Rheological Implications of the Internal Structure and Crystal Fabrics of the West Antarctic Ice Sheet as Revealed by Deep Core Drilling at Byrd Station.

摘要

Crystalline textures and fabrics of ice cores from the 2164-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 1200m. This progressive growth of an oriented crystal fabric is accompanied by a 20-fold increase in crystal sized between 56 and 600m, followed by virtually no change in crystal size between 6000 and 1200m. A broad vertical clustering of c-axes develops by 1200m. Between 1200 and 1300 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 1800m, 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 1800m. This transformation, accompanied 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 1200 and 1800 m also contains numerous layers of volcanic dust. Fabrics of the very fine-grained ice associated with these dust bands indicate the bands are actively associated with shearing in the ice sheet. Some slipping of ice along the bedrock 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.