Activity along the Osning Thrust in Central Europe during the Lateglacial: ice-sheet and lithosphere interactions

The Osning Thrust is one of the major fault systems in Central Europe, with a length of approximately 115 km. It underwent a polyphase tectonic evolution in the Mesozoic that ranged from extensional movements in the Jurassic to reverse faulting and thrusting during inversion in the Late Cretaceous. A series of complex metre-scale faults and related fold structures are developed within the Upper Pleistocene alluvial-aeolian complex of the Upper Senne, 1 km away from the thrust. The faults dip towards the north and show offsets in a range of several decimetres. Growth strata indicate a two-fold evolution of the structures. The faults began as normal faults and were later transformed into reverse faults, which resulted in the formation of small-scale inversion structures with a typical harpoon shape. Glaciotectonics can be ruled out as the driving mechanism for these structures, because new optically-stimulated luminescence (OSL) ages imply that the sedimentary succession was deposited during the late Pleistocene between 29.3 +/- 2.9 ka and 13.1 +/- 1.5 ka, at which time the ice margin was much further to the northeast. We believe the inversion structures were caused by movement on the Osning Thrust due to generation of a forebulge of the Late Pleistocene Weichselian ice sheet. This led to normal faulting, as a consequence of extension in the forebulge area during glaciation. The OSL ages for the normal fault-related growth strata are in a range of 16-13 ka. Later, reverse movements occurred during deglaciation, due to the N-S directed compressional stress field in northern Germany. Numerical simulations of the deglaciation seismicity point to seismic events with a thrust mechanism in the study area between 15.5 and 12.3 ka BP, although normal faulting is also possible in this time period. Numerous soft sediment deformation structures imply that movement on the Osning Thrust caused earthquakes with a significant magnitude. In the autumn of 1612, an earthquake took place in this area. It is very likely that this event is related to background seismicity on this fault, although movement due to the influence of the ongoing glacial rebound in Fennoscandia is also possible. (C) 2012 Elsevier Ltd. All rights reserved.