Preferred slip-band orientations and bending observed in the Dome Concordia (East Antarctica) ice core

Abstract

AbstractFabric analysis of the upper 1300 m of the Dome C (East Antarctica) ice core reveals a slight clustering tendency of c axes towards the vertical, which gradually enhances with depth from an initially isotropic orientational distribution of c axes at the free surface. Such a strain-induced anisotropy is compatible with the expected macroscale stress state in a dome, i.e. dominated by vertical compression. Yet, when one analyzes the orientational distribution of the visible gliding layers of individual crystallites (slip bands), the evidence is quite contrasting. Direct observation of slip bands in samples from the Dome C ice core taken from different depths (204–1291 m) indicates a higher slip activity in nearly horizontal planes, in such a manner that >60% of the detected slip bands have an inclination of <30˚ with respect to the horizontal. Furthermore, the observed slip activity is not symmetric, i.e. the number of slip bands discerned at 20˚, say, is usually not comparable with the number found at 160˚. Such features are not consistent with the predicted slip activity induced by compression and/or extension. In this work, we present evidence for this unexpected orientational distribution of slip bands and discuss some of the possible causes. Natural and artificial agents are investigated, together with their respective consequences for ice-sheet modeling and ice-core processing. Additionally, we show the occurrence of bent slip bands in certain crystallites. Such a bending represents an early stage of polygonization, and highlights the strong inhomogeneity of deformation at the crystal level. Moreover, it indicates that polygonization might be mathematically interpreted as a continuous process of rotation, characterized by the divergence of c axes from a common direction.