The Vercelli seamount is a submarine granitic intrusion of Miocene age (Barbieri et al., 1986) localized in the central portion of the Tyrrhenian Sea back arc basin. The seamount was interpreted as a granitic intrusion correlated to the initial immature stage of the Miocene volcanism which affected the northern Tyrrhenian Sea (Barbieri et al., 1986). The evolution of the Tyrrhenian back arc basin and in general the whole western Mediterranean Sea is related to the southeastward rollback of the Ionian Slab (Malinverno and Ryan, 1986, Royden, 1988, Patacca et al., 1990, Gueguen et al., 1997). The Vercelli seamount represents a major N–S arc-shaped structural high placed at the western of the central Tyrrhenian Sea along the 41st parallel zone, known as a major crustal discontinuity of the Tyrrhenian domain (Boccaletti et al., 1990, Spadini and Wezel, 1994, Cocchi et al., 2008).
Despite the tectonic setting of the Tyrrhenian Sea being well outlined in literature, the role held by the 41st parallel discontinuity during the geodynamic evolution of the central portion of the basin is still debated. Although a peculiar alignment of magnetic anomalies along the N41° latitude suggests the presence of an E–W discontinuity, the evidence of a crustal/lithospheric boundary between two Tyrrhenian domains is not fully supported by other geophysical evidences. To this aim, a new research project (Tysec-Prin Miur 2007) to investigate the Vercelli was proposed in 2007. The main purpose of this scientific proposal was to gather biological, geophysical, oceanographic features of the Vercelli seamount which represents a reference example in order to understand how this kind of structure can play a leading role in the deep sea environment. In this study, we present new high resolution ship-borne magnetic and swath bathymetry data over the Vercelli seamount and surrounding areas acquired during the recent VER2010 oceanographic cruise onboard R/V Aretusa, a hydro-oceanographic catamaran-shaped vessel of the Italian Navy. Analysis of newly acquired geophysical data allows the outlining of a new geomorphologic model of the seamount, mainly based on interplay between the local tectonic features and erosive processes affecting the whole structure.
Depth estimating analysis and forward modelling interpretation of the magnetic anomaly field of Vercelli and neighbouring basins highlights the presence of shallow magnetic causative sources not directly correlated to the seamount structure, but rather to post-Miocene intra-sedimentary lava sequences. The modelling of the Vercelli region can be adopted to interpret other magnetic anomalies along the 41st parallel zone, questioning thus the role of this magnetic lineament as chief evidence of a deep crustal bounda
The Tyrrhenian Sea is a back arc basin with triangular shape resulting from a simultaneous E–SE extension and shortening in the Apennine chain (Malinverno and Ryan, 1986, Royden, 1988, Faccenna et al., 2001). Since the Oligocene, the eastward retreating of the Africa–European subduction system has triggered a stretching of upper European crust evolving in step with back arc formations occurring first in the Valencia area, Provencal Basin and then in the Tyrrhenian Sea (Doglioni, 1991, Faccenna et al., 1997, Gueguen et al., 1997, Gueguen et al., 1998, Doglioni et al., 1999, Doglioni et al., 2004; Chiarabba et al., 2008; Argnani, 2009, Carminati et al., 2012).
Major tectonic differences are observed between the southern Tyrrhenian domain and the northern one, mainly because of a heterogeneous amount of subduction and related slab retreat along the belt (Doglioni et al., 1994, Faccenna et al., 1997). Lower Miocene syn-rift deposits of the Corsica and Sardinia margins (Tortonian) mark the initial stage of the Tyrrhenian rifting process. In this stage, the roll back movement triggered an extension of the eastern sector of the Tyrrhenian area with a slow extension rate (1–2 cm/y) forming a set of N–S and NE–SW oriented basins associated to magmatic intrusions and alkaline volcanism (Faccenna et al., 1997, Faccenna et al., 2001, Jolivet et al., 1998 Mauffret et al., 1999). Crustal shortening and basin formation were coeval with magmatism episodes, which first occurred in the Corsica region (Lomproite sills, 13.5 Ma), followed by younger products in central (i.e. Montecristo Isl. 7.3–7.1 Ma) and eastern margins (i.e. Elba Isl. – Monte Capanne, 6.8/6.2 Ma) (Jolivet et al., 1998, Savelli, 2000, Serri et al., 2001).
Subsequently, during the Pliocene, the eastward migration of the rollback shifted from E–W to NW–SE affecting the southern Tyrrhenian Sea (Doglioni et al., 1994), with a high deformation rate triggering an upper continental crust depletion with spurs of new oceanic crust which carpeted the seafloor of the Vavilov (5–5.5 Ma) (Kastens et al., 1990, Marani and Gamberi, 2004) and Marsili basins (2.0 Ma) (Faggioni et al., 1995, Marani and Trua, 2002, Cocchi et al., 2009, Caratori Tontini et al., 2010). A tectonic-structural boundary between the northern Tyrrhenian Sea and the southern basin runs along the 41st parallel zone which is interpreted as a major strike-slip fault system (Spadini and Wezel, 1994, Rosenbaum and Lister, 2004, Cocchi et al., 2008). Around the 41st parallel, the main geochemical and petrological features properly of peri-tyrrhenian margin change, suggesting the presence of a major crustal/lithospheric discontinuity across this E–W structure (Patacca et al., 1990, Serri, 1990, Serri et al., 1993). From a geologic point of view, the 41st parallel zone is characterized by a lineament of several morphological structures extending in an E–W direction for 300 km, connecting the offshore Campania to NW Sardinia. Along the western edge of this sector, close to the Sardinia coast, several magmatic evidences are associated to minor Pleistocene sub-volcanic bodies with tholeitic affinity (Bigi et al., 1990), while quaternary magmatic structures such as Ponza, Palmarola and Zenone are placed at the easternmost edge (Fig. 1). The Plio-Pleistocene genesis of these volcanic islands is generally linked with subduction-related magmatism, while Pleistocenic rhyolites of Palmarola with OIB affinity may be derived by an upwelling of deep asthenospheric material during the Tyrrhenian Sea opening (Cadoux et al., 2005). Geochemistry findings identified the Pontine islands as a transitional area where the mixing of subduction-related magmatism and rifting-related volcanism took place. Moving eastward along the 41st parallel zone, the offshore campanian margin, composed by Ischia and Procida volcanic islands, is mainly characterized by high K content typical of the Campanian Magmatic Province (Serri, 1990).
