The integration of magnetic data in the Neapolitan volcanic district

V. Paoletti M. Secomandi M. Fedi G. Florio A. Rapolla Dipartimento di Scienze della Terra, UniversitaÁ di Napoli Federico II, Largo S. Marcellino 10, 80138 ,

ABSTRACT volcanic structures. In those cases magnetic data set should enable the characterization of data can locate buried structures such as line- the main buried volcanic structures such as In this paper we present an example of aments, faults, and volcanic and intrusive vents and calderas, providing a better under- the integration of airborne and marine structures. As noted by Finn (1999), compos- standing of the connection between tectonics magnetic data sets measured in the Nea- ite data sets allow a more complete view of and volcanism in the Neapolitan volcanic politan area, southern Italy. The integra- patterns and trends that individual data sets district. tion involved detailed data measured re- may not provide. The author gives an over- cently in the Phlegrean Fields, in the view of the procedures for merging magnetic THE NEAPOLITAN VOLCANIC Somma-Vesuvis area and in the Bay of Na- surveys into a regional digital compilation and DISTRICT: GEOLOGICAL ples, that produced a high-resolution mag- shows, through the illustration of the case of FRAMEWORK AND GEOPHYSICAL netic map of the whole active volcanic dis- Washington State, the usefulness of these pro- STUDIES trict. The data sets partially overlapped cedures for providing synoptic views of re- and characterized varying ¯ight height and gional features. Chiappini et al. (1999, 2002) The Neapolitan volcanic district is located line spacing. Integration was therefore per- merged magnetic data acquired in the Antarc- on the Tyrrhenian margin of the Campanian formed through several procedures includ- tic region over the areas of the Ross Sea and Plain (Fig. 1A). This plain was formed during ing continuation between general surfaces. the Transantarctic Mountains in the frame- the Plio-Pleistocene as the result of the com- The integration produced a new, detailed, work of the Antarctic Digital Magnetic Anom- plex geodynamic events connected with the draped magnetic data set of the Neapolitan aly Project. They produced an integrated grid opening of the Tyrrhenian Sea and the anti- region characterized by a terrain clearance that resulted in a fundamental tool for regional clockwise rotation of the Italian Peninsula of 200 m, giving a meaningful overall view interpretation of the tectonic and geologic (Scandone et al., 1991). A tensile stress re- of the volcanic area. The study of the main characteristics of this area of Antarctica. Da- magnetic features of the area was carried gime thus affected the Tyrrhenian margin, maske (1999) gives another example of the out by computing the horizontal gradient of causing N-S and NNW-SSE normal faults, and compilation of an integrated aeromagnetic the pole-reduced draped data. The analysis then NW-SE and NE-SW normal faults and map from data sets ¯own over areas with very of the obtained map showed the presence of W-E strike-slip faults (e.g., Doglioni, 1991). different topographies in the Antarctic region lineaments of preferential magma upwell- Along the Campanian border, the Quaternary (Central Queen Maud Land). Golynsky et al. ing and buried volcanic structures and al- basin of the Bay of Naples was produced by (2002) merged airborne and marine magnetic lowed the delineation of a geovolcanological NE-SW±trending normal faults. Intense vol- observations in East Antarctica and adjacent and structural framework of the whole Ne- canism has characterized this area since the seas of the Indian Ocean and compiled an in- apolitan volcanic district. late Miocene. This volcanic activity seems to tegrated magnetic anomaly map that provided be in close spatial relation with the NE-SW Keywords: aeromagnetic survey, marine new insight on the tectonic features of the East faults (e.g., Bruno et al., 2003). The present magnetic survey, data sets integration, Antarctic. Campanian volcanism started 1±2 Ma (Capal- magnetic data analysis. In this paper we present and analyze a new, di et al., 1985). Currently the active areas are detailed magnetic map of the whole Neapoli- the island of Ischia (last eruption in 1302), the INTRODUCTION tan active volcanic district, southern Italy (Fig. Phlegrean Fields (last eruption in 1538), and 1). This was obtained by merging recently ac- the Somma-Vesuvius (last eruption in 1944). Magnetic anomaly maps provide insights quired airborne, land, and marine magnetic The products of the Campanian volcanism be- for a better understanding of the geologic, tec- measurements. long to two cycles: an older cycle (Miocene- tonic, and geothermal characteristics of an In this study we aim to gain insights into Pleistocene), evidences of which were only area. This is particularly true for volcanic the characteristics of the Neapolitan volcanic found in the Parete 2 Well (see Fig. 2), and a zones, where recent volcanic activity and vol- area from an overall view of its magnetic ®eld. second cycle related to the previously men- canoclastic deposits often cover important The analysis of the newly merged magnetic tioned Plio-Pleistocene extensional tectonics.

Geosphere; October 2005; v. 1; no. 2; p. 85±96; doi: 10.1130/GES00003.1; 7 ®gures.

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Figure 1. A: Location of study area. B: Neapolitan volcanic district, southern Italy.

