SEDIMENTARY PROCESSES RELATING TO A GIANT LANDSLIDE ON THE CANARY ISLANDS AT THE END OF THE LAST ICE AGE

Erosion of volcanic oceanic islands by large scale landsliding is a well known phenomenon. Many hundred km³ of rock can be removed in a single event, leaving towering headwall scars, amphitheatre shaped depressions (many previously regarded as calderas), and massive dumps of debris offshore. Here we summarise one single event which removed a substantial portion of the Canary Island of El Hierro about 12-17,000 years ago. The initial landslide involved dislocation of about 180 km³ of rock from the north-western flank of the island; about 50% of the material was derived from subaerial sources. Avalanche deposits extend to 4000 m waterdepth, cover an area of 1500 km², and have a thickness of 250-350 m. Blocks up to 1 km across occur on the seabed up to 65 km away from the island.

The debris avalanche overloaded the continental slope, causing failure of the sediments and triggering a debris flow with a volume of 400 km³. The boundary of the debris avalanche and debris flow is marked by a series of shallow faults related to the disintegration of the failed sediment. The debris flow contains rafted blocks and clasts ranging from a few hundred metres to a few tens of centimetres across. It travelled 600 km to the edge of the abyssal plain depositing a thin (average 10 m thick) debris flow sheet over an area of 40,000 km².

A giant turbidity current was also generated during the landslide. This initially followed the route of the debris flow but diverged to the north after about 3-400 km. A coarser bed load can be identified which was diverted directly to the plain via a series of lower slope channels, whilst the suspended load continued northwestwards, before ultimately entering the plain from the northeast. The turbidite has a volume of 125 km³.