The Canary Debris Flow: source area morphology and failure mechanisms

Abstract

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.

Growth and collapse of the Reunion Island volcanoes

“…These fields can cover large surfaces, as for example, to the east of the island where they are observed over at least 400 km 2 . La Réunion sedimentary ridges are comparable to features observed along the Hawaiian Ridge, but only on volcanoes west of Kauai and older than 5 Ma (Moore et al 1994), El Hierro (Masson et al 1998) or La Palma (Urgeles et al 1999). They are interpreted as mud or sediment waves.…”

The Gebra Slide: a submarine slide on the Trinity Peninsula Margin, Antarctica

“…The Verrill Canyon area on the Scotian Slope and successive landslides in the Canary Islands illustrate such multi-phase mass-wasting processes (Mosher et al, 1994;Masson et al, 1998;Gee et al, 2001). The Humboldt Slide, on the northern California continental margin, is interpreted as a large shear-dominated retrogressive slope failure (Gardner et al, 1999).…”

“…In recent years large-scale slope instabilities on continental margins have been the subject of many studies (Masson et al, 1998;Gardner et al, 1999;Laberg and Vorren, 2000a;Locat and Lee, 2000;Weaver et al, 2000;Locat, 2001). Such instabilities seem to a¡ect all types of submarine slopes although involving di¡erent types of mass-wasting processes and trigger mechanisms.…”

Rocky coast: geological constraints for hazard assessment

“…Whereas debris avalanches and slumps normally are phenomena that cut into volcanic and intrusive rocks on the flanks of volcanic edifices, debris flow affects only the sedimentary cover of submarine slopes and develops over much greater distances (up to 600-700 km; Gee et al 2001;Masson et al 2002). Debris flow may occur simultaneously with debris avalanche, as reported on the western slopes of El Hierro and La Palma in the Canary Islands (Masson et al 1998), or it develops after debris avalanche emplacement, leaving ridges and scours of debris on top of the blocky deposits. The transport of volcanically derived debris into the marine environment may continue downslope in the form of turbidite currents developing far from debris avalanches, which are able to transfer a complex sedimentary assemblage of volcaniclastic and pelagic slope sediments to abyssal plains ( Fig.…”

“…Some of these major events occurred in prehistoric times, such as at Ischia in the Bay of Naples (Chiocci & de Alteriis 2006;de Alteriis & Violante 2009), at Stromboli in the Aeolian Islands (Tinti et al 2000) and possibly at Etna on Sicily (Pareschi et al 2006). Nevertheless, the risk (c) Debris flow generated by flow transformation from a decelerating turbidity current (after Kim et al 1995;Masson et al 1998;Talling et al 2007). …”