TIMING AND GEOMORPHOLOGICAL IMPACT OF POSTGLACIAL FAULTING IN SOUTHWESTERN DENMARK

Analysis of high-detailed LiDAR-data has revealed surface deformations of a Late Weichselian glacial outwash plain in the southwestern part of Denmark. The deformations are clearly seen in an area of 4 X 6 km around the city of Tinglev, but several more subtle geomorphological irregularities can be seen in a much larger area. The deformations are typically seen as large depressions where the surface of the outwash plain lies up to 16 m lower than expected. In addition to this, the otherwise gently sloping surface of the plain is divided into a mosaic of polygons with varying slope orientation and slope magnitude. Underneath the deformations and with an overall corresponding orientation lies the Tønder Graben structure and the deformations of the outwash plain can be related directly to tectonic movements along the graben faults.

The outwash plain was formed at the time when the Late Weichselian ice sheet was residing at the Main Stationary Line just east of the study area (18.000 to 23.000 years ago) and therefore, any alterations to the surface of this outwash plain will be younger than this time interval. Soil samples from a lake in a depression on the outwash plain show that the earliest sedimentation started in early Boreal around 9.000 years after the ice sheet had melted away. This shows that the event that created the depressions most likely took place 9.000 years ago. A number of present-day streams on the outwash plain are deflected and display highly angular patterns that match the mosaic pattern of the deformed outwash plain showing that the tectonic event was short-lived.

We relate the tectonic event to stress-release following the weight-relief from the Scandinavian Ice Sheet and the timing corresponds with the majority of known deglaciation-induced faulting events in Northern Scandinavia. Denmark is at present a quiet area in terms of tectonics, but our investigations show that short-lived tectonic instability was introduced shortly after the deglaciation and that the event most likely was accompanied by earthquakes.