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Chapter

THE GEOPHYSICAL AND GEOLOGICAL SETTING OF NORTHERN ISCHIA ISLAND (NAPLES BAY, SOUTHERN TYRRHENIAN

SEA, ITALY) BASED ON HIGH RESOLUTION SEISMIC REFLECTION PROFILES

Gemma Aiello*

Institute for Marine and Coastal Environment (IAMC), National Research Council of Italy (CNR), Calata Porta di Massa,

Porto di Napoli, Napoli, Italy

ABSTRACT

Significant geophysical studies on Ischia Island have been recently produced based on the geological interpretation of Sparker data within marine geological projects. High resolution seismic reflection profiles (Sparker Multitip) have been presented. In this framework, new geological data are herein provided concealing the slide deposits and the

* * Corresponding Author. [email protected].

canyon systems in the Ischia offshore through seismo-stratigraphic interpretation aimed at improving the knowledge on the Ischia geologic evolution during Quaternary times. In this chapter, a stratigraphic and structural interpretation of the seismic profiles recorded in the northern Ischia Island is presented aimed at improving the knowledge of the northern Ischia coastal belt and its relationships with the emplacement of the slide deposits in the offshore surrounding the Ischia island, which has been controlled by the complex and different volcano-tectonic events.

The Casamicciola town is located in the northern Ischia sector and is characterized by an active seismicity and by the occurrence of the reworked deposits pertaining to the Ischia activity and suggesting a cyclical slope instability. The marine counter-part of this instability is represented by the wide submarine landslides which, in the Casamicciola offshore, correspond to the wide hummocky topographies observed on the Multibeam, Sidescan Sonar and seismic data.

The relationships among the seismicity and the structural elements in the Ischia Island have been studied through the macro-seismic data analysis and the structural geology. The structural analysis of the faults occurring in the volcanic deposits of the Casamicciola outcrops has evidenced a main tectonic extension, N-NNE trending in the oldest pyroclastic rocks, whose age exceeds 35 ky B.P. A secondary tectonic extension occurs along a NNW direction and seems to affect also the younger formations. The two extensional tectonic episodes have been recognized in correspondence with the E-W and WNW-ESE trending geological structures.

The hummocky area offshore the Northern Ischia Island in correspondence with the Casamicciola town has been singled out and mapped based on the Multibeam bathymetry. The hummocky deposits occupy a tongue extending from Lacco Ameno to Punta La Scrofa (northern Ischia Island) towards the Tyrrhenian sea, culminating in correspondence with the Punta Cornacchia submarine canyon’s head.

The hummocky topographies, located at water depths ranging between 20 m and 180 m, show the large blocks, cropping out at the sea bottom and partly buried by the Holocene sediments. These deposits form a wide field cropping out at the sea bottom, whose shape is sub-circular. This field is bounded by the morphologic reliefs represented by the depositional levees of turbiditic slope systems, triggered by the strong erosion and by the canyon development. In fact, the portion of the sea bottom occupied by the hummocky topographies is involved by a network of erosional gullies, shown by the Multibeam bathymetric data.

The hummocky deposits (H1 and H2) are distinguished from two different bodies, which have been identified on the seismic profiles. They are characterized by an acoustically-transparent seismic facies. The H1 body is overlain by the forced regression system tract deposits (FST), deposited on the continental shelf offshore the Casamicciola town. The

The Geophysical and Geological Setting of Northern Ischia Island

occurrence of the two different wedges, which have been detected based on the seismic data interpretation, suggests that two distinct volcano-tectonic events have probably controlled the emplacement of the Casamicciola slide deposits. Moreover, the seismo-stratigraphic data collected from other authors have suggested that the incised fan-shaped valley, which develops offshore the Lacco Ameno town, should be related with the emplacement of a recent landslide event.

INTRODUCTION

One aim of this chapter is to give new insights into the knowledge of the geology of the Northern Ischia Island through the seismic interpretation, focusing, in particular, on the hummocky and chaotic deposits located in the north-western sector of the island.

While the Naples and Pozzuoli bays have been deeply surveyed through marine geophysics during the last 30 years (Latmiral et al., 1971; Finetti and Morelli, 1974; Pescatore et al., 1984; Fusi et al., 1990; Aiello et al., 2001; Secomandi et al., 2003; D’Argenio et al., 2004; Aiello et al., 2004; 2005; Ruggieri et al., 2007; Aiello et al., 2008; 2012; 2016; 2017), the Ischia Island has been less explored, due to the scarce acoustic characterization of the igneous bodies (volcanic and volcaniclastic deposits, lava deposits) and to their inner chaotic structure.

