THE CANARY DEBRIS FLOW - SOURCE AREA MORPHOLOGY AND FAILURE MECHANISMS

The morphology of the source area of the Canary Debris Flow has been m apped using both GLORIA reconnaissance and TOBI high-resolution sidesc an sonar systems. West of approximate to 19 degrees W, the seafloor is characterized by a strongly lineated downslope-trending fabric. This fabric can be interpreted as being caused by streams of debris separat ed by longitudinal shears. Multiple flow pulses are indicated by a ser ies of asymmetrical lateral ridges which mark the northern boundary of the now. East of approximate to 19 degrees 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 acros s, surrounded by debris now material. These are interpreted as in situ areas of seafloor sediment which have survived the slope failure and debris flow event rather than transported fragments of a failed sedime nt slab. TOBI data from the best developed area of alongslope lineamen ts show a series of small faults downstepping to the west. This area o f seafloor is interpreted as one of partial sediment failure, where th e failure process became 'frozen' before total mobilization of the sea floor sediments could occur. The overall morphology of the failure are a indicates removal of a slab-like body of sediment, although we canno t 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 tra nsport. Immediately upslope from the debris flow source area, a seaflo or of characteristic rough blocky texture is interpreted as the surfac e of a debris avalanche derived from the slopes of the island of El Hi erro. The debris flow and avalanche appear to be simultaneous events, with failure of the slope sediments occurring while the avalanche depo sits were still mobile enough to fill and disguise the topographic exp ression of the debris flow headwall. Loading of the slope sediments by the debris avalanche most probably triggered the Canary Debris Flow.