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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B10105, doi:10.1029/2006JB004639, 2007

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Aeromagnetic anomalies reveal hidden tectonic and volcanic structures in the central sector of the Aeolian Islands, southern Tyrrhenian Sea, Italy Riccardo De Ritis,1 Guido Ventura,1 and Massimo Chiappini1 Received 20 July 2006; revised 21 June 2007; accepted 8 August 2007; published 30 October 2007.

[1] Salina Island (Italy) in the central Aeolian chain is the northernmost volcanic structure

of an elongated ridge on a regional shear zone with NNW-SSE strike-slip faults and second-order N-S and NE-SW faults. Modeling of high-resolution, low-altitude aeromagnetic data collected in 2003 and 2005 using volcanological data constraints reveals the inner structure of the Salina volcanoes and surrounding marine regions. Regional negative anomalies overlie E-W elongated sedimentary basins related to the early Pliocene-Pleistocene opening of the southern Tyrrhenian Sea. These negative anomalies flank positively magnetized crust of Salina and seamounts off its eastern coast where the shorter-wavelength anomalies map late Pleistocene-Holocene volcanic conduits, dikes, and faults. The magnetic and volcanological data indicate that the early (circa 168– 100 ka) basaltic to andesitic Salina volcanism developed along N-S and NE-SW tectonic faults, whereas the more recent andesitic lavas and rhyolitic pyroclastics (circa 100–13 ka) were emitted by vents related to the main NW-SE faults. The tectonic structures also controlled the location of the seamounts around the island and the geometry of the volcano-tectonic collapses. Salina volcanism was emplaced on a NNW-SSE tear fault of the presently developing roll-back slab in the southern Tyrrhenian Sea. Citation: De Ritis, R., G. Ventura, and M. Chiappini (2007), Aeromagnetic anomalies reveal hidden tectonic and volcanic structures in the central sector of the Aeolian Islands, southern Tyrrhenian Sea, Italy, J. Geophys. Res., 112, B10105, doi:10.1029/2006JB004639.

1. Introduction [2] Aeromagnetic surveying is widely employed in volcanic areas due to the recent improvements in data acquisition, precise GPS measurements, and accurate data processing [e.g., Finn et al., 2001; Le´nat et al., 2001]. In these areas, the aeromagnetic survey is particularly useful to detect buried or submerged eruptive centers, alignments of vents, faults [i.e., Rollin et al., 2000; Blanco-Montenegro et al., 2003], and more generally to analyze the subsurface distribution of crustal magnetic features at temperatures below the Curie isotherm [De Ritis et al., 2005]. [3] In the last 5 a, the Airborne Geophysics Science Team of Istituto Nazionale di Geofisica e Vulcanologia of Italy carried out various high-resolution, low-altitude aeromagnetic surveys of the Aeolian volcanoes in the Southern Tyrrhenian Sea. In this study, we present the aeromagnetic results for Salina Island (Figure 1) and surrounding marine areas from data collected in 2003 and 2005 that also supplement the more regional coverage of the magnetic anomaly map of Italy [Chiappini et al., 2000]. Salina is centrally located within the Aeolian Arc (Figure 1) and last

1

Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy.

Copyright 2007 by the American Geophysical Union. 0148-0227/07/2006JB004639$09.00

erupted at 13 ka. Fumaroles and seismicity suggest that the magmatic system is still active. The last eruption, which was the largest event of the Aeolian volcanoes, occurred at 24 ka (Pollara I eruption [Calanchi et al., 1993]). [4] The results of the aeromagnetic surveys here are discussed in light of the available volcanological, structural, and marine geology information. A high correlation between the local and regional structural trends and the direction and shape of the magnetic anomalies is found. The aeromagnetic data also constrain the extent of the Salina sector collapse structures, and allow us to recognize previously unknown eruptive centers presently buried by products of more recent activity and/or partly eroded. The data and results presented in this study help to clarify the complex relationships between the volcanism, tectonics and morphology in the central sector of the Aeolian Islands.