A large debris avalanche has been individuated in the Southern Ischia Island after a Tobi Sidescan Sonar survey of the continental slope south of Ischia (Chiocci et al., 1998). It starts from the southern flank of the island from the S. Angelo and Capo Grosso Promontories, covering an area of 400 km2 and extending up to water depths of 1000 m (Chiocci et al., 1998).

The lateral collapses of the volcanic edifices alternate with the periods of construction of volcanic edifices through the volcanic eruptions, as it has been studied in the case history of the Canary Islands (Watts and Masson, 1995). As a general rule, the lateral collapses produce a large amount of volcanic detritus, which is propagated along the slopes and is accumulated along the toe of slope of the volcanic edifices. In the case of insular volcanic edifices (such as the Ischia Island), the detritus takes the

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shape of an avalanche in the areas proximal to the slide scar, then evolving through turbiditic channelized flows. During the Tivoli Oceanographic Cruise (Chiocci et al., 1998) a large field of blocky detritus has been discovered offshore the southern shoreline of the Ischia Island and interpreted as a debris avalanche, extending for more than 35 kilometers, covering an area of 400 km2. The blocks (some of which exceed 50 m3) are densely-spaced up to 18 km from the southern coast of the Ischia Island; their density rapidly decreases proceeding seawards towards the Tyrrhenian Sea. The flux direction of the debris avalanche follows the gradient along the slope isobaths.

The Ischia Debris Avalanche (IDA of Chiocci and de Alteriis, 2006) is related to a set of collapses of the southern coast of the Ischia Island. The onshore morphology of the island shows a clear evidence of the large-scale instability, in particular an amphitheatre representing a slide scar or different coalescent slide scars. The volcano-tectonic event triggering the IDA seems to be quite recent in the Ischia Geologic History (whose last eruption happened a few centuries ago); in fact, the debris avalanche seems to overlie and fossilize the Magnaghi canyon (Chiocci et al., 1998; Chiocci and de Alteriis, 2006).

A large amount of Multibeam morphobathymetric data has been collected in the Ischia Island during the realization of the marine geologic maps (Budillon et al., 2003a; D’Argenio et al., 2004; Monti et al., 2012). The extension of the gravity flow deposits in the Ischia offshore based on Multibeam survey (D’Argenio et al., 2004) is shown in the Figure 1 (Figure 1).

During the 2000-2002 years the coastal belt located to the north and to the west of the Ischia Island has been explored in detail through the Multibeam bathymetry and the Sidescan Sonar in the frame of the research project for the institution of marine protected areas of “Regno di Nettuno” (Ischia, Procida and Vivara; Budillon et al., 2003a; Violante et al., 2003). On the other side, the southern sector of the Ischia Island has been explored in detail during the January-February of the 2000 year aimed at studying the Maronti littoral (Marsella et al., 2001; Budillon et al., 2003b). The geophysical studies on both the magnetic anomalies occurring

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westwards of the island and on the debris avalanches have been performed from the 2000 to the 2008 years (Bruno et al., 2002; de Alteriis et al., 2001; 2002; 2004; Violante et al., 2003; Passaro, 2005; Chiocci and de Alteriis, 2006; Aiello et al., 2008; 2009a; 2009b; 2012).

Figure 1. Extension of the gravity flow deposits in the Ischia offshore based on Multibeam survey (modified after D’Argenio et al., 2004).

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Figure 2. Sidescan Sonar photomosaics in the western Ischia offshore showing hummocky facies interpreted as debris avalanche deposits.

