DOES THRUST FAULTING OCCUR IN GLACIERS? A CASE STUDY OF A SWEDISH GLACIER

It has long been proposed that thrust faulting occurs in glaciers based on the presence of debris ridges at the ice surface in regions of longitudinal compression, primarily near glacier termini. However, recent modelling indicates that the development of faults is mechanically inhibited in glaciers, even for those having surge-type behavior – the stresses in glaciers are much lower than that required for shear failure. The mechanism for emplacement of the debris ridges that appear at the surface in the terminal zone of many polythermal valley glaciers is therefore open to question.

Storglaciären, a polythermal valley glacier in northern Sweden, has a series of debris ridges present on the surface near the terminus. This study assesses the origin of these debris ridges using field observations, microstructural analysis, grain size analysis and stable isotope data. The ridges are debris-covered and ice-cored causing differential ablation and a protruding surface expression. Underlying ice has variable debris composition and concentration, containing bands of poorly sorted sediment, ranging in grain-size from silt to cobbles, with the larger particles showing evidence of rounding. DGPS measurements indicate no differential movement of the ice above and below the ridges, and the limited lateral extent of the ridges violates an empirical law for faulting in rocks relating fault length to maximum displacement. Additionally, there is no evidence for the existence of active fractures with discrete discontinuities across them. At the microstructural scale, there is no evidence for brittle failure. Recrystallized grains are locally variable in size and shape, but an overall increase in grain size and grain boundary smoothness relative to the surrounding ice characterizes the bubble free ice within the ridge. Ice within the debris bands is enriched in δ¹⁸O and δD, which is characteristic of refreezing rather than ice derived directly from precipitation. We therefore propose that these debris bands originate at the base of the glacier by one of two mechanisms: 1) freezing of meltwater, and/or 2) injection into tensile fractures induced at the base from high pore fluid pressure. In either case the debris is then transported upwards due to longitudinal compression and revealed by surface ablation.