Three tectonic units can be distinguished on the island of Ikaria; they are from top to bottom: (a) the non-metamorphosed Fanari nappe; (b) the Messaria nappe (including the Kefala unit) and (c) the Ikaria nappe (Figs 4, 6). The general structure of Ikaria is dominated by a ~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 reminder of this field trip guide 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 4, 6, 7). The northwestern slopes of the island dip gently to the north, which mimics the shallow northern dip of the Messaria extensional fault system (Figure 7). The southern slopes of Ikaria Island dip much more steeply to the south (Figure 8).
Figure 6. Cross sections A-A’ and B-B’
Cross sections A-A’ and B-B’ illustrating domed architecture, which is also expressed by dip and plunge of foliations and stretching lineations on both sides of Ikaria Island.
Figure 7. I-type Raches granite
View on the I-type Raches granite showing gently sloping north side in the upper part of the photograph and abrupt drop at south coast in lower right. The plain area in the centre of the photograph essentially mimics the shallow dip of the Ikaria shear zone.
The Pliocene conglomerates of the Fanari nappe, which is part of the Upper unit of the Cycladic zone, contain pebbles of metamorphic rocks, which are not exposed on Ikaria Island (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). The marble in places contains small serpentinite lenses (Figure 9). Structural position and lithology indicate that the Messaria nappe correlates with the Ampelos nappe on nearby Samos Island (Ring et al. this volume). 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 ~15 kbar and ~500°C for the Ampelos nappe on Samos Island (see also Ring et al. this volume) and we regard these estimates as a reasonable approximation for high-pressure P-T conditions in the Messaria nappe. The Kefala unit is a klippe of marbles (with upper Triassic fossils) that makes up a small patch in the central part of Ikaria north of Evdilos (Figure 10).
Figure 10. Klippe of Kefala marble
Klippe of Kefala marble in centre of photograph. Note also marked asymmetry in morphology with gently sloping northern part of the island to the right and steeply dipping south side to the left.
The Ikaria nappe at the base is more than 1000 m 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 three synkinematic granites: the large I-type Raches granite in the west (Figure 7), the S-type Karkinagrion granite and the small S-type Xylosirtis granite in the southern part of the island (Figure 4). The metapelite contains the amphibolite-facies mineral assemblage garnet-kyanite-staurolite-biotite-plagioclase (Altherr et al. 1982). An important question is whether the Ikaria nappe shows evidence for pre-amphibolite-facies 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 to that of the Carboniferous basement of the Cycladic blueschist unit on Naxos Island 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.