De Alteriis and Toscano (2003) have published a synthesis on the morpho-bathymetry of the Ischia Island, showing that two wedges of hummocky deposits occur in the north-western sector of the island. They are genetically linked with the gravitational events (landslides) triggered by the earthquakes accompanying the uplift of the Epomeo block. The structure and the relationships of the debris avalanche deposits with the landslides occurring in the adjacent sector of Ischia have also been discussed (Budillon et al., 2003a; 2003b; Violante et al., 2003). The Sidescan Sonar interpretation has allowed for the individuation of different acoustic facies, including the hummocky facies interpreted as the debris avalanche deposits, whose heterometric blocks are included in a pelitic matrix. The hummocky facies have been interpreted on the Sidescan Sonar photomosaics (Figure 2) and are characteristic of the debris avalanche

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deposits occurring in the Western Ischia Island. On the photomosaics the heterometric blocks, fossilized and buried by the Holocene pelitic sediments, correspond to the more reflective areas The other Sidescan Sonar facies correspond with the blocks, the rocks cropping out at the sea bottom, the sandy pelites, the middle-coarse-grained sands, the organogenic sands, the gravels and the pebbles and the coverage with marine Phanerogams.

The Northern sector of the Ischia Island between the Punta Caruso and Punta La Scrofa Promontories is characterized by a strong morphologic complexity. Between the Punta Caruso and the Punta di Monte Vico Promontories it is distinguished by the high coastal cliffs incised in the rocks and shows a regular gradient, with a decrease in the grain-size of the sediments, as a function of both the depth and the distance from the coast. Between the Punta di Monte Vico and the Punta Cornacchia Promontories the physiography is very articulated and is characterized by different reliefs, with a distribution of the blocks on the mobile sea bottoms which is not coherent with the distance from the coast (Budillon et al., 2003a). Between the Punta Caruso and the Punta Cornacchia Promontories a wide outcrop of the acoustic basement occurs (“Secca del Bell’ommo e’ Terra”), having a lobate shape and representing the seawards prolongation of the Zaro lava flow.

Violante et al. (2003) have evidenced that the hummocky submerged topographies occurring in the North-Western sector of the Ischia Island well correlate with the strong slope instability of the Epomeo North-Western Rim. The most of the landslides are genetically related with the historical events (Alessio et al., 1996), but their stratigraphic location in the units younger than the caldera resurgence has evidenced older slope instabilities, probably coeval and recurrent with respect to the caldera resurgence.

The erosional slopes and the sedimentary structures which characterize the Northern Ischia Island suggest the occurrence of recent depositional events, while in the Western Sector of the Ischia Island the hummocky deposits are partly covered by the prograding depositional wedge. In both the cases, the morphologic characters suggest the recurrence of significant

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landslide events with catastrophic implications. The historical seismicity has been recognized as the main triggering factor of the instability processes along the Epomeo northern rim. The seismological analysis has evidenced a narrow epicentral area in correspondence with this rim, controlled by a main seismo-tectonic structure, having a ENE-WSW trending (Alessio et al., 1996; Violante et al., 2003).

The southern coastal sector of Ischia between the Punta del Chiarito and the Punta S. Pancrazio Promontories has been deeply studied through the morpho-bathymetry and the geology in the frame of the beach nourishment of the Maronti Bay (Marsella et al., 2001; Budillon et al., 2003b). This beach has been involved by a strong erosional retreatment due to the wave action (Cocco et al., 1987; 1995; De Pippo et al., 2000). The project on the beach nourishment of the Maronti Beach has been committed by the Region Campania and has covered different marine sectors, including the Cava Grado Area, the Maronti Bay and the Barano Bay, surrounding the “Scarrupata di Barano”, characterized by a slope having high gradients, probably tectonically-controlled.

The coastal geomorphology is characterized by some promontories (Punta del Chiarito, Punta S. Angelo, Capo Grosso, Punta S. Pancrazio), where trachytic rocks pertaining to the oldest eruptive cycle of the Ischia Island crop out (Gillot et al., 1982; Vezzoli, 1988). In the coastal sector between the Punta del Chiarito and the S. Angelo Promontories well-stratified volcanic deposits crop out, characterized by tuffs, pumice lapilli and pumice breccias interpreted as pyroclastic flux and surge deposits (“Faro di Punta Imperatore Formation”; Vezzoli, 1988).

Detailed stratigraphic studies have been carried out in areas located to the East and to the South-East of the Epomeo Mount at Ischia Island aimed at showing the close relationships between the caldera resurgence and the slope instability during the last 5.5 ka (De Vita et al., 2006). During this time span the reactivation of the faults and the related volcanic activity were accompanied by the deposition of sequences controlled by superficial gravitational movements. These sequences preceded and followed the successions of volcanic rocks, suggesting that the slope instability was controlled by the vertical movements. A strong slope instability was

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controlled by the caldera resurgence, which was accompanied by the activation of faults and the renewal of volcanism, controlling the over-steepening of the slopes and the onset of the seismicity, triggering the superficial gravitational movements. The landslide generation was favored from the availability of a large amount of loose materials, rapidly accumulating on the slopes during the volcanic eruptions (De Vita et al., 2006).

