Mesozoic Tectonics of the Aegean

Tectonique Mesozoïc de l'Egee

Konstantinos Kydonakis (1)

1 GR - Géosciences Rennes

The Aegean Sea started to form since middle Eocene back-arc extension aecting the previously formed Hellenic Thrust Wedge. The latter was made by Mesozoic - Cenozoic accretion of i) three continental fragments, namely Rhodopia, Pelagonia and External Hellenides, arranged in a SW-verging stack from NNE to SSW in the order listed, and ii) the closure of two intervening oceanic domains now forming, from north to south, the Vardar-Axios and the Pindos Suture Zones, respectively. The thickened crustal-scale wedge started to collapse, in a process driven by the Hellenic slab rollback, triggering the development of core complexes in the severely stretched Aegean crust. During the early stage of extension, deformation localised to the hinterland of the crustal-scale wedge in the area described as the Rhodope Metamorphic Province (or simply Rhodope). The Rhodope is an area of large-scale nappe tectonics and is considered unique for the geology of the Aegean as it recorded both the Mesozoic convergence-related deformation and the subsequent Cenozoic extension of the Aegean during which, part of the earlier fabrics were re-worked. During Cenozoic extension, the southwestermost part of the Rhodope, namely Chalkidiki block, escaped much of the related deformation as it remained in the hangingwall of the Kerdylion Detachment, a large structure related to the Aegean extension. As a result, we selected the Chalkidiki block to study the Mesozoic tectonics of the Aegean. This study is based around a multidisciplinary approach that aims to unravel dierent aspects of the tectonics of North Aegean. For that purpose we have selected to study the Chalkidiki block using a variety of geological methods. 1) We carried out several eld mapping campaigns measuring the geometry of structural fabrics. An overview of the regional geometry is given in the compiled regional maps that illustrate the attitude of the measured planar and linear structural fabrics. 2) We evaluated the intensity of the metamorphic conditions using isochemical phase diagram sections calculated by minimisation of the Gibbs free energy. Based on our model results, we inferred the existence of an early eclogite-facies event before the regional amphibolite-facies overprint. 3) We performed high- (U/Pb in zircon and monazite) and medium-temperature (40Ar/39Ar on micas) geochronology coupled with low-temperature thermochronology and inverse thermal history modelling using a Bayesian transdimensional inversion scheme. The modelling results delineated the complete thermal path (T-t) of the study area from Cretaceous heating to Eocene near surface exposure. 4) We modelled the Tertiary geological evolution of the North Aegean with reference to the exhumation of the lower crust during core complex formation. Using scaled laboratory experiments we tested whether the gravity spreading of a crustal-scale thrust wedge that undergoes extension is a suitable process for the development of the Rhodopean core complexes during the early opening of the Aegean. Our analysis suggests that strain localisation and core complex development near the backstop of the area aected by extension is intrinsic to the pre-collapse geometry of the orogenic wedge.

Les Hellenides, qui font partie integrale de la chaine Alpes-Himalaya, aeurent a l'est de la Mediterranee. Elles resultent de la convergence vers le nord et de l'accretion, a la marge Sud-Europeenne, de blocs continentaux appartenant au Gondwana. Dans cette region, les donnees de tomographie sismique mettent en evidence un seul panneau de lithosphere en subduction, ancre dans le manteau inferieur a 1600 km de profondeur. Les Hellenides resultent de l'empilement vers le Sud-Ouest, du Mesozoique au Cenozoique, de trois microplaques (Rhodopia, Pelagonia et les Hellenides Externes), avec fermeture de deux domaines oceaniques, formant maintenant du Nord au Sud, les zones de sutures du Vardar-Axios et du Pinde. Une phase d'extension arriere-arc contr^olee par le retrait vers le sud du panneau plongeant Hellenique a commence a l'Eocene moyen, aectant la cro^ute precedemment epaissie. Le systeme Egeen represente la partie la plus etiree du domaine arriere arc et resulte du retrait de la fosse oceanique, d'environ 600 a 700km vers le sud. Les Hellenides internes ont subit une extension de type "core complex" localisee dans la province metamorphique du Rhodope (ou Rhodope). Puis l'extension a migre vers la partie centrale de l'Egee, dans la region des Cyclades. Depuis le Miocene moyen, l'extension est passee a une extension de type rift large, donnant naissance a de nombreux bassins sedimentaires marins et continentaux. Le Rhodope est un domaine precieux du systeme Egeen car la tectonique chevauchante Mesozoïque de grande echelle y est bien conservee malgre les eets ulterieurs de l'extension Egeenne. C'est donc une localite privilegiee pour l'etude de la convergence Jurassique- Cretace des Hellenides. C'est aussi une region ideale pour etudier l'eondrement gravitaire de la cro^ute hellenique epaissie au debut de l'extension egeenne. Cette superposition des deformations liees a la convergence (epaississement) et de celles liees a l'extension (amincissement) dans un seul systeme geologique donne au Rhodope une signature particuliere dans l'evolution du systeme Egeen, tres favorable pour l'etude des mecanismes de formation et d'eondrement d'une cro^ute orogenique epaissie.

Mots clés: Tectonics; Aegean; Metamorphism

Mots clés: tectonique; Egée; Métamorphisme

Domaines: Sciences de la Terre

Konstantinos Kydonakis. Mesozoic Tectonics of the Aegean: Insights from analogue modelling, petrology, geo- and thermo-chronology. Earth Sciences. Universite Rennes 1, 2014. English. ⟨NNT : ⟩. ⟨tel-01137808⟩