SEISMO-STRATIGRAPHIC AND MARINE MAGNETIC DATA OF THE GULF OF POZZUOLI (NAPLES BAY, SOUTHERN TYRRHENIAN SEA): RELATIONSHIPS WITH THE GEOLOGY AND THE VOLCANOLOGY OF THE PHLEGREAN FIELDS VOLCANIC COMPLEX (CAMPANIA)

SEISMO-STRATIGRAPHIC AND MARINE MAGNETIC DATA OF THE GULF OF POZZUOLI (NAPLES BAY, SOUTHERN TYRRHENIAN SEA): RELATIONSHIPS WITH THE GEOLOGY AND THE VOLCANOLOGY OF THE PHLEGREAN FIELDS VOLCANIC COMPLEX (CAMPANIA) G. Aiello (1), E. Marsella, V. Di Fiore (1) (1) IAMC-CNR, Sede di Napoli, Napoli, Italy 1. INTRODUCTION Seismo-stratigraphic and marine magnetic data of the Gulf of Pozzuoli are here presented on the basis of the geological interpretation of recently collected Sparker profiles (Aiello et al., 2012a; 2012b). The geology and the volcanology of the Phlegrean Fields volcanic complex and its adjacent offshore (Gulf of Pozzuoli) are here improved through a new stratigraphic setting delineated through seismic stratigraphy. Offshore stratigraphic units have not been recently discussed in detail in an active volcanic area as the Gulf of Pozzuoli, where active volcanism is suggested by bradyseismic movements and small earthquakes. According to many geological interpretations, this area represents the submerged border of the Phlegrean Fields caldera, located in correspondence to the Nisida, Miseno and Pentapalummo banks (Pescatore et al., 1984; Milia and Giordano, 2002; Aiello et al., 2005). Deep drilling of the Phlegrean caldera volcanic units onshore is now in course at the Bagnoli quarter (Naples town) in order to clarify the nature of the deep volcanic units underlying the caldera (De Natale et al., 2007). Seismic image and rock properties of the Phlegrean Fields caldera have been recently discussed through joint analysis and tomographic images based on the Serapis experiment dataset, carried out in the Gulf of Pozzuoli (Dello Iacono et al., 2009). The results regard the identification and characterization of the volcaniclastic layer filling the shallowest part of the Phlegrean caldera. The tomographic analyses have revealed a correlation among P-velocity anomaly at 0.5-0.75 km in depth, that tends to split into two parallel arcs and the position of volcanic dykes, buried volcanic structures and offshore fumaroles detected in the Gulf of Pozzuoli (De Bonatatibus et al., 1970; Pescatore et al, 1984; Milia 1998; 2010; Milia and Torrente, 2003; Aiello et al., 2012a; 2012b). The occurrence of these geological structures has suggested a highly fractured area, through which the magma and gases rise towards the surface. The analysis of the vertical component CMP gathers located on the southern rim of the Gulf of Pozzuoli has revealed the occurrence of a strong amplitude reflection event at 0.6 - 0.7 sec (twt) and corresponds to an approximate reflector depth of 600-800 m. Concerning the origin of the strong velocity contrast generating the high amplitude reflection event at about 0.6-0.7 sec (twt), it is worth noting that the stratigraphic profiles of onshore drill sites in the Mofete area (Fig. 1), located on the northern border of the caldera, show the evidence of a thick trachitic and latitic lava layer at about 750 m under tuff and tuffites post-caldera (Dello Iacono et al., 2009). While the volcanic activity gradually became subaerial in the northern part of the caldera, in the southern sector (i.e. the present Gulf of Pozzuoli), the conditions of marine sedimentation persisted since the caldera collapse except for the Wurmian phase (18 ky B.P.), when the sea level reached - 120 m and all the area was emerged above the sea level (Pescatore et al., 1984). The thick and shallow volcaniclastic layer, filling the Gulf of Pozzuoli during the post-caldera phases of activity of the Phlegrean Fields, could therefore include incoherent pyroclastic rocks and chaotic tuffs, highly water saturated because of marine sedimentation. The stratigraphy and geology of volcanic areas is an intriguing research theme, particularly applied to the c