Tyrrhenian Area Research Articles

Block model versus thermomechanical model: new insights on the present-day regional deformation in the surroundings of the Calabrian Arc

Abstract A finite-element thermomechanical model is used to analyse present-day crustal deformation in the surroundings of the Calabrian Arc. The major structural complexities of the Tyrrhenian area are taken into account, along with the rheological properties of the rocks resulting from a thermal analysis. A comparison between the results obtained from a model composed of three wide rheologically uniform blocks and those obtained from the thermomechanical model allows us to better constrain the geophysical assumptions and shed light on the roles of the different active mechanisms acting in the Tyrrhenian. Our comparative analysis enlightens the crucial role played by lateral rheological heterogeneities when deformation is analysed at short wavelengths of a few hundred kilometres of the Tyrrhenian, driving the observed diffuse SW–NE extension within the regional context of active Africa–Eurasia convergence. Furthermore, a χ 2 analysis based on comparisons with GPS data confirms the hypothesis that a significant part of the Africa–Eurasia convergence is absorbed through the Calabrian subduction.

An improved seismicity picture of the Southern Tyrrhenian area by the use of OBS and land-based networks: the TYDE experiment

The problem of large location uncertainties for seismicity occurring in the Southern Tyrrhenian Sea have been partially exceeded during the implementation of the long-term scientific mission of the TYrrhenian Deep sea Experiment (TYDE), which allowed the installation of 14 wide-band Ocean Bottom Seismometers (OBS) and Hydrophones (OBH) in the period December 2000 – May 2001 on the sea-bottom floor around the Aeolian and Ustica Islands. Local events recorded at landstations have been observed also on the seismograms of the Ocean Bottom Seismic Network (OBSN). Moreover, some hundreds of low magnitude events, undetected from the land networks, have been recorded. We combined the readings of body wave arrival times from OBS-OBH with those from landstations to localise seismic events. We focused our study on three clusters of events representative of the seismic activity of the area: (i) “deep” events, (ii) Ustica (iii) NE-Sicily. The analysis of the integrated data set of the seismicity off-shore and on-shore, obtained from the combined land-OBS seismic network (Ustica sequence and Deep events), has improved locations in terms of RMS residuals, azimuthal gap, epicentral and hypocentral errors. Moreover, further classes of events have been analysed: the first one includes some local events that could be located only by integrating single trigger readings from the few available land-stations with the OBSN-data; the second one comprises local events that have been detected only by the OBS-OBH stations. In particular, the last cluster underlines the importance of an OBSN in the Tyrrhenian deep basin to reveal its unknown intense micro-seismicity, permitting to better understand both the tectonic and geodynamic picture of the area.

Historical biogeography of Tyrrhenian land snails: The Marmorana–Tyrrheniberus radiation (Pulmonata, Helicidae)

The few studies available on Tyrrhenian land snails support high diversification in the Italian Peninsula and groups structured mainly by vicariant events. Here we investigated the phylogeny of a conchologically diversified group of Tyrrhenian land snails assigned to the genera Marmorana and Tyrrheniberus. We constructed a molecular phylogeny by sequencing two commonly used mtDNA genes (cytochrome oxidase I and the large ribosomal subunit). We also carried out conchological and anatomical analysis. Morphological (shell and genitalia) and genetic data (mitochondrial genes) showed paraphyly of Marmorana. Plio-Pleistocene events in the Tyrrhenian area may have structured relationships between species at regional scale while isolation by distance probably played a role in diversification between populations on a local scale. Continental populations experienced dispersal during interglacial periods and fragmentation and reduction during the dry cold climatic phases. Areas inhabited by Apennine Marmorana could represent relict and/or glacial refugia, with extinction in some areas along the Apennines and survival in the south. The results support a reassessment of taxonomy both at genus and species level and call for further analysis.

