A GIANT LANDSLIDE ON THE NORTH FLANK OF TENERIFE, CANARY-ISLANDS

The extent of mass wasting along the north flank of Tenerife has been mapped using swath bathymetry, GLORlA side-scan sonar, and 3.5-kHz ech o sounder data. The marine surveys show that, north of Tenerife, a gia nt landslide is exposed over an area of 5500 km(2) of the seafloor, mo re than twice the surface area of the island. The landslide truncates an older ridge and valley topography that is associated with the shiel d building basalts on Tenerife. We interpret the ridge and valley topo graphy as the result of subaerial erosion. The landslide is estimated to have a length of 100 km, a width of up to 80 km, and a volume of ab out 1000 km(3) It extends onshore into the Orotava and Iced valleys wh ich have been interpreted as of landslide origin. K-Ar dating of basal tic flows in the steep headwall of Orotava suggests an age of formatio n for the valley is younger than 0.78 Ma and may even be younger than 0.27 Ma. The Iced valley is located immediately to the north of the mo st recent volcano on Tenerife, Las Canadas, and has been associated wi th the collapse of its caldera, between 1.2 and 0.2 Ma. A young age fo r the landslide is supported by the 3.5-kHz echo sounder data which sh ow that the landslide is draped by a thin (< 10 m) layer of younger se diment. The landslide did not form, however, during a single catastrop hic event but represents the amalgamation of a number of separate land slides. The occurrence of the ridge and valley topography in water dep ths of up to 2.5 km suggests that the shield-building basalts have sub sided by at least this amount since they formed, 3.3-8.0 Ma. We specul ate that this subsidence is caused by some form of stress relaxation t hat occurs in the underlying lithosphere. The giant landslide imaged i n our sonar data is associated with the late stages in the development of the most recent volcano on Tenerife, Las Canadas, which only began at about 1.8 Ma. Thus landsliding may be a particular feature of the time soon after emplacement when because of incomplete isostatic adjus tment, oceanic volcanoes have their greatest elevations above sea-leve l and therefore are most susceptible to slope failure.