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Evidence of Tectonic Control on the Geochemical Features of the Volatiles Vented along the Nebrodi-Peloritani Mts (Southern Apennine Chain, Italy)
Investigations carried out over the southernmost portion of the Apennine chain (Nebrodi-Peloritani Mountains, Sicily, Italy) reveal a close connection between the tectonic setting and the regional degassing of CO2-dominated volatiles. The geochemical features of the collected gases show that the pristine composition has been modified by gas-water interaction (GWI) and degassing processes. The 3He/4He isotopic ratio in the range of 0.7-2.8 Ra highlights variable contributions of mantle-derived helium, representing an unusual feature for the crustal regime of the study areas characterized by the widespread presence of 4He-producer metamorphic rocks. The degassing of mantle helium is coherent with the tectonics and related to the NW-SE extensional regime of the Calabro-Peloritan Arc (CPA). We propose that the degassing regime as well as the geochemical features of both the dissolved and bubbling gases is closely connected to the strain accumulation rate, inducing almost no temporal changes and insignificant deep-originated fluid contributions to the locked fault volumes. Investigations including discrete and continuous monitoring and degassing-rate estimations are useful tools to gain a better insight into the evolution of seismogenesis, considering the fault rupture as the final stage of a seismic cycle.
Results of studies on fluid/fault relationships have widely shown the role of fluids both as triggering agents of seismic shocks and as fast carriers of information on processes occurring at deep levels, making them useful tools in gaining a better insight into the evolution of seismogenic processes. The Apennines are well known to be a still-developing chain, with tectonic movements driven by a large number of active faults often recognized as seismogenetic [1, 2]. The Southern portion of the Apennines, the Calabro-Peloritan Arc (CPA), develops over an area that has been struck by some of the most destructive seismic events ever seen in Europe (e.g., Messina 1908 and Calabria 1905, 1783). Indeed, its complicated tectonic setting has been satisfactorily constrained only in recent times and remains under investigation. The CPA is a major tectonic structure running across northeastern Sicily and Calabria (Figure 1). The Nebrodi and Peloritani mountains stretch E-W along the Northern Ionian and Tyrrhenian coasts for about 100 km.
The presence of thermal springs besides the degassing occurring over some areas (evidenced by bubbling gases in thermal waters and shallow sea waters) is a clue to the close connection of the fluids’ circulation pattern with the local tectonic structures. This paper accounts for the results of a fluid collection carried out over the Nebrodi and Peloritani Mts over the period 2004-2006. More samples collected in 2002, 2003, and 2007 were added to the time series of some of the most important sites. The survey aimed at defining the origin as well as the interactions of the fluids circulating over the area in order to evaluate possible relationships with local tectonic structures.
Improving the knowledge of the geochemical features of the fluids and their behaviour in the geodynamic context of the area can provide new information leading to a better understanding of local tectonic settings along with new tools for an insight into the development of the seismogenesis (stress accumulation, deformation, strain release, etc.).
The study area is characterized by the venting of geothermal fluids over both the Tyrrhenian and Ionian coasts, as well as by gas emissions over areas crossed by the Aeolian-Tindari-Letojanni fault (ATLF) system (Figure 1). The gases are mainly dissolved in groundwater and vented as a separate phase only in a few sites.
2. Geologic and Tectonic Setting
The current tectonic framework of the Calabro-Peloritan Arc (CPA) results from the N-S Africa-Eurasia convergence during the Neogene-Quaternary at a rate of 1–2 cm/yr during the last 5–6 My [3–7]. Despite this, a rapid E to SE motion affected the CPA at a rate of 5–6 cm/yr, with an uplift between 0.5 and 1.2 mm/year, in the last 1–0.7 My, mainly accommodated by normal faulting [8–10]. This NE corner of the chain exhibits an uplift with the highest rate in proximity to the Messina Strait (along the Ionian coast) and lower uplift rates along the Tyrrhenian coasts .
The motion is related to the roll-back of the subjacent Ionian transitional to oceanic slab and back-arc expansion in the Tyrrhenian Sea [12–14]. During the middle-late Pleistocene, roll-back and subduction slowed to less than 1 cm/yr .
