MICRO-RELIEF AND DEBRIS COVER DEVELOPMENT IN POLYGONAL PATTERNED ICE IN ANTARCTICA: A MODELING PERSPECTIVE

Contraction cracks are ubiquitous in the Dry Valleys of Antarctica where they define extensive areas of polygonal patterned ground. Despite the hyper-arid conditions of the region, the contraction cracks reflect extensive ground ice and, in places, massive subsurface ice beneath a debris mantle. The micro-relief and characteristic of the debris layer provide useful clues about the nature of the subsurface, and because they can be detected remotely, they are likely to be of interest in studies of permafrost regions not only on earth but also on Mars.

Herein, we describe a modeling approach aimed at better understanding the micro-relief and debris cover that develop over a long time above massive debris-rich ice that is actively sublimating. Any reasonable quantitative description must account for the progressive growth of sand-wedges or ice-wedges at sites of contraction cracking, extensive sublimation of the ice and surface accumulation of debris formerly contained in the ice, modulation of ice sublimation rate by the protective debris cover, and lateral motion of the debris driven by surface slope as the micro-relief develops. The model unifies these processes to produce a slowly evolving micro-topography and a debris-layer thickness distribution that are comparable to those in actual polygons. It also calculates deformation fields in the ice and debris that are consistent with the spatially varying sublimation rate, steeply dipping foliation commonly seen in the ice at the periphery of polygons, and exposure age histories obtained from vertical profiles of cosmogenic isotopes in the debris cover, as well as in the ice. Our model results help define the time scale over which the ground surface would be completely reworked, which has rich implications for the age and interpretation of landscape surfaces in Antarctica.