Crystalline textures and fabrics of ice cores from the 2,164-m-thick ice sheet at Byrd Station, Antarctica, reveal the existence of an anisotropic ice sheet. A gradual but persistent increase in the c-axis preferred orientation of the ice crystals was observed between the surface and a depth of 1,200 m. This progressive growth of an oriented crystal fabric is accompanied by a twentyfold increase in crystal size between 56 and 600 m, followed by virtually no change in crystal size between 600 and 1,200 m depth. A broad vertical clustering of c axes develops by 1,200 m. Between 1,200 and 1,300 m, the structure transforms into a fine-grained mosaic of crystals with their basal glide planes now oriented substantially within the horizontal. This highly oriented fine-grained structure, which persists to 1,800 m depth, is compatible only with a strong horizontal shear deformation in this part of the ice sheet. Rapid transformation from single- to multiple-maximum fabrics occurs below 1,800 m. This transformation, accompanied also by the growth of very large crystals, is attributed to the overriding effect of relatively high temperatures in the bottom layers of old ice at Byrd Station rather than to a significant decrease in stress.
The zone of single-maximum fabrics between 1,200 and 1,800 m also contains numerous layers of volcanic dust. Fabrics of the very fine grained ice associated with these dust bands indicate that the bands are actively associated with shearing in the ice sheet.
Some slipping of ice along the bed rock seems likely at Byrd Station, since the basal ice is at the pressure melting point and liquid water is known to exist at the ice-rock interface. The textures and fabrics of the ice indicate that plastic deformation (intracrystalline glide) in the zone of strong single-maximum fabrics and movement of ice along discrete shear planes situated well above bed rock are also major contributors to the flow of the ice sheet. Any extensive shearing at depth could seriously distort stratigraphic records contained in the ice cores, such as climatic history as inferred from stable isotope analysis. Also, the common practice of using simplified flow models to approximate the depth-age relationships of deep ice-sheet cores may need to be revised in light of the deformational features and fabrics observed in the Byrd Station ice cores.