| dc.description.abstract | The rheology of the middle crust and the upper mantle dictates the rates at which geological processes occur. In these regimes, deformation predominantly occurs by ductile flow, meaning either dislocation creep or grain size sensitive creep. The mylonites that form as a consequence of ductile flow commonly consists of more than one mineral phase. This complicates the rheological behaviour of the system, as phases might deform by different mechanisms and at different strengths, or strain rates. Due to this complexity, the rheological strength is usually approximated by the mechanically weakest phase. In the middle crust, this weakest phase is quartz, and in the upper mantle it is olivine, when assuming deformation by dislocation creep. However, this approximation is not realistic for all mylonites, potentially leading to an underestimation of strain rates. In this study, the rheological behaviour of polyphase mylonites that highlight the importance of feldspar and orthopyroxene in mid-crustal and upper mantle rocks respectively is explored. To do so, sheared pegmatites from the Club Med shear zone on the Cap de Creus Peninsula and sheared ultramafic rocks from the Turon de Técouère body are researched. The deformation conditions for both these locations are well constrained in previous studies. Central in this study is the determination of deformation mechanisms in the major rock forming minerals. Polarized light microscopy is used to identify recrystallization processes, and to identify structures that indicate the relative competence of layers in the mylonites. The sheared pegmatite samples are unique as they show direct microstructural evidence of a competence contrast between stronger monomineralic quartz veins and a weaker albite rich matrix. The ultramafic rocks, in contrast, show no microstructural evidence for a competence contrast between olivine and orthopyroxene rich layers. Electron back-scatter diffraction microscopy (EBSD) is then used to quantify grain sizes, crystallographic preferred orientations (CPO), shape preferred orientations (SPO), and the interaction between mineral phases. Flow conditions in the Club Med shear zone are calculated using piezometers and a wattmeter. These results are then used in flow laws to mathematically quantify the flow behaviour. Subsequently, the mathematical results are tested against the microstructural results. For the Club Med sheared pegmatites, monomineralic quartz veins are found to deform by grain boundary sliding assisted dislocation creep. The fine grained feldspar (albite) rich matrix deforms by grain size sensitive creep. Flow laws confirm that the feldspar layers are mechanically weaker than the quartz veins, thus corresponding to the microstructural observations. Grain size sensitive creep in albite was enabled after grain size reduction by cataclasis, while quartz deformed by dislocation creep. In the Turon de Técouère ultramylonite, olivine (forsterite) and orthopyroxene (enstatite) both deformed by grain size sensitive creep. Although there is large variability between flow laws, the mechanical strength of olivine, and orthopyroxene are found to be similar, while having a similar grain size. This study therefore proves the importance of feldspar and orthopyroxene in controlling the behaviour of a shear zone in the middle crust and upper mantle. | |