The slope instability deposits of Ischia have been described by many authors, distinguishing the rock falls, the slides, the debris flows and the debris avalanches (Vezzoli, 1988; Guadagno and Mele, 1992; 1995a; 1995b; Mele and Del Prete, 1998; Del Prete and Mele, 1999; Tibaldi and Vezzoli, 2004). Many landslides of the Casamicciola town, including the rock falls, the debris flows and the debris avalanches have been triggered from the earthquakes during a time span from 1762 and 1910 years (Cubellis, 1985; Buchner, 1986; Guadagno and Mele, 1995a; 1995b; Cubellis and Luongo, 1998; Mele and Del Prete, 1998; Molin et al., 2003). The first phase of landslide generation (5.5-2.9 ka) is characterized by a debris flow after the emplacement of the Piano Liguori tephra (De Vita et al., 2006). During the second phase (2.9 ka) the superficial gravitational movements pre-dated the emplacement of the Chiarito Tephra in the Succhivo area (De Vita et al., 2006). The third phase (2.6-2.3 ka) has included deposits emplaced during the last 300 years, including the debris avalanches after the Chiarito Tephra eruption (De Vita et al., 2006). The fourth phase (2.3-1.9 ka) was characterized by the debris flow deposits (De Vita et al., 2006).

Brown et al. (2008) have presented a new stratigraphy of pyroclastic deposits of the Ischia Island, ranging a time interval between 75 and 50 ka. Different volcanic vents were active during this period, including at least ten explosive eruptions, whose deposits had a phonolitic, basaltic and trachy-andesitic composition (Brown et al., 2008). Based on the volcanological data the morphology of the Ischia Island was significantly different from the present-day one, with volcanic edifices to the South and West and a submerged depression in the centre. The largest volcanic event was the Monte Epomeo Green Tuff (MEGT), resulting in a caldera

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collapse across all or part of the island (Brown et al., 2008). The corresponding deposits are represented from two thick ignimbrite units, separated by volcaniclastic sediments. The extra-caldera deposits of the MEGT include a pumice fall deposit, emplaced during the opening phases of the eruption, some volcanic breccias (Ischia and Procida) and a distal ignimbrite to the South-West of the Phlegraean Fields (Brown et al., 2008).

The volcanic and deformative evolution of the Ischia Island during the last 10 ka has been recently reconstructed through stratigraphic, volcanologic and structural data (De Vita et al., 2010). The periods of quiescence have alternated with the periods of strong volcanism, concentrated at 5.5 ka and over the past 2.9 ka. The distribution of volcanic vents and the structural analysis of the resurgent block have suggested that the volcanism was strongly influenced by the resurgence, allowing for the magma uprising mainly towards the East (Orsi et al., 1991; Marotta, 2001; De Vita et al., 2010). An extensional regime is suggested from the distribution of the tectonic lineaments to the East of the most uplifted block. The deformation of the volcanic areas outside the resurgent block has been related to a local stress field reactivating pre-existing regional features (De Vita et al., 2010).

The integrated interpretation of the geological and geophysical data has shown that the gravitational movements have involved the marginal parts of the Epomeo block (Della Seta et al., 2012). These movements occurred during the last 3 ka and included the debris avalanches, the large debris flows (lahars), the smaller mass movements (rock falls, slumps, debris and rock slides) and the deep-seated gravitational slope deformations (Della Seta et al., 2012). The occurrence of submarine deposits genetically related with the subaerial ones has shown that the debris avalanches impacted on the sea. The largest mass movements could also flow into the sea, generating the tsunami waves that could impact on the island’s coast, as well as on the neighboring and densely inhabited coast of the Neapolitan area (Di Fiore et al., 2011).

