The Deformation Offshore of Mount Etna As Imaged by Multichannel Seismic Reﬂection Proﬁles

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

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Journal of Volcanology and Geothermal Research

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The deformation offshore of Mount Etna as imaged by multichannel seismic reﬂection proﬁles

A. Argnani a,⁎, F. Mazzarini b, C. Bonazzi a, M. Bisson b, I. Isola b a CNR, ISMAR-Bologna, Bologna, Italy b Istituto Nazionale di Geoﬁsica e Vulcanologia, Pisa, Italy article info abstract

Article history: Despite the clear evidence of active flank dynamics that is affecting the eastern side of Mount Etna, the con- Received 5 May 2011 tribution of tectonic processes has not been yet understood. So far, the various models proposed to explain Accepted 10 April 2012 the observed flank deformation have been based on onshore structural data, coming from the volcanic edi- Available online 20 April 2012 fice. The Ionian offshore of Mount Etna has been only recently investigated using multichannel seismic profiles, and offers the opportunity to image the structural features of the substrate of the unstable flank of Keywords: the volcano. This contribution aims at describing the deformation located offshore Mount Etna using multi- Mount Etna offshore fi fl Volcano flank instability channel seismic pro les recently acquired during three seismic surveys. The onshore ank deformation of Active tectonics Mount Etna appears to be laterally confined by two tectonic guidelines, trending roughly E–W, located to Multichannel reflection seismics the north and south of the deforming flank; the northern guideline, in particular, takes the surface expression Intrusive body of a sharp fault (Pernicana Fault). Though often assumed that these boundary structures continue offshore as linear features, connected to a frontal thrust ramp, the occurrence of this simple offshore structural system has not been imaged. In fact, seismic data show a remarkable degree of structural complexity offshore Mount Etna. The Pernicana Fault, for instance, is not continuing offshore as a sharp feature; rather, the deformation is expressed as ENE–WSW folds located very close to the coastline. It is possible that these tectonic structures 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 expressed onshore, and does not show up offshore; in fact, seismic data indicate that the Catania canyon, a remarkable E–W-trending feature, does not reflect a tectonic control. Seismic interpretation also shows the occurrence of a structural high located just offshore the edifice of Mount Etna. Whereas a complex deformation affects the boundary of this offshore bulge, it shows only limited internal deformation. Part of the topography of the offshore bulge pre-existed the constructional phase of Mount Etna, being an extension of the Hyblean Plateau. Only in the northern part, the bulge is a recent tectonic feature, being composed by Plio-Quaternary strata that were folded before and during the building of Mount Etna. The offshore bulge is bounded by a thrust fault 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 evidence of active flank dynamics that is affecting the eastern side of Mount Etna, the contribution of tectonic processes has not been yet un- Mount Etna is a large volcano that originated in its present form ca. derstood. So far, the different models proposed to explain the observed 15 kyr ago, following a long magmatic history that began ~500 kyr ago flank deformation have been based on onshore structural data, coming in an area of complex tectonics (Fig. 1; Branca and Del Carlo, 2004; from the volcanic edifice (Fig. 2). The flank deformation of Mount Etna Branca et al., 2004, 2008). A large set of geodetic and interferometric appears to be a complex system of tectonic blocks that are laterally data collected in the last two decades shows that the eastern flank confined by two main tectonic guidelines, trending roughly E–Win of Mount Etna is slowly moving eastward (e.g., Froger et al., 2001; the north and roughly WNW–ESE in the south of the deforming flank Puglisi and Bonforte, 2004; Bonforte et al., 2011), supporting longer- (e.g., Rust et al., 2005); the northern guideline, in particular, takes the term geological evidence (e.g., Borgia et al., 1992). Despite the clear surface expression of a sharp fault (Pernicana Fault). Though often assumed that these boundary structures continue offshore as linear features, connected to a frontal thrust ramp (i.e., Borgia et al., 1992, 2000a), the occurrence of this simple offshore structural system has ⁎ Corresponding author at: ISMAR-CNR, Via Gobetti 101, 40129 Bologna, Italy. Tel.: +39 0516398940; fax: +39 0516398940. not yet been proven. The Ionian offshore of Mount Etna has been only E-mail address: [email protected] (A. Argnani). recently 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 Mount Etna. 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 maximum 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 extensional and thrust symbols are present along the same line. Small arrows near a fault line indicate a strike–slip component (after Argnani, 2009). the opportunity to image the structural features of the substrate of the 2. Geological setting unstable ﬂank of the volcano. This contribution aims at describing the deformation located offshore Mount Etna using recently acquired mul- Mount Etna is the largest subaerial volcano in Europe and is locat- tichannel seismic proﬁles (Fig. 2). ed in an area of great tectonic complexity (Fig. 1; Hirn et al., 1997; 52 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

Pernicana F. N NE Rift 5 km

VdB RR

Ragalna F. Mascalucia-Tremestieri- Timpe F. Trecastagni F. ? CC

Malta Escarpment thurst fault trend extensional Catania anticline fault + fold axis

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 white dotted 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 (see regional 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, whereas the 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 indicate the 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 more detailed bathymetry for part of the offshore, is presented in the Supplementary material (Fig. S1, modiﬁed after Chiocci et al., 2011).

