Journal of Volcanology and Geothermal Research 251 (2013) 50–64

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The deformation offshore of Mount Etna as imaged by multichannel seismicreflection profiles

A. Argnani a,⁎, F. Mazzarini b, C. Bonazzi a, M. Bisson b, I. Isola b

a CNR, ISMAR-Bologna, Bologna, Italyb Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy

⁎ Corresponding author at: ISMAR-CNR, Via GobetTel.: +39 0516398940; fax: +39 0516398940.

E-mail address: andrea.argnani@ismar.cnr.it (A. Arg

0377-0273/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.jvolgeores.2012.04.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 May 2011Accepted 10 April 2012Available online 20 April 2012

Keywords:Mount Etna offshoreVolcano flank instabilityActive tectonicsMultichannel reflection seismicsIntrusive body

Despite the clear evidence of active flank dynamics that is affecting the eastern side of Mount Etna, the con-tribution of tectonic processes has not been yet understood. So far, the various models proposed to explainthe observed flank deformation have been based on onshore structural data, coming from the volcanic edi-fice. The Ionian offshore of Mount Etna has been only recently investigated using multichannel seismic pro-files, and offers the opportunity to image the structural features of the substrate of the unstable flank ofthe volcano. This contribution aims at describing the deformation located offshore Mount Etna using multi-channel seismic profiles recently acquired during three seismic surveys. The onshore flank deformation ofMount Etna appears to be laterally confined by two tectonic guidelines, trending roughly E–W, located tothe north and south of the deforming flank; the northern guideline, in particular, takes the surface expressionof a sharp fault (Pernicana Fault). Though often assumed that these boundary structures continue offshore aslinear features, connected to a frontal thrust ramp, the occurrence of this simple offshore structural systemhas not been imaged. In fact, seismic data show a remarkable degree of structural complexity offshoreMount Etna. The Pernicana Fault, for instance, is not continuing offshore as a sharp feature; rather, the defor-mation is expressed as ENE–WSW folds located very close to the coastline. It is possible that these tectonicstructures might have affected the offshore of Mount Etna before the Pernicana Fault system was developed,less than 15 ka ago. The southern guideline of the collapsing eastern flank of the volcano is poorly expressedonshore, and does not show up offshore; in fact, seismic data indicate that the Catania canyon, a remarkableE–W-trending feature, does not reflect a tectonic control. Seismic interpretation also shows the occurrence ofa structural high located just offshore the edifice of Mount Etna. Whereas a complex deformation affectsthe boundary of this offshore bulge, it shows only limited internal deformation. Part of the topography ofthe offshore bulge pre-existed the constructional phase of Mount Etna, being an extension of the HybleanPlateau. Only in the northern part, the bulge is a recent tectonic feature, being composed by Plio-Quaternarystrata that were folded before and during the building of Mount Etna. The offshore bulge is bounded by a thrustfault that can be related to the intrusion of the large-scale magmatic body below Mount Etna.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Mount Etna is a large volcano that originated in its present form ca.15 kyr ago, following a long magmatic history that began ~500 kyr agoin an area of complex tectonics (Fig. 1; Branca and Del Carlo, 2004;Branca et al., 2004, 2008). A large set of geodetic and interferometricdata collected in the last two decades shows that the eastern flankof Mount Etna is slowly moving eastward (e.g., Froger et al., 2001;Puglisi and Bonforte, 2004; Bonforte et al., 2011), supporting longer-term geological evidence (e.g., Borgia et al., 1992). Despite the clear

ti 101, 40129 Bologna, Italy.

nani).

rights reserved.

evidence of active flank dynamics that is affecting the eastern side ofMount Etna, the contribution of tectonic processes has not been yet un-derstood. So far, the different models proposed to explain the observedflank deformation have been based on onshore structural data, comingfrom the volcanic edifice (Fig. 2). The flank deformation of Mount Etnaappears to be a complex system of tectonic blocks that are laterallyconfined by two main tectonic guidelines, trending roughly E–W inthe north and roughly WNW–ESE in the south of the deforming flank(e.g., Rust et al., 2005); the northern guideline, in particular, takes thesurface expression of a sharp fault (Pernicana Fault). Though oftenassumed that these boundary structures continue offshore as linearfeatures, connected to a frontal thrust ramp (i.e., Borgia et al., 1992,2000a), the occurrence of this simple offshore structural system hasnot yet been proven. The Ionian offshore of Mount Etna has been onlyrecently investigated using multichannel seismic profiles, and offers

Fig. 1.Main structural features along the Malta Escarpment. The subsurface extent of the Hyblean Plateau is indicated in gray, and it is hypothesized to continue underneath MountEtna. Part of the folding and thrusting observed offshore north Etna could be possibly related to the deformation at the Gela Thrust front. Thick arrows indicate direction of max-imum compressional stress (Neri et al., 2005b). The dashed line near the coast of Sicily north of Etna is a tilted monocline. Contractional reactivation occurred where extensionaland thrust symbols are present along the same line. Small arrows near a fault line indicate a strike–slip component (after Argnani, 2009).

51A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

the opportunity to image the structural features of the substrate of theunstable flank of the volcano. This contribution aims at describing thedeformation located offshore Mount Etna using recently acquired mul-tichannel seismic profiles (Fig. 2).

2. Geological setting

Mount Etna is the largest subaerial volcano in Europe and is locat-ed in an area of great tectonic complexity (Fig. 1; Hirn et al., 1997;

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Fig. 2.Main tectonic features of Mount Etna taken from various sources (see text for details). The main tectonic features in the offshore, from this study, are also included. The whitedotted line indicates the approximate boundary of the high velocity body identified by tomographic studies between 23 and 9 km depth (Chiarabba et al., 2000; Patanè et al., 2006).Outcrops of Quaternary marine sediments are in white, whereas the Chiancone outcrops are in dotted pattern. The thrust on the lower left is the front of the Gela Nappe (seeregional map of Eastern Sicily). The two dashed lines with rectangles in the south are normal faults affecting the Hyblean plateau in the subsurface of the Catania Plain, whereasthe symbol + surrounded by a gray line is a high (b700 m depth) of Miocene volcanics in the Hybean plateau (from subsurface data in Longaretti et al., 1991). FC and CC indicatethe Fiumefreddo and the Catania canyons, respectively, whereas RR marks the Riposto Ridge. The boundary of the Valle del Bove (VdB) is marked by white solid lines. A moredetailed bathymetry for part of the offshore, is presented in the Supplementary material (Fig. S1, modified after Chiocci et al., 2011).

