• 2012-01-01T00:00:00+01:00 (literal)
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  • Aiello Gemma; Marsella Ennio; Passaro Salvatore (2012)
    in 31 Congresso Nazionale GNGTS, Potenza, 21-23 novembre 2012
  • Aiello Gemma; Marsella Ennio; Passaro Salvatore (literal)
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  • CNR IAMC Sede di Napoli (literal)
  • 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 continental lithosphere thinned and transitional towards the oceanic area which characterizes the Tyrrhenian bathyal plains, deep about 3500 m. The lithospheric extension of the Campania Tyrrhenian margin started since the Early Pleistocene and manifested in the upper crust as a strong vertical tectonics and uprising of magmas with volcanism. The Campania Plain may be considered as a depression resulting from the downthrowing of the Meso-Cenozoic carbonatic basement, representing the bulk of the Apenninic chain. Along the coastal profile this basement crops out in the Sorrento Peninsula as a NW dipping monoclinalic structure and crops out another time at the Massico Mt (to the north of the Volturno river). The magma uprising is happened along counter-Apenninic (NE-SW) directions, perpendicular to the general axis of the Apenninic chain, as in the case of the ridge Ischia-Procida-Phlegrean Fields. The Naples Bay is bounded eastwards by several monogenic volcanoes aligned along a ENE-WSW regional strike, running from the Ischia island to the Phlegrean Fields, passing towards the Procida island. The origin of the magmas is common with that one of the Neapolitan volcanoes, with alkali-potassic magmas, further subdivided in ultra-potassic and potassic ones. The ultra-potassic magmas are characterized by a high content of K20, translating as leucitic lavas (leucitites and leuco-tephrites) for the Somma-Vesuvius and siliceous lavas, as the trachytes, the alkali-basalts and the latites, which are characteristic of Ischia, Procida and Phlegrean Fields. In the local framework of the Phlegrean-Ischian magmatism the rate of differentiation may be more or less high depending on the time of resilience in the magmatic tanks, on their depth and on the immission of fluids. This has provoked the discrete heterogeneity of the petrographic types, including alkali-basalts, latites and hyperalkaline trachytes. If we exclude some great ignimbritic eruptions, happened in the Campania region each 20 ky about (Green Ignimbrites of Ischia, 55 ky B.P.; Campanian Ignimbrites, 35 ky B.P.; Neapolitan Yellow Tuff, 12-15 ky B.P.) the most part of the explosive events may be considered of middle-little entity. Most part of the eruptions has not produced lavas, excluding the uprising phenomena along intrusions (dykes) and domes, which a characteristic redundant and dominant of the volcanology of Ischia and rare lava domes at Procida and in the Phlegrean Fields, where, on the contrary, the hydromagmatic features predominate. 2.2 Striking of the main eruptive centres and vertical motions The alignment of eruptive centres giving origin to the Phlegrean islands fits to two main counter-Apenninic structural directions: the first one, which seems to be older, is E-W oriented, while the second one is ENE-WSW oriented, as the Ischia-Procida-Phlegrean Fields lineament. Also in the Ischia Channel and in the surrounding areas, both the trending of the marine topography and the distribution of the submerged and cropping out eruptive centres well fits to the same ENE-WSW regional trend. To the regional deformation it has to be added the interference of the local stress field, which, in the Ischia island, is dominated by the uplift of the Epomeo block, along perimetral faults, N-S, E-W and NW-SE trending (Acocella and Funiciello, 1999; Tibaldi and Vezzoli, 1998). Several geomorphologic indicators (uplifted marine terraces and fossil beaches; Cinque et al., 1997) testify phenomena of subsidence and uplift in several sectors of the Ischia island, which allowed for a whole uplift of the Epomeo block of about 700 m, both previous than successive to the emplacement of the Green Tuffs (55 ky B.P.). Based on geochronological data Gillot et al. (1982) have dated the uplift of the Epomeo Mt. at 33 ky B.P. 2.3 Marine geomorphology The Phlegrean Fields represent a physiographic division, more than geographic, among the Gaeta Gulf to the north and the Naples Gulf to the south. In fact, the volcanic complex Ischia-Procida-Phlegrean Fields is localized in correspondence to a main structural element, ENE-WSW trending, which has controlled the elongation of the main eruptive centres. In the study area three main physiographic domains are included, i.e. the continental shelf, located between the shoreline and the 140-150 m isobath; the upper continental slope, located between the shelf break and the - 600 m isobath; the lower continental slope, located at more than 600 m of water depth. These domains are joined by the main canyons (Magnaghi canyon, Cuma canyon) or their tributary channels (debris channels). The continental shelf has a low gradient and depositional shelf edge at water depths of 150-170 m, sometimes eroded. The elongation Ischia-Procida-Monte di Procida constitutes a morphological barrier between the continental shelf of the Gaeta Gulf and the Naples Bay, characterized by greater water depths. This morphological saddle is interrupted in correspondence to the Ischia and Procida islands. In these passages a great transit of water masses and sediments happen and high hydrodinamics on the submerged banks, as on \"Le Formiche\" saddle. In the Gaeta Gulf the continental shelf, supplied by clastic sediments coming from the Volturno river, is well developed. The shelf break results partially eroded