Monaco et al., 1997; Argnani, 2000; Nicolich et al., 2000; Doglioni et (Kieffer, 1985; Corsaro et al., 2002; Branca et al., 2004; Branca and Del al., 2001; Argnani, 2009). The volcano is sitting on top of the Ma- Carlo, 2005; Branca et al., 2008). Recent eruptions of Mount Etna are ghrebian fold-and-thrust belt, that runs W–E through Sicily, and is characterized by summit eruptions at the central craters, and by fis- located on the northern prolongation of the NNW-trending Malta sure eruptions and dike intrusions at the rift zones oriented NE, N–S Escarpment, a major morphological feature of the central Mediterra- and E–W. Several eruptions have occurred from both summit and nean, part of which has been tectonically reactivated in Quaternary flank vents in the last century (Romano et al., 1979; Behncke and time (Argnani and Bonazzi, 2005; Argnani, 2009). The Malta Escarp- Neri, 2003; Branca and Del Carlo, 2004). Additional information ment is separating the continental crust domain of the Hyblean region, about historical lava flows on Mount Etna, along with a daily update in SE Sicily, from the oceanic domain of the Ionian basin (e.g., Argnani of Mount Etna activity, is available on the Catania Istituto Nazionale and Bonazzi, 2005 and references therein), a setting that greatly affect- di Geofisica e Vulcanologia Web site (http://www.ct.ingv.it). ed the subduction regime underneath Calabria (e.g., Gvirtzman and Recent geodetic, InSAR and GPS data, clearly show that northern Nur, 1999; Argnani, 2009). Moreover, the Scicli–Ragusa fault system, and western sectors at Mt. Etna are mainly characterized by uplift a large NNE-trending fault zone that bounds to the west the Hyblean and radial pattern of ground deformation and by a widespread and plateau, is also converging toward Mount Etna. This location at a cross- structurally complex seaward deformation accommodated by move- roads of structural trends has been often called upon to explain the ment of different blocks in the eastern and southern flanks of the vol- origin of Mount Etna (e.g., Lo Giudice et al., 1982), although the relation- cano (Lanari et al., 1998; Froger et al., 2001; Lundgren et al., 2004; ships between tectonic processes and geodynamic regime have not Bonforte and Puglisi, 2006; Solaro et al., 2010; Bonforte et al., 2011). been yet fully unraveled (Gvirtzman and Nur, 1999; Doglioni et al., Particularly, geological, geodetic and volcanologic investigations have 2001; Argnani, 2009; Schellart, 2010). identified a short term shallow deformation, mainly linked with vol- Besides the volcanic activity of Mount Etna, the region appears to be canic activity (volcano inflation and emplacement of dikes), and a tectonically highly active: a remarkable late Quaternary uplift affected long term deep seated deformation due to an overall gravity instabil- the adjacent Peloritani and Calabrian domains (Monaco and Tortorici, ity by the volcano load (Neri et al., 2009; Solaro et al., 2010). Inver- 2000; De Guidi et al., 2003), and one of the largest earthquakes in sion of geodetic data shows that volcanic load (pressurized source Italy occurred in the Messina Straits in 1908 (e.g., Pino et al., 2009). at depth) acted at depths variable in the 3–9 km b.s.l. range (Puglisi The geologic history of Mount Etna includes phases of submarine vol- and Bonforte, 2004; Bonforte et al., 2008). A sub-vertical high velocity canism, shield building, formation of nested volcanic centers and calderas, body (HVB) has been imaged by seismic tomography (Fig. 2), with bot- and the final constructional phase of the stratovolcano in the last 15 ka tom at a depth of about 18 km and top at about 3 km (e.g., Hirn et al., A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64 53

1991; Chiarabba et al., 2000; Patanè et al., 2003, 2006). This volume mostly display sinistral strike–slip motion along the fault plane. How- with positive velocity anomaly has been interpreted as a large plutonic ever, although the Pernicana Fault is a sharp feature, the deformation body. Between 3 and 9 km depth the lateral extent of this body in- along the southern boundary of the Etna eastern flank is rather diffuse, creases to about 6 km, and the estimated volume is about 3 times larger as indicated by the field of GPS velocities (Bonforte et al., 2011), and the than the volcano pile, suggesting that its accretion could be responsible displacement appears also of lesser extent. The very limited seismicity for destabilizing the eastern flank (Allard et al., 2006). Then most of associated to this southern fault system further supports the lack of a the magma mass is actually cumulated at the shallower interface of clear boundary. Some remarkable seismicity is instead associated to the upper limit of the HVB and is detected by the ground deformation the Timpe fault system, where earthquakes occur down to 10–15 km, and gravity data analysis (Bonaccorso et al., 2011). with focal mechanisms that show dextral-oblique extensional motion Sector collapses also characterized the evolution of Mt. Etna, as (Allard et al., 2006; Neri et al., 2005a). Magmatic activity is also associ- testified by the Valle del Bove (VDB) scar and by the occurrence of ated with this fault system, and the connection with magmatic outpour Pleistocene to Holocene debris deposits (Chiancone) cropping out could be somewhat puzzling, as the magma-tapping faults should on the eastern flanks of the volcano (Fig. 2). The Holocene Chiancone cut through the decollement surface of the sliding flank of the volcano. deposits form a seaward bulge of the coastline and consist of fluvial It should be noted, however, that the Timpe magmatism was switched deposits resting upon debris avalanche deposits whose top has been off at about 130 ka (De Beni et al., 2005; Branca et al., 2008). dated back to about 8000 yr B.P. (Calvari and Groppelli, 1996; Calvari The huge bulge just offshore of Catania (Fig. 2), has been regarded et al., 1998, 2004). The results of magnetic surveys carried out offshore as a large antiform and thought to be the offshore expression of the of the Chiancone deposits have been interpreted as imaging volcano- seaward volcano's spreading (Borgia et al., 1992, 2000a). Occurrence clastic deposits and alluvium in a stretch a few km wide (Del Negro of widespread landslide deposits offshore of Mount Etna was proven and Napoli, 2002); this volcanic-derived deposit has been attributed by recent multichannel seismic surveys (Argnani and Bonazzi, 2005; to the Chiancone, and it would increase the estimate of the volume Pareschi et al., 2006), although major contractional structures were of material evacuated during the Valle del Bove collapse. not documented, so far. Following the observation that the eastern flank of Mount Etna The offshore prolongations of the Pernicana Fault and of the southern is moving eastward, several attempts have been made to identify boundary of the Etna sliding flank are represented by the Fiumefreddo a basal sliding plane, using the concepts of volcanic spreading (e.g., Canyon, to the north, and Catania Canyon, to the south, respectively Borgia et al., 1992, 2000b). Several different interpretations of the (Fig. 2). It should be remarked, however, that these are mainly morpho- basal sliding surface have been proposed on the basis of geological logical features and so far no clear tectonic structure has been shown, data. Borgia et al. (1992) proposed a detachment surface, located at although they have been interpreted as marking two regional tectonic about 5–6 km depth (below sea level) and slightly dipping westward, lineaments that accommodate the basinward movements of both the which is rooted in the plutonic complex below the volcano summit. submarine and subaerial flank of the volcano (Chiocci et al., 2011). The sub-Etnean clays have been taken as representing the detachment surface. A similar solution was subsequently adopted by several authors 3. Seismic data (Tibaldi and Groppelli, 2002; Neri et al., 2004; Rust et al., 2005). A west dipping surface located at 0–2 km a.s.l., which therefore does not extend The data from three seismic surveys have been used to map the offshore, has been also proposed (Lo Giudice and Rasà, 1992; Bousquet geology of the marine area offshore of Mount Etna (Fig. 3). and Lanzafame, 2001, 2004). Moreover, in an early work (Kieffer, 1983) The MESC 2001 and TAORMINA 2006 surveys were carried out an offshore subhorizontal detachment was linked to the extension oc- in order to investigate the regional geology and active tectonics of curring in the Timpe fault system, and was not directly related to the the eastern Sicily slope and of the Messinian Straits (Argnani et al., flank dynamics. These different interpretations show that the geometry 2002, 2008, 2009a; Argnani and Bonazzi, 2005). of the detachment plane lacks of solid constraints, despite its geological The INGV survey (Pareschi et al., 2006) was aimed at investigating appeal. More recently, a 12° east-dipping plane was defined by invert- the marine flank of Mount Etna, and seismic acquisition was planned ing GPS velocities (Bonforte and Puglisi, 2006; Bonforte et al., 2008; for a higher resolution, with respect to the other two surveys. The de- Puglisi et al., 2008); this plane would reach the coastline at a depth of tails of seismic acquisition and processing are given in Appendix A. ca. 4 km below sea level. With a similar approach, Bonaccorso et al. (2011) proposed the occurrence of an eastward dipping slide surface lo- 4. Interpretation and discussion 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 Pernicana Seismic profiles have been interpreted by applying the procedures Fault adjacent to the NE Rift has been constructed by applying a rollover of seismic stratigraphy (Payton, 1977) and the technique for identify- construction to (Ruch et al., 2010). However, the constraints on the ge- ing structural styles (Bally, 1983). ometry of the fault plane are rather loose, and the reconstructed fault For sake of clarity the interpretation of seismic profilesispresented plane flattens to a depth of ca. 2 km b.s.l. over a short distance (few by subdividing the study area in three sectors: northern, central, and kms), resulting shallower than the GPS-derived sliding plane. southern. The northern sector straddles the prolongation of the Perni- Overall, flank instability is widespread at Mt. Etna, regardless of cana Fault, the central and the southern sectors covers the frontal part volcanic activity, and remains by far the predominant type of deforma- of the bulge located offshore Etna, and its southern boundary, respective- tion on the eastern flank of the volcano. Based on geological and geodet- ly. The seismic profiles described in this section are located in Fig. 3. ic data, the main bounding structures of the unstable eastern flank are the E–W trending left-lateral strike–slip Pernicana Fault, to the north, 4.1. Northern sector with an average slip rate of 10–20 mm/yr, and the Trecastagni Mascalu- cia fault system, to the south, with average slip rate in the order of Seismic profiles show evidence of contractional structures in the 1–10 mm/yr (Fig. 2; Tibaldi and Groppelli, 2002; Acocella et al., 2003; region offshore northern Etna. In some instance, small-scale thrust Neri et al., 2004; Acocella and Neri, 2005; Rust et al., 2005). sheets are imaged as detaching at rather shallow level (Fig. 4). The Seismicity in the eastern flank is rather intense and associated seismic facies in the lower part of the line shows low frequency, var- with the main structural features (e.g., Azzaro et al., 2008). The Perni- iable amplitude and poor continuity reflections, recalling the seismic cana Fault is characterized by intense very shallow (b3 km from sur- facies of the Hyblean domain (see Central sector section, below). On face) seismicity, especially in the western sector of the fault (e.g., the other hand, the upper seismic facies, with high frequency, good Alparone et al., 2011a, 2011b-this issue), and focal mechanisms continuity reflections, resembles the response of the Plio-Quaternary 54 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