SEM-EDS characterization of western Mediterranean obsidians and the Neolithic site of A Fuata (Corsica)

During the Neolithic, obsidians of the Monte Arci (Sardinia) volcanic complex were by far more used in the northern Tyrrhenian area than those of the three other source-islands (Lipari, Palmarola, Pantelleria) in the western Mediterranean. It is shown that merely determinations of content for six major elements with a scanning electron microscope by energy dispersion spectrometry (SEM-EDS) are sufficient to distinguish the four types of Monte Arci obsidians. Because of the compositional similarities between these obsidian types, a multivariate analysis is recommended in provenance studies. Although SEM-EDS, electron microprobe-wavelength dispersion spectrometry (EMP-WDS) and particle induced X-ray emission (PIXE) give essentially concordant results in the determination of these six element contents, subtle technique-related biases prevent the combination of SEM-EDS, EMP-WDS and PIXE data on source samples for provenance purposes. An SEM-EDS test-study reveals the first occurrence of obsidians of Lipari for the A Fuata Middle to Late Neolithic site of NW Corsica (north of Sardinia), in addition to the usual Monte Arci obsidians. Similar to EMP-WDS, the SEM-EDS technique requires only millimeter-sized fragments.

Baseline Trace Metals in Seagrass, Algae, and Mollusks in a Southern Tyrrhenian Ecosystem (Linosa Island, Sicily)

Trace elements were analyzed in organisms collected at five sampling stations along coastal areas of Linosa Island, Sicily (southern Tyrrhenian Sea, Italy). Concentrations of Cd, Cr, Cu, Pb, and Zn were measured in Posidonia oceanica L. Delile tissues, the two brown algae Padina pavonica (L.) Thivy and Cystoseira sp., and the two gastropod mollusks Monodonta turbinata Born and Patella caerulea L. Seawater samples were also collected at each site to assess soluble metal concentrations and to gain relevant information on their bioaccumulation ability. Data were processed by multivariate techniques, such as principal component analysis (PCA) and linear discriminant analysis on PCA factors. The scoreplots obtained showed that the pollutant distribution is more significantly correlated with species than with sites. For seaweeds, P. oceanica was associated with higher Cd, Cu, and Zn levels; Padina species had higher Cr concentrations, and Cystoseira had higher Pb levels. For mollusks, Monodonta species had high concentrations of Cu and Cr and Patella species were associated with Cd. Some general metal bioaccumulation patterns are described but no one sampling site was more contaminated than the others. The hypothesis of Linosa island serving as a reference ecosystem for baseline trace metal levels in southern Tyrrhenian areas is indeed supported by the statistical comparison among other southern Tyrrhenian ecosystems performed with Kruskall Wallis and Mann-Whitney tests. For P. oceanica leaves, P. pavonica, M. turbinata, and P. caerulea, this study confirms their usefulness as possible cosmopolitan biomonitors of trace metals in marine Mediterranean areas.

Neogene-Quaternary tectonics and sedimentation in the Central Apennines

The Central Apennines consist of a fold and thrust belt developed during the Neogene, involving sedimentary successions of different paleogeographic domains. The geodynamic evolution of this sector of the chain was controlled by several important tectonic events, highlighted by numerous depositional sequences recognized in the Neogene marine successions. Time-scanning of the stratigraphic events has allowed us to reconstruct: the different stages of the eastward shift of the chain-foredeep-foreland system; the deformation chronology; the recent effects of the interactions between the regional uplift and extensional tectonics in the chain and the progressive eastward tilting of the Periadriatic domain. During the Miocene-Early Pliocene interval the Tyrrhenian area constituted a remarkable topographic swell, linked to an upwelling mantle dome. As a consequence, intense erosional processes developed on this area, together with crustal delamination and lateral gravitational spreading. The Tyrrhenian-derived siliciclastic detritus fed the Apennine foredeep and thrust-top basins; the continuous asthenospheric uplift in the Tyrrhenian margin favoured the formation of axial crest grabens, crust delamination and the emplacement of a stack of gravitational nappes in the Apennine sector. The maximum uplift of the mantle doming was recorded at the beginning of the Upper Messinian. The remarkable uplift of several asthenospheric domes, located in the Mediterranean area, favoured the closing of the sea-ways, connecting the Atlantic with Peri-mediterranean basins. Consequently the salinity crisis developed, with a relative diachrony: the evaporites sedimented at first in the crestal areas and later in the deep ones. The marine episodes recognized in the lago-mare sediments represent the end of uplift processes and a progressive dome deflation with sporadic connections with the Atlantic. During the Middle Pliocene-Quaternary, the intense thinning of the Tyrrhenian crust induced the collapse of the axial dome and subsidence of the rift belt.

