Englacial and proglacial glaciotectonic processes at the snout of a thermally complex glacier in Svalbard

Many glaciers in Svalbard, which tend to be thermally complex with both warm and cold ice, produced complex ‘push moraines’ or ‘thrust-moraine complexes’ at the time of their maximum neoglacial advance, and during their subsequent recession. Uvěrsbreen, which nearly reaches the coast of northwest Spitsbergen, has a push moraine 50 m high, comprising gravel and sand of fluvial origin, and diamicton derived from basal ice, as well as thrust masses of dead ice. The push moraine is almost entirely a product of the tectonic processes, especially thrusting, that occurred in the snout of the glacier and in the frozen sediments immediately in Journal of Quaternary Science front. Thrusting is probably initiated where the glacier switches from a sliding mode to a state of being frozen to the bed. Thrusting in the foreland is an extension of the same processes that occur within the ice itself, and a sole thrust probably extends from the frozen sediment that underlies the base of the glacier near its snout. Thrusting results in subglacial sediment being uplifted to a high level within the glacier or even to the surface. Subsidiary near-horizontal thrusts parallel to the bedding plane link thrusts dipping up-glacier, and result in local thickening of individual subglacial sedimentary units. Gravels incorporated in this way retain their integrity but sands are folded, whereas uplifted debris-rich basal ice is intensely folded. The distinctive tectonic style of low-dip bedding in gravels, truncated by high-angle (30–40°) thrusts, and only localised folding in less competent material, differs from most descriptions of such ridges, but may be typical of many push moraines composed principally of fluvial material.