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Passage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflection of flow pathways



Some of the Earth's largest submarine debris flows are found on the NW African margin. These debris flows are highly efficient, spreading hundreds of cubic kilometres of sediment over a wide area of the continental rise where slopes angles are often <1°. However, the processes by which these debris flows achieve such long run-outs, affecting tens of thousands of square kilometres of seafloor, are poorly understood. The Saharan debris flow has a run-out of ≈700 km, making it one of the longest debris flows on Earth. For its distal 450 km, it is underlain by a relatively thin and highly sheared basal volcaniclastic layer, which may have provided the low-friction conditions that enabled its extraordinarily long run-out. Between El Hierro Island and the Hijas Seamount on the continental rise, an ≈25- to 40-km-wide topographic gap is present, through which the Saharan debris flow and turbidites from the continental margin and flanks of the Canary Islands passed. Recently, the first deep-towed sonar images have been obtained, showing dramatic erosional and depositional processes operating within this topographic `gap' or `constriction'. These images show evidence for the passage of the Saharan debris flow and highly erosive turbidity currents, including the largest comet marks reported from the deep ocean. Sonar data and a seismic reflection profile obtained 70 km to the east, upslope of the topographic `gap', indicate that seafloor sediments to a depth of ≈30 m have been eroded by the Saharan debris flow to form the basal volcaniclastic layer. Within the topographic `gap', the Saharan debris flow appears to have been deflected by a low (≈20 m) topographic ridge, whereas turbidity currents predating the debris flow appear to have overtopped the ridge. This evidence suggests that, as turbidity currents passed into the topographic constriction, they experienced flow acceleration and, as a result, became highly erosive. Such observations have implications for the mechanics of long run-out debris flows and turbidity currents elsewhere in the deep sea, in particular how such large-scale flows erode the substrate and interact with seafloor topography.