (~40 mGal) in the FCBAS, suggesting a subducting lithosphere (unlike Tsokas and Hansen’s shallower Moho. Between these two regions, the previous work [35]). Tiberi et al. [34] used tomo- FCBAS in the Cyclades reaches lower values. These graphic data [36] and a linear relationship between observations are consistent with a thin crust in the P-wave velocity and density [37] to compute the slab Cretan Sea and the North Aegean Through and a effect. The modeled slab anomaly (Fig. 2b) is thicker crust in the Cyclades, as previously men- centred north of Crete with a maximum of 120 tioned [26–30,39]. mGal and decreases radially to vanish in the North Aegean domain. The SW–NE decrease of the gravity 2.2. Inversion procedure signal is observed both in the computed slab anomaly and in the CBA (Fig. 2a,b). This good The inversion used here is based on the direct correlation first validates the proposed slab geometry formula of Parker [40]. It calculates the gravity used for the computation and also shows that the signal Dg(x, y) of a layer having a density contrast African slab is responsible for the major part of the Dq with its underlying semi-infinite space. The CBA. contact between the two domains is non-flat, and The complete Bouguer anomaly free from the topography h(x, y) creates the gravity signal [41]. African slab effect (CBAS) is shown in Fig. 2c. Oldenburg [42] described and solved the inverse Compared to the CBA, the CBAS shows lower problem within the frequency domain using the variations of the gravity signal in the Aegean Sea Fourier Transform. The topography of the contact area. More specifically, the gravity signal in the between the two layers is obtained by iteratively Cretan Sea and the North Aegean Trough are now solving the direct problem, assuming a constant similar (+40 mGal; Fig. 2c), while part of the density contrast Dq. The following equation is then Cyclades is marked by a lower gravity anomaly. used: However, short wavelength variations of the CBAS are still present within the whole Aegean domain. FðÞDgxðÞ; y ejkjz0 FhxðÞ¼ðÞ;y These short wavelength variations are assumed to be 2pGDq related with intracrustal density variations, which Xl n1 jkj have thus to be discarded to compute the Moho ¼ FhðÞnðÞx; y ð1Þ n! depth. To remove most of the intracrustal high- n¼2 272 C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280

3 Fig. 3. (a) Moho depth from inversion of FCBAS (five iterations running, z0=26 km, Dq=0.4 g.cm ; isolines every 0.5 km). (b) Crustal thickness from Moho depth and bathymetry filtered with a bandpass 50–300 km (isolines every 0.5 km) and (c) the residual (CBAS-computed gravity signal after gravity inversion, isolines every 10 mGal). C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280 273 where F represents the Fourier Transform, G is the 3. Aegean crustal thickness gravitational constant, k is the wave number and z0 is the reference depth from which the variations h(x, 3.1. Results y) are calculated. The absolute Moho depth is obtained using the following relation: The Moho topography obtained by the inversion of the complete Bouguer anomaly corrected from the zxðÞ¼; y z0 þ hxðÞ; y ð2Þ African slab and crustal density effects (FCBAS) is presented in Fig. 3a. The Aegean Moho appears quite A value of 400 kg m3 is used for Dq which flat for the whole region with variations of only +2 km reflects the mean density contrast between the crust (near Continental Greece and Anatolia) and 2km (2800 kg m3, average density of the entire crust) and (North Aegean Trough and Cretan Sea) around an 3 the mantle (3200 kg m ). The reference depth z0 is average depth of about 25 km. calibrated with recent seismic data [26–30] and set to The generally good anticorrelation between the 26 km. It is worth noting here that the magnitude of bathymetry (Fig. 1a) and the Moho shape strongly the variations strongly depends both on the reference suggests an overall isostatic compensation of the depth and the density contrast. Increasing Dq or Aegean topography. More specifically, regions of low decreasing z0 (with constant z0 or Dq, respectively) bathymetry (less than 100 m), such as the Cyclades, both leads to a decrease in the magnitude of the are marked by a Moho depth of 25 km, whereas the variations. Cretan Sea (2000 m depth bathymetry) is marked by a The stability of the inversion is tested with respect to shallower Moho (22 km). However, a very simple the variation of the parameters. The tests made on z0 calculation shows that there is no local (airy sense show that the magnitude of crustal thinning or speaking) but regional isostasy in the present case. thickening depth is about the same. When increasing The Aegean crustal thickness (Fig. 3b) is computed the density contrast from 400 to 500 kg m3, the by substracting a 50–300 km filtered bathymetry magnitude of the crustal thickness decreases about 0.5– (computed from Fig. 1a) to the Moho depth (Fig. 3a). 