Crevasse patterns and the strain-rate tensor: a high-resolution comparison

Values of the strain-rate tensor r epresented at a 20 m length sca le a re found to explain the pattern and orientation of crevasses in a 0.13 km 2 reach of Worthington Glacier, Alaska, U.S.A. The flow fi eld of the reach is constructed from surveyed displacements of 110 markers spaced 2030 m apart. A velocity gradient m ethod is then used to calcul ate values of the principal strain-rate axes at the nodes of a 20 m x 20 m orthogonal grid. Crevasses in the study reach are of two types, splaying and transverse, and are everywhere norma l to the trajectories of greatest (most tensile) principal strain ra te. Splaying crevasses ex ist where the longitudinal strain rate (Ex ) is :'S 0 and transverse crevasses are present under longitudinally extending flow (i. e. Ex > 0). The orientation of crevasses changes in the down-glacier direction, but the calculated rota tion by the flow field does not account for this change in orientation. Observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time-scale of 12 yea rs; they are not persistent features that a re translated a nd rotated by flow. Crevasse patterns are thus found to be a useful tool for mapping the strain-rate ten or in this reach of a temperate valley glacier.