Δελτίο της Ελληνικής Γεωλογικής Εταιρίας, Τομ. XXXIV/1, 289-296, 2001 Bulletin of the Geological Society of Greece, Vol. XXXIV/1, 289-296, 2001  Πρακτικά 9ου Διεθνούς Συνεδρίου, Αθήνα, Σεπτέμβριος 2001 Proceedings of the 9th International Congress, Athens, September 2001  TECTONIC STRUCTURE AND VOLCANIC CENTRES AT THE EASTERN EDGE OF  THE AEGEAN VOLCANIC ARC AROUND NISYROS ISLAND.  D. PAPANIKOLAOU 1 AND P.NOMIKOU 2 ABSTRACT  The recent volcanic activity at the eastern edge of the Aegean Volcanic Arc is limited within a neotectonic  graben structure which is developed in an E-W general direction between the alpine basement of Kos Island to  the north and the alpine basement of Tilos Island to the south. In between the boundary faults of the neotectonic  graben there is an extended volcanic area comprising several individual volcanic centers, which penetrate through  the thick post-alpine sedimentary deposits of the graben.  KEY WORDS: Aegean Volcanic Arc, Nisyros, Volcanic Centres, Tectonic Structure.  l.INTRODUCTION The Aegean Volcanic Arc is the result of the subduction of the Eastern Mediterranean lithosphère below the active Hellenic margin of the European plate. The Volcanic Arc follows in parallel position successively the development of the Hellenic Trench, the Peloponnese-Crete-Dodecanese Island Arc and the Cretan back-arc molassic basin. The major recent or active volcanoes along the Aegean Volcanic Arc occur from Soussaki- Methana-Aegina and Poros in the west, through Milos and Santorini in the center, up to Kos and Nisyros in the east (Papanikolaou, 1986)(Fig. la). Submarine volcanoes should exist all along the volcanic arc, in between the Aegean volcanic islands, as this was first documented in the case of Paphsanias Submarine Volcano in Epidauros Basin within Western Saronikos Gulf (Pavlakis et al 1990) and later in the case of the submarine area around Nisyros Island (Nomikou & Papanikolaou, 1999, 2000). The understanding of the tectonic structure of the Kos-Nisyros volcanic area requires data both from on- shore and offshore studies, combining classical geology and marine geology investigations. In this paper, some answers on questions such as, which is the tectonic structure and how many volcanic centres exist in the area, will be given. 2. MORPHOLOGY The morphology of the area is given in Fig.l based on a digital elevation model, which has incorporated both onshore data from the Geographical Military Service and offshore data obtained from océanographie surveys, effected during 1997-1999. Additional morphological data of high accuracy, using two kind of multibeam system i) Sea Beam 1120, 20 KHz for depths >500m and ii) Sea Beam 1180, 180 KHz for depths <500m on R/V Aegaeo, have been obtained during 2000. A first set of these newly obtained data for the area around Nisyros are given on Fig.4 within the description of the volcanic centres. The main morphological features of the studied area are three zones of positive relief comprising Kos in the NW (843m elevation in Dikeos Mt), Nisyros (698m in Prophitis Ilias Mt.) and surrounding islets in the middle and Tilos (654m, Prophitis Ilias) in the SE. These zones subdivide the submarine area between Kos and Tilos in two basins with an average sea bottom depth of 600m. Consequently, the topographic differences between the Mountain ranges and the submarine basins are of the order of 1-1,5 Km. In those cases where there is a high morphological gradient from the top of the mountain ranges to the edge of the submarine basins which results in very steep slopes, the zone of shallow depths (0-300m) is very narrow, as in the case of the southern coast of Kos Island. On the contrary, the area of the islets around Nisyros is charac- terized by extended shallow water depths which, as it will be shown in the next chapter, are occupied by volcanic formations. 1. University of Athens, Department of Dynamic, Tectonic & Applied Geology, Panepistimiopolis, 15784 Athens, Greece. 2. National Centre for Marine Research, Agios Kosmas, 16604, Greece. -289-