Seismic and magnetic evidences of a possible presence of an igneous body related to very recent volcanic activity in the "Mergellina Roadstead" (Bay of Naples, Italy)

Volcanological and seismic stratigraphy studies (Cole et al., 1994; Milia et al., 1998a,b) provide evidence of Holocene intrusive and explosive volcanic activity, in the offshore area of the city of Naples. In this area the bedrock is formed by Neapolitan Yellow Tuff (NYT), which is faulted and eut by deeply eroded valleys and covered by Holocene marine-sediments. New seismic exploration of the "Mergellina Roadstead" (Bay of Naples) where Milia et al. (1998a) based on a single seismic profile, indicated the presence of a geological body interpreted like a dike, distinguished three seismic facies: two of which are associated with NYT and the overlying Holocene marine-sediments, the third, is restricted to a wedge-shaped igneous body, may be associated to a body because of the marginal flexures of the marginal sediments and the absence of secondary reflectors characteristically absent above massive and rigid bodies. Furthemore, the lack of a seismic facies which could be associated with deposits near the base of the body exclude the possibility that it is a morphological relict of bed-rock (NYT). In order to further cheek of the presence of a dike, a magnetometric survey it was carried out. The tuffs and lavas cropping out in the area, though having the same basic trachytical composition have different concentrations of magnetic minerals per unit volume creating a spatial anomaly in the Terrestrial Magnetic Field (TMF). The survey showed the existence of a positive magnetic anomaly whieh correlates with the surveyed body. Inserting into a mathematic model (Telford et al.,1981) magnetic susceptibility values, measured on samples of lava and NYT from samples collected in the vicinity of Naples and the geometric shape obtained from an interpretation of the seismic profiles, the calculated magnetic field is compatible with that measured. The data of this research strenghtens the possibility that it is an igneous body related to very recent volcanic activity.