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

Material markers such as deformed pegmatite and quartz veins have been used for strain analysis and quantitative assessment of the noncoaxiality of deformation. Passchier (1990) showed that it is sufficient to know the orientation of the boundaries between domains of material lines with different stretch histories to determine the parameters of finite deformation, i.e. the kinematic vorticity number, W^sub m^ the finite strain ratio, R^sub f^, and the rate of area change, A, if deformation has accumulated by approximately steady-state flow (Passchier 1988). Deformed veins were measured in sections perpendicular to the y-axis of the strain ellipsoid (principal x > y > z, finite-strain axes). The veins can be separated into: (1) those that have only been shortened; (2) those that were shortened and then extended; (3) those that have only been extended. The veins were plotted as vectors from a central point and cluster in four sectors separated by four material lines that lie parallel to the lines of no instantaneous longitudinal strain (L-axes) at the beginning of deformation and at the end of deformation (Passchier 1990) (Fig. 5a). After Passchier (1990), these four lines are La^sub 1^, La^sub 2^ (La lines indicate 'after'deformation) and Lb^sub 1^, Lb^sub 2^ (Lb lines indicate 'before' deformation). We used the Mohr circle for the stretch tensor, H (Fig. 5b), which relates particle positions in the deformed state back to the undeformed state, for calculation of finite-deformation parameters. The position and size of the circle can be defined by the parameters T, Q and R, which are also used for the calculation of W^sub m^ (W^sub m^ = Q/R) and R^sub f^ (T R)/(T - R). The angle ψ between La^sub 1^ and La^sub 2^ can be used to calculate A (A = cos(ψ/2)) (Fig. 5b).

Another method of estimating W^sub m^ is to measure the axial ratio, R, of a particle and the angle between the maximum elongation of the particle and the shear plane (blocked-object method of Passchier 1987). For W^sub m^ = 1 (i.e. simple shear), all particles that behave as active markers with R ≥ 1 will rotate freely as the shear strain increases and the rate of rotation equals the rate of stretching. If W^sub m^ is

Rb/Sr geochronology

Two samples were selected for Rb/Sr mineral analysis. Sample IK99XD is a very fine-grained, mylonitic quartzitic schist composed of quartz and K-feldspar, with minor albite, white mica, apatite and biotite. Obvious feldspar and mica textural relics are not observed. Sample IK02-4 is a mylonitically deformed metapegmatite. The mineral assemblage comprises quartz, K-feldspar, albitic plagioclase, white mica, apatite, tourmaline and garnet. Although most of the rock is synkinematically recrystallized, the pegmatitic nature of the protolith is still obvious from local megacrystic relics of phases with high shear strength (feldspar, garnet, tourmaline, white mica). Mica fish reach sizes of up to 1 cm. Rb/ Sr isotope systematics of texturally defined generations of white mica in metapegmatites can be used to define both deformation and protolith age (Glodny et al. 1998). We therefore analysed separated minerals from an apparently pervasively deformed, 5 cm3 domain as well as only slightly bent and kinked inner portions of two primary, pegmatitic white mica crystals. Analytical procedures are those of Glodny et al. (2002).

^sup 40^Ar/^sup 39^Ar geochronology

For ^sup 40^Ar/^sup 39^ Ar dating, potassic minerals were degassed with an argon laser probe using step heating or direct ablation of grains on thin rock sections. For step-heating analyses, single grains less than 1 mm in diameter were progressively degassed with a defocused argon laser beam until their fusion. An apparent age was calculated for each heating step and reported within an age spectrum where the successive ages are plotted against the cumulative amount of argon released. The data were also reported in ^sup 36^Ar/^sup 40^Ar v. ^sup 40^Ar/^sup 39^Ar correlation plots for which intercepts with the abscissa and ordinale axes correlate with age and ^sup 40^Ar/^sup 39^Ar initial ratio. ^sup 40^Ar/^sup 39^ Ar laser probe dating has been also applied to sections (c. 1 cm × 0.5 cm), which were polished on one side and afterwards cleaned in ethanol and distilled water. Both single grains and rock sections were packed in aluminium foils and irradiated for 70 h in the McMaster nuclear reactor (Canada) with MMHb hornblende neutron flux monitor dated at 520.4 ± 1.7 Ma (Samson & Alexander 1987). After irradiation, the samples were placed on a Cu-holder inside an ultrahigh vacuum gas extraction system and baked for 48 h at 200 °C. For step-heating analyses, the progressive degassing of each single grain included 40 s of laser exposure at variable beam power and 5 min of gas cleaning. For in situ spot ablation of mica in thin section, several spots (10-20) were made on the mineral surface with a focused laser beam using an exposure time of 30 ms for each spot. Each crater is a 30-40 µm hemisphere surrounded by a wall of melted material. After gas extraction and cleaning, argon was introduced into the mass spectrometer and 15 min were required for data acquisition by peak switching from mass 40 to 36, through 10 sets of data. System blanks were evaluated every three analyses and range around 2 × 10^sup -12^ cm^sup 3^ for ^sup 40^Ar and 3 × 10^sup -14^ cm^sup 3^ for ^sup 36^Ar. For each analysis, classical isotope corrections including blanks, mass discrimination radioactive decay of ^sup 37^Ar and ^sup 39^Ar and irradiation-induced mass interferences were applied to calculate the variable argon ratios. Step heating and spot fusion apparent ages were calculated assuming that all the initially trapped argon was atmospheric in composition.