The 41st parallel zone shows a complicated structural geologic setting where the effects of different geodynamic processes were combined. This evidence is well described by the deep-crustal seismic soundings along the lines M2A and M29B of the CROP project (Scrocca et al., 2003). Interpretation of seismic lines suggests the presence of a deep fault zone which played an active role during the Tortonian eastward migration of Sardinia continental margin. Despite several authors correlating this lineament with a main E–W mega shear-zone (Boccaletti et al., 1990, Lavecchia, 1988) due to an asymmetrical extension of the Tyrrhenian Sea, the 41st parallel line does not show particular E–W elongated morphostructural features, or any evidence of continental block rotation. Spadini and Wezel (1994) instead connected the magnetic and free air gravity lineaments of the 41st parallel line as an effect of inverse tectonics as linked to a regime of intraplate connection. Patacca et al. (1990) described this area as a lithospheric boundary across which the average slab retreat drastically changed. From the geophysical point of view, the central Tyrrhenian area shows an E–W lineament of several isolated magnetic anomalies that seem to be directly caused by the presence of main structural highs (i.e. seamounts) having high magnetization. A large depiction of these features is also pointed out by the distribution of regional anomaly field in distinct aeromagnetic anomaly maps of Italy (Chiappini et al., 2000; Caratori Tontini et al., 2004). The magnetic lineation represents just the direct geophysical evidence of a possible E–W structural boundary along the 41st parallel. In this context the detailed description and interpretation of the Vercelli seamount and its close surrounding basin may improve the knowledge of this intriguing but also controversial area.
The Vercelli Seamount
Vercelli is a seamount located in the central Tyrrhenian Sea, at geographical coordinates 41°06′36″N and 10°54′10″E about 110 km west of Sardinian east coast (Fig. 1). The edifice is 8 km long and 3.4 km wide, SW–NE oriented with its base at a depth of 1200 m and its top at 58 m b.s.l. (Fig. 2A and B).
The seafloor morphology of the Sardinian margin in the central Tyrrhenian Sea is mainly characterized by a sequence of N–S and NNE–SSE oriented submarine horsts, separated by intervening sedimentary flat basins. Vercelli seamount is one of the largest structural highs of this margin consisting of a sequence of seamounts such as Etruschi, Cialdi and Tiberino (Marani and Gamberi, 2004). The NNE–SSW horst and graben sequence are developed following a main E–W trend approximately along the 41st parallel (Fig. 1). South of this latitude, tectonic features and seafloor morphology change with the occurrence of major passive depositional sedimentary sequences as the flat Cornaglia terrace.
From a geological point of view, the Vercelli seamount is a granitic structure intruded during the late Miocene (Barbieri et al., 1986). The chronological evolution of the magmatic products of the Tyrrhenian Sea follows the eastward retreating of the subduction system. In the northernmost portion of the Tyrrhenian Sea this geodynamic evolution occurred during the Miocene time with the first evidence of calc-alkaline magmatism around Corsica. The eastward flexural migration of the subduction system has produced a sequence of magmatic centers that became younger from west to east. K/Ar analysis of granitic samples from the summit of Vercelli suggests the emplacement of the intrusion occurred about 7.2 Ma. During this period, magmatic episodes and manifestations affected other regions such as Montecristo, Elba and Capraia (Savelli, 2000, Sartori et al., 2004). The average granitic composition of Vercelli mismatches the granodioritic affinity of the other intrusive structures belonging in the northern Tyrrhenian domain (Serri et al., 1993, Savelli, 2000). In particular, the granitic geochemistry of the Vercelli seamount shows a calc-alkaline affinity with high value of K (HK-CA series from Peccerillo and Taylor, 1976) and high anomalies of Ba, Th, U. This peculiar geochemical pattern may indicate a deep magmatic root correlated to the mixing of different parental magmas coming from distinct geodynamic environments. The association of HK-CA affinity and high anomalies of incompatible elements may signify that the granitic upwelling occurred in a post collisional geodynamic scenario during the first stage of Tyrrhenian rifting. Although direct geochemical analysis highlighted the granitic nature of Vercelli, we can not exclude other possible interpretations i.e. a tilted continental block having Variscan-Alpine origin. The west to east distribution of magmatic intrusions, which become younger from Corsica to Elba island, leads us to interpret the Vercelli granites as the result of erosion of an in-situ intrusive manifestation rather than an allochthonous block.