52 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

Monaco et al., 1997; Argnani, 2000; Nicolich et al., 2000; Doglioni etal., 2001; Argnani, 2009). The volcano is sitting on top of the Ma-ghrebian fold-and-thrust belt, that runs W–E through Sicily, and islocated on the northern prolongation of the NNW-trending MaltaEscarpment, a major morphological feature of the central Mediterra-nean, part of which has been tectonically reactivated in Quaternarytime (Argnani and Bonazzi, 2005; Argnani, 2009). The Malta Escarp-ment is separating the continental crust domain of the Hyblean region,in SE Sicily, from the oceanic domain of the Ionian basin (e.g., Argnaniand Bonazzi, 2005 and references therein), a setting that greatly affect-ed the subduction regime underneath Calabria (e.g., Gvirtzman andNur, 1999; Argnani, 2009). Moreover, the Scicli–Ragusa fault system,a large NNE-trending fault zone that bounds to the west the Hybleanplateau, is also converging toward Mount Etna. This location at a cross-roads of structural trends has been often called upon to explain theorigin ofMount Etna (e.g., LoGiudice et al., 1982), although the relation-ships between tectonic processes and geodynamic regime have notbeen yet fully unraveled (Gvirtzman and Nur, 1999; Doglioni et al.,2001; Argnani, 2009; Schellart, 2010).

Besides the volcanic activity of Mount Etna, the region appears to betectonically highly active: a remarkable late Quaternary uplift affectedthe adjacent Peloritani and Calabrian domains (Monaco and Tortorici,2000; De Guidi et al., 2003), and one of the largest earthquakes inItaly occurred in the Messina Straits in 1908 (e.g., Pino et al., 2009).

The geologic history of Mount Etna includes phases of submarine vol-canism, shield building, formationof nested volcanic centers and calderas,and the final constructional phase of the stratovolcano in the last 15 ka

(Kieffer, 1985; Corsaro et al., 2002; Branca et al., 2004; Branca and DelCarlo, 2005; Branca et al., 2008). Recent eruptions of Mount Etna arecharacterized by summit eruptions at the central craters, and by fis-sure eruptions and dike intrusions at the rift zones oriented NE, N–Sand E–W. Several eruptions have occurred from both summit andflank vents in the last century (Romano et al., 1979; Behncke andNeri, 2003; Branca and Del Carlo, 2004). Additional informationabout historical lava flows on Mount Etna, along with a daily updateof Mount Etna activity, is available on the Catania Istituto Nazionaledi Geofisica e Vulcanologia Web site (http://www.ct.ingv.it).

Recent geodetic, InSAR and GPS data, clearly show that northernand western sectors at Mt. Etna are mainly characterized by upliftand radial pattern of ground deformation and by a widespread andstructurally complex seaward deformation accommodated by move-ment of different blocks in the eastern and southern flanks of the vol-cano (Lanari et al., 1998; Froger et al., 2001; Lundgren et al., 2004;Bonforte and Puglisi, 2006; Solaro et al., 2010; Bonforte et al., 2011).Particularly, geological, geodetic and volcanologic investigations haveidentified a short term shallow deformation, mainly linked with vol-canic activity (volcano inflation and emplacement of dikes), and along term deep seated deformation due to an overall gravity instabil-ity by the volcano load (Neri et al., 2009; Solaro et al., 2010). Inver-sion of geodetic data shows that volcanic load (pressurized sourceat depth) acted at depths variable in the 3–9 km b.s.l. range (Puglisiand Bonforte, 2004; Bonforte et al., 2008). A sub-vertical high velocitybody (HVB) has been imaged by seismic tomography (Fig. 2), with bot-tom at a depth of about 18 km and top at about 3 km (e.g., Hirn et al.,

53A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

1991; Chiarabba et al., 2000; Patanè et al., 2003, 2006). This volumewith positive velocity anomaly has been interpreted as a large plutonicbody. Between 3 and 9 km depth the lateral extent of this body in-creases to about 6 km, and the estimated volume is about 3 times largerthan the volcano pile, suggesting that its accretion could be responsiblefor destabilizing the eastern flank (Allard et al., 2006). Then most ofthe magma mass is actually cumulated at the shallower interface ofthe upper limit of the HVB and is detected by the ground deformationand gravity data analysis (Bonaccorso et al., 2011).

Sector collapses also characterized the evolution of Mt. Etna, astestified by the Valle del Bove (VDB) scar and by the occurrence ofPleistocene to Holocene debris deposits (Chiancone) cropping outon the eastern flanks of the volcano (Fig. 2). The Holocene Chianconedeposits form a seaward bulge of the coastline and consist of fluvialdeposits resting upon debris avalanche deposits whose top has beendated back to about 8000 yr B.P. (Calvari and Groppelli, 1996; Calvariet al., 1998, 2004). The results of magnetic surveys carried out offshoreof the Chiancone deposits have been interpreted as imaging volcano-clastic deposits and alluvium in a stretch a few km wide (Del Negroand Napoli, 2002); this volcanic-derived deposit has been attributedto the Chiancone, and it would increase the estimate of the volumeof material evacuated during the Valle del Bove collapse.

Following the observation that the eastern flank of Mount Etnais moving eastward, several attempts have been made to identifya basal sliding plane, using the concepts of volcanic spreading (e.g.,Borgia et al., 1992, 2000b). Several different interpretations of thebasal sliding surface have been proposed on the basis of geologicaldata. Borgia et al. (1992) proposed a detachment surface, located atabout 5–6 km depth (below sea level) and slightly dipping westward,which is rooted in the plutonic complex below the volcano summit.The sub-Etnean clays have been taken as representing the detachmentsurface. A similar solutionwas subsequently adopted by several authors(Tibaldi and Groppelli, 2002; Neri et al., 2004; Rust et al., 2005). A westdipping surface located at 0–2 kma.s.l., which therefore does not extendoffshore, has been also proposed (Lo Giudice and Rasà, 1992; Bousquetand Lanzafame, 2001, 2004). Moreover, in an early work (Kieffer, 1983)an offshore subhorizontal detachment was linked to the extension oc-curring in the Timpe fault system, and was not directly related to theflank dynamics. These different interpretations show that the geometryof the detachment plane lacks of solid constraints, despite its geologicalappeal. More recently, a 12° east-dipping plane was defined by invert-ing GPS velocities (Bonforte and Puglisi, 2006; Bonforte et al., 2008;Puglisi et al., 2008); this plane would reach the coastline at a depth ofca. 4 km below sea level. With a similar approach, Bonaccorso et al.(2011) proposed the occurrence of an eastward dipping slide surface lo-cated just at the top of the HVB at 3 km depth b.s.l. A listric fault con-necting the sliding detachment with the segment of the PernicanaFault adjacent to theNE Rift has been constructed by applying a rolloverconstruction to (Ruch et al., 2010). However, the constraints on the ge-ometry of the fault plane are rather loose, and the reconstructed faultplane flattens to a depth of ca. 2 km b.s.l. over a short distance (fewkms), resulting shallower than the GPS-derived sliding plane.

Overall, flank instability is widespread at Mt. Etna, regardless ofvolcanic activity, and remains by far the predominant type of deforma-tion on the easternflank of the volcano. Based on geological and geodet-ic data, the main bounding structures of the unstable eastern flank arethe E–W trending left-lateral strike–slip Pernicana Fault, to the north,with an average slip rate of 10–20 mm/yr, and the TrecastagniMascalu-cia fault system, to the south, with average slip rate in the order of1–10 mm/yr (Fig. 2; Tibaldi and Groppelli, 2002; Acocella et al., 2003;Neri et al., 2004; Acocella and Neri, 2005; Rust et al., 2005).