2. Regional Geological Setting of Salina Island [5] Salina Island (26 km2; 962 m above sea level (asl)) locates in the central sector of Aeolians and is composed of calcalkaline vulcanite. The occurrence of seismicity at depths up to 500 km in the southern Tyrrhenian Sea and Calabrian Arc reflects the active roll-back of a NW dipping slab related to the subduction of the Ionian plate beneath the Calabrian Arc (Figure 1a) [Carminati et al., 1998; Faccenna et al., 2001; De Astis et al., 2003].

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Figure 1. (a) Geodynamic setting of Italy and southern Tyrrhenian Sea (adapted from Mantovani et al. [1996], Carminati et al. [1998], Gvirtzman and Nur [2001], and De Astis et al. [2003]). (b) Simplified structural scheme of the Aeolian Islands (adapted from Mazzuoli et al. [1995], Ventura et al. [1999], Pepe et al. [2000], and De Astis et al. [2003]). (c) Structural scheme of the Salina-Lipari-Vulcano volcanic ridge (adapted from Romagnoli et al. [1989], Ventura et al. [1999], and Favalli et al. [2005]). [6] The central Aeolian Islands of Salina, Lipari, and Vulcano form a NNW-SSE elongated volcanic ridge emplaced along a major NNW-SSE dextral shear zone called the Tindari-Letojanni (TL in Figure 1b) fault system, which extends from Salina southward to the northern Sicily [Mazzuoli et al., 1995; Ventura et al., 1999]. This shear zone and the associated second-order N-S tensional and NESW striking faults separate the Aeolian volcanoes (1.3 Ma active) into two sectors with different crustal thicknesses. The western sector involves Moho depths at roughly 20– 25 km and includes the islands of Filicudi and Alicudi and the western Aeolian seamounts. The eastern sector involves thinner crust with Moho depths of about 15 –20 km and includes the islands of Stromboli and Panarea [De Astis et al., 2003]. [7] The volcanoes of the western sector were emplaced along WNW-ESE striking dextral to reverse faults and those of the eastern sector were affected by NE-SW striking normal faults (Figure 1b). Salina, Lipari, and Vulcano are located at the intersection of the TL shear zone and the WNW-ESE and NE-SW fault systems. Geodynamic models of the evolution of the Tyrrhenian Sea [e.g., Mantovani et al., 1996; Gvirtzman and Nur, 2001; Goes et al., 2004] indicate that in the early Pliocene to Pleistocene the WNWESE fault system was a strike-slip structure, which, on the south, bounded the southeastward opening of the Tyrrhenian Sea back-arc basin and the E-W Aeolian sedimentary

basins. The Pleistocene-Holocene TL faults developed as tears of the roll-back slab. [8] The evolution of the volcanism and the structural setting of Salina are well known and thus only the main features are summarized here [Keller, 1980; Bernasconi and Ferrini, 1989; Romagnoli et al., 1989; Barca and Ventura, 1991; Rossi et al., 1987; Calanchi et al., 1993; Critelli et al., 1993; Mazzuoli et al., 1995; Gertisser and Keller, 2000; Quareni et al., 2001; De Rosa et al., 1989, 2002, 2003; De Astis et al., 2003; Favalli et al., 2005; Ventura et al., 2006; Donato et al., 2006]. Salina includes (Figure 2) the RiviCapo volcanic complex, which consists of a NE-SW aligned eruptive fracture (Capo) and a younger central vent (Rivi), the Fossa delle Felci cone, the Porri cone, the Corvo volcano, and the Pollara depression. The island began to develop with the formation of the basaltic to andesitic Corvo volcano and the Rivi-Capo volcanic complex. The age of the partly eroded Corvo volcano is unknown, but relative dating of its basalt flows relates it to the earliest stages of activity on the island. [9] The Rivi-Capo volcanic complex developed between 87 and 168 ka in the northeastern sector of Salina (Figure 2). The northern flank of Rivi-Capo is cut by NE-SW striking normal faults, and consists of dikes striking NE-SW and hydrothermalized volcanics that represent the remnants of the upper portions of the Rivi-Capo conduits. The southern flank consists of lava flows and scoria layers of basaltic to andesitic composition. The two flanks of the volcano are

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Figure 2. (a) Aeromagnetic survey flight path. The 2003 profiles are reported in red, and those of 2005 are black. (b) Geological map of Salina Island (adapted from Keller [1980], Rossi et al. [1987], Barca and Ventura, [1991], and Mazzuoli et al. [1995]). The location of the profiles A-A0 and B-B0 (Figures 6 and 7) are also reported in (a) and (b).