The so-called Roman Comagmatic Province, to depths of a few thousand meters below sea A study of the distribution of the magneti- which includes the Vesuvian and the Phle- level (mbsl) (Carrara et al., 1973; Bruno et al., zations in the Vesuvian area by detailed air- grean volcanic districts, belongs to this second 1998). The Trecase 1 well, drilled inside the borne data is described in Paoletti et al. cycle (Fig. 1B). Vesuvius volcanic area, detected the sedimen- (2004). A boundary analysis of the data clear- The main characteristics of the Bouguer tary basement at ϳ1700 mbsl (Bernasconi et ly showed the southern rim of the caldera of anomaly map of the Campanian Plain (Florio al., 1981). Rock magnetism measurements Mount Somma. In the area surrounding the et al., 1999) are intense maxima correspond- from Somma-Vesuvius and the Trecase 1 well Somma-Vesuvius edi®ce, the authors detected ing to carbonatic reliefs bordering the plain (Cassano and La Torre, 1987) showed mag- the magnetic anomaly pattern of the lateral and a wide minimum area broken up by Ve- netization values ranging from 6.8 A/m (lavas boundaries of some buried small volcanic suvius and Parete highs. The horizontal gra- from Vesuvius) to 0.5 A/m (tuffs), and an in- structures and showed some magnetic trends dient of these data shows some maxima tense remanent magnetization, with a Koe- possibly consistent with trends detected by aligned in three E-W lineaments, two of which nigsberger ratio of ϳ8.6. other geophysical studies (Fedi et al., 2005b; border the depression to the north and The presence of a shallow magma chamber Bruno et al., 1998). the south, and the third is South of Vesuvius. (between 4 and 10 km bsl) was assumed from The Acerra depression is closed by two other a study of ¯uid inclusions of ejected nodules The Bay of Naples NE-SW alignments of maxima that seem to (Belkin and De Vivo, 1993). However, seis- cut the Vesuvius and Phlegrean Fields volca- mic studies (Zollo et al., 1996; De Natale et In the Bay of Naples, a morphologic struc- nic area (Fig. 2). al., 1998) seem to exclude the presence of a ture formed by a continental shelf, a continen- magmatic melt above 4±5 km bsl in the Ve- tal slope, and a basin can be identi®ed (Milia, The Vesuvian Area suvian area. Gravity studies (Cella et al., 1999; Aiello et al., 2001). In the northern area 2003) detected a deeper intracrustal low den- of the bay, the Phlegrean Fields offshore, the Somma-Vesuvius is a stratovolcano char- sity source that was interpreted as being the shelf is irregular and characterized by the acterized by products of both explosive and main magmatic reservoir of the volcanic ac- presence of monogenic volcanoes, small cal- effusive eruptions. The complex is formed by tivity of the whole Neapolitan region. The deras, tuff cones, and lava extrusion (Milia, an older volcanic center (Mount Somma), presence of such a deep magmatic source was 1999). The bay is dominated by two subma- which underwent a calderic collapse, and a also proposed by Rolandi et al. (2003) on the rine canyons: the Magnaghi and the Dohrn more recent one, . It is located basis of geochemical, stratigraphic, and struc- Canyons (Fig. 1B). The Magnaghi Canyon is in an area where a carbonate basement extends tural studies. ϳ15 km long, has a trilobate head, and has

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Figure 2. Geological sketch map of the Campanian Plain (Bonardi et al., 1988). Solid black lines indicate the main faults singled out by seismic studies (Bruno et al., 1998, 2003; Milia and Torrente, 1999), barbs are on the downthrown side. Dashed lines show fault location extrapolated from seismic data. Blue lines show the main gravimetric and magnetic lineaments identi®ed by Florio et al., 1999. Yellow lines indicate the faults in Ischia Island (quoted in Nunziata and Rapolla, 1987). MSFÐMagnaghi-Sebeto fault; VFÐVesuvius fault; 41 PLÐ 41st-N-parallel magnetic lineament; P2ÐParete 2 well; TC1ÐTrecase 1 well.

