Inferring Microtectonic Vorticity Axes from Crystallographic Orientation Dispersion

Rocks that deform by crystal-plastic mechanisms typically contain grains that exhibit some amount of crystallographic distortion (e.g., kinking, undulose extinction, subgrain development). Previous studies have shown a compelling link between the dispersion axis of a distorted crystalline lattice and the rotational axis (vorticity axis) of grain-scale deformation. This dissertation builds on these previous observations to develop a new method of microstructural analysis -- Crystallographic Vorticity Axis (CVA) analysis. CVA analysis can be used to quantify intragranular orientation dispersion and identify a best-fit dispersion axis. Importantly, the application of this approach shows that a best-fit axis of a single grain does not often match the inferred bulk vorticity. However, a population of dispersion axes aggregated from many grain-scale calculations can be used to calculate a preferred vorticity axis. Chapter 1 of this dissertation focuses on development of the CVA method and presents results from quartzites in three different monoclinic shear zones (Moine thrust zone, Gem Lake shear zone, and western Idaho shear zone). Chapter 2 summarizes results from CVA analyses in different lithologies along kinematically distinct segments of the western Idaho shear zone. Chapter 3 further explores the utility of CVA analysis for field-scale studies of fabric development in multiply deformed geologic terranes. This dissertation demonstrates that CVA analysis can be used to determine the orientation of vorticity axes in a range of deformed lithologies, including quartzite, marble, orthogneiss, and peridotite. Furthermore, when considering vorticity estimates from multiple locations within a field area, CVA analysis may be used to recognize zones of overprinting deformation with different kinematic signatures.