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separated by a NW-SE to NE-SW oriented rim that delimit the north facing collapse of the rim along a NE-SW structural discontinuity. This collapse occurred after 87 ka, which is the age of the younger outcropping lava flows, and dissected the inner parts of the volcanic complex (Figure 2). [10] The Fossa delle Felci volcano (962 m asl) formed between 108 and 59 ka in the eastern sector of the island. The products consist of lava flows and minor pumice and scoria deposits, and range in composition from basalts to dacites. Lava flows outcrop on the western and southern flanks, whereas pyroclastics and minor lavas cover the eastern flank. These latter products filled a preexisting basin of uncertain origin and produced a well-preserved summit crater. Minor dome-like structures, which represent parasitic vents, outcrop at the base of the southeastern flank of the edifice and between the summit crater and the Rivi-Capo rim (Figure 2). The eastern sector of Fossa delle Felci was affected by a N-S striking fault with oblique (dextral/ normal) movements. [11] The Porri volcano (860 m asl) occupies the western sector of the island. Its activity developed between 83 and 43 ka and started with the emplacement of lava flows that outcrop at the base of the southern flank. Wet surges and breccia deposits overlie these lavas. This basal succession of deposits is covered by scoria and lava flows that form the present-day Porri cone and summit crater (Figure 2). The Porri rocks range in composition from basaltic andesite to minor dacite. NW-SE striking oblique slip faults affected the southern and northern sectors of the Porri volcano, as well as the valley separating Porri from Fossa delle Felci. [12] The most recent activity on Salina developed in the northwestern corner of the island between 30 ka and 13 ka. The 1-km-wide Pollara depression to the northwest (Figure 2) is bounded by the 30 ka dacitic Perciato lava flows at the northern corner, and filled by lacustrine and reworked deposits related to the explosive eruptions at 24 ka (Pollara I eruption) and 13 ka (Pollara II eruption). The Pollara I sub-Plinian eruption was the highest recorded intensity event of the Aeolian Islands with mass discharge rates up to 8  106 kg/s [Calanchi et al., 1993]. [13] The Pollara pyroclastics consist of pumice and flow deposits, and range in composition from basaltic andesites to dacites and rhyolites. The northern and eastern rims of the Pollara depression are partly covered by these pyroclastics, whereas the southeastern and southwestern rims cut the northwestern flank and the crater of the Porri volcano, as well as the older Corvo lavas. NE-SW striking, northwest dipping normal faults affected the inner sector of the Pollara depression [Critelli et al., 1993], which originated by the collapse of the northwestern flank of the Porri volcano (Figure 2). This event occurred between 43 ka, which is the youngest age of the Porri lava flows, and 30 ka, which is the age of the oldest lava flows filling the depression. [14] Marine geology data reveal hummocky seafloor around the Rivi-Capo and Pollara sector collapses that reflects the deposits from these collapses [Favalli et al., 2005]. Major seamounts around the island [Romagnoli et al., 1989] include the two larger seamounts with cone radii of about 2 km located roughly 5 km north of the Rivi-Capo volcanic complex (Figure 1c). Two more with cone radii of about 0.8 –1 km occur off the southeastern shore of the island and another two are located about 4 km east of the

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Fossa delle Felci and Rivi-Capo volcanoes. Submarine fumaroles occur off the shores of the Porri volcano to the southeast and the Pollara depression (Figure 2). Wide erosional shelves, which correspond to low stands of the sea level, surround Salina and the other Aeolian islands.