Direct observation of Vercelli’s morphology was achieved through several dives by “Argus” submarine as reported in the pioneering study by Barbieri et al. (1986). The Vercelli edifice can be distinguished in two main portions: the large basal sector and the summit portion (Fig. 2). This latter is represented by a small NE–SW elongated granitic cone, 3 km long, which rises up to the minimum depth of 58 m (Fig. 2B). The crest of seamount is placed over a large flat platform localized at the depth of 200–250 m b.s.l. This flat area is featured by a sedimentary sequence mostly made of incoherent sands having a costal/littoral provenance (Barbieri et al., 1986). The peculiar morphology of the upper portion of the seamount may be related to erosive process triggered by the eustatic sea-level variation during the passage from warm-glacial periods. High amplitude erosive features affecting the flanks of Vercelli can be directly associated to the peculiar hydrodynamic processes present in the area. In fact, the Vercelli seamount is localized between a cyclonic gyro acting in the north portion and an anti-cyclonic one to the south. Different hydrodynamics of two gyros has been recently interpreted as strongly driven by the physiography of the seafloor, in particular by the presence of sharp bathymetry gradients such in the case of Vercelli (Vetrano et al., 2010, Misic et al., 2012).
The magnetic anomaly field of the central Tyrrhenian Sea correlates well with its structural tectonic pattern. In this area, the tectonic movement has driven a low rate extension affecting a thickened crustal wedge (Royden, 1988, Patacca et al., 1990, Faccenna et al., 1997) resulting in an alternation of N–S elongated basins and structural highs, which recalls the tectonic style of the Basin and Range province (Carmignani and Kligfield, 1990; Marani and Gamberi, 2004). Previous magnetic studies in this area were based on aeromagnetic surveys (AGIP, 1981; Cassano, 1984, Caratori Tontini et al., 2004), and highlighted a low amplitude and low-frequency anomalies mainly associated with the presence of thick continental crust. The aeromagnetic anomaly over the Vercelli seamount shows low values ranging from −10 to 25/35 nT (Caratori Tontini et al., 2004), with a maximum positive overlapping most of the morphologic high. This smooth low-frequency magnetic pattern is mostly related to the decrease of magnetic signal due to the high flight altitude. In this context, the magnetic airborne investigation is not fully able to detect the finest crustal features as those related to the small edifice of Vercelli.
Previous paleomagnetic studies (Barbieri et al., 1986) focused on rock samples dredged from the summit area suggest a very low magnetization for the granitic rocks forming the Vercelli (average susceptibility K = 70.4 × 10−6 SI), with a low remnant contribution (Koenigsberger ratio from 1.6 to 7.4) and a dispersion of the magnetization vector directions. Barbieri et al. (1986) questioned the low susceptibility of Vercelli as due to the lacking of Fe-bearing minerals present in the granites, and more speculatively, also related to the effect of incipient hydrothermal alteration.
Materials and methods
In May 2010, the VER-10 cruise onboard R/V Aretusa performed new mapping of the Vercelli seamount (Fig. 1) using dense shipborne magnetic lines and high resolution multibeam coverage. The vessel, a 40 m long and 12.6 m wide fibre-glass catamaran is equipped with differential GPS, single and multibeam bathymetric systems and oceanographic instrumentation (sound velocity profiler and CTD rosette). In addition, high resolution swath bathymetry data were collected on Vercelli’s summit portion using a small hydrojet vessel (10 m long) deployed from the mother ship.
The geophysical survey was performed in collaboration with the personnel of Italian Navy who provided logistic and scientific support during the campaign. The VER10 cruise was conducted within the “TySec – Tyrrhenian Seamounts ecosystems: an Integrated Study” project (2007 PRIN project). The main purpose of this research project was the integration of a set of multi-parametric data such as geologic, biologic, oceanographic information in order to outline a new interpretative model for the Vercelli seamount.
Seafloor morphologic features of the Vercelli edifice and part of the N–S elongated basin were achieved using a Kongsberg EM300 multibeam echosounder (operational frequency of 30 kHz) coupled with a Kongsberg MRU-5 (Motion recording Unit) installed onboard of the main vessel. Navigation was performed using QPS Qinsy and Kongsberg SIS. Swath bathymetry data were geo-referenced using DGPS Fugro 3200LR12 applying OmniSTAR-VBS corrections. The bathymetry acquisition was conducted following, in the first instance, the same line-path of the magnetic survey (black tracks in Fig. 3A) and subsequently providing additional track lines to accomplish full-coverage with a minimum 40% overlap (Fig. 3B). The multibeam calibration was performed before and after the survey execution, while the sound speed profiles were collected every 4 h using an Idronaut Ocean Seven 316 CTD multiparameter probe. The raw multibeam dataset was processed using Caris Hips&Sips software package incorporating motion sensor corrections (pitch, roll and yaw) and sound speed variations in the water column (post-acquisition raytracing using closest-in-time sound speed data). Finally, manual despiking (outliers along the single swath) and statistical filtering were applied to minimize the noise component. The processed data were thus used to create a 50-m resolution Digital Terrain Model (DTM) of the Vercelli seamount and neighbouring abyssal basin (Fig. 2A).
