In situ evidence for icefacies rheological variability

A range of field evidence from various environments indicates that different ice facies found contiguously, and therefore in identical or similar stress conditions, deform at very different rates in response to similar stresses (Fig. 63.3). Competence-contrast boudinage in the basal zone of temperate Variegated Glacier, in which dispersed facies basal ice was boudinaged within layers of debris-laden stratified facies basal ice (Fig. 63.3a), clearly indicated the relatively rapid deformation of the stratified facies compared with the dispersed facies. At cold conditions, velocity profiles through the basal zone of Suess Glacier in Antarctica indicated that a 0.8 m thick layer of dispersed-facies ice experienced much more rapid deformation than either the overlying englacial ice or the underlying stratified (solid subfacies) ice (Fig. 63.3b; Fitzsimons et al., 2000). Similarly, various types of borehole evidence have shown that ice with enhanced debris concentrations near the base of ice cores deforms more readily than the overlying clean ice (Shoji & Langway, 1984; Fisher & Koerner, 1986; Simoes et al., 2002).

The rapid rate of deformation of debris-laden, stratified-facies ice is such that deformation of these basal layers can account for large proportions of surface motion: for example, Echelmeyer & Wang (1987) found that deformation in a basal debris-laden ice layer 35 cm thick accounted for up to 60% of surface motion at Urumqi Glacier No 1 in the Tienstshan Mountains. Brugman (1983) reported similar importance of deformation across debris-laden ice in motion of glaciers on Mount St Helens.

It is clear then, that debris-laden, basal-ice facies in situ have a lower effective viscosity than relatively debris-free ice facies. It is also clear that the processes involved in the formation of the debris-laden ice, and in particular regelation (Fig. 63.1), produce ice with a range of distinctive characteristics as well as the physical presence of debris. In the next section, we attempt to understand the cause of the lower effective viscosity of debris-laden ice facies, by reviewing what is known about the effect of various physical and chemical ice characteristics on ice rheology. These ice facies also have distinctive crystal characteristics. The effects of crystal fabrics on ice deformation are relatively well understood (Budd & Jacka, 1989) and discussion below focuses on impurity effects.

Figure 63.2 Englacial ice facies overlying an erosional unconformity with distinctively layered debris-laden basal stratified-ice facies in the basal zone at Taylor Glacier, Antarctica.

Figure 63.3 Examples of field evidence for the lower effective viscosity of different ice facies in situ: (a) Competence-contrast boudinage at Variegated Glacier, showing the relatively high effective viscosity of the cleaner boudinaged dispersed facies ice, with stratified facies ice layers deformed around it. (b) Velocity profiles in the basal zone at Suess Glacier, showing relatively rapid deformation in the 'amber' (dispersed) facies basal ice (from Fitzsimons et al., 2000). (c) Displacement profile in a marginal cliff at Taylor Glacier, Antarctica, showing rapid displacement rates in the lower part of the debris-laden basal ice section.

Figure 63.3 Examples of field evidence for the lower effective viscosity of different ice facies in situ: (a) Competence-contrast boudinage at Variegated Glacier, showing the relatively high effective viscosity of the cleaner boudinaged dispersed facies ice, with stratified facies ice layers deformed around it. (b) Velocity profiles in the basal zone at Suess Glacier, showing relatively rapid deformation in the 'amber' (dispersed) facies basal ice (from Fitzsimons et al., 2000). (c) Displacement profile in a marginal cliff at Taylor Glacier, Antarctica, showing rapid displacement rates in the lower part of the debris-laden basal ice section.

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