A series of volcanic edifices discovered a few kilometers off the coast of SW Sicily

Marine Geology 416 (2019) 105999

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A series of volcanic edifices discovered a few kilometers off the coast of SW Sicily

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Emanuele Lodolo , Dario Civile, Massimo Zecchin, Luigi Sante Zampa, Flavio Accaino Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: NW Sicilian Channel Volcanic edifices Multibeam swath bathymetry High-resolution seismic profiles Late Quaternary volcanic activity

The Graham and Terrible banks, located about 35 km from the south-western coast of Sicily, host a large number of volcanic constructs, the most famous being the ephemeral Ferdinandea Island. These volcanoes occur along two N-S trending strike-slip lineaments that constitute the lithospheric-scale Capo Granitola-Sciacca Fault Zone. Here we present recently acquired swath bathymetric data and magnetic measurements, in conjunction with high-resolution seismic profiles, which reveal the presence of another six volcanic edifices located very close to the Sicilian coasts, one of which is only 7 km away. Three of these volcanic constructs have been previously identified only on the basis of available seismic profiles, but their morphology and their volcanic nature had so far not been documented. Two edifices to the north show a possible tuff cone/ring-like morphology modified by crater breaching, while the three southernmost volcanoes have a truncated-conical shape. The northwesternmost volcano (here called Actea) shows a more complex morphology, probably representing the remnants of a previous crater rim. Seismic data analysis suggests that the six volcanic edifices were generated during a pre-Last Glacial Maximum (LGM, ca. 20 ka B.P.) magmatic phase associated with a tectonic event. Only the Actea volcano shows indications of magmatic reactivation, possibly between the LGM and the initial post-LGM transgressive phase. This reactivation is evident by the emplacement of a prominent young lava flow. The discovery of submerged volcanoes so close to the populated coast of Sicily demonstrates that there are large submerged areas near the littoral that are still little known and studied, and underlines how crucial it is to analyze the issue of volcanic risk for densely inhabited coastal areas like Sicily.

1. Introduction The northwestern sector of the Sicilian Channel hosts extensive magmatic activity that mainly developed during the Quaternary, and is characterized by a series of submerged volcanic edifices, which were identified and mapped in the course of several geophysical campaigns (Falzone et al., 2009; Coltelli et al., 2016). Most of these volcanoes are located within two shallow banks known as Graham and Terrible banks (Fig. 1A). Both banks are crossed by a regional, N-S trending lithospheric shear zone - the Capo Granitola-Sciacca Fault Zone (CGSFZ) which extends for about 200 km in the western part of the Sicilian Channel, from the Sicily coast to the volcanic island of Linosa (Argnani, 1990; Calò and Parisi, 2014; Civile et al., 2018). Among the volcanic edifices present in the Graham Bank, there is the ephemeral Ferdinandea Island, formed by a surtseyan-type eruption in 1831 CE (Washington, 1909), which represents the only volcanic activity documented in historical time in this sector of the Sicilian Channel. Other volcanic manifestations have also been recognized on the

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Adventure Plateau (Calanchi et al., 1989) and in the area of the Nameless Bank (Beccaluva et al., 1981). More recently, the presence of another three submerged volcanic edifices, situated within 24 km of the Sicilian coast between Capo Granitola and the town of Sciacca, was suggested on the basis of available seismic data (Lodolo et al., 2017; Civile et al., 2018; Lodolo et al., 2019). The lack of high-resolution bathymetric data coverage and magnetic profiles has so far not allowed us to unambiguously identify the nature of these constructs. In order to analyze their shape, geometry, morphology and lithology, a new dataset consisting of swath bathymetric data, magnetic profiles, and highresolution seismic lines (Chirp), was acquired between 2017 and 2018 near the coast between Capo Granitola and Sciacca by the R/V OGS Explora. These campaigns also had the purpose of identifying the presence of additional volcanic edifices closer to the coast, where the available bathymetric maps (both nautical charts and the recently released EMODnet map - http://www.emodnet.eu/new-high-resolutiondigital-terrain-model) were insufficient or ambiguous. In addition, this study aimed to investigate the possible age of the magmatic activity in

Corresponding author. E-mail address: [email protected] (E. Lodolo).

https://doi.org/10.1016/j.margeo.2019.105999 Received 2 April 2019; Received in revised form 18 July 2019; Accepted 19 July 2019 Available online 20 July 2019 0025-3227/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (A) General physiographic map of the Sicilian Channel. Bathymetry derived from EMODnet Digital Terrain Model (1/16*1/16 arc minutes), and extracted from the following link: http://www.emodnet-bathymetry.eu/. Abbreviations: GB, Graham Bank; TB, Terrible Bank; NB, Nerita Bank; NaB, Nameless Bank; SMTF, Sicilian-Maghrebian Chain Frontal Thrust. Black rectangle indicates the study area. The dotted black segments indicate the position of the main tectonic lineaments associated with the Capo Granitola and Sciacca fault system (adapted from Civile et al., 2018). Box in the upper-right corner locates the Sicilian Channel within the central Mediterranean Sea. Abbreviations: SMC, Sicilian-Maghrebian Chain; CGSFZ, Capo Granitola-Sciacca Fault Zone. (B) Shaded-relief swath bathymetric map (Multibeam) of the study area off the SW coast of Sicily, superposed onto the EMODnet Digital Terrain Model to fill the areas not covered by the multibeam measurements. The six volcanic edifices described in this study are labelled. Black dotted segments indicate the corresponding figures (seismic profiles). Thin white lines are the faults associated with the two strike-slip fault systems identified in the study area (from Civile et al., 2018): the Capo Granitola Fault System (CGFS) and the Sciacca Fault System (SFS). The positions of the available exploration wells, made available by the project “Visibility of Petroleum Exploration Data in Italy” (ViDEPI, http:// www.videpi.com), are also shown.

