Studies of submarine slope failures in the North Atlantic: Causes, timing and consequences

Submarine landslides are a significant geohazard to offshore infrastructure and coastal areas. They occur worldwide on the slopes of volcanic islands and continental margins. In the NE Atlantic, many large-scale Holocene and Pleistocene submarine landslides are located at the mouth of cross-shelf troughs and were probably triggered by earthquakes. Discussions on critical preconditioning processes for slope failure relate to cyclic sedimentation patterns during glacial periods, gas hydrate dissociation caused by changing pressure and temperature condition, and over-steepening due to toe erosion or uneven sedimentation patterns. However, the significant geological destabilizing processes leading to slope failure are still not fully understood and require further studies. The main objective of this thesis is to gain new insights about the initiation of submarine landslides and to identify which particular destabilizing preconditioning processes make slopes prone to failure. This aim is addressed by the reconstruction of the failure chronology of the newly discovered Fram Slide Complex, and by a numerical modeling approach that investigates a new preconditioning process related to gas hydrates. This thesis shows that low-sedimentation-rate slopes on glacial continental margins can bear significant geohazards related to slope failure. The Fram Slide Complex covers an area of ~5500 km2 at 850 to 4200 m water depth ~35 km NW off the Svalbard shelf. This distal location relative to proximal plume deposits rules out a causal relationship of the slope failure in this area to overpressure generation caused by rapidly deposited glacial sediments. Nevertheless, the Fram Slide Complex underwent repeated slope failure since 5 Ma without obvious limitation or concentration to a certain period. Spatial variation of recurrence frequencies and volume of past failures of different parts of the Fram Slide Complex are significant but do not present a simple explanation for destabilization. The absence of evidence for destabilization linked to tectonics, climate and oceanographic conditions emphasizes the impact of local destabilizing processes. The combination of toe erosion and slope shape might have caused repeated slope failure in the southern part of the study area. Another possible local destabilizing process could be related to overpressure generation by gas hydrates. Numerical modeling shows that critical overpressure from below the gas hydrate stability zone can transfer through pipe structures to inherently weak layers in the shallow subsurface. The pressure can build up laterally in the weak layer and can initiate retrogressive submarine slope failure. The results of this thesis show that the investigated region is highly instable. Potential future slope failures are a threat to offshore infrastructure but the limited volumes of evacuated material and water depth below 1000 m of most recent landslides suggest a rather small tsunami potential. The investigation could not identify a significant reason for slope failure which demonstrates that hazard assessments for the glaciated NW European continental margins are more complex than the results of many studies suggest that analyzed major slope failures adjacent to trough mouth fans. The focus on the glacial cycles has to be reconsidered and expanded. To further improve the understanding of slope stability and destabilizing processes in the future, a compilation of in situ and laboratory measurements and experiments could provide information on the detailed sediment composition. Theses integrated and multi-disciplinary investigations of geophysical and geotechnical data could be based on drilling and gravity cores of failed and intact sediments to reveal shear strength, environmental in situ stresses or pore pressure. These findings could contribute to a new assessment of slope stability in the North Atlantic.