Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy)

GEXPLO-05455; No of Pages 10 Journal of Geochemical Exploration xxx (2014) xxx–xxx

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Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy) A. Sciarra a,⁎, A. Fascetti b, A. Moretti c, B. Cantucci a, L. Pizzino a, S. Lombardi d, I. Guerra e a

INGV, Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata, 605-00143 Rome, Italy DOF Subsea, Thormøhlens Gate 53C, 5006 Bergen, Norway Life, Health and Environmental Sciences Department, University of L'Aquila, Via Vetoio (Coppito 1), Coppito, 67100 AQ, Italy d Earth Science Department, Sapienza Università di Roma, Piazzale Aldo Moro, 5-00185 Roma, Italy e Physics Department, University of Calabria, Via P. Bucci, Arcavacata di Rende, 87036 CS, Italy b c

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 6 June 2014 Accepted 31 August 2014 Available online xxxx

Geochemical and geophysical surveys were carried out in the Cagno valley (Sila massif, central-northern Calabria, Italy) to investigate the gas bearing properties of a seismogenic fault (Lakes Fault, LF), discovered by paleoseismological analysis. Soil gas measurements (N2, O2, Rn, CO2, CH4 and light hydrocarbons) and exposure to γ radiations were performed along two detailed profiles (about 150 m long), trending almost parallel to a trench crossing the LF. The highest values of Rn, γ radiation, CO2, CH4 and light hydrocarbons were detected in the area around the LF and 100 m far away. In the central part of the profiles, where a hanging valley is present, geo-gas distribution is likely controlled by both lithology (colluvial deposits and peaty silt deposits, characterized by medium to low permeability) and the presence of a local cold aquifer. In particular, water table influences the circulation of the gas species in the sub-surface environment, as well as their distribution at the surface by playing a sort of sealing effect for the gas migration. In the area located about 100 m westward of the fault, characterized by soils originated from altered granodiorites, the occurrence of a previously unknown blind fault is supposed. The multidisciplinary approach of this work allows to better understand the relationship between geochemical and geophysical analyses linked to migration processes of deep fluid through preferential leakage pathways providing some hints on the spatial influence of active tectonic. © 2014 Elsevier B.V. All rights reserved.

Keywords: Soil gas survey γ radiations measurements Multidisciplinary approach Leakage pathways Sila massif

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . Geological and seismotectonic setting Geochemical and geophysical methods Results and discussion . . . . . . . 4.1. Cagno 1 profile . . . . . . . 4.2. Cagno 2 profile . . . . . . . 5. Conclusions . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . References . . . . . . . . . . . . . . .

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1. Introduction Several studies have observed anomalous gas concentrations over active faults and have defined these gases as fault indicators (Baubron et al., 2002; Ciotoli et al., 1999, 2005; Fu et al., 2005; Klusman, 1993; ⁎ Corresponding author. Tel.: +39 651860748; fax: +39 651860507. E-mail address: [email protected] (A. Sciarra).

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Lombardi et al., 1996; Quattrocchi et al., 2012; Zhang and Sanderson, 1996; Zhiguan, 1991). According to Toutain and Baubron (1999), fault gases display a very wide range of geochemical signatures, even on a single fault. In a first attempt, it is possible to relate this feature to the contrasted characters and sources of the respective leaking gases. Indeed, thermal, radiogenic and geodynamic processes are involved in earth degassing at active faults, therefore inducing complex patterns of degassing from the

http://dx.doi.org/10.1016/j.gexplo.2014.08.015 0375-6742/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

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A. Sciarra et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