continental plate, and its present-day tectonic setting is the product of two independent geodynamic processes (e.g., Jongsma et al., 1985; Boccaletti et al., 1987; Reuther et al., 1993; Corti et al., 2006): (i) the Neogene formation of the Sicilian-Maghrebian Chain in the north and northwestern sectors of the Sicilian Channel; (ii) the Pliocene-Quaternary rift-related processes that generated the Pantelleria, Malta and Linosa troughs in the central part of the Sicilian Channel. Widespread, anorogenic volcanic activity has accompanied the rifting process, with the formation of the islands of Pantelleria and Linosa. The lithosphericscale CGSFZ separates sectors in the offshore part of the SicilianMaghrebian Chain. These sectors appear to have different deformation ages, distinct structural trends and tectonic evolution (e.g., Argnani, 1993a, 1993b; Grasso, 2001). The CGSFZ was probably responsible for the present-day Sicilian Channel rifting configuration (Argnani, 1990; Civile et al., 2014). The two main strike-slip tectonic lineaments

relation to the structures associated with the CGSFZ, which are considered tectonically active (Civile et al., 2018; Ferranti et al., 2019). The recognition of the horizon associated with the Last Glacial Maximum (LGM, ca. 20 ka B.P.), as identified in the Chirp profiles, allowed us to interpret the age of the volcanic activity. The discovery of submerged volcanoes so close to the populated coast of Sicily demonstrates that there are large areas of the submerged coastal zone, even close to the coast, which are still poorly known and studied. This is particularly relevant when it comes to volcanic manifestations proximal to the shoreline that may represent potential hazards to the densely populated coastal areas.

2. Geological framework The Sicilian Channel occupies the northern part of the African 2

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composing the CGSFZ (i.e., the Capo Granitola Fault System “CGFS” and the Sciacca Fault System “SFS”; Fig. 1B) cross the Graham Bank to the west and the Terrible and Nerita banks to the east, respectively, forming a complex deformation system which includes sub-vertical faults, flower structures, and arrays of subsidiary faults (Fedorik et al., 2018; Civile et al., 2018). In addition to the two volcanic islands of Pantelleria and Linosa, the continental shelf occupied by the Sicilian Channel is punctuated by many submerged volcanic edifices, mostly concentrated on the Terrible and Graham banks (Falzone et al., 2009; Coltelli et al., 2016). Other volcanic constructions are present in the vicinity of the western margin of the Adventure Plateau (Fig. 1A), a shallow platform occupying the northwestern sector of the Sicilian Channel (Colantoni et al., 1975; Calanchi et al., 1989; Civile et al., 2014, 2015). Bathymetric and seismic data have shown that most of the volcanic constructions identified on the Graham Bank are truncated-cone edifices (Coltelli et al., 2016) developed along a sub-vertical master fault associated with the Capo Granitola Fault System (Civile et al., 2018). The poorly evolved alkali basalts collected in this area (Rotolo et al., 2006) suggest the direct rise of primitive magmas to the surface along the master fault, without the presence of intermediate magma chambers. In the Terrible Bank the volcanic manifestations are smaller in size with respect to those in the Graham Bank (Coltelli et al., 2016; Civile et al., 2018), and may represent relict volcanic necks. Gravity data and modelling have suggested that a magma chamber is present below the Terrible Bank, at about 6 km depth (Lodolo et al., 2019), which could be the feeding system for magma to rise along the structures associated with the Sciacca Fault System (Civile et al., 2018). The volcanism of the Sicilian Channel primarily occurred during the Plio-Pleistocene (Rotolo et al., 2006), but significant activity took place more recently, leading to the birth of the island of Ferdinandea on the Graham Bank (Colantoni et al., 1975), and to the submarine eruption that occurred in 1891 CE about 5 km NW of the Pantelleria island (Conte et al., 2014).

Shipborne magnetic data were acquired using a SeaSpy® Marine Magnetics magnetometer, towed 180 m astern of the vessel. Raw magnetic data were collected at a sample rate of 1 s, resulting in an average spacing between each measurement of about 4 m. Most of the lines were acquired along the multibeam tracks, and some tie lines were acquired above the prominent volcanic structures. This provides a better resolution of the magnetic dipoles (approximately N-S oriented), less biased by the diurnal variations which affect the survey lines (E-W oriented). Magnetic anomalies were derived by removing: (i) the International Geomagnetic Reference Field (IGRF), and (ii) the diurnal effects generated by the variations of the external magnetic field. The diurnal effects were determined by using the data of the Lampedusa Magnetic Observatory. Before applying the correction, data were also filtered with a median moving window of 60 s. This provides the same temporal resolution of the Lampedusa station, avoids the aliasing, and removes most of the noise spikes. The reduction-to-the-pole was applied to obtain a better qualitative understanding of location and shape of the anomalies. This operation transforms the total field anomaly, using the same distribution of sources and considering a total vertical magnetization, as if the sources were located at the North Pole. The operation was accomplished by using the same average values of declination (D = 3°) and Inclination (I = 53°) as input parameters for the whole study area, assuming a negligible residual magnetization with respect to the effect of induced magnetization. This assumption can be considered correct only for “relatively young” igneous bodies, which have not experienced large variations of the Earth's magnetic field. The reduced anomaly was further smoothed with 2-D Hanning and Gaussian filters within a square moving window of 167 × 167 m2 that removed most of the remaining high frequency noise. The high-resolution seismic profiles (Chirp profiles along survey lines totaling about 950 km) were acquired along the multibeam tracks with a hull-mounted Benthos Teledyne® Chirp III DSP-665, characterized by sweeps ranging from 2 to 7 kHz (the vertical resolution is in the order of decimal centimeters). The SwanPro® software was used to collect the data, which were processed with the Paradigm® software and Seismic Unix®, applying absorption compensation, migration and the Hilbert transform. Finally, a seismic reflection profile (line FASTMIT-01) crossing the two southern volcanic edifices was also used in this study. This line, together with another 5 multichannel seismic reflection profiles (for a total length of 200 km), were acquired during the “FASTMIT” survey of 27–31 August 2017. The acquisition system includes an acoustic source of two GI-guns shooting every 25 m at 120 bar, and a digital streamer of 1500 m length with 120 channels. The recording system was a Sercel® Sentinel Seal 428. Sampling rate was 1 ms. Data processing (which includes pre-stack depth migration) was performed using the freely distributed Seismic Unix® software and the commercial software Echoes and Geodepth (Paradigm®).

