CREVASSE PATTERNS AND THE STRAIN-RATE TENSOR - A HIGH-RESOLUTION COMPARISON

Values of the strain-rate tensor represented at a 29 m length scale ar e 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 field of t he reach is constructed from surveyed displacements of 110 markers spa ced 20-30 m apart. A velocity gradient method is then used to calculat e 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, spl aying and transverse, and are everywhere normal to the trajectories of greatest (most tensile) principal strain rate. Splaying crevasses exi st where the longitudinal strain rate ((epsilon)over dot (x)) is less than or equal to 0 and transverse crevasses are present under longitud inally extending flow (i.e. (epsilon)over dot (x) > 0). The orientatio n of crevasses changes in the down-glacier direction, but the calculat ed rotation by the flow field does not account for this change in orie ntation. Observations suggest that individual crevasses represent loca l values of the regional flow field and are transient on the time-scal e of 1-2 years; they are not persistent features that are translated a nd rotated by flow Crevasse patterns are thus found to be a useful too l for mapping the strain-rate tensor in this reach of a temperate vall ey glacier.