Relationship between Bore-hole Closure and Crystal Fabrics in Antarctic Ice Core from Cape Folger

AIlSTRACT. Two holes were drilled to dept hs g rea ter than 300 m in the Antarctic ice sheet, nea r Cape Folger on the Law Dome. The holes underwent consid erable closure below 250 111 \·"ilh localized stra in-rates as hig h as I to 1.5 x 106 S I The closure obse n 'ed in the holes was non-uniform a nd occurred in zo nes 0.5 to 3 m wide. These zones parallel the now plane in the ice mass and are associated w ith a distinct domainal structure. High-clos ure zo nes are characterized by inte rlocking and irregul ar-shaped ice grains with man y sub-horizontal c-axes and onl y occasio nal c-axis clusters a t a high angle to the flow pl ane. Low-closure zones cOI1[ain tabula r grains with the long dim ension parallel to the now pla ne, a bundant d eform at ion features and a predominance ofc-axes ori en ted at a hig h a ng le to the now plane. The re la tionship between clos ure rate and c-axis fabric is attributed to ma rk ed plasti c now by intrarrys ta lline slip on the basal plane to produce higher closure in areas where there is a grea ter va ria tion in c-axis orientation. This de formation is att ributab le la overburd en pressure a nd hence is related to depth , and is independ ent of shear with in the main body of the ice mass .