1 km while the wavelength content remains remarkably Similarly to the CBAS, the bathymetry was filtered to stable. remove short wavelengths. Crustal thickness (Fig. 3b) For stabilization in the Fourier domain, the data seems to increase westwards from approximately 25 are mirrored prior to the inversion and a low-pass km (Turkish Coast) to about 27 km (Continental filter is used (a cosine taper) to withdraw high- Greece), with a sharper gradient in the western edge of frequency anomalies arising from shallow crustal the Aegean Sea. Despite the poor constraint of marine structures, if any. Five iterations were run, but in gravity data near coastal areas, this smooth westward general, the convergence appeared after only two increase of the crustal thickness is in agreement with iterations, with a final root-mean-square (rms) of previous studies [7] and with the estimate of about 30 2.2104 mGal. The results are remarkably stable km for the continental crustal thickness near the even when changing the cosine taper, which reflects Corinth Gulf obtained from inland gravity surveys a good wavelength coherence for the sources. A [34]. Unfortunately, the lack of data precludes the residual map is deduced from the inversion (Fig. 3c). same type of comparison for the Turkish side of the It reflects the difference between the observed CBAS Aegean. anomaly and the one computed with the predicted As previously seen from the filtered complete Moho topography. The residual signal generally Bouguer map (FCBAS; Fig. 2d), the Cretan Sea and ranges between F20 mGal, with maxima located the North Aegean domains are marked by a thinner around the coasts (+30–35 mGal near the Pelopon- crust (22 and 23 km, respectively). In the North nesus or within the Corinth Gulf, for instance), Aegean domain, the minimum crustal thickness where the data are the less constrained. Most of the defines an elongate zone trending in a NE–SW residuals come from short wavelength components direction, and whose minimum is located beneath and thus reflect intracrustal sources that the inversion the North Aegean Trough (Figs. 1c and 3a). Mascle is unable to fit. and Martin [25] already identified this orientation and 274 C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280 extended it through the Cyclades region. More the Cyclades region, Makris and Vees [26], Vigner recently, Goldsworthy et al. [43] proposed a similar [29] and Li et al. [30] describe a regularly flat Moho at trend from a study of fault systems, but that ends a depth of 25–26 km, in agreement with our before the Cyclades, in Central Aegean along an calculations. However, some discrepancies of Moho Andros–Tinos line (see Fig. 1a). depth estimates must be noted, particularly in the In the Cyclades, between the two above quoted Cretan Sea, where our estimate of 22 km is larger than regions of minimum crustal thickness (North Aegean values of 15–20 km proposed by Makris and Vees and Cretan Sea), the Moho has a rather uniform depth [26], Makris [27] and Bohnhoff et al. [28] (Table 1). of c.a. 25 km. Note that the minimum SW–NE This apparent inconsistency could be explained by the horizontal dimension of the Cyclades area is at least presence of sedimentary basins, which were not taken 100–150 km (Fig. 1a), which is much larger than the into account for in our inversion. lower value of the bandpass filter used prior to the Two main differences with the previous gravity inversion. This rules out a flat Moho coming from any study of Tsokas and Hansen [35] must be pointed artefact of filtering. out. The first difference occurs along the Evvia In terms of crustal thickness, the Aegean Sea can profile (Fig. 1a). Makris and Vees [26] show an thus be divided into three main regions: (1) the increasing Moho depth along this profile from 32 km Cyclades with a flat Moho at 25 km depth, (2) the depth at Mandouthi, in North Evia, to 26 km at Cretan Sea and (3) the North Aegean domains, both Amorgos (East Cyclades). This is confirmed by with a thinner crust. Vigner [29] and our study. However, the same It is worth noting here that the variations of the profile using Tsokas and Hansen’s data [35] shows Moho depth remain very similar and stable through all a strong gradient of down-dipping Moho between the tests performed for each modelling parameters (z0, Andros and Amorgos. The second discrepancy Dq, cosine