Relating Lattice Preferred Orientation to Deformational Process Using Statistical Analysis of Symmetry in Orientation Distribution Space
Christopher Thissen, Yale University
Mark Brandon, Yale University
We introduce and apply a new method for analyzing the symmetry of lattice preferred orientation (LPO) fabrics in deformed rocks. At present, these fabrics are usually analyzed empirically, through comparison with experimental results and inspection for asymmetry in pole figures. The symmetry elements in the structures of LPO fabrics are related to the symmetry of the deformational processes that created the LPO and can be used to interpret the symmetry of the deformation, such as pure shear vs. simple shear, plane strain vs. flattening vs. constriction, and the sense of shear in noncoaxially deformed rocks. Our analysis focuses on finding the set of statistically significant 2-fold rotation axes in the LPO fabric. We expect a simple-shear deformation to contain only a single two- fold rotation axis, perpendicular to the shear direction and parallel to the shear plane. In contrast, a coaxial flattening deformation (Sx=Sy>Sz) would have a great circle distribution of rotation axes, with the pole of the great circle equivalent to the maximum shortening direction. To estimate the two-fold rotation symmetry elements of an LPO data set, we search a grid of rotation axis orientations in the lower hemisphere of a stereonet, and rotate the LPO distribution around this axis to create a second, rotated LPO distribution. The original and rotated distributions are compared in orientation distribution space using a Kolmogorov-Smirnov test to measure the probability that the distributions are similar, save for random variation. This analysis allows us to simultaneously consider the symmetry of all of the measured crystallographic directions. Here, we apply our technique to analyze the deformation symmetry from a suite of samples collected across the Moine Thrust, Scotland.