Tectonostratigraphic models of the Alpine terranes and subduction history of the Hellenides
Highlights
► Tectonostratigraphy and geodynamic stages.. ► Detachment of the Hellenides from the subducting slab during oceanic and continental subduction. ► Or Tectonostratigraphy and geodynamic stages. ► Detachment from the subducting slab during oceanic and continental subduction.
Introduction
The paleogeographic organisation of the Hellenides within the Tethyan system in the Eastern Mediterranean has been a matter of discussions and different views depending on the change of the general concepts of geotectonics. Thus, early syntheses following the geosynclinal concept have proposed the existence of a number of different paleoenvironments, both shallow neritic and deep pelagic, in a simplified model of alternating ridges and furrows forming isopic zones, which after their orogenic deformation, were transformed to geotectonic zones across the belt (Aubouin, 1959, Aubouin, 1965, Brunn, 1956, Brunn, 1960, Philippson, 1898, Philippson, 1959, Renz, 1940, Renz, 1955). After the development of geological models within the plate tectonics theory in the 1970s the paleogeography of the Hellenides has been revisited, focusing on the identification of the ophiolite suture zone(s) representing the lost oceanic basin(s) of Tethys and the attribution of the geotectonic zones to the northern/Eurasian or to the southern/African continental margin. However, this simple Atlantic type paleogeographic organisation could not be accepted because more than one ophiolite belt occurred within the pelagic sediments, alternating with shallow water carbonate platforms (Aubouin, 1976, Aubouin et al., 1977, Dercourt, 1970, Dercourt, 1972, Dewey et al., 1973, Dimitrievic, 1974, Jacobshagen, 1979, Le Pichon and Angelier, 1979, Smith, 1971).
Several tectonic and paleogeographic models of the Hellenides have been proposed involving a different number of oceanic basins along the belt and also different types of geodynamic settings, such as ophiolites formed within spreading centres along mid-ocean ridges, above supra-subduction zones, etc. (Biju-Duval et al., 1977, Dercourt et al., 1985, Dewey and Şengör, 1979, Garfunkel, 2006, Robertson, 2002, Robertson, 2004, Robertson and Dixon, 1984, Robertson et al., 1991, Smith, 1993, Smith and Rassios, 2003, Stampfli and Borel, 2004). However, in between the ophiolite suture zones of the Hellenides outcrops of pre-Alpine continental crust of Pre-Cambrian and/or Paleozoic age covered by shallow-water carbonate platforms of Mesozoic–early Cenozoic age, were thought to represent microcontinents like the Cimmerian/Pelagonian blocks (Jacobshagen, 1986, Mountrakis, 1986, Papanikolaou, 1984, Papanikolaou, 1986b, Robertson and Dixon, 1984, Sengor, 1979, Sengor, 1984, Sengor, 1989, Sengör et al., 1984a, Sengör et al., 1984b, Şengör et al., 1988).
The concept of tectonostratigraphic terranes was applied in the Mediterranean mainly within the IGCP project 276, aiming to analyse the pre-Alpine basement rocks within the Mesozoic organisation of Tethys and to identify their provenance from Gondwana or Eurasia (Papanikolaou and Ebner, 1997, Papanikolaou and Sassi, 1989). In the Hellenides this project resulted with the distinction of nine terranes, five continental and four oceanic (Papanikolaou, 1989, Papanikolaou, 1997, Papanikolaou, 2009, Papanikolaou et al., 2004) (Fig. 1). Some authors have questioned the use of the terrane concept in favour of the term “microcontinent” and the use of the “tectonic facies” concept (Robertson, 2004). The main difference in the terminology and notion of the microcontinent concept from the terrane concept lies in the overall aspect, which is rather static in the microcontinent but highly dynamic in the terrane. In the first case, it is a paleogeographic element separating different parts of the Tethys ocean with extremely complex tectonic scenarios resulting from a variety of tectonic facies whereas in the second case it is a dynamically evolving element with differentiating tectono-stratigraphy following the entire movement and history from the break of the African margin through rifting in the early Mesozoic, to the drifting within the oceanic basins of the changing Tethys and the final accretion to the European margin in the late Mesozoic–Tertiary (Papanikolaou, 1989, Papanikolaou, 1997). Earlier events of terrane rifting, drifting and accretion can be detected within the continuation of the more internal part of the Hellenides north of Rhodope in the basement of the Balkanides and the Dinarides (Haydoutov, 2002, Haydoutov et al., 1997, Karamata et al., 1997, Yanev, 1993). Tectonostratigraphic terranes have also been described in the Pontides–Taurides with events ranging from the Variscan orogeny to the Late Cretaceous and Early Tertiary (Goncuoglu et al., 1997, Moix et al., 2008). Paleogeographic representations and stratigraphic columns of the Hellenic terranes show a continuous change both as far as their autonomous existence as microplates and their relative position in the Tethyan organisation are concerned. Thus, the nine terranes described in the Hellenides (H1 to H9) never co-existed, because by the time the younger ones in the south were drifted northwards and new oceanic basins were opened, the early ones in the north were already accreted to Europe and the older oceanic basins were closed. The tentative paleogeographic reconstruction of the Hellenic terranes (Papanikolaou, 1989, Papanikolaou, 2009) is an overall scheme which corresponds to a synthesis of paleogeographic schemes during different time intervals. Correlation of the pre-Alpine basement rocks with their usually detached sedimentary cover, made of shallow-water carbonate platforms, is the main tool for deciphering the Tethyan paleogeographic organisation of the Hellenides by the distinction of the drifting continental terranes within the adjacent oceanic basins (opening behind and closing in front of each terrane) (Papanikolaou, 1989, Fig. 6).
