Magnetic fabric of sheared till: A strain indicator for evaluating the bed deformation model of glacier flow

Abstract

[1] Wet-based portions of ice sheets may move primarily by shearing their till beds, resulting in high sediment fluxes and the development of subglacial landforms. This model of glacier movement, which requires high bed shear strains, can be tested using till microstructural characteristics that evolve during till deformation. Here we examine the development of magnetic fabric using a ring shear device to deform two Wisconsinage basal tills to shear strains as high as 70. Hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility (AMS) develops during shear due to the rotation of primarily magnetite particles that are silt sized or smaller. At moderate shear strains ($6-25), principal axes of maximum magnetic susceptibility develop a strong fabric (S 1 eignevalues of 0.83-0.96), without further strengthening at higher strains. During deformation, directions of maximum susceptibility cluster strongly in the direction of shear and plunge ''up-glacier,'' consistent with the behavior of pebbles and sand particles studied in earlier experiments. In contrast, the magnitude of AMS does not vary systematically with strain and is small relative to its variability among samples; this is because most magnetite grains are contained as inclusions in larger particles and hence do not align during shear. Although processes other than pervasive bed deformation may result in strong flow parallel fabrics, AMS fabrics provide a rapid and objective means of identifying basal tills that have not been sheared sufficiently to be compatible with the bed deformation model.

Magnetic fabrics of drumlins of the Green Bay Lobe, southeastern Wisconsin

“…A1). Hooyer et al (2008) observed this same behavior in ring-shear experiments on two other basal tills and provided an explanation based on the observation that the silt-sized magnetite grains that dominated AMS resided mostly in larger rock particles. Although these larger host particles rotate during shear, their long-axis orientations do not reflect orientations of the magnetic particles within them, which cannot align with strain.…”

“…4) for this till is very similar to that measured for two basal tills from ice lobes that were on either side of the Green Bay lobe (Superior and Lake Michigan lobes, Hooyer et al, 2008) during the late-Wisconsinan. For these two tills, silt-sized magnetite grains were independently determined to be largely responsible for their AMS , using hysteresis experiments and the dependence of susceptibility on temperature.…”

Experiments on the dynamics and sedimentary products of glacier slip

“…This lack of common understanding provides clear motivation for experimentally studying particle and magnetic fabric evolution as a function of shear-strain magnitude and orientation (Hooyer and Iverson, 2000b;Thomason and Iverson, 2006;Hooyer et al, 2008;Iverson et al, 2008;Shumway and Iverson, 2009). To date, experiments have been carried out with five different tills and a linear-viscous putty subjected to a state of strain in the ISU ring-shear device close to simple shear.…”

“…The direction of shearing is along the x axis, and the sense of shearing in the stereonets is bottom north and top south. Stereonet labels indicate experiment number and the number, n, of AMS samples (fromHooyer et al, 2008).…”

“…AMS fabric strength, based on k 1 orientations, as a function of shear strain for the (A) Douglas till and (B) Batestown till (seeHooyer et al, 2008, for till descriptions), as determined in ring-shear experiments carried out to the indicated shear strains. Lines of the form y = y 0 + a(1 − e bx ) were fitted to the data.…”