Relationship Between Bore-Hole Closure and Crystal Fabrics in Antarctic Ice Core from Cape Folger

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Abstract
Two holes were drilled to depths greater than 300m in the Antarctic ice sheet, near Cape Folger on the Law Dome. The holes underwent considerable closure below 250m with localized strain-rates as high as 1 to 1.5×10−6s−1. The closure observed in holes was non-uniform and occurred in zones 0.5 to 3 m wide. These Zones parallel the flow plane in the ice mass and are associated with a distinct domainal structure. High-closure zones are characterized by interlocking and irregular-shaped ice grains with many sub-horizontalc-axes and only occasionalc-axis clusters at a high angle to the flow plane. Low-closure zones contain tabular grains with the long dimension parallel to the flow plane, abundant deformation features and a predominance ofc-axes oriented at a high angle to the flow plane. The relationship between closure rate andc-axis fabric is attributed to marked plastic flow by intracrystalline slip on the basal plane to produce higher closure in areas where there is a greater variation inc-axis orientation. This deformation is attributable to overburden pressure and hence is related to depth, and is independent of shear within the main body of the ice mass.