Structures at the Base of an Ice Fall

Abstract

The zone of renewed flow at the base of an ice fall on the Gulkana Glacier, in the eastern Alaska Range, lies below the firn limit, and thus the structures in the perfectly exposed ice can be studied in detail. Mapping of both major and minor structures demonstrates a characteristic style of progressive deformation which leads to an increased understanding of the relationship between flow and structural evolution of the ice, and by analogy, to similar processes in rocks. At the base of the fall, and particularly where the transition between fall and flow is rapid, a series of small cone-shaped topographic surfaces develops. Each is a distinct flow unit, and in each the first and dominant evidence of flow is the formation of penetrative planar structures in the ice. At the ice surface, this foliation forms with an arcuate pattern; in three dimensions, single foliation surfaces approximate inverted cones with steep axes. During continued flow, the initial foliation is rotated, and a new set of arcuate foliation planes develops which cuts the older structures. A number of interesting minor structures are associated with these crosscutting events, two of which are particularly important in working out the mechanism of foliation formation: (1) thin zones of intense shearing parallel to the active foliation, and (2) minor folds, developing in sequence from incipient similar folds to mushroom-shaped folds, always with the active foliation as the axial plane. An interpretation of this progressive fold development leads to the concept of similar folds by single-sense shearing, and the subsequent buckling of these folds by a component of shortening across the axial plane. The dominant role of shear in the formation of these structures demonstrates that foliation in ice is fundamentally a shear phenomenon. The small flow units at the base of the ice fall are, in effect, tributaries to the main ice stream, and as they merge with the larger units, and finally with the main stream, the foliation patterns concomitantly develop larger and larger arcs. Evidently the entire sequence of formation and deformation of the foliation and associated structures, on both small and large scale, is the principal process by which the differential flow of the ice mass is accomplished.