Abstract
AbstractBasal ice of glaciers and ice sheets frequently contains a well-developed stratification of distinct, semi-continuous, alternating layers of debris-poor and debris-rich ice. Here, the nature and distribution of shear within stratified basal ice are assessed through the anisotropy of magnetic susceptibility (AMS) of samples collected from Matanuska Glacier, Alaska. Generally, the AMS reveals consistent moderate-to-strong fabrics reflecting simple shear in the direction of ice flow; however, AMS is also dependent upon debris content and morphology. While sample anisotropy is statistically similar throughout the sampled section, debris-rich basal ice composed of semi-continuous mm-scale layers (thestratified facies) possesses well-defined triaxial to oblate fabrics reflecting shear in the direction of ice flow, whereas debris-poor ice containing mm-scale star-shaped silt aggregates (thesuspended facies) possesses nearly isotropic fabrics. Thus, deformation within the stratified basal ice appears concentrated in debris-rich layers, likely the result of decreased crystal size and greater availability of unfrozen water associated with high debris content. These results suggest that variations in debris-content over small spatial scales influence ice rheology and deformation in the basal zone.
Highlights
A basal zone of ice that interacts with the glacier bed is common to many glaciers and ice streams
The specimen was cooled to 20 K in the absence of a field, a low-temperature saturation isothermal remanent magnetization (SIRM) of 2.5 T was applied, and the remanence was measured upon warming to 300 K
anisotropy of magnetic susceptibility (AMS) fabric analysis within stratified basal ice of Matanuska Glacier supports the occurrence of simple shear within the basal zone, as expected physically and as observed for many other glaciers and ice streams
Summary
A basal zone of ice that interacts with the glacier bed is common to many glaciers and ice streams. The basal zone commonly possesses internal structures, such as banding or zonation at various scales, which frequently display folding, boudinage or other deformational features Basal ice such as that observed at Matanuska Glacier is derived from subglacial waters – both glacial melt and surface-derived – which freezes and incorporates debris by processes of regelation or hydraulic supercooling (Alley and others, 1998). The mechanism by which this stratification arises is poorly understood, and might either reflect pseudo-seasonal hydraulic fluctuations at the glacier bed or be a foliation resulting from post-freeze-on segregation and deformation within the basal zone (Alley and others, 1998) The origin of this stratification is of interest because of its implications for ice flow and processes of subglacial sediment entrainment and transport. In situ melting of stratified basal ice contributes to the formation of melt-out till, a not-uncommon type of till that until recently has been both variably and poorly understood (Larson and others, 2016)
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