THE VOLCANIC COMPLEX OF THE ISCHIA ISLAND: NEW GEOLOGICAL DATA ON SOUTHERN ISCHIA CANYON SYSTEM BASED ON SPARKER SEISMIC STRATIGRAPHY

THE VOLCANIC COMPLEX OF THE ISCHIA ISLAND: NEW GEOLOGICAL DATA ON SOUTHERN ISCHIA CANYON SYSTEM BASED ON SPARKER SEISMIC STRATIGRAPHY G. Aiello (1), E. Marsella, S. Passaro (1) (1) IAMC-CNR, Sede di Napoli, Napoli, Italy 1. INTRODUCTION New geological data on the volcanic complex of the Ischia island (Naples Bay, southern Italy) are here presented in order to constrain the Ischia geologic evolution during Quaternary times. The south-eastern offshore has been mainly investigated, among the southern continental slope of Ischia and Ischia Channel. A strictly-spaced grid of seismic sections has been recently acquired and interpreted in the frame of research programs of marine cartography (CARG Project) financed by the Campania Region (Sector of Soil Defence, Geothermics and Geotechnics) during the mapping of the marine areas of the geological sheet n. 464 "Isola d'Ischia" at the scale 1:25,000. Some of the collected seismo-stratigraphic data are here interpreted and discussed (Aiello et al., 2009a; 2009b; 2012). The seismic grid was recorded using a Sparker Multitip seismic source. The marine data acquisition and the related mapping around the Ischia island were performed down to the 200 m isobath (Geological Map n. 464 "Isola d'Ischia"; scale 1: 25,000; 2010; in press; Aiello et al., 2009b). The marine geological mapping offshore the Campania region was accomplished under the direction of Prof. B. D'Argenio and in part Dott. E. Marsella. Marine geological survey of the southeastern Ischia offshore at water depths ranging between 30 m and 200 m has been carried out by Dott. Gemma Aiello on the basis of data collected at sea. The seismic grid is composed of 13 dip seismic lines in the southern Ischia offshore, running perpendicularly to the shoreline and 2 tie lines, parallel to the shoreline (Fig. 1). The geological interpretation of seismic lines that cover the southeastern offshore of the island, from Punta Imperatore (southwestern Ischia) to the Aragonese Castle (eastern Ischia) has been carried out (Fig. 1). Seismic acquisition was carried out using a multielectrode Sparker system (SAM96 model). The advantages of the Multitip Sparker include shorter pulse lengths for an equivalent energy discharge, as well as a increase in peak pressure, i.e. the amplitude of the outgoing acoustic wave. The sparker source used in this survey generated 200 J in the 200-2,000 frequency range. Ship positioning was determined using a GPS system with an accuracy of 1 m. All the seismic sections were recorded graphically on continuous paper sheets with a vertical recording scale of 0.25 s. The best vertical resolution was approximately 1 m for the sparker data. The proposed stratigraphy derives from the type of data utilized in marine geology (reflection seismics) and from the methods of seismic interpretation (high-resolution sequence stratigraphy). The geological structures recognized through the seismic interpretation are acoustically transparent seismic units, representing the volcanic acoustic basement and the system tracts of the Late Quaternary depositional sequence (Fabbri et al., 2002). The depositional systems, referred respectively to the sea level fall (FST; Helland-Hansen and Gjielberg, 1994), sea level lowstand (LST) and related internal subdivisions (Posamentier et al., 1991), transgressive phase (TST; Posamentier and Allen, 1993; Trincardi et al., 1994) and sea level highstand (HST) have been identified in the Late Quaternary depositional sequence of the Ischia offshore through seismo-stratigraphic analysis (Catuneanu, 2006; Catuneanu et al., 2009). 2. GEOLOGICAL SETTING OF THE PHLEGREAN ISLANDS (ISCHIA AND PROCIDA) 2.1 Geology and marine geology The study area pertains to the Eastern Tyrrhenian margin, located on a con