A high resolution investigation of the shallowest portion (58 m b.s.l.) of the seamount was achieved from a dedicated survey (red track lines in Fig. 3A). This investigation was conducted by using a hydrojet survey boat equipped with a 300-kHz Kongsberg EM 3002 MBES, paired with a Seapath 300 as attitude sensor, which is particularly effective for shallow-water application. In this case, the statistical raw data processing was performing CUBE (Combined Uncertainty and Bathymetry Estimator) surface algorithm (Calder and Mayer, 2003). A detailed digital elevation model of Vercelli’s summit was then produced using a 10-m grid cell size (Fig. 2B).
Ship-borne magnetic data were collected using a SeaSpy Marine Magnetics magnetometer towed about 70 m astern of the vessel. Raw magnetic data were sampled at 1 Hz by using the specific SeaLink software suite which also provides a synchronous GPS lay-back-corrected position for the tow-fish.
The layout of the magnetic survey was previously planned knowing the local morphology and also the spectral characteristics about the magnetic anomaly field (low-frequency aeromagnetic map from Caratori Tontini et al., 2004). The magnetic investigation was thus conducted following a set of 17 roughly E–W (heading N20°) parallel track lines and 3 orthogonal control tie lines (Fig. 4A). The resulting dataset counts about more than 125,000 data records collected along 525 linear km.
Total-field magnetic data were processed removing spikes and outliers. Additional heading correction was required in order to minimize the magnetic effect generated by the vessel. Cross-over estimation and then a levelling procedure were applied by means statistical approach. The diurnal correction was performed using data provided by the L’Aquila observatory (belonging to the InterMagnet observatories). Final results were levelled by statistical analysis of cross-over errors. The magnetic anomaly field (total intensity) (Fig. 4B) was calculated by subtracting the IGRF 2010 model (Finlay et al., 2010). The intrinsic dipolar behaviour of magnetic anomaly field has been corrected applying the reduction to the pole (RTP) transformation, based on a data phase shifting using the present day values of inclination and declination of Earth’s magnetic field (derived from IGRF model) (Fig. 9A).
The DTM of the Vercelli region is reported as a shaded relief map in Fig. 2. The granitic edifice shows an ENE–WSW elongation (average heading of N40°) rising from 1432 up to 58 m b.s.l. (Fig. 5, profile E–E′). In the east, it is bounded by a minor seamount structure which extends from 1250 m (approximately the same depth of the Vercelli’s base) up to 495 m showing the similar tectonic trend of Vercelli. The two highs are connected by a structural low which may be interpreted as an intervening downward shifted block trending along a NE–SW direction (the same structural alignment observed for the two seamounts). At the north of the Vercelli edifice, the seafloor morphology changes with the development of an E–W trending flat sedimentary basin located at an average depth of 1200 m (profile A–A′ in Fig. 5). This may be considered an example of an intraslope sedimentary sequence between a set of morphological highs as those observed also between Cialdi and Tiberino seamounts (northern Tyrrhenian domain, Marani and Gamberi, 2004).
The Vercelli seamount shows a slight arcuated shape with a north eastern portion which tends to bend from the main ENE–WSW direction to E–W. In contrast, the neighbouring smaller seamount located at the east of Vercelli shows an opposite bending from the ENE–WSW to NNE–SSW direction. This deformation pattern can be explained by N–S tectonic movement which modelled the entire western sector of Tyrrhenian Sea during the Tortonian stage of the rifting. The Vercelli edifice can be divided in two main sectors: a pseudo elliptic base having a complicated morphologic pattern due to the tectonic and erosive submarine processes, and a small summit portion formed by a sharp crest placed over the semi-flat erosive platform.
3D representation of DTM and bathymetry gradient analysis (Fig. 6A) suggest that the shape of the flanks of Vercelli was modified over time by erosive processes, such as submarine landslides with the displacement of large blocks, as observed at the northern and south-eastern sectors of the edifice (highlighted in Fig. 6B). The eastern/south-eastern sector has steep flanks with an average slope value of 18°, it is distinguished from the opposite side where the morphology follows a gentler profile (average slope of 14°) associated with minor submarine slides and scars. In order to quantify the different morphologic style of the Vercelli seamount, a set of graphs of cumulative surface gradient was computed (Mitchell et al., 2002). The slope gradient vs. depth plots outline the variability of the average steepness around the edifice distinguishing, in a quantitative way, distinct sectors featured by common or uncommon morphology evolution. The Vercelli seamount was thus divided into four sectors considering a main NE–SW structural alignment. Fig. 7 reports the median and inter-quartile range of cumulative surface gradients computed for Vercelli and neighbouring areas.
Considering the results of the analysis of slope gradient vs. depth plots, the different sectors (NE–SE–NW–SW) of the main edifice can be gather in two main groups: sectors A and D (see inset in Fig. 7) that can be associated by common variation of the slope gradient with the depth; and sectors B and C, showing steep gradient for the shallower water area (200–600 m) and a gentler slope (semi-flat distribution) on the deeper portion of the flanks. A common deformation and erosive pattern is also observed on the neighbour structural high close to east of Vercelli, which shows the displacement of large blocks and submarine scars particularly focused (labelled in Fig. 6A and B) on the southern sector.