15˚10' 15˚20' 15˚30' 15˚40' 38˚00' 38˚00' MCS Surveys Calabria

TAORMINA 2006 MESC 2001 INGV 2005

37˚50' Tao16 N 37˚50' 10 km Tao14 17 2 3a S3

37˚40' 37˚40' ETNA 9W

37˚30' 16bis 37˚30'

composite

15˚10' 15˚20' 15˚30' 15˚40'

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 location of seismic profiles shown in this paper is indicated with a bold black line. succession (e.g., Argnani and Bonazzi, 2005). The seismic data show that a cluster of small NNE–SSW scarps, between 80 and 110 m water that the strata describing these contractional structures are truncated depth, are present on the prolongation of the Pernicana Fault (Chiocci by an erosional surface. It appears that the erosional surface has been et al., 2011). How the Pernicana Fault continues offshore is a critical ques- subsequently folded with a wavelength larger than that of the thrust tion. The interpretation of seismic profiles shows that the Pernicana Fault sheets. More than one episode of contraction is therefore recorded in does not continue offshore as a sub-vertical strike–slipfaultwithanE–W the area. Considering the thickness of the folded Plio-Quaternary suc- trend (Figs. 6 and 7). Therefore, models of distributed brittle deformation cession (over 400 m), and comparing it to the thickness in the adjacent above a decoupling layer given by Etnean clays (Groppelli and Tibaldi, areas, the older thrust faulting can have occurred sometimes within 1999) seem not likely. Rather, it seems that two broad anticline and syn- the Quaternary. The thrust faults trend NNE–SSW, but detailed mapping cline, trending ENE–WSW and at least 8 km long, interrupts the trend of of their lateral extent is inhibited by their small scale and by the great the Pernicana Fault (Fig. 5). complexity of the area (Fig. 5). It seems, however, that the easternmost extent of the Pernicana The erosional surface that cut the thrust and folded strata is in turn Fault onshore is more similar to what observed in the offshore (small folded with a much larger wavelength. The effect of this later large- NE-trending extensional faults), and one may wonder whether the scale folding on sea-floor topography can be appreciated on a ca. N–S large folds mapped offshore, that underline the superficial extensional profile (Fig. 6) where folded strata are well imaged. Fold axes trend faults, can continue inland, underneath the coastal plain. The change ENE–WSW (Fig. 5). Interestingly, minor extensional faults with roughly from a clear strike–slip Pernicana Fault and a different kind of deforma- the same trend and dipping downslope occur in this area; they are likely tion to the east could, therefore, be located onshore. The coastline, as driven by a combination of folding and gravity. Small extensional faults it is often the case, is just an ephemeral and apparent boundary. with the same trend have been imaged on multibeam bathymetry, very The left-lateral Pernicana Fault accommodates the sliding of the close to the coast (see also Chiocci et al., 2011). Onshore prolongations eastern flank of Mount Etna; therefore, the displacement along the of these features might be responsible for historical episodes of mud fault is either absorbed as contractional deformation (as previously sug- blow out in the coastal lagoons, known as La Gurna mud volcano gested, e.g., by Borgia et al., 1992), or is linked to a broader extensional (Fig. 5; Carveni et al., 2006). In this respect, it is interesting to observe system that marks the large-scale collapse of the volcanic edifice and that the Pernicana Fault loses its continuity toward the coastline, and its substrate toward the depressed region of the Ionian Sea. As the is present with short NE-trending segments. Here the faults have almost Pernicana Fault appears kinematically linked to the Rift of NE, its activity normal component, and displacement is much reduced; the morpho- is likely comprised within the last 20 ka (last phases of Ellittico volcano logical expression of the faults and the associated seismicity are also re- and inception of Mongibello activity; Branca, 2003; Del Carlo et al., duced (Groppelli and Tibaldi, 1999; Acocella and Neri, 2005). High 2004). Moreover, with the estimated (maximum) current slip rate of resolution multibeam bathymetry, very close to the coast, also shows 15–20 mm/yr along the Pernicana Fault, the total amount of displacement A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64 55 line 3a WSW ENE 0.00 erosion at sea floor folded erosional surface intense mass-wasting along the slope (a) 0.50

