Sea gravity data in the Gulf of Naples: a contribution to delineating the structural pattern of the Vesuvian area

Three active volcanoes are located in the Neapolitan volcanic region of the Campanian Plain i.e. Somma–Vesuvius, Campi Flegrei and Ischia. Bathymetric and gravity surveys in the Gulf of Naples were made since 1987 in order to complete the off-shore Bouguer map. Eight hundred sixty-two stations were surveyed within the 400 m isobath and the project was concluded in 1994. The data have been processed with 2000 gravity values available on land to obtain a Bouguer anomaly map of the whole volcanic region. The global set of gravity data has been referred to the absolute gravity station established in Naples in 1986 and remeasured in 1994. The gravity anomalies have been computed according to the 1980 geodetic reference system; a density value of 2200 kg/m3 has been used in the computation of the Bouguer and terrain effects. The anomaly map shows positive values ranging from 7 to 102 mGal with a mean value of 41 mGal. The Vesuvian area is characterized by a short wave length Bouguer anomaly, located at the southeastern edge of a large gravity minimum, north of Naples, interpreted as a tectonic depression (Acerra depression). A strip of high gravity gradient runs south of Somma–Vesuvius with an E–W trend on land changing to a NE–SW trend in the Gulf of Naples. This strip borders two gravity minima located southwest of the Somma–Vesuvius and south of the Bay of Pozzuoli, respectively. A well defined minimum spans the Campi Flegrei and strong gravity gradients border the islands of Ischia and Procida. The interpretation of the gravity anomalies is based on 2.5D modelling along selected profiles crossing the investigated area. The models have been constrained with data obtained from deep wells and other geophysical information. Three main bodies have been modelled, i.e. the polygenic filling of the Campanian Plain, the sedimentary basement and the crystalline basement. The interpretative sections suggest that in the Neapolitan area the sedimentary basement is strongly displaced. Its top reaches the maximum depth of 3 km in the central part of the Gulf of Naples and in the Acerra depression. In the Vesuvian area the sedimentary basement is about 11 km thick with the top at a depth of 2 km. A complex system of faults surrounds the Somma–Vesuvius.