Flow fabrics around porphyroclasts in a calcite marble shear zone
Michel Bestmann, David J. Prior
Department of Earth Sciences, Liverpool University, L69 3GP, UK
Mylonites can contain porphyroclasts that occur as relatively rigid inclusions embedded in a fine-grained, ductile matrix even though both are made up of the same mineral. These rigid objects affect the crystal-plastic flow in shear zones and cause flow heterogeneity. Porphyroclasts and their fine-grained surroundings allow insights into flow properties of mylonite zones. In this study we present lattice orientation analyses of flow fabrics around two different shaped calcite porphyroclasts within a calcite marble shear zone by means of electron backscatter diffraction (EBSD) technique.
The calcite shear zone, located in the Thassos-Marble (Greece), is characterized by pervasive dynamic recrystallization with shear planes parallel to the shear zone boundary due to dominant non-coaxial deformation (Bestmann et al., 2000). The general crystallographic preferred orientation (CPO) is characterized by a single c-axis maximum perpendicular to the SZB and distinct a-axis maxima within the shear plane. It is the shape, in particular, of the rigid inclusions that controls their rotational behaviour during deformation and therefore the development of specific flow fabrics. Whereas elongated porphyroclasts seem to be non-rotating obstacles resulting in a s-geometry, the nearly round inclusions seem to be rotated towards a kind of d-type porphyroclast system (Passchier and Simpson, 1986). Changes in the orientation maps illustrate for both analysed microstructures distinct CPO heterogeneity of the crystal-plastic flow fabric around the porphyroclasts (Fig.1). The general CPO of the ultramylonite is rotated about an axis approximately oriented in the shear plane and perpendicular to the shear direction (kinematic rotation axis of the shear zone). Also the crystal-plastic deformation within the porphyroclasts and the initial process of recrystallization-accommodated dislocation creep within the fine-grained ultramylonite matrix is controlled by the bulk shear kinematic. Misorientation axes of low angle boundaries cluster around the kinematic rotation axis of the shear zone.
Fig. 1. EBSD orientation maps generated from one of the Euler angles (f1). Change in grey level illustrate CPO heterogeneity of the fine-grained calcite matrix around the different shaped porphyroclasts. Bulk sense of shear is given.
Bestmann, M., Kunze, K., Matthews, A., 2000. Journal of Structural Geology 22, 1789-1807.
Passchier, C.W., Simpson, C., 1986. Journal of Structural Geology 8, 895-910.