A study of microfabric and structures in glacial ice: insights from Storglaciären, northern Sweden
Better understanding of the flow of ice is essential for helping to predict and adapt to the global mean sea level rise in the coming decades and centuries. Flow produces internal structures and fabrics in glaciers that in turn influence the flow. Documenting and understanding the internal structure of glaciers is thus important for understanding how glaciers behave and respond to changing conditions. This thesis focuses on establishing the significance of commonly described but enigmatic crystallographic ice fabrics and associated structures, with specific examples taken from Storglaciären, a small, polythermal valley glacier in northern Sweden. Characteristic microstructures in deformed ice in valley glaciers and at the base of ice sheets include large, irregularly shaped interlocking grains with no clear shape-preferred orientation, little internal distortion, and c-axis crystallographic preferred orientations (CPOs) defined by several maxima. Such fabrics develop in warm ice (T> -10 o C). They are difficult to quantify and interpret in part because the large grain size and interlocking texture creates issues with quantifying the true size and shape of individual crystals, and thus with determining a representative CPO. All but one previous study have analyzed these fabrics using techniques that can only measure c-axis orientations and thus partial CPOs. A newly developed sample preparation method allows for the first use of electron backscatter diffraction (EBSD) on such coarse-grained ice, which then provides high accuracy measurements of full crystallographic orientations (c- and a-axis measurements). Both EBSD and the older Rigsby stage optical technique (c-axis only) are used in this study and this allows for comparison of the two methods and the results of this study with the results of previous work. In all samples analyzed from Storglaciären, the ice has been entirely recrystallized by dynamic recrystallization accommodated by grain boundary migration. The degree to which a grain is irregularly shaped is related to the density of bubbles, which are a secondary phase and which act to slow or stop grain boundary migration. There is no consistent angular relationship between the orientations of the a-axes and c-axes of two adjacent grains that might indicate a twin relationship and thus provide an explanation for the multimaxima c-axis fabric, as had been previously suggested. In nearly all cases c-axis maxima themselves lie in a cluster at a high angle to a plane of high shear stress or shear strain and to the foliation plane. As the number of grains included in a fabric is increased by combining data from several samples, thus creating a more representative sample, the multimaxima nature of the patterns diminishes, and in some cases the fabrics can be understood as being consistent with a simpler interpretation than previously proposed, specifically with fabrics developed under simple shear. Additionally, small bands of ice that are entirely pinned by bubbles and characterized by finer grains display a more clearly defined CPO reflecting simple shear. These may represent zones of localized high strain. On the microscale as on the macroscale, many structures in ice are comparable to those developed in rocks undergoing high temperature deformation. These include fractures, foliation, and folds. In many glaciers, including Storglaciären, ridges of basally-derived debris present on the surface in the ablation zone are interpreted as having being emplaced along thrust faults, even though thrust faulting has not been demonstrated (except in one surging glacier) and should be mechanically inhibited in ice at natural strain rates. An alternative hypothesis is presented that calls on a combination of high and variable subglacial water pressure and tensile fracturing, based on field observations, CPO, sediment and isotopic analyses, supported by simple modeling. The debris ridges on the surface were likely incorporated into the ice by refreezing processes or injection into fractures opened during times of high water pressure near the slip/no slip (frozen near the toe) transition. Once the debris is incorporated into the glacier, the bands of debris features are transported forwards and upwards in the ice due to basal shear and longitudinal compression and vertical extension in the ablation zone.
Crystallographic preferred orientation (CPO), electron backscatter diffraction, faulting, glacier, microstructure, Storglaciären
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