Back-arc extension in the Aegean, which was driven by slab rollback since 45 Ma, is described here for the first time in two stages. From Middle Eocene to Middle Miocene, deformation was localized leading to (i) the exhumation of high-pressure metamorphic rocks to crustal depths, (ii) the exhumation of high-temperature metamorphic rocks in core complexes, and (iii) the deposition of sedimentary basins. Since Middle Miocene, extension distributed over the whole Aegean domain controlled the deposition of onshore and offshore Neogene sedimentary basins. We reconstructed this two-stage evolution in 3D and four steps at Aegean scale by using available ages of metamorphic and sedimentary processes, geometry, and kinematics of ductile deformation, paleomagnetic data, and available tomographic models. The restoration model shows that the rate of trench retreat was around 0.6 cm/year during the first 30 My and then accelerated up to 3.2 cm/year during the last 15 My. The sharp transition observed in the mode of extension, localized versus distributed, in Middle Miocene correlates with the acceleration of trench retreat and is likely a consequence of the Hellenic slab tearing documented by mantle tomography. The development of large dextral northeast–southwest strike-slip faults, since Middle Miocene, is illustrated by the 450 km long fault zone, offshore from Myrthes to Ikaria and onshore from Izmir to Balikeshir, in Western Anatolia. Therefore, the interaction between the Hellenic trench retreat and the westward displacement of Anatolia started in Middle Miocene, almost 10 Ma before the propagation of the North Anatolian Fault in the North Aegean

The North Aegean core complexes developed in middle Eocene soon after the end of continental block convergence and piling up of the Hellenic Thrust Wedge. They formed during back-arc extension, driven by the Hellenic slab rollback, at the back of the thrust wedge. A series of scaled laboratory experiments was performed to test whether the gravity spreading of a thrust wedge is a suitable process for the development of the North Aegean core complexes during back-arc extension. Wedge-shaped sand-silicon models with variable boundary displacement velocities and different geometries of the upper sand layer were used to study the effects of variations in wedge rheology on the pattern of extension. The models exemplify that extension, either distributed (wide rift mode) or localized (core complex mode), is always located at the wedge rear. Core complex development was favored in models with thicker brittle layer (higher frictional strength) and low stretching rate (lower ductile strength). Both core complex location at the wedge rear and detachment location and dip are interdependent and intrinsically related to the initial wedge shape of the extending system. The experimental model displays striking similarities with the extensional pattern of the North Aegean in terms of (i) location, size, and shape of core complexes as well as their sequence of development and (ii) detachment location and dip. We conclude that it is the initial wedge geometry of the system and the weak nature of the crust at the onset of extension that controlled the extensional pattern of the North Aegean domain.

International audienceThe Chalkidiki block is a major domain in the North Aegean that, contrary to other domains in the region, largely escaped thermal perturbations during Tertiary extension. As a result, the Chalkidiki block is an ideal candidate to glean information related to the timing of Mesozoic thermal events using appropriate geochronological techniques. We have undertaken a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study (U-Th-Pb on monazites and U-Pb on zircons) coupled with 40Ar/39Ar dating on nine samples from various structural levels within the thrust system of the Chalkidiki block. The eastern, and structurally lower part of the system revealed a complete isotopic reset of Carboniferous – Early Triassic monazites coeval with partial monazite destruction, REE-mobilisation and formation of apatite-allanite-epidote coronas at ~ 132 Ma, a reaction that is commonly observed in amphibolite-facies rocks. These coronas formed after crystallisation of garnet (i.e., at T > 580 °C) and, in all probability, either close to the peak-temperature conditions (~ 620 °C) on a prograde path or during retrogression between the peak-temperature and the low-temperature boundary of the amphibolite facies. Cooling of these rocks and arrival at mid-crustal levels occurred at 95–100 Ma. By contrast, the western, and structurally uppermost part of the system went through the same event by 120–125 Ma. Further structural considerations with respect to medium-temperature geochronology data imply that syn-metamorphic thrusting must have ceased by early Late Cretaceous. We emphasize that, with the sole exception of the Chalkidiki block, no pre-45 Ma medium-temperature geochronology data are preserved in other North Aegean domains, a feature that is clearly related to the extension-induced thermal perturbation of the region during the Tertiary

