Multimaxima crystallographic fabrics (CPO) in warm, coarse-grained ice: New insights

Multimaxima crystallographic c-axis fabrics (CPOs) commonly observed in coarse-grained ice have been interpreted in several different ways but remain enigmatic. We review previous interpretations of these fabrics and present new data from Storglaciären, northern Sweden, for comparative purposes using both U-stage methods (c-axes only) and electron backscatter diffraction (EBSD) (both c-axis and a-axis orientations). Consistent with most previous studies, microstructures in our study indicate that all grains have been dynamically recrystallized with grain boundary migration as the dominant process. The CPO is characterized by grains oriented favorably for easy glide on the basal plane, given the local kinematics. When a representative sample is obtained, areas on the glacier margin undergoing non-coaxial deformation have a CPO generally consistent with simple shear. Multimaxima CPOs developed in the center of the glacier, which is associated with more coaxial deformation, likely represent a small circle girdle or partial girdle fabric associated with flow-parallel compression. In both locations, sampling bias, resulting from repeat measurements of single parent grains appearing several times as “island grains” in a 2D thin section, is likely responsible for the observed multiple maxima that, with bias removed, simplify to patterns expected for simple shear or coaxial deformation. An earlier study suggesting that multimaxima CPOs may result from twinning is not borne out from our data combining c-axes and a-axes that do not show the symmetry required of twinning.