3. Rock Magnetic Properties [15] The extensive tectonism since the Pliocene has greatly modified the magnetic properties of the crust that produce the magnetic anomalies of the study region. In general, magnetic anomalies reflect the integrated effects of three-dimensional magnetization variations that record crustal evolution. However, because of the nonuniqueness of potential field sources, magnetic property and other geological constraints are necessary for effective interpretation of magnetic anomalies. Magnetization attitudes and intensities of the lithological units are especially necessary to developing reliable interpretations of subsurface crustal features. [16] Rock magnetization depends on mineral composition and texture, and can be highly variable even within the same lithologic unit. With respect to sedimentary rocks, volcanic rocks generally have higher magnetizations due to their greater concentrations of ferromagnetic iron oxides. Because magnetization is a trace property, direct measurements of the magnetic properties of outcropping rocks may not reflect the properties of the subsurface rocks. [17] At Salina Island, measurements of the volume magnetic susceptibilities and the natural remanent magnetizations (NRM) were carried out on the main outcropping rock units. These measurements were integrated with previously available data [Barca and Ventura, 1991; Critelli et al., 1993; Barberi et al., 1994; Zanella, 1995] for the shallower rock magnetic properties evaluated in Table 1. These results include the remanent, induced, and total magnetizations for the sampled units, as well as the Koenigsberger ratios between the remanent and induced magnetizations. [18] Because of the small number of paleomagnetic measurements from Salina Island, we considered the paleomagnetic data from near-by Lipari Island [Zanella, 1995] collected on rocks of the same ages and compositions as those from Salina. The paleomagnetic samples of andesites of the Porri and Fossa delle Felci volcanoes spanned ages between 43 and 108 ka with averaged inclination and declination of hIi = 51.6° and hDi = 5.5°, respectively. The mean remanent attitudes measured on the Lipari samples of the last 223 ka were nearly identical and gave hIi = 52.1° and hDi = 3.0°. These values were all close to the presentday magnetic field directions (I = 54°, D = 2°) from the Geomagnetic Reference Field (IGRF) model [International Association of Geomagnetism and Aeronomy (IAGA), 2005]. Thus we assumed that the attitudes from the IGRF were effective for modeling the magnetization attitudes of these rocks because they differed only marginally from the observed paleomagnetic attitudes. [19] The averaged magnetization intensities in Table 1 are relatively rough estimates considering the limited data available. They are generally high and reflect the high volume ratios of basaltic andesite-to-andesite (80%) and rhyolite-to-dacite (20%) for the outcropping rocks of the island [Barca and Ventura, 1991]. Magnetizations for the

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Table 1. Mean Rock Magnetic Properties at Salina Island Compiled Using the Paleomagnetic Data From Barberi et al. [1994] and Zanella [1995] and Field Measurementsa Volcano

Age, ka

Corvo Rivi-Capo

87 – 168

Fossa delle Felci

59 – 108

Porri

43 – 83

Pollara

30 – 13

Lithology

Deposits

JI, A/m

JR, A/m

Q

JToT, A/m

basalts, basaltic andesites basalts, basaltic andesites basaltic andesites, andesites, high-K dacites basaltic andesites, andesites, dacites basaltic andesites, andesites, dacites, high-K dacites, rhyolites

lava flows and scoria

1

4.0

4

5.0

lava flows and scoria

0.7

2.8

4

3.5

lava flows, scoria, pumice falls and flows, domes lava flows and scoria

0.9

2.7

3

3.6

0.9

2.1

2.3

3.0

0.3

1. 7

5.6

2.0

lava flows, pumice falls and flows

a JI is the induced magnetization (calculated assuming an ambient magnetic field of 35 A/m), JR is the natural remanent magnetization, Q is the Koenigsberger ratio (JR/JI), and JTOT is total magnetization (calculated on the assumption that JR and JI are parallel).

offshore volcanic units were not available, but the early volcanic phases on Aeolian Islands were characterized by basaltic magmas [De Astis et al., 2003]. Therefore we assumed that the average magnetization intensities of the offshore rocks were those of the older Salina rocks. [20] Below the Aeolian volcanics, the crystalline basement rocks belonging to the Calabrian Arc were inferred from the gneisses, granites and sediments found in the lavas and pyroclastics. The magnetic susceptibilities of the Calabrian Arc rocks were generally less than 7  107 SI and thus close to zero [Barberi et al., 1994]. Therefore the magnetic sources in the central sector of the Aeolian Islands are probably located in the uppermost 3 km of the crust, which is the maximum thickness of the volcanic cover over the crystalline basement [Barberi et al., 1994; Ventura et al., 1999].