The Vertiskos Unit of northern Greece is an elongated basement belt with a complex poly-metamorphic history. It extends from Greece (Chalkidiki peninsula), to the south, up to Serbia, in the north, and arguably represents the westernmost part of the Rhodope Metamorphic Province (northern Greece to southern Bulgaria). The Vertiskos Unit experienced a medium pressure lower amphibolite-facies metamorphic overprint during the Alpine Orogeny. The available medium-temperature geochronology implies that it remained at temperature of approximately 300°C (or slightly higher) during Lower Cretaceous. In order to constrain its post-Lower Cretaceous thermal history, until near-surface exposure, we applied apatite fission track analysis. The central ages obtained range from 68.5 ± 3.8 to 46.6 ± 3.6 Ma (uppermost Cretaceous to Middle Eocene) and mean track lengths between 13 and 13.5 μm. We applied two inverse thermal modeling approaches using either each sample independently (high degree of freedom in the thermal history, better data fit) or all samples together interpreting them as a vertical profile (simpler thermal history, worse data fit). Irrespective of the modeling approach, we conclude that the bulk thermal history of the Vertiskos Unit crosses the high-temperature limit of the apatite partial annealing zone by the uppermost Cretaceous and reaches near-surface conditions as early as lower/middle Eocene. These results contrast with the thermal history of the other domains of the Rhodope Metamorphic Province further east (namely the Southern Rhodope Core Complex and the Northern Rhodope Complex) and establish the Vertiskos basement complex as the oldest exhumed coherent basement fragment of the Rhodope Metamorphic Province and Greece.

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International audienceThe Chalkidiki block in Northern Greece represents the southwesternmost piece of theultrahigh-pressure Rhodope and has played an important role in the evolution of the NorthAegean. The eastern part of the Chalkidiki block is a basement complex (Vertiskos Unit) thatis made largely of Palaeozoic granitoids and clastic sediments metamorphosed during theMesozoic. This basement is traditionally considered as part of the Rhodopean hanging-wall,an assignment mainly supported by the absence of high-pressure mineral indicators and thepresence of a regional medium-pressure/medium-temperature amphibolite-facies Barrovianmetamorphic imprint. Toward the west, the basement is juxtaposed with meta-sedimentary(Circum-Rhodope belt) and arc units (Chortiatis Magmatic Suite) that carry evidence of aMesozoic high-pressure/low-temperature event. In this study, garnet-staurolite-mica schistsfrom the eastern part of the basement were examined by means of micro-textures, mineralchemistry and isochemical phase-diagram sections in the system NCKFMASHMn(Ti)[Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-MnO-(TiO2)]. The schists represent formerMesozoic sedimentary sequences deposited on the Palaeozoic basement. We document thepresence of a relict eclogite-facies mineral assemblage (garnet + chloritoid + phengite +rutile) in an amphibolite-facies matrix composed of garnet + staurolite + phengite ± kyanite.Model results suggest the existence of a high-pressure/medium-temperature metamorphicevent (1.9GPa / 520°C) that preceded regional re-equilibration at medium-pressure/mediumtemperatureconditions (1.2GPa / 620°C). Clearly, the eastern part of the Chalkidiki block(basement complex) retains memory of an as yet unidentified Mesozoic eclogiticmetamorphic event that was largely erased by the later Barrovian overprint. In the light of ourfindings, the basement complex of the Chalkidiki block shares a common tectonometamorphicevolution with both the high-pressure units to the west, and the high-gradeRhodopean gneisses further to the northeast. Our results are consequential for the geodynamic reconstruction of the Rhodope since they require participation of the Chalkidikiblock to the well-established Mesozoic subduction system

Aegean extension is a process driven by slab rollback that

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