Sill to dyke transition beneath a caldera: the competition between local stress and regional extension. Insights from analogue experiments applied to Campi Flegrei caldera, Italy.

Unrests at calderas are usually characterized by surface uplift, which is often driven by the pressurization of a sill-like reservoir. If an unrest ends up with an eruption, the location and timing for the opening of the eruptive vent are difficult to predict. In fact, when a reservoir fails, a magmatic dyke nucleates and starts propagating towards the surface, following a direction that results from the interplay between magma pressure, local stress, and regional tectonic. Where and how a sill reservoir will fail is one of the most uncertain factors in such a pre-eruptive scenario. In order to study the transition between an inflating sill and a dyke intrusion, we developed an original analogue model set-up: We shaped the surface of a solidified gelatin block, reproducing a simplified topography of Campi Flegrei caldera (Italy). This provides our model with the local unloading stress due to the presence of the caldera. In addition, we introduced a variable horizontal extension by expanding the gelatin block in one direction, providing a regional extension. We placed a sill-type reservoir below the caldera, scaling its dimensions based on previous deformation studies at Campi Flegrei. In our experiments, the reservoir was progressively pressurized through the injection of air from the bottom of the gelatin block, simulating a process of shallow sill-reservoir activation by a deeper “feeder dyke”. Depending on the ratio between the local unloading stress and the regional extension, we observed two main behaviors for the nucleation of a shallow dyke: I) if the local stress dominates over the regional extension - when the sill overpressure reaches a critical value - we observed the lateral growth of the sill, followed by the progressive re-orientation of the intrusion towards vertical, thus forming a dike which fed a circumferential vent on the rim of the caldera; II) if the extension dominates, the sill-to-dyke nucleation still occurs at the edge of the sill, but with a vertical dyke opening in the direction of the regional extension (on the same plane as the feeder dyke). The intrusion grows towards the surface, leading to a radial fissure located at the edge of the caldera.Previous estimates for the stress state at Campi Flegrei caldera from Rivalta et al. (2019) would suggest that the most relevant mechanism for Campi Flegrei may be the one dominated by the local stress rather than the regional extension (type I).