A magnetic fabric study of the origin of englacial debris bands at Fláajökull, Southeast Iceland

Abstract

Two debris bands associated with an overdeepened basin in the terminus region of the Fláajökull glacier, Iceland, have been sampled for their ice and debris. Here the kinematics of deformation are investigated through anisotropy of magnetic susceptibly (AMS), microstructural analyses, and c-axis preferred orientations. Comparison of the crystallographic orientations of 180 grains and the AMS shape orientations of mineral inclusions in polycrystalline ice reveals a multi-maximum girdle oblique to the planar stratification and a record of near vertical triaxial ellipsoids. These two distinct fabrics can be attributed to both pure and simple shear suggesting a complex deformational history. In addition, AMS from an ice-free sediment ridge on the glacier surface indicates pure shear locally dominated strain. The physical properties of the debris bands, sedimentology and field observations suggest that they were sourced from the glacial bed and emplaced by highly pressurized water into localized fractures.