The submerged border of the Phlegrean Fields volcanic complex (Bay of Naples, Southern Tyrrhenian sea, Italy): constraints from high resolution Multibeam bathymetry and multichannel seismics

The submerged border of the Phlegrean Fields caldera (Bay of Naples, Southern Tyrrhenian sea, Italy) has been identified based on combined high resolution magnetic anomaly map superimposed on Digital Terrain Model of the Naples Bay. High resolution bathymetric and magnetic data have been recorded during several oceanographic cruises carried out by the CNR-IAMC Institute of Naples, Italy onboard of the R/V Urania, National Research Council of Italy. A Total Magnetic Field (EMF) survey was recently carried out in the Bay of Naples, having an advanced spatial coverage with respect to previous measurements, such as by airborne magnetic survey recorded by AGIP in the 1981. Moreover, the data were acquired at a shorter distance from the source and at a lower velocity, giving more precise sampling and improved field restoration. The main magnetic anomaly fields recognised on the map have been constrained by geologic features based on geologic interpretation and correlation with features identified based on the interpretation of Digital Terrain Model. Two main belts of sharp magnetic anomalies have been identified, the first one located offshore the Somma-Vesuvius volcanic complex and the second offshore the Phlegrean Fields volcanic complex. The latter is here interpreted as the submerged border of the Phlegrean Fields resurgent caldera. The caldera border shows a NE-SW structural elongment, parallel to the Sarno/Sebeto - Dohrn canyon structural elongment. It is located in correspondence to the outer shelf of the Gulf of Pozzuoli and to the submerged volcanic banks of Nisida, Miseno and Pentapalummo. It represents a relatively complex magnetic anomaly area, characterized by several magnetic anomaly fields having a different intensity. Two dipolar anomalies, E-W oriented, have been recognised, indicating magnetised bodies and/or volcanic edifices. The first anomaly, located northwards, is characterized by a minimum of about - 200 nT, associated to a maximum of + 185 nT. Such a values should indicate small buried volcanic bodies. The second one, NW-SE oriented and located eastwards is characterized by a maximum-minimum couple having intensities similar to those of the previous field. Other non-dipolar anomalies, having weaker intensities, ranging between 40 and 135 nT are due to the occurrence of small volcanic centres. As revealed by the combined interpretation of DTM and magnetic anomaly map, the shape of magnetic anomalies is not directly related to the submarine topography of large volcanic banks, flat and terraced. It is probable that, after the formation of the bank, the intrusion of minor volcanic bodies occurred; the latter ones occur as lava domes cropping out at the sea bottom and elevated with respect to the flat top of the banks, terraced during the Late-Quaternary glacio-eustatic oscillations of sea level.