taper...). Thus, the relative variations of concerns the Cretan Sea, where an elongate zone the crustal thickness described above cannot be of shallower Moho (15 to 20 km) runs parallel to the numerical artefacts of the inversion. However, as north of Crete. This tendency is exemplified in most previously mentioned, the absolute value of the Moho studies, including ours. However, in Tsokas and depth is dependent on the choice of the inversion Hansen [35], the Moho depth variations display a parameters, and this trade-off induces nonuniqueness. rather different pattern with no elongate minimum of For example, setting z0 to 30 km (instead of 26 km) crustal thickness. These differences should come yields to Moho depth variations from 27 to 33 km either from their modelling method (multiple-source (instead of 23–28 km). At this stage, previous Werner deconvolution) or, more probably, from the geophysical studies will help to solve this uncertainty. fact that they do not consider the African slab gravity effect. Based on tomographic imaging of the 3.2. Comparison with previous geophysical studies African slab [36], and following Tiberi et al. [34], we have estimated more precisely and fairly the The comparison of the above results with previous shape and thus the gravity effect of the subducting geophysical studies shows very similar overall relative lithosphere. This has a strong effect on the complete variations of Moho depth (Table 1). In particular, in Bouguer anomaly (CBAS; Fig. 2c), as shown for

Table 1 Comparison between the estimates of Moho depth of the present study and previous 2D local geophysical studies Authors Method Crete Cretan Sea minimum Cyclades North Aegean Makris and Vees Makris [26,27] Refraction 30–32 20 26 – Tsokas and Hansen [35] Gravity 28–30 26 19–32 26–28 Bohnhoff et al. [28] Reflection refraction 24–32.5 15 – – Vigner [29] Vertical reflection – – 26–25 25 Li et al. [30] Receiver function 31–39 – 25 – This study Gravity inversion 28–31 23 25 24–26 C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280 275 example by the positive free-air gravity anomaly has undergone two successive stages of extension centred in South Aegean (Fig. 1b) that disappears since Oligocene times. The first stage of Oligo– after removing the African slab effect (Fig. 2a). Miocene age is related to the southward migration of the African slab [5–9], and the second stage (Plio- 3.3. Role of sedimentary basins Pleistocene) is related to the combined effects of the still active migration of the African slab and of the The gravity inversion tool used in this study to westward extrusion of Anatolia [4,10–14]. In this compute Moho depth and crustal thickness did not work, we identify in the Aegean Sea three regions take into account sedimentary basins. The negative (Fig. 4) that suffered either only the first stage of gravity signal due to sedimentary basins (negative extension (the Cyclades) or the two successive stages intracrustal density variation) has thus been disre- (the North Aegean and the Cretan Sea). Because we garded, leading to an overestimate of the Moho depth cannot have access to the crustal thickness prior to and crustal thickness in regions where basins are extension, we assume, following McKenzie [44] and present. For a maximum estimation, we calculate that Gautier et al. [4], the amount of crustal thickness prior a sedimentary layer at the surface with a thickness of 3 to the extension to be c.a. 50 km by reference to km yields a negative anomaly of 40 mGal. This could continental Greece and Anatolia [26,45]. be misinterpreted as a crustal thickening if one is not The Aegean Moho appears rather flat for the whole aware of the presence of sediments in the area. region, with variations of only +2 km (near Con- The presence of low density sediments in the tinental Greece and Anatolia) and 2 km (North Cretan Sea can in particular explain our overestimate Anatolian Trough and Cretan Sea), around an average of the Moho depth (22 km) compared to that proposed depth of about 25 km (Fig. 3a). Such a regional-scale by Makris and Vees [26], Makris [27] and Bohnhoff et flat Moho is commonly observed in domains of wide al. [28] (15–20 km; Table 1). Sedimentary thickness in rifting like the Basin and Range, in particular, beneath the Cretan Sea might reach values close to 3 km [26], core complexes domains [46,47]. This requires a leading to shallower Moho depths than those mapped lower crust viscosity, low enough to flow rapidly, in Fig. 3a. Similarly, in the North Aegean, sediment allowing the surface and the Moho to remain thicknesses of about 5–6 km in the Orfanos Gulf and relatively flat during dome