Lead


Lay summary
It is proposed to investigate in details the internal structure of awell-exposed caldera, the Las Cañadas caldera of Tenerife, in the Canaryarchipelago. Geophysical techniques using electromagnetic waves arethought to be particularly convenient in such volcanic areas with strongcontrasts of the electrical resistivity. The contemplated methods are:audio-magnetotellurics, VLF-R, VLF-EM, VLF-GRAD and Slingram (HLEM).

The Las Cañadas caldera makes up the central part of Tenerife. In the deepdepression (average elevation: 2,200 m) the highest volcanic edifice(3,718 m) of the Atlantic ocean is standing (Teide volcano). In thecaldera, evolved phonolitic magmas indicate the presence of shallow,high-level magma chambers. The caldera structure is the object of intensedebates about its buildup, either by multiple vertical collapses or bygiant sector collapse.

Marine surveys along the northern coast of Tenerife showed that a giantlandslide is exposed over an area of 5,500 km2 of the seafloor, more thantwice the area of the island. The landslide truncated an older ridge andvalley topography that is associated with the shield building basalts onTenerife. The landslide is also though to have caused the disappearance ofthe northern margin of the calderas.

Stratigraphic, structural, volcanological, petrological, geochronologicaland geophysical data suggest that the Las Cañadas caldera resulted frommultiple vertical collapse episodes that occurred during the constructionof the Las Cañadas youngest edifice, the Upper Group. Three long-term(200 ka) cycles of phonolitic explosive activity, eachculminating with a caldera collapse, have been identified in the UpperGroup. During this process, the focus of felsic volcanism migratedeastward, concurrently with the migration of the shallow magmachamber.

The exact development of the events described in the last two paragraphsis still controversial, largely due to the hiding effect from recentpyroclastic deposits. Although some relics from the first caldera wallsare still visible (Roques de Garcia, a large spur of pre-caldera rocks),most of the detailed structure is now hidden. The purpose of this study ismultiple:

to reveal the shallow (<200 m) caldera structures buried underthe pyroclastics.
to detect the hidden continuation of the most recent caldera rim(Diego Hernández)
to circumscribe the extension of the three successive calderasand estimate the depth to the impermeable floor (hydrothermalizedlavas).