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®rst a N-S, then a NE-SW trend. The Dohrn domes are present, mainly connected to the the presence of hydrothermally altered and de- Canyon, with a preferential NE-SW trend, is early stages of activity. The apex of the activ- magnetized rocks at the caldera ¯anks. The ϳ25 km long and formed by two branches, ity was reached ϳ37 ka (dated by Deino et southern rim of the caldera was clearly iden- which merge into a main branch northwest of al., 1994), with the eruption of the Campanian ti®ed by Paoletti et al. (2004) on the basis of Capri Island. A structural high formed by a Ignimbrite. The eruption of the Neapolitan a boundary analysis on an integrated high- horst of the carbonate basement (Banco di Yellow Tuff, which occurred ϳ12 ka, is the resolution aeromagnetic data set. Fuori) is extended in a NE-SW direction in second in order of importance because of its The gravity high near Parete located in the the central area of the bay, between the Capri lower volume. In the last 12,000 yr the Phle- northern sector of the Phlegrean Fields by and Ischia Islands, with a minimum depth of grean Fields have been subsiding, except for Cassano and La Torre (1987) was interpreted 130 m (Milia, 1999; Aiello et al., 2001). Seis- local uplifts localized inside the caldera. as due to a volcanic structure (Barbieri et al., mic re¯ection data from the Bay of Naples The Phlegrean Fields are also an area of 1976) or as a buried carbonate horst with mag- (Finetti and Morelli, 1974), recently repro- particular geothermal interest. Boreholes matic intrusions along its bordering faults. cessed by Bruno et al. (2003), show a struc- drilled in 1987 by the Agenzia Generale Ital- This latter hypothesis was supported by the tural pattern of normal faults cutting Pleisto- iana Petroli (AGIP) and Ente Nazionale Ener- local magnetic signature appearing on a land cene sediments with a prevailing NE-SW gia Elettrica show high temperature gradients based magnetic pro®le (Carrara et al., 1973). strike. This trend is named the Magnaghi-Sebeto in all of the Phlegrean area. The 200 ЊC iso- The study carried out by Paoletti et al. (2004) line (see MSF in Fig. 2) (Bruno et al., 2003) therm is at a depth of 500 m in the Baia area, identi®ed a number of volcanic structures that and divides the Bay of Naples into two areas: then deepens, reaching 1000 m southeast of seem to be aligned along an E-W trend, show- a western area, characterized by several vol- the Astroni Volcano. This strong thermal ing the presence of diffused volcanic activity canic banks, and an eastern one, characterized anomaly probably caused thermal mineral al- all over the Campanian Plain. by a NE-dipping monoclinal structure made of teration in the rocks (Rapolla et al., 1989). Bruno et al. (2000) performed a seismic sedimentary rocks. In the Vesuvian area, we The analysis of well logs (Cassano and La study of the northern sector of the Phlegrean recognized a NE-SW normal fault that seems Torre, 1987) shows a stratigraphic difference Fields and showed that the so-called 41st-N- to continue onshore (Bruno et al., 1998; Ju- between the internal and external part of the parallel magnetic lineament is characterized denherc and Zollo, 2004). This is known as Phlegrean Fields. In the central area, the suc- by ESE-WNW and E-W strike-slip faults the Vesuvius fault (VF in Fig. 2) and was also cessions are poorer in lavas, and the thermal whose activity developed during the Pliocene± located by a boundary analysis of the Bouguer alteration of rocks is deeper than in the exter- early Pleistocene. The authors interpreted the anomaly ®eld (Florio et al., 1999), which nal areas. This analysis and other stratigraph- 41st-parallel line as a deep-seated transfer highlighted a NE-SW±oriented fault running ic, volcanological, and geophysical studies fault system formed as a consequence of the both onshore and offshore. pointed out the existence of collapsed struc- different rates of opening of the Tyrrhenian Secomandi et al. (2005) performed a study tures, interpreted as calderas, which formed as Sea in the frame of the NW-SE extension of of the magnetic and gravity data of the Bay a consequence of the two Phlegrean major the Appenines belt. of Naples and characterized some of the most eruptions. The origins and limits of these meaningful marine anomalies of the area. The structures are an object of debate. According Ischia Island study demonstrated that while the Magnaghi to some authors (Rosi and Sbrana, 1987; Fish- Canyon is correlated to gravimetric highs and er et al., 1993), a caldera was produced by the Ischia Island is the oldest volcanic complex magnetic structures, and can therefore be in- Campanian Ignimbrite eruption, while accord- in the Neapolitan area. It is entirely composed terpreted as a lineament of magma uprising, ing to other authors (Scandone et al., 1991; of volcanic rocks with a chemical composition most of Dohrn Canyon is not characterized by De Vivo et al., 2001), the Ignimbrite erupted of lavas ranging from trachybasalts to alkali- volcanic activity and does not correlate to any in other areas and the caldera collapse was due trachyte and phonolite (Capaldi et al., 1985). gravimetric or magnetic structures. Finally, to the Neapolitan Yellow Tuff eruption. Re- Its geological structure was ®rst described by the analysis showed the presence in the Ve- cently, Orsi et al. (1996, 1999) proposed a Rittmann (1948), who interpreted Mount Epo- suvian area of some intense circular anoma- new scheme including a complex structure meo (the highest Ischia mountain) as a volcanic- lies, aligned in the NW-SE direction possibly formed by two nested calderas. Judenherc and tectonic horst uplifted because of magma in- connected to buried vents. Those structures Zollo (2004), however, suggested a single cal- trusion in a shallow magmatic chamber. Orsi were also identi®ed by seismic data (Aiello et dera rim common to both eruptions. et al. (1991) showed the existence of a shallow al., 2002). The gravity map of the Phlegrean area has magmatic chamber and presented a dynamic its main features in a circular gravity low in model of the Mount Epomeo uplift, which The Phlegrean Fields the Bay and a gravity high in the should be the highest part of a block bounded northern Phlegrean Fields, near Parete (Cas- by subvertical faults inclined toward inner. The volcanic district of the Phlegrean Fields sano and La Torre, 1987). In regard to the area From the last eruption in 1301, the only (see Figs. 1B and 2) is located west of the of the Pozzuoli Bay, a boundary analysis of volcanic activity in the island is fumarolic and town of Naples and is characterized by a num- both gravity and aeromagnetic data on land hydrothermal. High temperatures of 106 ЊC ber of small volcanoes. The oldest outcrop- and coastal areas (Florio et al., 1999) showed have been measured at a depth of 15 m (Bar- ping strata of the area are pyroclastics and a curved structure, which was interpreted as beri et al., 1979). lava domes from 50 ka (Cassignol and Gillot, the border of the Phlegrean Field Caldera (see A gravity and magnetic study of Ischia Is- 1982), while the main products are pyroclas- Fig. 2). These borders are less clear in the land (Nunziata and Rapolla, 1987) de®nes tics with a variable composition from trachy- magnetic map than in the gravity map. In par- shallow structures of pyroclastic nature with basalts to phonolitic alkali-trachytes (e.g., ticular, the limits of the caldera correspond to local domes and lava ¯ows of high density Rosi and Sbrana, 1987). Some lava ¯ows and a ring of magnetic minima, very likely due to and susceptibility. The Ischia basement was

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Figure 3. Different data sets used in integration and contouring of the relative magnetic ®elds. Data sets A, B, C, D, and F have the same color scale. See text for details about the data sets.

