Search

Abstract: The rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field, experimental and numerical modelling studies, often centred on the question whether objects rotate or not in non-coaxial deformation. With numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations. Originally, shape and orientation were investigated, while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century. Due to improved numerical techniques, anisotropic rheology has become the focus of most recent studies, indicating that it is a primary factor in the rotation behaviour of objects. We present new simulations investigating the role of anisotropy on different scales relative to the object, and show how this influences the rotation rate, as well as the inclusion patterns in case of syntectonically growing porphyroblasts. These simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects. The variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field. [Copyright &y& Elsevier]

Abstract: The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (W m) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating W m. The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used W m plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating W m, the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones. [Copyright &y& Elsevier]

Abstract: This paper studies the flow heterogeneity around porphyroclasts associated with greenschist facies deformation of a calcite marble shear zone. Microstructural data from electron backscatter diffraction analyses (EBSD) are used to constrain the flow mechanics of this dominantly non-coaxial type of deformation. The microstructure of the undisturbed ultramylonite (grain-size range 5–100 μm, mean ∼40 μm) is interpreted to represent steady-state (time-independent) flow conditions with flow planes parallel to the shear zone boundary. Single calcite porphyroclasts (grain-size 1–3 mm) caused flow perturbation in the fine-grained marble ultramylonite. It is the shape, in particular, of these rigid porphyroclasts that controls their rotational behaviour during deformation and, therefore, the development of specific flow fabrics. The flow planes around elongated-rhomboidal, stable porphyroclasts change the orientation to become roughly parallel to the porphyroclast margin, whereas the geometry of flow planes around nearly equant, rotating porphyroclasts describes a δ-type flow pattern. We infer that to some extent decoupling at the clast–matrix interface has occurred to guarantee a stable orientation of elongated porphyroclasts, but was not sufficient to reduce the rotation rate of equant clasts to zero. According to the flow deflection, the general crystallographic preferred orientation (CPO) with its single c-axis maximum perpendicular to the flow plane is rotated about an axis which is (sub)parallel to the kinematic rotation axis of the shear zone. Ultramylonite microstructures, CPOs and misorientation data are best explained by the dual operation of grain-size-insensitive (dislocation creep with recovery and recrystallization) and grain-size-sensitive (diffusion creep) mechanisms. The limited grain-size reduction around porphyroclasts suggests that the grain-size-insensitive mechanisms controlled rheology. [Copyright &y& Elsevier]

Within a greenschist facies calcite marble shear zone, isolated quartz grains change shape across the shear zone profile. Whereas quartz grains have a spherical to elongated shape in the coarse grained marble protolith and protomylonite, they are asymmetric σ-shaped porphyroclasts with wedge shaped appendages (wings) in ultramylonites. In all cases quartz grains are single-crystals. They are sometimes twinned but never recrystallized. Stress-induced dissolution–precipitation creep is the favourable shape-controlling process during deformation providing a relatively undeformed core, truncation of pre-existing cathodoluminescence (CL) patterns and syntaxial precipitation of wings. The necessary fluids may have been released from fluid inclusions during dynamic recrystallization of the calcite matrix. The development of σ-shape is not related to the crystallographic orientation of the quartz porphyroclasts. Crystallographic orientation analysis by electron backscatter diffraction (EBSD) and CL analyses exclude crystal-plasticity as a shape-controlling mechanism of quartz grains during mylonitisation. However, in ultramylonites quartz clasts have a strong crystallographic preferred orientation (CPO), with c-axes (sub)parallel to the shear direction. This fabric is uncommon for quartz under greenschist facies conditions. It might be explained by a strengthening of a pre-existing weak CPO during mylonitisation by rigid particle rotation of elongated quartz grains. [Copyright &y& Elsevier]