New high-resolution bathymetric and magnetic data from the western Aeolian sector, southern Tyrrhenian Sea, provide insights into structural and volcanic development of the area, suggesting a strong interaction between volcanism and tectonics. The analysis of these data combined with relocated earthquake distribution, focal plane solutions and strain rate evaluation indicates that the dextral strike-slip Sisifo-Alicudi shear zone is a complex and wide area of active deformation, representing the superficial expression of the deep seated lithospheric tear fault separating the subduction slab below Sicily and Calabria. Most of the observed volcanic features are aligned along a NW–SE trend, such as the Filicudi island-Alicudi North Seamount and Eolo-Enarete alignments, and are dissected by hundred-metre-high scarps along conjugate NNE–SSW trending fault systems. The magnetic field pattern matches the main trends of volcanic features. Spectral analysis and Euler deconvolution of magnetic anomalies show the existence of both deep and shallow sources. High-amplitude, high-frequency anomalies due to shallow sources are dominant close to the volcanic edifices of Alicudi and Filicudi, while the main contribution on the surrounding Eolo, Enarete, Alicudi North and Filicudi North seamounts is given by low-amplitude anomalies and/or deeper magnetic sources. This is probably related to different ages of the volcanic rocks, although hydrothermal processes may have played an important role in blanketing magnetic anomalies, in particular at Enarete and Eolo seamounts. Relative chronology of the eruptive centres and the inferred deformation pattern outline the Quaternary evolution of the western Aeolian Arc: Sisifo, Alicudi North and Filicudi North seamounts might have developed in an early stage, following the Late Pliocene–Early Pleistocene SE-ward migration of arc-related volcanism due to the Ionian subduction hinge retreat; Eolo, Enarete and Filicudi represent later manifestations that led volcanoes to develop during Mid-Late Pleistocene, when the stress regime in the area changed, due to the SSE-ward propagation of the subduction slab tear fault and the consequent reorientation and decrease of trench migration velocity. Finally, volcanic activity occurred in a very short time span at Alicudi, where an almost conical volcanic edifice emerged, suggesting negligible interactions with regional fault systems.
The Aeolian Volcanic Arc, in the southern Tyrrhenian Sea, is related to the NW-directed subduction of the Ionian oceanic lithosphere below the Calabrian Arc (Malinverno & Ryan 1986; Doglioni 1991; Doglioni et al. 1994; Carminati et al. 1998; Gvirtzman & Nur 1999; Faccenna et al. 2001; Goes et al. 2004). It can be subdivided into western, central and eastern sectors (Fig. 1), characterized by different structural and tectonic evolution (De Astis et al. 2003). The study area extends from Sisifo seamount to Filicudi Island, in the western Aeolian sector, and includes Enarete and Eolo seamounts, Alicudi, and a number of minor volcanic edifices (Figs 1 and 2).
Studies carried out in last years on the islands of Alicudi and Filicudi allowed the reconstruction of the chronostratigraphy of outcropping units and their correlation with volcanic successions on other Aeolian islands (Tranne et al. 2002; Lucchi et al. 2008). Conversely, researches on the wide submarine portions of the volcanic edifices and on surrounding areas are still scarce or based on low-resolution bathymetry (Calanchi et al. 1995; Favalli et al. 2005). Recent marine geophysical surveys have been focused on more active sectors of the Aeolian Archipelago (i.e. eastern and central ones), except studies on Eolo and Enarete reporting the morpho-bathymetric features (Marani & Gamberi 2004) and the petrochemical characters of related rocks (Beccaluva et al. 1982; Trua et al. 2004, 2007; Dekov et al. 2007, 2009).
In 2007, new multibeam bathymetry and magnetic data were collected by Istituto di Scienze Marine (ISMAR) around Filicudi and Alicudi (R/V Urania, Cruise PANA_07, Bortoluzzi et al. (2007)). Further multibeam data were collected on 2009 August aboard R/V Urania on top of the Enarete Seamount. These data have been integrated with previously recorded multibeam and magnetic data (Gamberi et al. 1998; Bortoluzzi et al. 1999; Marani et al. 2004), single (30 kJ Sparker) and multichannel seismic profiles. The study benefits of the regional magnetic data (Chiappini et al. 2000; Caratori Tontini et al. 2004), and takes into account the GPS derived stress field and the distribution of seismicity occurred in the area in the last 30 years (Gasparini et al. 1985; Bottari et al. 1986; Falsaperla & Spampinato 1999; Falsaperla et al. 1999; Neri et al. 2003; Billi et al. 2007; Serpelloni et al. 2007; Giunta et al. 2009), including relocated earthquake epicentres from additional data provided by on-land and Ocean Bottom Seismometers and Hydrophones (OBS and OBH) deployed in the area (Dahm et al. 2002; Sgroi et al. 2006).