Chert and obsidian procurement of three Corsican sites during the 6th and 5th millenniums BC

Provenance studies of Corsican siliceous raw materials used during, the Neolithic focused mainly on obsidian. They showed an almost systematic use of Monte Arci sources (Sardinia). Chert studies have long been dodged, whereas the multiplicity of potential origins, in Sardinia and in continental areas, may provide complementary spatial information about diffusion patterns and interaction phenomenon in the Middle Tyrrhenian during the Neolithic. We studied obsidian and chert industries of three Corsican sites: Renaghju (Early Neolithic), Monte Revincu and Vasculacciu (both Middle Neolithic). In order to assign a provenance, 2241 chert archaeological samples were characterized by petrographic approaches and 100 obsidian artefacts were submitted to geochemical analyses. Each site provides a specific trend regarding relative abundances of raw materials, provenance and consumption patterns. Considered in the wider perspective of the Neolithic Corsican context, results reveal procurement variations from a chronological as well as a geographical point of view. Those variations may echo economic and social evolutions undergone in Neolithic societies of the Tyrrhenian area.

NEW DATA ON THE VERTEBRATE ASSEMBLAGE OF FIUME SANTO (NORTH‐WEST SARDINIA, ITALY), AND OVERVIEW ON THE LATE MIOCENE TUSCO‐SARDINIAN PALAEOBIOPROVINCE

Abstract: This paper reports the results of a study of more than 2400 specimens in the collection of fossil vertebrates from Fiume Santo in north‐west Sardinia. This locality represents the westernmost documentation of the Tusco‐Sardinian palaeobioprovince, which was in existence during the late Tortonian in the North Tyrrhenian area. During the Tortonian, the region that presently corresponds to southern Tuscany and the Sardinian Massif (Central Italy) was occupied by a complex of large islands characterised by endemic vertebrate populations. Morphological features, adaptations and trophic structure (e.g. low diversity and scarce carnivore taxa), mainly in the mammal faunas, attest to a long period of isolation from the continental palaeobioprovinces of the Mediterranean and Central Europe. The greatest numbers of fossil remains of the well‐known endemic Oreopithecus fauna of the so called V0–V2 assemblages, are found at several sites in southern Tuscany (e.g. Montebamboli, Casteani, Baccinello). In Sardinia this endemic fauna occurs only at the Fiume Santo locality, from which 11 taxa have been recognized: Crocodylia indet., Chelonii indet., Oreopithecus bambolii, Mustelidae indet., Eumaiochoerus cf. E. etruscus, Umbrotherium azzarolii gen. et sp. nov., Tyrrhenotragus gracillimus, Bovidae gen. et sp. indet. (?Neotragini), Maremmia cf. M. lorenzi, Etruria viallii gen. et sp. nov., Turritragus casteanensis gen. et sp. nov. Analysis of these fossils and comparison with material from localities in Tuscany has led to a re‐evaluation of the latter and to the description of three new endemic taxa among the ruminants.

