Stratigraphic architecture of the Plio-Pleistocene infill of the Corinth Rift: Implications for its structural evolution
Introduction
The Gulf of Corinth is a WSW–ENE trending graben structure between the Peloponnesus peninsula and continental Greece (Fig. 1). This graben is located in the Aegean region where the tectonic scheme is well known: back-arc extension behind the Hellenic subduction zone and complex westward propagation of the North Anatolian Fault (Armijo et al., 1999, Jolivet, 2001) (Fig. 1). However, the relative role of each phenomenon is difficult to quantify and kinematic models and plate boundaries are not yet completely validated by the scientific community. The principal remaining questions for the Gulf of Corinth, in term of tectonic setting, are (1) its evolution during the period between the main Hellenic compression and the current extension (roughly between 25 and 2 Ma) (2) the link between regional Aegean extension and the recent N–S extension in the Gulf and (3) the cause of the uplift of the northern Peloponnese peninsula. Sedimentary basins document the complex interactions between deformation and external processes (sea-level change and/or climate variation). The spectacular outcrops of the southern margin of the Gulf of Corinth, where syn-rift sediments are well preserved, can help to constrain the tectono-stratigraphic evolution of the Gulf. However, while the southern coast of the Gulf of Corinth has been extensively studied and mapped (e.g. Doutsos et al., 1988, Ori, 1989, Doutsos and Piper, 1990, Doutsos and Poulimenos, 1992, Flotté et al., 2005), the structural and stratigraphic details of these outcrops are still not precisely defined. The geometry of the fault system and the relationship between the different segments of the margin are also not completely understood. A tectonic model proposing the existence of a north-dipping crustal detachment beneath the Corinth Rift was proposed by Sorel (2000) and tested by Chéry (2001). This model is still supported (Flotté et al., 2005) although it has been questioned (e.g. Westaway, 2002, Moretti et al., 2003). Ori (1989) was the first to propose a synthetic stratigraphic column for the syn-rift series of the southern coast of the Gulf. Recently, Ghisetti and Vezzani (2005) propose synthetic stratigraphic columns not completely in accordance with Ori (1989). Lateral facies variations and thickness changes are only schematically sketched and were not interpreted using recent sequence stratigraphic concepts, and thus no clear stratigraphic scheme exists. Moreover, only a few biostratigraphic and chronostratigraphic data are available to constrain the timing of deformation and sedimentation. Therefore, sedimentary architecture and sequence organisation of the syn-rift sediments have never been placed within a clear structural framework. The purpose of this paper is to present (1) a description of syn-rift stratigraphic sections logged in the Akrata–Derveni region, (2) a detailed description of the faults, the syn-rift deposits and the key geomorphological features and (3) a restoration of the stratigraphic architecture of the rift along a N–S structural transect through the central part of the southern coast of the rift. This work has been done as a part of the Corinth Rift Laboratory, a cluster of European projects associated to create a permanent observatory of the Gulf's seismic activity (Cornet et al., 2004 and papers in this special issue).