Seismicity in the eastern flank is rather intense and associatedwith the main structural features (e.g., Azzaro et al., 2008). The Perni-cana Fault is characterized by intense very shallow (b3 km from sur-face) seismicity, especially in the western sector of the fault (e.g.,Alparone et al., 2011a, 2011b-this issue), and focal mechanisms

mostly display sinistral strike–slip motion along the fault plane. How-ever, although the Pernicana Fault is a sharp feature, the deformationalong the southern boundary of the Etna eastern flank is rather diffuse,as indicated by the field of GPS velocities (Bonforte et al., 2011), and thedisplacement appears also of lesser extent. The very limited seismicityassociated to this southern fault system further supports the lack of aclear boundary. Some remarkable seismicity is instead associated tothe Timpe fault system, where earthquakes occur down to 10–15 km,with focal mechanisms that show dextral-oblique extensional motion(Allard et al., 2006; Neri et al., 2005a). Magmatic activity is also associ-ated with this fault system, and the connection with magmatic outpourcould be somewhat puzzling, as the magma-tapping faults shouldcut through the decollement surface of the sliding flank of the volcano.It should be noted, however, that the Timpe magmatism was switchedoff at about 130 ka (De Beni et al., 2005; Branca et al., 2008).

The huge bulge just offshore of Catania (Fig. 2), has been regardedas a large antiform and thought to be the offshore expression of theseaward volcano's spreading (Borgia et al., 1992, 2000a). Occurrenceof widespread landslide deposits offshore of Mount Etna was provenby recent multichannel seismic surveys (Argnani and Bonazzi, 2005;Pareschi et al., 2006), although major contractional structures werenot documented, so far.

Theoffshore prolongations of the Pernicana Fault andof the southernboundary of the Etna sliding flank are represented by the FiumefreddoCanyon, to the north, and Catania Canyon, to the south, respectively(Fig. 2). It should be remarked, however, that these are mainly morpho-logical features and so far no clear tectonic structure has been shown,although they have been interpreted as marking two regional tectoniclineaments that accommodate the basinward movements of both thesubmarine and subaerial flank of the volcano (Chiocci et al., 2011).

3. Seismic data

The data from three seismic surveys have been used to map thegeology of the marine area offshore of Mount Etna (Fig. 3).

The MESC 2001 and TAORMINA 2006 surveys were carried outin order to investigate the regional geology and active tectonics ofthe eastern Sicily slope and of the Messinian Straits (Argnani et al.,2002, 2008, 2009a; Argnani and Bonazzi, 2005).

The INGV survey (Pareschi et al., 2006) was aimed at investigatingthe marine flank of Mount Etna, and seismic acquisition was plannedfor a higher resolution, with respect to the other two surveys. The de-tails of seismic acquisition and processing are given in Appendix A.

4. Interpretation and discussion

Seismic profiles have been interpreted by applying the proceduresof seismic stratigraphy (Payton, 1977) and the technique for identify-ing structural styles (Bally, 1983).

For sake of clarity the interpretation of seismic profiles is presentedby subdividing the study area in three sectors: northern, central, andsouthern. The northern sector straddles the prolongation of the Perni-cana Fault, the central and the southern sectors covers the frontal partof the bulge located offshore Etna, and its southern boundary, respective-ly. The seismic profiles described in this section are located in Fig. 3.

4.1. Northern sector

Seismic profiles show evidence of contractional structures in theregion offshore northern Etna. In some instance, small-scale thrustsheets are imaged as detaching at rather shallow level (Fig. 4). Theseismic facies in the lower part of the line shows low frequency, var-iable amplitude and poor continuity reflections, recalling the seismicfacies of the Hyblean domain (see Central sector section, below). Onthe other hand, the upper seismic facies, with high frequency, goodcontinuity reflections, resembles the response of the Plio-Quaternary

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Tao16

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Fig. 3. Seismic data set. The grid of multichannel seismic profiles is composed by three different surveys, carried out between 2001 and 2006 (see text for more details). The locationof seismic profiles shown in this paper is indicated with a bold black line.

54 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

succession (e.g., Argnani and Bonazzi, 2005). The seismic data showthat the strata describing these contractional structures are truncatedby an erosional surface. It appears that the erosional surface has beensubsequently folded with a wavelength larger than that of the thrustsheets. More than one episode of contraction is therefore recorded inthe area. Considering the thickness of the folded Plio-Quaternary suc-cession (over 400 m), and comparing it to the thickness in the adjacentareas, the older thrust faulting can have occurred sometimes withintheQuaternary. The thrust faults trendNNE–SSW, but detailedmappingof their lateral extent is inhibited by their small scale and by the greatcomplexity of the area (Fig. 5).

The erosional surface that cut the thrust and folded strata is in turnfolded with a much larger wavelength. The effect of this later large-scale folding on sea-floor topography can be appreciated on a ca. N–Sprofile (Fig. 6) where folded strata are well imaged. Fold axes trendENE–WSW(Fig. 5). Interestingly, minor extensional faults with roughlythe same trend and dipping downslope occur in this area; they are likelydriven by a combination of folding and gravity. Small extensional faultswith the same trend have been imaged onmultibeam bathymetry, veryclose to the coast (see also Chiocci et al., 2011). Onshore prolongationsof these features might be responsible for historical episodes of mudblow out in the coastal lagoons, known as La Gurna mud volcano(Fig. 5; Carveni et al., 2006). In this respect, it is interesting to observethat the Pernicana Fault loses its continuity toward the coastline, andis presentwith shortNE-trending segments. Here the faults have almostnormal component, and displacement is much reduced; the morpho-logical expression of the faults and the associated seismicity are also re-duced (Groppelli and Tibaldi, 1999; Acocella and Neri, 2005). Highresolution multibeam bathymetry, very close to the coast, also shows

that a cluster of small NNE–SSW scarps, between 80 and 110 m waterdepth, are present on the prolongation of the Pernicana Fault (Chiocciet al., 2011). How the Pernicana Fault continues offshore is a critical ques-tion. The interpretation of seismic profiles shows that the Pernicana Faultdoes not continue offshore as a sub-vertical strike–slip fault with an E–Wtrend (Figs. 6 and 7). Therefore, models of distributed brittle deformationabove a decoupling layer given by Etnean clays (Groppelli and Tibaldi,1999) seem not likely. Rather, it seems that two broad anticline and syn-cline, trending ENE–WSW and at least 8 km long, interrupts the trend ofthe Pernicana Fault (Fig. 5).

It seems, however, that the easternmost extent of the PernicanaFault onshore is more similar to what observed in the offshore (smallNE-trending extensional faults), and one may wonder whether thelarge folds mapped offshore, that underline the superficial extensionalfaults, can continue inland, underneath the coastal plain. The changefrom a clear strike–slip Pernicana Fault and a different kind of deforma-tion to the east could, therefore, be located onshore. The coastline, asit is often the case, is just an ephemeral and apparent boundary.