This paper aims at clarifying the interactions between volcanism and tectonics in the western sector of the Aeolian arc, within the geodynamic framework of the southern Tyrrhenian Sea, and at updating available information, with regard to the stress regime and the morpho-structural setting. The analysis of the structural elements and the definition of volcano-tectonic constraints by integration and comparison of different data sets, depict with unprecedented detail the geological setting of the area.
Geological and Geophysical Framework
The present-day tectonic setting of the Tyrrhenian–Apenninic–Maghrebian system is the result of the complex, long lived (∼65 Ma) collision between Africa and Eurasia that led to the complete consumption of the Thetis and part of the Ionian oceanic lithosphere. The Tyrrhenian Sea formed as a consequence of rifting and backarc extension of the Alpine/Apennine suture above the subducting Ionian oceanic slab (Malinverno & Ryan 1986; Kastens et al. 1988). Crustal thinning has affected the Tyrrhenian area since Late Miocene and oceanic accretion began during Pliocene in the Vavilov basin (4.3–3.5 Ma) and migrated SE-ward during Late Pliocene–Quaternary time, resulting in the formation of the Marsili backarc basin. Simultaneously, arc-related volcanism migrated southeast, from Sardinia in the west to the currently active Aeolian arc, developing the present-day arc and backarc configuration of the southern Tyrrhenian area. Rollback of the subducting Ionian slab might account for the SE-ward migration of arc volcanism and backarc basin opening. Thus, the southern Tyrrhenian region is characterized by the transition from a nearly oceanic crustal domain (Marsili Basin, Fig. 1) to a continental margin offshore Sicily and Calabria in a complex geodynamic framework governed by the sinking and rollback of the Ionian subducting slab (Doglioni 1991; Patacca et al. 1992; Mantovani et al. 1996; Argnani 2000, 2009). The western Aeolian arc is located among areas characterized by different stress regimes (Neri et al. 2003, 2005; Billi et al. 2006): N–S compression to the W, NNW–SSE dextral strike-slip to the E and extension to the N, where crustal thinning produced the oceanic domain of the Marsili Basin, dominated by the NNE–SSW trending Marsili volcano (Marani & Trua 2002).
The western sector of the Aeolian arc (Fig. 2) includes the Islands of Alicudi and Filicudi, 675 m and 774 m a.s.l., respectively, and several major seamounts (Sisifo, Enarete, Eolo, Alicudi North and Filicudi North). The oldest dated rocks (∼1.3 Ma) are from Sisifo seamount (Beccaluva et al. 1982). Rocks sampled from Enarete and Eolo yield an age comprised between 0.85 and 0.64 ± 0.08 Ma, respectively, and related products show calc-alkaline (CA) and high-K-CA to shoshonitic magmatic character (Beccaluva et al. 1982; Trua et al. 2004). Recent chronostratigraphic reconstructions indicate that Filicudi island (and associated submarine eruptive centres) developed mainly during the Middle-Late Pleistocene, in a time span comprised between about 420–330 and <56 ka ago (Tranne et al. 2002). Alicudi island is much younger, and developed between 120–110 and 41–26 ka ago (Lucchi et al. 2008).
Volcanic activity at Filicudi is accompanied by a progressive SE-ward shifting of eruptive vents along N120°E direction, while Alicudi maintained a central-type activity, with no substantial migration of the feeding conduit; this different evolution has been interpreted as an indication of tectonic control and of negligible interactions with the regional fault systems, respectively (Calanchi et al. 1995; Lucchi et al. 2008). Geochemistry of erupted lavas also suggests that Alicudi and Filicudi are independent volcanic systems (Calanchi et al. 1995).
Eolo and Enarete are relatively large-volume (about 40 km3), NW–SE (N125°E) elongated seamounts. Eolo is a flat-summit edifice, its top is about 800 m deep and appears as a closed depression, bounded by several culminations. This morphology and the wide irregular base suggest the possible occurrence of a gravitational collapse of a previously larger edifice (Marani & Gamberi 2004). Enarete has a subconical shape with a basal diameter of about 10 km, and a smaller parasitic eruptive centre lying about 3 km to the west (WES in Fig. 2).
From a structural point of view, the main lineaments in the western Aeolian sector are related to the ‘Sisifo-Alicudi’ system (SA), reported in literature (Finetti & Del Ben 1986; De Astis et al. 2003) as a NW–SE oriented dextral strike-slip shear zone passing through Alicudi and Filicudi and accommodating the SE-ward migration of the southern Tyrrhenian lithosphere during the opening of the Marsili Basin (Fig. 1). In this basin seafloor spreading initiated ∼2 Ma (Kastens et al. 1988) and oceanic crust has been produced with continuity at an average full spreading rate of 32 mm a−1, although spreading rates decreased through time, as well as SE-ward retreating velocity of the subduction hinge (Cocchi et al. 2009).