Obsidians in the Rio Saboccu (Sardinia, Italy) campsite: Provenance, reduction and relations with the wider Early Neolithic Tyrrhenian area

Technologic analyses of Neolithic obsidian assemblages are quite rare in Sardinia, like in the wider Western Mediterranean. Such an approach is presented here in conjunction with a visual/instrumental provenance study for the Rio Saboccu Early Neolithic (EN) site, which yielded more than 1000 obsidian artefacts. It is shown that this mostly expedient industry was realized from a non-opportunistic exploitation of the four obsidian types of the nearby Monte Arci (Sardinia) volcanic massif. The choice of the raw materials was chiefly guided by their intrinsic knapping qualities and in function of the maximum size of the expected final products; hence some selection in the (primary and/or secondary) sources exploited. The obsidian industries of EN sites from the northern Tyrrhenian area present clearly some affinities with that of Rio Saboccu and of other EN Sardinian sites, mainly those located in its vicinity. This suggests a regional influence of the EN communities settled in the proximity of the Monte Arci.

From speciation to introgressive hybridization: the phylogeographic structure of an island subspecies of termite, Reticulitermes lucifugus corsicus

BackgroundAlthough much research has been carried out into European Reticulitermes taxonomy in recent years, there is still much discussion about phylogenetic relationships. This study investigated the evolution from intra- to interspecific phylogeny in the island subspecies Reticulitermes lucifugus corsicus and threw new light on this phenomenon. An integrative approach based on microsatellites and mitochondrial and nuclear DNA sequences was used to analyze samples taken from a wide area around the Tyrrhenian sea and showed how the subspecies evolved from its origins to its most recent form on continental coasts.ResultsAccording to mitochondrial phylogeny and molecular clock calculations, island and continental taxa diverged significantly by vicariance in the Pleistocene glacial period. However, more recently, numerous migrations, certainly human-mediated, affected the structure of the populations. This study provided evidence of direct hybridization and multiple introgressions which occurred in several hybrid areas. Analysis using STRUCTURE based on microsatellite data identified a population in Provence (France) which differed considerably (Fst = 0.477) from populations on the island of Corsica and in Tuscany in the Italian peninsula. This new population, principally distributed in urban areas, is highly heterogeneous especially within the ITS2 regions where homogenization by concerted evolution does not appear to have been completed.ConclusionThis study provides an unusual picture of genetic interaction between termite populations in the Tyrrhenian area and suggests that more attention should be paid to the role of introgression and human impact on the recent evolution of European termites.

Seismotectonics of the Nubia plate compressive margin in the south Tyrrhenian region, Italy: Clues for subduction inception

Seismological and structural data are used to constrain the active tectonics of the Nubia plate compressive margin in southern Italy. In this region, compressional displacements have resumed at the rear of the Maghrebian orogenic wedge since about 700–500 ka and are presently accommodated along a seismic belt in the south Tyrrhenian area, where a passive margin has developed during late Neogene‐Quaternary times. Earthquake data from the south Tyrrhenian belt are analyzed with Bayloc, a probabilistic nonlinear location method. Results show that the seismic belt is segmented and involves a series of NE‐SW and NW‐SE elongated clusters. The NE‐SW clusters are interpreted as high‐angle reverse fault zones verging toward the southeast, whereas the perpendicular clusters are interpreted as possible strike‐slip fault zones transferring the contractional displacements to advanced segments of the belt in the southeast. Brittle deformations observed in the Quaternary volcanic island of Ustica are consistent with the geometry and kinematics of the seismic belt deduced from the seismological data. The south Tyrrhenian active belt may constitute an early stage of subduction of the Tyrrhenian oceanic crust beneath Sicily. A similar scenario has been hypothesized also for the westward prolongation of the south Tyrrhenian belt off the Algerian coast. By integrating instrumental and historical seismic data with tectonic evidences, we infer that the south Tyrrhenian belt is probably capable of producing earthquakes with a maximum size close to magnitude 7. The analyzed data suggest a multifault system behavior for the studied compressional belt. This inference is important to estimate and mitigate the seismic and tsunamic hazards in such a densely populated region.