The current structural framework of northern Sicily is the result of the Plio-Pleistocene activation of a complex network of fractures related to a W-E trending right-lateral regional shear zone extending from the Pantelleria Rift to the Aeolian Islands [16–20]. Some of these structures are still active and responsible for the shallow seismicity occurring both in inland Sicily and in the northern offshore in the Tyrrhenian Sea. Focal mechanisms are typically characterized by strike-slip and oblique kinematics consistent with low-dip NW-SE to NNW-SSE trending -axes [21–26], roughly consistent with the global convergence direction between the European and African plates [11, 27–31].
The seismological and geodetic data depict two main crustal domains marked by different stress regimes: a compressive domain in the northern Sicilian offshore and an extensional domain in northeastern Sicily and southern Calabria [24, 32–36]. The transition between the two domains occurs along the Aeolian-Tindari-Letojanni fault (ATLF) system which has been interpreted as a transfer crustal zone between the northern Sicily offshore thrust belt in the Tyrrhenian Sea and the accretionary wedge offshore the eastern Calabria in the Ionian Sea [29, 32, 37] or as a lithospheric tear fault bounding the western edge of the subducting Ionian slab [13, 38–41]. Extension, however, although at an immature tectonic stage, also occurs in a narrow band to the west of the ATLF in northern Sicily as documented by seismology and structural studies [25, 42]. In the field, the ATLF is formed by NW-SE-oriented en echelon segments characterized by prevailing right-transpressional movements in the Aeolian sector [43, 44] and by transtensional motion in the northern Sicily sector . Seismological, geological, and geodetic data evidence that the ATLF is very active with more than 1500 earthquakes () occurring in the past 30 years in its northern portion. Fault plane solutions reveal prevailing normal faulting coupled with dextral solutions along the inland part of the ATLF . Figure 1 summarizes the above-mentioned information alongside the distribution of the sampling sites.
Recent geodetic observations coupled to geological constraints have served to better elucidate the interplay of crustal blocks of the Nebrodi-Peloritani area . The ATLF juxtaposes north-south contraction between Sicily and the Tyrrhenian block with northwest-southeast extension in northeastern Sicily and Calabria (Nebrodi-Peloritani chains). As a matter of fact, the Africa-Eurasia convergence in Sicily and southern Calabria is nowadays expressed by two different tectonic and geodynamic domains: a roughly N-S compression over the western region caused by continental collision and a NW-SE extension to the east (Calabro-Peloritan Arc) related to the S-E-directed expansion. The ATLF right-lateral shear zone accommodates the different deformation patterns of these two domains from the Ionian Sea (north of Mt. Etna) to the Aeolian Islands across the Peloritani chain, thus crossing our study area.
3.1. Field Investigations and Sample Collection
A suite of 158 samples taken at 67 different sites has been collected along the Nebrodi-Peloritani chain, the southernmost portion of the Apennines. They include natural springs, fountains, and boreholes as well as bubbling gases from thermal and cold ponds spread over an .
Table 1 lists the sample locations, the coordinates (in WGS84 notation), and the field data; Table 2, the analytical results for the dissolved gases (43 from the Nebrodi sector and 19 on the Peloritani sector); and Table 3 (8 different sites), the bubbling (free) gases. The samples are listed using ID numbers as well as the site names. The ID number identifies the site; thus, bubbling and dissolved gases taken at the same site display the same ID number but are listed in different tables.
To carry out dissolved gas analyses, water samples were collected and stored in 240 ml Pyrex bottles sealed in the field using silicon/Teflon septa and purpose-built pliers, following the methodology and instrumentation described in Italiano et al. [47–49]. All of the samples were collected taking care to avoid even the tiniest bubbles in order to prevent atmospheric contamination.
To recover a pure gas sample, we collected gas bubbles using an inverted funnel placed on top the bubbles, driving them towards a Pyrex bottle of about 50 ml in volume with two vacuum-type valves at both ends. The sampling bottle was washed by the gas coming from the funnel and the sample taken by closing the two valves after a volume of at least one order of magnitude larger than that of the sampling bottle had been passed through.