4. Magnetic Data Acquisition and Processing [21] To study Salina Island and surrounding marine areas, two low-altitude helicopter-borne magnetic surveys were carried out at constant barometric height. The first campaign was conducted in October 2003 along NE-SW oriented profiles with a line spacing of 1500 m (Figure 2a). The second campaign in December 2005 was flown with the same line spacing, but in between the lines of the 2003 survey. The two digital data sets were integrated and reduced to the same geomagnetic epoch to produce the magnetic anomaly map in Figure 3 with the profile spacing of 750 m. [22] Total intensity anomaly data were acquired with an optically pumped cesium magnetometer at the sampling rate of 10 Hz. The survey was conducted mainly at constant barometric altitude of 500 m, except for the few profiles that intercepted higher topographic relief. These few lines over the summits of the Porri, Fossa delle Felci, and Rivi Capo volcanoes were flown in draped mode at the constant terrain clearance of 200 m. A GPS receiver in differential mode was used for accurate survey navigation, whereas a laser altimeter at 0.3 Hz recorded the flight altitude. A proton precession magnetometer located on neighboring Vulcano Island served as the magnetic base station to record at 1 Hz the Earth’s magnetic field variations. [23] Data processing included the base station data to remove the external magnetic field variations. Both data sets

were reduced to the same geomagnetic epoch of 2003.0 using the L’Aquila Geomagnetic Observatory (central Italy) datum as the reference value. The total intensity magnetic anomaly field was obtained by removing the Geomagnetic Reference Field (IGRF) [IAGA, 2005]. The data sets were merged and gridded at the spacing of 200 m using minimum curvature to produce the total field magnetic anomaly map in Figure 3. [24] To facilitate geological interpretation, the total field anomalies were reduced-to-pole (RTP [Baranov and Naudy, 1964]) in Figure 4. This analytical transformation centers the magnetic anomalies on their crustal sources including the remanent sources because their magnetization attitudes are very close to the induction attitudes of the present-day IGRF. The positive RTP anomalies, for example, are now centered on Salina and the seamounts off its northeastern coast. Thus these terrains apparently are the actual sources of the positive total field anomalies that in Figure 4 were shifted to the S-SE due the low inclination of the geomagnetic field. [25] The large negative total field anomaly along the northern coast of Salina was due mostly to low geomagnetic field inclination because it is substantially attenuated in the RTP anomalies. Thus suppressing the distorting effects of the low-inclination geomagnetic field with the RTP anomalies unmasked numerous crustal magnetic sources that we consider in section 5.

5. Anomaly Analysis [26] The RTP anomalies in Figure 4 are dominated by the positive anomaly over Salina that is flanked by elongated negative anomalies to the N-NW (NA1 in Figure 4) and S-SE (NA2). We hypothesize that these negative anomalies map underlying linear nonmagnetic sedimentary basins [Pepe et al., 2000; Favalli et al., 2005]. [27] To test this hypothesis, we modeled the magnetic effects of the crustal terrain with superposed basins using the algorithm of Blakely [1981]. However, the available terrain elevation data covered only the portion of the magnetic survey shown in Figure 5. The total field terrain magnetic effects in Figure 5 were based on modeling the terrain data to the depth of 2500 m with two 10-km-wide, zero magnetization intensity crustal bands elongated E-W.

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Figure 3. Shaded relief map of total field aeromagnetic anomalies of Salina Island and offshore areas. 6 of 13

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Figure 4 7 of 13

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Figure 5. Magnetic effects modeled using the algorithm of Blakely [1981] and Blakely and Grauch [1983]. The sources for the negative anomalies NA1 (TF) and NA2 (TF) were modeled as nonmagnetic sedimentary basins, whereas the central positive anomaly was modeled from the available terrain coverage with the magnetization intensity of 2.6 A/m within the terrain delineated by the dashed white line.

Figure 4. Reduced-to-pole aeromagnetic anomaly map assuming geomagnetic inclination of 54° and declination of 2°. The labeled RTP anomalies are discussed in the text. The structural and morphological features were adapted from Romagnoli et al. [1989], Ventura et al. [1999], and Favalli et al. [2005] as also given in Figure 1c. The locations of crustal profiles A-A0 and B-B0 are also shown. 8 of 13

