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Abstract

In most current ice sheet models the rheological properties of ice are usually
assumed to be either isotropic or some effects of anisotropy are taken into account
by the use of a simple constant enhancement factor. Studies of ice cores have
shown that in natural ice sheets approximately isotropic ice only exists at shallow
depths typically less than several hundred metres. The large amounts of deeper
ice have developed strong anisotropic crystal orientation fabrics. The results from
both field and laboratory studies have indicated that the anisotropy due to flowinduced
crystal orientations has a considerable effect on ice flow rate, therefore,
the assumption of isotropic ice is no longer appropriate when anisotropy of the ice
crystal fabric develops and we need to take account of the interaction between
crystal orientation and rates of deformation. This project aims to incorporate into
an ice sheet rnpdel the effect of ice fabric anisotropy on the flow of ice.
A model for anisotropic ice flow in a polar ice sheet is developed. It is based on
laboratory measurements of ice rheology including ice cores and combined with
borehole measurements in the ice sheet. In the model the shear flow of ice is
enhanced as the ice passes through a range of stress regimes from predominantly
vertical compression to predominantly vertical shear stress, as it flows down
through the ice sheet. The enhanced flow, characterised by an increase in strainrates
compared to the secondary creep rate for isotropic ice, is primarily based on
laboratory measurements of tertiary creep under combined compression and shear
stresses.
To test this model two different flow lines in the Antarctic ice sheet have been
studied. One flow line is from Law Dome which is an isolated ice cap with
shallower and warmer ice. For comparison, the second, longer flow line, the
I.A.G.P traverse line inland of Casey in the interior of East Antarctica, was
studied to explore different conditions for deeper, colder ice and larger scale
bedrock features. A simple application of the model to a number of boreholes
with ice cores in Law Dome has been made, including a new borehole drilled in Law Dome Summit South where shear strain rates were measured as well as ice
core fabrics through the 1200 m depth to the bed. In the study areas a large
amount of glaciological work involving field surveys and ice core drilling
provides sufficient data for model inputs and for model verification. The results
from the model were compared with observations.
The implications of this work for other regions, including the whole Antarctic ice
sheet, were also investigated.
It is concluded that in regions of high bedrock roughness the occurrence of a high
shear layer of strongly anisotropic ice above the predominant bedrock
perturbations needs to be taken into account.

Item Type:	Thesis - PhD
Authors/Creators:	Wang, WL
Keywords:	Glaciology, Ice sheets
Copyright Holders:	The Author
Copyright Information:	Copyright 2000 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (PhD.)--University of Tasmania, 2000. Includes bibliographical references
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