1.00 T.W.T. (s)

(b) 1.50

2.5 km 2.00

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 thrust sheets 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 surface that 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. in the last 20 kyr, 300–400 m, is much less than what observed at the suggesting that the age of the submarine slide can be taken as older than thrust front, as described below. any known historical tsunamis (Argnani et al., 2009b). The ENE–WSW-trending system of folds affects the sea floor mor- Moreover, the high resolution seismic profiles and morpho- phology, suggesting a recent activity, perhaps coeval with the activity bathymetric data on the northern part of our study area support pre- of the Pernicana Fault. The ENE–WSW fold trend, however, is at odds vious arguments, based on geophysical data (Argnani et al., 2009b), and with a left lateral strike–slip along the Pernicana Fault, undermining allow to rule out the occurrence of a recent large-scale landslide located the simple view of a collapsing volcanic flank where two lateral strike– offshore Giardini–Naxos (Fig. 5). This observation undermines the slip faults are linked to a frontal thrust (i.e., Borgia et al., 1992). On the proposal of Billi et al. (2008) that the 1908 tsunami was originated by other hand, it is possible that complexity related to regional active tec- aover20km3 large landslide in this sector of the eastern Sicily slope. tonics played a role (e.g., Argnani, 2009). Finally, it has been recently proposed that the Etnean volcanism The thrust fault at the front of the structural high is well imaged by extends eastward, for about 20 km into the Ionian Sea (Patanè et al., seismic data (Fig. 8). The thrust plane cuts through the stratal surface 2009). The authors further hypothesize that the morphology of the which represents the base of a large-scale landslide and reaches the volcanic apparatus is not directly observed because it is buried sea floor. Although the sea floor shows evidence of submarine erosion under the debris deposits of the Chiancone unit. The supposed volca- and mass wasting at the location where it is intersected by the thrust nic apparatus is located in the Riposto Ridge, a marked east–west- plane, it is likely that shortening is currently occurring. However, trending features that bounds to the north the offshore bulge. This the observed displacement along the thrust is larger than 1 km, and proposal, however, does not find support in our data, which show therefore much larger than the maximum horizontal displacement that the Riposto Ridge is composed by folded sedimentary strata, in accrued along the Pernicana Fault since its onset (300–400 m). More- the seismic facies of which there is no evidence of volcanic material over, it seems that the large scale landslide has been triggered by (Fig. 6). Moreover, as discussed more in details below, the deposits the uplift caused by the initial activity of this thrust. belonging to the Chiancone unit do not extend as far as the locality A remarkable character of this offshore area is the widespread occur- where the volcanic apparatus is supposed to be. rence of erosional features that testify the intense mass wasting along the slope (see also Chiocci et al., 2011). The scars left by mass wasting 4.2. Central sector processes 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 is The area located offshore of central and southern Mount Etna is the large scale slide at the northern boundary of the offshore bulge characterized by a marked topographic bulge that extends ca. 10 km (Fig. 5). The body of such submarine slide is composed by stratified sed- from the coastline. On seismic profiles this region appears as a structural iments that were deposited on the submarine slope (Figs. 8 and 9)and high that displays a thin sedimentary cover, characterized by high fre- that, therefore, cannot be directly related to volcanic collapse. A regional quency, good continuity reflections, above a poorly reflective substrate profile (Fig. 9) shows the broad view of the thrust fault located at the (Fig. 10). On the other hand, well stratified and thicker sedimentary front of the structural high, and illustrates also the large-scale slide packages occur on either side of this high, on a N–S cross section body and the extent of mass-wasting processes which have been active (Fig. 9). Stratigraphic relationships, still partly visible on the southern along the offshore slope. The details of the large submarine slide will be and eastern sides (Figs. 9 and 10), suggest that the sedimentary strata given elsewhere, nevertheless, the slide deposit has been deeply eroded onlapped a pre-existing topographic relief. As the strata span at least on the northern side, where it faces the large Fiumefreddo Canyon the whole of the Quaternary, it is believed that the structural high (Figs. 2 and 10). In fact, the flanks and the head of the wide Fiumefreddo pre-existed the stabilization of the volcano feeder system and the canyon are deeply incised by smaller canyons; the dense network of change from fissural to central eruptions, that led to the formation of gullies and canyons indicates an erosionally mature submarine drainage, Mount Etna (120 ka; Branca et al., 2004). Whereas the strata on the 56 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

15˚10' 15˚20' 15˚30' 15˚40' 38˚00' 38˚00' Calabria Etna offshore simplified structural map −

Taormina flexure 10 km N Riposto Ridge Giardini-Naxos 37˚50' approximate extent of 37˚50' Pernicana Fault large-scale gravity slide

boundary of structural high

trace of 37˚40' erosional scar 37˚40'

Timpe Faults

Catania Canyon

La Gurna mud volcano

37˚30' thurst fault 37˚30'

extensional Malta Escarpment fault tren d fold axis sea-floor lineamen t 15˚10' 15˚20' 15˚30' 15˚40'

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 and folded (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 to folding (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. southern side show little or no deformation, the sedimentary package In the northern part, the sedimentary strata have been folded and to the north has been heavily folded. It is worth noting that although the eastern edge of the structural high appears clearly as thrust fault the folded strata to the north, which include the Riposto Ridge, contrib- (Figs. 6 and 8). The same thrust fault, although less evident, can be ute to the topographic bulge, this area differs substantially from the rest followed further to the south on seismic profiles, till the Catania can- of the topographic high. yon, where it turns from NNE–SSW to ca. E–W(Fig. 5).

line 17 NNW SSE 0.00 minor extensional faults folded strata mass wasting scars

0.50

1.00 T.W.T. (s)

1.50 multiple

2.5 km 2.00

Fig. 6. Seismic proﬁle 17 illustrating the sea-ﬂoor topography created by large-scale folding. Fold axes are shown in the structural map (Fig. 5). Note also the minor extensional faults, likely driven by a combination of folding and gravity. Location is in Fig. 3. A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64 57

prolongation of the Pernicana Fault Tao 16 SSW multiple NNE 0

1 multiple T.W.T. (s) 2 multiple continuous reflection package

3 5 km

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 southern portion of the proﬁle belong to the system of folds of the Riposto Ridge (Fig. 5). Location is in Fig. 3.

