ICE FLOW VECTORS ON THE DEBRIS-MANTLED TASMAN GLACIER, 1957-1986

A photogrammetric survey of the movement of supraglacial boulders toge ther with field survey of surface markers has revealed the spatial and temporal change in velocity on the debris-mantled tongue of Tasman Gl acier, New Zealand. Between 1957 and 1986 a general reduction of glaci er velocity is recorded, and towards the terminus the local effects of thermokarst erosion on flow vectors became more influential as the do wnvalley component of velocity decreased. Only ca. 2% of the debris-co vered glacier area had no measurable movement, and mature thermokarst sink-holes formed on ice moving at >24 ma(-1). Increases in ice veloci ty and surface elevation 10 km above the terminus were not recorded 2. 5 km above the terminus in 1986. Changes in ice volume have been manif ested by changes to ice levels and velocity, not by fluctuations of th e terminus. The study shows that extensive supraglacial debris mantles and thermokarst forms can exist on actively-moving ice in alpine regi ons of high debris supply and ablation, and that shear-zone formation is not necessary to explain debris-mantling of such glaciers.