REGIONAL TECTONICS AND 40Ar/39Ar TIMING OF CRUSTAL EXTENSION IN THE EASTERN RHODOPE MASSIF, BULGARIA-GREECE, AND MESOZOIC-CENOZOIC GEODYNAMIC EVOLUTION OF THE NORTH AEGEAN REGION

We synthesize here the Mesozoic-Cenozoic evolution of the eastern Rhodope Massif, and provide a tectonic framework and 40Ar/39Ar age constraints for the processes involved during crustal extension in the north Aegean region. Triassic rifting has its magmatic expression in proto-ophiolitic metamafic rocks of the Serbo-Macedonian and the western Rhodope massifs, whereas it is scarcely represented by Permo-Triassic near-margin sediments in the eastern Rhodope Massif. Triassic-Jurassic rift propagation into the Carpatho-Balkan and Hellenic domains of the Maliac-Meliata oceanic basins and its southward intra-oceanic subduction has created near to the Rhodope margin an Early-Middle Jurassic intra-oceanic arc and established on the upper plate the Vardar ocean in a back-arc setting. Both island arc and associated Triassic-Jurassic sedimentary units were thrust northwards over the Rhodope high-grade basement in the late Jurassic (⁴⁰Ar/³⁹Ar ages 154-157 Ma) during an arc-continental margin collision. This event contributed to the Mesozoic, paleoalpine crustal thickening. The late Jurassic thrust sheet forms the uppermost Mesozoic low-grade unit (greenschist to sub-greenschist facies) that we correlate with the Circum-Rhodope Belt. Underlying are an upper, high-grade basement unit lithologically heterogeneous unit (diverse in age) and the lower unit of high-grade basement composed of orthogneisses with Permo-Carboniferous protoliths. These two latter units are involved in south-directed ductile nappe stacking in amphibolite facies and related to the subduction (of the Vardar ocean) reversal towards the north since the Middle Cretaceous. Cretaceous nappe stacking further contributed to crustal thickening in the region. The Cenozoic tectonics is dominated by the extensional collapse of the Alpine orogen and the formation of detachment-bounded core-complex type metamorphic domes in Bulgaria and Greece. The domes expose the lower high-grade basement unit in the footwall, with the hanging wall consisting of the upper high-grade basement unit and the Mesozoic low-grade unit. Unmetamorphosed syn-tectonic Paleocene-Eocene sedimentary rocks and post-tectonic Oligocene sedimentary and volcanic units represent cover successions. The Kesebir-Kardamos dome displays NNE-directed ductile to brittle shear fabrics overprinted by brittle extensional shearing in the footwall mylonites and the bounding detachment. The Byala reka-Kechros dome exhibits the same structural pattern as the Kesebir dome in the footwall mylonites and the bounding detachment, but with a SSW-directed extension. This structural feature reveals a partitioned kinematic flow direction, likely related to a transfer fault linking the opposite sense detachment systems in both domes at a high crustal level. In the Byala reka-Kechros dome extension direction inherited the kinematic direction of the Cretaceous nappe stacking. In both domes, the 40Ar/39Ar mica cooling ages span 42.1-34.1 Ma in the hanging walls, whereas ages of the footwalls fall within the interval 38.1-35.5 Ma. The hydrothermal adularia 40Ar/39Ar ages in the range of 34.99-34.71 Ma are indistinguishable from the 40Ar/39Ar ages ranging between 34.69-33.91 Ma for the paleovolcano lavas at the northern Kesebir-Kardamos dome. The 40Ar/39Ar age of 36.5 Ma characterizes hydrothermal adularia in the prospect low-sulfidation mineralization in the Byala reka-Kechros dome. The acid dykes within the domes yield 40Ar/39Ar ages of 32.9 Ma and 31.82 Ma. The dykes of alkaline basalts at 28-26 Ma record latest magmatic pulse of enriched mantle source in the extensionally thinned crystalline basement of both domes. The new geochronology data and the reginal tectonic constraints allow us to delineate the following temporal evolution for the Mesozoic contraction and the Cenozoic extension and exhumation history of the region. Following the Late Jurassic-Cretaceous thrusting and thickening, Cenozoic crustal extension started as late as in the Eocene (youngest Ypresian syn-tectonic sediments) with the initiation of the low-angle detachment faults. Progressive extension led to the regional cooling and exhumation of the hanging wall between 42-34 Ma, followed by cooling and exhumation of the footwall within the interval of 38-35.5 Ma. Continued brittle extension and high-angle faulting in supra-detachment grabens of the hanging wall was followed by hydrothermal alteration at 36-35 Ma and formation of epithermal ore prospects and mineralization that are coeval in their waning stage with the onset of volcanism at 35-34 Ma. The data reveal interplay and overlapping tectonic, hydrothermal and magmatic processes within a narrow time interval with feedback from crustal and mantle (e.g. ductile-brittle basement thinning, hydrothermal activity, magmatism) processes involved in the late-stage extensional history of the eastern Rhodope Massif.

Acknowledgments: The support by NSF grant no. VU-NZ-02/06, SNSF SCOPES grant no. IB7320-111046/1 and US Department of State Fulbright Research grant to NB is gratefully acknowledged.