The left-lateral Pernicana Fault accommodates the sliding of theeastern flank of Mount Etna; therefore, the displacement along thefault is either absorbed as contractional deformation (as previously sug-gested, e.g., by Borgia et al., 1992), or is linked to a broader extensionalsystem that marks the large-scale collapse of the volcanic edifice andits substrate toward the depressed region of the Ionian Sea. As thePernicana Fault appears kinematically linked to the Rift of NE, its activityis likely comprised within the last 20 ka (last phases of Ellittico volcanoand inception of Mongibello activity; Branca, 2003; Del Carlo et al.,2004). Moreover, with the estimated (maximum) current slip rate of15–20 mm/yr along the Pernicana Fault, the total amount of displacement

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T.W

.T. (

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intense mass-wastingalong the slope

folded erosional surfaceerosion at sea floor

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Fig. 4. Seismic profile 3a located in the northern sector offshore Mount Etna. The line displays a high structural complexity, which is only partly highlighted; small-scale thrustsheets detached at shallow level are a remarkable feature indicated by bold black lines. The seismic facies in the upper part (a) show high frequency and good continuity reflections,whereas the seismic facies of the lower part (b) shows low frequency, variable amplitude and poor continuity reflections. Note that the strata are truncated by an erosional surfacethat has been subsequently folded. Subsequent erosion and mass wasting have heavily modified the sea-floor topography. T.W.T. is two-way-traveltime. Location is in Fig. 2.

55A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

in the last 20 kyr, 300–400 m, is much less than what observed at thethrust front, as described below.

The ENE–WSW-trending system of folds affects the sea floor mor-phology, suggesting a recent activity, perhaps coeval with the activityof the Pernicana Fault. The ENE–WSW fold trend, however, is at oddswith a left lateral strike–slip along the Pernicana Fault, underminingthe simple view of a collapsing volcanic flank where two lateral strike–slip faults are linked to a frontal thrust (i.e., Borgia et al., 1992). On theother hand, it is possible that complexity related to regional active tec-tonics played a role (e.g., Argnani, 2009).

The thrust fault at the front of the structural high is well imaged byseismic data (Fig. 8). The thrust plane cuts through the stratal surfacewhich represents the base of a large-scale landslide and reaches thesea floor. Although the sea floor shows evidence of submarine erosionand mass wasting at the location where it is intersected by the thrustplane, it is likely that shortening is currently occurring. However,the observed displacement along the thrust is larger than 1 km, andtherefore much larger than the maximum horizontal displacementaccrued along the Pernicana Fault since its onset (300–400 m). More-over, it seems that the large scale landslide has been triggered bythe uplift caused by the initial activity of this thrust.

A remarkable character of this offshore area is thewidespread occur-rence of erosional features that testify the intense mass wasting alongthe slope (see also Chiocci et al., 2011). The scars left by mass wastingprocesses are well imaged in many of the profiles (e.g., Figs. 4 and 6).The most relevant feature related to mass wasting along the slope isthe large scale slide at the northern boundary of the offshore bulge(Fig. 5). The body of such submarine slide is composed by stratified sed-iments that were deposited on the submarine slope (Figs. 8 and 9) andthat, therefore, cannot be directly related to volcanic collapse. A regionalprofile (Fig. 9) shows the broad view of the thrust fault located at thefront of the structural high, and illustrates also the large-scale slidebody and the extent of mass-wasting processes which have been activealong the offshore slope. The details of the large submarine slide will begiven elsewhere, nevertheless, the slide deposit has been deeply erodedon the northern side, where it faces the large Fiumefreddo Canyon(Figs. 2 and 10). In fact, theflanks and the head of thewide Fiumefreddocanyon are deeply incised by smaller canyons; the dense network ofgullies and canyons indicates an erosionally mature submarine drainage,

suggesting that the age of the submarine slide can be taken as older thanany known historical tsunamis (Argnani et al., 2009b).

Moreover, the high resolution seismic profiles and morpho-bathymetric data on the northern part of our study area support pre-vious arguments, based on geophysical data (Argnani et al., 2009b), andallow to rule out the occurrence of a recent large-scale landslide locatedoffshore Giardini–Naxos (Fig. 5). This observation undermines theproposal of Billi et al. (2008) that the 1908 tsunami was originated bya over 20 km3 large landslide in this sector of the eastern Sicily slope.

Finally, it has been recently proposed that the Etnean volcanismextends eastward, for about 20 km into the Ionian Sea (Patanè et al.,2009). The authors further hypothesize that the morphology of thevolcanic apparatus is not directly observed because it is buriedunder the debris deposits of the Chiancone unit. The supposed volca-nic apparatus is located in the Riposto Ridge, a marked east–west-trending features that bounds to the north the offshore bulge. Thisproposal, however, does not find support in our data, which showthat the Riposto Ridge is composed by folded sedimentary strata, inthe seismic facies of which there is no evidence of volcanic material(Fig. 6). Moreover, as discussed more in details below, the depositsbelonging to the Chiancone unit do not extend as far as the localitywhere the volcanic apparatus is supposed to be.

4.2. Central sector

The area located offshore of central and southern Mount Etna ischaracterized by a marked topographic bulge that extends ca. 10 kmfrom the coastline. On seismic profiles this region appears as a structuralhigh that displays a thin sedimentary cover, characterized by high fre-quency, good continuity reflections, above a poorly reflective substrate(Fig. 10). On the other hand, well stratified and thicker sedimentarypackages occur on either side of this high, on a N–S cross section(Fig. 9). Stratigraphic relationships, still partly visible on the southernand eastern sides (Figs. 9 and 10), suggest that the sedimentary strataonlapped a pre-existing topographic relief. As the strata span at leastthe whole of the Quaternary, it is believed that the structural highpre-existed the stabilization of the volcano feeder system and thechange from fissural to central eruptions, that led to the formation ofMount Etna (120 ka; Branca et al., 2004). Whereas the strata on the

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N10 km

Fig. 5. Structural map of the region offshore of Mount Etna. The thrust fault defining the boundary of the offshore morphological high merges to the north into a region of ca. NE-trending folds, at the prolongation of the Pernicana fault. The morphologic high is composed by a northern part, where a thick package of sedimentary strata has been thrust andfolded (dotted pattern), and by a less deformed southern part, which is indicated as “boundary of structural high”. In the northern part, only one of the extensional faults related tofolding (Line 17) has been mapped, because of their small scale. Structures are from this study and, in the periphery of the offshore bulge, from previous work (Argnani and Bonazzi,2005; Argnani et al., 2009a, 2009b). The boundary of the main erosional scar is also indicated. The approximate location of the La Gurna mud volcano is after Carveni et al. (2006).The sea-floor lineament is taken from Chiocci et al. (2011) and commented in the text.

56 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

southern side show little or no deformation, the sedimentary packageto the north has been heavily folded. It is worth noting that althoughthe folded strata to the north, which include the Riposto Ridge, contrib-ute to the topographic bulge, this area differs substantially from the restof the topographic high.

folded strata0.00

0.50

1.00

1.50

2.00

linNNW

T.W

.T. (

s)

Fig. 6. Seismic profile 17 illustrating the sea-floor topography created by large-scale foldinfaults, likely driven by a combination of folding and gravity. Location is in Fig. 3.

