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 case history of the Gulf of Pozzuoli (Naples Bay), representing the submerged border of the Phlegrean caldera. The state-of-the-art methodological aspects for mapping volcanic areas, applied also to case histories of Naples Bay, have been recently delineated, highlighting recent studies on the stratigraphy, structure and evolution of active volcanic terranes (Groppelli and Goette, 2010). New stratigraphic and volcanological studies, mainly onshore, dealing with the definition of volcanological units during the field survey and specific volcanological features, petrographic, geochemical and petrological studies and geophysical models have been recently presented (Lucchi et al., 2010); Tibaldi, 2010; Bonomo and Ricci, 2010; Palladino et al., 2010; Milia, 2010; Perrotta et al., 2010; De Vita et al., 2010). These studies allowed us to delineate the problems related to stratigraphy and volcanology of active volcanic districts of southern Italy, referring in particular to the Naples Bay. Apart the new stratigraphic setting of the offshore units, another aim of this paper is to produce a qualitative correlation of the seismo-stratigraphic data with marine magnetics already recorded both in the Gulf of Pozzuoli (Galdi et al., 1988) and in the Phlegrean offshore (Secomandi et al., 2003; Aiello et al., 2004) better constraining the volcanic nature of some seismic units. The integrated interpretation of Sparker profiles and magnetic anomaly maps will contribute to a better understanding of volcanic structures in the Gulf of Pozzuoli. Moreover, a correlation with the terrestrial geology of the Phlegrean Fields has been attempted in order to highlight tectonic and magmatic implications in the geological evolution of the Phlegrean Fields volcanic complex, intensively studied through field geology (Morhange et al., 2005; Bellucci et al., 2006; De Natale et al., 2007; Bodnar et al., 2007). 2. GEOLOGICAL SETTING The Gulf of Pozzuoli, offshore the Phlegrean Fields volcanic complex (Naples Bay, southern Tyrrhenian sea) is an inlet with limited dimensions, bounded seawards by several submerged volcanic banks (Pentapalummo Bank, Nisida Bank, Miseno Bank; Aiello et al., 2005). A morphological sketch map is shown in Fig. 2, representing several offshore morphological units (Aiello et al., 2012a). These units include the inner continental shelf, the central basin, the submerged volcanic banks and the outer continental shelf (Fig. 2). The inner shelf is located in the northern sector and grades into a central basin, located at 100 m of water depth through a -50 m deep shelf break and a steep slope. Proceeding seawards, the Pozzuoli central basin is limited by a belt of submarine volcanic highs and then grades into the outer shelf, located at water depths of - 140/-160 meters. The Phlegrean Fields are a volcanic district surrounding the western part of the Gulf of Naples, where volcanism has been active since at least 50 ky B.P. They correspond to a resurgent caldera with a diameter of 12 km (Phlegrean caldera; Rosi and Sbrana, 1987), resulting from the volcano-tectonic collapse induced from the eruption of the pyroclastic flow deposits of the Campanian Ignimbrite (35 ky B.P.). Coastal sediments ranging in age from 10 to 5.3 ky B.P. crop out 50 m above the sea level in the marine terrace of La Starza (Gulf of Pozzuoli), indicating a volcano-tectonic uplift of the caldera center (Dvorak and Mastrolorenzo, 1991; Morhange et al., 2005). The Quaternary volcanic area of the Phlegrean Fields is located in a central position within the graben of the Campania Plain (D'Argenio et al., 1973; Bigi et al., 1992). The pre-calderic volcanic activity developed in correspondence with small and scattered volcanic centers, erupting trachytic pyroclastics and lavas. The corresponding outcrop located at the Monte di Procida coastal cliff exhibits a thick subaerial pyroclastic sequence with several interbedded paleosols. It includes four main pumice fall beds, a welded pyroclastic flow deposit and dark lapilli and ashes erupted by the Fiumicello volcanic center (Procida island). The post-calderic volcanic activity developed during four main phases, individuated through radiometric age determinations and palaeogeographic evolution, in a time interval spanning from 35 ka to historical times, as established by deep wells located in the Mofete and San Vito geothermal areas (Rosi and Sbrana, 1987). 