Mechanisms and longevity of strain localization during dynamic recrystallization of olivine
The mechanisms that lead to localized deformation within Earth’s lithosphere are critical to the formation and longevity of tectonic plate boundaries. Large-scale geodynamic models that incorporate parameters for rheological ‘damage’ and ‘healing’ are most successful at reproducing self-consistent plate-like behavior on Earth and other terrestrial planets, but the microphysical processes that maintain localization are not well understood. While the preservation of fine grain sizes in crustal and mantle shear zones suggests that grain size reduction (damage) by dynamic recrystallization is one of the most important mechanisms of strain localization, subsequent grain growth (healing) may inhibit localization. In this dissertation, I describe three projects focused on evaluating these processes and their contribution to the persistence of plate boundaries. In Chapter 2, I test the consistency of existing stress-grain size piezometers that are used to estimate the strength of the lithosphere. Using quartz–feldspar- and olivine–orthopyroxene-bearing mylonites, which are common mineral pairs in crustal and mantle rocks, I quantify new piezometers for feldspar and orthopyroxene that are consistent with observations from natural rocks, and can be used to estimate stress magnitudes in rocks such as granulites and pyroxenites, for which the quartz and olivine piezometers are unsuitable. In Chapter 3, I quantify the rate of syn- and post-deformation grain growth in experimentally deformed, wet natural olivine aggregates, and compare it to that predicted by the existing olivine grain growth law to highlight the influence of microstructurally stored strain energy on grain growth. I derive a new olivine grain growth law that predicts significantly slower growth than previously reported for wet olivine, suggesting that grain size reduction by dynamic recrystallization plays an important role in the longevity of strain localization. In Chapter 4, I compare the mechanical and microstructural evolution of dry olivine aggregates that I experimentally deformed under both constant strain rate and constant stress conditions, and use detailed microstructural analysis to identify the mechanisms that accommodate strain weakening during and after grain size reduction. The results suggest that strain localization in monophase olivine does not occur at constant strain rate conditions but is common under constant stress boundary conditions.