In the northern part, the sedimentary strata have been folded andthe eastern edge of the structural high appears clearly as thrust fault(Figs. 6 and 8). The same thrust fault, although less evident, can befollowed further to the south on seismic profiles, till the Catania can-yon, where it turns from NNE–SSW to ca. E–W (Fig. 5).

minor extensional faults

multiple

mass wastingscars

e 17SSE

2.5 km

g. Fold axes are shown in the structural map (Fig. 5). Note also the minor extensional

SSW NNE

5 km

prolongation of thePernicana Fault

multiple

multiplecontinuous reflection package

multiple

Tao 16

3

2

1

0

T.W

.T. (

s)

Fig. 7. Southern part of Line Tao 16 showing the lack of subvertical tectonic features at the seaward prolongation of the Pernicana Fault. The folded sediments on the southernportion of the profile belong to the system of folds of the Riposto Ridge (Fig. 5). Location is in Fig. 3.

57A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

The origin of this topographic high, which pre-existed the forma-tion of Mount Etna, is not fully understood. However, subsurface datain the Catania plain, south of Etna, show that a narrow stripe of unitsbelonging to the Hyblean plateau is entering below the eastern part ofthe volcano edifice (Longaretti et al., 1991). In fact, Miocene volcanicsbelonging to the Hyblean succession have been found at depth lessthan 700 m near Catania (Fig. 2). It seems, therefore, that the high cor-responding to part of the bulge located offshore of Mount Etna is anorthern stretch of the Hyblean plateau (Fig. 5).

4.3. Southern sector

The southern prolongation of the Timpe Faults system has beenassumed by several authors (e.g., Monaco and Tortorici, 2000), mak-ing the Timpe faults as part of the Malta Escarpment fault system.

LinWNW

T.W

.T. (

s)

0

1

3

2

thrustcollapsedsediments

onlapping strata

Fig. 8. Line 2. Thrust fault at the front of the structural high. The thrust cuts through the surfalarge scale-slide. The intense sea floor erosion is evident on the profile. The minimum thrualong the Pernicana Fault (300–400 m). Note that the basinal strata (right side) are onlapp

A physical continuity, however, has never been proven, and previousdata suggested that it was not likely (Argnani and Bonazzi, 2005). Thecurrent seismic coverage allows to better define this issue (Fig. 3).The east–west-trending seismic profiles south of Mount Etna showa thick and undeformed sedimentary package that likely spans thewhole of Quaternary and possibly older sediments. This sedimentarypackage covers and sutures pre-existing structures (W-dipping ex-tensional faults) but it is not affected by faulting (Fig. 11); consideringthat if the Timpe Faults were continuing southward they should passthrough this seismic profile, a direct connection between the Timpefaults and the Malta Escarpment fault system should be rejected.Although, it is possible that the Timpe fault system represents a W-ward (left) stepping of the Malta Escarpment fault system, it shouldbe noted that the final morphology produced by this fault systemis rather different from the morphology resulted from the Malta

e 2ESE

2.5 km

slideerosion at sea floor

ce above which a thick package of sediments (500–600 m) have slid down, originating ast displacement (ca. 1 km) is larger than the horizontal displacement possibly accrueding to the west a pre-existing high.

mass wasted slope

onlapping strata

Etna landslide

SW NETao 14

5 kmmultiple

0

1

2

3

4

TW

T (

s)

Fig. 9. Seismic profile Tao 14 showing a broader view of the thrust fault at the front of the structural high. The large-scale slide body and the effect of mass-wasting processes alongthe slope are also well imaged. Note that the basinal strata (right side) are onlapping to the west a pre-existing high. Location is in Fig. 3.

58 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

Escarpment fault system, as the Timpe fault system does not show asteep scarp toward an eastern deep water domain. High resolutionbathymetric data show that the Timpe faults offset the coastline by10–15 m throw in their southern termination (Chiocci et al., 2011).

A large submarine canyon, the Catania Canyon, represents a majormorphological feature in this last area (Fig. 5). Because of its E–Wrough-ly linear trend in the 15 km near the coastline, the Catania Canyon hasbeen often considered as a tectonically-controlledmorphological feature.

However, our seismic data suggests that no E–W trending struc-ture occur in that area, where, instead, extensional faults with trendsfrom NW to N have been mapped (Fig. 5). The thrust bounding theoffshore bulge is passing to the north of the Catania Canyon, and thecanyon is incising a subhorizontal sedimentary succession (Fig. 10).Although a more than 30 km long, WNW–ESE lineament can pickedout in detailed bathymetries (Marani et al., 2004; Chiocci et al., 2011;Fig. 5), seismic data (Argnani and Bonazzi, 2005 and this study) showthat the lineament is just amorphological feature that has no relationshipwith deep structures; in fact, a fault of the Malta Escarpment system iscrossing the lineament, remaining unaffected. This evidence underminesthe interpretation of Chiocci et al. (2011), that consider the Catania Can-yon lineament as crustal feature bounding the offshore bulge to thesouth. It seemsmore likely that the Catania Canyon is controlled by amor-phology that is inherited from previous tectonics (Fig. 10).

Line CoSSW

T.W

.T. (

s)

0

1

2

3

structural Cataniacanyon

sedimentsof the

Catania plain

Fig. 10. Composite NNE–SSW seismic profile crossing the sectors offshore of Mount Etna. Treflective substrate. Well stratified sedimentary packages occur on either side of the highonlapped a pre-existing high. The sedimentary package to the north has been heavily folde

4.4. The Chiancone unit

An apron of prograding sediments has been observed offshore of thearea where Chiancone unit is cropping out, and it may be of interest tofigure out its relationship with the Chiancone deposit (Fig. 2). A N–Sprofile close to the coastline shows an erosional platform located justoff the Chiancone outcrops (Fig. 12); this erosional surface is coveredby little or no sediments, without any trace of the chaotic facies thatcan be attributed to the Chiancone deposits. Rather, judging from its lo-cation, this erosional surfacemight represent the base of Chiancone unitcropping out onshore. This widespread erosional surface can be fol-lowed all along the shelf and can be attributed to the last sea level low-stand (ca. 20 ka). Moreover, the prograding unit imaged by E–Wprofiles (Fig. 13) is affected by the erosion that originated the subhori-zontal platform, indicating that the prograding unit was older thanthe Chiancone unit, from which it may be physically detached(Fig. 14). Although it is possible that the sedimentarywedge located off-shore has been fed by the dismantling of the Etna eastern flank, there isno relationshipwith an abrupt event such as the collapse of the Valle delBove, and deposition appears to be due to regular and continuous sedi-mentation. This observation questions some results obtained from ma-rine magnetic survey offshore Mount Etna (Del Negro and Napoli,2002), where the Chiancone unit has been inferred to be extensively

mpositeNNE

5 km

high Riposto ridge(folded strata)

?

thin sedimentary cover

he region marked as structural high displays a thin stratigraphic cover above a poorly. Stratigraphic relationships on the southern side suggest that the sedimentary stratad. Location is in Fig. 3.

