GNGTS 2013 - Atti del 32° Convegno Nazionale

New morphobathymetric and seismostratigraphic results on the north-western Ischia offshore (Naples Bay, Southern Tyrrhenian sea, Italy) and the emplacement of submarine slides in a volcanic setting G. Aiello, E. Marsella, S. Passaro Istituto per l’Ambiente Marino Costiero (IAMC), Consiglio Nazionale delle Ricerche (CNR), Napoli, Italy Introduction. New morphobathymetric and seismostratigraphic results on the north- western Ischia offshore (Naples Bay, Southern Tyrrhenian sea) are here discussed based on high resolution Multibeam bathymetry and Sparker seismic profiles recently collected in the frame of research programmes on marine geological mapping (Aiello et al. , 2012a; 2012b). The Phlegrean islands (Ischia, Procida and Vivara) represents a physiographic domain occurring among the Gaeta Gulf to the north and the Naples Bay to the south. This can be seen with the use of large-scale bathymetric maps, obtained by satellite data (Amante and Eakins, 2008). An elevation versus average slope plot (Moore and Mark, 1993), highlighting where steep and flat areas occur in terms of elevation (Fig. 1) has been constructed in order to describe the main morphological features of Ischia, including both the onshore volcanic edifice and the offshore surrounding the island. This plot, derived by the marine Digital Elevation Model of Ischia (DEM) and the related slope map, constructed through an opportunely built routine, helped to identify Ischia morphological macro-areas, since they individuate groups of elevation/ slope pairs related to specific physiographic and morphologic settings. Geologic setting. Systematic studies carried out on the sea bottoms around the Ischia island (Naples Bay, Southern Tyrrhenian sea) having a notable, up to date scientific content have been recently carried out and are, in any case, mainly geophysical. The first marine geological study concealing the whole Neapolitan area has been carried out by Walther (1886) and has furnished, with the old methods available at that times, an overall framework on the trending, constitution and structure of the sea bottoms between the Capri island and the Naples town (Aiello et al. , 2001). The Ischia island represents an alkali-trachytic volcanic complex, whose eruptive activity lasted from the Late Pleistocene up to historical times (Vezzoli, 1988). The activity of the island is characterized by the occurrence of a resurgent caldera, where volcanic eruptions coupled to tectonic activity gave rise to the Mount Epomeo block (Orsi et al. , 1991). Five eruptive cycles have characterized the magmatic system of Ischia-Procida-Phlegrean Fields, ranging in age from 135 ky B.P. up to prehistorical and historical times. Several volcanic edifices, both dismantled and preserved or buried below Quaternary deposits crop out onshore at Ischia (Forcella et al. , 1981; Gillot et al. , 1982; Luongo et al. , 1987; Vezzoli, 1988). Landslide deposits, cropping out onshore and deriving from the accumulation and fragmentation of pre- existing volcanic rocks, extensively crop out (Guadagno and Mele, 1995; Mele and Del Prete, 1998; Calcaterra et al. , 2003; De Vita et al. , 2006, 2007; Di Maio et al. , 2007; Di Nocera et al. , 2007). Many geo-volcanologic studies have been carried out on the island, starting from the syntheses of Rittmann (1930, 1948) and then on the specific aspects of the eruptive activity of the island and related geological processes (Forcella et al. , 1981; Gillot et al. , 1982; Chiesa et al. , 1985, 1987; Poli et al. , 1987, 1989; Civetta et al. , 1991; Orsi et al. , 1991; Luongo et al. , 1997). A particular meaning is accomplished by the aspects concerning the geochronology of the volcanic deposits in the island (Orsi et al. , 1996) and the time evolution of the magmatic system (Luongo et al. , 1997). The geologic and volcanologic history of the Ischia island has been characterized by a main event, represented by the eruption of the Green Tuff of the EpomeoMt, which verified 55 ky B.P. ago, allowing for the down throwing of the central sector of the island consequent to a caldera 225 GNGTS 2013 S essione 1.3