MEASURING FABRICS OF GLACIAL FLUTES USING ANISOTROPY OF MAGNETIC SUSCEPTIBILITY, MÚLAJÖKULL, ICELAND

Macrofabrics have long been used to infer deformational patterns of subglacial till in glacial flutes. Flute macrofabrics, derived from the aggregate orientation of elongate pebbles, are either parallel to glacier flow or form a “herringbone” pattern along the flute’s central axis. Fabrics based on orientation of magnetic particles using anisotropy of magnetic susceptibility (AMS) have not been applied to flutes. AMS fabrics provide more complete, three-dimensional data on deformation kinematics than particle fabrics, since all principal susceptibility vectors, maximum [k₁], intermediate [k₂] and minimum [k₃], are measured. Laboratory experiments using a ring-shear device indicate that all principal susceptibility vectors develop distinct, strong fabrics at moderate strains (7-30) and that those fabrics remain strong to higher strains. k₁ fabrics form in the direction of shear and plunge mildly (<30˚) up-glacier. Under the same conditions k₂ fabrics form perpendicular to the direction of shear, and k₃fabrics form normal to the shear plane.

We are studying flutes in the forefield of Múlajökull, a glacier in central Iceland, using intact samples of the basal till to measure AMS fabrics. Thermomagnetic, hysteresis and FORC experiments show that magnetic susceptibility of the Múlajökull tills is controlled by silt-sized pseudo single domain and single domain titanomagnetite. Within the interior of the flute, all principal susceptibility fabrics are strong and similar to simple shear fabrics observed in ring shear experiments. k₁ fabrics are near-parallel to the flute longs axis and plunge < 25˚ upglacier. k₂ fabrics are perpendicular to the flute long-axis and plunge slightly away from the center of the flute. Near the surface of the flute, k₂ and k₁ fabrics are weak and have no distinct orientation, but do form a low-angle girdle. k₃ fabrics are strong and near vertical both at the surface of the flute and in its interior. Marked increase of fabric strength with depth is an effect not observed in studies of flute macrofabrics. The increase in k₁ and k₂ susceptibility fabric strengths with depth, in addition to constant minimum susceptibility fabric strengths may represent consolidation of the fluted till following shearing.