The origin of this topographic high, which pre-existed the forma- A physical continuity, however, has never been proven, and previous tion of Mount Etna, is not fully understood. However, subsurface data data suggested that it was not likely (Argnani and Bonazzi, 2005). The in the Catania plain, south of Etna, show that a narrow stripe of units current seismic coverage allows to better define this issue (Fig. 3). belonging to the Hyblean plateau is entering below the eastern part of The east–west-trending seismic profiles south of Mount Etna show the volcano edifice (Longaretti et al., 1991). In fact, Miocene volcanics a thick and undeformed sedimentary package that likely spans the belonging to the Hyblean succession have been found at depth less whole of Quaternary and possibly older sediments. This sedimentary than 700 m near Catania (Fig. 2). It seems, therefore, that the high cor- package covers and sutures pre-existing structures (W-dipping ex- responding to part of the bulge located offshore of Mount Etna is a tensional faults) but it is not affected by faulting (Fig. 11); considering northern stretch of the Hyblean plateau (Fig. 5). that if the Timpe Faults were continuing southward they should pass through this seismic profile, a direct connection between the Timpe 4.3. Southern sector faults and the Malta Escarpment fault system should be rejected. Although, it is possible that the Timpe fault system represents a W- The southern prolongation of the Timpe Faults system has been ward (left) stepping of the Malta Escarpment fault system, it should assumed by several authors (e.g., Monaco and Tortorici, 2000), mak- be noted that the final morphology produced by this fault system ing the Timpe faults as part of the Malta Escarpment fault system. is rather different from the morphology resulted from the Malta

Line 2 WNW ESE 0 collapsed 2.5 km sediments thrust erosion at sea floor 1 slide T.W.T. (s) 2

onlapping strata

Fig. 8. Line 2. Thrust fault at the front of the structural high. The thrust cuts through the surface above which a thick package of sediments (500–600 m) have slid down, originating a large scale-slide. The intense sea ﬂoor erosion is evident on the proﬁle. The minimum thrust displacement (ca. 1 km) is larger than the horizontal displacement possibly accrued along the Pernicana Fault (300–400 m). Note that the basinal strata (right side) are onlapping to the west a pre-existing high. 58 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64 Tao 14 SW NE 0

mass wasted slope Etna landslide

2 TWT (s)

onlapping strata 3 5 km multiple 4

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 along the 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.

Escarpment fault system, as the Timpe fault system does not show a 4.4. The Chiancone unit steep scarp toward an eastern deep water domain. High resolution bathymetric data show that the Timpe faults offset the coastline by An apron of prograding sediments has been observed offshore of the 10–15 m throw in their southern termination (Chiocci et al., 2011). area where Chiancone unit is cropping out, and it may be of interest to A large submarine canyon, the Catania Canyon, represents a major figure out its relationship with the Chiancone deposit (Fig. 2). A N–S morphological feature in this last area (Fig. 5). Because of its E–Wrough- profile close to the coastline shows an erosional platform located just ly linear trend in the 15 km near the coastline, the Catania Canyon has off the Chiancone outcrops (Fig. 12); this erosional surface is covered been often considered as a tectonically-controlled morphological feature. by little or no sediments, without any trace of the chaotic facies that However, our seismic data suggests that no E–W trending struc- can be attributed to the Chiancone deposits. Rather, judging from its lo- ture occur in that area, where, instead, extensional faults with trends cation, this erosional surface might represent the base of Chiancone unit from NW to N have been mapped (Fig. 5). The thrust bounding the cropping out onshore. This widespread erosional surface can be fol- offshore bulge is passing to the north of the Catania Canyon, and the lowed all along the shelf and can be attributed to the last sea level low- canyon is incising a subhorizontal sedimentary succession (Fig. 10). stand (ca. 20 ka). Moreover, the prograding unit imaged by E–W Although a more than 30 km long, WNW–ESE lineament can picked profiles (Fig. 13) is affected by the erosion that originated the subhori- out in detailed bathymetries (Marani et al., 2004; Chiocci et al., 2011; zontal platform, indicating that the prograding unit was older than Fig. 5), seismic data (Argnani and Bonazzi, 2005 andthisstudy)show the Chiancone unit, from which it may be physically detached that the lineament is just a morphological feature that has no relationship (Fig. 14). Although it is possible that the sedimentary wedge located off- with deep structures; in fact, a fault of the Malta Escarpment system is shore has been fed by the dismantling of the Etna eastern flank, there is crossing the lineament, remaining unaffected. This evidence undermines no relationship with an abrupt event such as the collapse of the Valle del the interpretation of Chiocci et al. (2011), that consider the Catania Can- Bove, and deposition appears to be due to regular and continuous sedi- yon lineament as crustal feature bounding the offshore bulge to the mentation. This observation questions some results obtained from ma- south. It seems more likely that the Catania Canyon is controlled by a mor- rine magnetic survey offshore Mount Etna (Del Negro and Napoli, phology that is inherited from previous tectonics (Fig. 10). 2002), where the Chiancone unit has been inferred to be extensively

Line Composite SSW NNE 0 sediments Riposto ridge Catania structural high of the (folded strata) canyon Catania plain thin sedimentary cover

? T.W.T. (s) 2

5 km 3

Fig. 10. Composite NNE–SSW seismic profile crossing the sectors offshore of Mount Etna. The region marked as structural high displays a thin stratigraphic cover above a poorly reflective substrate. Well stratified sedimentary packages occur on either side of the high. Stratigraphic relationships on the southern side suggest that the sedimentary strata onlapped a pre-existing high. The sedimentary package to the north has been heavily folded. Location is in Fig. 3. A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64 59

Timpe F. southern Line 16bis W prolongation E 0 unfaulted sedimentary 2.5 km succession