A submerged, NW–SE (N120°E) elongated volcanic belt, is present for over 35 km from Filicudi island to the Alicudi North Seamount (ANS) (Calanchi et al. 1995), as well as along Eolo-Enarete alignment. Lineaments on Filicudi island strike NW–SE, NNW–SSE, NNE–SSW and NE–SW (Manetti et al. 1995b); furthermore NW–SE and NNE–SSW oriented features have been recognized at the submerged base of the volcanic edifice and in the area between Filicudi and Salina, respectively (Calanchi et al. 1995). Conversely, on the island of Alicudi, fault and fracture distribution shows a radial pattern (Manetti et al. 1995a), although preferential WNW–ESE oriented lineaments were recognized on the submerged flanks by Belderson et al. (1974) on the base of a long-range side-scan sonar survey.
Earthquake data collected since 1988 by Istituto Nazionale di Geofisica e Vulcanologia (INGV), accurately relocated and integrated with seafloor stations data (Dahm et al. 2002; Sgroi et al. 2006), highlights the main seismogenic features of Southern Tyrrhenian Sea (Fig. 1), such as the SA system and the NNW–SSE striking Tindari–Letoianni (TL), corresponding to the Vulcano–Lipari–Salina alignment. Earthquakes occur mostly in the upper 20 km of the crust, with higher magnitude events (M > 5) located at a depth close to the crust–mantle transition (Gasparini et al. 1985; Bottari et al. 1986; Neri et al. 1991, 1996; Caccamo et al. 1996; Falsaperla et al. 1999; Falsaperla & Spampinato 1999; Neri et al. 2003, 2005; Pepe et al. 2005; Billi et al. 2006, 2007; Giunta et al. 2009). Stress tensor computation in the western sector of Aeolian Arc, evaluated from events with M > 5 is consistent with a NNE–SSW striking compression and an WNW–ESE striking extension (Neri et al. 1996; Falsaperla & Spampinato 1999; Falsaperla et al. 1999).
Between the islands of Alicudi and Vulcano, earthquake epicentres arrange into a NW–SE oriented seismic belt showing contrasting fault plane solutions on short distances, with normal faulting for the southeasternmost events and compressional mechanisms for the others (Neri et al. 2003, 2005). West of Alicudi (Fig. 1), focal solutions of the 1957 and 1980 ‘Alicudi’ earthquakes (Gasparini et al. 1985; Agate et al. 2000; Vannucci & Gasperini 2004) and of the 2002 ‘Palermo seismic sequence’ (Pepe et al. 2005) show oblique (strike-slip/reverse) and reverse mechanisms, respectively, with a subhorizontal P-axis striking NW–SE. Fault plane solutions indicate also that seismicity to the W of Ustica Island is mainly related to E–W trending compressive belt (Fig. 1). Fault slip data and focal mechanisms suggest therefore that the western Aeolian sector is affected by an oblique (reverse/strike-slip with dextral components) deformation related to NNW–SSE compression; this strain field is also considered responsible for the cessation of volcanism in the area (De Astis et al. 2003; Neri et al. 2003).
Velocity vectors (Fig. 1) by non-permanent GPS stations relative to the ITRF2000 reference frame (Altamini et al. 2002; Serpelloni et al. 2007), highlight the present-day velocity field in the Aeolian Islands and northern Sicily with respect to Eurasia. The vectors show that no significant shortening is occurring between the Hyblean Plateau (SE Sicily) and Alicudi, both moving towards NNW, whereas Filicudi and Salina move towards NNE at slower rate (Argnani et al. 2007). The present day stress field from GPS-derived strain rate axes show a complex and wide area of deformation where compressional, strike-slip and extensional regimes coexist over short distances in response to a NNW–SSE oriented compressive stress (Serpelloni et al. 2005, 2007; Argnani et al. 2007), whereas seismicity is connected to a heterogeneous stress field due to a transtensional domain between compressional and extensional environments (Billi et al. 2006).
Data Acquisition and Analysis
New bathymetric data were collected by R/V Urania using Reson's 8160-PDS-2000, and Kongsberg's EM710 SIS MBES systems (DGPS positioning). Sound velocity profile data were collected by a SBE probe. Processing was done using the Kongsberg's Neptune package, jointly with pre-existing data collected since 1994 in the area (Gamberi et al. 1998; Bortoluzzi et al. 1999). The new bathymetric compilation obtained from grids at spatial resolution of 25–50 m (UTM, zone 33) or ∼1 arcsec is shown in Fig. 2.