Section snippets
Regional tectonic setting of the Corinth Rift
The Corinth Rift corresponds to one of the most recent extensional features in the Aegean area (Fig. 1). Extension in the Aegean Sea started in Miocene times (Le Pichon and Angelier, 1979, Jolivet et al., 1994, Armijo et al., 1996) and was caused by a combination of gravitational collapse of the thick crust inherited from the Hellenic thrust belt (Horvath and Berckhemer, 1982, Jolivet, 2001), lithospheric thinning in the Aegean back-arc region (Papazachos and Comninakis, 1971, Doutsos et al.,
Dataset and methodology
The Akrata–Derveni region was chosen for this work as the relationships between tectonics and sedimentation can be studied in spectacular outcrops of syn-rift deposits (Fig. 1). Regional transects were constructed between the uplifted southern part of the margin in which proximal depositional systems are still preserved, and the modern Gulf of Corinth. The area has already been surveyed by Doutsos et al. (1988), Ori (1989), Doutsos and Poulimenos (1992) and Zelilidis (2000), who compiled
Structural framework of the Akrata–Derveni area and fault geometry
A series of faults exposed along the southern coast of the Gulf of Corinth have been well described and studied, especially in the areas where pre-rift basement is exposed, thus making possible the estimation of fault throw (e.g. Doutsos and Poulimenos, 1992, Roberts et al., 1993, Koukouvelas et al., 1999, Goldsworthy and Jackson, 2001, Micarelli et al., 2003). However, such detailed studies are more difficult where faults cut only syn-rift sediments at surface because of the lack of reference
Regional stratigraphy setting of the syn-rift infill
The mean thickness of the syn-rift sequence of the southern coast of the Corinth Rift has been previously estimated to be 1200 m westward of the studied area (Doutsos and Poulimenos, 1992) but can in fact reach more than 2800 m in the Mavro Oros region. In the Derveni area, the maximum thickness reaches 2100 m. Syn-rift deposits of the Akrata–Derveni region can be subdivided into three main lithostratigraphic units, which are in stratigraphic order: (1) a Lower Group mostly made of
Sedimentology and depositional systems of the southern coast of the Gulf of Corinth in the Akrata–Derveni area
A composite section logged in the Derveni area (Fig. 4) represents the syn-rift infill on a tilted block. The geometrical relationships between the different units are summarised in Fig. 5. We have defined for the first time three unnamed groups (Fig. 4). The Lower Group can be subdivided into three main formations, clearly distinguished in the Derveni area. These are in stratigraphic order: the Exochi Formation, the Valimi Formation and the Aiges Formation (Fig. 4). These formations can be
Geomorphological features
Specific geomorphological features have been identified in the field. They correspond to the upstream structures, mainly erosive, of depositional systems such as alluvial fan of Gilbert-type delta. They may be used to constrain the morpho-structural evolution of the rift of Corinth, especially for the most recent stages of rift evolution (Middle and Upper Groups) (Dufaure et al., 1979, Doutsos and Piper, 1990, Zelilidis, 2000). These features are well known on a regional scale and speculative
Timing of the deformation
Three geological transects were compiled from vertical outcrop sections (indicated on Fig. 9), mapping and panoramic view interpretation to illustrate the structural and stratigraphic architecture of the basin fill (Fig. 9). Section 1 is located in the western part of the study area and shows syn-rift infill across several tilted fault blocks. Section 2 is located in the southern part of the study area, cutting through the syn-rift stratigraphic architecture of the Killini Fault block. Section
Evolution of the southern coast of the Gulf of Corinth (Akrata–Derveni region)
Without accurate timelines within syn-rift stratigraphy across the southern coast of the Gulf, the evolutionary model proposed here can only be based on the relative succession of each depositional phase from the Exochi Formation, to the Upper Group in the eastern part of the study area (Fig. 10). We have no control on the position of the northern margin of the rift and thus can give only minimum estimates of rift width.
Progressive onlap of conglomerates onto tilted block crests, thickness
Comparison with previous syn-rift stratigraphic schemes
This is the most detailed syn-rift stratigraphic scheme published so far for the Xylokastro–Akrata area. Ori (1989) proposed a more general two phase rifting history for the same area in which his first and second sedimentary cycles correspond roughly to our Lower and Middle Groups. However, Ori (1989) identified a 1000 m thick basinwide fanglomerate at the top of his first sedimentary cycle, which we do not recognise. We speculate that it may locally be identified as the Mavro alluvial fan,
Conclusion
In this paper we document a fault controlled syn-rift stratigraphy up to 2800 m thick in the Derveni–Akrata area of the Corinth Rift. The syn-rift succession is divided into three lithostratigraphic groups, in which facies and thickness changes have been correlated across an area of ∼ 700 km2. These groups record a three phase rift history.
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The Lower Group records an early wide rift. The initial stages of rifting were contemporaneous with a lowstand stage in the relative sea-level curve (Exochi
Acknowledgements
This paper forms part of a PhD thesis carried out by the first author for which the “Institut Français du Pétrole” and the “Université de Rennes 1” are gratefully acknowledged for their financial and technical support. J.-P. Colin is particularly thanked for the ostracod identification. We would like to thank Prof. D.J.W. Piper and Dr. S. Kokkalas for their constructive comments.
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