Holocene mass movements in west and mid-Norwegian fjords and lakes

In this study, identified mass transport deposits (MTDs) in previously analyzed as well as new sediment cores and seismic profiles from 22 fjord systems and six lakes in west and mid-Norway are compiled in order to close knowledge gaps in the understanding of how mass movements relate to seismic activity and climatic changes affecting the sedimentary systems. A total of 125 postglacial MTDs have been identified and dated in the studied systems. Mass movement frequency plots suggest high mass movement activity for two periods in the Early Holocene (11,000-9700 and 8300-7700 cal. yrs BP), a quiescent period in the mid-Holocene, and a mass movement reactivation at similar to 4200 cal. yrs BP. The evaluation of trigger mechanisms makes us infer that some of the mass movements identified are most likely related to climatic processes and tsunamis. However, most of the events seem to be caused by regional seismic activity, resulting in failure of climatically-preconditioned slopes. We suggest that at least 36 individual regional earthquakes (M > 6) have occurred in west and mid-Norway through the Holocene. Earthquake recurrence rates of 1/80 years are estimated for the period directly after the last deglaciation (similar to 12,800-11,600 cal. yrs BP), 1/200 years for the Early Holocene and 1/200 years for the last 4000 years of the Holocene time period. Comparisons with studies from other previously glaciated settings indicate that both Scandinavia and the Alps show similar trends as west and mid-Norway, but not Eastern Canada. This most likely reflects a later deglaciation of the Laurentide Ice Sheet.