3. Methods The results presented in this paper derive from the interpretation of high-resolution swath bathymetric data, high-resolution Chirp seismic profiles and magnetic measurements collected on 27–31 August 2017 and 12–19 February 2018 by the R/V OGS Explora during two surveys as part of the “FASTMIT” research project, coordinated by the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) and funded by the Italian Ministry of University and Research (MIUR). Moreover, the stratigraphic data of two offshore exploration wells (Onda 1 and Orione est., see Fig. 1A, which contain 500 and 440 m of Quaternary succession, respectively) have been used, along with literature information (Civile et al., 2018; Ferranti et al., 2019) to define the stratigraphy of the area and calibrate the Chirp profiles. The well data were made available by the Italian Ministry of the Economic Development in the framework of the project “Visibility of Petroleum Exploration Data in Italy (ViDEPI)” (http://www.videpi.com). The morpho-bathymetric data were acquired with a hull-mounted Reson® SeaBat 8111. It operates at a frequency of 100 kHz and illuminates a swath on the sea floor that is 150° across track and 1.5° along track. To ensure the best data coverage, each swath overlapped the adjacent ones by about a third. Navigation was ensured by three integrated D-GPS systems. The collected data, logged through the PDS2000® acquisition software, were fully corrected for ship's motion, navigation, sound velocity, and tides. Sound velocity profiles were collected daily with a Sound Velocity Probe to correct for density, salinity, and temperature variations in the water columns. The resulting Digital Terrain Model (with a grid cell size of 5 × 5 m) was then edited (removal of residual spikes) and filtered. Data visualization was done using the GlobalMapper® software and the freely available GMT® (Generic Mapping Tools; Wessel and Smith, 1991).

4. Results and interpretation The combined interpretation of high-resolution swath bathymetric data, magnetic measurements and high-resolution seismic profiles has allowed us to define the morphology and shallow structure of six small, isolated volcanic edifices at 60–200 m water depth on the continental shelf off Sicily, between Capo Granitola and the town of Sciacca, and located from 7 to 23 km from the coast. Following Calanchi et al. (1989), the identified volcanoes were named for the Greek mythological figures of the Nereid sea nymphs (i.e., Actea, Climene, Nesea, Doride, Ianeira and Ianassa, Fig. 1B). These volcanoes were recognized on the basis of: (i) morphology shown by high resolution swath bathymetry data, (ii) high-resolution (Chirp) profiles, in which the volcanic edifices and their products generally show an opaque seismic facies without internal reflectors, (iii) multichannel seismic profiles, and (iv) magnetic measurements. The three southern edifices, as mentioned, were previously identified by scattered and sparse seismic 3

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Fig. 2. (A) Reduced-to-the-pole, 3-D magnetic anomaly map of the study area, showing the occurrence of localized and circumscribed magnetic anomaly peaks at the identified volcanoes. (B) Selected bathymetric and magnetic profiles across the identified volcanic edifices.

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5

b

0.21 0.14 0.31 0.62 0.37

b

0.165 0.169 0.068 0.033 0.035 0.045 90 101 397 262 200

b b

AB: Basal area; PB: Basal perimeter; Zmin: Minimum depth below actual sea-level; Hmax: Maximum height of crest from base; MAxB: Major basal axis; mAxB Minor basal axis; As: Summit area; Ps: Summit perimeter; MAxs: Major summit axis; mAxs: Minor summit axis; WB: Basal width; WS: Summit width; H/WB: Height/basal width ratio; WS/WB: Summit width/basal width ratio; SFL: Average slope of the flank. a Height of the entire volcanic edifice considering both the outcropping and buried portions. b MAxs and mAxs, and their derived parameters Ws and WS/WB, have not been computed for the Actea volcanic edifice because a crater-like feature is not recognizable.

412 710 1265 420 537 935 22 20 14 6 5 2-30 65 62 370 245 190 115 140 425 280 210

330 380 1500 843 679 1.69 0.008 0.0077 0.17 0.054 0.032 0.1 422 750 1430 500 550 1200 37°22′04.86”N 37°22′11.04”N 37°25′15.90”N 37°25′26.31”N 37°28′43.77”N 37°29′3’0.33”N Ianassa Ianeira Doride Nesea Climene Actea

12°50′05.87″E 12°47′43.78″E 12°45′05.90E 12.54′47.43″E 12.48′08.65″E 12°39′57.11″E

0.13 0.39 1.23 0.14 0.19 0.8

1320 2260 4100 1480 1640 3300

-133 −96 −73 −65 −63 −33

68–170a 120–205a 87–130a 16 18 43

401 670 1100 340 525 670

WS/WB H/WB WS (m) WB (m) SFL (°) mAXs (m) MAXs (m) Ps (m) AS (km2) mAxB (m) MAxB (m) Hmax (m)

4.2.1. Ianeira and Ianassa volcanic edifices The two southernmost volcanic edifices (Ianeira and Ianassa) are located at ca. 23 km from the Sicily coast and are 3.5 km away from each other (Fig. 1B). Both exhibit a conical-shaped morphology and are surrounded by a depression produced by currents eroding the surrounding sediments (a current-eroded moat) (Fig. 3A and B). The seismic profile FASTMIT-01 (Fig. 4), the interpretation of which derived from Civile et al. (2018), and Chirp profiles (both interpreted by Civile et al., 2018) highlight that part of these edifices and their volcanic products are buried under the upper part of the sedimentary section, which does not appear deformed. The buried base of these cones, which is located at about 300 m (400 ms two-way traveltime, TWT, converted to depth with an average acoustic velocity of 1500 m/s) below the present-day sea-level, is surrounded by a body characterized by chaotic and/or opaque seismic facies with a strongly irregular top (Fig. 4). These bodies are interpreted as consisting of volcanic and landslide deposits, the latter produced by gravitational flank collapses. Their similar morphological features and the comparable depth of the paleoseabed allow us to infer probable coeval activity for these two volcanic edifices. The western truncated, partially buried cone (Ianeira) (Figs. 3A, 4) rests on the seabed between 180 and 200 m water depth, whereas the moat that surrounds it is up to 216 m deep along the southwestern side of the volcano. This cone has a sub-circular shape in plan view, and its unburied part has a maximum height of about 120 m and a minimum depth of 96 m (Fig. 3A and Table 1). The summit area consists of an almost flat surface, weakly inclined towards the east. The flanks appear steep with maximum slope values up to 28–30°. A clear crater morphology is not present. Considering the same paleo-seabed of the Ianassa volcano, the total height of the edifice might have been

Zmin (m)

Shape and size of the volcanic edifices may provide valuable information on the constructional and destructive processes that acted during a volcano's life, and for this reason several morphometric characteristics and derived parameters (Porter, 1972; Wood, 1980; Favalli et al., 2009; Grosse et al., 2009, 2012; Sánchez-Guillamón et al., 2018) were measured on the volcanic edifices, as reported in Table 1. The average width values or basal width of the volcanoes (WB) were calculated as the arithmetic mean of the maximum and minimum widths of the outline.

