STRAIN PATTERNS IN THE RUBJERG KNUDE GLACIOTECTONIC COMPLEX, DENMARK: A STUDY USING ANISOTROPY OF MAGNETIC SUSCEPTIBILITY

The Rubjerg Knude glaciotectonic complex along the northwestern coast of Denmark is a 6 km long fold-and-thrust belt consisting of glacial mud and sand. An enduring question, with paleoclimatological implications, is the extent to which sediments were frozen during their deformation. In the late Wisconsin, the bed of the southern margin of the Laurentide Ice Sheet is thought to have been locally cold-based at higher latitudes, and local evidence of glaciotectonic deformation is sometimes cited in support of that idea. Brittle failure of frozen sediments would have been dominated by fault slip and rigid-body rotation, with seemingly little pervasive strain. Glaciotectonically deformed sediments, however, are commonly devoid of macroscopic structures that allow the state of strain to be inferred, and hence strain patterns are usually poorly known.

This is the first study to determine strain patterns in a glaciotectonic complex by using fabrics based on the anisotropy of magnetic susceptibility (AMS) of intact sediment samples. To measure fabrics based on orientations of principal susceptibility axes, field samples of glaciolacustrine mud were collected in transects normal to fault surfaces dividing imbricated, stacked thrust sheets. Samples from horizontal layers in the forefield of the primary thrust complex provided pre-thrusting reference fabrics. AMS fabrics 0–35 cm from fault surfaces indicate simple-shear patterns consistent with experimental data collected from ring-shear experiments. In contrast, AMS fabrics farther from fault surfaces indicate rotation of principal susceptibility axes from those of the sampled horizontal layers, but these fabrics do not indicate pervasive deformation of mud slabs during thrusting. Mud samples subjected to hybrid states of strain between simple shear and pure shear provide support for this interpretation. These data indicate that strain was accommodated primarily by brittle deformation, except very near fault surfaces where sediments were sheared to high strains. This result, together with the coherence of alternating thrusted slabs of unlithified mud and sand over length scales of 100s of meters, suggest that sediments were largely frozen during thrusting.