extensional Messaria shear zone and associated brittle detachment faults, Aegean Sea, Greece, The

Journal of the Geological Society, Jul 2005 by Kumerics, Christine, Ring, Uwe, Brichau, Stéphanie, Glodny, Johannes, Monié, Patrick

The metamorphic evolution of the Cycladic blueschist unit includes an Early Tertiary high-pressure event (c. 15-20kbar and 500 ±50 0C) at c. 55-45 Ma followed by a greenschistfacies overprint (c. 4-7 kbar and 400 ± 50 0C on most islands; the amphibolite-facies overprint on Naxos is exceptional) (Altherr et al. 1982; Wijbrans et al. 1990; Ring & Layer 2003; Tomaschek et al. 2003). High-pressure metamorphism in the Basal Unit on Evia and Samos is dated at 24-21 Ma (Ring et al. 2001a; Ring & Reischmann 2002).

In the Mid- to Late Miocene, the Cyclades became part of the magmatic arc of the southward retreating Hellenic subduction zone as shown by arc-related volcanic rocks ranging from c. 11 to 6 Ma (Fytikas et al. 1984; Weidmann et al. 1984) and granites spanning an age range from c. 14 to 10Ma (Keay 1998). The granites were emplaced synkinematically during extensional faulting (Faure et al. 1991; Lee & Lister 1992).

Structure and geology of Ikaria and Samos

The island of Ikaria belongs to the Cycladic zone. Three tectonic units can be distinguished; they are from top to bottom: (1) the non-metamorphic Fanari nappe; (2) the Messaria nappe; (3) the Ikaria nappe (Fig. 2). The general structure of Ikaria is dominated by a c. 300-500 m thick ductile extensional shear zone, the Messaria shear zone, and two associated brittle detachment faults, the Messaria and Fanari detachments. The Messaria detachment is the upper crustal expression of the ductile Messaria shear zone. In the remainder of the paper we refer to this detachment-shear-zone system as the Messaria extensional fault system. The Fanari detachment is not associated with an underlying ductile shear zone. The Messaria and Ikaria nappes are separated from one another by the Messaria detachment; the Messaria shear zone developed in the upper parts of the Ikaria nappe. The Fanari detachment separates the Messaria nappe from Pliocene conglomerates of the Fanari nappe. The island has an asymmetric dome-shaped architecture (Figs 2 and 3). The northwestern slopes of the island dip gently to the north, which mimics the shallow northern dip of the Messaria extensional fault system. The southern slopes of Ikaria dip more steeply to the south.

The Pliocene conglomerates of the Fanari nappe, which is part of the Upper Unit of the Cycladic zone, contain pebbles of metamorphic rocks that are not exposed on Ikaria (Dürr et al. 1978). The Messaria nappe comprises metabauxite-bearing marble, graphite-rich calc-mica schist, chloritoid-kyanite-bearing phyllite, quartzite and greenschist (Altherr et al. 1982). Structural position and lithology indicate that the Messaria nappe correlates with the Ampelos nappe on nearby Samos (Ring et al. 1999e) (Fig. 4). Both nappes are part of the passive-margin sequence of the Cycladic blueschist unit. Will et al. (1998) estimated peak pressure-temperature (P-T) conditions of c. 15 kbar and c. 500 °C for the Ampelos nappe on Samos and we regard these estimates as a reasonable approximation for high-pressure P-T conditions in the Messaria nappe. The Ikaria nappe at the base is more than 100Om thick and consists of a huge succession of metapelite as well as minor marble, calcsilicate rocks, amphibolite, quartzite and metapegmatite. The Ikaria nappe was intruded by two synkinematic granites: a large I-type granite in the west and a small S-type granite in the southern part of the island (Altherr et al. 1982). The metapelite contains the amphibolitefacies mineral assemblage garnet-kyanite-staurolite-biotite-plagioclase (Altherr et al. 1982). An important question is whether the Ikaria nappe shows evidence for pre-amphibolitefacies high-pressure metamorphism, which is typical for the nappes of the Cycladic blueschist unit. If so, the P- T evolution of the Ikaria nappe would generally be comparable with that of the Carboniferous basement of the Cycladic blueschist unit on Naxos with its exceptional amphibolite-facies Miocene metamorphism. Alternatively, the Ikaria nappe might be an exotic non-high-pressure unit in the Aegean and would probably correlate with the Menderes nappes of western Turkey.