Mechanics of Folding in Glaciers: A Numerical Study

Iowa State University, Ames IA

Investigators

Abstract

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.