PB (m)

4.2. Morpho-bathymetric data

AB (km2)

Magnetic data are often helpful for discriminating the lithological nature of outcropping or buried geological bodies. The presence of magnetic anomalies detected in these bodies indicates they have volcanic affinity. In our case, as seen from the map of the magnetic data (Fig. 2A), six magnetic anomaly peaks are found just above the morphological highs identified by the high-resolution bathymetric data (Fig. 2B). This map complements the general magnetic map of the western sector of the Sicilian Channel (Lodolo et al., 2012). Magnetic anomalies in the studied area range from −240 to up to +790 nT. The background magnetic anomaly is in the range from −120 to +80 nT. The largest anomaly (+792 nT) is found on the Actea volcanic edifice, whereas the other two larger anomalies were recorded on Nesea (+340 nT) and Climene (+210 nT). Doride, Ianeira and Ianassa exhibit smaller anomalies of +110, +45 and + 120 nT, respectively. It is worth noting that the small feature located at 1.5 km to the east of the Actea volcano, which rises about 10 m above the surrounding sea-floor as seen from the high-resolution bathymetric map (Fig. 1A), does not show any magnetic anomaly. This suggests that it is not volcanic in nature, but rather may represent a mud volcano or a simple hard sedimentary outcrop.

Longitude

4.1. Magnetic data

Latitude

profiles, but their morphology and volcanic nature were unclear.

Volcano

Table 1 Morphometric measurements of the six identified volcanic edifices (morphometry based on swath-bathymetry data and high-resolution seismic profiles). Data reported in Table concern only the outcropping part of the volcanic edifices.

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Ianeira

Ianassa -100 m -100 m

-150 m -150 m

-200 m -200 m current-eroded moat

current-eroded moat

500 m

500 m

A

B

Doride

Nesea -50 m

-50 m volcanic outcrop

wide terraced surface sand ridge

-100 m

-100 m volcanic deposits

-150 m volcanic neck or secondary cone volcanic blocks or volcanic outcrops

-200 m

volcanic deposits covered by sand ridge

current-eroded moat

300 m

C

300 m

D

Actea

Climene -20 m

-50 m

-50 m sand ridge

scar

-100 m

sand ridge -100 m

lava flow

scarp of volcanic deposits

possible mud-volcano

scar lava flow prominent lava flow

volcanic blocks or volcanic outcrops gas escape

scar

300 m

potential dyke

E

1 km

gas escape

F

Fig. 3. 3-D shaded-relief images derived from high-resolution swath bathymetry of the six volcanic edifices described in this study. The main sedimentary and volcanic-related morphological features are labelled, along with the occurrences of gas escapes (Indicated by a black circles with grey plume icons).

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Seismic line FASTMIT-01 Depth (m)

0

WNW

volcanic and landslide deposits

ESE

Ianeira

Ianassa

200

400

volcanic conduit

volcanic conduit

sill reflector

Plio-Quaternary succession

sill reflector

sill reflector 600

800

Terravecchia Fm. (Tortonian-Messinian)

1 km

Fig. 4. Part of the migrated multichannel seismic reflection profile FASTMIT-01 crossing the two volcanic edifices of Ianeira and Ianassa. The position of the subhorizontal sill horizon has been indicated. The interpretation of the Plio-Quaternary and Tortonian-Messinian (Terravecchia Formation) successions is based on the information derived from Civile et al. (2018).

consisting of volcanic and landslide deposits (Fig. 5A). This body locally extends for over 1.5 km from the volcanic edifice. The interpretation of the multichannel seismic profile suggests that the paleo-seabed was about 200 m below the present-day sea-level, for a total height of the cone of about 130 m. Finally, some buried secondary cones were identified on the Chirp profiles (Fig. 5A).

about 205 m. The eastern - and smallest - truncated cone (Ianassa) (Figs. 3B, 4), lies in water depths between 172 m and 189 m. The surrounding current-eroded moat is up to 200 m deep. The unburied part of this edifice is 68 m high and shows a sub-circular shape in plan view (Fig. 3B and Table 1). Considering the paleo-seabed associated with this volcanic edifice, lying at the base of the volcanic and landslide deposits in the seismic sections, its total height might be around 170 m. The summit of the cone, where the minimum water depth is 133 m, shows an ellipsoidal shape and an irregular morphology. The flanks of the edifice are very steep, up to a maximum value of 27°, and are smooth.

