Rate controlling processes in the creep of polar ice, influence of grain boundary migration associated with recrystallization

Information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from Greenland and Antarctica. It is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration (gbm). A deformation model based on an equilibrium between work-hardening and recovery processes has been developed. The decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization. The value of the dislocation density along the Byrd and GRIP ice cores obtained thanks to the model is about 1x10(11) m(-2). This value is in agreement with data from synchrotron X-ray diffraction on samples taken along the core. This deformation model can account for the deformation of polar ice at low stresses. It is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process. (C) 2000 Elsevier Science B.V. All rights reserved.