interpreted as a horst elongated in the E-W Data set A. The helicopter-borne survey in monitor the external ®eld activity during the direction. the Vesuvian area (A in Fig. 3) was carried ¯ights and a global positioning system (GPS)± out in 1999 (Supper et al., 2001; Paoletti et reference station used for the differential cor- SURVEY LAYOUTS AND DATA al., 2005). The ¯ight lines, with a N-S azi- rection of satellite data. The ¯ight section con- PREPROCESSING muth, were spaced ϳ600 m apart. The cross- sisted of: (1) a cesium magnetometer having track tie lines were spaced ϳ2 km apart. The a precision of 0.01 nT, which was contained Logistical Characteristics of the Surveys sample spacing along each ¯ight line was in a ``bird'' ¯own 30 m below the helicopter, ϳ4m. (2) a GPS sensor for the horizontal positioning The integrated map of the Neapolitan re- The survey was ¯own at a constant terrain of the helicopter, having a precision of Ϯ 1m gion was obtained by merging ®ve different clearance of ϳ200 m. The instrumentation after the differential correction, (3) a laser- magnetic data sets. Four of them are from used for the survey was supplied by the Geo- altimeter for the vertical positioning of the he- aeromagnetic surveys and one is from a ma- logical Survey of Austria and consisted of a licopter, and (4) a computer for data rine survey (Fig. 3). They have the following ground and a ¯ight section. The ground sec- acquisition. characteristics: tion contained two magnetometers used to Data set B. The magnetic data in the Bay

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of Naples (B in Fig. 3) were acquired during which consisted in the removal of wrong co- to a land survey and therefore was character- a Consiglio Nazionale delle Ricerche (CNR)± ordinates and double records, differential cor- ized by different altitudes. The data sets in- Istituto per l'Ambiente Marino Costiero rection of the GPS data, and check of the tegration was performed through several pro- (IAMC) oceanographic survey performed in ¯ight altitude; (3) Earth's magnetic ®eld di- cedures including continuation between 2000 onboard the R/V Urania (Marsella et al., urnal variation corrections, which were per- general surfaces. More speci®cally, the inte- 2002; Secomandi et al., 2005). The survey formed using the local base station and, in gration of the different data sets was made consisted of 32 survey lines, which were 400 some cases, the data from the magnetic ob- through the following steps: m apart and trended northwest-southeast, and servatory of L'Aquila, Italy; (4) removal of the ● Downward and upward continuations of 20 tie lines, which trended northeast-southwest International Geomagnetic Reference Field data sets B, C, D, and E from ¯at surfaces, and were 800 m apart. The sampling time was (IGRF), performed using the Italian Geomag- corresponding to the ¯ight altitudes or to the 3 seconds. netic Reference Field updated for 2000 (De sea surface, to an arbitrary surface parallel to Acquisition was made by the EG&G Geo- Santis et al., 2003); (5) statistical leveling, the topography and/or to the bathymetry of metrics proton magnetometer G-811 with an consisting in a minimization of the differences the area. For data set F, an upward continua- instrumental resolution of 0.5 nT. The mea- between the ®eld values measured at the tion between arbitrary surfaces was per- sured data were integrated with the magnetic crossing points between ¯ight lines and tie formed. This led to draped data sets all placed data acquired in 1998 during oceanographic lines; and (6) decorrugation, a directional ®l- at a distance of 200 m from the ground. The cruise GMS98±01 to ®ll the data gap in the tering to allow the removal of the directional continuation between general surfaces was southwest area of the bay. Siniscalchi et al. anomalies still present along the ¯ight lines. performed by an algorithm based on the Con- (2002) describe other data acquisition details. The Ischia Island land magnetic data were pre- tinuous Wavelet Transform (CWT) (Ridsmill- Data set C. The aeromagnetic data set in processed following the steps 1, 3, and 4. Smith, 2000). the Pozzuoli Bay (C in Fig. 3) was measured For the marine survey, the preprocessing ● Determination of the mismatch between by AGIP in 1985 along N-S ¯ight lines ϳ250 phase included the same ®rst four steps of the data values of the data sets across the bound- m apart and at an altitude of 700 m above sea aeromagnetic data treatment, while the level- ary of the different surveyed areas; this mis- level (masl). More details about this survey ing procedure was performed in a different match can be due to imprecision of the ®eld are quoted in Ente Nazionale Idrocarburi way. Aeromagnetic surveys are generally pro- around the boundaries, to inaccuracy in the (ENI) (1985). grammed with only a few tie lines and, after compilation of data for contouring, and to re- Data set D. This helicopter-borne survey in the leveling corrections, only the survey lines moval of inaccurate IGRF around the edges of the northern Phlegrean area (D in Fig. 3) was are used to obtain the magnetic map. In ma- an area (Bhattacharyya et al., 1979). carried out in 1999 and 2001 at ϳ70 m above rine surveys, however, a great number of both ● Removal of the mismatch between the two ground level (Paoletti et al., 2004). The ¯ight survey lines and tie lines may be available as data sets; this mismatch is a function of the lines (having a W-E azimuth) were spaced 400 the magnetic survey is often performed to- spatial coordinates and may be represented by m apart, while the tie-lines (having a N-S az- gether with other kinds of surveys (e.g., seis- a quadratic surface matching the ®elds on both imuth) were ϳ2.5 km apart. The instrumen- mic) that need many lines in both directions. sides of the boundary (Bhattacharyya et al., tation used for this survey was also supplied This is the case of the marine survey in the 1979). In the speci®c case presented here, we by the Geological Survey of Austria and con- Bay of Naples, where a predominance of sur- used a polynomial surface of zero order. sisted of the same sections used for the ac- vey lines exists in some areas, while in other ● Merging of the separate grids into one grid quisition of data set A. areas the tie lines are predominant and it may performed using Kriging and an interval con- Data set E. This aeromagnetic data set in be dif®cult to choose a single data set to ob- sistent with all the different data sets (200 m). the area north of Vesuvius (E in Fig. 3) was tain a map without losing useful data. There- measured in the frame of a regional survey fore, in order to improve the signi®cance of THE NEW INTEGRATED DRAPED carried out by AGIP in 1981. The survey was our magnetic data, we used both type of lines MAGNETIC MAP OF THE performed at a constant altitude of 1460 masl by ®rst performing a reciprocal leveling and NEAPOLITAN VOLCANIC REGION with N-S survey lines 2 km apart. More de- then by charting the data in order to consider tails about this survey are quoted in AGIP all of the lines measured. The integration of the different data sets and (1981). their continuation between arbitrary surfaces Data set F. The data set of Ischia Island (F INTEGRATION OF THE DIFFERENT led to a new high-resolution draped magnetic in Fig. 3) was acquired during a land survey DATA SETS map of the whole volcanic Neapolitan region. by the Department of Geophysics and Volca- The obtained magnetic map was then pole re- nology of the University of Naples Federico The integrated map was obtained by merg- duced (I ϭ 60Њ,Dϭ 0Њ) and overlaid to the II. The total magnetic ®eld intensity was mea- ing the high-resolution data sets relative to the topography and bathymetry of the area (Fig. sured in 725 randomly located stations. For new surveys (in the Somma-Vesuvius area, in 4A). Please note that the map is draped, i.e., more details about the survey see Nunziata the Bay of Naples, and in the northern Phle- relative to different heights above and below and Rapolla (1987). grean Fields) with previous data sets (in Ischia sea level, with a constant clearance of 200 m. Island, in the Pozzuoli Bay, and north of the This should minimize the effect of magnetized Data Preprocessing Vesuvian area). The data sets were partially terrain on the data (Bhattacharyya and Chan, overlapping and characterized by different de- 1977). The NE section of the map lacks detail The preprocessing relative to the aeromag- tails and altitudes. Furthermore, while data because it has been obtained from a regional netic magnetic data included the following sets B, C, D, and E were measured at constant data set (data set E in Fig. 3). In Figure 4B steps: (1) removal of spikes and gaps in the altitudes, data set A was characterized by a the data are overlaid by the outcropping vol- data; (2) ¯ight path check and repositioning, draped acquisition and data set F was relative canic rocks of the area in order to better eval-