4.2.3. Nesea and Climene volcanic edifices The two northeastern volcanic edifices, named Nesea and Climene (Fig. 1B), exhibit a different morphology compared to the above-described volcanoes. The Nesea volcano, which is located at 17 km from Sciacca (Fig. 1B), shows a wide and sub-circular horseshoe-shaped crater, with a maximum height of about 16 m, in water depths between 73 m and 81 m (Fig. 3D and Table 1). The crater is breached to the SW due to the collapse of a part of the summit. A prominent rocky structure inferred to be a secondary cone, slightly higher than the crater-rim, is present in the NE sector of the crater (Fig. 3D). The maximum diameter of the crater is about 280 m, and the summit is at about 65 m water depth. The crater rim is 8 to 13 m higher than the surrounding seabed and it is up to 7 m higher than the crater floor. Its external slope is affected by several modest scars. Two main lobate bodies can be seen N and SSW of the volcanic edifice in multibeam data (Fig. 3D) and Chirp profiles. The seismic facies of these raised bodies is characterized by a totally opaque aspect without internal reflectors and suggests a volcanic nature (lava flows). The southern body, which extends for less than 1 km in length and is 800 m wide, is buried under a modest thickness (up to about 10–15 m) of sandy sediments (dunes/sand ridges). These features are characterized by a dome shape with a semi-transparent seismic facies that contains inclined internal reflectors, which we interpret as foresets (Fig. 5B, C). In contrast, the northern body, which is over 1 km wide and is 1.3 km long, is partially outcropping to the N-NE of the volcanic edifice or covered by very thin deposits (Fig. 5B). The entire Nesea volcanic zone, including the crater and the surrounding volcanic deposits, covers a total area of ca. 1.9 km2. Some rocky structures, probably composed of decameter-scale blocks produced by collapses of the volcanic edifice, are present to the NW of the crater within a depression, and also to the NE. Alternatively, these rocky structures might represent volcanic outcrops. The Climene volcanic edifice, which is located at about 11 km from the coast (Fig. 1B), exhibits an irregular morphology, quite similar to that of Nesea. It consists of a low relief (maximum height of 18 m above the surrounding sea-floor), hosting a small sub-circular crater-like feature breached towards the SW (Fig. 3E and Table 1) and probably produced by the collapse of part of the summit. The entire edifice covers an area of ca. 0.17 km2, is about 450 m wide, and lies at water depths between 75 and 81 m. Its minimum water depth is ca. 63 m. The crater, located in the southern part of the edifice (Fig. 3E), occupies an

4.2.2. Doride volcanic edifice The third volcanic edifice, named Doride, is located at less than 7 km to the NW from the two volcanoes described above and at about 16.5 km off the coast of Capo Granitola at water depths between 125 m and 150 m (Fig. 1B). This truncated volcanic cone, whose summit is at a water depth of 73 m, is larger than Ianassa and Ianeira but exhibits a similar sub-circular shape in plan view with a N-NE trending, 1430 m long maximum axis, and a maximum height of the rimmed crest from the base of about 87 m (Fig. 3C and Table 1). This volcanic edifice, like Ianassa and Ianeira, is surrounded by a current-eroded moat, which is deeper (up to 160 m) along its southern side, and wider along the western side. Its summit is characterized by a horseshoe-shaped crater, opened to the east. Some small rocky structures visible on the floor of the crater, which is about 5–6 m lower than the rim, might represent collapsed blocks of the ancient volcanic rim. The present-day rim exhibits a smooth surface up to 170 m wide, and it is externally bounded by a scarp showing a maximum slope of up to 17°. The volcanic edifice shows a concentric pattern with evident breaks in the slope probably associated with relatively flat terraced surfaces (Fig. 3C). The deeper and thin terraced surface lies at water depths between 107 and 113 m. The wider terrace is almost flat and lies at water depths between 92 and 100 m. Another slope break, just below the summit area, occurs at water depths between 82 and 86 m along the western side of the edifice. The two uppermost concentric levels that occur along the western side of the volcano might represent different eroded volcanic rims. The summit region may have been flattened by wave action during the postLGM relative sea-level rise. Some small landslide scars affect the summit crater. The seismic line CROP-M23A (see Civile et al., 2018) and Chirp profiles (Fig. 5A) document that this edifice is also partially buried under the upper part of the sedimentary succession, the latter showing a current-controlled internal geometry (Marani et al., 1993; Verdicchio and Trincardi, 2008; Rebesco et al., 2014; Pepe et al., 2018). This is suggested by sedimentary thinning towards the volcano (which is an obstacle to bottom-water circulation) with draping and/or thickening in correspondence of the underlying basement high. Moreover, the buried base of this edifice is surrounded by a body possibly 7

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Chirp profile 54

meters

W

E

Doride

75

TWT (ms)

0.10

partially buried volcanic cone current-eroded moat

LGM surface

0.15

current-eroded moat

112.5

secondary cone 0.20

150

Capo Granitola positive flower structure

VDB

1 km

volcanic and landlslide deposits

A

Multiple

Chirp profile 77 TWT (ms)

0.05

NE

SW

187.5

meters 37.5

Nesea crater

dunes

0.10

75

lava flow LGM SU+WRS

LGM SU+WRS 0.15

112.5

truncated LGM prograding wedge 500 km

Multiple

B

Chirp profiles 47-48

meters

W

E

75

LGM SU+WRS

TWT (ms)

0.10

dunes current-eroded moat

0.15 truncated LGM prograding wedge

0.20

Nesea volcanic deposits (lava flow ?)

112.5

150

VDB Doride volcanic deposits 1 km

C (caption on next page)

area of about 0.032 km2. Its maximum diameter is less than 200 m, whereas the depth of the floor is between 67 and 71 m. The rim is up to 3–4 m high with respect to the crater floor. The maximum slope of the edifice is 11°. An area of voluminous gas escape, with 50 m high acoustic flares, is clearly seen both in Chirp data (Fig. 6A) and

multibeam data, emanating from the center of the crater. Another minor gas seep, recorded by multibeam sonar in the water column and on Chirp profiles, is present at the base of the SE flank of the volcano (Fig. 3E). The edifice seems to be affected by several scars. Potential large blocks probably produced by the collapse of the edifice, occur 8

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Fig. 5. (A) High-resolution seismic profile (Chirp data) crossing the Doride volcano and showing a current-eroded moat close to the cone. The upper Quaternary succession covering part of the volcanic edifice is inferred to be associated with bottom-current deposits. The LGM surface and the VDB unconformity, are also labelled. The buried base of the volcanic edifices is surrounded by an irregular reflector characterized by several convex-up structures potentially interpreted as the result of interbedded volcanic and landslide deposits. The upper part of the positive flower structure associated with the Capo Granitola fault system is visible on the left side of the profile. (B) High-resolution seismic profile (Chirp data) crossing the Nesea volcano and showing volcanic deposits below the LGM SU + WRS unconformity along the NE flank of the volcano. The buried truncated LGM prograding wedge and the LGM SU + WRS are visible on the left side of the profile. (C) High-resolution seismic profiles (Chirp data) showing the architecture of the truncated LGM prograding wedge. Dunes associated with the post-LGM transgressive phase are found above the LGM SU + WRS. Buried volcanic deposits associated with Doride and Nesea are clearly visible, and they lie on the VDB unconformity. Onlapping reflectors are recognizable in the upper right part of the figure. Location of the corresponding profiles in Fig. 1B. Abbreviations: LGM, Last Glacial Maximum; SU, Subaerial Unconformity; VDB, Volcanic Deposits Base unconformity; WRS, Wave-Ravinement Surface.

