New 3·5 kHz profiles and a series of piston cores from the north‐west African margin provide evidence that the Saharan debris flow travelled for more than 400 km on a highly fluid, low‐friction layer of poorly sorted sediment. Data suggest that the Saharan debris flow is a two‐phase event, consisting of a basal, volcaniclastic debris flow phase overlain by a pelagic debris flow phase. Both phases were emplaced on the lower continental rise by a single large debris flow at around 60 ka. The volcaniclastic flow left a thin deposit less than 5 m thick. This contrasts with the much thicker (over 25 m) deposit left by the pelagic debris flow phase. We suggest that pelagic sediment, sourced and mobilized as debris flow from the African continental margin, loaded and destabilized volcaniclastic material in the vicinity of the western Canaries. When subjected to this loading, the volcaniclastic material appears to have formed a highly fluid sandy debris flow, capable of transporting with it the huge volumes of pelagic debris, and contributing to a runout distance extending over 400 km downslope of the Canary Islands on slopes that decrease to as little as 0·05°. It is likely that the pelagic debris formed a thick impermeable slab above the volcanic debris, thus maintaining high pore pressures generated by loading and giving rise to low apparent friction conditions. The distribution of the two debris phases indicates that the volcaniclastic debris flow stopped within a few tens of kilometres after escaping from beneath the pelagic debris flow, probably because of dissipation of excess pore pressure when the seal of pelagic material was removed.
The morphology of the source area of the Canary Debris Flow has been mapped using both GLORIA reconnaissance and TOBI high-resolution sidescan sonar systems. West of »19°W, the sea¯oor is characterized by a strongly lineated downslope-trending fabric. This fabric can be interpreted as being caused by streams of debris separated by longitudinal shears. Multiple¯ow pulses are indicated by a series of asymmetrical lateral ridges which mark the northern boundary of the¯ow. East of »19°W, GLORIA data show only a weak fabric of irregular patches and alongslope lineaments. The TOBI data show the patches to be coherent sediment blocks up to 10 km across, surrounded by debris¯ow material. These are interpreted as in situ areas of sea¯oor sediment which have survived the slope failure and debris¯ow event rather than transported fragments of a failed sediment slab. TOBI data from the best developed area of alongslope lineaments show a series of small faults downstepping to the west. This area of sea¯oor is interpreted as one of partial sediment failure, where the failure process became`frozen' before total mobilization of the sea¯oor sediments could occur. The overall morphology of the failure area indicates removal of a slab-like body of sediment, although we cannot distinguish between retrogressive and slab-slide failure mechanisms. If the latter mechanism is applicable, fragmentation of the failing slab' must have commenced concurrently with the onset of downslope transport. Immediately upslope from the debris¯ow source area, a sea¯oor of characteristic rough blocky texture is interpreted as the surface of a debris avalanche derived from the slopes of the island of El Hierro. The debris¯ow and avalanche appear to be simultaneous events, with failure of the slope sediments occurring while the avalanche deposits were still mobile enough to ®ll and disguise the topographic expression of the debris¯ow headwall. Loading of the slope sediments by the debris avalanche most probably triggered the Canary Debris Flow.
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