Abstract
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/m3 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/m3 basement (carbonate
in places) and of the overlapping denser polygenic layer on which
the inferred main structures was drawn.
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