Abstract Subduction of seamounts at destructive sedimented plate margins results in spectacular deformation of the overriding plate. High-resolution sidescan sonar imagery from the Costa Rica margin show the tracks of five individual seamounts, of which four are described in this paper. These were subducted at various times during the last 690 ka and each represents a different stage in the subduction process. Each subducted seamount leaves a parallel-sided depression in its wake, that can be traced for up to 55 km landward of the deformation front. This wake is created by deformation and uplift of the continental slope as the seamount passes beneath it, followed by collapse due to landsliding as support for the uplifted area is withdrawn. Areas of uplift above seamounts are characterized by complex normal and strike-slip fault patterns. Collapse of the uplift along the trailing edge of the seamount creates a zone of slope failure (landsliding) that migrates upslope (or landward) with the seamount. Landslide processes are dominated by debris flow, but also include sliding of coherent blocks and debris avalanche. Erosion occurs by repeated landslides, which produce a series of overlapping debris flows. Downslope sediment transport typically extends over limited distances, resulting in partial ‘backfilling’ of the scar as its headwall moves up slope. The amount of margin material disrupted by seamount subduction is four to five times the volume of the subducting seamount, of which about three quarters seems to be recycled downslope, backfilling the scar, and nearly one quarter is subducted with the seamount.

Abstract Submarine debris flows show highly variable mixing behaviour. Glacigenic debris flows travel hundreds of kilometres along the sea floor without undergoing significant dilution. However, in other locations, submarine slope failures may transform into turbidity currents before exiting the continental slope. Rates and processes of mixing have not been measured directly in submarine flow events. Our present understanding of these rates and processes is based on experimental and theoretical constraints. Significant experimental and theoretical work has been completed in recent years to constrain rates of shear mixing between static layers of sediment and overlying turbulent flows of water. This work was driven by a need to predict transport of fluid mud and the erosion of cohesive mud beds in shallow water settings such as estuaries, docks and shipping channels. These experimental measurements show that the critical shear stress necessary to initiate shear mixing (around 0.1 to 2 Pa) is typically several orders of magnitude lower than the yield strength of the debris. Shear mixing should initiate at relatively low velocities (about 10–200 cm s −1 ) on the upper surface of a submarine debris flow, at even lower velocities at its head (about 1–10 cm s −1 ), and play an important role in mixing over-ridden water into the debris flow. Addition of small amounts of mud (approximately 3% kaolin) to a sand bed dramatically reduces the rate of mixing at its boundary, and changes the processes by which sediment is removed. Estimates are presented for rates of shear mixing at a given flow velocity, and for the critical velocity necessary for hydroplaning or a transition from laminar to turbulent flow. Although these estimates are crude, and highlight the need for further experimental work, they illustrate the potential for highly variable mixing behaviour in submarine flow events.