Age and geometry of an aborted rift flank collapse: the San Andres fault system, El Hierro, Canary Islands

The catastrophic slope failures and landslides that occur in the final stages of lateral collapses of volcanoes destroy much of the evidence for precursory deformation and the early stages of the collapses concerned. Aborted or incomplete collapse structures, although rare, are rich sources of information on these stages of development of catastrophic collapses. The San Andres fault system, on the volcanic island of El Hierro, is a relatively young (between about 545 and about 261-176 ka old) but inactive lateral collapse structure. It appears to represent an aborted giant landslide. It is developed along the flank of a steep-sided volcanic rift zone, and is bounded by a discrete strike-slip fault zone at the up-rift end, closest to the centre of the island. This geometry differs markedly from that of collapse structures on stratovolcanoes but bears some similarities to that of active fault systems on Hawaii. Although the fault system has undergone little erosion, cataclasites which formed close to the palaeosurface are well exposed. These cataclasites are amongst the first fault rocks to be described from volcano lateral collapse structures and include the only pseudotachylytes to have been identified in such structures to date. Their development at unusually shallow depths is attributed to large movements on the fault in a single event, the inferred aborted landslide, and a lack of pressurized pore water. The absence of pressurized fluids in the slumping block may have caused the San Andres fault system to cease moving, rather than develop into a giant volcanic landslide. The recognition that the San Andres fault system is inactive greatly reduces the estimated volcanic hazard associated with El Hierro. However, the lack of evidence for precursory deformation prior to the aborted land-slide event is disturbing as it implies that giant lateral collapses can occur on steep-sided oceanic islands with little warning.