A map of the Monte Arci (Sardinia Island, Western Mediterranean) obsidian primary to secondary sources. Implications for Neolithic provenance studies

Four types of obsidians from the Monte Arci volcanic complex (Sardinia) were used by Neolithic men in the North Tyrrhenian area of the western Mediterranean. A map of their occurrences from primary sources (mother rocks) to distant secondary deposits in the surrounding plains is presented. Some 1457 specimens were macroscopically characterized and in addition ~15% of them fingerprinted from their elemental compositions as determined by electron microprobe, neutron activation analysis or ion beam analysis. The results show that secondary sources, up to now largely neglected in provenance studies of ‘archaeological’ obsidians will have from now to be taken into account. To cite this article: C. Lugliè et al., C. R. Palevol 5 (2006) .

Lands and endemic mammals in the Late Miocene of Italy: Constrains for paleogeographic outlines of Tyrrhenian area

The Pre-Messinian land mammal localities of Italy contribute substantial information for reconstructing the complex history of the land masses of the central Mediterranean. At the present state of knowledge the available paleogeographic maps do not have the adequate time resolution for providing an effective picture of the evolution of the physiographic and of the oceanographic characteristics of the area. This can only be accomplished through detailed comparative analyses of the geology and of the land and marine faunas in the course of the Miocene. The Pre-Messinian Late Miocene land mammals localities of Italy document the existence of three distinct bioprovinces. Two of the latter are characterised by faunas with manifestly endemic features, thus attesting to the occurrence of isolated emerged areas. One of these areas is called the Abruzzi–Apulia paleobioprovince; it was located on the Adriatic side of Apennines. The other one is the so-called Tusco–Sardinian paleobioprovince and was located in the peri-Tyrrhenian side of Italy. A third bioprovince, testified by sites in Calabria and Sicily, is characterised by non-endemized mammals, counterparts of which were identified in North Africa and Europe. This area was therefore, at least in part, a northern extension of the Late Miocene Mediterranean border of the African plate. These three areas belong to completely different tectonic domains and kept separated for a considerable time span and each one has a peculiar biogeographic and tectonic history.

Crustal magnetism of the Southern Tyrrhenian Sea from aeromagnetic surveys

Abstract Aeromagnetic data from the Southern Tyrrhenian Sea have been analysed, to remove the contributions of shallow and deep sources to the magnetic anomalies by applying a processing technology that increases the informative potential of the magnetic data acquired by AGIP with great accuracy, uniform distribution and high density. The aeromagnetic anomaly field has been reduced to the bottom topographic surface by the bottom reduction method. Spectral signal analysis has been carried out to separate low- and high-frequency components. The spectral reference field (SRF), obtained from the cotransformation of low frequencies, gives information about deep magnetic structures. The high-frequency components, computed from the difference between the magnetic anomaly field and the SRF, provides the spectral anomaly field (SAF). The bottom reduced magnetic anomaly field is derived by merging the high- and low-frequency bands. The resulting map provides information about geotectonic features in the Southern Tyrrhenian area and shows the effectiveness of the adopted approach over traditional procedures.

Relationships between organic carbon and microbial components in a Tyrrhenian area (Isola del Giglio) affected by mucilages

