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 echo sounder data. The marine surveys show that, north of Tenerife, a giant landslide is exposed over an area of 5500 km(2) of the seafloor, more than twice the surface area of the island. The landslide truncates an older ridge and valley topography that is associated with the shield building basalts on Tenerife. We interpret the ridge and valley topography 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 about 1000 km(3) It extends onshore into the Orotava and Iced valleys which have been interpreted as of landslide origin. K-Ar dating of basaltic flows in the steep headwall of Orotava suggests an age of formation 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 most recent volcano on Tenerife, Las Canadas, and has been associated with the collapse of its caldera, between 1.2 and 0.2 Ma. A young age for the landslide is supported by the 3.5-kHz echo sounder data which show that the landslide is draped by a thin (< 10 m) layer of younger sediment. The landslide did not form, however, during a single catastrophic event but represents the amalgamation of a number of separate landslides. The occurrence of the ridge and valley topography in water depths of up to 2.5 km suggests that the shield-building basalts have subsided by at least this amount since they formed, 3.3-8.0 Ma. We speculate that this subsidence is caused by some form of stress relaxation that occurs in the underlying lithosphere. The giant landslide imaged in 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 adjustment, oceanic volcanoes have their greatest elevations above sea-level and therefore are most susceptible to slope failure.