Palaeoseismic structures in Quaternary sediments, related to an assumed fault zone north of the Permian Peissen-Gnutz salt structure (NW Germany) – Neotectonic activity and earthquakes from the Saalian to the Holocene

Abstract

This study reports on neotectonic deformation represented as faults, graben, and horst structures, which have been found in the uppermost 10 m of Quaternary sediments, consisting mainly of Saalian glaciofluvial and Weichselian periglacial material in northern Germany that is generally considered as a low-seismicity area. An area with up to 35 faults in a 60 m wide zone exhibits mainly extensional structures. Most of those are SE-NW-oriented and in some cases NE-SW. Soft sediment deformation structures include sand blows, blowout-related infill structures, and growth faults. Small-scaled, soft-sediment deformation structures are also found. Based on the large size of the sand blows and the widespread occurrence of small-scaled, soft-sediment deformation structures, the magnitude of the earthquakes is estimated in the order of M ≥ 5.5. Single faults exhibit a displacement of a few metres. Because some faults are overlain by unfaulted sandur sediments the system has been active over at least ~200 ka (Saalian). Several faults extend through the Weichselian periglacial layer to the present ground surface, proving ongoing deformation during the Weichselian Lateglacial and/or the Holocene. This major activity during the Saalian and the Weichselian points to the activation of old tectonic structures. A possible process can be the glacial loading and unloading associated with the isostatic rebound. Holocene and late Holocene seismic events are shown by the penetration of the Weichselian-Lateglacial periglacial layer by faults and sand blows as well as a possible blowout-related bowl with organic material and corresponding young 14C dates of <300 a cal 14C. The documented fault zone could possibly be attributed to an intraplate pre-Quaternary fault within the central Glückstadt Graben. Concurrent adjacent faults are assumed from the morphology.