The chemical and biological properties of the water column at a Tyrrhenian site (Isola del Giglio) were studied during a 3-year period. The results highlighted the oligotrophic features of the site, characterised by quite low concentrations of organic carbon (on average DOC 102 μmol/L and POC 9 μmol/L). Relevant bacterial biomass (on average 42.1 μg C/L) and a notable activity (in terms of frequency of dividing cells, on average more than 5%) were observed. However, remarkable changes for these parameters were seasonally recorded. The cyclic occurrence, generally during the late spring–summer period, of benthic mucilage indicated that localised distrophic processes may occur. In particular, the benthic mucilage events of 2000 and 2001 were investigated, although some comparative information was available also for 1999 and 2002. The mucilage aggregates generally showed high bacterial colonisation, which have remarkable effects on the organic matter cycle both inside the aggregates and in the surrounding seawater. During the benthic mucilage development, an increase of DOC and POC concentrations was observed (up to 129 and 18 μmol/L, respectively, in June 2000 and up to 145 and 10 μmol/L, respectively, in May and June 2001) in the water column adjacent to the bottom. However, a general decrease of the trophic value of particulate matter (in terms of C/N ratio) was also observed, especially in 2000 after the disappearance of the mucilage. The available energy and organic matter during the mucilage events led to an increased presence of bacteria in the bottom waters of the Isola del Giglio, with maximum biomass values in 2001. Similarly, the replicative activity of bacteria was higher in 2001 (frequency of dividing cells about 5% vs. 3% of 2000). The lower activity of 2000, in addition to the lower trophic value of organic matter and different environmental conditions (namely lower temperature), might be involved in the persistence of mucilage in 2000 with respect to the rapid disappearance observed in 2001.

Crustal velocity and strain-rate fields in Italy and surrounding regions: new results from the analysis of permanent and non-permanent GPS networks

SUMMARY We present a new geodetic velocity solution for Italy and the surrounding areas, obtained from an analysis of continuous and survey-mode Global Positioning System observations collected between 1991 and 2002. We have combined local, regional and global networks into a common reference frame velocity solution, providing a new detailed picture of the regional-scale deformation field in the central Mediterranean. Our velocity estimates are computed with respect to a new stable Eurasian reference frame, constraining the kinematics of the greater African–Eurasian plate boundary system in the study area. We provide strain-rate values for the main seismotectonic districts, which are in good agreement with the seismic deformation inferred from earthquake focal mechanisms. The southern Tyrrhenian area, Calabria, the Apennines, the southeastern Alps, the southern Dinarides and the Albanides display deformation rates at the order of 20–30 nanostrain yr−1. The Corsica–Sardinia block moves according to Eurasian Plate motions, and there is no indication that the opening of the Tyrrhenian is still active. The Pelagian and Sicilian domains are separated by a northwest–southeast discontinuity, the Sicily Channel rifted area, and marginally significant relative motion between the Pelagian Plateau and the African Plate is also observed. The southern Tyrrhenian is affected by north–south compression and accommodates up to 50 per cent of the Africa–Eurasia relative plate motion, whereas the Calabrian Arc exhibits ∼2 mm yr−1 northwest–southeast extension. The observed deformation pattern suggests the presence of a major approximately north–south tectonic discontinuity separating the Sicilian and Calabrian domains. An extensional boundary observed along peninsular Italy coincides with the distribution of seismogenic faults along the axis of the Apennines. This boundary separates a Tyrrhenian and an Adriatic domain with diverging velocities, orientated north–NNW-ward and northeastward, respectively. The Apennines are extending perpendicularly to the chain axis at rates of less than 3 mm yr−1, and only in the outer northern Apennines indications of active shortening are observed. Insignificant deformation is observed in the western Po Plain and the western Alps, while the central and eastern Alps display north–south shortening. The eastern Adriatic domain is shortening perpendicular to the Dinaric front with strain rates increasing from north to south. The locus of collision between the Aegean/Balkan system and the Adriatic and Ionian domains is marked by the Kephalonia fault system, which accommodates up to 20±1 mm yr−1 of right-lateral motion. The deformation pattern observed in the peri-Adriatic domain is well described by a counter-clockwise rotation of the Adriatic microplate around a pole located in the western Alps.

