Folding in glaciers disrupts stratigraphic layering and changes flow trajectories of ice and any sediment entrained therein. Folds appear at scales ranging from centimeters to hundreds of meters, and are particularly evident near glacier margins where ice containing sediment bands undergoes longitudinal compression. While descriptions of folds in this context are plentiful, a complete understanding of the mechanical conditions that produce them is lacking. A common assumption is that layers become distorted passively as the glacier passes through homogeneous stress fields. However, ice-sediment mixtures have significantly different mechanical properties than clean ice, suggesting that stresses should be far from homogeneous. Resulting inhomogeneous strains, and therefore sediment transport paths, can be dictated by the rheological differences between sediment-bearing ice and surrounding clean ice. This project will evaluate the impact of mechanically inhomogeneous ice layering on fold kinematics in glaciers using established rheological formulations for sediment-rich ice and clean ice. A commercial finite difference code (FLAC, Itasca Consulting Group) will be used to simulate deformation to large strains relevant in glacial environments. Key results will tested against available analytical and numerical solutions.
This project will provide a physically-grounded framework for interpretation of folds in glaciers containing sediment layers. Folded layers preserve a composite record of the stresses experienced by the ice as it passes through the glacier. Changes in stresses are often due to temporal or spatial changes in the glacier's coupling to the bed. A better understanding of how layered, sediment-bearing ice responds to imposed stresses will allow more confident interpretation of the glaciological circumstances that gives rise to folds. The results will also broaden the basis for prediction of how stratigraphic layering in ice sheets can be disturbed along flow, complicating the task of dating layers in ice cores. Furthermore, the study can contribute to the analysis of folding in rock formations, which occur over longer time spans that cannot be observed, This project will produce a hands-on teaching module integrating glacial processes and structural geology that will be made widely available on the internet.
The overarching goal of this project was to develop a physicall-sound framework to describe deformation in glaciers containing layers of dirty ice. Among the key findings of this work has been an integrated review of dirty ice rheology, incorporating theoretical, field, and laboratory results from glaciology, permafrost mechanics, planetary science, volcanology, granular physics, metallurgy, and materials science, among other fields. A key behavioral transition occurs when the volumetric concentration of solid particles equals about 40%. Above this value, the debris-ice mixture has a yield strength, above which it behaves like a frictional material like soil. When the frictional strength is exceeded, it transiently gets stronger with increased strain rate until a critical shear or volumetric strain is achieved and connectivity of interstitial ice disappears. Below 40% volumetric debris content the ice strength depends on competing effects of the temperature of the ice and the grain-size distribution of the debris and ice crystals, producing either strengthening or weakening (relative to clean ice at the same temperature) as a complex function of these variables. Applying this complex rheology to modeling of folding in debris-bearing ice layers required some generalization of the rheological rules that were optimized according to the following rules: 1) Densely-dirty ice was treated as a frictional material with a nonlinear strain-rate hardening rule that decayed with post-yield strain; 2) Sparsely-dirty ice was treated as a power-law fluid with constant or variable flow index (power-law exponent) and a fluidity parameter dependent on the number and volumetric concentration of debris particles and the temperature (and thus unfrozen water content) of the system. Because generalizations of this sort applicable to glacial systems were previously unavailable, these two relationships are a key finding of the project. Numerical glacier simulations with varying distributions of dirty ice layers confirmed that a buckling instability could develop near ice margins subjected to layer-parallel compression, particularly for cold, sparsely dirty ice or densely-dirty ice prior to strain weakening. In addition, reference runs with arbitrary layer rheologies showed that flow over a subglacial obstacle in an overall longitudinally-compressive strain regime can generate englacial folds even in the absence of a mechanical instability. Flow of compressing, debris-bearing basal ice over an unfrozen sediment bed shows promise as a mechanism for entraining and transporting basal materials to the surface because of the ineraction between rigid, dirty basal ice and adjacent deformable sediment.
