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The aim of this paper is to contribute to deciphering the evolutionary history of the Hellenides by the study of a large sector of the chain located between the front of the ophiolitic units and the external zones classically attributed to the continental margin of Adria. In particular, the tectonic units located in Boeotia – a key area located in Central Greece at the boundary between the Internal and External Hellenides – were studied from structural, stratigraphic and biostratigraphic points of view. Addressing the main debated aspects concerning the origin of the ophiolite nappe(s), the tectonic evolution of the Hellenic orogen was revised with a particular emphasis on the period between obduction and continental collision. New findings were compared with consolidated data concerning the main metamorphic events recorded in the more Internal Hellenides, geochemistry and age of the ophiolites and main stratigraphic constraints obtained in other sectors of the belt. Finally, a new reconstruction of the tectonic evolution of this area was introduced and, in the context of the dispute concerning the origin of the ‘ophiolitic belts’ as a possible record of multiple oceanic basins, we put forward for consideration a ‘single ocean’ tectonic model spanning from Triassic up to Tertiary times, and valid for the whole Hellenic–Albanian sector.

The Koziakas massif, located at the western boundary of the Thessaly plain, consists of a nappe pile, which includes from bottom to top: 1– the Paleocene-Eocene Pindos Flysch; 2– the Western Thessaly Units (WTU); 3– the Ophiolitic Nappe, which consists of four minor tectonic units: a) the ophiolite-bearing Koziakas Mélange; b) the Fourka Unit; c) a thin discontinuous level of amphibolites (Metamorphic sole); d) a “Peridotite Unit”. This work focuses on the study of both the radiolarian assemblages of cherts situated at the top of basalts and the geochemistry of the latter. Our data show that in the Fourka Unit the cherts overlying the various basalts have diverse ages: a) latest Anisian (associated to WPB-OIB), almost coeval to the trachyandesites effused onto the continental rifts to the west; b) Early Carnian-Middle Norian and Late Carnian-Early Norian (associated to WPB-OIB); c) Early Norian (associated to E-MORB). In the Koziakas Mélange the age is Early-Middle Bathonian (associated to WPB-OIB). These data confirm the geodynamic evolution hypothesized for all the other ophiolitic massifs of the southern portions of the Dinaric-Hellenic belt, which can be synthesized as follows: a) birth of the Vardar-“Maliac” ocean in the Middle Triassic, coeval to continental basins to the west (Pindos basin, etc.); b) ocean spreading until Late Jurassic with intraoceanic subduction at the beginning of Middle Jurassic, followed by intraoceanic obduction; c) closure of the ocean (latest Jurassic-earliest Cretaceous) and thrusting of the Ophiolitic Nappe onto the eastern continental margin of Adria (the Pelagonian); d) westward displacement of the Ophiolitic Nappe testified by the presence of ophiolitic detritus first in the WTU (latest Jurassic to Late Cretaceous) and then, to the west, in the Pindos Flysch (Paleocene-Eocene).

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