This project investigates the role of grain size reduction by dynamic recrystallization in promoting strain localization in dry olivine aggregates using a suite of laboratory experiments coupled with detailed microstructural analyses. There are two primary goals of the project. The first is to identify the ways in which dynamic recrystallization can lead to localization under different conditions of stress, temperature, grain size, and strain magnitude. The processes that are most commonly proposed in the literature include a) geometric softening, b) a switch to grain size- sensitive deformation mechanisms, either diffusion creep or grain boundary sliding, and c) recovery by grain boundary migration. One or all of these mechanisms may contribute to the pronounced weakening observed in olivine under specific experimental conditions and in nature, and each should exhibit unique mechanical and microstructural signatures. The investigators will integrate detailed microstructural and mechanical observations over a range of experimental conditions, to systematically determine which of these mechanisms are dominant. The second goal is to examine the conditions under which dynamic recrystallization will result in permanent, as opposed to transient localization, by evaluating the role of syn-deformational grain growth. The project uses several unique ways of quantifying the role of grain growth in dynamically recrystallizing olivine aggregates, and of distinguishing between surface energy and strain energy driven grain boundary migration for a range of experimental conditions. The results of this research will have several important implications for geodynamics and mantle rheology, including the following. 1) The mechanical data will improve current deformation mechanism maps for dry olivine and can be incorporated into both large-scale mantle convection models and smaller-scale models of transient instabilities and mantle seismicity. 2) The processes identified as contributing to the development of olivine lattice preferred orientation can be input into models of olivine olivine lattice preferred orientation evolution and associated seismic anisotropy. 3) Distinguishing the conditions under which grain growth will counteract grain size reduction will allow evaluation of theoretical descriptions of the relationship between stress and grain size (the piezometric relationship). 4) The identification of microstructural criteria that are diagnostic of specific localization processes can be extrapolated to naturally deformed rocks and used to infer localization mechanisms and associated mechanical behavior of rocks under natural conditions.
Understanding why deformation in Earth's rigid outer shell, the lithosphere, is commonly localized into faults and shear zones, rather than distributed over wide distances, is a fundamental question in geodynamics. These localized faults and shear zones are unique to planet Earth and are the reason parts of Earth exhibit rigid, plate-like behavior, in contrast to the more distributed deformation typically observed on other planets. Decades of observation of faults and shear zones where they cut the crust and upper mantle reveals that they are almost always associated with a significant reduction in grain size, which suggests that grain size reduction may be one of the most efficient mechanisms of localizing deformation. This project investigates mechanisms of grain size reduction in mantle rocks. Specific questions to be addressed include: under what conditions does grain size reduction lead to localization in mantle rocks? By what processes does the grain size reduction cause localization? And how long will the localization last on geological timescales? These questions will be addressed through integrated rock deformation experiments on olivine as well as detailed microstructural analysis of the experimentally deformed products.
