New structural, hydrogeological and hydrothermal insights on Cumbre Vieja (La Palma, Canary Islands, Spain)

Determining the link between shallow structure and volcanism is a topic of interest in Volcanology. We carried out a geophysical study devoted to the characterization of the major structural limits influencing volcanic hydrothermal activity and underground meteoric circulations in La Palma (Canary Islands). Since 1 Ma volcanic activity concentrates at the southern half of the island, on the Cumbre Vieja volcanic rift-zone. During the 1949 eruption a N-S fault system, facing west, developed in the summit area of Cumbre Vieja. This was interpreted as the surface expression of an incipient deformation zone on the western flank. The distribution of the recent activity and faulting indicate that a discontinuity may be present beneath the western flank of Cumbre Vieja, along which a future collapse may occur. Our study, which combines the application of self-potential and soil temperature measurements, provides new information to characterize and locate the limits guiding upward or downward fluid circulation and possibly associated to future failures and potential landslides on the Cumbre Vieja rift-zone. We found a clear asymmetry of the self-potential signal between the eastern and the western flanks of Cumbre Vieja. Strong infiltration of meteoric water seems to affect most of the summit axis while we observe poor variation in the self-potential values on the western flank. This could be explained by a constant thickness of the vadose zone on this flank, i.e., the presence of an impermeable layer at constant depth. This layer could correspond to the landslide interface associated to the previous Cumbre Nueva edifice destruction, on which Cumbre Vieja is currently growing. This constant self-potential pattern is interrupted by at least one other main infiltration zone, near Tajuya volcano, at 1200 m a.s.l. on the western flank, where field observation allowed identifying a small horst-graben system. However it is not clear if this fault system results from local extension or from larger scale instability. Funding: this study is part of the ESTEPA project funded by the Spanish Ministerio de Economía y Competitividad (ayuda del para la realización de Acciones Complementarias a proyectos de Investigación fundamental no orientada 2011; Ref CGL2011-15139-E (subprograma BTE))