All of the samples (dissolved and free gases) were analyzed for the chemical and isotopic composition of carbon (CO2) and He.
3.2. Analytical Methods
Field measurements of temperature, pH, redox potential (Eh), and electrical conductivity (EC) were performed by a multiparameter device (Multi 350i, Weilheim) (Table 1).
In the laboratory, the chemical and isotopic composition (He and C) of the bubbling and dissolved gases were determined using the same analytical equipment. The dissolved gases were extracted after equilibrium was reached at constant temperature with a host-gas (high-purity argon) injected in the sample bottle through the rubber septum (for further details, see Italiano et al. [47, 49]). Chemical analyses were carried out by gas chromatography (PerkinElmer Clarus 500 equipped with a double TCD-FID detector) using argon as the carrier gas. Typical uncertainties are within ±5%.
Helium isotope analyses were performed on gas fractions extracted following the same procedure as for the gas chromatography and purified following methods described in the literature [50–52]. The purified helium fraction (either of dissolved or of bubbling gases) was analyzed by a static vacuum mass spectrometer (GVI5400TFT) that allows the simultaneous detection of 3He and 4He ion beams, thereby keeping the 3He/4He error of measurement to very low values. Typical uncertainties in the range of low-3He samples are within ±1%. During the same analytical procedure, the 4He/20Ne ratio was measured by peak intensities on the mass spectrometer.
The isotopic composition of the total dissolved carbon (δ13CTDC) was measured in a sample of 2 ml of water introduced into containers injected with high-purity helium to remove atmospheric CO2. The water samples were acidified with phosphorus pentoxide in an autosampler to ensure complete release of CO2 from acidified waters. CO2 was then directly admitted to a continuous flow mass spectrometer (AP2003). The results are reported in δ‰ units relative to the V-PDB (Vienna Pee Dee Belemnite) standard; standard deviation of the 13C/12C ratio was ±0.2‰.
4.1. Chemical and Isotopic Composition
Table 2 shows the chemical and isotopic data of the bubbling gases as well as their He and C isotopic compositions and 4He/20Ne and CO2/3He ratios.
CO2 is by far the main component of the bubbling gases with concentrations always above 90%, but sample Rodì Milici (after ) which has a composition dominated by N2 (96.1%) with CO2 content is as low as 0.01% with a large amount of O2. The sample is largely air contaminated and also suffered from gas-water interactions as shown by the high helium and negligible CO2 content (Table 2). The composition of the dissolved gas phase was calculated from the gas-chromatographic analyses, combining the solubility coefficients (Bunsen coefficient “β,” ccgas/mlwater STP) of each gas species, the volume of gas extracted (cm3), and the volume of the water sample and the equilibration temperature, as shown in the following equation: where is the concentration of the selected gas specie, is its concentration measured by gas chromatography (vol%), and and represent the extracted and introduced gas volumes, respectively, while is the volume of the analyzed water sample (see also [47, 49] for further details). All volumes are carefully measured at the equilibration temperature. Data for dissolved gas compositions are expressed in cm3STP/LH2O and listed in Tables 3(a) and 3(b), as well as the amount of dissolved air (air% in Tables 3(a) and 3(b)). The air content estimation is based on the oxygen content and represents the minimum amount of dissolved air at the sampling site. Oxygen, in fact, can be consumed because of bacterial activity as well as oxidation reactions likely occurring during the water circulation; the percentage of atmospheric components during infiltration might be much higher. As we assume that the deep volatiles are oxygen-free, we recalculated the gas composition removing the atmospheric components (since atmospheric nitrogen is calculated from the oxygen content, for some sites, it might be underestimated). The gas analyses recalculated in vol%, allowing us a comparison with the analytical results of the bubbling gases, are listed in Tables 4(a) and 4(b) together with the helium and carbon isotopic compositions and the He/Ne ratios.