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Figure 4. Map of the reduced-to-the-pole (rtp) integrated magnetic data set of the whole volcanic Neapolitan region. A: The draped data (clearance 200 m) are overlain on the topography and bathymetry of the area. See text for de®nitions. B: The rtp map overlaid by a sketch map of the outcropping volcanic rocks (Bonardi et al., 1988) and by the main mapped faults (Bruno et al., 1998, 2003; Milia and Torrente, 1999; Nunziata and Rapolla, 1987) of the area. 41 PLÐ 41st-N-magnetic parallel lineament; MSFÐMagnaghi- Sebeto fault; VFÐVesuvius fault.

uate the correlation between anomalies and gent part the volcano, below the Mount Ve- that anomalies F and G do not have any to- sources and locate volcanic rocks beneath suvius cone. This evidence seems to indicate pographic evidence and coincide fairly well cover units (Finn and Morgan, 2002). The the presence of igneous rocks plugging the with mapped faults. As regards the reversed comparison shows, indeed, that some outcrop- volcanic conduit. Magnetized rocks appear to anomalies of F, a possible pre-ignimbritic or- ping volcanics of the area, such as pyroclastic extend down to a depth of ϳ2 km bsl, while igin of their sources was hypothesized by Pa- fall units, do not have a distinctive magnetic magnetization values below this level are oletti et al. (2005). The authors interpreted signature. We notice, on the contrary, that a negligible. those anomalies as very likely connected to few anomalies are placed above sedimentary The area surrounding the edi®ce is charac- reversely magnetized rocks originated from units. terized by several high-frequency anomalies the activity of small local vents during one of The analysis of Figure 4A allows us to note (E, F, and G in Fig. 4A). A study of the mag- the polarity excursions recorded in the Brun- netic ®eld in the Vesuvian area (Paoletti et al., that all of the main anomalies of the area, ex- hes epoch. The analysis of the power spectra cept the anomalies marked with F in Figure 2004) showed that anomalies E and F, whose of the F anomalies resulted in a depth-to-the- 4A, seem connected to a positive contrast of amplitude is ϳ400 nT, may be due to the su- top-of-the-sources measurement of ϳ210±230 magnetization. The most remarkable feature of perimposition of antropic and geological mbsl (Paoletti, 2002). the map is the anomaly related to the Somma- sources. More speci®cally, they seem partly With regard to the anomalies in the Vesu- Vesuvius complex (anomaly D in Fig. 4A). connected to the presence of railway lines and This anomaly, whose amplitude is ϳ3100 nT, partly related to buried geological sources, vius offshore (G in Fig. 4A), they have an ϳ aligns in the direction of the inducing ®eld and which were already identi®ed by gravity amplitude of 830 nT, and Secomandi et al. is characterized by a roughly elliptical shape methods (Florio et al., 1999; Fedi et al., (2005) showed that they correspond to buried elongated toward the southeast. A three- 2005b). The correlation between the anoma- dome shaped bodies singled out by seismic dimensional magnetic model of the Somma- lies and the outcropping volcanics of the area studies (Aiello et al., 2002). These are inter- Vesuvius edi®ce (Paoletti, 2002; Fedi et al., (Fig. 4B) shows that these anomalies are in preted as buried vents, possibly of historical 2002) reveals the presence of a heteroge- correspondence with pyroclastic fall units and/ age, as suggested by the warping of the thin neously magnetized edi®ce, whose highest or alluvial units, and this let us hypothesize late Pleistocene±Holocene sedimentary cover magnetizations (of 6±7 A/m) are in the emer- that their sources are buried. We also noticed (Aiello et al., 2002). The analysis of Figure

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Figure 5. Three-dimensional inversion of the two main anomalies measured in the northern Phlegrean Fields, marked with A and B in Figure 4A. A: Close-up of the reduced-to-the-pole anomalies in the Parete area. B: Magnetization model obtained from inversion. See text for details.

