Volcano spreading and fault interaction influenced by rift zone intrusions: Insights from analogue experiments analyzed with digital image correlation technique

Large volcanoes tend to be structurally unstable and subject to various forms of deformation and mass wasting. Although morphological changes associated with flank deformation and flank instability are similar on many islands, the details of the structural expression, the fault geometry and the size of the mobile flanks vary significantly. Flank instability on ocean island volcanoes is thought to be related to two principal mechanisms: gravitational spreading and intrusions into a rift zone. In this paper, we first summarize typical structures observed in nature, and then experimentally investigate structures characteristic of volcanic spreading and rift zone intrusions, in order to better understand the interplay of these two mechanisms.We reproduced unstable, intruding and deforming volcanic flanks in a sand-box and systematically studied faults developing during (i) gravitational spreading, (ii) rift zone intrusion, and (iii) combined spreading and intrusion. We recorded the experiments with a digital camera and measured deformation by applying a digital image correlation technique. This enables us to study high resolution strain fields, locate faults and analyze their related activities. The end-member models show that typical spreading structures are listric normal faults, whereas typical rift zone intrusion structures include a graben at the surface and a subhorizontal detachment at depth. Concurrent or alternating simulations of spreading and rift zone intrusions reveal the interplay of the developed faults; some are hindered while others are more encouraged to slip. For instance, while the pure spreading end-member promotes internal faulting within the unstable flank, the occurrence of rift zone intrusions hinders this type of faulting and hence may even stabilize part of the volcanic flank. We apply this understanding to Kilauea (Hawaii) and Piton de la Fournaise (La Réunion) in an attempt to better identify the related mechanisms of intrusions, flank instability and sector collapse. Important implications arise for geological and geophysical field data, such as seismicity or geodetic measurements.