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Figure 6. Crustal 2.75-D magnetic model for profile A-A0 that crosses Salina Island as shown in Figure 4. Alphabetical identifiers mark anomaly features shown in Figure 4 and discussed in the text. Faults are indicated by red lines that are dashed where inferred. These bands represent the sedimentary basins flanking Salina to the north and south [Pepe et al., 2000; Finetti, 2005; Favalli et al., 2005], and were positioned using the RTP anomalies in Figure 4. They represented the westward extensions into the magnetic survey area of known sedimentary basin units in the terrain elevation data [Favalli et al., 2005]. [28] The central positive regional anomaly over Salina and its submerged flanks was modeled in Figure 5 with the constant magnetization of 2.6 A/m. In general, these results are consistent with the anomaly features in Figures 3 and 4 and strongly support the westward extensions of nonmagnetic Plio-Pleistocene sedimentary basins as the sources of the NA1 and NA2 anomalies. [29] In Figures 6 and 7, we present detailed models of the possible magnetic properties of crust for two key profiles that cross the study region as shown in Figures 2 and 4. Profile A-A0 (Figure 6) crosses the high-intensity anomalies centered on the Rivi, Porri and Corvo edifices, as well as offshore volcanic structures. This profile is orthogonal to the graben-like structure on which the Vulcano, Lipari, and Salina islands are emplaced. Profile B-B0 (Figure 7) crosses profile A-A0 at Rivi and includes the Rivi sector collapse structure and the Fossa delle Felci volcano. The B-B0 profile also intersects the inferred basin north of Salina, as well as the submarine platform of Lipari island.

[30] Figures 6 (top) and 7 (top) show the negligible error of the match between the observed RTP anomalies and the vertically polarized effects calculated for the crustal magnetic models in Figures 6 (bottom) and 7 (bottom). The crustal models were constructed using the 2.75-D modeling algorithm from Won and Bevis [1987] based on the analyses of Talwani and Heirtzler [1964] and Rasmussen and Pedersen [1979]. [31] We constrained the crustal modeling using seismic and gravity results on the depth and shape of crystalline basement [Barberi et al., 1994] and the thickness of the volcanic and sedimentary blanket of the Aeolians [Barberi et al., 1994; Pepe et al., 2000; Finetti, 2005]. The outcropping units were taken from the surface geology maps of Keller [1980] and Barca and Ventura [1991] and with magnetization intensity variations referenced to the values generalized in Table 1. [32] Figures 4, 6, and 7 provide an overview of the main crustal modeling results for the aeromagnetic anomaly data considered in this study. Section 6 investigates in detail the geological implications of these modeling results.

6. Discussion [33] The modeling of the anomalies NA1 and NA2 in Figure 5 and both ends of profile B-B0 in Figure 7 are

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Figure 7. Crustal 2.75-D magnetic model for profile B-B0 that crosses Salina Island as shown in Figure 4. Alphabetical identifiers mark anomaly features shown in Figure 4 and discussed in the text. Faults are indicated by red lines that are dashed where inferred. consistent with the negative magnetic effects of the sedimentary basins located between northern Sicily and the Aeolian Islands in Figure 1b [Caratori et al., 2004]. The basin underlying the NA1 negative anomaly (Figure 4) is separated from two weakly magnetic seamounts on its northern boundary by E-W to NW-SE striking faults. The negative anomaly NA2 south of Salina overlaps the western flank of the Salina-Lipari-Vulcano volcanic ridge and includes two weakly magnetic seamounts located 2 – 3 km south of the southeastern corner of Salina (Figures 1c and 4). Thus the negative NA1 and NA2 anomalies may mark the boundaries between nonmagnetic sediments and volcanoclastic deposits with thickness up to 3 km and adjacent strongly magnetized basaltic units. The weakly magnetic seamounts in the basins (Figure 4) are consistent with marginally magnetized volcanites of dacitic or rhyolitic composition, or reversely magnetized rocks. The latter interpretation, however, implies that the seamounts are older than about 0.7 Ma (Bruhnes epoch). Unfortunately, geochronological data on the seamounts are lacking and thus we cannot completely exclude this hypothesis. However, the well-preserved dome-like shape of the seamounts, along with the occurrence of dacitic and rhyolitic volcanites on Salina and neighboring Lipari, and the young age (circa  223 ka) of the Salina-Lipari-Vulcano ridge [De Rosa et al., 2003] suggest that the negative anomalies