1 slide T.W.T. (s) 2

“older” tectonics

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 Escarpment system) suturing any deformation. If the Timpe Faults were continuing southward they should pass through this seismic profile. Note also the minor gravitational instability affecting slope sediments. Location is in Fig. 3. present in the offshore. In this respect, it is interesting to note that a 20 ka). The Chiancone units was deposited following the Valle del similar study based on potential methods comes to a different conclu- Bove collapse (loosely comprised between 15 ka, age of the final activity sion, reducing the offshore extent of the Chiancone unit (La Delfa et of Ellittico volcano (Branca, 2003; Del Carlo et al., 2004), and ca. 8 ka, al., 2011). age of the upper part of the Chiancone deposits), likely by fluvial Our results in the offshore region led to reconsider the volume es- reworking of debris avalanches, as suggested by the facies of the de- timates of the Chiancone unit, that is assumed to be indicative of the posits (Calvari and Groppelli, 1996; Calvari et al., 1998). The prograding volume of material evacuated from collapse of the Valle del Bove. Al- unit terminates toward the coast, against the erosional surface which though mass wasting is a widespread process in the area, the related represents the base of Chiancone, and can be interpreted as deposited deposits do not record a large unique event (Pareschi et al., 2006). during the last sea level fall. In this scheme, therefore, the Chiancone In this respect it might be interesting to note that late Miocene can be interpreted as a retrogressive deposit during the last sea level volcanics have been reported in the subsurface, south of Catania rise (Fig. 14). (Longaretti et al., 1991). If the morphologic high observed in the central sector offshore represents the northern prolongation of the Hyblean Plateau, late Miocene volcanics might be present here as well, and can 4.5. Sequence of deformational events be responsible for the observed magnetic signal. An attempt to frame our observation in a stratigraphic setting can Direct dates of sediments and rocks are lacking in the area, and the be made by using the geological constraints. The offshore erosional age of the sedimentary units identified on seismic profiles is often platform can be taken as due to the last sea level lowstand (about only loosely constrained by regional correlation.

Line S3 SSW Line 9w NNE extent of Chiancone onshore 0 erosional surface (a)

(b) 1 T.W.T. (s) approximate base of prograding unit

2.5 km 2

Fig. 12. Line S3, NNE–SSW-trending, located just offshore the Chiancone outcrops. The line is located above the structural high if the central sector and shows a well developed erosional platform. Note the upper seismic facies (a) that presents reflections with higher frequency greater continuity, with respect to the facies below (b) characterized by low amplitude, low frequency and poor continuity reflections. The same seismic facies are present in Line 9W, that is perpendicular but terminates ca. 1 mile to the east (projected intersection is indicated in figure). Location is in Fig. 3. 60 A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64

W Line 9W E 0 base prograding unit prograding unit eroded sea floor (a)

1 T.W.T. (s) multiple (b)

2.5 km 2

Fig. 13. Line 9W, E–W-trending, showing a prograding unit located just offshore the erosional platform present in Line S3, and therefore physically disconnected by the Chiancone unit. Note that 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.

Nevertheless, from the analysis of the whole seismic data set, some can be followed from offshore of the Chiancone northward, to the key events can be identified and correlated across the study area, allow- Riposto Ridge area. ing a sequence of deformational events to be worked out (Fig. 15). The onset of Mount Etna magmatism, leading to the intrusion of Intense erosion represents a remarkable morphological feature of the large magmatic body, started at about 120 ka (Branca et al., the region offshore of Mount Etna, and is characterized by several 2004), and we infer that the intrusion-related deformation could be incised canyons and slide scars (see also Chiocci et al., 2011). Recent responsible for the initiation of shortening at the bulge frontal thrust. assessment of Holocene uplifted coastlines in the Peloritani and In fact, this frontal thrust is kind of outlining the large-scale intrusion Southern Calabria indicates an increase, of about 0.5 mm/yr, in the located below Mount Etna (Fig. 2). upliftratessincelateHolocene(ca.6ka)(Antonioli et al., 2009). The main structural features which have been identified offshore Seismic data show that the remarkable erosion has been a fairly re- of Mount Etna are summarized in Fig. 5. The thrust fault defining the cent event (e.g., Fig. 8), and we take the intense erosion as related boundary of the offshore bulge merges to the north into a region of to the late Holocene increase in regional uplift. It is worth noting ca. NE-trending folds, at the prolongation of the Pernicana fault. that the intense erosion and mass wasting operating along the sub- The morphologic high is composed by a northern part, where a marine slope have masked the morphologic expression of deeper thick package of sedimentary strata has been thrust and folded (see structures, making it difficult to detect them (e.g., Chiocci et al., Figs. 8 and 10), and by a less deformed southern part, which is inter- 2011). preted as the northern prolongation of the Hyblean Plateau (Fig. 10). The geological evolution of Mount Etna suggests that the initiation Folding in the northern part promoted the gravitational instability of the Pernicana Fault is not older than 15 ka, the age of the present that originated the large-scale submarine landslide (Figs. 5 and 9). day volcanic edifice (Mongibello) that grows above a previous edifice As previously mentioned, the onset of thrusting preceded the activi- (Ellittico) partly dismantled by the Valle del Bove collapse (Branca et ty of the Pernicana fault, and was therefore occurring before the al., 2004, 2008). main building stage of the present volcanic edifice. The erosional platform that is present, at about 100 m water Pleistocene NNE-trending thrust faults are also present in this depth, near the coast adjacent to Mount Etna (Fig. 12) can be related northern area (Fig. 4). The strata involved in this deformation have to the last sea level lowstand, dated at ca. 20 ka. This erosional surface been subsequently affected by the previously described folding.

NWChiancone SE fluvial reworking eroded platform of volcanic debris last Sea-Level lowstand (?) (<8 ka) (ca. 20 ka)

Present Sea Level

relative ? prograding sediment apron Sea Level

Valle del Bove collapse (ca. 15 ka) time

Fig. 14. Scheme of the possible stratigraphic relationships between the Chiancone unit and the offshore prograding apron, as inferred from marine seismic data. The prograding unit is capped by an erosional surface and can be interpreted as deposited during or before the last sea level lowstand (ca. 20 ka), whereas the deposition of the Chiancone unit followed the collapse of the Valle del Bove, likely marking a backstepping of the ﬂuvial system during sea level rise. A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64 61

Key Events Southern Central Northern

thick sedimentary 1.8 Ma base Pleistocene unit thick sedimentary (Riposto Ridge) unit offshore Catania “older” thrusting

220 ka

Timpe magmatism

130 ka Timpe faulting

120 ka onset of Etna ? magmatism (central edifice) prograding erosion unit Riposto Ridge 20 ka SL low stand folding intruded body

15 ka large-scale Valle del Bove slide

collapse frontal thrust 8 ka Chiancone deposits ext. faults 6 ka regional uplift increase Catania Fiumefreddo canyon increased erosion Pernicana Fault canyon Present

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 not to 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 the sedimentary 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.

