Talk by Professor Ed. D. Waddington – Niels Bohr Institute - University of Copenhagen

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Talk by Professor Ed. D. Waddington

Folding in the ice sheets: why does it happen, and what can we learn from the new Greenland radar profiles?

ABSTRACT Back in 1988, we thought that layers near ice divides would just get thinner and thinner, but stay in the correct stratigraphic order, because incipient folds would be suppressed by the horizontal stretching expected there. The only layer disturbances might be boudins, or pinch-and-swell structures on alternating stiff and soft layers. Even extreme boudins could remove, but not duplicate record sections. In 1992, some glaciologists (who will not be named) predicted that a 250 ka record would be obtained at both GRIP and GISP2. That didn’t happen, because of unexpected flow disturbances in the deep ice. 

Subsequent analysis showed that passive recumbent folds could be expected close to divides, even with very simple flow assumptions, when strong basal shear strains overwhelm stretching effects.  However, perturbations to steady-state layer architecture are necessary. Transients in divide location or flow direction could leave layers in vulnerable positions from which they can be overturned.  The new CReSIS/IceBridge radar data from NW Greenland illuminate deep layers for the first time, and show large folds in progress.

With crystal anisotropy, inclined shear zones or slow faults may also be active, and may create the layer offsets needed for folding.