5.1. Fluid Geochemistry
The chemical composition of the bubbling gases (Table 2) shows that CO2 is the most abundant component with the concentration always above 95% by vol. The only exception is represented by the sample #67 (Rodì Milici, Table 2; after ), whose CO2 content is the lowest of the entire dataset. Considering the high He concentration, this sample is a very fractionated gas, which lost almost all the CO2 likely for intense GWI. It is noteworthy that it was possible to recover a free gas phase only over the Peloritani chain. The main difference in the gas chemistry is related to the amount of CH4 and N2 (Table 2) that besides CO2 are the main components in crustal gases. All the CO2-dominated volatiles are here classified as of deep origin, where the term “deep” may indicate an origin from either crustal or mantle/magmatic environments.
The composition of the dissolved gases (Tables 3(a) and 3(b)) shows the presence of air-derived gases (N2 and O2) along with the nonatmospheric gases CO2 and CH4. The estimated amount of dissolved air is significantly lower in the samples collected from the Peloritani chain than in those from the Nebrodi Mts. (Tables 3(a) and 3(b)): it ranges between 8.94 and 84.53% for the latter samples and from 0.01 to 53% for the former (Peloritani chain) with the exception of two samples (#62 and 63, Table 1(b)) showing air content as high as 87 and 88%, respectively. The relationships between atmospheric (O2) and nonatmospheric gas species in the dissolved gases (Tables 3(a) and 3(b)) are shown in Figure 2.
The arrows show the trends produced by the addition of CO2 and CH4 to an atmospheric gas assemblage as well as the effect of gas-water interactions (GWI) on the CO2 dissolution. The graph clearly shows the presence of fluids from a crustal and/or a mantle source over the whole study area that interact with the groundwater changing their original atmospheric-derived gas assemblage. Gas-water interactions (GWI) allow dissolution of deep gas species as a consequence of the solubility coefficients of each of them, the temperature, and the flux intensity towards the surface.
The recalculated gas analyses show that even in the case of dissolved gases, the deep volatiles are always made by a CO2-dominated gas assemblage despite a widely different extent of atmospheric contamination, namely, the air-derived/deep-originated volatile mixing proportion (Tables 3(a) and 3(b)).
It is worth noting that the investigated area is from 30 to more than 60 km away from the volcanic districts of Mt Etna and the Aeolian Islands and there is no evidence of melt intrusions in shallow crustal levels supporting the release of mantle-derived volatiles as already observed in other portions of the Southern Apennines .
5.2. Helium and Carbon Isotopes
The helium and carbon systematics may provide the necessary information to constrain the origin of the CO2-dominated volatiles. It is well accepted that the isotopic ratio of helium is a very sensitive tracer of volatile mixing in volcanic and tectonic systems located near the Earth’s surface. During earth evolution and differentiation, the production of radiogenic 4He (alpha particles) as a function of U and Th concentrations modified the pristine 3He/4He ratio leading to a wide range of isotopic ratios both in the crust (from less than 0.01 to 0.05 Ra, where Ra is the atmospheric 3He/4He value of ) and in the mantle (e.g., ; ~8 Ra; [55–58]; 6.5 Ra in the Sub Continental European Mantle (SCEM) ).
The helium isotopic ratio observed in the gases dissolved in the groundwater of the Nebrodi and Peloritani chains, ranging from 0.5 to 1.85 Ra (Tables 4(a) and 4(b)) and increasing to 0.67-2.54 Ra (Table 2) in the bubbling gases of the Peloritani area, indicating significant 3He injections, thus pointing to a mantle origin for those volatiles.
Figure 3 plots the 3He/4He ratios (expressed as R/Ra values) versus the 4He/20Ne ratio and includes results of both dissolved (Tables 4(a) and 4(b)) and bubbling (Table 2) gases. Assuming all neon to be of atmospheric origin, the 4He/20Ne ratio provides an indication of the presence of an atmospheric-derived component in the gas assemblage.
The plot shows that the dissolved gases, although extracted from groundwater equilibrated with the atmosphere, display 4He/20Ne ratios remarkably higher than the ASW. In particular, the ratios over the Peloritani Mts are higher than those from the Nebrodi Mts, most likely as a consequence of a larger contribution of CO2-dominated, He-rich gases. Moreover, the isotopic composition of helium in both dissolved and bubbling gases of the Peloritani samples denotes a broadly higher contribution of mantle-type helium that might seem surprising, considering the geology of this portion of the chain, with its outcrops of high-grade metamorphic rocks (micaschists, gneiss) normally enriched in radioactive, 4He-producing elements.
