Deformation of the substratum of a large shield volcano: Triggering factor for past flank collapses in the old volcanic edifice of La Gomera, Canary Islands

The Basal Complex of La Gomera (Canary Islands) has been analyzed with the aim of deciphering its structural evolution, spanning the middle to late Miocene. Detailed structural maps and cross sections, and the results of a systematic measurement of dike orientation and fault-slip data are presented in this work. The main structural features found in the Basal Complex of La Gomera are four dike sets and two large, normal faults (Guillama and Montana de Alcala faults). Several extensional episodes have been identified. The main deformation phase corresponds to a NNW-SSE– to NW-SE–directed extension that generated first a dense swarm of mafic dikes, and then a large-scale collapse of the Basal Complex with displacement along the Guillama and Montana de Alcala faults and associated rotation of large blocks of La Gomera basement. It is interpreted that the supposed staircase geometry of the Guillama fault is responsible for the observed arrangement of rotated dikes due to development of kilometric-scale fault-bend folds with rounded hinges. Deformation of the Basal Complex shows a complete geometric, kinematic, and chronologic consistency with the large volcanic flank collapses that affected the units belonging to the early growth stages of La Gomera subaerial shield volcano. The results of this work support the models that invoke the importance of the large-scale geodynamic setting on volcano destabilization. More attention should be paid to the structural and geophysical characteristics of volcano basements in order to better evaluate the danger of large, catastrophic volcanic landslide events.