Strain dependent variation of microstructure and texture in naturally deformed Carrara marble

Abstract

This study investigates the microstructure and texture variations across a mm-scale shear zone in Carrara marble of the Alpi Apuane (Italy). The microstructures have been investigated for grain size, texture, and shape fabrics. Textures have been measured with Computer-Integrated Polarization Microscopy (CIP) and Electron Back Scattered Diffraction (EBSD) separating porphyroclast and recrystallized grains. The deformation, which post-dates an earlier deformation phase and subsequent annealing, is strongly localized. The microstructures and textures change across the shear strain gradient and are interpreted to preserve a time sequence of progressive stages of deformation. The bulk shear strain rate is estimated to be about 10-11 sec-1 at deformation temperatures of approximately 325 °C ± 30 °C. The protomylonite is characterized by a core mantle structure with a bimodal grain size distribution which changes gradually to a completely dynamically recrystallized microstructure with a unimodal grain size distribution in the mylonitic center of the shear zone. Core-mantle-structures are produced by dominant rotation recrystallization accompanied by some grain boundary migration. The microstructural transition from protomylonite to mylonite coincides with a change in texture. With increasing strain the single c-axis maximum of an earlier inherited texture in the protomylonite is replaced by a similar texture in a different orientation (maximum normal to the shear plane) which is consistent with dominant basal 〈a〉 and r 〈-2201〉 slip. The microstructural and textural variations depend on the proportion of recrystallized grains. As dynamic recrystallization progresses with finite strain the texture development is finite strain-dependent. The comparison of the microstructures and textures to other natural and to experimental examples explains the progressive change of the texture and demonstrates the texture evolution produced by dynamic recrystallization.