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GEOLOGIC SETTING

The geologic and volcanologic studies have been carried out on the Ischia Island, starting from the geological synthesis of Rittmann (1930; 1948) and following with the punctual aspects of the eruptive activity of the island and the related geological processes (Ducci et alii, 1967; Forcella et alii, 1981; Gillot et alii, 1982; Chiesa et alii, 1985; 1987; Poli et alii, 1987; 1989; Crisci et alii, 1989; Civetta et alii, 1991; Orsi et alii, 1991; Luongo et alii, 1997). The aspects regarding the geochronology of the volcanic deposits of the island (Orsi et alii, 1996) and the space and time evolution of the magmatic system (Luongo et alii, 1997) are particularly significant.

The Ischia Island represents an alkali-trachytic volcanic complex, whose eruptive activity lasted from the Late Pleistocene up to historical times (Vezzoli, 1988). The oldest rocks date back to about 150 ky and crop out in several sectors of the coastal belt of the island, mainly in correspondence to the “Scarrupata di Barano”, a steep slope located in the South-Eastern sector of the island (Vezzoli, 1988; De Vita et al., 2006; 2010; Brown et al., 2008; Sbrana and Toccaceli, 2011; Della Seta et al., 2012). The geological evidence has suggested the occurrence of a resurgent caldera, about 10 km wide, where the eruptive activity coupled with the tectonic activity gave rise to the uplift along faults of the Mount Epomeo Block (Orsi et alii, 1991). Five eruptive cycles have been suggested by the succession of the eruptive events, ranging in age from 135 ky to pre-historical and historical times. Moreover, the extensional tectonics related to the tectonic uplift of the Mount Epomeo Block has been interpreted as intra-calderic resurgence (Acocella & Funiciello, 1999) and genetically related to shallow seismicity, triggering gravity instabilities on the Epomeo relief. The significant geophysical studies on the Ischia Island have been recently provided based on the geological interpretation of Sparker data within marine geological projects (Biagio, 2007; Corradi et alii, 2009; Aiello et alii, 2012; Giordano et alii, 2013; Aiello & Marsella, 2014; 2015b).

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The alignment of the eruptive centers giving origin to the Phlegraean Islands, including Ischia, fits into two main NE-SW trending structural directions. The first one, which is the older one, is E-W trending, while the second one is ENE-WSW trending, as the Ischia-Procida-Phlegrean Fields lineament. The sum of different stress fields must be added to the regional deformation, which is dominated by the uplift of the Epomeo Block, controlled by the normal faults (Acocella & Funiciello, 1999; Tibaldi & Vezzoli, 1998). Several geomorphologic hints including uplifted marine terraces and fossil beaches (Cinque et alii, 1997) have testified the phenomena of subsidence and uplift in the different sectors of Ischia. These phenomena have allowed for a whole uplift of the Epomeo Block at about 700 m during the phases older and younger than the emplacement of the Epomeo Green Tuffs, happened at 55 ky B.P. (Vezzoli, 1988). The uplift of the Epomeo Block has been dated back to 33 ky B.P. (Gillot et al., 1982).

The Ischia offshore is characterized by several monogenetic edifices aligned along a NW-SE trending fault system, connecting the south-eastern sector of the Ischia Island to the Procida Island and to the Phlegraean Fields. The volcanic edifices have been formed through subaqueous explosive eruptions, documented by hialoclastites fed by a potassic magma of latite-basaltic and latitic composition (Di Girolamo et alii, 1984). The geophysical data have shown the occurrence of the collapsed volcanic edifices located to the West and to the South-West of the Ischia Island, a sector of the sea bottom characterized by a strong residual magnetic anomaly field.

The Ischia Island is dominated by the Epomeo structure, resulting from a prominent resurgence, mainly if compared with the relatively small size of the calderas. The total uplift of 900 m has been accompanied from the reactivation of faults, the seismic activity and the renewal of volcanism. The geologic and volcanologic history of the Ischia island has been controlled by a main event, represented by the eruption of the Epomeo Green Tuffs, verified at about 55 ky B.P., allowing for the down throwing of the central part of the Island. After the formation of the caldera of the Epomeo Green Tuffs the volcanic activity of the Island has been

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conditioned by the caldera resurgence, starting from 30 ky B.P., which allowed for a gradual uplift and emersion of the rocks deposited in the caldera itself, which were submerged below the sea level. The total uplift, indicating the caldera resurgence, has been evaluated in the order of 800-1100 m (Barra et al., 1992).

