Date of Award
Master of Science
Geological and Atmospheric Sciences
The Møns Klint and Rubjerg Knude glaciotectonic complexes along the SE and NW coasts, respectively, of Denmark are fold-thrust belts that outcrop over distances of four and six kilometers. An enduring question, with implications for paleo-glacier dynamics, regarding this complex and similar sediments thrusted near glacier margins elsewhere is the extent to which sediments were frozen during their deformation. Brittle failure of frozen sediments would have been dominated by fault slip and rigid-body rotation, with little penetrative shear strain. Sediments of glaciotectonic complexes, 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 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 extending from fault surfaces that either divide mud from overlying chalk at Møns Klint or divide imbricate thrust sheets at Rubjerg Knude. Samples from a horizontal layer at the distal end of the Rubjerg Knude complex provide reference fabrics for strain that indicate layer-parallel shortening in the absence of thrusting. AMS fabrics 0-35 cm from fault surfaces at Rubjerg Knude indicate simple-shear patterns consistent with experimental data collected from ring-shear experiments. No fabrics at Møns Klint, including some from samples collected as close as 3 cm to the fault surface, indicate simple shear, suggesting that shear strain was focused even nearer to the fault surface. In contrast, most AMS fabrics more distant from fault surfaces at both locations indicate only rotation of principal susceptibility axes from those of the sampled horizontal layer, with no penetrative strain other than the layer-parallel shortening that preceded thrusting. Thus, these data indicate that thrust sheets rotated but behaved rigidly during thrusting, except very near fault surfaces where sediments were sheared to high strains. This conclusion, together with the coherence of alternating long but thin thrusted blocks of unlithified mud and sand at Rubjerg Knude, suggest that sediments were largely frozen during thrusting, except at or very near thrust surfaces where resistance to shear was presumably low. Friction on thrust surfaces at Rubjerg Knude would have been sufficient to significantly heat and hence weaken the mud-ice mixture in a sheared zone a few decimeters thick adjacent to faults, based on the reconstructed range of fault-slip displacement.
Warbritton, Matthew, "Strain patterns in the Møns Klint and Rubjerg Knude glaciotectonic complexes, Denmark, inferred from anisotropy of magnetic susceptibility" (2019). Graduate Theses and Dissertations. 17119.