Quaternary stratigraphy of the Lake Thun area: Implications for neotectonics and glacial history

Abstract

The Alps and its adjacent foreland have been dominated by the interplay of tectonic (topography building) and erosional (destructive) processes since the onset of Alpine orogeny in the Early Paleogene. In the more recent geological past (Quaternary), the landscape was predominantly shaped by several glacial-interglacial cycles, contributing significantly to the evolution of the modern expression of the landscape. Relicts of these partially ongoing tectonic and erosional processes exist in the form of present seismicity and overdeepened valleys. Perialpine Lake Thun, located at the northern margin of the Bernese Alps, represents an ideal natural laboratory to investigate these fundamental processes. Paleoseismological off-fault evidence shows that prehistoric seismic events with moment magnitudes Mw >6 occurred on a regular basis of every 1’000–2’000 years during the last 15 ka. However, corresponding seismogenic fault structures and on-fault evidence seems to be absent. The first part of this thesis applies a multidisciplinary approach combining offshore swath bathymetry and multi-channel reflection seismic data with onshore geomorphological, geological and ground penetrating radar data, which provides multiple lines of evidence for an active fault zone. The ~4 km long strike-slip fault with a normal faulting component experienced its last activity less than 11’000 years ago and is ideally oriented in today’s NW-SE compressional stress field to be reactivated at any time. The second part of the thesis explores the sedimentary infill and morphology of the overdeepened bedrock trough covered by perialpine Lake Thun. Apart from the significant overdeepening of more than 750 m with respect to the current fluvial base level (i.e. lake level), the seismic stratigraphy details the internal architecture of a subaquatic moraine complex illustrating the slightly fluctuating behavior associated with the intermediate halt of the retreating Aare Glacier during its overall recessional phase following the last glaciation. The third part of the thesis discusses complex deformational patterns unraveled by the detailed seismic stratigraphic characterization of the sedimentary infill of Lake Thun. Sets of dominating basin-parallel strike-slip faults and perpendicular thrust faults are primarily the results of neotectonic processes and are therefore compared to the different configurations of extracted lineament sets of the surrounding Helvetic and Pennininc nappe stacks. Additionally, these faults may be subject to other driving forces, such as glacio-tectonics and post-glacial rebound.