The Ischia graben is located in the central part of the Island and represents a depression triangular in shape, located between the Casamicciola, Barano and Ischia Ponte towns and bounded southwards by a system of normal faults (Rittmann, 1930; Forcella et alii, 1981; Vezzoli, 1988; Mattera, 1995). It is divided into two sectors by a normal fault following the valley of Rio Corbore. Several faults and fractures round the Epomeo structure (Acocella et alii, 1997; Acocella & Funiciello, 1999) with NE-SW (counter-Apenninic) directions.

The Ischia Island is characterized by a strong hydrothermal activity in correspondence to the warm sources, showing the temperatures ranging between 20° and 80°. These sources are particularly abundant at the Forio d’Ischia, Casamicciola, Maronti, Carta Romana, Punta del Chiarito, Bagnitiello, Sammontano and Ischia harbor localities. They should coincide with the points of the potential opening of the eruptive vents.

Many fumaroles have been documented in the Ischia island (Penta, 1940; 1949; 1954; 1963; Penta and Conforto, 1951; Chiodini et al., 2004; Di Napoli et al., 2009) and more than one hundred thermal sources, representing a rich hydrothermal setting, whose benefits are known since ancient times. The mineral waters come from the thermal sources, having temperatures ranging between 30°C and 85°C (Penta, 1940; 1949; 1954; 1963; Penta and Conforto, 1951; Chiodini et al., 2004; Di Napoli et al., 2009). Many source waters are radioactive due to the occurrence of radon. The shallow manifestations have been supplied by the meteoric and marine waters and by the thermal fluids deriving from the two distinct hydrothermal tanks, in which the deep gas, probably magmatic in origin, arrives (Di Napoli et alii, 2009).

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MATERIALS AND METHODS

In the Ischia Island the seismic grid has been recorded by using a Sparker Multitip seismic source. The marine data acquisition has been realized during the technical activities for the realization of the geological map n. 464 Isola d’Ischia at the 1:25.000 scale (Monti et alii, 2012) down to the 200 m isobath. The seismic lines have been plotted on a sketch map in order to show their location (Figure 3).

The seismic grid is made up of several seismic lines (Figure 3). The seismic sections have been recorded by using a multi-electrode Sparker system (SAM96 Sparker; Aiello et al., 2016). This seismic source shows short pulse lengths for an equivalent energy discharge, coupled with an increase in the peak pressure, representing the amplitude of the outgoing acoustic wave. The vertical resolution of the seismic data was about 1 m. The Sparker seismic lines have facilitated the stratigraphic correlations between the seismic sections and have revealed the differences in the structural and stratigraphic setting between the seismic lines.

Figure 3. Location map showing seismic lines in the northern Ischia offshore interpreted in this chapter.

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Figure 4. Sparker line L26 (northern Ischia offshore; see Figure 3 for location).

RESULTS

Several seismic sections located in the Northern Ischia Offshore have been interpreted through the seismo-stratigraphic analysis, allowing for the distinction of several seismic units, both volcanic and sedimentary in origin. A particular attention has been paid to the recognition of the chaotic facies interpreted as hummocky deposits, which probably were emplaced during two distinct episodes and were controlled by the volcano-tectonic movements and the gravitational processes.

The L26 seismic line has a SSW-NNE trending and runs from the Western Casamicciola Offshore to the Tyrrhenian Offshore northwards of Ischia Island (Figure 4). A weak multiple reflection relative to the sea bottom has prevented for seismic interpretation below the depth of about 230 msec (twt). The high resolution interpretation has suggested the

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occurrence of six seismic units (Figure 5). An undetermined volcanic substratum has been recognized, distinguished from an acoustically-transparent seismic facies with scattered and discontinuous seismic reflectors. It is bounded upwards by a highly continuous seismic reflector, representing also the base of the overlying seismic unit, probably composed of marine deposits. It is characterized by a package of continuous and sub-parallel seismic reflectors and represents the first unit of the basin filling (1 in Figure 5). The second and third seismic units have similar seismic characteristics and interpretation (2 and 3 in Figure 5). The fourth seismic unit of the basin filling directly crops out at the sea bottom and is established from highly continuous and parallel seismic reflectors, interpreted as recent marine sediments (Figure 4). Hummocky deposits are represented by two distinct and superimposed bodies (H1 and H2 in Figure 5).

Figure 5. Geological interpretation of the seismic line L26 (see the text for further details).

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Figure 8. Sparker line L27 and corresponding geologic interpretation (modified after Aiello et al., 2009a).