Approximately 800 km of new magnetic data were collected by a Marine magnetic Sea-Spy magnetometer towed 180 m from stern and acquired at 1 Hz by the Geometric's MagLog software (Fig. 3). Total magnetic field data were processed for correcting spikes and navigation errors. Removal of diurnal variations and transient magnetic events was achieved using the data of L'Aquila geomagnetic observatory (Central Italy, INGV, INTERMAGNET). In addition, a statistical levelling procedure was applied to reduce the cross over errors between the lines (errors ranged between –160 and 214 nT). Magnetic anomaly field was computed using the International Geomagnetic Reference Field model of 2005 (IAGA 2009). The new data set was merged with data acquired in 1996 by ISMAR (Bortoluzzi et al. 1999), were gridded using a cell size of 500 m, and reduced to the pole (RTP) using values for declination and inclination of 2.2° and 54°, respectively. Depth distribution of magnetic bodies and structures were computed by conventional Euler deconvolution (Reid et al. 1990; Fitzgerald et al. 2004), that is, applying Euler's homogeneity equation to the magnetic anomaly data (Thompson 1982) using a structural index 1 (dykes and/or intrusions). The results have been windowed to the area of interest and referred to the seafloor depth, filtering out suspicious data. Shallow and deep magnetic sources were defined for the ranges 0–1000 m and 1000–3000 m below seafloor (bsf), respectively. Source depths were analysed within subareas, and statistical results are reported in Table 1. Fig. 3 shows RTP high resolution magnetic anomaly map and Euler deconvolution solutions.
Crustal seismicity occurred in the Southern Tyrrhenian sea between 1988 January and 2007 December has been analysed. In particular, we relocated 1725 crustal earthquakes with magnitude ranging between 1.5 and 5.6 using the HYPOELLIPSE code (Lahr 1989), starting from arrival times collected in the INGV Bulletins (http://iside.rm.ingv.it). Only earthquakes recorded at a minimum of four stations (with a total number of P- and S-wave arrival times not less than 7) were used. Moreover, we included arrival times recorded during the Tyrrhenian Deep-Sea Experiment (TYDE, 2000 December–2001 May), by a network of 14 OBS and OBH, deployed in the Southern Tyrrhenian Sea, around the Aeolian and Ustica Islands, at depths ranging from 1500 to 3500 m (Dahm et al. 2002; Sgroi et al. 2006). The 17 events exceeding the magnitude threshold of 3.7, and 13 major local events, recorded also by the OBS and OBH deployed during TYDE (Table 2), were relocated combining body waves arrival times from the seafloor stations with those of the land stations. In addition, we determined epicentral locations of hundreds of low-magnitude events (mostly undetected by land networks) recorded by the OBS–OBH network. The final data set we used consists of 1906 local earthquakes.
The geodetic strain rate was computed from the GPS velocity vectors of Serpelloni et al. (2007) solving the 2-D tensor equations of the velocity gradient (Calais et al. 2000, 2002), on a mesh obtained from Delaunay triangulation (Fig. 4). Results show that shortening is mostly distributed offshore northern Sicily, while extension becomes dominant moving ESE-ward from Ustica towards Calabria.
Bathymetry and Magnetics
Bathymetric data depict the submerged portions of Alicudi and Filicudi volcanic edifices, Eolo and Enarete seamounts, and a number of minor volcanic features (Fig. 2). In the following paragraphs we report the morphology, the magnetic anomalies (Fig. 3) and the structural setting (Fig. 5) of each volcanic edifice and of surrounding areas.
The seaward extension of Filicudi Island rises from a depth of ∼1300–1400 and 1800 m on the southern and northern flanks. Its base has an elliptical shape elongated towards WNW–ESE (N115°E), with major and minor-axis lengths of ∼21 and 15 km, respectively (Figs 2 and 5). The submarine flanks of the volcano have dips ranging from 30° to 5° and are characterized by a series of radial ridges and gullies, mostly in continuity with subaerial features. An important, morphologically independent, eruptive centre, the Banco di Filicudi (BdF), is located few kilometres NW of the Island. It has a subrounded shape (Fig. 2), with a basal diameter of ∼7 km and a flat summit with a diameter of ∼1.3 km, that deepens towards NW from ∼70 to 120 m of depth. The flat top of BdF is due to wave abrasion during past sea level low stands (Calanchi et al. 1995). The BdF is connected to La Canna eruptive centre (LC in Fig. 2) trough a narrow saddle (depth <200 m), carved at both sides by wide erosive channel heads. The upper part of LC cone has been completely disrupted by wave action, except for the lava neck, impressively emerging from the seafloor as a 71 m high pinnacle, just to the W of the Island (Tranne et al. 2002).