This paper aims to:
- (i)
Present two tectonostratigraphic models for the two distinctive types of terranes in the Hellenides, continental versus oceanic, as they result from the general rifting, drifting — ocean opening and accretion paleogeographic/geodynamic stages. This first “theoretical” part can be applied to any other case in space and time where similar geodynamic phenomena have occurred inside or outside the Tethyan belt.
- (ii)
Apply the two tectono-stratigraphic models in the five continental (H1, H3, H5, H7 and H9) and four oceanic terranes (H2, H4, H6 and H8) of the Hellenides.
- (iii)
Describe the subduction history of the Hellenides through the chronological constraints that can be deduced for each terrane, according to its own history as this is unravelled by its tectono-stratigraphy.
Section snippets
Stages of tectonostratigraphic terrane evolution and resulting tectonostratigraphic models in the Hellenides
The overall history of each continental tectonostratigraphic terrane of the Hellenides can be divided in three stages (Papanikolaou, 1989, Papanikolaou, 1997) (Fig. 2a, b and c): 1) The rifting stage, which initiates the creation of the new diverging plate boundary between the new crustal element/terrane and the African continent (part of the Gondwana megacontinent). 2) The drifting stage, which represents the autonomous motion of the continental terrane within the Tethyan domain, which is
Continental terranes with shallow-water carbonate platforms
The timing of the successive geodynamic and paleogeographic stages of each continental terrane may indicate its history within the paleogeographic organisation of Tethys and its final subduction/accretion beneath the European margin. Thus, the previously described tectonostratigraphic model (Fig. 5a) is exemplified for each continental terrane/platform of the Hellenides (Fig. 6), using all available chronological data from the literature, including the published geological maps at scale
Oceanic basins with ophiolites and pelagic sequences in the Hellenides
The opening and closure of the oceanic basins of the Hellenides can be determined by the dating of the transition between the stages of rifting and opening 1 and 2 and by the transition between the stages of opening and closure 2 to 3, respectively (Fig. 7). The transition from stage 1 to stage 2 in the Pindos basin of H2 occurred in the Carnian with the deposition of Halobia-bearing pelagic limestones over the Carnian clastics around 220 myears ago (Aubouin, 1959, Fleury, 1980, Lekkas, 1986,
The subducted slab and subduction history of the Hellenides
The results of seismic tomography in the Hellenides have permitted the visualisation of the subducted slab for almost 2000 km beneath the Aegean Sea with a possible extension up to 3300 km total length (Kárason and van der Hilst, 2001, Spakman et al., 1993, Suckale et al., 2009, Wortel and Spakman, 2000). These data have confirmed the existence of a rather continuous subduction process since Mesozoic, that was proposed on the basis of geological and tectonic data from the successive orogenic arcs
Discussion
The previous description of the subduction history of the Hellenides is based on a continuous process of convergence between Europe and Tethyan paleogeographic elements since Jurassic. During this long period the European plate was diverging for some time spans and converging in others towards the African plate (e.g. Savostin et al., 1986). Nevertheless, until recently in late Miocene–early Pliocene, the two large continental plates were not in immediate contact but only through the
Conclusions
The paleogeography of the Hellenides within the Tethyan domain can be described by two tectono-stratigraphic models: the continental terrane with shallow-water carbonate platform model and the oceanic basin model. Both models are characterised by their crustal rocks, pre-Alpine continental crust in one case and Mesozoic ophiolite complex in the other. Their stratigraphic columns are similar at the base with Permo-Triassic volcano-sedimentary complexes corresponding to the rifting stage and at
Acknowledgements
A lot of information and ideas regarding the application of the tectonostratigraphic terrane concept in the Mediterranean region have been developed during the 10 years of the IGCP project No 276 “Paleozoic geodynamic domains and their Alpidic evolution in the Tethys” especially during the elaboration of the Terrane maps and tectonostratigraphic columns of the studied area, published in the Annales Geologiques des Pays Helleniques in 1996–1997. Ever since, a number of basic problems have been
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