in correspondence to some active canyon axes, as the Cuma canyon and to WNW of Punta Cornacchia, a canyon head located next to the Forio Bank. Along the southern Ischia offshore the continental shelf is very reduced or absent. Several sub-horizontal terraced surfaces with eroded breaks in slope in correspondence to canyon heads are localized to non-standard water depths for the volcano-tectonic movements of the island. Marine exploration of the Ischia offshore showed great improvement in the last 10 years, due to surveys carried out by the IAMC-CNR of Naples in the frame of the GNV (National Group of Volcanology) and CARG projects. These surveys resulted in a Multibeam coverage that allowed the exploration of the whole area surrounding the island (Chiocci and de Alteriis, 2006; Aiello et al., 2009a, b). The Ischia offshore is characterized by several monogenic edifices aligned along the NE-SW system of faults that connects the southeastern sector of Ischia to the island of Procida and to the Phlegrean Fields (de Alteriis and Toscano, 2003; de Alteriis et al., 2006). These edifices have been mainly formed through subaqueous explosive eruptions, documented by the hialoclastites from a potassic magma of latitebasaltic-latitic composition (Di Girolamo and Rolandi, 1976; Di Girolamo et al., 1984). Marine geophysical surveys revealed collapsed volcanic edifices located to the W and SW of the Ischia island, a sector of the seafloor characterized by strong residual magnetic anomaly fields. A westward submerged continuation of the Ischia volcanic structure, representing a remnant of the emerged top of a larger, E-W trending volcanic complex, has been also inferred, based on marine geophysical data (Bruno et al., 2002). 3. SEISMO-STRATIGRAPHIC RESULTS Here we will focus on some seismo-stratigraphic results on the southern Ischia canyon system (seismic profile L50; Fig. 2). Some geological considerations on canyons are also delineated. The canyons observed onshore are some morphological incisions controlled by a strong erosion on an underlying substratum and characterized by steep walls. As a general rule their origin is due to a sharp lowering of the erosional base level, which can be interpreted as the level of minimum potential (generally the sea level), where all the rivers converge. This realizes with a sea level lowering or a relative uplift of the mainland, for example due to the tectonics, or to isostatic adjustements of the earth's crust. The origin of the submarine canyons is similar to the terrestrial one, i.e. to a relative variation of the sea level, also if their morphology is not so harsh. Another important factor which induces the formation of canyons along the submerged continental shelves and slopes apart the sea level oscillations, is represented by a variation of the sedimentary regime due to the increase of solid supply from the rivers and the great eruptions. In the Tyrrhenian sea the main canyons have been often encountered in correspondence to tectonically active areas, such as the Calabrian Arc, which is in uplift of several hundred of meters from almost 120 ky B.P. In the Naples Bay the origin of Dohrn and Magnaghi canyon is quite different (Di Fiore et al., 2011). In fact, also if we cannot exclude a component surely linked to the tectonism (in particular for the deepest part of the Dohrn canyon), the birth of these two incisions has been mainly controlled by the volcaniclastic aggradation during the Late Pleistocene-Holocene. All the southern Procida offshore is dominated by the occurrence of the Magnaghi canyon (Aiello et al., 2001). Its trilobate head starts from the shelf break at water depths of 130-140 m. The canyon has an average width of 700-800 m, which enlarges towards the confluence up to 2 kilometres. The seismic profile L50 crosses the south-western offshore of the Ischia island, from the continental shelf, west of Punta Imperatore, to the continental slope located southward of Capo Negro (Figs. 1 and 2). The line meets three main submarine canyons (Fig. 2), pertaining to the southern Ischia canyon system. The canyons (Fig. 2) are respectively located at water depths of 173 m (canyon 1), 240 m (canyon 2) and 285 m (canyon 3). The three canyon thalwegs are bounded by levees that may evolve into extensive channel-levee complexes. A volcanic acoustic basement has also been identified (Fig. 2), dipping from the continental shelf off Punta Imperatore toward the southwestern Ischia offshore. Based on land-sea correlation, this basement could be linked to the lavas cropping out at Punta Imperatore (southwestern Ischia), represented by alkali-trachytic grey lavas, dating to about 116-123 +/- 6 ky (Vezzoli, 1988). The top of the volcanic acoustic basement appears strongly incised, probably in a subaerial environment. A wide pyroclastic edifice has been recognized, buried in the continental shelf (Fig. 2). This edifice is interstratified in the FST deposits, recognized on the continental shelf off Punta Imperatore, where Quaternary sediments are truncated by an erosional unconformity, overlain by thin highstand deposits. A relic prograding wedge has also been identified in correspondence to the western canyon, offshore southwestern Ischia, where it overlies the volcanic acoustic basement corresponding to the Punta Imperatore lavas. This wedge is overlain by a marine seismic unit, which seems to be syntectonic, since it appears to be deposited during the activity of the normal fault downthrowing the volcanic acoustic basement. REFERENCES Acocella V. and Funiciello R. (1999) The interaction between regional and local tectonics during resurgent doming: the case of island of Ischia, Italy. 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