Thrust faulting in glaciers? Re‐examination of debris bands near the margin of Storglaciären, Sweden

Thrust faulting has been suggested as a viable mechanism of debris transport at many glaciers, often inferred from the presence of up-glacier dipping bands of debris that emerge at the ice surface to form ridges of basally derived material. However, modelling indicates that the development of thrust faults is mechanically inhibited because stresses are much lower than that required for shear failure, a prerequisite for thrust faulting, and field measurements fail to detect thrust-related displacement. The mechanism for the emplacement of these ridges that appear at the surface of many polythermal valley glacier termini remains open to question. This study re-examines the origin of debris ridges on the surface of Storglaciären, a polythermal valley glacier in northern Sweden, using field observations, ice microstructural analyses, sediment grain size analysis, stable isotope composition of the ice, and modelling. We find no evidence of discrete displacement across the debris bands that produce the ridges, nor do we find evidence that folding might be responsible. We propose that the bands originate at the base of the glacier by one of two mechanisms, perhaps in combination: (i) refreezing of meltwater near the thermal transition in basal ice, and (ii) injection into tensile fractures periodically opened at the base due to high fluid pressure and then freezing. In either case, separation from the base occurs due to high fluid pressure and freezing introduces ice below the debris bands, which are then transported forwards due to basal shear and upwards due to longitudinal compression, and revealed by surface ablation.