Microtomography investigation of ultramylonites from crustal scale shear zones and implications on the rheology of the continental lithosphere

Recognition of different constituting phases and their volumetric distribution in different portions of the same deformed rock are crucial constraints to develop models for rock rheology. A 3D microstructural study of ultramylonites sheared under opposite deformational regimes (compressional vs extensional) developed within different crustal scale shear zones has been performed. Two sites have been selected as natural laboratories for investigating the mylonitic evolution of rocks during the shearing event: Montalto Shear Zone (MSZ, Italy; Fazio et al., 2015; 2018a) and Kavala Shear Zone (KSZ, Greece; Punturo et al., 2014; Fazio et al., 2018b). These rocks are characterized by extremely fine grain size of matrix as well as by small porphyroclasts. Matrix-constituting minerals, such as quartz, feldspars and micas form an interconnected network testifying high strain conditions, which affects the rheological behavior of the shear zone at depth. Therefore, our aims are to investigate the mylonitic fabric at the micro-scale by quantifying the appearance of new mineral phases at increasing matrix/ porphyroclasts ratio as well as the 3D development of matrix network and the variations in Shape Preferred Orientations (SPO) of constituent minerals, with particular regards to dynamically recrystallized new grains, few microns sized. These features can be well investigated by 3D analysis of their morphology and distribution by X-ray computed tomography (CT) technique. On investigated samples, the microtomography analyses provided detailed 3D information about clasts (volumetric distribution, grain sizes, phase interconnectivity), permitting to fully investigate the 3D distribution and arrangement of minerals, with particular regards to grains (few microns sized) of the matrix. Further investigations and 3D data rendering elaborations will allow us to formulate realistic models about grainsize reduction processes induced by deformation within such rocks.