5. Discussion and conclusions

around the western part of the volcano (Fig. 3E).

The presence of volcanic edifices near the coastlines is not unusual. There are several examples in the central Mediterranean of submerged (and emerged) volcanic edifices close or relatively close to the coast. Some examples include underwater constructs which are part of the Aeolian Islands archipelago (Casalbore et al., 2016), some volcanoes off the coast of the southern Tyrrhenian Sea (Cocchi et al., 2017), and off the coasts of western Sardinia (Conforti et al., 2016). Most of these submerged volcanic edifices have been known for a long time, while others have been identified only in more recent times, through the use of modern high-resolution bathymetric instruments that are able to detect morphological features of sizes smaller than tens of meters. Our study area, which is located a few kilometers from the coast of SW Sicily, has been crossed since remote times by countless boats of all kinds because of its strategic position for trade, traffic and travel. Despite this, there are many elements of the sea floor that are still unknown to us today. The discoveries of new volcanic edifices described in this work are examples to this. They were not reported in any of the commonly used nautical charts or in any available bathymetric maps. Some of these features, however, were known by local fishermen as morphological highs, but nothing was known about their shape, size and geological nature. The high-resolution bathymetric data presented here, integrated with seismic profiles at different vertical resolution, have allowed us to image in detail their morphologic characteristics, and through the magnetic profiles to deduce their volcanic nature. The northwestern-most volcanic edifice - Actea - lies on the northern part of the Capo Granitola Fault System, where available seismic data have revealed the presence of a prominent positive flower structure (Civile et al., 2018) affecting an already deformed Mesozoic-Cenozoic carbonate succession and Upper Miocene deposits. To the south, the fault system changes its structural setting, and is characterized by the presence of a strike-slip master fault (Civile et al., 2018) along which the volcanic cones of the Graham Bank are aligned broadly N-S. The magmatism observed along the Capo Granitola Fault System was likely driven by a mechanism of non-plume origin. Magmas have used the faults that cut the whole lithosphere and reached the asthenosphere as open conduits, leading to partial melting by simple pressure release (Civile et al., 2018). The presence of a volcanic edifice within a transpressional context (i.e., Actea) is not unusual. Several investigations have showed that volcanism can occur in compressional tectonic settings associated with reverse and strike-slip faulting (Tibaldi et al., 2010). The other five volcanic edifices are located between the two fault systems of Capo Granitola and Sciacca (Fig. 1B), within a relatively undeformed area where a Plio-Quaternary sedimentary succession overlies sub-vertical normal faults of Late Miocene age (Civile et al., 2018). The volcanism observed in this sector might be related to the presence of a shallow sill complex (lying at depths of ca. 800–1000 m), which may have fed dykes that locally reached the surface (Civile et al., 2018). In turn, the sill complex might be fed by magmas that have migrated upward along the lithospheric faults of Capo Granitola and Sciacca, and then they expanded laterally to the buried, Late Miocene normal faults present in this area. The presence of a sill complex can facilitate magma transport over significant vertical and horizontal

4.2.4. Actea volcanic edifice Actea is the northwestern-most volcanic edifice, and is the one closest to the coast (only 7 km off Capo Granitola) (Fig. 1B). It has an irregular and complex morphology, in which a crater-like structure is not recognizable from the swath bathymetric data (Fig. 3F). The edifice is elliptical in shape and has relatively flat relief, with a major axis (1.2 km) oriented NW-SE that occupies an area of about 0.8 km2 (Fig. 3F and Table 1). This edifice lies at water depths between 62 and 70 m. Several decametre-size rocky blocks are present around the southern margins of the relief. The central part of the Actea volcanic edifice hosts a ridge that extends for about 500 m in the NW-SE direction and is bounded by very steep scarps on either side (Figs. 3F, 6B) with a maximum slope up to 25°–30°. The ridge might be the remnant of a previous crater rim affected by widespread volcanic collapses and eroded by wave action. The summit ridge, reaching a minimum water depth of 33 m, seems to be affected by small scars. A large, EW-oriented lobate feature, interpreted as a lava flow and located SW of the volcanic edifice, is well imaged by the high-resolution bathymetry data and Chirp profiles (Figs. 3F, 6C). This flow, which is elongated to the west and covers an area exceeding 4 km2, is ca. 4 km long and up to 1.3 km wide. Other inferred lava flows are located at the base of the SW and S sides of the elliptical edifice and are elongated towards the SSW; these flows are 350–800 m long and 250–500 m wide (Fig. 3F). An evident morphological scarp is present along the eastern side of the volcano (Figs. 3F, 7A). The area including all described volcanic features extends for about 6 km2. A NE–SW trending linear structure, 80–90 m wide and about 2 km long, closes this area to the south (Fig. 3F). On the basis of its shape and seismic character (an opaque acoustic response), it is interpreted as an outcropping dyke. Some gas emissions have also been imaged from Chirp profiles and multibeam data along the SW part of the volcanic edifice (Figs. 3F, 6B).

4.2.5. Stratigraphy of the deposits surrounding the volcanic edifices A recurrent feature in the Chirp profiles is represented by a toptruncated clinoformal body, which locally lies on the flanks of the shallower volcanic edifices (Figs. 5B, C, 6B, C, 7A, C). It is up to 20 m thick and is present in water depths of ca. 70 to over 100 m. This body exhibits a relatively flat, gently seaward-inclined top overlain by irregular sedimentary bodies interpreted as large dunes (Fig. 5B, C), although in places it is deeply eroded (Figs. 5C, 6C, 7A, C). The observed features suggest that this clinoformal body is a prograding shallowmarine wedge associated with the last glacial phase, later eroded by subaerial processes and then by wave processes (the LGM unconformity, Figs. 5–7). Another unconformity, the VDB (Volcanic Deposits Base) unconformity, is present in the study area (Figs. 5–7), and surrounds the Doride, Nesea, Ianassa and Ianeira volcanoes. Its formation seems to be related with a tilting of the substrate due to a tectonic event linked to movement along the CGSFZ. This angular unconformity is locally onlapped by pre-LGM deposits and truncates the underlying reflectors (Figs. 5C, 7B, 8), while at the Actea volcano it represents the base of the LGM prograding marine wedge (Figs. 6B, 7A). 9