From a magnetic point of view, a main RTP positive anomaly, aligned along WNW–ESE direction, dominates this sector (Fig. 3). Two peaked sharp anomalies of 900 and 1300 nT culminate along the positive lineament, corresponding to Filicudi and the BdF-LC centres, respectively. Both anomalies, in terms of spectral components, and as inferred by the Euler deconvolution results, may be connected with shallow sources (Fig. 3 and Table 1, average ∼700 m bsf), including clusters of deeper sources, as well as on the NE oriented ridge towards the Filicudi North Seamount (FNS).
Filicudi North Seamount
The FNS, located NE of Filicudi, is a volcanic high made up by different coalescent secondary centres (Calanchi et al. 1995). Besides a central sub-circular high (diameter of ∼4.5 km; top at a depth of 1070 m), to the west it includes a subrounded, NNE–SSW slightly elongated parasitic cone (top at a depth of ∼1500 m) and to the south-east, a number of smaller subconical edifices (Fig. 2). The FNS is dissected by some NNE–SSW (N030°E) oriented >200 m high scarps, that can be followed to the Island of Filicudi (Fig. 5). In particular, a straight, up to 300 m-high scarp cuts the western flank of the volcanic ridge on the NE edge of Filicudi Island. The submarine volcanic ridge is associated with a narrow positive magnetic anomaly striking NNE–SSW with Euler's depth distribution that clusters in two groups: shallow to the north and deep to the south (Fig. 3).
The magnetic pattern shows a wide and weak positive anomaly centred on the FNS. Euler's deconvolution results indicate the presence of deep magnetic sources (Table 1, average ∼2000 m bsf). A wide negative magnetic anomaly with a minimum at −190 nT characterizes the almost flat triangular basin (Filicudi Basin) separating Filicudi from Salina (Figs 2 and 3), lying at a depth of 1360–1430 m and filled by a 250–300-ms thick pile of sediments. The wedge shaped geometry of the sedimentary layers within the basin, as imaged on seismic profiles (Fig. 6), suggests that its development may be related to extensional activity along a major NNE–SSW striking normal fault, in agreement with previous studies (Calanchi et al. 1995), GPS vectors (Fig. 1) and strain rate analysis (Fig. 4).
Alicudi North Seamount
A NW–SE (N125°E) elongated belt characterizes the morphology of the northwestern sector of Filicudi, extending seaward the Filicudi-BdF alignment (Calanchi et al. 1995). It lies at a depth of 1200–1400 m showing an irregular morphology and culminates northward into the Alicudi North Seamount. This belt includes subrounded volcanic structures and a few km-long, ∼100 m high narrow ridges striking from NNW–SSW to N–S direction. The ANS arises from a depth of ∼1450 m and has an irregular shape (Fig. 2). Two morphological highs, representing single volcanic eruptive centres, are present on the eastern and western side of a central structure, where a depth of 1120 m is reached. The eastern high has a subconical shape with an average basal diameter of ∼2.5 km and the top reaching the minimum depth of 915 m; the western high is a deeper and smaller edifice with the top at a depth of 1860 m and an average base diameter of 4 km. The western side of the ANS is delimited by NNE–SSW-oriented fault scarps (Fig. 5). The main one is >500 m high, raises from 2000 m (Fig. 7) and prolongs for about 12 km to the area NW of Alicudi (Fig. 5).
A wide and low-intensity positive RTP magnetic anomaly (ranging from 200 to 300 nT) is centred on the ANS structure (Fig. 3); the main spectral contribution is attributed to low-frequency components of the magnetic anomalies, with depth of sources averaging ∼1900 m bsf (Fig. 3 and Table 1). A small perched basin with depths of 1420–1450 m, the Alicudi Basin, is enclosed among the morphological highs of the Filicudi-ANS belt and the Alicudi volcanic edifice (Figs 2 and 5). On seismic profiles it appears filled by a maximum of 200 ms of sediment with subhorizontal bedding (Fig. 7) and it corresponds to a low-amplitude negative magnetic anomaly (–165/–80 nT, Fig. 3).
The submerged portions of Alicudi show a quite regular subconical shape with a basal diameter of ∼14 km and slopes between 30° and 4°. Volcanic and erosive structures are radially distributed, without any preferential directions of development. Few morphological irregularities, such as those mapped by Belderson et al. (1974), are recognizable on the lower submerged slope and are interpreted as due to lava flows and/or radial dykes.
