Evidence from seismites for glacio-isostatically induced crustal faulting in front of an advancing land-ice mass (Rugen Island, SW Baltic Sea)

It is hitherto commonly thought that earthquakes triggered by geologically rapid changes in the pressure caused by the fluctuating extent of thick land-ice masses occur exclusively during ice retreat. Our study of deformed sediment layers within an undeformed glaciolacustrine and glaciofluvial succession exposed in a coastal cliff on Rugen Island, south-western Baltic Sea, challenges this widespread idea. Based on their structural and deformational features, the layers under investigation are interpreted as seismites which formed as a result of seismically-induced liquefaction. The stratigraphic context aided by optically stimulated luminescence (OSL) dating indicate that the documented seismites must have formed shortly before the front of the Pleistocene Scandinavian Ice Sheet reached the study area during the Last Glacial Maximum, thus during a stage of ice advance. This implies that the flexural isostatic response of the Earth's crust as a consequence of the ice load during ice advance was accompanied by earthquakes probably due to local re -activation of pre-existing faults. The crustal response to an increasing ice load was consequently less gradual than previous studies about glaciation-related tectonics suggest.