Shear resistance and continuity of subglacial till: hydrology rules

Abstract

ABSTRACT. The field observations of G.S. Boulton stimulated widespread interest in deformable beds. Shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure. During unsteady deformation, pseudo-viscous shear resistance can be caused by dilation of consolidated tills and resultant pore-pressure decline. This effect is probably uncommon, however, because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective-pressure transients. Stick-slip motion at Whillans Ice Stream, Antarctica, indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip. Recurrence intervals for rapid-slip episodes there (6-18 hours) indicate that till-strength variations, if driven by changes in pore pressure either related or unrelated to basal freezing, are focused in the uppermost several centimeters of the bed. Ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate-weakening during rapid slip, with pore-pressure decay causing strengthening between slip episodes. By promoting shallow, sluggish subglacial water flow and low effective pressure, soft beds may help sustain themselves by slowing their own transport. Soft-bed shear resistance, kinematics and continuity are problems rooted in subglacial hydrology.

Emergent drumlins and their clones: from till dilatancy to flow instabilities

“…However, once it became clear that such a viscous rheology might not be appropriate (e.g. reviewed in Clarke, 2005, and see Iverson, 2010), analysis was performed by Schoof (2007), Hindmarsh (in Dunlop and others, 2008) and Fowler (2009), all concluding that the presence of unstable states was not dependent on the choice of rheology (from viscous to plastic). All that is required by the theory is that there is basal ice motion and a mobile till and that their movement depend sensibly on both basal shear stress and the effective pressure at the ice-till surface (Fowler, 2009).…”

Tidal pacing, skipped slips and the slowdown of Whillans Ice Stream, Antarctica

“…Such 'healing' behavior, strengthening the interface over time, is observed in laboratory studies (Marone, 1998;Zoet and others, 2013) and is inferred for repeating earthquakes beneath the head of David Glacier, Antarctica (Zoet and others, 2012). Beneath WIS, a strengthening mechanism could be related to sediment properties, such as pore-pressure diffusion or regelation into the bed (Alley, 1993;Iverson, 2010) or perhaps to thermal processes (Zoet and others, 2013). A time-dependent yield stress has been proposed to explain the observations that slip events that initiate following longer interevent periods result in higher displacements due to the larger stress drops (Fig.…”

Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland

“…Although glaciers themselves are clearly good approximations to half-spaces, subglacial tills frequently contain metre-(or sub-metre-) scale contrasts (Smith, 2007), such as the transitional boundaries between dilatant and lodged till (e.g. Clarke et al, 1984;Echelmeyer and Wang, 1987;Iverson et al, 1988Iverson et al, , 1994Truffer et al, 2001;Boulton et al, 2001;Porter and Murray, 2001;Evans et al, 2006;Iverson, 2011;Reinardy et al, 2011). It would therefore be prudent to consider how thin layering affects a seismic AVA response whenever surveys are conducted over till substrates.…”