The initiation of fractures and fast flow in floating regions of Antarctica have the poten-

7 The initiation of fractures and fast flow in floating regions of Antarctica have the poten8 tial to destabilize large regions of the grounded ice sheet, leading to significant sea-level 9 rise. While observations have shown rapid, localized deformation and damage in the mar10 gins of fast-flowing glaciers, there remain gaps in our understanding of how rapid defor11 mation affects the viscosity and toughness of ice. Here we derive a model for dynamic 12 recrystallization of ice that includes a novel representation of migration recrystallization. 13 This mechanism is absent from existing models and is likely dominant in warm areas un14 dergoing rapid deformation, such as shear margins in ice sheets. While solid earth stud15 ies find fine-grained rock in shear zones, here we find elevated ice grain sizes (> 10 mm) 16 due to warmer temperatures and high strain rates activating migration recrystallization. 17 Large grain sizes implies that ice in shear margins deforms primarily by dislocation creep, 18 suggesting a flow-law stress exponent of n ≈ 4 rather than the canonical n = 3. Fur19 ther, we find that this increase in grain size results in a decrease in tensile strength of 20 ice by ∼ 75% in the margins of glaciers. Thus, this increase in grain size softens the mar21 gins of fast-flowing glaciers and makes ice shelf margins more vulnerable to fracture than 22 previously supposed. These results also suggest the need to consider the effects of dy23 namic recrystallization in large-scale ice-sheet modeling. 24