The plasticity of the Arctic ice pack depends on its granular nature, in particular on the size and distribution of areas of thin ice and open water surrounding multiyear ice floes. The paper begins with construction of a mesoscale (10--100 km) granular model of the central Arctic ice pack. The mesoscale model is based on a dynamic particle simulation in which individual multiyear ice floes and surrounding parcels of first-year ice are explicitly modeled as discrete, convex polygons in a two-dimensional domain. Deformation of the domain produces areas of localized failure and areas of open water. The areas of localized failure are modeled as pressure ridging events using the results of numerical experiments performed with a computer simulation of the ridging process. The paper focuses on the results of numerical experiments performed with the mesoscale model. In the experiments the model ice pack is biaxially deformed at constant strain rates. The principal strain rates are varied to create deformation states ranging from pure shear to uniform compression. The results define the shape and magnitude of the plastic yield surface, the strain rate vectors associated with points on the yield surface, the partition of energy dissipation between ridging and in-plane sliding, and the changes in the ice thickness distribution associated with various deformation states. ¿ American Geophysical Union 1996