Ice sliding over weak, fine-grained tills: Dependence of ice-till interactions on till granulometry

Abstract

Two fundamental aspects of ice-till interactions, the strength of the ice-till coupling and the vertical distribution of deformation in till, may be strongly dependent on till granulometry. In particular, results of theoretical analysis of several physical processes involved in such interactions suggest the following hypotheses: (1) finegrained tills facilitate ice sliding with ploughing and little distributed deformation, and (2) coarse-grained tills facilitate strong ice-till coupling and relatively deep till deformation (~0.1 m). The theoretical analysis is limited to Coulomb-plastic tills under low subglacial effective stresses (0-100 kPa). Fine-grained tills are represented in the analysis by a clay-rich till from beneath Ice Stream B (ISB), West Antarctica, and a silty Pleistocene till from Ohio. For comparison, two coarse-grained, clast-rich tills are also considered (from beneath the Trapridge Glacier, Yukon, and the Breidamerkurjökull Glacier, Iceland). The mechanical condition for ice sliding over till is defined as the situation in which the strength of the ice-till interface is lower than the strength of the till itself. Model calculations predict that this condition is more likely to be met in fine-grained rather than coarse-grained tills because of (1) lower abundance of ploughing clasts (clast fraction ~0.01 vs. ~0.1), (2) widespread submergence of fine matrix particles even by a very thin basal water film (~10 -6 m), and (3) greater susceptibility to interface smoothing due to ice-water surface tension. In addition, the theoretical analysis of ice-till interactions considers three potential mechanisms for distribution of deformation in tills of Coulomb-plastic rheology: (1) plastic deformation of till around a ploughing clast, which may affect till to depth of c. 2.7 to c. 4.5 times the clast diameter; (2) particle/clast bridging, which is typically observed to result in a shear-zone that is 10 times greater than the characteristic clast/particle diameter; and (3) vertical shear-zone migration due to water-pressure fluctuations. Combined, these three effects may result in distribution of a significant fraction of ice motion throughout ~0.1 m thickness of a coarse-grained, clast-rich till. However, lower clast abundance and smaller hydraulic diffusivity of a fine-grained till makes it a less favorable environment for significant strain distribution (predicted shear zone thickness ~0.01 m).Tulaczyk, S., 1999, Ice sliding over weak, fine-grained tills: Dependence of ice-till interactions on till granulometry, in Mickelson, D. M., and Attig, J. W., eds.

Distributed shear of subglacial till due to Coulomb slip

“…Our analysis also indicates that neither dilatant hardening (Iverson and others, 1998) nor pore-pressure diffusion into the bed from the ice^till interface (Tulaczyk, 1999), two proposed mechanisms for distributing deformation in Coulomb models, is necessary to account for the observed distribution of displacement.…”

A groove-ploughing theory for the production of mega-scale glacial lineations, and implications for ice-stream mechanics

“…The strong appeal of the groove-ploughing theory is that relief amplification is so readily explained: it is simply inherited from the ice bumpiness, which in turn is inherited from the natural bumpiness of bedrock surfaces. There is no requirement for the till to behave viscously, which has been a recent source of debate (Hindmarsh, 1997;Murray, 1997;Tulaczyk andothers, 2000a, b, 2001). …”

“…This has far-reaching implications with regard to the regulation and stability of ice streams. Although still hotly debated (Hindmarsh, 1997;Murray, 1997;Tulaczyk andothers, 2000a, b, 2001), if till does behave plastically at the scale of ice-stream beds, there are a number of problems with our understanding of ice-stream functioning with regard to their underlying soft beds. These are addressed below and we propose that a bumpy ice base and the groove-ploughing mechanism reconcile many of these.…”

Basal till and subglacial conditions at the base of the Upper Odra ice lobe (southern Poland) during the Odranian (Saalian) Glaciation

“…Pre sum ably the ef fec tive pres sure reached low val ues (which re sulted in a low shear strength of the de pos its), but as a rule higher than zero. These con di tions en abled the trans fer of the shear stress from the sole of the mov ing ice sheet to the un der ly ing sed i ments and con se quently their de for ma tion (see Brown et al, 1987;Iverson et al, 1995Iverson et al, , 2003Piotrowski and Tulaczyk, 1999;Tulaczyk, 1999;Bennett, 2003). The dif fer ent scales of fi nite shear strain in the ver ti cal Sec tions in all pho to graphs ori ented ap prox i mately par al lel to the for mer ice sheet move ment di rec tion, which was from left to right; A -sharp con tact of the till with the un der ly ing sandy-grav elly de pos its; B -small de for ma tion struc ture (re cum bent fold) in the con tact zone; C -sin gle elon gated sand in clu sion in the bot tom part of the till; D -gran ite boul der in the basal part of the till, the boul der dips into the di rec tion of ice sheet move ment, the lower bound ary of the till just be low the boul der is slightly bent down wards A -in the de for ma tion till, as a re sult of clast ro ta tion the bot tom till sur face be low the clast is bent down wards; B -in the lodge ment till, clast plough ing pre ceded its de po si tion, the lower bound ary of the till be low the plough ing clast is flat pro file of till re flect the pe ri odic vari a tion of its in ten sity.…”

“…The small thickness of the de formed ho ri zon was partly a re sult of the fine-grained char ac ter of the de posit. The lack of a grain framework was the main rea son for the lim ited down ward trans fer of the shear stress (Tulaczyk, 1999). As a re sult, the clay was inten sively de formed only in a thin layer (<20 cm) di rectly be low the ice sheet base.…”