rise and continuing in the Sporades basin have been measured [29,39]. extension. Crustal thickening during Cretaceous and We estimate the Moho depth to be 22 km after a basin Eocene times created high thermal conditions (Moho correction, instead of the 24 km previously estimated. temperature higher than 700 8C) suitable for such a Consequently, we expect the North Aegean region to mode of extension in the Aegean lithosphere [48,49]. be marked by a more pronounced NE–SW trending of Gravitational collapse has likely been triggered by the crustal thinning because of the numerous sedimentary southward retreat of the Hellenic subduction zone basins reported in the region (Fig. 1d). [2,4], as represented by Fsr in Fig. 4. Following the Because a complete map of the basement depth is above arguments, it is therefore likely that after the not available for the whole Aegean Sea, we can only first stage of Aegean extension, characterized by the give some insights on the change of Moho depth development of metamorphic core complexes, the induced by the presence of large basins in regions Moho should have had a rather flat geometry at a where the basement depths is well documented as in mean depth of around 25 km at the scale of the whole the North Aegean Trough. Aegean, from the Rhodope to Crete. Crustal thinning during this first stage of extension is thus approximately 100% (reduction of crustal thickness from ~50 4. Geodynamical implication km to 25 km), an estimate consistent with previous studies [4,44]. As extension must lead to significant crustal Variations of Moho depths around the mean value thinning, the above results bring information on the of 25 km obtained from gravity modelling (Fig. 3a) stretching accumulated since the beginning of exten- should therefore represent variations in crustal thin- sion and its regional variations. The Aegean domain ning related to more recent extension. The westward 276 C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280

Fig. 4. Schematic map showing the three main regions of the Aegean domain inferred by our gravity inversion and marked by different crustal thickness: (1) The Cyclades with a flat Moho at 25 km, (2) the Cretan Sea and (3) the North Aegean domain, both with shallower Moho. Since Oligo–Miocene, the Hellenic slab retreat triggers a gravitational collapse, which led to a ~25 km thinned Aegean crust. More recently, since 5

Ma, the extrusion of Anatolia ( Fext) changes the pattern of deformation in the North Aegean, giving NE–SW trending narrow zones of crustal thinning. The strongly thinned crust of Cretan Sea is mostly due to the slab retreat ( Fsr). Thick lines correspond to the volcanic arc and the southwestern limit of the NE–SW trending narrow zones of thinning. extrusion of Anatolia, which started around 5 Ma ago Messinian deformation [29], whereas no major Plio- [4,10–14], modified the kinematics of extension in the Quaternary basins are present in the Cyclades. This North Aegean. The North Aegean domain (NW–SE second stage of deformation induces an additional trending zone of thinning) and the Cretan Sea show thinning of around 10% (reduction of crustal thickness crustal thicknesses smaller than 25 km. These two from 25–26 to 22 km) in restricted regions (the North regions are also marked by Plio-Quaternary basins Aegean and the Cretan Sea). Crustal thinning during (Fig. 1d), such as the North Aegean Trough, the this second stage of deformation is mostly governed Edremit Trough, the Lesbos–Psara Trough [25,29,39] by the extrusion of Anatolia in the North Aegean for the North Aegean domain and the Mirthes Basin domain ( Fext in Fig. 4) and by the still active slab and the Iraklion Central basin in the South Aegean retreat in the Cretan Sea ( Fsr in Fig. 4) [4,9,10,24, [25]. Each of these basins shows evidences of post- 50,51]. This interpretation is confirmed by the follow- C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280 277 ing geophysical features in the three main regions. In recent deformation in the Cyclades are however still a the North Aegean domain, three major active strike- matter of debate [53,55,56] that is beyond the scope slip zones (North Aegean Trough, Edremit Trough of the present paper. and Lesbos–Psara Trough) have been recognized by a In summary, a two-stage model for the Aegean ex- strongly localized seismicity [15,18] (Fig. 1c). The tension could well explain the observed crustal thick- observed NE–SW trend of thinning (Fig. 3a) is also ness variation within the whole Aegean region. First, well marked by these active strike-slip zones, during Oligo–Miocene, the southward migration of the indicating a