crust. In particular, radon, often together with other gas like CO2 and CH4 is successfully used as pathfinder component to discover buried fault and fracture fields, both in soils and in groundwater, mostly when other conventional structural-geology methods are lacking or unreliable (Abumurad and Al-Taminmi, 2001; Annunziatellis et al., 2003; King et al., 1993; Lombardi and Voltattorni, 2010; Pizzino et al., 2004a, b). Generally speaking, the spatial distribution of soil gases in faulted areas appears to be a suitable tool for identifying active tectonic structures, also in areas characterized by thick clay covers whose plastic behavior and pervious feature could mask the identification of faults by means of other geological and geophysical methods (e.g. Baubron et al., 2002; Ciotoli et al., 1998, 1999, 2005; Quattrocchi et al., 2012). Furthermore, the γ radiation, originated from long-life radioactive natural elements (e.g. 232Th, 238U, and 222Rn), gives a radioactivity value site specific due to the minerals contained in the soil. The gamma radiation study coupled with radon measurements can give some hints to discriminate radon origin. Indeed, soil 222Rn activities depend mainly on: i) the uranium and radium distribution, ii) rocks and soils emanating power, iii) the permeability and porosity of the host rock and soil, iv) the soil moisture conditions, and v) the carrying gas flow (e.g. Ball et al., 1991; Buccianti et al., 2009; Morawska and Phillips, 1993; Tanner, 1964; Yang et al., 2003). In a wide range of geological settings, carrier gases such as CO2 and CH4, may play a dominant role for non-diffusive transport and redistribution of trace gases (radon, He and H2) toward the Earth's surface (e.g. Ciotoli et al., 1999; Etiope and Lombardi, 1995; Hermansson et al., 1991a,b; Malmqvist and Kristiansson, 1984; Sugisaki et al., 1983; Toutain and Baubron, 1999). Moreover, the measured chemical compounds can be selected for the source information they are expected to provide: superficial for radon, deep for association of radon, CH4 and CO2, and mixed for single species (e.g. CO2). In this way, soil gas measurements can distinguish whether or not fractures are leaking (Baubron et al., 2002). In 2001, the Italian Civil Protection Department carried out paleoseismological analyses in the Sila massif (central-northern Calabria, Italy) to study the primary source of the catastrophic 8 June 1638 Crotonese earthquake (Me = 6.78; Working Group CPTI, 1999).

In the framework of these geological and geomorphological surveys, Galli and Bosi (2003) individuated a previously unknown fault (Lakes Fault, LF onward), whose footwall dams small and large streams, creating lakes and ponds. To find evidence of this fault, they opened four trenches across the tectonic structure (Fig. 1), identifying several displacement events dated back to the 1638, corresponding to previous unknown earthquakes (Galli and Bosi, 2003). Coupled with paleoseismic surveys, we performed detailed geophysical and geochemical analyses on two profiles perpendicular to the LF, in the northern part of the Cagno basin, in correspondence of trench 3 (Fig. 1). This area, still today called “a colla u terremotu” (the sag of the earthquake, in the local Calabrian dialect), was selected for our surveys due to evidence of tectonic activity dated to XVII century. Indeed, a piece of glass, probably belonging to an oil lantern felt in the chasm opened during the 1638 earthquake, was found in the trench 3. In particular, on the two profiles parallel to trench 3, we measured the exposure to γ radiation, 1 m above ground level, and the soil gas concentrations (N2, O2, Rn, CO2, CH4 and light hydrocarbons) at 0.6 m depth. In view of these considerations, the main objective of this study is to apply soil gas spatial distributions coupled with γ radiation exposure to: i) individuate enhanced permeability sectors possibly linked to preferential leakage pathways such as fault and/or fracture systems, and ii) discriminate the migration processes and the carrier role of the various gaseous species.

2. Geological and seismotectonic setting The investigated area is located in the Sila massif, in the southern segment of the Apennine chain known as the Calabrian Arc (hereafter CA). It lies on the upper plate of the Tyrrhenian–Ionian subduction system (Fig. 2; e.g. D'Agostino et al., 2011), where the Ionian lithosphere subducts beneath the CA and dips steeply at 75°–80° beneath the Tyrrhenian Sea down to a depth of 450–500 km (Selvaggi and Chiarabba, 1995).

Fig. 1. North Ampollino Lake area. The central strand of Lakes Fault is drawn together with the trench sites, the sampling profiles and the earthquake sag location.

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

A. Sciarra et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

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Fig. 2. W-E geological cross section through central-northern Calabria, from the Tyrrhenian Sea to the Ionian Sea (A). Black stars indicate the possible hypocenters of the two 1638 main shocks, occurred within the extending upper crust of the Calabrian arc (modified after Van Dijk and Scheepers, 1995).

