Multiple related flank collapses on volcanic oceanic islands: Evidence from the debris avalanche deposits in the Orotava Valley water galleries (Tenerife, Canary Islands)

Abstract

Catastrophic flank collapses are common on oceanic islands of volcanic origin and often recognizable from the resulting morphology as large U-shaped embayments. However, post-collapse volcanic activity can infill such features, thereby obscuring them. This study takes advantage of a dense network of long sub-horizontal galleries for groundwater extraction in the flank collapse structure of the Orotava Valley (Tenerife, Canary Islands). This impressive landform is located in the overlapping zone of the NE Rift and the Cañadas Edifice of this volcanic island. Three debris avalanche deposits (DADs) have been identified inside the waterworks bored into the valley's western sector. The deeper layer, the Lower-DAD (L-DAD) was previously described under the local name of mortalón. This deposit lies unconformably over older volcanic rocks, where a prominent shear-zone is developed, here interpreted as the detachment plane of a massive rockslide. The L-DAD was therefore produced during the large failure event, the Orotava Landslide (OL), which carved the depression. Two younger DADs, much smaller in volume and not previously described, were also identified in the underground strata: the Intermediate- (I-DAD) and Upper-DAD (U-DAD). Both correlate well with the thick breccia deposits cropping out at the base of the marine cliff along the western coast of the valley. The I-DAD and U-DAD are conformably intercalated between the lava flows and other volcanics infilling the depression, and their bases are erosive/depositional features without structural deformation zones below them. Their stratigraphic setting and geometrical reconstruction indicate that both deposits were emplaced after the valley was formed by the OL event, in two successive failures here called Western Orotava Landslides (WOLs). The younger, called WOL-2, was dated at 494 ± 22 ka using the 40Ar/39Ar method. These two subaerial failures affected the upper eastern flank of the Cañadas III Edifice, previously destabilized when the OL failure event carved a steep wall at its foot, the western lateral escarpment of the Orotava Valley.