At the southern base of the volcanic apparatus (between depths of 1000 and 1500 m), the seabed is irregular and marked by subrounded features with sizes of 300–500 m (Fig. 2), locally aligned downslope. They characterize a ∼30° wide sector of hummocky terrains, slightly bulging down to a depth of 1600 m. A slight fan-shaped positive magnetic anomaly (100–120 nT, Fig. 3) related to shallow sources (Table 1, average ∼500 m bsf) is present in correspondence of this sector. A high-reflectivity seismic unit with little lateral continuity of reflectors (‘volcanogenic deposit’ in Fig. 8), embedded into the Late-Quaternary sedimentary succession of the Cefalu’ Basin and in continuity with the hummocky terrains, led previous authors to suggest that it might represent the deposit of a lateral collapse event (Romagnoli & Tibaldi 1994; Calanchi et al. 1995). However, no morphological evidence of a wide collapsed sector is found on the submarine and emerged southern flank of Alicudi. Recent field studies show only the occurrence of three successive small concentric collapses on the island summit (Lucchi et al. 2008). In this view, the seismic highly reflective unit with clear Alicudi southern flank provenance and interfingered with the sedimentary layers of the Cefalu' Basin (Fig. 8), might be related to submarine lava flows during the early stages of growth of the Alicudi volcano.
In correspondence of Alicudi, the magnetic anomaly field shows a well-developed circular distribution, correlated to the shape of the emerged portion of the island (Figs 2 and 3). The maximum anomaly value of ∼1300 nT is slightly shifted SW of the island summit, in the sector where the oldest volcanic products outcrop (Lucchi et al. 2008). Amplitudes and spectral components of the magnetic anomalies in this sector are mainly due to shallow sources, as suggested by Euler deconvolution estimates with an average depth of ∼1000 m bsf, similar to those observed in the Filicudi sector (Table 1).
Eolo is a large seamount rising from a depth ∼1600 m; it is NW–SE (N125°E) aligned for about 12 km with a width of about 8 km (Figs 2 and 9). Its western and southern flanks are quite steep with dips of 15–20°; in particular, the western one is almost rectilinear (with a slight NW-ward concavity) along the NNE–SSW (N025°E) direction, being likely controlled by a major structural lineament that prolongs far to the SW (Fig. 5). Similarly, the southern flank appears aligned along the main regional NW–SE direction. Both the southern and the eastern flanks are affected by wide (in the order of 2 km) and deep scars (Fig. 9), often organized as retrogressive failures. In particular, a marked horse-shoe shaped scar is present on the E flank, with the main scarp that develops between 680 and >1000 m depth. Within and at the base of the collapsed sector are present three cone-shaped structures that can be interpreted as primary volcanic features, as suggested by recent sampling (Trua et al. 2007). The seamount summit is, instead, almost flat and characterized by a depression (depths of 750–800 m) bounded by culminations (Marani & Gamberi 2004), except for the SW side where it has been likely affected by retrogressive erosion due to sliding (Fig. 9). On seismic profile, Eolo flat summit is covered by at least 100 ms of sediments (Fig. 10), indicating a relative old age for this volcanic edifice.
Magnetic signature of Eolo seamount is mostly characterized by a round-shape positive anomaly of about 350 nT centred on the NW sector of the edifice. This anomaly prolongs northwards with a narrow NNW–SSE anomaly of about 250 nT that follows a main tectonic feature (Figs 3 and 5). This trend is also well represented by the distribution of Euler's solutions indicating shallow sources with an average depth of ∼500 m bsf (Fig. 3 and Table 1).
Enarete is a subconical seamount rising from depths of 1600 and 2000 m (on the NW side) to the summit <300 m (Figs 2 and 11). It shows a regular shape, quite symmetrical about a NW–SE (N125°E) elongation, with a length of 12 km of and a width ∼10 km. Both its summit and its flanks appear covered by sediments (Fig. 12), in agreement with its relatively old age. The eastern flank shows an angular lowered sector, with apex at about 400 m, enclosed between two NW–SE (N125°E) and ENE–WSW (N060°E) oriented lateral scarps (Fig. 11); downslope, it appears delimited by a NNW–SSE (N150°E) oriented scarp crossing the flank at depth of 1070–1400 m. On the whole, this lowered sector is about 3 km2; its peculiar morphology is very similar to the failure geometry obtained through analogue modelling by Acocella (2005) for sector collapse of volcanic cones. In particular, the wide arcuate shape of the scar (Fig. 11) should be due to the intersection between the failure plane and the conical shape of the edifice; the NNW–SSE oriented scarp represents an ‘antithetic’ failure with opposite dip. West of Enarete, a small eruptive centre, 500 m high (West Enarete seamount, WES), is present; it has a subconical shape with average diameter of 2.5 km, although it shows a preferential NW–SE elongation (Fig. 13).
The Enarete seamount magnetic anomaly pattern reveals a slight positive high of about 150 nT centred on the volcanic edifice, whereas the WES shows a positive value of 250 nT (Fig. 3). The magnetic anomaly has a rounded shape with little elongation southwards. Magnetic sources are located at shallow depths (average ∼600 m bsf, Table 1).
