The experimental basis for interpreting particle and magnetic fabrics of sheared till

Abstract

Particle fabrics of basal tills may allow testing of the bed-deformation model of glacier flow, which requires high bed shear strains (> > > > >100). Field studies, however, have not yielded a systematic relationship between shear-strain magnitude and fabric development. To isolate this relationship four basal tills and viscous putty were sheared in a ring-shear device to strains as high as 714. Fabric was characterized within a zone of shear deformation using the long-axis orientations of fine-gravel and sand particles and the anisotropy of magnetic susceptibility (AMS) of small (~5·8 cm 3 ) intact samples. Results indicate that till particles rotate toward the plane of shearing with long-axis orientations that become tightly clustered in the direction of shear (0·78 < < < < < S 1 < < < < < 0·94 for three-dimensional data). These strong, steadystate fabrics are attained at shear strains of 7-30, with no evidence of fabric weakening with further strain, regardless of the specific till or particle-size fraction under consideration. These results do not support the Jeffery model of particle rotation, which correctly describes particle rotation in the viscous putty but not in the tills, owing to fluid-mechanical assumptions of the model that are violated in till. The sensitivity of fabric development to shearstrain magnitude indicates that, for most till units where shear-strain magnitude is poorly known, attributing fabric variations to spatial differences in other variables, such as till thickness or water content, will be inherently speculative. Attributing fabric characteristics to particular basal till facies is uncertain because shear-strain magnitude is unlikely to be closely correlated to till facies. Weak or spatially variable fabrics, in the absence of postdepositional disturbance or major deviations from unidirectional simple shear, indicate that till has not been pervasively sheared to the high strains required by the bed-deformation model. Strong flow-parallel fabrics are a necessary but insufficient criterion for confirming the model.

Subglacial clast behaviour and its implication for till fabric development: new results derived from wireless subglacial probe experiments

“…In contrast, other researchers have suggested that the clasts will initially rotate into the shear plane and then remain there (Hooyer and Iverson, 2000;Larsen and Piotrowski, 2003;Thomason and Iverson, 2006;Iverson et al, 2008). Thus the March (1932) model, which treats clasts as passive strain markers, is more appropriate.…”

Effect of strain localization on frictional behavior of sheared granular materials

“…Similar connections between strain localization and frictional properties are emerging from studies of glacial till [e.g., Iverson et al, 2008;Rathbun et al, 2008]. However, most studies of till do not include direct information on friction constitutive behavior or stick-slip.…”

“…[45] Laboratory studies of till localization using anisotropy of magnetic susceptibility (AMS) fabric indicate that localization evolves until g on the order of 100 [Larsen et al, 2006;Thomason and Iverson, 2006;Iverson et al, 2008]. Unfortunately, these studies do not include data on the friction constitutive parameters for sheared till, and are all conducted in ring shear apparatuses.…”

“…Recent studies show that glaciers exhibit stick-slip motion in some cases [Anandakrishnan and Bentley, 1993;Ekstrom et al, 2003Ekstrom et al, , 2006 and physical models have been proposed [Tsai et al, 2008;Winberry et al, 2009]. Basal tills are often characterized by zones of localized shear, [e.g., Truffer et al, 2000;Kamb, 2001;Evans et al, 2006;Menzies et al, 2006] and laboratory studies indicate that till rheology is governed, in part, by the distribution of strain localization [Larsen et al, 2006;Thomason and Iverson, 2006;Iverson et al, 2008;Rathbun et al, 2008]. However, there are relatively few detailed laboratory studies of strain localization and its effect on friction constitutive properties of till.…”

Multiple subglacial till deposition: A modern exemplar for Quaternary palaeoglaciology

“…At the same time field based observations thought to be indicative of deformation 50 style and cumulative shear strain need to be reconciled with apparently contradictory laboratory based 51 measurements of shearing style and magnitude (e.g. Iverson et al 1994Iverson et al , 1998Iverson et al , 2008 …”