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

The Cumbre Vieja volcano is the youngest component of the island of La Palm a. It is a very steep-sided oceanic island volcano, of a type which may und ergo large-scale lateral collapse with little precursory deformation. Recon figuration of the volcanic rift zones and underlying dyke swarms of the vol cano 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 Cumbr e Vieja volcano was characterised by a triple ("Mercedes Star") volcanic ri ft zone geometry. The three rift zones were unequally developed, with a hig hly productive south rift zone and weather NE and NW rift zones: the dispar ity in activity was probably due to topographic gravitational stresses asso ciated 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 th e NW volcanic rift zone diminished and the intersection of the rift zones m igrated 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 th e volcano, and by E-W-trending en echelon fissure arrays on the western fla nk of the volcano. The NE rift zone has become completely inactive. This st ructural reconfiguration indicates weakening of the western flank of the vo lcano. The most recent eruption near the summit of the Cumbre Vieja, that o f 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 zo ne of deformation and seaward movement within the western flank of the Cumb re Vieja: the volcano is therefore considered to be at an incipient stage o f flank instability. Climatic factors or strain weakening along the Cumbre Nueva collapse structure may account for the recent development of this ins tability. (C) 1999 Elsevier Science B.V. All rights reserved.