Also in this case, the southern portion of the submarine edifice shows a steep gradient limited for shallower level of the flank, while the opposite side is featured, as observed at Vercelli, by minor variation of the slope vs. the depth (see profiles in Fig. 6). The structural low located between the two highs (sector E, highlighted in light green in the inset map of Fig. 7) shows cumulative surface gradient distribution which completely differs from those observed in the neighbouring areas. In this sector, the gradient decreases uniformly with the depth and tends to become flatter approaching the sedimentary basin. The morphologic evolution of the two structural highs seems to be correlated by a common erosive-tectonic process which affected the two elements with a similar mechanism and intensity. In contrast, the intervening structural low (sector E, Fig. 7) seems to be a distinct morphologic element having a different geologic nature which could be not correlated to the Miocene magmatic/intrusive manifestation.
The high gradient vertical development of the Vercelli edifice is interrupted by a large and flat upper portion localized at a depth of 200–225 m. This sector was already interpreted by Barbieri et al. (1986) as an erosive platform resulting by the eustatic sea-level changing during the last glacial period. In addition, morphologic profiles along the entire Vercelli structure outline the distinct slope break from the flat erosion platform (1°–5°) to >20° slope flanks (Figure 5, Figure 6A). However, 2D profiling (Fig. 5) does not highlight the presence of any evaluable marine terraces correlated to the sea-level still-stand episodes as those observed in other Tyrrhenian submarine edifices (e.g., Palinuro seamount, Passaro et al., 2011; Volcano island, Romagnoli et al., 2013). In addition, the erosive platform of Vercelli is placed deeper than the estimated sea-level position during the last low-stand (i.e., about 125 m below the present–day sea level, at 19 ka, Bintanja et al., 2005). The lacking of marine terraces and the misfit between the depth location of marine erosive platform and level of the sea during the last warm period could indicate an active tectonic process affecting the entire region during the Quaternary.
The top of the seamount consists of a very sharp ridge which rises up to a minimum depth of 58 m b.s.l. (Fig. 2). This morphologic element represents an erosive relict of the uppermost portion of the granitic intrusion. The geologic nature of the crest was previously investigated by Barbieri et al. (1986), who provided pioneering information about the chemistry and susceptibility signature of rock samples dredged at 41°06′52″N and 10°54′50″E (Fig. 8A).
Analysis of the morphologic gradient (Fig. 8A) reveals the presence of steep flanks with slope gradients comparable with those observed along the entire edifice (maximum slope of about 24°). Joint analysis of the seafloor morphologic features and slope gradient distribution allows an interpretation of the structural setting of Vercelli’s top area as made of two main separate blocks, having both different orientation and distinct fault systems. The ENE–WSW elongated block is associated and partially deformed by S1 (average heading N70°) and S2 (average heading N15°) fault systems; north of this block, a N–S elongated massive rock portion is placed and partially tilted by the S2 fault system (Fig. 8A). The two blocks are aligned along two main pre-existing structural lineaments (faults) and/or preferable pathways along which the granitic intrusion took place. In addition, the two faults/intrusive lineaments S1 and S2 show peculiar alignments which recall the main N–S and ENE–WSW tectonic trends of the entire edifice (Fig. 6). The narrow crest of Vercelli can be interpreted as the innermost portions of the granitic intrusion which remained as relicts of the erosive activity during the last sea-level eustatic low stand. Morphologic profiles along NW–SE and E–W directions outline in a detailed way the bathymetric features of this small sector. Both profiles highlight the presence of two distinct morpho-structural domains: a flat eroded area ranging from 180 to 220 m b.l.s. and the sharp massive granitic blocks. Although on observation of profiles of Fig. 8B these two morphologic elements are clearly distinguishable, the transition from one to another occurs without any intervening erosive features like as marine terraces or gullies. This lack of evidence could be related to the resolution of multibeam investigation which is not able to detect the small morphologic features. On the other hand, the high rate of sedimentation observed over the flat platform (Barbieri et al., 1986) could have blanketed the fine scale erosive features. The flat summit area is covered by incoherent material deriving from the mixing of erosive in-situ granitic products and incoherent sediments (organogenic sands coming from the neighbouring coasts).
The magnetic investigation of the Vercelli seamount and adjacent basins, carried out during the VER10 cruise aimed to study the deep geometry of the granitic intrusion related to the magmatic episodes which affected the entire 41st parallel zone during the Miocene.
