Internal structure of the western flank of the Cumbre Vieja volcano, La Palma, Canary Islands, from land magnetotelluric imaging

Large-scale mass wasting is a natural part of the evolution of volcanic islands, where deformation and indications of flank instability, such as large-scale faulting and seismic and aseismic slip are common. The Cumbre Vieja volcano on the island of La Palma (Canary Islands) provides an ideal setting to address fundamental questions about the structure, evolution and stability of island volcanoes. The island of La Palma is still in a shield-building stage, and it has been postulated that the western side of the island lies over a pre-existing zone of weakness that can nucleate ruptures. We undertook an audiomagnetotelluric (AMT) survey over the proposed unstable western flank to try to image structures that may be associated with the zone of weakness. The magnetotelluric method (MT) is a geophysical technique used to map the presence of fluids or image important structural contrasts. The goals of this study were (1) to delineate the unstable flank, (2) to map the structures underneath and (3) to determine the presence and geometry of fluids. The results show a 1 km thick top resistive layer overlaying an area of reduced resistivity (enhanced conductivity), interpreted as a layer consisting of an alteration zone and also fluids. Our results confirm previous studies that suggested the existence of a western flank lying over collapse debris material and hyaloclastites, and also they allow us to map part of the subaerial southern extent of the Cumbre Nueva units that lie beneath the more recent Cumbre Vieja rocks. In addition, dimensionality analysis maps the rotation of the dike emplacement off ridge, along the western flank in an en echelon fashion.