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

Samos, to the east of Ikaria, is made up from top to bottom by the non-metamorphic Kallithea nappe (Upper Unit), which is underlain by the Selcuk (ophiolitic mélange), Ampelos (passivemargin sequence) and Agios Nikolaos (Carboniferous basement) nappes, all three of which belong to the Cycladic blueschist unit. Below the Cycladic blueschist unit is the Kerketas nappe, which is part of the Basal Unit. Middle Miocene to Pliocene sediments with intercalated arc-related volcanic rocks ranging in age from 11 to 6 Ma occur in three graben (Weidmann et al. 1984). At 10Ma a granodiorite was intruded in westernmost Samos (Ring et al. 1999e).

Three extensional fault systems occur on Samos (Fig. 4); they are from top to bottom: (1) the top-to-the-north Kallithea detachment, which separates the Kallithea nappe from the Cycladic blueschist unit and the Kerketas nappe; (2) the top-to-the-ENE Selçuk extensional system between the Ampelos nappe and the Selçuk nappe; (3) the top-to-the-ENE Kerketas detachment between the Kerketas nappe and the overlying Cycladic blueschist unit. The Kerketas detachment is associated with the formation of the Mid-Miocene graben (Weidmann et al. 1984; Ring et al. 1999e). A critical question of regional tectonic significance is how these extensional faults relate to those exposed on nearby lkaria.

Methods and sampling

Pressure-temperature estimates

Samples IK02-6a and IK02-16 from the lkaria nappe (Fig. 2) were studied to constrain P-T conditions of the lkaria nappe, especially for unravelling a possible pre-amphibolite-facies high-pressure metamorphism. Sample IK99-31 is a mylonite from the Messaria shear zone (Fig. 2) and should constrain P-T conditions during mylonitization and thus the depth from which the shear zone was exhumed. A complete set of P- T estimates is available online at http://www.geolsoc.org.uk/SUP18219. A hard copy can be obtained from the Society Library.

The mineral analyses were obtained with a Jeol Superprobe (JXA 8900RL) at Johannes Gutenberg-Universitat, Mainz. A complete set of mineral analyses and abbreviations used is available as a Supplementary Publication (see above). Operating conditions were 15 kV acceleration voltage, 15nA beam current and 20s counting time per element. Standards used were wollastonite for Si and Ca, corundum for Al, pyrophanite for Ti, hematite for Fe, MgO for Mg, albite for Na, orthoclase for K, tugtupite for Cl, F-phlogopite for F, Cr^sub 2^O^sub 3^ for Cr, rhodochrosite for Mn, and ZnS for Zn. The mineral analyses are considered to be accurate within a range of c. 3% (relative) on any given grain. P- T pseudosections were calculated with the thermocalc software (version 3.2.1) (Powell et al. 1998). Structural formulae were calculated from microprobe analyses using the ax software (Holland 2000).

Strain and rotation analysis

To quantify finite strain, quartz aggregates from deformed granites were analysed by the R^sub f^/φ and Fry methods (Ramsay 1967; Fry 1979; Ramsay & Huber 1983). The granites were emplaced during Miocene extension (see below) and therefore record extension-related finite strain. For R^sub f^/φ analysis, the long and short axes of up to 40 grains per section were measured and mean aspect ratios for each section were calculated. Tectonic strains were determined from the χ^sup 2^ minima of the R^sub f^/φ analyses (Peach & Lisle 1979). For Fry analysis, the central points of more than 100 quartz and feldspar grains per section were used to calculate strain. The strain estimates were used to calculate the finite-strain ellipsoid according to the modified least-squares technique of Owens (1984).