3. SEISMO-STRATIGRAPHIC AND MARINE MAGNETIC RESULTS The seismic profile L68_07 running from the western continental shelf of Pozzuoli Gulf and Nisida island has been interpreted to show the main stratigraphic and structural features of the Pozzuoli Gulf (Fig. 3). A sketch stratigraphic table, representing the key to the geological section reported in Fig. 3, describes the main characteristics and the possible chronostratigraphic attribution of the seismic units based on their stratigraphic relationships (Aiello et al., 2012a; 2012b). Large compressional features have been individuated on the seismic section, i.e. the Punta Pennata anticline, the central syncline of Pozzuoli Gulf and the Nisida anticline (Fig. 3). The structural lineaments involved in deformation of the volcaniclastic unit V3 (Fig. 3) have been produced by compressional events genetically related to the tectonic and magmatic events involving the Phlegrean Fields during the Late Quaternary. The Pozzuoli marine and volcanic sequences have been deformed by kilometer-scale folding during a main compressional event, probably due to tectonic inversions of the basin, already documented on the Tyrrhenian margin (Sartori et al., 2004; Jolivet et al., 2006; Gaullier et al., 2010). Based on seismo-stratigraphic evidence the uplift of the marine terrace of "La Starza", on which the town of Pozzuoli is located and the formation of an erosional platform on the inner Pozzuoli continental shelf are linked to an anticlinal crest. On the other side, the present-day basin depocenter is located on a syncline (Fig. 3). These folds formed during the deposition of the seismic unit G3 (Fig. 3), as evidenced by the wedging geometries of the seismic unit and the overall thinning towards the hinge of the anticline. Several magnetic studies have been carried out in the Neapolitan volcanic district (Nunziata and Rapolla, 1987; Secomandi et al., 2003; Paoletti et al, 2005). In particular, Paoletti et al. (2005) have presented a new detailed magnetic map of the whole Neapolitan active volcanic district, obtained by merging recently acquired airborne, land and marine magnetic measurements. The analysis of this magnetic dataset has enabled the characterization of the main buried volcanic structures, providing a better understanding of the connection between tectonism and volcanism in the study area. In the Gulf of Pozzuoli four magnetic sections have been constructed, NE-SW and NW-SE trending respectively (Galdi et al., 1988; Aiello et al., 2012a). The first one runs from Punta Pennata (Gulf of Pozzuoli) to the town of Pozzuoli (Via Napoli). The total magnetic intensity shows a trending with a magnetic minimum of - 80 nT in the central basin of the gulf (water depth of - 90 m) and a magnetic maximum of 70 nT near the Pozzuoli shoreline. The second one, translated of 2.4 km towards the southwest, shows a strong increase of magnetic gradient, due to the occurrence of the industrial system of Bagnoli. It has provoked the release in the environment mainly of metals derived by the burning of fossil oil coals and by remnants of industrial production. The third section, running from the Gulf of Pozzuoli to Punta dell'Epitaffio, shows a magnetic maximum at Bagnoli and a minimum near Nisida. The fourth one, running from Nisida to the historical centre of Pozzuoli (Rione Terra), shows a minimum at Nisida and a maximum at Rione Terra. In conclusion, the Phlegrean Fields are a volcanic area featuring a shallow Moho, a high thermal gradient, bradyseismic movements and shallow seismicity. The offshore portion of the Phlegrean Fields (continental shelf of the Pozzuoli Bay) has been investigated through high resolution seismic reflection profiles and volcanic and marine units are herein documented. An up-to-date geologic and stratigraphic setting of the seismic units is here furnished and discussed. Marine magnetic in the study area are interpreted. Some implications of the interpreted data on the tectonics and magmatism of the Phlegrean Fields volcanic complex are discussed. Regional geological concepts on the Phlegrean Fields in the framework of the Campania continental margin are recalled to support interpretations of the volcanic and tectonic context. 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