significant extensional component along South Hellenic subduction zone triggers the gravita- these fault zones. The present day extensional strain tional collapse of a previously thickened crust, leading axes calculated by Kalhe et al. [19] are indeed oblique to an overall crustal stretching of the whole Aegean to the strike-slip fault trend, in agreement with the domain by a factor of two [4,44] and to a flat Moho elongate zones of thinning observed below the strike geometry at regional scale. Second, in addition to the slip fault systems (Fig. 1c). Note that this obliquity of still active southward migration of the Hellenic stretching axes indicates that the strain is not only subduction zone, the Anatolian westward extrusion controlled by the NE–SW strike-slip faults but also by has recently changed the pattern of extensional submeridian stretching related to the southward deformation in the North Aegean domain. This second retreat of the subduction zone. In the Cretan Sea, no phase of extension, which probably began about 5 Ma significant seismicity is recorded, which apparently ago [4,10–14], is responsible for ~10% additional contradicts the observed strong and recent crustal thinning mostly located in South and North Aegean. thinning. However, strain rate deduced from GPS The Cretan Sea thinning is mainly controlled by the velocity [19,20] reveals a significant stretching rate back-arc extension, while the North Aegean extension within the Cretan Sea and, more specifically, near the is due to the combined effects of the extrusion of Peloponnesus and Rhodes, which is consistent with Anatolia and back-arc extension. Between these two the presence of normal faults in this region (Fig. 1c). regions, the Cyclades likely behave as a rigid plateau. However, more recent strain estimates [52] do not seem to indicate major deformation in the Cretan Sea. This discrepancy between strain rate estimates of 5. Conclusions these studies in the Cretan Sea has to be related with a poor constrain on GPS measurement (less than three A simple gravity inversion was used in this paper points of measurement). The only evidence of post- to compute a new map of the Aegean crustal thick- Messinian deformation in the Cretan Sea is therefore ness. The Aegean crust thickness is homogeneous and given by the presence of Plio-Quaternary basins and relatively thin within the whole region, with variations normal faults. Moreover, Plio-Quaternary normal of only +2 km (near Continental Greece and Anatolia) faults in the Cretan Sea indicate a recent extensional and 2 km (North Anatolian Trough and Cretan Sea) deformation despite the lack of seismicity. Opposite around an average depth of about 25 km. Our results to the Cretan Sea and North Aegean Trough situation, are consistent with local 2D geophysical studies and strain rates and GPS velocities in the Cyclades are show a regional isostatic compensation. low [10,19,52] and seismicity is scarce and scattered In this study, we emphasize the potential of gravity [18]. Located between the two recently thinned analysis in the understanding of the extensional regions of North Aegean and Cretan Sea, the processes in the Aegean region. An important result Cyclades domain is thus likely translated as a rigid is the identification of three domains of different block towards the South, as already suggested crustal thickness at a regional scale (Cyclades, North [4,10,53,54]. As quoted by Walcott and White [54], Aegean Trough, Cretan Sea), each of them being the Cyclades block seems to be isolated from the rest related to the two-stage evolution of Aegean exten- of the Aegean domain when the Anatolian extrusion sion. The first stage took place in Oligo–Miocene started (late Miocene–early Pliocene). The Cyclades times, when gravitational collapse of a previously do not undergo major extensional deformation since thickened Aegean crust led to a crustal stretch of Oligo–Miocene times. The reasons for the absence of about 100%. During the second stage, the westward 278 C. Tirel et al. / Earth and Planetary Science Letters 228 (2004) 267–280 extrusion of Anatolia modified the kinematics of Parra, Midcrustal shear zones in postorogenic extension: ex- extension and led to more localized deformation ample from the northern Tyrrhenian Sea, J. Geophys. Res. 103 (1998) 12, 123–12, 160. within specific areas. The North Aegean Trough and [4] P. Gautier, J.P. Brun, R. Moriceau, D. Sokoutis, J. Martinod, L. the Cretan Sea show maximum thinning with a crustal Jolivet, Timing, kinematics and cause of Aegean extension: a thickness of 24 and 22 km, respectively. 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