glaciotectonics

glaciotectonics Glaciotectonic deformation may be defined as ‘structural deformation as a direct result of glacier movement or loading’ (INQUA Work Group on Glacier Tectonics 1988). This topic has received a great deal of study in recent years because glaciotectonic processes have been related to glacial dynamics (Fig. 1). There are two types of glaciotectonic deformation formed by the action of a moving glacier: 1. Subglacial deformation, which takes place beneath the glacier and is characterized by simple shear and extensional tectonics, i.e. attenuated folds, boudins, and augens, and results in the formation of deforming bed till and/or flutes and drumlins;2. Proglacial deformation, which takes place at the glacier margin and is characterized by pure shear and compressional tectonics, i.e. open folds, thrusts, and nappes. This results in the formation of push moraines.Additionally, deformation also occurs within the glacial environment as a result of gravitation instabilities associated with stagnant ice and is known as dead-ice tectonics. Within the progracial area, features that are associated with melting ice form, such as ice-collapse structures in the outwash plain and debris-flow mobilizations of till. In the subglacial environment, ‘crevasse infill’ structures are formed when saturated tills are extruded up through crevasses on to the glacier surface.

Subglacial glaciotectonic deformation

Many research workers have shown that when a glacier moves over an unconsolidated bed there is a coupling between the glacier and the underlying bed. This leads to an increase in the velocity or a decrease in the slope angle of the glacier, or both, and the deformation of the sediments below. This deformation occurs because the water produced at the base of a temperature glacier cannot drain away well enough to prevent the build-up of pore-water pressures and the associated reduction in the strength of the subglacial sediment.

Subglacial deformation has been recorded beneath a number of modern glaciers, in particular BreiDhamerkurjökull, in Iceland, and Ice Stream B, in western Antarctica. It has also been inferred from sedimentological studies of the deposits from several neoglacial surging glaciers and Pleistocene glaciers (where the ice moved over unconsolidated rocks associated with the European (and British) and Laurentide (North American) ice sheets).

Subglacial deformation occurs within a subglacial shear zone. This is a thin layer undergoing high ductile shear in which rocks are broken and ground up by mechanical processes (cataclastic mylonization). The resultant till (a deforming bed or deformation till), is a primary till formed from a combination of both deformation and deposition.

At relatively low shear strains, deformation is apparent from the slight deformation of strain markers, such as the overturning of ice-wedge casts. At higher shear strains, criteria for the recognition of subglacial deformation include folds, tectonic laminations, boudins, augen-like features, and rotated clasts and boulder pavements. At very high shear strain (or sites with homogeneous bedrock), the deformation processes result in the homogenization of the till, and so macroscale structures may not be visible. Instead, criteria such as a specific till fabric (low strength associated with a thick deforming layer, high strength associated with a constrained deforming layer), specific micromorphology structures (e.g. rotation of clasts, shears), and a high fractal dimension can be used.

It has also been argued that subglacial streamline bedforms (flutes and drumlins) are a product of subglacial deformation. These features form because of the presence of more competent masses (or cores) within the deforming layer, which act as obstacles to flow.

Proglacial glaciotectonic deformation

Proglacial glaciotectonic deformation is generally characterized by large-scale compressional folds and thrusts. These are very common and are associated both with modern and with Pleistocene glaciers.

Proglacial deformation spans a continuum of compressive deformation styles, from folding (ductile deformation) to stacking (brittle deformation), both of local and of fartravelled sediment or rock masses, and the final result depends upon the rheology and competence of the bed and the behaviour of the ice sheet.

The usual result of the proglacial deformation processes is to produce a topographic ridge transverse to the ice margin, called a push moraine. There is often a basin up-glacier from which the material of the ridge has been removed. These features do not, however, always have a topographic expression because many proglacial structures have been subsequently overridden by ice and so have become incorporated into drift deposits.

Thrust blocks (within push moraines) can be modelled rather like nappes in hard-rock terrains; they move along incompetent rock units or planes of weakness resulting from high pore-water pressures, and are formed by lateral push, gravity gliding, or gravity spreading. Some push moraines form simply by lateral push, and need not necessarily be associated with the deforming bed. In contrast, where a deformable bed is present, the push moraine forms by a combination of frontal pushing and stresses transmitted through a subglacial deformable wedge (the latter being a direct result of the gravity spreading of the ice sheet). Small-scale versions of this phenomenon are common in modern glacial environments associated with deforming beds. Here the push moraine is composed of subglacial till that has been moved out into the foreland while the slower-moving sediment remains behind in the form of flutes.

Conclusions

There have been many studies of glaciotectonics in recent years because it has been shown that glaciotectonic deformation is a fundamental process within the glacial environment. It is not ‘special’ or unusual, but occurs in association with glacier movement, deposition, and deformation. Very few modern-day glaciers do not have push moraines currently being formed at their margins. Evidence for deformation (both on a macro- and microscale) within tills is similarly very common, and is now being reported regularly. Future research in glaciotectonics is, however, needed in the field of thermal properties, and the geotechnical behaviour of tills and their relationship to ice dynamics.

J. K. Hart

Bibliography

Hart, J. K. and and Martinez, K. (1997) Glacial analysis. CD-ROM, Routledge, London.
Wateren, van der F. M. (1995) Processes of glaciotectonism. In Menzies, J. (1995) Modern glacial environments: processes, dynamics and sediments, pp. 309–33. Butterworth-Heinemann, Oxford.