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The Geodynamic Evolution of Pindos Foreland Basin in SW Greece

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The Geodynamic Evolution of Pindos Foreland Basin in SW Greece 501 by Panagiotis Konstantopoulos 1 and Avraam Zelilidis 1 The geodynamic evolution of Pindos foreland basin in SW Greece University of Patras, Department of Geology, 26500, Patras, Greece. E-mail: [email protected]; [email protected] The distribution of Eocene-Oligocene turbidite facies basins is a matter of question, such as eustasy, orogenic loading, in the Pindos foreland and the paleocurrent directions erosion and deposition, dynamic topography, intraplate stress, basement reactivation. The sedimentary fill records interplay between of submarine fan deposits, of the Peloponnesus area, regional tectonics, eustasy and normal sedimentary processes document the proximal part of an underfilled foreland (Beaumont, 1981; Allen et al., 1986). Much effort has gone into the basin. The definition of Pindos foreland basin attributes modelling of foreland basins using various techniques (e.g. Allen et sediment accommodation solely to flexural subsidence al., 1986; Stockmal and Beaumont, 1987; Sinclair, 1997; Allen et driven by topographic load of the thrust belt and sediment al., 2001; Rafini and Mercier, 2002; Clevis et al., 2004; Bera et al., 2008; Bera et al., 2010; Roddaz et al., 2011). The resulting models loads in the foreland basin. The restriction of the coarse have focused on the proximal part of foreland basins, where the effects grained deposits and the basin underfilled conditions of lithospheric loading are pronounced close to the overthrust load, are related to the Pindos foreland evolution, and and where deep-water ‘flysch’ and late-orogenic ‘molasse’ especially to the internal thrusting and produced predominate. Sublithospheric “loading” of the overriding plate is primarily caused by the drag force generated by viscous mantle corner intrabasinal highs. The presence of strike-slip faults can flow coupled to the subducting plate, especially where subduction is affect the geometry of a basin by causing changes in rapid and/or takes place at a shallow angle beneath the retroarc foreland depth and width, resulting in the transformation from a basin (Mitrovica et al., 1989; Gurnis, 1992; Holt and Stern, 1994; uniform to non-uniform configuration. This change has Burgess et al., 1997). Excepting for dynamic loading, all other types an intensive impact on depositional environments along of supra- and sublithospheric subsidence mechanisms relate to the gravitational pull of “static” loads represented by the subducting slab, the basin axis. Strike-slip faults that cross-cut the orogen, or the sediment-water mixture that fills the foreland intrabasinal highs produce pathways for the sediment accommodation created by lithospheric flexural deflection. The static distribution on both sides of the highs. Distributary tectonic load of the orogen and the sublithospheric dynamic loading channels that discharge into the basin are perpendicular are most often invoked as the primary subsidence mechanisms that control accommodation and sedimentation patterns in retroarc foreland to its axis and shift axially at the basin floor. The strike- settings (Beaumont et al., 1993; DeCelles and Giles, 1996; Pysklywec slip and thrust faulting operated contemporaneously for and Mitrovica, 1999; Catuneanu et al., 1999a,b, 2002). However, much of their active periods, although it appears that proximal parts of foreland basins are commonly deformed or partly thrust faulting, initiated slightly earlier than strike-slip obscured by the overthrust load. Also, the relative effects of tectonics faulting. versus eustatic sea-level change and normal (i.e. autocyclic) sedimentation may not be easy to untangle in such deep-water “flysch” basins. Introduction This paper provides a case study for the sedimentation patterns that may develop in an underfilled forearc foreland system, under a The interplay of base level changes and sediment supply controls unique combination of factors that control the dynamics of the basin the degree to which the available accommodation is consumed by and the amounts of available accommodation. We focus on the Eocene- sedimentation. This defines the underfilled, filled and overfilled stages Oligocene sedimentation of the Pindos foreland basin of south-west in the evolution of the foreland system, in which depositional processes Greece, which we interpret as part of the proximal area of a foreland are dominated by deep marine, shallow marine, or fluvial basin related to the final closure of the Pindos Ocean. The Pindos sedimentation, respectively (Sinclair and Allen, 1992). The change foreland basin is an example of Tertiary turbidite basin fill, segmented from underfilled to overfilled stages is best observed in the foredeep, during