Masters Thesis

EBSD and microstructural analysis of quartzofeldspathic rocks from the South Mountains, Arizona: an evaluation of flow laws and crustal rheology

The strength of the middle and lower continental crust is approximated by experimentally-derived flow laws that describe plastic flow of single mineral phase or polyphase aggregates, but there is little consensus about which aggregate compositions may best characterize this part of the crust. In this thesis, I use microstructural and electron backscatter diffraction (EBSD) analyses to evaluate the strength of naturally-deformed quartzofeldspathic middle crust exposed in the South Mountains metamorphic core complex, Arizona. These results are compared to published quartz and feldspar experimental flow law data to evaluate the applicability of these flow laws for assessing the strength of middle continental crust. Microstructural analysis of the constituent minerals in the South Mountains granodiorite shows that strain is accommodated by crystal plastic deformation and dynamic recrystallization in quartz, limited crystal plasticity in orthoclase feldspar, and brittle microfaulting in plagioclase feldspar. The microstructures suggest that quartz is the weakest phase in the rock, and the crystal plastic deformation of the abundant quartz dominates the strain localization process during mid-crustal shearing. EBSD analyses reveal that dynamically recrystallized quartz grains exhibit lattice preferred orientation (LPO) indicative of basal and rhomb slip, and that orthoclase and plagioclase feldspar exhibit limited evidence for LPO and an absence of LPO, respectively. Together, the microstructural and EBSD data suggest that the operative deformation mechanism in the interconnected network of abundant quartz is dislocation creep, whereas the dominant deformation mechanism in the feldspars is brittle fracturing. I use the ‘wet’ quartz flow laws of Hirth et al. (2001) and Rutter and Brodie (2004) to create deformation mechanism maps in which I compare the predicted strength of quartz aggregates to the observed strength of the granodiorite mylonites of the South Mountains metamorphic core complex. Given the deformation conditions of the South Mountains mylonites, the strength of these rocks are best fit by the quartzite flow law of Hirth et al. (2001) at temperatures ~500ºC, and the quartzite flow law of Rutter and Brodie (2004) at temperatures of ~600ºC. The results suggest that the strength of quartz may be a proxy for the bulk strength of the rock during ductile deformation, and that quartz flow laws may provide the best estimate of the strength of deforming continental crust where quartz is abundant, interconnected, and deforming by dislocation creep.

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