Relationship between structural, petrological and petrophysical features on strain evolution in the Kavala shear zone (Rhodope Massif, north-eastern Greece)

Mylonitic shear zones represent the binary which drives the structural evolution of mountain belts; for this reason they can be considered as “natural laboratories” where it is possible to observe and concentrate the investigation on microstructural and textural characteristics related to the changes in physical and mineralogical properties of progressively deformed rocks. Our attention focussed on the Kavala (Symvolon) pluton, which crops out in North-Eastern Greece and is mainly composed of early-Miocene granodiorite deformed at various extents during syn-shearing emplacement. According to the Authors, it represents the South-Western termination of the Rhodope Core Complex, which was exhumed as a result of large scale extension from mid-Eocene to mid-Miocene times. Structural features of mylonitic textures highlighted the presence of a NE-SW monotone stretching lineation consistent with the shape elongation of the outcropping pluton, with intensity in deformation increasing towards the contact with the host gneiss. Despite gneiss rocks experienced a longer deformation history, any previous fabric was completely reworked by the Alpine mylonitic shearing activity. Such a peculiar outcrop conditions set aside samples to be collected by taking into account progressive deformation. Bulk rock geochemical and petrographic investigation allowed the characterisation and selection of representative samples for both Kavala granitoid and country gneiss rocks. They mostly classify as metaluminous to weakly peraluminous granodiorite and strongly peraluminous granite, respectively, and define distinct parallel trends in specific major and trace element variation diagrams. Geochemical trends have been used to check for any possible modification of the different rock types associated with mylonitization, as well as to identify original magmatic compositions. Mineral chemistry investigation accompanied by multivariate statistical analysis of multispectral X-ray maps highlighted well preserved original porphyroclast zoning within low strain domains, whereas a complete compositional re-homogenisation was observed in widespread highstrain domains. Moreover, several generations of syn-mylonitic mineral growth inducing local strength-softening have been detected. Quantitative microstructural investigation was performed on thin sections cut parallel to the stretching lineation. Optical assisted-image technique analysis on quartz grains (800 ca. grains per sample) revealed a mean axial ratio (AR, i.e. the ratio between major and minor axis of particles) ranging between 2.2 and 3.2 (AR max = 15) and average grain size of 20 μm2. Calculated flow-law equation describing the rheological properties of quartzite suggests a mean shear strain rate value of 4.87 × 10-12 (s-1) for the studied samples. Finally, petrophysical investigation on progressively deformed granodiorite at confined pressure conditions (up to 400 MPa) highlighted that Vp are distributed with the highest and lowest values orientated within and normal to the main foliation plane, respectively. Vp related seismic anisotropy ranges from 2.16 to 6.92%. Vs related acoustic birefringence shows its maximum along the stretching lineation direction. Vp/Vs ratios are in the range of 1.67-1.70. Preliminary correlation between petrophysical and quantitative microstructural features opens new perspectives in the calculation of strain rate estimates of progressively natural sheared rocks.