probably reflect the weakly magnetic compositional properties of the seamounts. [34] A number of smaller-scale magnetic anomaly features in Figure 4 may be interpreted in the context of available geological constraints and the detailed crustal models given in Figures 6 and 7. For example, the positive anomaly A extends offshore from the northwestern corner of Salina some 7 km and overlaps a NW-SE elongated topographic ridge [Favalli et al., 2005]. Near the coastline, the southeastern tip of the anomaly overlaps the erosional shelf located in front of the Pollara depression. Anomaly A is consistent with the effect of a NW-SE striking, probably dike-like body from the crystallized plumbing system. The adjacent, more circular positive anomaly B overlies the vent area of strongly magnetic lavas from the 30 ka Perciato and Pollara I sub-Plinian eruption (Figures 2 and 4). [35] The negative anomaly C overlies the Pollara depression filled with nonmagnetic lacustrine deposits [De Rosa et al., 1989], and the vent area of the Pollara II eruption (Figure 4). The anomaly minimum D may reflect the effects of chaotically emplaced materials from the Corvo and Pollara collapses that extend off the northwestern coast of Salina. [36] The positive anomaly E modeled in profile A-A0 (Figure 6) includes the summit crater of the Porri volcano, the southern boundary of the Pollara depression, and the Corvo vent area (Figure 2b). This anomaly was interpreted

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in Figure 6 as the superimposition of the effects of the plumbing systems of the Porri and Corvo volcanoes. The relatively isolated character of the positive anomaly F and the lack of overlapping topographic structures suggest that it may reflect an intrusive body or buried eruptive center as modeled in Figure 6. [37] The positive anomaly G in Figure 4 is the southern tip of the larger, N-S elongated anomaly that extends 5 km southward from the Porri crater. In the offshore, the G anomaly overlaps a N-S elongated morphological ridge [Favalli et al., 2005]. The elongated shape of this anomaly suggests that it may be due to a buried N-S elongated, intrusive body. [38] The negative anomaly H, which was modeled in profile B-B0 (Figure 7), extends 4 km offshore from the northern coast of Salina. This linear N-S anomaly is bounded to the west by the positive A, B, and E anomalies and to the east by the anomaly maxima I, J, and K. The southern tip of anomaly H overlaps the collapsed sector of the Rivi-Capo volcanic complex, which also includes the positive I (Rivi) and J (Capo) anomalies. Therefore anomaly H is consistent with the magnetic expression of collapsed Rivi-Capo materials (Figures 2 and 4) as evidenced by the hummocky seafloor in front of the Rivi-Capo scar [Favalli et al., 2005]. [39] The positive I and J anomalies may mark dike systems of the Rivi and Capo volcanic structures, respectively. Anomaly I is modeled in both profiles A-A0 and B-B0 in Figures 6 and 7, respectively. The linear N-S positive K anomaly just north of Rivi-Capo overlies bathymetry with essentially no volcanic relief. However, hydrothermalized rocks and dikes testify to the occurrence of an eroded vent outcropping along the northern coastline of Salina where anomaly K partly overlaps these volcanics [Barca and Ventura, 1991]. This anomaly may reflect the effects of the shallow plumbing system of this vent that was eroded by the sea as the erosional shelf was created. [40] The L and M anomalies form a NW-SE elongated magnetic high that overlaps the Fossa delle Felci crater (L) and the secondary vent (M) located at the southeastern corner of the volcano (Figures 4 and 7). Thus the two anomalies may mark shallow conduits of the Fossa delle Felci volcano. The NW-SE striking alignment of the L and M anomalies is also the strike of the major TL fault segments affecting the island. Thus the NW-SE elongated anomaly affecting the southeastern sector of Fossa delle Felci may reflect the effects of the buried NW-SE dike system. These observations suggest that the TL faults may have controlled the development of the Fossa delle Felci plumbing system. [41] The N-S striking boundary between the negative and positive magnetic values south of anomaly M is also consistent with the strike of the fault affecting the eastern flank of the Fossa delle Felci volcano. Therefore this N-S magnetic gradient may represent the offshore prolongation of this fault as shown by the dashed line in Figure 4. [42] The N and O maxima align along a N-S trend where the N anomaly overlaps two seamounts located eastward of the Fossa delle Felci volcano [Romagnoli et al., 1989]. As shown in crustal profile A-A0 (Figure 6) we interpret this anomaly for the magnetic effects of the two seamounts. The positive O anomaly overlies and appears to be due to a

