Tectonic processes in a surge-type glacier

The 1982–1983 surge of Variegated Glacier involved the development, growth and downglacier propagation of a velocity peak associated with rapid basal sliding facilitated by high subglacial water pressures. Passage of the velocity peak through the glacier was preceded by an episode of longitudinal shortening and followed by an episode of elongation. The deformation history of the glacier ice was dependent upon location relative to the surge nucleus and the final position reached by the propagating velocity peak. Ice above the surge nucleus experienced continuous and cumulative elongation; ice below the final position of the velocity peak experienced continuous and cumulative shortening; ice between these two points experienced shortening followed by elongation and low cumulative strain. The large-scale pattern of ice structure development reflects these deformation histories. Surging is equivalent to thrust sheet emplacement by a combination of gravity gliding over a weakened basal layer and ‘push from behind’, with the gravity-driven motion of the surging part of the glacier providing the push which allows the surge front to propagate. The relationships established between the deformation history of surging glaciers and the development of ice structures may facilitate the interpretation of structures in thrust sheets.