New chronological and geochemical constraints on the genesis and geological evolution of Ponza and Palmarola Volcanic Islands (Tyrrhenian Sea, Italy)

A new geochronological and geochemical study of the volcanic rocks of the Ponza and Palmarola Islands, Pontine Archipelago, has been carried out. This archipelago is located along the boundary between the Italian continental shelf and the opening Tyrrhenian basin. It is a key area to study volcanism related to the opening of the Tyrrhenian Sea. Ponza is the oldest felsic magmatic manifestation in the central Tyrrhenian area. Previous studies suggested that Ponza volcanic activity began before 5 Ma. Twenty-five new K–Ar ages constrain the volcanic activity (rhyolitic hyaloclastites and dykes) to the last 4.2 Ma, with two episodes of quiescence between 3.7 and 3.2 Ma and between 2.9 and 1.0 Ma. A new volcanic episode dated at 3.2–2.9 Ma has been identified on the central and southern Ponza, with emplacement of pyroclastic units. At 1.0 Ma, a trachytic episode ended the volcanic activity. The near island of Palmarola exhibits rhyolitic hyaloclastites and domes dated between 1.6 and 1.5 Ma, indicating that the island was entirely built during the Early Pleistocene in a short span of time of ca. 120 ka. Although only 6–8 km apart, the two islands display significantly different geochemical signatures. Ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction/collision zones: high-K calc-alkaline and metaluminous rhyolites (Agpaitic Index [AI] and Alumina Saturation Index [ASI] <1), high LILE/HFSE (Th/Ta=16–21) and LREE/HFSE ratios (La/Nb>3), and Nb–Ta negative anomalies. In Palmarola, the orogenic character is also present, but much less marked than in Ponza: rhyolites have a peralkaline character (AI>1), lower LILE/HFSE (Th/Ta=11–15), low LREE/HFSE ratios (La/Nb=1–2) close to those of anorogenic lavas, and the Nb–Ta negative anomalies are almost absent. Y/Nb ratios indicate different magmatic sources, one similar to island-arc or active continental margin basalts for Ponza rhyolites, and the others probably involving an OIB type component for Palmarola rhyolites and Ponza trachytes. Palmarola volcanics represent a transitional magmatism: although a preserved collisional geochemical imprint, they show geochemical features approaching those of anorogenic lavas erupted in a within-plate context. The change of magmatism evidenced in this study can be related to the tectonic evolution of the area. Indeed, Hf, Ta and Rb contents suggest that the oldest Pliocene rhyolites of Ponza would emplace in a syn- to late-collisional setting, while the younger Pleistocene rhyolites of Palmarola would be emplaced in a post-collisional setting in which the orogenic character (Th/Ta) decreases and mantle influence (Nb/Ta) increases. Geochemical modeling strongly suggests that the Palmarola rhyolites represent the waning stages of a subduction-related magmatism. The K–Ar datings allow us to estimate precisely the transition of magmatism to last less than 1.3 Ma. The transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources. This upwelling could occur during the extension of the Tyrrhenian basin, caused by the slab retreat and steepening, or during a process of slab break-off starting in the Pliocene.

Crustal features along a W–E Tyrrhenian transect from Sardinia to Campania margins (Central Mediterranean)

The Tyrrhenian Sea is a Neogene back-arc basin formed by continental extension at the rear of the eastward migrating Apennine subduction system. Its central part, generated from Tortonian to Pliocene, includes the Sardinia rifted margin to the west, an area with large volcanoes in the deep central sector, and the Campania rifted margin to the east. A reprocessing of some 2000 km of MCS lines, a new swath bathymetric map, and a review of previous geological and geophysical data allow to analyse the nature and distribution of continental vs. oceanic crust in this area, which evolved in a short time span. The central portion of the southern Tyrrhenian Sea is characterized by MOHO at about 10 km depth. North of Magnghi and Vavilov Smts, this thinned crustal domain include a wide continent–ocean transition, with the occurrence of extensional allochthons and of serpenitinzed sub-continental mantle, recalling other well known rifted margins, as the Iberia one. Sectors floored by oceanic crust should occur, mainly in the southern part of the study area, but they do not appear related to discrete spreading ridges. The continent–ocean boundary cannot be drawn unequivocally in the area, due also to the occurrence of widespread and huge magmatic manifestations not related to oceanic spreading. These portions of the southern Tyrrhenian Sea represents therefore a complex oceanic back-arc basin surrounded by magma-rich rifted continental margins. The abundant igneous manifestations and the very high stretching rates observed in the area may be related to the fact that the present Tyrrhenian area was occupied by an orogenic domain affected by shortening until middle Miocene times, which is just before the Late Miocene onset of back-arc extension. The lithosphere in the region had then to be rheologically weak. Abundant generation and ascent of magmas, mostly of Ocean Island Basalt type, was favoured by the large lithospheric permeability induced by strong extensional deformations.

