Journal of Geodynamics 36 (2003) 323–340 www.elsevier.com/locate/jog The Gulf of Corinth: an active half graben? Isabelle Morettia,*, D. Sakellarioub, V. Lykousisb, L. Micarellia aIFP Av de Bois Pre´au 92852 Rueil Malmaison, Paris, France bNCMR, Athens, Greece Abstract The Gulf of Corinth is often considered as a typical example of a more or less simple half graben with major border faults to the south and a flexure of the northern shore. This paper reviews new data, espe- ciallysubsurface data, from both onshore and offshore, compiled or acquired through the Corinth Rift LaboratoryEEC project. This data indicate that (1) the Gulf of Corinth is bordered both north and south byactive faults; (2) there is a lot of them, and not onlythe one bordering the Peloponnese coastline are still active; and (3) distinct opening phases maybe recognized on the area. During the first phase, the depo- center was located near the northern shore, whereas during the most recent phase, and onlyin the centre and the eastern sector, the depocenter moved towards the southern shore. Furthermore, active tectonic, in this western sector, is characterised bya general uplift of the Peloponnese that leads to the formation of new faults, i.e. the Doumena, Aigion and Helike faults and to selective reactivation of older ones such as the Pirgaki fault. We suggest that this current tectonic phase started about 150–120 000 years ago in the Aigion area and about 350 000 years ago eastward (Corinth-city) and that it represents a third opening phase during the growth of the Gulf of Corinth. # 2003 Elsevier Ltd. All rights reserved. 1. Introduction The Gulf of Corinth area (Fig. 1) has been studied for a long time, since its southern shore exposes spectacular outcrops and because it is the most seismicallyactive zone of the European Union. However, until recently, subsurface data, from both offshore and onshore, were not available. Consequently, because synrift deposits crop out only along the southern shore of the Gulf, the models about the extensional processes in the area were build up on a small amount of data. Subsurface data have been collected offshore byHellenic Petroleum, the National Centre of * Corresponding author. Tel.: 33-1-4752-6191; fax: +33-1-4752-7067. E-mail address: [email protected] (I. Moretti). 0264-3707/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0264-3707(03)00053-X 324 I. Moretti et al. / Journal of Geodynamics 36 (2003) 323–340 Fig. 1. Structural map of the Gulf of Corinth area, modified from Armijo et al. (1996), Sakellariou et al. (2001) and Stefanos et al. (2002). The onshore structural pattern in the area of Aigion is modified from the map of Ghisetti et al. (2001). Water depth in metres. I. Moretti et al. / Journal of Geodynamics 36 (2003) 323–340 325 Marine Research (NCMR) and the Patras University, and more recently onshore by the cluster of European projects called ‘‘CRL’’ (Corinth Rift Laboratory, Moretti et al., 2002). This new data have lead to a better-constrained structural model of the Gulf that will be discussed in the following section. Through the CRL project, extensive field works have also been carried out around the Aigion area, together with detailed analyses of cuttings and cores collected on land as well as offshore. The CRL also has set-up the installation of few networks (seismic, GPS, strain meters, geochemical...) which have allowed the monitoring of the area and the recollection of a large mass of information. Our work suggests that the Gulf of Corinth is the result of a longer and more complex deformation historythan the simple half graben, bordered southward byan unique active fault, often described in the literature. 2. Geological setting Extension in the Aegean Sea started in Miocene times (Le Pichon and Angelier, 1979; Jolivet et al., 1994; Armijo et al., 1996). This extension is thought to be due to both gravitational collapse of the thick crust inherited from earlier mountain building (Jolivet, 2001) and lithospheric thin- ning in the Aegean back arc region (Doutsos et al., 1988). The extension rate is fast, about 3 cm/ year, with respect to Eurasia (Kahle et al., 2000; Jolivet, 2001) and progressivelymigrates to the south. Within this framework, the Corinth Rift represents one of the most recent extensional features although the relationships between the Aegean Miocene extension and the evolution of the Gulf of Corinth are still unclear. The possible multiple causes for the fast and intense tectonic activityin the Gulf of Corinth itself are debated. In addition to gravitational instabilityof the Neogene mountain belt and lithospheric thinning in the back arc region of the subduction mar- gin, the Evia and Corinth grabens mayalso be considered as accommodation sites at the western- propagating tip of the North Anatolian Fault (NAF). The propagation of the NAF towards the north Aegean Sea during the late Pliocene time (Dinter, 1998) has been recorded on seismic data (Roussos and Lyssimachou, 1991) and suggested bythe focal mechanism solution of major earthquakes on the area (Mc Kenzie, 1972, 1978). The Corinth Rift, which separates the Peloponnese from continental Greece, is a N100E oriented elongate graben, 105 km long, which is bounded bysystemsof veryrecent roughlyE–W normal faults (Fig. 1). These faults are though to be younger than 2 MY and are arranged en echelon, thus accommodating the gap between their orientation and the graben direction. The Gulf of Corinth is the most seismicallyactive zone in Europe, and the fastest opening area of continental break-up, with up to 1.5 cm/year of north–south extension rate, and more than 1 mm/year of uplift rate of the southern shore (Tselentis and Makropoulos, 1986; Billiris et al, 1991; Collier et al., 1992; Davies et al., 1997). The high rates of tectonic faulting and uplift lead to the exposure of veryrecent faults with large offsets which are quite impressive. Eastward, the Gulf of Corinth is wider, and the syn-rift sediments are thicker. However, the current opening rate is faster westward between Aigion and Rion-Antirion, where the width of the Gulf is at the minimum (Fig. 1). In the eastern part of the Corinth Gulf, at present, the basin reaches water depths of 860 m (Brooks and Ferentinos, 1984) and an average width of 30 km. The synrift sedimentary sequence is up to 2.4 km thick (Cle´ ment, 2000, Fig. 2). These values are representative of the central and eastern part of the Gulf (from 22E25 to 23E). Westward, the 326 I. Moretti et al. / Journal of Geodynamics 36 (2003) 323–340 water depth decreases up to 60 meters at the Rion-Antirion sill, and to about 400 m between Aigion and Trizonia, where the Gulf is 6 km wide. Due to the shallow depth of the Rion-Antirion sill and to the Late Pleistocene sea-level fluctuations, the Gulf has been periodicallytransformed into a lake. This continuous alternation, since the beginning of the sedimentaryrecord of exten- sion, is also clearlyrecorded bythe outcropping sediments that waver between lacustrine and marine facies. For the most recent period, this alternation has been proven for the western part of the Gulf by the core analyses of the sediment aged for the time at least from 11 800 years to about 41 000 years BC collected in the Aigion harbor (Lemeille et al., 2002), and bythe sequence stra- tigraphyanalysesof the prodelta sequences of the upper northern slope of the Gulf, offshore Eratini (Lykousis et al. 1998). The lacustrine facies have been sampled in the center of the Gulf, thank to the Marion Dufresne vessel piston cores (Lykousis et al., 2002), and described eastward, in the Alkyonides sub-basin, by Perissoratis et al. (1993, 2000) and Collier et al. (2000). These last authors suggest an age of about 70 000 years for the start of the (last) so-called Corinth lake, and 12 000 years for the last marine transgression. Up to about 300 000 years BP, the Gulf was also open eastward to the Saronicos Gulf. The marine deposits of that time are now uplifted and exposed along the Corinth Canal where theyhave been dated at 312 000 yearsold ( Collier et al., 1992). Fig. 2. Interpretation of the seismic lines (A and B) from Hellenic Petroleum. The time-depth conversion is from Clement (2000), but we have substantiallymodified the interpretation. I. Moretti et al. / Journal of Geodynamics 36 (2003) 323–340 327 Progressive east-to-west opening of the Gulf of Corinth has been proposed byvarious authors (Sakellariou et al., 2001). However, offshore there is no age available for the bottom of the synrift deposits to confirm this model, and, onshore available data are sporadic. Nevertheless, there is clear evidence of a lower cumulative amount of extension and a current faster opening and uplift rates westward. Based on GPS data, Briole et al. (2000) measured 1.5 cm/year of opening west- ward and 0.5 eastward. The coastal uplift rate in the Perachora peninsula is 0.75 mm/year, whereas it reaches 1.5 mm/year in the Aigion area (Stewart and Vita Finzi, 1996). Crustal thick- ness under the Gulf also displays a strong east-west variation: the Moho is 20 km deep below Corinth and 40 km deep in the Aigion area (Makris et al., 2001). Gravitydata indicate the same west to east decrease of crustal thickness under the Gulf of Corinth but theyalso show north- south variation. The crustal thickness increases to the north from the Gulf of Corinth and decreases again below the Evia graben (Tiberi, 2000; Tiberi et al., 2001). If the east-west evolution mayeasilybe related to the extensional phase (from Miocene up to now), the north-south evo- lution is likelyto be partiallyinherited from the previous phases.