Exploring the Rheological Properties of the Upper Mantle

Book excerpt: The ductile deformation and flow of the upper mantle controls a wide variety of geologic phenomena, including the flexure of plates below glaciers, the formation of new plate boundaries, and the movement of tectonic plates. Olivine, the main mineral of the upper mantle, controls mantle deformation behavior, responding differently to applied stresses depending on the deformation conditions (including temperature, pressure, and composition). This dissertation provides constraints on the rheological properties of the Earth's upper mantle across multiple deformation regimes. In Chapter 1, I use nanoindentation experiments to show that the strength of olivine deforming at low temperatures and high stresses is controlled by its grain size, with smaller grains being stronger than larger grains. As the lithospheric mantle has a relatively large grain size, the mantle is likely weaker than many previous studies have suggested. Deformation throughout the lithosphere is sensitive to small concentrations of water, down to the parts per million level in olivine. Chapter 2 contains a comprehensive description of the methodology necessary for measuring water in the nominally anhydrous minerals of the upper mantle: olivine, orthopyroxene, and clinopyroxene. Using this methodology, I measured water in samples from ductile shear zones in the Josephine Peridotite. In Chapter 3, I examine the relationship between water content and the lattice preferred orientation (LPO) of olivine. Previous experiments found that olivine LPO, and thus the dominant active slip system, changes with increasing water content. However, my measurements suggest that the olivine LPO in the shear zones does not vary as a function of water content, and I use micromechanical modeling to show that deformation kinematics can drive the observed LPO changes. Chapter 4 then examines the role of water in shear localization in the same set of shear zones. While water concentrations are elevated relative to other peridotite settings like mid-ocean ridges, my geochemical data indicate that melt was the initial factor that led to shear localization.