Discussion and Conclusions
Integration and analysis of bathymetric, seismic and magnetic data indicate that tectonic and volcanic activity in the western Aeolian sector are strictly connected. The NW–SE elongated volcanic belt represented by the alignment Filicudi-BdF-ANS, oriented along the main regional trend, as well as the overall regular shape of Alicudi volcanic edifice were previously reported (Calanchi et al. 1995; Favalli et al. 2005). However, the new high resolution bathymetric data, defining with greater detail the submerged setting of the area, better clarify the relationships between tectonic and volcanic features, whose nature is supported by magnetic anomaly data.
Filicudi Island and its submerged portions, as well as the system BdF-LC, are characterized by high-frequency magnetic anomalies correlated with subaerial and submarine shallow structures and depicting a narrow NW–SE oriented volcanic chain (Figs 2, 3 and 5). The distribution of eruptive centres and magnetic anomalies suggests a tectonic control on the development of the composite volcanic edifice. Recent NNE–SSW oriented extensional faults occur NE of Filicudi, between Filicudi and Salina (Fig. 5), and NW of Salina where the faults show large throws accommodating vertical displacement (Argnani et al. 2007). Fault activity is also evidenced by earthquake epicentres and GPS-derived strain field (Fig. 14).
Alicudi is characterized by a main positive magnetic anomaly centred on the island and dominated, in terms of amplitude and shape, by its emerged portion (Fig. 3). Radial distribution of volcanic structures and erosional features on its submerged flanks confirms the negligible influence of regional fault systems on the development of this volcanic apparatus (Calanchi et al. 1995; Lucchi et al. 2008). In addition, GPS velocity vectors suggest little or no shortening between Alicudi and the foreland, instead focal mechanisms and strain rates show extension and transtension between the Island and the Sicily shoreline in the Cefalu' Basin (Figs 1 and 14).
An evident NW–SE alignment is shown by Eolo and Enarete seamounts (Fig. 14), characterized by sub rounded-slightly elongated magnetic anomalies related to shallow sources (Table 1). In fact, the observed high-frequency low-amplitude magnetic anomalies suggest decay in volcanic rock magnetization related to mineral alteration due to the hydrothermal activity being observed on Enarete and Eolo (Lupton et al. 2008; Dekov et al. 2007, 2009). In contrast, ANS and FNS seamounts, lined up along WNW–ESE directions, are characterized by weak and wide positive magnetic anomalies with the prevalence of low-frequency components associated to deeper magnetic bodies with a depth of magnetic sources ranging from 2000 to 3000 m bsf. This indicates a clear depth separation of magnetic sources with respect to Eolo, Enarete, Filicudi and Alicudi suggesting a different spatial and temporal evolution of volcanism within the sector. The lack of petrochemical data on ANS and FNS prevents any discussion about magmatological aspects. However the occurrence of a thick sedimentary cover draping their summits (Calanchi et al. 1995), suggests that both seamounts (possibly coeval to Sisifo, the oldest dated volcanic edifice in the area) developed in an early stage of arc-related volcanism, when melt migration and extraction was driven by a different stress regime or by the activation of different tectonic structures with respect to those active in more recent times. Eolo and Enarete developed later; when volcanism ceased (as suggested by the thin sedimentary cover draping their tops) magmatic activity moved to Filicudi and lately to Alicudi, leading both volcanoes to emerge in the Late Quaternary (Fig. 14).
The morpho-structural characterization of western Aeolian volcanic centres highlights their relationships with tectonics, mainly acting through WNW–ESE/NW–SE and NNE–SSW systems. Geodetic measurements indicate that roll-back of the Ionian subduction plate is almost halted (Goes et al. 2004; Serpelloni et al. 2007; Argnani 2009; Cocchi et al. 2009) and that the western Sicily, including the island of Alicudi, moves together with the Nubian Plate (Serpelloni et al. 2007). Earthquake locations and focal fault plane solutions, in this sector of the southern Tyrrhenian Sea, show that the 5 mm a−1 Nubia-Eurasia convergence is absorbed along a E–W oriented belt located north of western Sicily and south of Ustica (Goes et al. 2004). Our analysis reveals a more complex fragmentation of the Sisifo-Alicudi fault system than previously proposed; the compressive segment is interrupted east of Ustica by a wide zone of shear deformation that transfers the convergence front SE-ward towards the Ionian Sea (Figs 1 and 14). Moreover, GPS velocities indicate that Filicudi and Salina are moving away from Alicudi east and east-south–eastward, respectively (Fig. 14), at a rate of ∼3.3 mm a−1 (Serpelloni et al. 2007).