These thrust faults are therefore the oldest features recognized in the 2011). It has been inferred that instability was triggered along the mar- area; they are interpreted as the northern prolongation of the Gela gin because abundant magma intrusion (starting at ca.120 ka) caused a thrust front, that likely continues underneath the Etna edifice (Fig. 1). lateral bulging toward the deep Ionian basin. Large-scale mass wasting If this interpretation is correct, thrusting can be as old as middle Pleisto- originated from the base of the unstable slope and propagated upslope, cene (Argnani, 1987; Patacca and Scandone, 2004). to the present configuration, with faults in a semicircular pattern that dissect the coastal area. The Timpe fault system is considered as part 4.6. Flank instability: Comments on triggering mechanisms of this large-scale retrogressive sliding. In spite of the relatively superficial nature of the sliding, two WNW–ESE crustal lineaments, bounding Three actors are playing a role in the deformation that affected the the bulge to the north (Fiumefreddo lineament) and to the south (Catania eastern flank of Mount Etna: i) tectonics, as Mount Etna is located in a Canyon lineament), are thought to be critical in assisting the large- position where several tectonic structures are currently active, ii) grav- scale mass wasting. A relationship certainly exists between the offshore itational instability, promoted by the Ionian depressed topography, and bulge and Mount Etna; nevertheless, some of the inferences that Chiocci iii) magmatic intrusion, which gave rise to the volcanic edifice (Fig. 1). et al. (2011) have made are not supported by our data. For instance, the Some of the tectonic features observed in the study area formed be- Riposto Ridge has been considered as made up by relatively strong tec- fore the onset of the magmatic system that led to the Etna central edifice tonic units (Chiocci et al., 2011); however our seismic profiles (Fig. 10) (ca. 120 ka), and can be attributed to the regional tectonics. In the off- show that it is composed by folded strata deposited on the margin, shore area, the shallow thrust faults observed in the western part and it is actually weaker than the structural high located to the south. of the Riposto Ridge are an example. The Timpe faults, onshore, also Although extensional faults affect the uppermost sediments, they ap- formed before the development of the Etna edifice, as testified by the pear to be related to the combined contribution of folding and gravity. associated magmatic activity (220–130 ka; e.g., Branca et al., 2008). Extensional faults, therefore, are mostly superficial features, and are The link with magmatism and the current seismicity, recorded down not reflecting regional extension; in fact, folds and thrust faults are ob- to 10 km or more, indicates that the Timpe faults are crustal-scale tec- served in several localities within the bulge. In the same line, the eastern tonic features. Altogether, the contribution of these tectonic structures slope is affected by intense mass wasting, but this process is the most in shaping the bulge offshore of Mount Etna appears rather limited. recent operating in the area, being preceded, and sometimes promoted, Gravity played an important role, particularly in the late stages of by tectonic deformation. The northern branch of the semicircular pat- evolution, but it was not enough by itself to drive the whole of the tern of faults is partly visible near the Riposto Ridge, but it is likely over- deformation. It certainly promoted the marked asymmetry that caused emphasized in the southern branch, where faults are mainly visible the preferential dismantling of the eastern flank of the volcano, but it near the coast and may be interpreted as the southern extent of the seems that gravity was superimposed to tectonic and volcano/tectonic Timpe fault system. The magmatism and relatively deep seismicity as- processes. sociated to the Timpe faults suggest an origin different from gravitation- It has been recently proposed that the gravitational instability of al sliding, and their age (onset at ca. 220 ka) contrasts with the upslope the eastern flank of Mount Etna, that is facing the deep Ionian basin, progression of the sliding. Finally, the Catania Canyon and the Fiume- is the main controlling factor for the volcano evolution (Chiocci et al., 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 offshore mechanism for the origin of the offshore bulge is the magmatic intru- bulge (Fig. 5) are not crustal features that extend further east; they sion, rather than the gravitational collapse of the volcanic edifice. only contributed to the development of the bulge. The interpretation of seismic data indicate that part of the offshore Magmatism is a specific feature of the Etnean area, which is super- bulge pre-existed the construction of Mount Etna, unlike what was imposed to the complex tectonic setting of the region. Seismic tomog- proposed by Chiocci et al. (2011); part of the offshore bulge was raphy has revealed the occurrence of a large intrusive body below build up by thrusting, rather than simply being an inflation response the volcanic edifice. The estimated volume of the upper part of the in- to magmatic intrusion. Moreover, we did not find any evidence for truded body, between 3 and 9 km, is over 650 km3. It has often been the occurrence of crustal-scale E–W-trending faults bounding the assumed that the intrusion of the large magmatic body is responsible bulge on its northern and southern sides. The extensional faults ob- for the origin of the offshore bulge, irrespective of the deformation served in the Riposto Ridge result from a combination of large-scale mechanism (cfr. Borgia et al., 1992; Chiocci et al., 2011). We also be- folding and downslope gravity. Although gravity contributed signifi- lieve that the deep magmatic intrusion can account for part of the cantly to the recent dismantling of the offshore slope of Mount Etna, observed large-scale deformation, with the topographicaly depressed there is more than just gravity in shaping the offshore bulge. Ionian area being responsible for the eastern structural asymmetry. From the interpretation of our data we propose that the frontal Acknowledgments thrusting followed the large intrusion below Mount Etna, but its inception preceded the development of the Pernicana Fault, which de- The research activity was framed in and funded by the INGV-DPC veloped much later (Fig. 15). 2007–2009 project “FLANK” and greatly benefitted from discussion We consider the sliding of the eastern flank, that is currently mea- with the colleagues participating in the project, and particularly with sured by GPS velocities (e.g., Bonforte et al., 2011), as a minor contrib- the project leaders G. Puglisi and V. Acocella. G. Groppelli and an anon- utor to the observed shortening at the front of the offshore bulge. ymous reviewer are kindly acknowledged for their helpful comments On the other hand, shallow magma emplacement within the vol- and suggestions. canic edifice can amplify the deformation near the central craters, and can trigger favorably oriented faults, such as the Pernicana Fault Appendix A. Details of seismic acquisition and processing and the north-eastern Rift (e.g., Currenti et al., 2008). The instability of the eastern flank of Mount Etna has likely affected the magmatic A1. Acquisition evolution of the upper part of the volcano, and it seems that the inter- play between magmatic intrusion/eruption, and the sliding of the The TAORMINA multichannel seismic survey, carried out in eastern flank, guided by the Pernicana Fault, characterized the most September 2006 onboard of the R/V Urania of the National Council recent history (last 15 ka) of the volcanic edifice. of Researches (CNR), has led to the acquisition of ca. 700 km of seismic profiles. Together with seismic data, Sub Bottom Profiles were acquired by a 16 transducers hull-mounted DATASONICS CHIRP-II 5. Conclusions profiler, with operating frequencies ranging between 2 and 7 kHz, in order to investigate the near bottom sediments. Seismic data show a remarkable degree of structural complexity off- The seismic survey has been carried out with two different sys- shore Mount Etna (Fig. 5). The Pernicana Fault, for instance, is not con- tems, according to the operation conditions encountered during the tinuing offshore as a sharp feature; rather, the deformation is expressed survey: as a set of ENE–WSW folds located very close to the coastline. Seismic 1) 48-channel Teledyne seismic streamer with 600 m of active sec- data indicate that these tectonic structures have affected the offshore tion and 12.5 m group interval. The streamer was kept at an oper- of Mount Etna before the Pernicana Fault system was developed, less ation depth of ca. 10 m. Shot interval was 18.75 m to get a 16-fold than 15 ka ago. The southern guideline of the collapsing eastern flank coverage. A seismograph StrataVisor (Geometrics) was used for of the volcano is poorly expressed onshore, and does not show up off- acquisition, with 1 ms sampling and 5 s of record length. shore; in fact, seismic data indicate that the Catania canyon, a remark- 2) 24-channel Teledyne seismic streamer with 120 m of active section able E–W-trending feature, does not reflect a tectonic control. In a and 5 m group interval was used in the narrower part of the Messina similar way, no structural guideline has been observed offshore, at the Straits. The streamer was kept at an operation depth of ca. 1 m. Shot prolongation of the Pernicana Fault. Moreover, the Timpe Fault system interval varied between 12.5 and 20 m to give a coverage from 3 to does not continue toward the Malta Escarpment. 4.8. A seismograph DAQ Link II (Seismic Source) was used for acqui- Seismic interpretation also shows the occurrence of a structural sition, with 1 to 2 ms sampling and 3 to 4 s record length. In both high located just offshore the edifice of Mount Etna. Whereas a com- cases the energy source was a G.I. Gun Sodera in Harmonic mode plex deformation affects the boundary of this tectonic element, it shows (105+105 ci), fed by a 3500 l electrically-driven compressor only limited internal deformation. Preliminary results led to interpret Bauer, and operating with a pressure of 140 bars. The Sure Shot sys- this structural high as the northernmost extent of the Hyblean foreland. tem (Real Time Micro Systems) was used to control the shots. The The role of this element in the volcano-tectonic evolution of the region depth of the energy source was ca. 8 m. appears rather complex and requires further investigations. We consider that part of the topography of the offshore bulge pre- The seismic survey Malta Escarpment (MESC) 2001 was carried out existed the constructional phase of Mount Etna, being an extension in July–August 2001, on board the R/V Urania and has led to the acqui- of the Hyblean Plateau. Only in the northern part, the bulge is a recent sition of 2500 km of seismic profiles. Data acquisition has been carried tectonic feature, being composed by Plio-Quaternary strata that were out using a 48-channel Teledyne streamer and a Sodera generator- folded before and during the building of Mount Etna. The offshore injector gun in harmonic mode (105+105 in3). The group interval of bulge is bounded by a thrust fault that can be related to the intrusion the acquisition streamer was of 12.5 m for a total active length of of the large-scale magmatic body below Mount Etna. 600 m. Shot interval was 50 m giving a coverage of 600%. Seismic data Borgia et al. (1992) and followers present a model which links the were collected by a Teledyne 48-channel streamer and digitized and currently observed sliding of the eastern flank of the volcano to the off- recorded by a Geometric's Stratavisor seismograph, with sampling shore thrust faults. Our seismic data indicate that the sliding of the east- rate of 1 ms and record length varying from 8 to 12 s. The streamer ern flank is younger than the frontal thrusting initiation, and its was kept at an operation depth of ca. 10 m, whereas the depth of the contribution to overall shortening is only minor. Therefore, the driving energy source was about 6 m. A. Argnani et al. / Journal of Volcanology and Geothermal Research 251 (2013) 50–64 63