The seismic line L28 has roughly a N-S trending and is located in the Eastern part of the Casamicciola Offshore (Figure 6), extending towards the Tyrrhenian Offshore of Northern Ischia Island. The different seismic units have been distinguished through the seismic interpretation (Figure 7). These seismic units are similar to those ones previously described for the seismic profile L26 (V1, 1, 2, 3, 4, H1, H2). These units can be mainly recovered as the volcanic substratum, the sedimentary units of the basin filling and the hummocky deposits, represented by two different bodies, probably emplaced during successive volcano-tectonic and gravitational events (Figure 7).

These two seismic lines have been compared with the results obtained from the geologic interpretation of the seismic profile L27, located at an average distance among the two newly interpreted seismic lines (Aiello et al., 2009). The previously obtained results are herein mentioned for a comparison and a correlation. The seismic profile L27 has been recorded offshore the Northern Ischia Island through a Sparker Multitip seismic source. The line runs from the continental shelf off the Casamicciola town towards the Northern Ischia Island. A qualitative sketch stratigraphic diagram of Northern Ischia Island based on the geological interpretation of the seismic profile L27 has been constructed (Aiello et al., 2009a; Figure 8). The forced regression prograding wedges characterize the stratigraphic architecture of the continental shelf off Northern Ischia Island (FST). The two superimposed wedges of the chaotic deposits suggest a multi-phase event for the evolution of the debris avalanche (H1 and H2). Three seismic units, separated by the regional unconformities occur in the basin filling (respectively units 3, 4 and 5). On the contrary, the seismic lines L26 and L28 have shown the occurrence of four seismic units, instead of three. The volcanic unit of the Casamicciola town is an undetermined volcanic unit, probably corresponding to a volcanic acoustic substratum, eroded at its top by an unconformity, probably subaerial. It well correlates with the V1 volcanic unit recognized on the seismic lines L26 and L28. Another volcanic unit is represented by the volcanic domes of Casamicciola town (Figure 8).Buried volcanic edifices, characterized by an acoustically-transparent acoustic facies and a dome-shaped external geometry are interstratified in the lowest seismic units of the basin filling (units 3 and 4). The unit 3 is characterized by parallel-to-sub-parallel seismic reflectors

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and, locally, by prograding clinoforms (on the left in the profile; Figure 8). The unit is strongly down thrown in correspondence to a normal fault. The unit 4 represents the intermediate seismic unit of the basin filling, characterized by parallel to sub-parallel seismic reflectors. The unit shows a strong wedging in correspondence to a normal fault, which is buried, in turn, by an erosional unconformity. The unit 5 represents the upper seismic unit of the basin filling, characterized by parallel seismic reflectors and, locally, by bidirectional onlaps in correspondence to depressions eroding the top of the underlying seismic unit. The hummocky deposits (H1 and H2) are characterized by a wedge-shaped external geometry and by chaotic facies. H1 and H2 occur on the continental slope and in the upper part of the basin and grade seawards to the marine deposits of the unit 5. The forced regression prograding wedges of the Late Quaternary depositional sequence (unit 7) deposited on the continental shelf off the Casamicciola Town.

The L29 seismic line is parallel to the Tyrrhenian shoreline (Figure 3). Important canyon systems have been crossed from this seismic line, genetically related with the “Testata di Punta Cornacchia” canyon system, located in the Northern Ischia Offshore and identified on the Ischia marine DEM (Digital Elevation Model; Aiello et al., 2009a; 2009b; 2012; Monti et al., 2012). The main canyon's heads are located in the North-Western Ischia Offshore ("Testata di Punta Cornacchia"; Canalone di Forio) and are partly covered by this seismic line (Figures 9 and 10).

It is important to speak about the Cuma canyon and about its role to controlling the erosional lineaments of the Northern Ischia Offshore in order to improve the comprehension of this seismic line. The Cuma canyon is a wide canyon system which starts northwards of the Ischia Island, in the areas next to the Phlegraean Fields and ends in a submarine valley, reaching a maximum depth of 850 m, located between the Ischia and the Ventotene Islands. The main canyon axis is NE-SW oriented. Different tributary channels start from this axis, including the Punta Cornacchia canyon, the Forio erosional channel and the Punta del Chiarito canyon. The canyon’s morphology increases the velocity of the upwelling (uprising of deep waters). It represents also a conduit for the waters of the deep basin.