The information is consistent with that from the geochemical features of the dissolved helium and its isotopic composition. The affinity of the heavy 13C for the liquid phase is responsible for the observed C fractionation depicting the increasing δ13C trend of the dissolved carbon species (total dissolved carbon (TDC)). The process is commonly observed in CO2-rich waters (e.g., Eastern Alps  and Southern Apennines ) with the exception of waters with the presence of gas bubbling (gas oversaturated) where C isotopic fractionation is induced by CO2 escaping (e.g., East Anatolian Fault Zone (EAFZ) ; Figure 5).
To evaluate the extent that mantle volatiles contribute to the dissolved gas phase and to the soils, as well as elemental fractionations between He and CO2, the correlations between CO2/3He versus CO2 (Figure 6(a)) and He (Figure 6(b)) were evaluated.
The CO2/3He ratios along the Nebrodi-Peloritani chain spans over five orders of magnitude (from to ) covering the range proposed for mantle ( for MORBs ) and crustal continental fluids (1014 ). The samples from the two sections of the chain, however, span over different ranges: 1010-1012 for the Nebrodi area and the whole range for the Peloritani chain.
Since He isotope ratios can only be modified by the admixture of He with a different isotopic signature, i.e., from a different reservoir, the coexistence of low 3He/4He ratios (namely, high crustal He component) and low, mantle-like CO2/3He ratios suggests that the ratio is modified by a mixing of crustal and mantle-derived helium, or alternatively, the crustal component ratios span over a wide range due to chemical CO2 fractionation.
The data are plotted on Figures 6(a) and 6(b) where the arrows in (a) display two concomitant trends here interpreted as (1) CO2 addition to the ASW-type waters because of regional degassing and (2) increase in helium concentration because of CO2 loss during GWI. Almost all the samples from the Nebrodi Mts and a group of samples from the Peloritani show CO2/3He ratios in the range of 1010-1012 broadly lower than the ratios detected for the dissolved gases from the Peloritani area.
It is of note that only samples from the Peloritani Mts show an antirelated CO2/3He-He relationship as a consequence of GWI processes with CO2 loss due to dissolution. In addition, the CO2/3He ratio vs. [CO2] content (Figure 6(b)) demonstrates a correlation trend indicating that whatever the GWI-induced CO2 loss, samples from the Nebrodi Mts are characterized by broadly lower CO2 content. The group of samples from the Peloritani area, marked by low He content with high 3He/4He ratios, denotes a different composition of the pristine gas phase.
6. Fluid/Fault Relationships
The active degassing of CO2-dominated volatiles is considered responsible for the deep-originated dissolved gases we detected in the groundwater collected over the Nebrodi-Peloritani area. Although it was almost unexpected in the geologic context of the Southern Apennines, the evidence that a large amount of CO2-dominated volatiles feeds the groundwater circulating over the study area indicates the close connection of the fluids’ geochemistry with the ATLF tectonic structure.
There is no evidence of melt intrusions in shallow crustal levels capable of releasing 3He-enriched fluids as already detected in other Apennine areas (e.g., ); thus, we propose that the presence of mantle fluids is related to lithospheric structures that, enhancing the vertical permeability, allow depressurization at the level of the upper mantle and the degassing of deep-originated fluids. A possible circulation model can be summarized as follows: CO2-dominated fluids are produced by the partial melting of the upper mantle induced by depressurization due to lithospheric faults; the mantle-originated fluids are driven toward the surface through the 25-30 km thick crustal layers by the ATLF zone; the mantle volatiles permeate shallow crustal levels where crustal-type helium (4He) is originated by a wide range of metamorphic rocks; a further mixing occurs as the volatiles move across the groundwater circulating at relatively shallow depths where they suffer GWI (e.g., gas dissolution as a function of Bunsen coefficients and boundary pressure and temperature conditions) and mix with the atmospheric components (mainly represented by O2 and N2).