4B shows that these sources align along a fault align in the Magnaghi Canyon. In particular, are characterized by the presence of large mapped by Milia and Torrente (1999). L and P are located on the canyon axis, while anomalies both in the southern and northern The magnetic ®eld in the Bay of Naples is I and O are on the canyon border. Phlegrean Fields. The southern sector is char- characterized by the presence of anomalies A three-dimensional inversion of anomalies I acterized by a curved belt of anomalies (C in both in the NW and NE sectors of the bay, and P (Secomandi et al., 2005) shows the pres- Fig. 4A), connected to the Phlegrean Field while the central area seems magnetically qui- ence of two body sources whose depth to the Caldera, that follow the Astroni volcano and et. The anomalies in the NW area of the bay bottom ranges from 0.5 to 1.5 km bsf and whose Torregaveta relief inland and do not corre- correspond to several volcanic edi®ces. More magnetization of 0.8 A/m is consistent with the spond to any bathymetric high in the Pozzuoli speci®cally, the H anomalies have an ampli- magnetization of the tuffs of the area (Cassano Bay. The Astroni and Torregaveta anomalies tude of ϳ600 nT and are related to volcanic and La Torre, 1987). Those sources were inter- have amplitudes ϳ250 nT and ϳ300 nT, re- banks, which have an age of 37±14 ka (Orsi preted by the authors as being related to buried spectively, and are placed above pyroclastic et al., 1996). The analysis of Figure 4B shows vents, which were possibly activated along the ¯ows (Fig. 4B). The lack of anomalies in the that no mapped fault cut these banks. Magnaghi Canyon. area of Baia could be due to thermal alteration The I, L, R, and Q anomalies correspond to The area of Dohrn Canyon is not charac- related to the high temperatures measured in volcanic edi®ces, and except for L, which is terized by any magnetic anomaly. The lack of this sector of the bay (Rapolla et al., 1989). buried, are located along the Magnaghi-Sebeto anomaly in the southeastern sector of the bay The offshore anomalies partially correspond to fault. is due to the presence of sedimentary units known faults and have an amplitude of ϳ200 Anomalies I, L (with an amplitude of (Secomandi et al., 2005). nT. ϳ150), O, and P (with an amplitude of ϳ250) The data in the Phlegrean sector of the map The ®eld measured in the Ischia area is

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Figure 6. A: Map of horizontal derivative of the reduced-to-the-pole draped data set of the Neapolitan region. See text for de®nitions. B: Same map as in A overlain with the main lineaments from seismic studies (in red) (Bruno et al., 1998, 2003; Milia and Torrente, 1999), the outline of Phlegrean caldera from gravimetric horizontal-gradient-maxima (in white) (Florio et al., 1999), and the Ischia faults (Nunziata and Rapolla, 1987). The axes of the two canyons of the Bay of Naples are in yellow. 41 PLÐ 41st-N-magnetic parallel lineament; MSFÐMagnaghi-Sebeto fault; VFÐVesuvius fault.

characterized by the presence of four main composition start from a depth of ϳ1200 m. ANALYSIS OF THE MERGED DATA anomalies (N in Fig. 4A). They have ampli- Figure 5A shows a close-up of the magnetic SET tudes of ϳ300 nT and are located above al- anomalies considered for the inversion. We kali-trachytic domes and lava ¯ows with a used a nonparametric discretization of the in- The study and identi®cation of the lateral magnetization of ϳ2.3 A/m (Nunziata and Ra- verse problem and assumed a source volume boundaries of the magnetic sources of the re- polla, 1987). While the anomaly in the south- of speci®ed depth and horizontal extent, in gion were carried out by computing the max- eastern area is crossed by a NE-SW fault, the which the solution is piecewise constant with- imum horizontal gradient of the reduced-to- one in the southwestern edge of the island is in a three-dimensional grid of prisms (see, the-pole (rtp) draped data (Fig. 6A). Cordell bounded by a NW-SE fault (Fig. 4B). e.g., Fedi et al., 2005a). The discretization and Grauch (1985) showed that the maxima In the northern sector of the Phlegrean used for the inversion is composed of 13 ϫ of the horizontal derivative of gravity or rtp Fields, we noticed two anomalies in the areas 13 ϫ 13 prisms in the x, y, and z directions, magnetic anomalies are located above changes of Patria Lake and Parete (A and B in Figure respectively, while the dimension of the of density or magnetization. Since the ϳ 4A, with amplitudes 300 nT and 250 nT, prisms is 1400 m in the x and y direction and horizontal-gradient method assumes that the respectively) and two small anomalies in the boundaries are single, near-vertical, and sharp, 250 m in the z direction. Volturno River Plain (S and T in Figure 4A, the location of the gradient maximum can be The solution (Fig. 5B) shows the presence with amplitudes of ϳ50±100 nT). They are all offset from the boundary when the contact is of two body sources, a bigger one relative to buried and placed in correspondence with not vertical or when several boundaries are the anomaly measured near the Patria Lake (A pyroclastic-fall and alluvial units (see Fig. close together (Grauch and Cordell, 1987). in Fig. 4A) and a smaller one corresponding 4B). The amount of this offset depends on the In order to study the geometric and mag- to the anomaly measured above Parete (B in depth of the top edge of the boundary below netization characteristics of the two main vol- Fig. 4A). The depth-to-top of these sources the observation and on the dip of the ϳ canic sources of the northern Phlegrean sector, ( 500 m) is compatible with the characteris- boundary. we carried out a three-dimensional inversion tics of the basaltic and andesitic lavas found In Figure 6B, the structures mapped by seis- of the magnetic anomalies measured in the Pa- in the Parete 2 well, as well as their magne- mic studies (Bruno et al., 1998, 2003; Milia tria Lake and Parete areas, which are charac- tization of ϳ3.5 A/m (Carmichael, 1989). The and Torrente, 1999) and the Phlegrean caldera terized by a remarkable thickness of volcanic inversion results obtained below ϳ2500 m located by a boundary analysis on gravimetric rocks. About 1.5 km of basaltic and andesitic cannot be trusted because of the loss of reso- data (Florio et al., 1999) are overlaid on the lavas, starting from a depth of ϳ300 m, were lution with depth. The widths of these bodies horizontal gradient maxima (hgm) map in or- found in the Parete 2 well (P2 in Fig. 2) (Baldi are ®nally comparable with the lateral dimen- der to study the correlation between magnetic et al., 1976; Barbieri et al., 1976). More spe- sions shown by the horizontal gradient map structures and known faults and calderas. ci®cally, the lavas with a prevailing basaltic anomalies (see following section). In the Vesuvian region the map of the hor-

