Englacial tephra layers in the East and West Antarctic ice sheets: indicators of glacial flow patterns on ice sheet margins.


 N W Dunbar, RP Esser, W.C. McIntosh, P.R. Kyle (E&ES Department, New Mexico Tech, Socorro, NM, 87801)
G.A. Zielinski (Climate Change Research Center, University of New Hampshire, Durham, NH, 03824)
 

Englacial volcanic ash (tephra) layers exposed in ablation areas on the margins of ice sheets can offer insight into the local and regional ice flow processes. These layers can be exposed in stratigraphic section when deep ice is pushed against a topographic barrier and ablated away. Seven such tephra-bearing blue ice field sites on the East and West Antarctic ice sheets have been mapped using GPS techniques and sampled in detail. The tephra layers from all of these sites appear to be true depositional units, based on the lack of contamination, distinct upper/lower contact nature with surrounding ice, and the unabraded nature of the fine volcanic glass shards.

Tephra layers at a number of the field areas (Allan Hills, 76o43S, 159o47E; Brimstone Peak,75o48S, 158o30E; Mt. DeWitt 77o11S, 159o52E) describe a series of sweeping curves, or folds that appear to correlate with areas of fast and slower ice flow. These folds correspond to known basement topography at Allan Hills and Brimstone Peak (Delisle and Seivers, 1991). Ice thinning of up to 90%, or more, has been observed in these areas, based on tephra layers spacing from fold limbs to noses. Tephra layers at Horseshoe Peak ( 77o30'S , 160o E ), on the margin of an outlet glacier of the East Antarctic ice sheet are highly thinned and aligned adjacent to the local outcrop, due to the inferred high ice flow rate and simple flow geometry. A thick and complex englacial tephra section at Mt. Waesche ( 77o25'S, 126o52'W) is highly folded and boudined, as a result of flow of the West Antarctic ice sheet around the 3292m volcano.