2.5 km

W E

unfaulted sedimentarysuccession

“older” tectonics

slide

Timpe F. southernprolongation

Line 16bis

T.W

.T. (

s)

0

1

2

3

Fig. 11. Line 16bis. A thick and undeformed sedimentary package (whole of Quaternary and possibly older) covers pre-existing structures (extensional fault of the Malta Escarpmentsystem) suturing any deformation. If the Timpe Faults were continuing southward they should pass through this seismic profile. Note also the minor gravitational instability affectingslope sediments. Location is in Fig. 3.

59A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

present in the offshore. In this respect, it is interesting to note that asimilar study based on potential methods comes to a different conclu-sion, reducing the offshore extent of the Chiancone unit (La Delfa etal., 2011).

Our results in the offshore region led to reconsider the volume es-timates of the Chiancone unit, that is assumed to be indicative of thevolume of material evacuated from collapse of the Valle del Bove. Al-though mass wasting is a widespread process in the area, the relateddeposits do not record a large unique event (Pareschi et al., 2006).

In this respect it might be interesting to note that late Miocenevolcanics have been reported in the subsurface, south of Catania(Longaretti et al., 1991). If themorphologic high observed in the centralsector offshore represents the northern prolongation of the HybleanPlateau, late Miocene volcanics might be present here as well, and canbe responsible for the observed magnetic signal.

An attempt to frame our observation in a stratigraphic setting canbe made by using the geological constraints. The offshore erosionalplatform can be taken as due to the last sea level lowstand (about

LineSSW

T.W

.T. (

s)

0

1

2

extent of Chia

Line 9w

approximate base of prograding unit

(a)

(b)

Fig. 12. Line S3, NNE–SSW-trending, located just offshore the Chiancone outcrops. The lineerosional platform. Note the upper seismic facies (a) that presents reflections with highelow amplitude, low frequency and poor continuity reflections. The same seismic facies are pintersection is indicated in figure). Location is in Fig. 3.

20 ka). The Chiancone units was deposited following the Valle delBove collapse (loosely comprised between15 ka, age of thefinal activityof Ellittico volcano (Branca, 2003; Del Carlo et al., 2004), and ca. 8 ka,age of the upper part of the Chiancone deposits), likely by fluvialreworking of debris avalanches, as suggested by the facies of the de-posits (Calvari and Groppelli, 1996; Calvari et al., 1998). The progradingunit terminates toward the coast, against the erosional surface whichrepresents the base of Chiancone, and can be interpreted as depositedduring the last sea level fall. In this scheme, therefore, the Chianconecan be interpreted as a retrogressive deposit during the last sea levelrise (Fig. 14).

4.5. Sequence of deformational events

Direct dates of sediments and rocks are lacking in the area, and theage of the sedimentary units identified on seismic profiles is oftenonly loosely constrained by regional correlation.

S3 NNE

2.5 km

erosional surface

ncone onshore

is located above the structural high if the central sector and shows a well developedr frequency greater continuity, with respect to the facies below (b) characterized byresent in Line 9W, that is perpendicular but terminates ca. 1 mile to the east (projected

Line 9WW E

T.W

.T. (

s)

2.5 km2

1

0

prograding unit

multiple

base prograding unit

eroded sea floor(a)

(b)

Fig. 13. Line9W, E–W-trending, showing a prograding unit located just offshore the erosional platformpresent in Line S3, and thereforephysically disconnected by theChiancone unit. Notethat the prograding unit presents a seismic facies characterized by high frequency, high amplitude and good continuity reflections (a), resting above the seismic facies with low frequency,low amplitude and poor continuity reflections (b), typical of the structural high. Sedimentary strata heavily eroded at the sea floor are also imaged toward the slope. Location is in Fig. 3.

60 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

Nevertheless, from the analysis of the whole seismic data set, somekey events can be identified and correlated across the study area, allow-ing a sequence of deformational events to be worked out (Fig. 15).

Intense erosion represents a remarkable morphological feature ofthe region offshore of Mount Etna, and is characterized by severalincised canyons and slide scars (see also Chiocci et al., 2011). Recentassessment of Holocene uplifted coastlines in the Peloritani andSouthern Calabria indicates an increase, of about 0.5 mm/yr, in theuplift rates since late Holocene (ca. 6 ka) (Antonioli et al., 2009).Seismic data show that the remarkable erosion has been a fairly re-cent event (e.g., Fig. 8), and we take the intense erosion as relatedto the late Holocene increase in regional uplift. It is worth notingthat the intense erosion and mass wasting operating along the sub-marine slope have masked the morphologic expression of deeperstructures, making it difficult to detect them (e.g., Chiocci et al.,2011).

The geological evolution of Mount Etna suggests that the initiationof the Pernicana Fault is not older than 15 ka, the age of the presentday volcanic edifice (Mongibello) that grows above a previous edifice(Ellittico) partly dismantled by the Valle del Bove collapse (Branca etal., 2004, 2008).

The erosional platform that is present, at about 100 m waterdepth, near the coast adjacent to Mount Etna (Fig. 12) can be relatedto the last sea level lowstand, dated at ca. 20 ka. This erosional surface

Chianconefluvial reworkingof volcanic debris

(<8 ka)

erodelast Sea-Le

(c

NW

time

relativeSea Level

Valle del Bovecollapse

(ca. 15 ka)

?

Fig. 14. Scheme of the possible stratigraphic relationships between the Chiancone unit and this capped by an erosional surface and can be interpreted as deposited during or before the lathe collapse of the Valle del Bove, likely marking a backstepping of the fluvial system durin

can be followed from offshore of the Chiancone northward, to theRiposto Ridge area.

The onset of Mount Etna magmatism, leading to the intrusion ofthe large magmatic body, started at about 120 ka (Branca et al.,2004), and we infer that the intrusion-related deformation could beresponsible for the initiation of shortening at the bulge frontal thrust.In fact, this frontal thrust is kind of outlining the large-scale intrusionlocated below Mount Etna (Fig. 2).

The main structural features which have been identified offshoreof Mount Etna are summarized in Fig. 5. The thrust fault defining theboundary of the offshore bulge merges to the north into a region ofca. NE-trending folds, at the prolongation of the Pernicana fault.The morphologic high is composed by a northern part, where athick package of sedimentary strata has been thrust and folded (seeFigs. 8 and 10), and by a less deformed southern part, which is inter-preted as the northern prolongation of the Hyblean Plateau (Fig. 10).Folding in the northern part promoted the gravitational instabilitythat originated the large-scale submarine landslide (Figs. 5 and 9).As previously mentioned, the onset of thrusting preceded the activi-ty of the Pernicana fault, and was therefore occurring before themain building stage of the present volcanic edifice.

