Seismites derived from glacio-isostatic rebound

What is it about?

Soft-sediment deformation structures are abundantly present in two levels within Warthanian/Eemian lacustrine sediments at study site, near the Polish/German border. The two ‘event horizons’ show intense folding, collapse, sag and load structures, indicative of liquidization andfluidization. The structures must have been caused by sudden shocks, most probably resulting from earthquakes that were induced by glacio-isostatic rebound, probably after the Warthanian deglaciation. Such seismites have not been previously recognized in Polish Quaternary sediments.

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Why is it important?

Fluctuations in the position of the ice-sheet margin due to alternat-ing phases of advance and retreat of the Warthanian ice mass caused changes in the pressure exerted by the ice weight on the substratum. Extensional stresses of up to10 MPa are assumed for a forebulge related to an ice cover of roughly 1 km thick, and the glacio-isostatic rebound when such an ice cap melts away is several hundreds of meters. It is therefore most likely that the Lower Odra Fault Zone, which is present in the vicinity of study site, was reactivated during the Pleistocene as a response to the loading/unloading cycles. This leads to the following conclusions: The two levels at study site with abundant soft-sediment deformation structures must be ascribed to earthquake-induced liquefaction of fine-grained lacustrine sediments; The earthquakes in the study area were most probably triggered by glacio-isostatic rebound at the end of the Warthanian; this is more likely because study site is situated outside of the limits of the Last (Weichselian) Glacial Maximum. To our knowledge, the earthquake-triggered soft-sediment deformation structures at study site are the first features in NW Poland that indicate neotectonics that can be ascribed to glacio-isostatic rebound.