Sea gravity data in the Gulf of Naples. A contribution to delineating the structural pattern of the Phlegraean Volcanic District

摘要

Bathymetric and gravity surveys were carried out from 1988 to 1994, in the Gulf of Naples (Southern Italy) to offshore extend the already existing Bouguer anomaly map. In order to improve the knowledge of the structural setting beneath the active Neapolitan volcanoes (Vesuvio, Campi Flegrei and Ischia), 862 stations were surveyed within the isobath of 400 m; at the same time, and about 2000 on-land gravity values were also collected. A new Bouguer anomaly map spanning the whole volcanic region was drawn from the final data set. Gravity anomalies were referred to the new absolute gravity station in Naples and computed according to 1980 Geodetic Reference System. Finally, a density value of 2200 kg/m~3 was used in the computation of the Bouguer and terrain effects. We carried out the inversion of the gravity anomalies adopting a 2.5-D modelling along selected profiles crossing the investigated area. The interpretative models were constrained to data obtained from deep wells and other geophysical investigations. In a previous paper, we investigated in detail the area around the Vesuvio. Here we focus on the Phlegraean Volcanic District (PVD), formed by Campi Flegrei, and the islands of Procida and Ischia. Our interpretation permits to outline two separate caldera-like structures in Campi Flegrei and their rims, mainly in the offshore sector. Moreover, it highlights a sharp separation between the island of Ischia and the rest of the PVD while showing Procida as well integrated, at least from the structural point of view. Ischia appears to be a block delimited all around and stepped by a series of volcano-tectonic structures. Both the qualitative description and the quantitative interpretation of the Bouguer anomaly in the PVD area, together with the comparison with the gravity features of the rest of the Neapolitan area, would suggest that the uprising basement beneath the island of Ischia, inducing most of the gravity signal, could be the carbonate ones. The certainty and the discrimination between a carbonate or lava basement beneath Campi Flegrei is not yet deducible. In addition, for this area, the presence of a magma chamber from the Bouguer anomaly, taking into account the information provided by further geophysical and geochemical researches, was checked. The add-on body improves the model as a whole and doesn't impact on the presence of a residual magma chamber. Finally, the quantitative interpretation also permitted to define the shape and the depth of the 2600 kg/m~3 basement (carbonate in places) and of the overlapping denser polygenic layer on which the inferred main structures was drawn.