The inferred deformation pattern in the western Aeolian sector shows a grid of high-angle conjugate fault systems represented by WNW–ESE/NW–SE and NNE–SSW trending lineaments (Fig. 14), in agreement with the structural setting depicted for the southern Tyrrhenian area by Guarnieri (2006) and Giunta et al. (2009). This fault pattern represents the superficial expression of the Sisifo-Alicudi lineament accommodating the right lateral displacements related to the SE-ward Ionian subduction trench migration. The WNW–ESE/NW–SE-trending fault system acts a structural control on location of eruptive centres, given that main composite volcanic edifices develop along this direction. The fault system would be transcurrent/transtensive, although no clear evidence of right-lateral shear deformation is present in the sector, probably obscured by slow slip rate and high magmatic productivity occurring along the faults. The NNE–SSW fault system is mostly extensional and is expressed by major fault scarps (over 500 m high), such as the one which extends from ANS western flank to the area NW of Alicudi, and those affecting the western flank of Eolo and FNS, where they extend to the NE portion of Filicudi (Fig. 14). Evidence of extensional tectonics along a NNE–SSW oriented structure was already recognized by Calanchi et al. (1995) in the Filicudi Basin; moderate seismicity along the same NNE–SSW alignment has been recorded at present in this area (Bottari et al. 1986; Neri et al. 1991).
The circum-Tyrrhenian shorelines have been hit by several tsunami events in historical times (Tinti et al. 2004). The observed fault activity and the collapses of portions of Eolo and Enarete seamounts (Figs 5, 9 and 11) suggest that the western Aeolian arc may represent a potential for tsunami sources, given that active tectonics and hydrothermal alteration weaken volcanic rocks favouring the instability of the flanks of volcanoes and massive gravitational collapses represent a major risk for tsunami generation.
Relative chronology of the eruptive centres and the deformation pattern inferred from stress-field and morpho-structural analysis outline the Quaternary evolution of the western Aeolian sector. During Late Pliocene-Early Pleistocene a SE-ward fast retreat of the subduction hinge occurred by tearing and differential roll-back of the Ionian slab (Gvirtzman & Nur 1999; Doglioni et al. 2001). In the same time a complex system of WNW–ESE trending transcurrent/transtensive and E–W, NE–SW extensional structures became active offshore the northern Sicily, leading to the collapse of the Sicilian margin towards the Tyrrenian Sea and led to the development of Sisifo, Alicudi North and Filicudi North arc-related seamounts along the WNW–ESE-trending dextral strike slip system. During Middle-Late Pleistocene the retreat velocity of the subduction hinge and the accompanying fast subduction slowed down to a minor fraction of the 60 mm a−1 roll-back velocity during the early stages of the Marsili basin opening (Cocchi et al. 2009). This caused a reduction in the spreading rate of the backarc basin that evolved from pure horizontal spreading to the superinflated Marsili seamount (Marani & Trua 2002; Cocchi et al. 2009). The new structural setting linked to a re-orientation of the subduction hinge retreat direction from SE to SSE and related to the deep seated Tindari-Letoianni tear fault (Lanzafame & Bousquet 1997; Billi et al. 2006; Guarnieri 2006; Rosenbaum et al. 2008), promoted the formation of a diffuse transfer zone with the S-ward development of a new NW–SE trending dextral transcurrent/transtensive fault system and of NNE–SSW-trending extensional faults. Thus, the western Aeolian arc volcanism shifted southward forming the composite edifices of Eolo, Enarete and Filicudi that preferentially developed along the transtensive NW–SE fault system. This fault system joins in the proximity of Salina the NNW–SSE oriented Tindari-Letojanni fault system that favoured the development of the central Aeolian sector (De Astis et al. 2003). Finally, during late Pleistocene, volcanic activity occurred within a short time interval at Alicudi, where an almost conical volcanic edifice emerged, suggesting negligible interactions with regional fault systems.
The work around Alicudi and Filicudi was done during the return of R/V Urania from the sinking site of R/V Thetis in Mazara del Vallo, where ship was routed immediately after the event, during cruise PANA_07. We wish to dedicate this work to the memory of the researcher Petr Mikheychik who died and to all people that were on board R/V Thetis. Prof E. Bonatti and M. Marani of ISMAR are acknowledged for the bathymetric data of the CNR's Strategic Project ‘Tyrrhenian Sea: high Resolution morphology and Structure of a Backarc basin’. The Italian ‘Ministry of the Industry’ funded the 1994 R/V J. Charcot cruise that collected bathymetric data on Eolo Seamount. During Cruise PRIMI09 R/V Urania collected multibeam data on the Enarete Seamount. We thank the chief-of-expedition, Dr. F. Bignami, the Captain and officers, and technicians on board, among them M. Tola, for their help. We thank A. Argnani for fruitful discussions. Dr T. Braun and an anonymous reviewer are kindly acknowledged for suggestions and paper improvements.