The Storegga Slide complex: repetitive large scale sliding with similar cause and development

Seven large pre-Holocene slides on the mid-Norwegian continental margin between 62 and 67°N have been investigated. Most of the slides are located in the area of the Holocene Storegga Slide and in the North Sea Fan. The largest of the pre-Holocene slides are comparable in size to the Storegga Slide. With the exception of one slide, the sliding took place after the onset of continental shelf glaciations at 0.5 Ma. Despite limited chronostratigraphic resolution, at least one large slide apparently occurs during every 100 ky, following glacial-interglacial cyclicity. The slides have several common characteristic features, and they detach in fine-grained, seismically stratified hemipelagic deposits, commonly developed as contourite drifts. The instability is most likely created by pore overpressure built up in the hemipelagic deposits by rapid loading from glacial deposits. In some cases fluid flow from underlying oozes, may be a cause of overpressure. Earthquakes are the most likely trigger. The slide morphologies and other characteristic features favour a retrogressive slide development, as also suggested for the Storegga Slide. The area of the Storegga Slide Complex is particularly slide prone because of its structural setting and the preference for fine grained drift deposition in this region.