Journal Article

Formation Mechanisms of Large‐Scale Folding in Greenland's Ice Sheet

Li, Dian, Zhang, Yu, Franke, Steven, Bons, Paul D., Yang, Haibin, Weikusat, Ilka, Sachau, Till

2024

Overview

Radio‐echo sounding (RES) shows large‐scale englacial stratigraphic folds are ubiquitous in Greenland's ice sheet. However, there is no consensus yet on how these folds form. Here, we use the full‐Stokes code Underworld2 to simulate ice movements in three‐dimensional convergent flow, mainly considering ice anisotropy due to a crystallographic preferred orientation, vertical viscosity and density gradients in ice layers, and bedrock topography. Our simulated folds show complex patterns and are classified into: large‐scale folds (>100 m amplitude), small‐scale folds (<<100 m) and basal‐shear folds. The amplitudes of large‐scale folds tend to be at their maximum in the middle of the ice column or just below, in accordance with observations in RES data. We conclude that ice anisotropy amplifies the perturbations in ice layers (mainly due to bedrock topography) into large‐scale folds during flow. Density differences between the warm deep ice and cold ice above may enhance fold amplification. Plain Language Summary Polar ice sheets are composed of compacted former snow layers deposited at the ice surface. If not distorted or deformed, these layers are flat or adapt to the underlying bedrock topography. However, vertical radar scans of Greenland's ice sheet show large‐scale folds of up to hundreds of meters in height. To investigate how these large‐scale folds form, we set up a three‐dimensional numerical ice‐sheet model and simulate fold growth. Our modeling emphasizes the distinctive physical properties of ice required for fold formation, notably its anisotropy (the direction dependency of the flow strength) and power‐law rheology (when ice becomes softer with increasing strain rate). These findings help to better explain ice flow dynamics. Key Points Large‐scale fold formation in polar ice sheets is mainly controlled by ice anisotropy and bedrock topography Buoyancy of deep warm ice can further enhance fold amplification The implementation of ice anisotropy should be included in large‐scale ice flow modeling

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Amplification / Amplitudes / Anisotropy / bedrock topography / Cold flow / Crystallography / Density gradients / Echo sounding / Echosounding / fold / Glaciation / Ice / Ice formation / ice modeling / Ice sheets / non‐linear viscosity / Physical properties / polar ice sheet / Preferred orientation / Radar / Rheological properties / Rheology / Sheet modelling / Strain rate / Stratigraphy / Topography / Vertical orientation / Viscosity