Pleistocene NNE-trending thrust faults are also present in thisnorthern area (Fig. 4). The strata involved in this deformation havebeen subsequently affected by the previously described folding.

d platformvel lowstand (?)

a. 20 ka)

prograding sediment apron

SE

Present Sea Level

e offshore prograding apron, as inferred frommarine seismic data. The prograding unitst sea level lowstand (ca. 20 ka), whereas the deposition of the Chiancone unit followedg sea level rise.

increased erosion

6 ka

8 ka

15 ka

20 ka

120 ka

130 ka

220 ka

1.8 Ma

Present

Key Events Southern Central Northern

Chianconedeposits

regional upliftincrease

Valle del Bovecollapse

SL low stand

onset of Etnamagmatism

(central edifice)

Timpe magmatism

base Pleistocene

intr

uded

bod

y

fron

tal t

hrus

t

Tim

pe fa

ultin

g

progradingunit

thick sedimentaryunit offshore Catania

Cataniacanyon P

erni

cana

Fau

lt

“older” thrusting

large-scaleslide

Riposto Ridgefolding

thick sedimentary

unit(Riposto Ridge)

?

ext.

faul

ts

Fiumefreddocanyon

erosion

Fig. 15. Scheme illustrating the main events that can be partly correlated over the sectors of the study area, and their approximate age. Note that the ages in the vertical axes are notto scale. The time interval covering the evolution of Etna volcanic center is in dotted pattern, whereas the time interval of increased slope erosion is marked in gray. The age of thesedimentary successions and of the “older” thrusting is estimated, as described in the text. The thrust bounding the offshore bulge is referred to as Frontal Thrust.

61A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

These thrust faults are therefore the oldest features recognized in thearea; they are interpreted as the northern prolongation of the Gelathrust front, that likely continues underneath the Etna edifice (Fig. 1).If this interpretation is correct, thrusting can be as old asmiddle Pleisto-cene (Argnani, 1987; Patacca and Scandone, 2004).

4.6. Flank instability: Comments on triggering mechanisms

Three actors are playing a role in the deformation that affected theeastern flank of Mount Etna: i) tectonics, as Mount Etna is located in aposition where several tectonic structures are currently active, ii) grav-itational instability, promoted by the Ionian depressed topography, andiii) magmatic intrusion, which gave rise to the volcanic edifice (Fig. 1).

Some of the tectonic features observed in the study area formed be-fore the onset of themagmatic system that led to the Etna central edifice(ca. 120 ka), and can be attributed to the regional tectonics. In the off-shore area, the shallow thrust faults observed in the western partof the Riposto Ridge are an example. The Timpe faults, onshore, alsoformed before the development of the Etna edifice, as testified by theassociated magmatic activity (220–130 ka; e.g., Branca et al., 2008).The link with magmatism and the current seismicity, recorded downto 10 km or more, indicates that the Timpe faults are crustal-scale tec-tonic features. Altogether, the contribution of these tectonic structuresin shaping the bulge offshore of Mount Etna appears rather limited.

Gravity played an important role, particularly in the late stages ofevolution, but it was not enough by itself to drive the whole of thedeformation. It certainly promoted the marked asymmetry that causedthe preferential dismantling of the eastern flank of the volcano, but itseems that gravity was superimposed to tectonic and volcano/tectonicprocesses.

It has been recently proposed that the gravitational instability ofthe eastern flank of Mount Etna, that is facing the deep Ionian basin,is the main controlling factor for the volcano evolution (Chiocci et al.,

2011). It has been inferred that instability was triggered along themar-gin because abundantmagma intrusion (starting at ca.120 ka) caused alateral bulging toward the deep Ionian basin. Large-scale mass wastingoriginated from the base of the unstable slope and propagated upslope,to the present configuration, with faults in a semicircular pattern thatdissect the coastal area. The Timpe fault system is considered as partof this large-scale retrogressive sliding. In spite of the relatively superfi-cial nature of the sliding, two WNW–ESE crustal lineaments, boundingthe bulge to the north (Fiumefreddo lineament) and to the south (CataniaCanyon lineament), are thought to be critical in assisting the large-scale masswasting. A relationship certainly exists between the offshorebulge andMount Etna; nevertheless, some of the inferences that Chiocciet al. (2011) havemade are not supported by our data. For instance, theRiposto Ridge has been considered as made up by relatively strong tec-tonic units (Chiocci et al., 2011); however our seismic profiles (Fig. 10)show that it is composed by folded strata deposited on the margin,and it is actually weaker than the structural high located to the south.Although extensional faults affect the uppermost sediments, they ap-pear to be related to the combined contribution of folding and gravity.Extensional faults, therefore, are mostly superficial features, and arenot reflecting regional extension; in fact, folds and thrust faults are ob-served in several localitieswithin the bulge. In the same line, the easternslope is affected by intense mass wasting, but this process is the mostrecent operating in the area, being preceded, and sometimes promoted,by tectonic deformation. The northern branch of the semicircular pat-tern of faults is partly visible near the Riposto Ridge, but it is likely over-emphasized in the southern branch, where faults are mainly visiblenear the coast and may be interpreted as the southern extent of theTimpe fault system. The magmatism and relatively deep seismicity as-sociated to the Timpe faults suggest an origin different fromgravitation-al sliding, and their age (onset at ca. 220 ka) contrasts with the upslopeprogression of the sliding. Finally, the Catania Canyon and the Fiume-freddo lineaments are only morphologic features that have no

62 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

expression at depth. The tectonic structures that bound the offshorebulge (Fig. 5) are not crustal features that extend further east; theyonly contributed to the development of the bulge.

Magmatism is a specific feature of the Etnean area, which is super-imposed to the complex tectonic setting of the region. Seismic tomog-raphy has revealed the occurrence of a large intrusive body belowthe volcanic edifice. The estimated volume of the upper part of the in-truded body, between 3 and 9 km, is over 650 km3. It has often beenassumed that the intrusion of the large magmatic body is responsiblefor the origin of the offshore bulge, irrespective of the deformationmechanism (cfr. Borgia et al., 1992; Chiocci et al., 2011). We also be-lieve that the deep magmatic intrusion can account for part of theobserved large-scale deformation, with the topographicaly depressedIonian area being responsible for the eastern structural asymmetry.From the interpretation of our data we propose that the frontalthrusting followed the large intrusion below Mount Etna, but its in-ception preceded the development of the Pernicana Fault, which de-veloped much later (Fig. 15).

We consider the sliding of the eastern flank, that is currently mea-sured by GPS velocities (e.g., Bonforte et al., 2011), as a minor contrib-utor to the observed shortening at the front of the offshore bulge.

On the other hand, shallow magma emplacement within the vol-canic edifice can amplify the deformation near the central craters,and can trigger favorably oriented faults, such as the Pernicana Faultand the north-eastern Rift (e.g., Currenti et al., 2008). The instabilityof the eastern flank of Mount Etna has likely affected the magmaticevolution of the upper part of the volcano, and it seems that the inter-play between magmatic intrusion/eruption, and the sliding of theeastern flank, guided by the Pernicana Fault, characterized the mostrecent history (last 15 ka) of the volcanic edifice.