The newly acquired high resolution magnetic data provided with unprecedented details a clear overview of magnetic properties of the seafloor improving information already achieved from previous investigations (Caratori Tontini et al., 2004, Cocchi et al., 2008). The area of interest shows a low magnetic anomaly pattern ranging from −60 to 15 nT with a maximum positive value localized in between the two structural highs (Fig. 4A). The total intensity magnetic anomaly dataset was reduced to the pole applying a phase shifting in the FFT domain (Baranov and Naudy, 1964, Blakely, 1995), addressing a direct correlation between the maximum of the anomaly field and the geographical position of the causative sources. The RTP anomaly map suggests a high magnetized body (60 nT, Fig. 9A) localized beside the Vercelli edifice over the intervening low between the two morphologic highs. The RTP map (Fig. 9A) shows a complicated pattern where an N–S trending high frequency anomaly extends north of the Vercelli into the flat sedimentary basin. Quantitative interpretation of the magnetic anomaly field was performed by computing the three dimensional analytic signal (Fig. 9B). This procedure is a combination of the horizontal and vertical gradient of the magnetic anomaly and is useful for enhancing the edges of the magnetic sources (Nabighian, 1984, Roest et al., 1992). The three-dimensional analytic signal results in a bell-shaped distribution having maximum amplitude along the lateral edges of the causative body. Distribution of the analytic signal indicates a NNE–SSW trending distribution of the magnetic causative bodies which are preferentially aligned between the two structural highs extending also close to the flat sedimentary basin. A quantitative analysis of the buried magnetic sources provides an estimation of the average depth location of the causative bodies. The depth distribution of the centroids of the magnetic sources was computed approaching the Euler’s deconvolution technique (Reid et al., 1990, Fitzgerald et al., 2004), based on a least-squares inversion of the Euler’s homogeneity equation to the magnetic anomaly data (Thompson, 1982) using a structural index (SI) equal to 1 (tabular structures, sill and other intrusive features). The structural index is correlated to the geometry of the sources present in the region of interest. In the case of study, several tests were performed using different SI values (from 0 to 3). The optimal SI value was determined to be 1 because it provided the highest clustering of the solutions. The Euler’s deconvolution was applied interactively using fixed-size dynamic windows that covered the entire magnetic anomaly grid. The size of the dynamic windows is a discriminating parameter that can lead to ambiguous results. The windows should be large enough to include the anomalies of interest and, at the same time, avoid the effects of multiple anomalies. In the case of study area, considering the spectral behaviour of the magnetic anomaly field and the spatial dimension of the survey area, the preferred Euler’s solution was achieved using a 1000-m window. The geographical position and depth distribution of the centroids are represented in Fig. 9B using proportional sized symbols. The pattern of distribution of Euler’s solution follows the same pattern of the Analytic signal, but in addition this analysis permits the distinction of sectors of the study area characterized by the same depth distribution of the causative bodies. Following this approach, the southern sector of survey area is characterized by a set of shallow-buried magnetic sources located at 1200–2000 m, which are in contrast with the northern portion where the depth distribution of the Euler’s solution seems to become deeper, reaching the maximum depth of 3700 m b.s.l. This data confirms the presence of buried magnetic sources below the flat sedimentary basin located north of the Vercelli seamount.
Direct information about the crustal setting and the depth distribution of different causative bodies were achieved with a magnetic forward modelling (Fig. 10). The observed data was sampled from the interpolated grid along the key profile A–A′, oriented NE–SW (heading N47°) and crossing the entire area of interest. The geographical position of this profile (track in Fig. 9A) was chosen considering the magnetic anomaly pattern and the distribution of the morpho-structural features. 2.75-D forward model was computed using the analytic algorithm based on methods of Talwani and Heirtzler, 1964, Won and Bevis, 1987 and Rasmussen and Pedersen (1979). The reconstruction of specific crustal features along the profile of interest is based on the best fitting between the observed magnetic anomaly profile and the synthetic signal generated by a defined distribution of causative bodies having shape and susceptibility features directly chosen by the user. In the 2.75 modelling, the geometry of prismatic blocks may be also constrained along the strike directions adding independent extensions (+/− Y axis) and/or defining the relative strike angle (the strike direction of the model can be skewed). The susceptibility value of the granitic rock of Vercelli was derived from direct data published in Barbieri et al. (1986), while the magnetic properties of the other modelled bodies were achieved from literature data (Telford et al., 1990).
The reconstructed geometry in the 2.75 forward modelling (Fig. 10) has been outlined following the depth positions of Euler’s solution (red diamonds) mapped in Fig. 9B. As previously stated, the Euler’s deconvolution allows the identification of the edges and contacting surface between adjacent causative bodies. Towards this point, the Euler’s solutions belonging along the profile A–A′ were used in order to calculate slopes and dipping of the contacts between different bodies. The introduction of additional information obtained through the Euler’s deconvolution tends to decrease the free degrees of the forward magnetic modelling procedure, justifying better the effectiveness of its results.
In addition, the accuracy of magnetic modelling was improved, defining the adequate elongation in the strike direction for every prisms forming the model. Unfortunately the interpretative model was not supported by additional information about the deep geological and structural setting because the lacking of any available local seismic sounding or other crustal imaging.
The new high resolution geophysical investigation of Vercelli area allows an improvement of the interpretative model about the deep crustal setting of the granitic seamount. The results of modelling and interpretation of this dataset can be useful to understand the main peculiar features of this small sector of the Tyrrhenian Sea, notably the relationship between seafloor morphology and manifestations of Miocene and post-Miocene magmatism. New insights carried out from the geophysical modelling represent a key interpretative element of the entire crustal discontinuity of the 41st parallel zone, one of the major tectonic elements of the Central Tyrrhenian region, whose evolution is not completely understood yet.
Interpretation of geophysical features of Vercelli seamount
2.75-D forward modelling (Fig. 10) shows the geometry, depth location and susceptibility signature of the magnetic causative bodies responsible for the anomalous variation of the magnetic field observed in the area of interest (Fig. 4B). The forward modelling was focused on an E–W profile crossing the Vercelli seamount and intersecting the flat sedimentary area. Along this profile the magnetic anomaly ranges from −120 to −10 nT with a mean value of −45 nT. The long wavelength and low amplitude magnetic pattern addresses the presence of a deep low susceptibility geologic basement, mostly formed by metamorphic-crystalline units (Finetti and Del Ben, 2005). The geometry of the Vercelli seamount was reconstructed considering a large granitic body, about 8.5 km long with a depth extension of about 1.5 km. In concomitance of this structure the magnetic anomaly field shows a negative semi-flat pattern having an average value of −65 nT, which has been modelled considering the susceptibility value obtained from direct measurement of in-situ dredged rocks (Barbieri et al., 1986). In the east of the seamount the pattern of magnetic anomaly changed following a local relative positive, with peaks having maximum value of +15 nT. This magnetic pattern was modelled by a N–S elongated high susceptibility body (2 × 10−2 SI) interpretable as a 1 km-thick layer of volcanic products (the susceptibility value is referable to Fe-rich lavas). This high magnetized body extends westward for 8 km dipping up to a maximum depth of 3.1 km just below the sedimentary sequence and intervening acid intrusive bodies forming the eastern portion of the Vercelli region. The overall misfit between observed data and synthetic model ranges from −12 to +8 nT with a mean value of −0.11 nT and a standard deviation of 6.35 nT. A better fit could be achieved but it would need a set of scattered small sources providing a more complicate model far from the real local geological setting.