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submarine lava flow field [Favalli et al., 2005], possibly from the northernmost seamount. [43] In general, the modeling results for profile A-A0 (Figure 6) are consistent with the superposition of the less magnetic andesitic-to-dacitic Porri scorias and lavas on the more magnetic basaltic Corvo lavas. In addition, the model includes the trace of the Pollara and Corvo collapses, which were filled by highly magnetic submarine lavas. The top of the Rivi edifice was modeled by rocks with relatively low magnetizations consistent with the hydrothermal alterations that have been observed in diffuse outcrops. The model also accommodates the observed superimposition of the younger (87 ka) Rivi edifice on the older (168 ka) Capo volcano. In addition, the decreased anomaly field at the marine margins of Salina is consistent with nonmagnetic sedimentary and minor lava successions overlapping the crystalline rocks of the Calabrian Arc. [44] The modeled profile B-B0 (Figure 7) accommodates a cone-like body with low magnetization capping the Fossa delle Felci volcano. This body corresponds to altered material (oxidized scoriae) forming the top of the edifice. In addition, the decreasing anomaly pattern south of the M anomaly is consistent with nonmagnetic materials filling the submarine valley between Salina and Lipari. Here, the profile can reflect the subvertical contact between the more magnetized Salina products (Fossa delle Felci) and the relatively weaker magnetic Lipari materials. This feature is consistent with tectonic faulting in the context of the available bathymetric data [Favalli et al., 2005].

7. Conclusions [45] The magnetic data and modeling results from this study suggest that the regional negative anomalies NA1 and NA2 are related to sedimentary basins, whereas shorterwavelength anomalies mark volcanic and tectonic structures. Accordingly, Salina volcanism older than about 100 ka appears to have developed along N-S and NE-SW tectonic structures, whereas volcanism younger than 100 ka seems related to the main NW-SE faults. In particular, linear NWSE positive anomalies appear to map out dike-like structures of the southeastern sector of Fossa delle Felci and the Pollara marine region (Figures 1c and 4). The Rivi-Capo plumbing system, by contrast, aligns along NE-SW trends, whereas the other offshore positive anomalies O, N, G and K align mostly along N-S trends. In the context of the ages of the Salina volcanoes, the early (168 ka) activities producing the Rivi-Capo volcanic complex were characterized by plumbing systems controlled by the antithetic NESW and tensional N-S striking structures. More recent activities such as the Fossa delle Felci and Porri volcanoes at circa 43– 108 ka and the Pollara vents at circa 13– 30 ka developed along the TL fault and other NW-SE striking structures. This tectonic scheme highlighted by the elongation and/or alignment of the positive anomalies is in agreement with the results from experimental models on the evolution of shear zones [Bartlett et al., 1981]. These models show that older structures developed in a shear zone are the antithetic and tensional fractures followed by the appearance of the main fault. [46] On Salina, the negative subcircular magnetic signatures (C and H in Figure 4) mostly reflect the effects of the

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collapsed deposits (Rivi-Capo), and/or the geometry of the morphological Pollara depression. [47] In general, the modeling results for the two profiles A-A0 and B-B0 can be also extended for interpreting the other anomalies. The magnetic anomalies clearly are useful for the characterizing the subsurface properties of volcanoes and their plumbing systems, as well as the interactions between the magmatism and related crustal tectonic features. Although the results are consistent with available geological and geophysical constraints on the crust of Salina, they must be used with caution because they are not unique and subject to the classical source ambiguity of potential anomaly field analysis. [48] Acknowledgments. We thank Elena Zanella for providing the magnetic rock property measurements for this study and Iacopo Nicolosi for discussions about the Aeolian magnetism. The Salina airborne magnetic survey was funded by INGV. Ralph von Frese provided very helpful discussions and useful editorial comments on the manuscript. We also benefited from the review comments of D. Ravat and an anonymous referee, and the encouragement of INGV’s President, Enzo Boschi.

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M. Chiappini, R. De Ritis, and G. Ventura, Istituto Nazionale di Geofisica e Vulcanologia, Vigna Murata 605, I-00143, Rome, Italy. ([email protected])

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