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Fig. 6. (A) High-resolution seismic profile (Chirp data) crossing the Climene volcano and showing a prominent gas bubble plume above the crater. (B) Highresolution seismic profile (Chirp data) crossing the irregular ridge forming the summit of the Actea volcano, which probably represents the remnant of an ancient crater rim. Another gas bubble plume is also labelled. The innermost part of the truncated LGM prograding wedge, lying on the western flank of the Actea volcano, is recognizable on the left side of the profile. The base of the wedge is the VDB unconformity. (C) High-resolution seismic profile (Chirp data) crossing the prominent Actea lava flow, which is locally covered by a very thin Holocene deposits. The truncated LGM prograding wedge is recognizable towards the west. The LGM SU + WRS, bounding the top of the wedge, is covered by chaotic deposits probably associated with landslides. In contrast with the smoothed shape exhibited by the LGM SU + WRS on the left side of the profile, that surface rises close to the front of the lava flow to form a topographic step, which is inferred to be the result of the relatively high resistance of the solidified lava to wave action. Locations of the corresponding profiles are shown in Fig. 1B. Abbreviations: LGM, Last Glacial Maximum; SU, Subaerial Unconformity; VDB, Volcanic Deposits Base unconformity; WRS, Wave-Ravinement Surface.

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Fig. 7. (A) High-resolution seismic profile (Chirp data) crossing the southern part of the Actea volcano. The truncated LGM prograding wedge lies above the volcanic deposits of Actea. Also in this case, the base of the wedge is the VDB unconformity. Part of the Capo Granitola positive flower structure is recognizable on the right side of the profile. (B) High-resolution seismic profile (Chirp data) showing the VDB unconformity, on which the Doride volcanic deposits and onlap several reflectors lie. The VDB also truncates the underlying reflectors. (C) High-resolution seismic profile (Chirp data) crossing the northern side of the prominent lava flow associated with the Actea volcano. The lava flow lies on the truncated LGM prograding wedge, which is eroded and covered by landslide deposits on the left side of the profile. Location of the corresponding profiles is shown in Fig. 1B. Abbreviations: LGM, Last Glacial Maximum; SU, Subaerial Unconformity; VDB, Volcanic Deposits Base unconformity; WRS, Wave-Ravinement Surface. 11

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is a stratigraphic marker generally recognizable in the Chirp profiles analyzed in this study. It truncates the underlying LGM prograding shallow-marine wedge and older deposits, and is locally very irregular (Figs. 5B, C, 6C, 7). The volcanic activity of the Nesea, Climene, Doride, Ianeira, and Ianassa volcanoes certainly predated the LGM, as these edifices are partially buried by the Late Quaternary succession underlying the LGM erosional surface (see Fig. 5 and see Fig. 12 in Civile et al., 2018). Each of these volcanic edifices was probably generated by a single explosive eruption (i.e., they represent monogenic volcanoes) in the same period of time, as their deposits lie on the same horizon (VDB unconformity). The formation of the VDB, as well as the activity of these five volcanoes, was probably related to a Quaternary tectonic reactivation of the CGSFZ. The VDB becomes shallower up-dip and rapidly merges with the LGM erosional surface (Figs. 5C, 7A, B), so that the volcanic edifices and associated magmatic deposits located closer to the coast tend to be shallower. This is evident in the case of Actea, where the LGM prograding wedge lies on the flanks of the edifice (Fig. 7B), highlighting also in this case a relatively older (pre-LGM) initial magmatic activity. However, the prominent lava flow associated with the Actea volcano, visible in the multibeam and high-resolution seismic data, clearly overlies the LGM prograding wedge (Figs. 6C, 7C). A marked step of the WRS is present just at the front of the lava flow (Figs. 6C, 7C), which was probably due to the higher resistance of lava to wave erosion during the post-LGM relative sea-level rise. In summary, a main pre-LGM magmatic phase might have generated all the monogenic volcanic edifices recognized in the study area. Afterwards, possibly between the LGM and the initial post-LGM transgressive phase, a second eruptive episode involved only Actea, generating the lava flow overlying the LGM prograding wedge. Actea seems to be a peculiar volcano with respect to the other five edifices for the following reasons: (i) it might be a polyphase volcano; (ii) it generated a prominent lava flow covering an area of over ca. 4 km2; (iii) it is located above the Capo Granitola Fault System. The presence of a series of volcanoes discovered a few nautical miles from the coasts of Sicily raises two important considerations:

distances (Magee et al., 2016). Volcanoes fed by sill complexes may thus be laterally offset, even by several kilometers from the melt source. Since the volcanic edifices and related volcanic deposits involve only the uppermost part of the sedimentary succession (Figs. 4-8), a Quaternary age for the volcanic activity is inferred. On the basis of their morphological characteristics, two groups of volcanoes can be considered. The first group includes the three southern truncated cones (Ianassa, Ianeira and Doride), which are very similar to those found elsewhere on the Graham Bank. These steep, truncated conical edifices are partially buried by the uppermost Quaternary succession. Their paleo-seabed lies at about 200–300 m below the present-day sea-level. Moreover, they appear partly eroded and surrounded by current-moat depressions, and do not show evidence of recent lava flows. The morphological aspects of these edifices suggest that they may be classified as cinder or scoria cones, following Kereszturi and Németh (2012). The second group includes the two northern edifices of Nesea and Climene; they have relatively low elevations and gentle slopes characterized by horseshoe-shaped craters, breached to SW. These edifices might be classified as partially collapsed and eroded tuff cones/rings, adopting the same terminology of Kereszturi and Németh (2012). The Actea volcano presents a more complex morphology that cannot be immediately associated with the other volcanic edifices. It might represent the remnant of a wide edifice affected by widespread collapse, possibly generated by explosive activity and subsequently eroded by wave action. The Actea and Nesea volcanoes show evidence of lava flows at their bases. In particular, a wide lava flow to the SW of Actea is a very unusual feature if compared to the modest size of the volcanic edifice and with respect to the known volcanoes of the Graham Bank area where a 1 km-long lava field was documented only along the western flank of the main volcanic edifice of the Graham Bank (Coltelli et al., 2016). Due to the absence of samples, the age of the volcanic activity cannot be directly determined, although some considerations can be made on the basis of the recognition on Chirp profiles of the surface that is interpret as associated with the LGM. This subaerial unconformity was generated by the global glacio-eustatic sea-level fall that culminated ca. 20 ka B.P., when sea-level was ca. 120–130 m lower than today (Siddall et al., 2003; Clarke et al., 2009). Wave action during the post-LGM marine transgression produced a wave-ravinement surface (WRS; Zecchin et al., 2019) that completely reworked the LGM subaerial unconformity (SU) (see Fig. 5C). This composite erosional surface