The pattern of analytic signal and the distribution of Euler’s solution (Fig. 9B) depict a mapping of the overall buried magnetic sources coherent with the results of forward modelling. The distribution of 3D analytic signal shows a preferential NNE–SSW trending alignment of the causative magnetic sources which runs along the east flank of Vercelli seamounts reaching also the eastern portion of the flat sedimentary basin. Analysis of Euler’s solutions improve the interpretative model suggesting how the ensemble of magnetic sources tends to dip northward passing from average depth of 1700–2000 m in the southern sector to 3200 m in the north area. The deepest magnetic sources are observed at 3700 m, under the thick sedimentary cover filling the basin structure. This pattern is also confirmed considering the magnetic forward model which outlines the presence of a shallow sequence of volcanic products dipping eastward up to a depth of 3400 m. Interpretation of the 3D magnetic gradients and associated solutions of Euler’s deconvolution confirmed the presence of a high magnetized volcanic deposits running in the middle of the two structural highs, following the same NE–SW trend. The two seamounts seem to represent two tectonic boundaries that have constrained the lateral spreading of the volcanic occurrence. These high magnetized crustal structures can be interpreted as lava products or other intra-sedimentary volcanic manifestations, which occurred after the emplacement of the Miocene granitic intrusions, and following pre-existing tectonic features. North of the Vercelli seamount, the analytic signals show a large positive peak interpreted as the occurrence of volcanic products in a not–boundary condition. This suggests the presence of the sedimentary basin has allowed the lateral development of lava products. Morpho-structural analysis of the Vercelli seamount and neighbouring areas has permitted the outlining of a new highly detailed description of the granitic structure.
The top of the seamount is featured by a large flat platform (Fig. 8) resulting from the erosive process occurred during the last 19 ka sea level still-stand. Over this platform we observe two granitic blocks partially tilted and deformed by local faults running along the same regional tectonic lineaments. These two blocks can be interpreted as the most massive parts of the edifice which survived the erosive activity. Passage from a sharp high gradient flank to a flat erosive platform occurs following a gentle profile without marine terraces and other erosive elements (Fig. 8B). This could be explained as consequence of a strong masking-effect due to the high rate of sedimentation in the area. The Vercelli structure is characterized by a specific hydrodynamic pattern known as the “seamount effect” (Misic et al., 2012), with a high energetic cyclonic current triggered by wind coming from Sardinia. The water mass movement follows this hydrodynamic pattern and in proximity of the Vercelli peak tends to accumulate allochthonous sediments coming from the neighbouring coasts (organogenic coarse-grained sand). In addition, comparing the actual depth position of the erosive platform and the estimated depth of the last low-stand (approximately 125 m, Bintanja et al., 2005), the entire Vercelli structure could be affected by tectonic movement (subsidence) acting a downward displacement of further 80–90 m. The cumulative gradient surface analysis (Fig. 7) revealed a peculiar morphologic pattern of the seamount mostly characterized by a clear separation between the northern sector and the southern one. Most of the erosive processes are focused on the southern side as revealed by the presence of large scars and correlated submarine slide deposits and erosive channels (Fig. 6). In this sector, the flanks of the edifice are very steep with a maximum variation of the slope gradient occurring between 600 and 800 m. The opposite flanks show a gentler bathymetry profile with the presence of minor slides and gullies. The same morpho-structural pattern is also observed at the neighbouring structural high, located east of Vercelli, suggesting thus a common evolution of the intrusive granitic structures. The asymmetric morphology of the flanks of the seamounts can be related to the interplay between the main ENE–WSW tectonic trending of the structural highs and flowing direction of the water current present in the area (the dominant current flows northward, Misic et al., 2012). In addition, it is also possible to speculate that part of the flank instability could be triggered by the shallow manifestation of post-Miocene volcanism.
In this context, the structural low located in between the two seamounts shows peculiar morphology patterns, as revealed by the cumulative gradient surface (sector E, Fig. 6). Within this sector, the seafloor depth ranges from 1200 to 1000 m (South to North) with a gentle slope (profile A–A′, Fig. 5) and without any evidence of erosive features as those observed in the proximal sectors (Fig. 6). This intriguing area could be interpreted as an independent structural element juxtaposed between the two highs and thus not correlated to their formation and evolution. These results match the independent interpretation of the magnetic data, indicating the presence of a N–S elongated high-magnetized shallow volcanic deposit occurring after the emplacement of the Miocene granitic intrusion of the Vercelli seamount.
