DISTINGUISHING DISTINCT DEFORMATION DOMAINS WITHIN A GRANITIC STRUCTURAL DOME USING EBSD-DERIVED CRYSTALLOGRAPHIC VORTICITY AXIS (CVA) ANALYSIS

Electron Backscatter Diffraction (EBSD) and Anisotropy of Magnetic Susceptibility (AMS) data are integrated to identify domains of distinct deformation within the Paleoarchean Mt Edgar dome, Western Australia. The Mt Edgar dome is an archetypal multicomponent TTG archean dome, and the crustal processes of its formation provide context for early Earth tectonic processes. The approach taken in this presentation addresses a key challenge for mapping deformation domains in granitic domes. Strain information — orientation of foliation and lineation as well as the shape of fabric (prolate to oblate) and degree of anisotropy — does not include information about kinematics or strain path (e.g., coaxial or non-coaxial), and so lack critical information about the flow history of dome rocks. In weakly deformed granitic rocks such as those found in the Mt Edgar dome, microstructural evidence for kinematics is often not observable at the thin section scale. The lack of kinematic or strain path data prevents robust interpretation of strain in terms of dome flow. This study uses EBSD-derived crystallographic vorticity axis (CVA) analysis to document variations in strain path across the dome. CVA analysis computes a vector of best fit intragranular distortion for thousands of grains across a thin section. The mean of these vectors is considered to be the bulk vorticity axis of the thin section, and a density contoured pole figure shows the variability of vorticity within the thin section. The CVA results are analyzed in two reference frames. First, they are compared across the map in the geographic reference frame to identify areas with parallel vorticity orientations. Second, the CVA results are analyzed in the strain (XYZ strain axes) reference frame to distinguish between coaxial, non-coaxial, and 2D vs. 3D strain paths.