(1) The imperative need to produce detailed maps of the coastal areas, which are still only partially covered by high resolution data. These have applications in various fields of interest such as navigation, fishing, management of coastal and port infrastructures, defense of coastlines from the often devastating effects of extreme phenomena 12

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generated by climate change, etc. (2) Volcanoes produce multiple primary and secondary hazards (Blong, 1984; Papale and Schroeder, 2014) that must each be recognized and assessed in order to mitigate their impacts. Depending upon volcano type, magma composition, eruption style, scale and intensity at any given time, these hazards will have different characteristics and may occur in different combinations at different times. Monitoring active volcanoes to provide early warnings of anticipated volcanic activity to responsible authorities and the public represent key factors in building resilience and reducing risk. This is particularly relevant for densely inhabited areas.

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Acknowledgements This study was carried out as part of the “FASTMIT” project (Seismogenic and Tsunamigenic Faglie in the IT Seas), funded by the Italian Ministry of Education, University and Research (MIUR). We are very grateful to the crew and the technical group on board the R/V OGS Explora for their assistance and advice in acquiring data at sea. Daniela Accettella (OGS - Trieste) carried out the processing of swath bathymetric data. We warmly thank the Editor Edward Anthony, and the reviewers William Chadwick and Marc De Batist for their invaluable suggestions that have contributed greatly to improving the manuscript. Special thanks go to Luca Gasperini (ISMAR, CNR - Bologna) for kindly providing us with the magnetometer used in the surveys. The EMODnet Bathymetry Consortium is recognized for the use of bathymetric data presented in this work. References Argnani, A., 1990. The Strait of Sicily rift zone: foreland deformation related to the evolution of a back-arc basin. J. Geodyn. 12, 311–331. Argnani, A., 1993a. Neogene tectonics of the Strait of Sicily. In: Max, M.D., Colantoni, P. (Eds.), Geological Development of the Sicilian-Tunisian Platform, Proceedings of International Scientific Meeting, University of Urbino, Italy, 4–6 November, 1992. Unesco Report in Marine Science. 58. pp. 55–60. Argnani, A., 1993b. Neogene basins in the Strait of Sicily (Central Mediterranean): tectonic settings and geodynamic implications. In: Boschi, E., Mantovani, E., Morelli, A. (Eds.), Recent Evolution and Seismicity of the Mediterranean Region. Kluwer Academic Publication, Dordrecht, pp. 173–187. Beccaluva, L., Colantoni, P., Di Girolamo, P., Savelli, C., 1981. Upper Miocene submarine volcanism in the Strait of Sicily (Banco Senza Nome). Bullettin of Volcanology 44, 573–581. Blong, R.J., 1984. Volcanic Hazards: A Sourcebook on the Effects of Eruptions. Academic Press, North Ryde, New South Wales, Australia (424 pp). Boccaletti, M., Cello, G., Tortorici, L., 1987. Transtensional tectonics in the Sicily Channel. J. Struct. Geol. 9, 869–876. Calanchi, N., Colantoni, P., Rossi, P.L., Saitta, M., Serri, G., 1989. The Strait of Sicily continental rift systems: physiography and petrochemistry of the submarine volcanic centers. Mar. Geol. 87, 55–83. Calò, M., Parisi, L., 2014. Evidence of a lithospheric fault zone in the Sicily Channel continental rift (southern Italy) from instrumental seismicity data. Geophys. J. Int. 199 (1), 219–225. Casalbore, D., Bosman, A., Romagnoli, C., Di Filippo, M., Chiocci, F.L., 2016. Morphology of Lipari offshore. Journal of Maps 12 (1), 77–86. https://doi.org/10.1080/ 17445647.2014.980858. Civile, D., Lodolo, E., Alp, H., Ben-Avraham, Z., Cova, A., Baradello, L., Accettella, D., Burca, M., Centonze, J., 2014. Seismic stratigraphy and structural setting of the Adventure Plateau (Sicily Channel). Mar. Geophys. Res. 35, 37–53. Civile, D., Lodolo, E., Zecchin, M., Ben-Avraham, Z., Baradello, L., Accettella, D., Cova, A., Caffau, M., 2015. The lost Adventure Archipelago (Sicilian Channel, Mediterranean Sea): morpho-bathymetry and Late Quaternary palaeogeographic evolution. Glob. Planet. Chang. 125, 36–47. Civile, D., Lodolo, E., Accaino, F., Geletti, R., Schiattarella, M., Giustiniani, M., Fedorik, J., Zecchin, M., Zampa, L., 2018. Capo Granitola-Sciacca Fault Zone (Sicilian Channel, Central Mediterranean): structure vs magmatism. Mar. Pet. Geol. 96, 627–644. Clarke, P.U., Dyke, A.S., Shakum, J.D., Carlson, A.E., Clark, J., Wohlfarth, B., Mitrovica, J.X., Hostetler, S.W., McCabe, A.M., 2009. The last glacial maximum. Science 325, 710–714. Cocchi, L., Passaro, S., Caratori Tontini, F., Ventura, G., 2017. Volcanism in slab tear faults is larger than in island-arcs and back-arcs. Nat. Commun. 8, 1451. https://doi. org/10.1038/s41467-017-01626-w. Colantoni, P., Del Monte, M., Gallignani, P., Zarudzky, E.F.K., 1975. Il Banco Graham: un vulcano recente nel Canale di Sicilia. Giorn. Geol. 40 (1), 141–162. Coltelli, M., Cavallaro, D., D’Anna, G., D’Alessandro, A., Grassa, F., Mangano, G., Patanè,

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