The 3-dimensional structure and flow field of a temperate ice mass: Surface and borehole deformation studies on Worthington Glacier, Alaska

The fully 3-dimensional structure and flow field of Worthington Glacier, Chugach Mountains, Alaska, are investigated using a combination of surface and borehole measurements. Four separate projects are presented: (1) an investigation of the englacial structure of the ice mass using borehole video observations in 16 boreholes totaling 2800 m in length; (2) a study of spatial and temporal variations in surface velocity using displacement measurements made at 64 locations spaced 20-30 m apart; (3) examination of the relationship between crevasse patterns and the strain field using calculated values of the principal strain rate axes and mapped crevasses; (4) an investigation of the fully 3-dimensional flow field in a {dollar}6.2times 10 msp3{dollar} block using deformation measurements made in a dense grid of 27 boreholes.; The glacier is found to be relatively isotropic and homogeneous with discrete features composing less than 3% of the bulk ice mass. Surface velocity varies diurnally and seasonally, but the summer average velocity field is similar from year to year. Regional velocity gradients show important structure at a 10-20 m length scale. However, local velocity anomalies caused by motion on crevasses are present over this same length scale, making local anomalies difficult to distinguish from regional gradients without spatial and temporal smoothing. Crevasse patterns closely match trajectories of the principal axes throughout the study reach, and show a rotation across the reach that is considerably more than the rotation caused by the flow field. This suggests that the crevasses are transient features which are not advected through the entire reach. The 3-dimensional flow field shows little deformation from the surface to a depth of about 110-120 m and that about 60-70% of the total surface motion is due to basal slip.