Re-orientation of individual crystal glide planes as isotropic surface
ice is deformed during its passage to depth in an ice sheet creates a
fabric and associated anisotropy. A simple macroscopic description is
that these material glide planes are rotated towards planes normal to
an axis of compression, and away from planes normal to an axis of ext
ension, inducing an instantaneous orthotropic viscous response with re
flexional symmetries in the planes orthogonal to the current principal
stretch axes. An associated orthotropic viscous law expresses the str
ess in terms of the strain-rate, deformation, and three structure tens
ors based on the principal stretch axes. General frame indifferent for
ms are analysed to determine the fabric induced during differential st
retchings along fixed principal axes. Then, freezing the fabric by rem
oval of the stress, and hence strain-rate, the instantaneous simple sh
ear responses in different planes are determined and compared. The cor
responding instantaneous viscosities are expressed in terms of the res
ponse coefficients of the constitutive model. A simple model is adopte
d to illustrate the evolution of the viscosities during axial stretchi
ngs and shearing.