its evolution by propagating thrusts. The distribution of because the forebulge may be subject to erosion in the absence of turbidite facies in the Pindos foreland and the palaeocurrent directions (sufficient) dynamic loading, or, at most, it may accommodate shallow of submarine fan development show that the southern part of the marine to fluvial environments even when the foredeep is underfilled. Pindos foredeep, from the late Eocene to the early Oligocene, was Appropriate ways to describe the stratigraphic fill and study the basin an example of underfilled foreland basin. The study area is an evolution in foreland basins have been looking for several decades. excellent example of an underfilled foreland basin in order to study The principal control on stratigraphic stacking patterns of foreland processes affecting the proximal part of a foreland basin and its Episodes Vol. 35, no. 4 502 development through time. It has been shown that the Eocene- system (sensu Boyer and Elliott, 1982). The external Hellenides Oligocene succession in the southern Pindos foreland basin, consist from east to west of the Pindos, Gavrovo, Ionian and Pre- Peloponnesus peninsula, consists of several-million-year tectonically- Apulian geotectonic Zones (Aubouin, 1959). These zones are driven cycles comprising strata deposited in an underfilled basin with generally bounded by east- to NE-dipping Tertiary thrust faults. Pindos a prominent forebulge zone. The Late Eocene strata and the early foreland is a tertiary turbiditic basin-fill trending parallel to the external Oligocene strata were deposited in underfilled conditions and could Hellenides (Aubouin, 1959) and occupies Gavrovo and Ionian be interpreted in terms of the expected syntectonic subsidence pattern geotectonic zones. The basin is bounded to the east by the Pindos of a foreland basin system, respectively. Isopach values increasing thrust and to the west by the Ionian thrust (Figure 1A, B). Apart of towards the Pindos Mountain fold-thrust belt may be interpreted as these two major thrusts, minor thrusts (Gavrovo, internal and middle representing sedimentation in the foredeep zone. A linear trend of Ionian thrusts) separate the basin into linear narrow sub-basins, low values indicates the presence of the forebulge zone. Therefore, trending parallel to the basin axis. the three studied sub-basins in southern Greece provide an opportunity Turbiditic accumulation resulted from the deformation of the for an in-depth study of the mechanisms of the forebulge migration external Hellenides which migrated westwards. During the Eocene/ and stratigraphic fill of the forebulge depozones in Pindos foreland Oligocene, eastward subduction of the Pindos Ocean resulted in deep- basin. In the following we proposed a qualitative model for the Pindos sea sedimentary lithologies being detached from their oceanic igneous forebulge migration and the stratigraphic fill of this underfilled basement as an accretionary prism and emplaced westwards onto the foreland basin. adjacent carbonate platform, ending up as a series of thin-skinned thrust sheets (Degnan and Robertson, 1998). Studied basin during Geological setting Miocene changed to a backstop basin (Le Pichon et al., 2002) due to outward migration of the Hellenic nappes between 20 and 15Ma. The convergence and collision of the Eurasian and African Plate The present accretionary wedge then would be the post -Middle margins in the Paleocene - early Tertiary formed the Hellenide orogen Miocene. (Robertson and Dixon, 1984; Dercourt et al., 1986; Savostin et al., The Pindos thrust activity formed a foredeep during Middle 1986). The Hellenides are generally interpreted as a thrust stack (Smith Eocene (IGSR & IFP, 1966), in the pre-existing area of Gavrovo and and Moores, 1974) resulting from a SW-propagating fold-and-thrust Ionian zones (Underhill 1985, Brooks et al., 1988; Underhill, 1989; Figure 1. A Geological map of Greece showing the study area, in Peloponnesus peninsula, modified after Bornovas and Rontogianni- Tsiabaou (1983). B Simplified geological map of the three studied sub-basins showing major geological units where the turbidite deposits were mapped. December 2012 503 Clews, 1989; Alexander, 1990), whereas due to the internal thrusting, the Hellenides in southern Greece, including the studied area, during the Late Oligocene the foreland changed to a complex type underwent regional tectonic rotation, associated with the formation foreland basin, in its northern part (Avramidis et al., 2002), and during of the southward-convex Aegean arc (Kissel and Laj, 1988). As a Early Pliocene to a piggy-back basin, in the southern Zakynthos area result, the present-day configuration of nearly N-S-trending tectonic (Zelilidis et al., 1998). The Pindos foredeep, during the middle Eocene units was orientated more east-west during Mesozoic-early
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