Previous research has distinguished several tectonic zones in the Hellenides characterised by rock type, stratigraphy, tectonometamorphic history and pre-orogenic palaeogeography (Robertson et al. 1991). The Cycladic blueschist unit is the dominant tectonic unit in the central Aegean and represents the most deeply exhumed unit in the entire region. The Cycladic blueschist unit comprises from top to bottom three composite nappes: (1) an ophiolitic mélange; (2) a Permo-Carboniferous to latest Cretaceous passive-margin sequence, and (3) a Carboniferous basement, which also occurs as slices in the passive-margin sequence (see Ring et al., this volume, for schematic of structural pile). The Cycladic blueschist unit is overlain on some islands by the Upper unit, which is a non- to weakly metamorphosed ophiolitic nappe that contains Pliocene sediments. In some windows in the Cycladic zone, the Basal unit, as part of the External Hellenides, crops out below the Cycladic blueschist unit (Godfriaux 1968; Avigad & Garfunkel 1989). The Cycladic blueschist unit and the External Hellenides were part of the Adriatic microcontinent. A major difference between the Aegean and adjacent western Turkey is that in the latter the Menderes nappes instead of the External Hellenides form the lowermost tectonic unit. In contrast to parts of the External Hellenides, the Menderes nappes do not show Tertiary high-pressure metamorphism (Gessner et al. 2001; Ring et al. 2001; Régnier et al. 2003). The Menderes nappes are part of the Anatolian microcontinent, which collided with Eurasia further east than Adria (Figure 5). A distinctive feature of the Anatolian microcontinent is that its basement formed at ~550 Ma (Gessner et al. 2001, 2004; Ring et al. 2004).
Figure 5. Palaeogeographic sketch
Palaeogeographic sketch outlining the complex geometry of the Adriatic and Anatolian plates.
The metamorphic evolution of the Cycladic blueschist unit includes an Early Tertiary high-pressure event (~15-20 kbar and 500 ± 50°C) at ~55-45 Ma followed by one or more greenschist-facies overprints (~4-7 kbar and 400 ± 50°C on most islands; the amphibolite-facies overprint on Naxos Island is exceptional) (Altherr et al. 1982; Wijbrans et al. 1990; Tomaschek et al. 2003; Ring & Layer 2003). High-pressure metamorphism in the Basal unit on Evia and Samos islands is dated at 24-21 Ma (Ring & Reischmann 2002).
In the Middle to Late Miocene, the Cyclades became part of the magmatic arc of the southward retreating Hellenic subduction zone as evidenced by arc-related volcanic rocks ranging from ~11-6 Ma (Fytikas et al. 1984; Weidmann et al. 1984) and granites spanning an age range from ~14-10 Ma (Keay 1998). The granites were emplaced synkinematically during extensional faulting (Faure et al. 1991; Lee & Lister 1992).