The upraising of large amounts of mantle-originated volatiles (as shown by the 3He content) at Alì Terme (Ionian Sea) and Capo Calavà (Tyrrhenian Sea) coincides with areas where the ATLF zone crosses the E-W-trending normal faults  on the Tyrrhenian Sea and the NE-SW normal structure of the Ionian Sea. The latter area that includes the eastern Peloritani area (Messina Strait and Ionian coasts, Figure 1) is shaped by a complex network of normal and transtensional faults  with extensional and right-lateral transtensional tectonics in the southern Messina Strait. This complex tectonic network looks to be able to generate high permeability zones where mantle volatiles can be driven to the surface, in full agreement with the obtained results.
On the other hand, the area is characterized by high strain rate (SR) , where the contemporary creeping of the upper and the lower crust may produce that high permeability zone where the mantle fluids move towards the surface without significant temporal changes, in full agreement with our findings. In terms of the escape of fluids, we found the highest CO2 flow rates at the eastern side of the area (Peloritani chain). By contrast, the Nebrodi area, to the west of the ATLF, seems to accumulate strain at lower rates acting as a locked fault marked by lower CO2 degassing rates and lower mantle volatile content. The ATLF zone cannot be simply described as a boundary line; however, it is a zone where many different fault segments separate locked fault volumes from areas where dilatancy goes on. Following the model proposed by Riguzzi et al. , the elastic energy accumulates in those areas where faults are locked and the strain rate (SR) is lower. High SR areas can be interpreted as regions where both the upper and the lower crust are creeping or alternatively where tectonic loading is more effective. Vice versa, lower strain rates suggest the presence of locked faults in a later stage in the seismic cycle . Doglioni et al.  proposed that during an interseismic period, the brittle-ductile transition zone separates the brittle upper crustal layers, where the faults are almost locked, from the deeper shear zones where the ductile deep crust/shallow mantle layers are constantly creeping. In terms of the geochemical features of the fluids and their temporal behaviour, this model implies that deep, mantle-originated volatiles (marked, for example, by high helium isotopic ratios) are continuously released at the level of the ductile, upper mantle level. They cannot easily move toward the surface over areas marked by very low SR due to absence of creeping that is the lack of discontinuities where fluids can move across. Contrastingly significant contribution of mantle-type fluids and high degassing rates can be expected over areas undergoing dilatancy, namely, where high SR produce measurable crustal deformation due to creeping activity that allows a fast and, in terms of masses, significant contribution of mantle-derived volatiles.
The CPA is one of the highest seismic risk-prone territories in Italy and includes areas with both high and low strain rate accumulation , thus representing a suitable test site to better understand the behaviour of the fluids with respect to the tectonic setting.
The investigations carried out on the volatiles vented over the CPA show that a huge amount of CO2 is daily dissolved in the groundwater. The dissolved gases carry helium with a typical, although variable, mantle signature here interpreted as induced by the lithospheric character of the ATLF.
During the evolution of the seismic cycle, deep lithospheric faults are able to drive mantle-derived fluids to the surface and to change the mixing proportions with the shallow fluids. Following the model proposed by Doglioni et al. , we may detect significant contribution of deep fluids over an area accumulating strain at a high rate in full agreement with the geochemical features of the fluids vented over the Peloritani and Nebrodi mountains. Due to high seismicity of the area, further investigations, including discrete and continuous monitoring activity as well as accurate estimations of the degassing rates, may provide a better insight in defining the role of the fluids during the final stage of a seismic cycle. Overall, an integrated approach coupling fluid geochemistry with accurate mapping of the actual SR analysis could represent an effective tool in order to better constrain those areas exposed to high seismic risk.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
We acknowledge the thorough and constructive reviews of two anonymous referees, which greatly improved the manuscript. The authors are also indebted to Prof. Alessandro Tripodo and Dott. Giuseppe Sabatino for their help during the field work. Mauro Martelli and Andrea Rizzo are kindly acknowledged for their support during the laboratory work. The research work was supported by the Istituto Nazionale di Geofisica e Vulcanologia (INGV)-DPC grants, S2 and V5 projects, Research Unit-Italiano.
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