A topographic surface reduction of aeromagnetic anomaly field over the Tyrrhenian sea area (Italy)

This paper shows the results of a detailed reprocessing of aeromagnetic data, obtained by the downward projection to the seabed. The area of interest is centered over the Tyrrhenian Basin, whose bathymetric–topographic lay-out is characterized by a somewhat irregular trend. The origin of the intense depth variations depends on the Tyrrhenian structural setting, that is associated with the presence of several tectonic lineaments, seamounts or volcanic islands. The data were characterized by good quality and dense sampling, but they have been reprocessed in order either to solve some problems in the original compilation, and to reduce the distortion of the geomagnetic anomaly field caused by the difference of distance between the survey level and the magnetic source. The reprocessed magnetic map is proposed as an effective analysis tool for the Tyrrhenian area that is characterized by high susceptibility lithotypes. Downward projection of the aeromagnetic data by BTM algorithm increases the definition of the anomalous magnetic signal without distortions in the geometric pattern of the field, thus showing a more stable and effective association between the magnetic anomalies and their geological sources. This effect is particularly true for high frequency anomalies that are directly comparable after the topographic projection because the depth filtering effect is attenuated. Moreover, the BTM method has been applied for the first time to a regional scale survey that shows substantial advantages because no fictitious anomalies in the high frequency sector of the spectrum were generated. This has been a typical effect of the traditional downward projection methods widely used before. The final result is a BTM anomaly map that is able to show the structural connections between the geological magnetic sources of the Tyrrhenian Sea area.

A biomonitoring study: trace metals in algae and molluscs from Tyrrhenian coastal areas

Marine organisms were evaluated as possible biomonitors of heavy metal contamination in marine coastal areas. Concentrations of Cd, Cr, Cu, Pb, and Zn were measured in the green algae Ulva lactuca L., the brown algae Padina pavonica (L.) Thivy, the bivalve mollusc Mytilus galloprovincialis Lamarck, and the two gastropod molluscs Monodonta turbinata Born and Patella cerulea L. collected at six coastal stations in the area of the Gulf of Gaeta (Tyrrhenian Sea, central Italy). The coastal area of the Regional Park of Gianola and Monte di Scauri (a “Protected Sea Park” area) was chosen as a control site. Seawater samples were also collected in each site to assess soluble and total metal concentrations and to gain additional information on both the environmental conditions of the area and possible bioaccumulation patterns. Metal concentrations detected in algae and molluscs did not show significant differences among all stations studied. Moreover, statistical analyses (ANOVA, multiple comparison tests, cluster analysis) showed that the Sea Park station was not significantly different from the others. The hypothesis that the Protected Sea Park would be cleaner than the others must therefore be reconsidered. Data from this study were also compared with those previously obtained from uncontaminated sites in the Sicilian Sea, Italy. The results show clearly differences between these two marine ecosystems. The species examined showed great accumulations of metals, with concentration factors (CFs) higher than 10,000 with respect to the concentrations (soluble fractions) in marine waters. Metal concentrations recorded in this area may be used for background levels for intraspecific comparison within the Tyrrhenian area, a body of water about which information is still very scarce.