5. Conclusions

Seismic data show a remarkable degree of structural complexity off-shore Mount Etna (Fig. 5). The Pernicana Fault, for instance, is not con-tinuing offshore as a sharp feature; rather, the deformation is expressedas a set of ENE–WSW folds located very close to the coastline. Seismicdata indicate that these tectonic structures have affected the offshoreof Mount Etna before the Pernicana Fault system was developed, lessthan 15 ka ago. The southern guideline of the collapsing eastern flankof the volcano is poorly expressed onshore, and does not show up off-shore; in fact, seismic data indicate that the Catania canyon, a remark-able E–W-trending feature, does not reflect a tectonic control. In asimilar way, no structural guideline has been observed offshore, at theprolongation of the Pernicana Fault. Moreover, the Timpe Fault systemdoes not continue toward the Malta Escarpment.

Seismic interpretation also shows the occurrence of a structuralhigh located just offshore the edifice of Mount Etna. Whereas a com-plex deformation affects the boundary of this tectonic element, it showsonly limited internal deformation. Preliminary results led to interpretthis structural high as the northernmost extent of the Hyblean foreland.The role of this element in the volcano-tectonic evolution of the regionappears rather complex and requires further investigations.

We consider that part of the topography of the offshore bulge pre-existed the constructional phase of Mount Etna, being an extensionof the Hyblean Plateau. Only in the northern part, the bulge is a recenttectonic feature, being composed by Plio-Quaternary strata that werefolded before and during the building of Mount Etna. The offshorebulge is bounded by a thrust fault that can be related to the intrusionof the large-scale magmatic body below Mount Etna.

Borgia et al. (1992) and followers present a model which links thecurrently observed sliding of the eastern flank of the volcano to the off-shore thrust faults. Our seismic data indicate that the sliding of the east-ern flank is younger than the frontal thrusting initiation, and itscontribution to overall shortening is only minor. Therefore, the driving

mechanism for the origin of the offshore bulge is the magmatic intru-sion, rather than the gravitational collapse of the volcanic edifice.

The interpretation of seismic data indicate that part of the offshorebulge pre-existed the construction of Mount Etna, unlike what wasproposed by Chiocci et al. (2011); part of the offshore bulge wasbuild up by thrusting, rather than simply being an inflation responseto magmatic intrusion. Moreover, we did not find any evidence forthe occurrence of crustal-scale E–W-trending faults bounding thebulge on its northern and southern sides. The extensional faults ob-served in the Riposto Ridge result from a combination of large-scalefolding and downslope gravity. Although gravity contributed signifi-cantly to the recent dismantling of the offshore slope of Mount Etna,there is more than just gravity in shaping the offshore bulge.

Acknowledgments

The research activity was framed in and funded by the INGV-DPC2007–2009 project “FLANK” and greatly benefitted from discussionwith the colleagues participating in the project, and particularly withthe project leaders G. Puglisi and V. Acocella. G. Groppelli and an anon-ymous reviewer are kindly acknowledged for their helpful commentsand suggestions.

Appendix A. Details of seismic acquisition and processing

A1. Acquisition

The TAORMINA multichannel seismic survey, carried out inSeptember 2006 onboard of the R/V Urania of the National Councilof Researches (CNR), has led to the acquisition of ca. 700 km of seis-mic profiles. Together with seismic data, Sub Bottom Profiles wereacquired by a 16 transducers hull-mounted DATASONICS CHIRP-IIprofiler, with operating frequencies ranging between 2 and 7 kHz,in order to investigate the near bottom sediments.

The seismic survey has been carried out with two different sys-tems, according to the operation conditions encountered during thesurvey:

1) 48-channel Teledyne seismic streamer with 600 m of active sec-tion and 12.5 m group interval. The streamer was kept at an oper-ation depth of ca. 10 m. Shot interval was 18.75 m to get a 16-foldcoverage. A seismograph StrataVisor (Geometrics) was used foracquisition, with 1 ms sampling and 5 s of record length.

2) 24-channel Teledyne seismic streamer with 120 m of active sectionand 5 mgroup intervalwas used in the narrower part of theMessinaStraits. The streamer was kept at an operation depth of ca. 1 m. Shotinterval varied between 12.5 and 20 m to give a coverage from 3 to4.8. A seismograph DAQ Link II (Seismic Source) was used for acqui-sition, with 1 to 2 ms sampling and 3 to 4 s record length. In bothcases the energy source was a G.I. Gun Sodera in Harmonic mode(105+105 ci), fed by a 3500 l electrically-driven compressorBauer, and operating with a pressure of 140 bars. The Sure Shot sys-tem (Real Time Micro Systems) was used to control the shots. Thedepth of the energy source was ca. 8 m.

The seismic survey Malta Escarpment (MESC) 2001 was carried outin July–August 2001, on board the R/V Urania and has led to the acqui-sition of 2500 km of seismic profiles. Data acquisition has been carriedout using a 48-channel Teledyne streamer and a Sodera generator-injector gun in harmonic mode (105+105 in3). The group interval ofthe acquisition streamer was of 12.5 m for a total active length of600 m. Shot interval was 50 m giving a coverage of 600%. Seismic datawere collected by a Teledyne 48-channel streamer and digitized andrecorded by a Geometric's Stratavisor seismograph, with samplingrate of 1 ms and record length varying from 8 to 12 s. The streamerwas kept at an operation depth of ca. 10 m, whereas the depth of theenergy source was about 6 m.

63A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

In May 2005, about 480 km of high resolution MCS profiles werecollected offshore Catania, in front of Mount Etna, by FUGRO OCEAN-SISMICA S.p.A., operating on the M/V Pehlivan II. The survey was car-ried out for INGV using a 1200 m long Litton Industries quick couplerstreamer, with 96 traces and a group interval of 12.5 m. The streamerwas connected to a TTS-II digital data recording system with a sam-pling interval of 1 ms and a record length of 3 s. The seismic sourcewas an airgun array of total size of 120 in3 operated at a pressure of2000 psi. The shooting interval was 12.5 m. The source and streamerwere towed approximately 2.5 m below sea level.

A2. Processing

The MESC-2001 and TAORMINA 2006 seismic data have been pro-cessed using the software Disco/Focus by Paradigm, following a standardsequence (Yilmaz, 1987) up to timemigration. Themain processing stepsare:

1) Resampling every 2 ms of the original record,2) Spherical divergence gain to recover signal amplitude,3) Editing and CDP sorting,4) Velocity analysis every 200 CDP,5) Normal Move Out correction,6) Stack,7) Muting to remove noise in the water column, and8) Finite-Difference Time Migration.

The processing of ca. 360 km of INGV seismic lines was completedusing the Fugro Seismic Imaging Uniseis package. The major steps ofthe processing sequence were: trace editing, gain recovery, sortingin the cmp domain, velocity analysis, pre-stack time migration,NMO correction and cmp stacking. Muting in the CDP domain wasdone to remove NMO stretch. The mute was applied with 2 tracesremaining at the seabed for shallow areas (WBb1000 ms) or 4 tracesremaining for deeper areas. Full-fold data (no traces muted) was usedafter WB+800 ms. A post-stack time variant filter was applied nearthe end of the processing sequence.

Appendix B. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.jvolgeores.2012.04.016.

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