Morphology and origin of smaller-scale mass movements on the continental slope off northern Norway

Little attention has been paid to smaller-scale mass movements on continental slopes, even though they occur much more frequently than their large-scale counterparts. Swath bathymetry, side-scan sonar, sub-bottom profiler and seismic data from the continental slope offshore the Lofoten Islands, northern Norway, reveal evidence of repetitive smaller-scale translational sliding, involving spreading and multi-phase retrogression, in water depths between 1100 and 2500 m. Three styles of failure have been identified, occurring in close proximity. Style 1 is characterized by a 4.7 km wide and up to 100 m deep amphitheater shaped headwall, a relatively deep glide plane (± 130 mbsf), detached sediment ridges and a run-out area with rafted sediment blocks. Style 2 consists of a staircase pattern of secondary escarpments, caused by the activation of several glide planes between ± 30 and 110 mbsf. Headwalls and secondary escarpments have a height of up to 30 and 70 m, respectively. The run-out area shows an almost complete sediment evacuation. Style 3 is more subtle, as it is only identified on the side-scan sonar data due to its higher spatial resolution. This style shows different phases of on-going evolution, illustrating the gradual disintegration of a slab of sediments moving over a shallow glide plane at ± 13 mbsf. Zones with sediment slabs are up to several hundreds of meters wide and are sharply delineated by shear margins or escarpments. The spatial variation in the failure style is inferred to have been caused by the activation of different glide planes, which is probably a result of the thinning of contouritic sediments towards the south-west. In the north-east, the mounded contouritic sediments contain more potential glide planes and higher slope angles. The smaller-scale mass movements are suggested to have been triggered by undercutting and removal of support at the foot of the slope due to large-scale mass movements that have occurred immediately south of the study area, such as the Trوnadjupet or Nyk slides. The geophysical data indicate that relatively recent mass movement processes have taken place in this area through the formation of shallow cracks and graben structures by the movement of ± 13 m thick slabs of sediments (Style 3), which could have severe implications for offshore infrastructure.