Recent structural evolution of the Cumbre Vieja volcano, La Palma, Canary Islands: volcanic rift zone reconfiguration as a precursor to volcano flank instability?

The Cumbre Vieja volcano is the youngest component of the island of La Palma. It is a very steep-sided oceanic island volcano, of a type which may undergo large-scale lateral collapse with little precursory deformation. Reconfiguration of the volcanic rift zones and underlying dyke swarms of the volcano is used to determine the present degree of instability of the volcano. For most of its history, from before 125 ka ago to around 20 ka, the Cumbre Vieja volcano was characterised by a triple ("Mercedes Star") volcanic rift zone geometry. The three rift zones were unequally developed, with a highly productive south rift zone and weather NE and NW rift zones: the disparity in activity was probably due to topographic gravitational stresses associated with the west facing Cumbre Nueva collapse structure underneath the western flank of the Cumbre Vieja. From 20 ka to about 7 ka, activity on the NW volcanic rift zone diminished and the intersection of the rift zones migrated slightly to the north. More recently, the triple rift geometry has been replaced at the surface by a N-S-trending rift zone which transects the volcano, and by E-W-trending en echelon fissure arrays on the western flank of the volcano. The NE rift zone has become completely inactive. This structural reconfiguration indicates weakening of the western flank of the volcano. The most recent eruption near the summit of the Cumbre Vieja, that of 1949, was accompanied by development of a west facing normal fault system along the crest of the volcano. The geometry of this fault system and the timing of its formation in relation to episodes of vent opening during the eruption indicate that it is not the surface expression of a dyke. Instead, it is interpreted as being the first surface rupture along a developing zone of deformation and seaward movement within the western flank of the Cumbre Vieja: the volcano is therefore considered to be at an incipient stage of flank instability. Climatic factors or strain weakening along the Cumbre Nueva collapse structure may account for the recent development of this instability. (C) 1999 Elsevier Science B.V. All rights reserved.