New insights into the formation of postglacial fracture valleys in Denmark using LiDAR, and towed Transient Electromagnetic Methods

The presence of postglacial fracture valleys in Denmark has been known since the early 20th century. Their mode of formation has never been fully understood, but they are often considered to originate from the reactivation of deep-seated faults in postglacial time in relation to glacio-isostatic adjustments. This study investigates fracture valleys from Denmark regarding their morphology, age, and mode of formation using high-resolution LiDAR, and towed Transient Electromagnetic Method (tTEM). Eleven areas in central Jutland have been found to host 371 fracture valleys. The c. 0.04–8 km long fracture valleys are in most places located parallel to tunnel valleys, which are cut into Paleogene clays. Based on the cross-cutting relationship between the fracture valleys and glacial- and periglacial landscape features, our study confirms that they were formed in postglacial time, most likely between c. 18–15 ka BP. Our findings suggest that the fracture valleys in Denmark are unrelated to deep tectonic faults and instead developed on the flanks of tunnel valleys cut into Paleogene clays by differential compaction by glacial loading and unloading during the last glaciation. The differential compaction led to local failures along tunnel valleys that are expressed in the present land surface as 5–10 m deep fracture valleys. Our findings may have important societal implications concerning the risk assessment of neotectonic activity in Denmark.