Sillimanite deformation mechanisms within a Grt-Sil-Bt gneiss: effect of pre-deformation grain orientations and characteristics on mechanism, slip-system activation and rheology
Sandra Piazolo, Paulina Jaconelli, 2014. "Sillimanite deformation mechanisms within a Grt-Sil-Bt gneiss: effect of pre-deformation grain orientations and characteristics on mechanism, slip-system activation and rheology", Deformation Structures and Processes within the Continental Crust, S. Llana-Fúnez, A. Marcos, F. Bastida
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Sillimanite-bearing metapelites are common in the middle to lower crust and play an important role in the rheology of these crustal levels. We report deformation microstructures in a metasedimentary gneiss (Greenland) produced during simple shear at c. 700 °C and c. 4.5 kbar. Electron backscatter diffraction analysis shows activation of the (001) and (001) slip systems in large grains. Additionally, (100) and (010) are activated in needles with their c-axes parallel to lineation. Medium-grained sillimanite aggregates mixed with biotite show a weak crystallographic preferred orientation (CPO) suggesting activation of (001) and (001) together with grain boundary sliding (GBS). GBS is the dominant deformation mechanism in the outer parts of fine-grained clusters of sillimanite stemming from andalusite replacement. Associated features are phase mixing, clockwise dispersion of c-axis orientations and change of misorientation axes from original c-axis rotation to rotation close to the kinematic Y axis. The latter features which result in strain localization around these clusters allow their ‘preservation’. Rheologically, the medium-grained aggregates together with zones around clusters take up most of the deformation. Our study shows that deformation mechanism and slip-system activation, and CPOs are strongly dependent on the orientation relationship between stress axes and pre-deformation grain orientations and characteristics.
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Deformation Structures and Processes within the Continental Crust
This volume presents a selection of papers showing the current focus of studies of deformation structures and processes within the continental crust. The selected contributions use a large range of analytical techniques suited to the full range of structure sizes and fine-tuned to the physical process that controls the deformation, from the grain boundary at the micro-scale, the lithological contact at the meso-scale to the plate boundary at the global scale.
The papers in the volume are grouped into three sections relating to specific lines of research within the analysis of rock deformation structures and processes, in particular in respect to the continental crust: structures within shear zones and faults; magmatic structures, and microstructures and rheology. These sections include papers describing field studies, experimental rock deformation and numerical modelling of deformation processes.