Deformation of the Ross Ice Shelf, Antarctica

Abstract

The structural features in the Camp Michigan area on the northeastern margin of the Ross Ice Shelf, Antarctica, include anticlines, synclines, drag folds, crevasses (some similar to strike-slip faults, others to joints) and rifts (extraordinarily wide crevasses). Strain rates were measured on the surface of the ice shelf by determining the relative positions of stakes in the ice at various times by triangulation. Nye9s flow law for ice is obtained by a systematic simplification of the generalized flow law proposed by Glen. Glen9s laboratory data on ice creep were used to determine the necessary constants in Nye9s flow law, and the stresses were calculated on this basis. A fracture criterion for ice is formulated on theoretical grounds. This criterion may be described as a surface of revolution about the hydrostatic stress line in stress space. The stresses calculated from strain rates measured in actively crevassing areas are considered “unstable” and are greater than the actual stresses present. They would cause rupture in the material if they were actually present. Stresses in noncrevassing areas are designated as “stable.” A plot of stable versus unstable stresses shows good agreement with the theory. Folding in the area was analyzed in accordance with the method of Biot. Even though an exact stability analysis is used, the predicted wave lengths agree only with the wave length of the youngest folds. The fracture criterion is extended to rocks, but the available data are insufficient to definitely establish its validity for rocks. The principle of least work indicates that faults do not necessarily include the intermediate principal stress axis in the plane of the fault, but the unavailability of data precludes formulation of a usable criterion. Folding in rocks is amenable to study by Biot9s theory, but lack of data prohibits wave-length prediction for geologic examples.