Microstructural, compositional and petrophysical properties of mylonitic granodiorites from an extensional shear zone (Rhodope Core complex, Greece)

At the southern boundary of the Rhodope Massif, NE Greece, theKavala Shear Zone (KSZ)represents an example of the Eastern Mediterranean deep-seated extensional tectonic setting. DuringMiocene time, extensional deformation favoured syntectonic emplacement and subsequent exhumationof plutonic bodies. This paper deals with the strain-related changes in macroscopic, geochemical andmicrostructural properties of the lithotypes collected along the KSZ, comprising granitoids fromthe pluton, aplitic dykes and host rock gneisses. Moreover, we investigated the evolution of seismicanisotropy on a suite of granitoidmylonites as a result of progressive strain. Isotropic compressional andshearwave velocities (Vp, Vs) and densities calculated frommodal proportions and single-crystal elasticproperties at given pressure–temperature (P–T) conditions are compared to respective experimentaldata including the directional dependence (anisotropy) of wave velocities. Compared to the calculatedisotropic velocities, which are similar for all of the investigated mylonites (average values: Vp 5.87 km s−1, Vs 3.4 km s−1, Vp/Vs = 1.73 and density = 2.65 g cm−3), the seismic measurementsgive evidence for marked P-wave velocity anisotropy up to 6.92% (at 400 MPa) in the most deformedrock due to marked microstructural changes with progressive strain, as highlighted by the alignmentof mica, chlorite minerals and quartz ribbons. The highest P- and S-wave velocities are parallel tothe foliation plane and lowest normal to the foliation plane. Importantly, Vp remains constant withinthe foliation with progressive strain, but decreases normal to foliation. The potential of the observedseismic anisotropy of the KSZ mylonites with respect to detectable seismic reflections is brieflydiscussed.