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Interesting results on the geology of the Northern Ischia Island have been obtained from the seismic interpretation of the seismic line L29, suggesting the occurrence of different volcanic and sedimentary units, which have not been singled out in previous papers. Four different volcanic units have been detected (V, V1, V2 and V3). The V unit represents an old, undetermined volcanic edifice underlying the undetermined volcanic substratum of the Casamicciola Town (V1). The V2 unit corresponds with mounded-shaped volcanic edifices, cutting the marine deposits. The youngest volcanic unit is represented by a thick volcanic deposit (V3 unit), which seems to be related with the Zaro lava flow and is engraved by the Punta Cornacchia canyon (Figure 10). Three main units are composed of the marine deposits of the Casamicciola basin filling. The base of the marine sequence is marked by an erosional unconformity, representing also the top of Late Pleistocene prograding wedges, similar to those ones detected in the Naples Bay shelf based on seismic interpretation (Aiello et al., 2001).

DISCUSSION AND CONCLUSIONS

The Ischia Offshore is characterized by alkali-potassic volcanic rocks (trachytes, latites and alkali-basalts) pertaining to a volcanic complex emplaced during the last 55 ky B.P. (Chiesa et al., 1985; Poli et al., 1987). The Ischia geologic evolution realized through four main phases.

The first phase was older than 150 ky B.P. and included the emplacement of the pyroclastic products and the interstratified lava flows.

The second phase, ranging in age from 150 to 75 ky B.P. was characterized by a strong extensional activity, giving rise to the growth of lava domes and to the eruption of pyroclastic materials.

The first and the second phases suggest the presence of an ancient volcanic complex, probably larger than the present-day one and now almost completely dismantled, pre-dating the eruption of the ignimbritic tuffs of the Mt. Epomeo.

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The third phase, ranging in age from 55 to 20 ky B.P., was characterized by the emplacement of the Epomeo Green Tuffs, an ignimbrite which covered the coastal relief and filled the Ischia graben. The volcanic center opened in the Southern Sectors of the island and the eruptive activity continued in several volcanic centers (Citara-Serrara Fontana Formation; Rittmann, 1930). The volcanic products of the last activity are represented by the Campotese lavas.

The fourth phase, ranging in age from 10 ky B.P. to 1302 A.D. was concentrated in the Ischia graben, identified both by the faults generated during the rapid uplift of the volcano-tectonic horst of the Epomeo Mt. and by the NE-SW (counter-Apenninic) trending normal faults. The Zaro lava flow (1302 A.D.) was the only eruption outside the Ischia graben.

The main seismic sequences of the South-Eastern Ischia Offshore have been already restored through the geological interpretation of Sparker profiles (Aiello et al., 2012). The regional seismic sections in the canyon system have been constructed based on the seismic interpretation in order to improve the understanding of the submarine features (Aiello and Marsella, 2014). New data on the Northern Ischia Offshore are herein provided, focusing on the hummocky deposits and on the canyon systems (Figures 4-10). The volcanology and the stratigraphy of the onshore sectors have also been taken into account (Vezzoli, 1988; Orsi et al., 2004; Brown et al., 2008; Della Seta et al., 2012; Monti et al., 2012).

In the Ischia Island the gravitational movements are coeval with the volcanic activity, whereas the uplift events are related to the long-lasting resurgence of the Epomeo Block (Molin et al., 2003; Della Seta et al., 2012). These movements are represented by the debris avalanches, the debris flows (lahars), the smaller mass movements and the deep-seated gravitational slope deformations (Della Seta et al., 2012). The occurrence of the submarine deposits linked with the subaerial deposits of the most voluminous mass movements clearly shows that the debris avalanches had a significant impact on the sea (Chiocci and de Alteriis, 2006; De Alteriis and Violante, 2009).

The results obtained in this paper have confirmed the emplacement of important volcanic deposits in the Northern Ischia Offshore and their

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probable role in controlling the deposition of hummocky facies corresponding to gravitational deposits, widely occurring in the Casamicciola Offshore (Figures 4-10). The important role of erosional processes in correspondence to the main canyon’s heads in the Northern Ischia Offshore has also been revealed from the seismic interpretation. The canyon systems have been observed in the Northern Ischia Offshore, such as the “Testata di Punta Cornacchia” and the “Canalone di Forio”.

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