ORIGIN AND DEVELOPMENT OF TEXTURES AND FABRICS IN BASAL ICE AT SUMMIT, CENTRAL GREENLAND

Detailed textural and crystallographic characteristics of the bottom 6 m of the GRIP core (Summit, Central Greenland) are presented. The results are discussed in the light of present-day fabric analogues. Ice fabrics are shown to retain the mark from the entrainment of locally formed ice by the growing ice sheet. This mark is only partly overprinted by the present-day stress configuration at the ice divide. These results corroborate the model put forward by Souchez et al. [1] on stable isotopes and gas composition grounds, in which the basal silty ice appears as a remnant of a growing stage of the ice sheet. During this stage, which probably represents the original build-up, ice of local origin formed at the ground surface was overridden by the main ice sheet and incorporated into its base. Preservation of remnants of ancient fabric patterns and occurrence of annealing recrystallization fabrics just above the first layer of silty ice are used, amongst other arguments, to demonstrate that this basal ice is not presently subject to large cumulative strains. The implications for flow and dating models are discussed.