Imaging and modelling the subsurface structure of volcanic calderas with high-resolution aeromagnetic data at Vulcano (Aeolian Islands, Italy)

In this paper, we present a magnetic model of the subsurface structure of Vulcano island based on highresolution aeromagnetic data. Three profiles across the most intense magnetic anomalies over the Piano and Fossa calderas were selected for the magnetic modelling, which was constrained by structural and volcanological data, previous geophysical models, paleomagnetic data, and
borehole stratigraphy obtained from two deep wells. The interpretation of the magnetic sources represents a significant contribution to the understanding of the Piano and Fossa calderas’ underlying structure, providing us with evidence of the lateral discontinuity between them at depth. We propose that the positive magnetic anomalies in the Piano caldera area are caused by: (a) the remnants of an early submarine volcano; (b) an outcropping dyke swarm related to the feeding system of the Primordial Vulcano phase (beneath Mt. Saraceno); and (c) the presence of a non-outcropping dyke system intruded along a NE–SWoriented intra-caldera fault (beneath the eastern part of the Piano caldera). Offshore, to the west, the magnetic anomaly map suggests the presence of a submarine volcanic structure, not revealed by bathymetric data, which represent the eruptive centre, the presence of which has
been indirectly deduced from the outcrop of eastern-dipping lavas on the western seashore. Magnetic modelling of the Fossa caldera points to the presence of a highly magnetized
cone-like body inside the Fossa cone, centred beneath the
oldest crater rims. We interpret this body as a pile of
tephritic lavas emplaced in an early phase of activity of the
Fossa cone, suggesting that the volume of mafic lavas that
erupted at the beginning of the construction of the Fossa
edifice was more significant than has previously been
deduced. Furthermore, the presence of a magnetized body
inside the Fossa cone implies that high temperatures are
contained in very limited spaces, do not affect its bulk inner
structure, and are restricted to fumarolic conduits and vents.
In addition, structures beneath the western and northern part
of the Fossa caldera are revealed to have null or low
magnetization, which can be ascribed to the presence of
pyroclasts and hyaloclastites in this area as well as to a
large volume of hydrothermally altered materials. This
suggests that the hydrothermal system, with a very limited
extension at present, affected a larger area in the past,
especially beneath the western part of the caldera.