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Figure 7. A: Comparison between the horizontal-gradient-maxima map and the location of vents of different ages (Orsi et al., 1996) in the southern Phlegrean Fields. B: Main structures identi®ed in this work: the buried magnetic sources of volcanic origin are in white, the magnetic sources of volcanic origin characterized by surface evidence are in pink, and dashed yellow lines separate the magnetized volcanic areas of the Bay of Naples from the magnetically quiet sedimentary sector. See text for de®nitions.

izontal gradient maxima shows that the Somma- H, O, M, and P are bounded by these faults. grean caldera is clearly shown by the hgm Vesuvius is placed at the intersection of dif- This evidence suggests that, similarly to the map, its northern borders are not clearly iden- ferent systems of faults and shows the pattern fault in the Vesuvius offshore, the Magnaghi- ti®ed from the analysis of magnetic data and of the Mount Somma caldera (D in Fig. 6A), Sebeto fault is a preferential way of magma are not characterized by distinctive topograph- whose southern sector lacks surface evidence. upwelling. The lack of magnetic structures be- ic evidence. This lack of magnetic signature This structure corresponds fairly well to the tween the R structure and the Sebeto fault is likely due to the presence of hydrothermally southern caldera rim of Mount Somma in- seems due to the low amount of magnetic data altered rocks at the caldera ¯anks (Florio et ferred by Rosi et al. (1987). In the area sur- in this area of the bay. al., 1999). rounding the edi®ce, the map points out the The presence of the magnetic structures O In Ischia Island, the main signatures on the lateral boundaries of several small sources and P in the trilobate head of the Magnaghi hgm map (N in Fig. 6A) are an elongated whose pattern is quite complex. The compar- Canyon lets us hypothesize the presence of a structure in the northwestern edge of the is- ison between these structures and the mapped lineament of magma upwelling not only in the land, located on a lava ¯ow, and three quasi- faults (Fig. 6B) shows that some of them (E, southwestern portion of the canyon, but also circular bodies, which correspond to small F, and G in Fig. 6A), not characterized by any in its northeastern part. craters (Nunziata and Rapolla, 1987). topographic or bathymetric high, align along In the southeastern area of the bay, the In the northern Phlegrean Fields area, the faults located by seismic studies. These faults mapped faults do not coincide with any mag- map highlights a subcircular structure, with a can therefore be interpreted as lineaments netic boundaries. The Dohrn Canyon is, in- diameter of ϳ10 km, in the Patria Lake area along which the magmatic activity developed. deed, not characterized by magnetic activity, In the Bay of Naples, the map shows the and the faults in this area cut sedimentary (A in Fig. 6A) and a number of complex struc- boundaries of the main volcanic structures and rocks, as also observed by Bruno et al. (2003). tures in the Parete area (B in Fig. 6A). These calderas of the bay. These structures are only In regards to the Pozzuoli Bay, the faults structures are related to buried sources, like placed in the northwestern part of the bay and located by seismic studies partly follow the those shown by the inversion of the magnetic in most cases correspond to bathymetric magnetic structure, marked with C, that high- ®eld (see previous section), and seem aligned highs. Their comparison with the mapped lights the southern rim of the Phlegrean Cal- along an E-W trend that may be viewed as an faults shows that some of the magnetic sourc- dera. This magnetic structure doesn't have eastward prolongation of the so-called Tyrrhe- es coincide with the lineaments located by bathymetric evidences and coincides well with nian 41st-N-parallel magnetic lineament (Lav- seismic studies. More speci®cally, while the the structure located by gravity data (Florio et ecchia, 1988). Finally, the presence of buried magnetic structures I, L, Q, and R are crossed al., 1999) (see Fig. 6B). It is worthwhile to sources in the Volturno River Plain (S and T by the Magnaghi-Sebeto fault, the structures note that while the southern rim of the Phle- and other small structures in Fig. 6A) suggests