New insight about the crustal discontinuity of the 41st parallel zone
The Vercelli seamount represents one of the several structural highs aligned along the E–W crustal discontinuity of the central Tyrrhenian known in the literature as the 41st parallel zone. Several authors interpret the E–W discontinuity at N41° latitude as a crustal and lithosphere major boundary between the northern and southern Tyrrhenian sectors (Patacca et al., 1990; Serri, 1990, Serri et al., 1993). The discontinuity of the 41st parallel zone is well identifiable considering the sequence of several magnetic anomalies oriented in an E–W direction (Chiappini et al., 2000, Caratori Tontini et al., 2004).
Previous interpretations of the E–W magnetic lineament of the 41st parallel zone were based on airborne magnetic surveys recorded at the end of 1970 (Cassano, 1984). Nevertheless, the very high resolution of this dataset (improved by the new processing techniques by Caratori Tontini et al., 2004) permits the description of the long wavelength magnetic anomaly field, which results to be more suitable for regional studies (see Fig. 11).
The magnetic anomaly pattern of this sector of the Tyrrhenian Sea has suggested the presence of a main E–W trending tectonic-structural discontinuity with a direct relationship between seamount/structural highs and isolated magnetic anomalies (Cassano, 1984, Boccaletti et al., 1990, Spadini and Wezel, 1994). In the light of our results, this latter interpretation should be reconsidered. In fact, the magnetic modelling here discussed suggests that the intrusive bodies forming the Vercelli Seamount show a very low susceptibility with a local magnetic positive (Figure 9, Figure 10), mainly associated to a shallow volcanic sequence emplaced below sedimentary sequences of a N–S elongated basin between the two structural highs. Morphology analysis and quantitative modelling of the analytic signal suggested that the emplacement of shallow volcanic sequence occurred after the Miocene granitic manifestation which worked as tectonic/structural boundaries favouring a N–S elongation. Although the new geophysical analysis of Vercelli could be representative just for the local environment, its interpretative model can be easily applied for the entire central Tyrrhenian sector which presents the same geophysical pattern where graben structures are always associated with low-amplitude magnetic anomalies (Fig. 11). A relevant result of this study is to provide a radical change in interpretation of the magnetic pattern of the 41st parallel zone. The magnetic alignment of the Central Tyrrhenian Sea seems thus not to be correlated with an E–W tectonic shear zone, but just associated to shallow intra-sedimentary volcanic bodies (lava products) as a result of the tectonic-deformation pattern responsible for the series of N–S trending horst and graben structures typical of the North Tyrrhenian domain.
The present study outlines new morphologic and geophysical patterns related to the edifice of the Vercelli seamount located in the Central Tyrrhenian Sea. The geophysical dataset collected during VER2010 cruise shows in a detailed way the magnetic pattern of the study area, clarifying the relationship between tectonic-structural setting and the magnetic anomaly pattern.
Magnetic and morphologic modelling of Vercelli area leads to these major conclusions:
(1) The actual morphology of the Vercelli seamount is mostly due to effects of erosion rather than a contribution of local tectonics. The flanks of the seamount are largely deformed by erosion and submarine slides ranging at different scales.
(2) Cumulative gradient surface analysis indicates that erosive processes focused mostly on the south-eastern sector of Vercelli probably result from the interplay between the NE–SW development of the intrusive structure and the direction of erosion activity driven by a main northward flowing water current.
(3) The top area of the Vercelli seamount is formed by two granitic blocks aligned along the same direction of the two main ENE–WSW and NE–SW tectonic/structural trends. These blocks represent the relict portion of the magmatic intrusion resulting after the action of erosive processes since the last sea level fluctuation.
(4) The 2.75-D interpretative forward model identifies a very low magnetized body compatible with a granitic nature directly below the seamount structure. High amplitude positive anomalies are modelled as shallow Fe-rich lava products extending westward for 8 km dipping up to a maximum depth of 3.1 km just below thin sedimentary sequences.
(5) Depth distribution of Euler’s solutions suggests that a high magnetized causative body is mainly developed in an N–S direction dipping northward up to 3.7 km under a thick sedimentary sequence and it is laterally bounded by the two seamount structures, indicating that the volcanic manifestation occurred after the Miocene intrusion following pre-existing and preferable tectonic/structural gateways.
The modelling of the Vercelli region presented in this work is in contrast to the previous interpretation of the 41st magnetic lineament as a shallower expression of an E–W crustal discontinuity. In fact, the data presented in this study outline that the E–W magnetic alignment existing along latitude N41° is the result of a geometrical juxtaposing of N–S trending volcanic bodies intervening between the horst-graben structures (Fig. 11), created by the typical tectonic evolution of the North Tyrrhenian Sea.
Along the 41st parallel zone, we observe a systematic presence of shallow magnetic sources in concomitance of N–S graben structures. This indicates the occurrence of magmatic/volcanic manifestations as resulting from an eastward crustal stretching during the rifting stage. Evidence of a crustal transition between northern and southern Tyrrhenian domains are clearly distinguishable south of the 41st parallel, where the master detachment fault named Selli Line (Fig. 1) bounds the passive continental margin (Cornaglia Terrace) from the Pliocene–Pleistocene rifting areas, as those represented by Vavilov and Marsili back arc basins.
Source: Geophysical mapping of Vercelli Seamount: Implications for Miocene evolution of the Tyrrhenian back arc basin
Authors: Luca Cocchi, Giuseppe Masetti, Filippo Muccini, Cosmo Carmisciano