In May 2005, about 480 km of high resolution MCS profiles were Argnani, A., Bonazzi, C., 2005. Tectonics of eastern Sicily offshore. Tectonics 24, TC4009, http://dx.doi.org/10.1029/2004TC001656. collected offshore Catania, in front of Mount Etna, by FUGRO OCEAN- Argnani, A., Bonazzi, C., MESC 2001 Crew, 2002. Tectonics of eastern Sicily offshore: SISMICA S.p.A., operating on the M/V Pehlivan II. The survey was car- preliminary results from the MESC 2001 Marine Seismic Cruise. Bollettino di Geo- ried out for INGV using a 1200 m long Litton Industries quick coupler fisica Teorica e Applicata 43, 177–193. streamer, with 96 traces and a group interval of 12.5 m. The streamer Argnani,A.,Brancolini,G.,Rovere,M.,Accaino,F.,Zgur,F.,Grossi,M.,Fanzutti,F., Visnovic, P., Sorgo, D., Lodolo, E., Bonazzi, C., Mitchell, N., 2008. Hints on active tectonics was connected to a TTS-II digital data recording system with a sam- in the Messina Straits and surroundings: preliminary results from the TAORMINA- pling interval of 1 ms and a record length of 3 s. The seismic source 2006 seismic cruise. Bollettino di Geofisica Teorica e Applicata 49, 163–176. was an airgun array of total size of 120 in3 operated at a pressure of Argnani, A., Brancolini, G., Bonazzi, C., Rovere, M., Accaino, F., Zgur, F., Lodolo, E., 2009a. The results of the Taormina 2006 seismic survey: possible implications for active 2000 psi. The shooting interval was 12.5 m. The source and streamer tectonics in the Messina Straits. Tectonophysics 476, 159–169. were towed approximately 2.5 m below sea level. Argnani, A., Chiocci, F.L., Tinti, S., Bosman, A., Lodi, M.V., Pagnoni, G., Zaniboni, F., 2009b. Comment on “On the cause of the 1908 Messina tsunami, southern Italy” by Andrea Billi et al. Geophysical Research Letters 36, L13307, http://dx.doi.org/ A2. Processing 10.1029/2009GL037332. Azzaro, R., Barbano, M.S., D'Amico, S., Tuve, T., Albarello, D., D'Amico, V., 2008. First The MESC-2001 and TAORMINA 2006 seismic data have been pro- studies of probabilistic seismic hazard assessment in the volcanic region of Mt. Etna (southern Italy) by means of macroseismic intensities. Bollettino di Geofisica cessed using the software Disco/Focus by Paradigm, following a standard Teorica e Applicata 49, 77–91. sequence (Yilmaz, 1987) up to time migration. The main processing steps Bally, A.W., 1983. Seismic expression of structural styles: Three Volumes, AAPG Studies are: in Geology Series #15. Behncke, B., Neri, M., 2003. Cycles and trends in the recent eruptive behaviour of 1) Resampling every 2 ms of the original record, Mount Etna (Italy). 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