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a diffused volcanic activity all over the Cam- some hypotheses about the evolution of the Toro, B., 1976, Geothermal research in western Cam- pania (Southern Italy): A revised interpretation of the panian Plain. area. -Parete structure: Athens, Proceedings of the The presence of several magnetic struc- International Congress on Thermal Waters, Geother- tures, interpreted as lava and/or pyroclastic mal Energy, and Volcanology of the Mediterranean DISCUSSION AND CONCLUSIONS Area. domes and local vents, aligned along known Barberi, F., Innocenti, F., Luongo, G., Nunziata, C., and faults suggests that these faults are preferential Rapolla, A., Rivvi., A., and Scandone, P., 1979, Anal- The integration of different remote-sensing ysis and synthesis of geological, geophysical and vol- ways of magma upwelling and predate the magnetic data sets measured in the active vol- canic data about the Neapolitan area and its geother- volcanism of the region. This evidence con- mal potential: European Commission, Report EUR canic Neapolitan region led to a new high- ®rms that the Neapolitan volcanism in the Bay 6386, General Direction for Research, Science and resolution, draped magnetic data set of the re- Education, p. 1±39. of Naples is controlled by both a pre- gion, allowing an overall view of this volcanic Barbieri, M., Di Girolamo, P., Locardi, E., Lombardi, G., Pleistocenic NE-SW± and a more recent Stanzione, D., and Nicoletti, M., 1976, Geothermal area (Figs. 4 and 6). NW-SE±regional-stress regime, as also ob- research in western (Italy): Stratigraphy of The study of the rtp and hgm magnetic the Parete exploratory well and new data on the vol- served by Judenherc and Zollo (2004). canic sequence: Athens, Proceedings of the Interna- maps provided insights into the characteriza- The lack of correspondence between mag- tional Congress On Thermal Waters, Geothermal En- tion of the main, buried, and outcropping vol- netic boundaries and faults in the southeastern ergy, and Volcanism of the Mediterranean Area. canic structures of the area. Belkin, H.E., and De Vivo, B.D., 1993, Fluids inclusion sector of the Bay of Naples suggests that the studies of ejected nodules from plinian eruptions of With regard to the Phlegrean Caldera, our lineaments bounding the magnetized areas Mt. Somma-Vesuvius: Journal of Volcanology and analysis led to the identi®cation of its southern Geothermal Research, v. 58, p. 89±100, doi: 10.1016/ (Fig. 7B) also limit the volcanic Neapolitan 0377-0273(93)90103-X. rim, while the northern boundary was not district from an area characterized by sedi- Bernasconi, A., Bruni, P., Gorla, L., Principe, C., and clearly located. For a more comprehensive in- mentary units and by lack of volcanic activity. Sbrana, A., 1981, Risultati preliminari vestigation of the Phlegrean caldera area, we dell'esplorazione geotermica profonda nell'area vul- The analysis of Figure 7B, which summa- canica del Somma-Vesuvio: Rendiconti SocietaÁ Geo- compared the magnetic structures with the lo- rizes the results of our study, allows the lo- logica Italiana, v. 4, p. 237±240. cation of volcanic vents of different ages (Orsi Bhattacharyya, B.K., and Chan, K.C., 1977, Reduction of cation of the lateral boundaries of buried and magnetic and gravity data on an arbitrary surface ac- et al., 1996) (Fig. 7A). This comparison shows diffused volcanic structures, suggesting the quired in a region of high topographic relief: Geo- that the vents older than 12 ka, i.e., pre± presence of volcanic ®elds not only in the physics, v. 42, p. 1411±1430, doi: 10.1190/ Yellow Tuff, do not seem characterized by any 1.1440802. Phlegrean area, but also in the Vesuvian area. Bhattacharyya, B.K., Sweeney, R.E., and Godson, R.H., magnetic signature except for the vents in the 1979, Integration of aeromagnetic data acquired at dif- Torregaveta and Pozzuoli offshore areas. This ACKNOWLEDGMENTS ferent times with varying elevations and line spacings: can be due to the difference in their products Geophysics, v. 44, p. 742±752, doi: 10.1190/ 1.1440974. that are mainly pyroclastics, except for the The authors thank Carol A. Finn for her construc- Bonardi, G., d'Argenio, B., and Perrone, V., 1988, Carta Torregaveta lava ¯ows. The correspondence tive help in improving the manuscript. The authors geologica dell'Appennino Meridionale: 74th Congres- acknowledge the support of the Istituto Nazionale so della SocietaÁ Geologica d'Italia, , Septem- between these lava ¯ows and the magnetic di Geo®sica e VulcanologiaÐOsservatorio Vesuvi- ber 13±17 1988, Dipartimento di Scienze della Terra, anomalies suggests that similar products can ano grants to A. Rapolla and to R. Pece (Programma Naples, Consiglio Nazionale delle Ricerche, Rome, be found in the Pozzuoli offshore area. As re- Quadro 2000±2002). The authors also acknowledge scale 1:250,000, 1 sheet. Bruno, P.P., Cippitelli, G., and Rapolla, A., 1998, Seismic gards the vents younger than 12 ka, they have I. Giori (Agenzia Generale Italiana PetroliÐEnte Nazionale Idrocarburi), for the availability of the study of the mesozoic carbonate basement around Mt. a clear magnetic signature except in the north- Phlegrean aeromagnetic data set from 1985, and E. Somma-Vesuvius volcanic complex (Italy): Journal of western sector of the area. 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