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Abstract

Metamorphism is intrinsically linked to the long-term strength of middle to lower crust, which in turn controls the extent and distribution of regional deformation. Our collective understanding of the strength of metamorphic rocks, namely the rheological behavior and deformation mechanisms of individual minerals, relies upon decades of carefully controlled rock deformation experiments. As these experiments are conducted over short time frames and often with either single phase or previously equilibrated multi-phase aggregates, by design they typically do not capture the chemical processes (i.e. metamorphism) that operate in the lithosphere over million year timescales. To better understand how the interaction between metamorphism and ductile deformation affects the strength of the mid- to lower crust, we need to: (1) compare mineral microstructures from deformation in both natural and experimental settings to unravel how the deformation mechanisms of metamorphic minerals evolve alongside metamorphic processes; and (2) obtain empirical measurements directly from metamorphic minerals to estimate the bulk stresses within a polymineralic crust. In this thesis, I investigated how subgrain-size piezometry can be applied to the mineral assemblages of metamorphic rocks from a range of geodynamic environments to better understand the interplay of metamorphism and mineral deformation. In each chapter I apply a range of characterization techniques to document mineral chemistry and microstructures, and on the basis of these data infer the deformation mechanisms responsible for their formation. Using a subset of amphibole-bearing rocks from a subduction zone setting, I demonstrate how the higher inherent effective diffusivity and greater abundance of chemical reactions within polymineralic rocks may impact the active deformation mechanisms. I also demonstrate how stress estimates from subgrain-size piezometry can be coupled with thermodynamic modelling and single-mineral thermometry to infer the stress that rocks underwent at specific portions of their metamorphic histories. I find that the stress estimates derived from multiple mineral phases in a single rock are most consistent with isostress conditions in shear zones, wherein the bulk strength of ductile crust is defined by the strength of the weakest interconnected phase. My thesis and the approaches documented herein serves as a guide for petrologists to accurately attribute subgrain-size piezometric stress estimates to metamorphic conditions as well as for researchers to test theoretical models of the strength of the metamorphic crust using polymineralic rocks deformed in shear zones.