Since the Middle Miocene, over-thrusting, combined with the progressive south-eastward migration of the CA along a NW-SE to WNWESE-trending regional strike–slip fault system (Ghisetti and Vezzani, 1982; Tansi et al., 2005, 2007), was associated with the opening of the Tyrrhenian basin (Amodio-Morelli et al., 1976; Ghisetti and Vezzani, 1982). A zone of crustal disequilibrium between the uplifting Aspromonte–Sila mountain chain (25–45 km thick continental crust) (Ghisetti and Vezzani, 1982) and the subsiding Tyrrhenian basin (10 km thick crust of sub-oceanic composition) (Fig. 2; Finetti and Morelli, 1973) was induced. Consequently, several N-S and NE-SW trending basins formed on the inner margin of the belt (Galli and Bosi, 2002; Tortorici et al., 1995).

From middle Pleistocene, Calabria was rapidly uplifted, favoring the deposition of thick sandy and conglomeratic bodies of deltaic or littoral environment directly over the marine clays (Moretti and Guerra, 1997). These deposits are found at elevation of 400 m asl within the extensional basins (i.e., Crati half-graben basin), whereas Pliocene–Pleistocene sediments and contemporary continental paleo-surfaces may reach 1400 m asl in the raised blocks of Sila, Serre, and Aspromonte (Galli and Bosi, 2003). The Sila massif consists of the Calabride Complex rocks (pre-Alpine metamorphic and igneous rocks with Meso-Cenozoic cover), structurally arranged to form imbricated km-scale thrust sheets (e.g., Bonardi et al., 2001; Caggianelli and Prosser, 2001; Ghisetti and Vezzani, 1982;

Fig. 3. Sketch of trench 3, opened in the northern Cagno basin and short description of found lithologies. In the sediments belonging to Unit 2 the ancient lamp glass was found (after Galli and Bosi, 2003).

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

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A. Sciarra et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

Lanzafame and Tortorici, 1980; Van Dijk et al., 2000). The Sila basement is made up of medium- to high-grade metamorphic rocks (amphiboliteto granulite-facies) intruded by late Hercynian granitoids (e.g., Borsi and Dubois, 1968; Graeßner and Schenk, 2001; Graeßner et al., 2000; Gurrieri et al., 1978; Lorenzoni et al., 1978; Messina et al., 2004). The studied LF is a W-dipping normal fault, which elongates for 15 km in the NNW-SSE direction, dislocating alteration deposits of batholitic unit with a slip lower to 100 m (Messina et al., 1994). In Cagno locality, the LF makes a small hanging valley (Fig. 3). In its middle part, colluvial deposits and peaty silt are interbedded, testifying periodically marsh or lake deposits presence, during several reactivations of dislocation. Across the LF structure Italian Civil Protection Department opened four trenches (Fig. 1), to find evidence of this fault. Geochemical and geophysical surveys were carried out in trench 3. This, 7 m long, 3 m deep, shows a branched fault system, dipping 60° SW, with several subvertical-to-reverse splays, affecting almost all the exposed deposits with a dominant normal component (Fig. 3; Galli and Bosi, 2003). Crystalline rocks outcrop in the footwall (Unit 13 of Galli and Bosi, 2003; Figs. 3 and 4) and a squeezed wedge of gray lacustrine clay is trapped along fault B (Unit 12; Figs. 3 and 4). In the hangingwall, alternating sand–gravel–silt alluvial deposits (Units 3–11; Fig. 3) are present, with a wedge of lacustrine clay (Unit 12; Galli and Bosi, 2003). In these deposits the ancient oil lantern felt in the open chasm. The crystalline substrate is often highly weathered in its upper part, forming weathering products up to 100 m thick. From a geodynamical point of view, Calabria is one of the most seismic areas of the Mediterranean region, and is characterized by strong and destructive earthquakes (Basili et al., 2008; CPTI Working Group, 1999, 2004). Active crustal deformation is documented by the intense historical seismicity generally associated with the active normal faults (Galli and Bosi, 2003; Tortorici et al., 1995) which, following the trend of the arc, accommodate arc-perpendicular extension all along the CA. The distribution of historical seismicity and paleoseismological studies in central-northern Calabria shows that active deformation (scattered M b 4.5 events) is distributed between two main fault systems trending approximately north–south: the Crati Valley (Tortorici et al., 1995) and the LF system (Galli and Bosi, 2003).

3. Geochemical and geophysical methods Two detailed geophysical and geochemical profiles were carried out in the northern part of the Cagno basin (trench 3, Figs. 1 and 5a), crossing the LF. The two profiles, distant 40 m each other, were performed

Fig. 4. Picture of NE sector of trench 3. Gray lacustrine clays (Unit 12, in Fig. 3) are trapped along fault B, into the crystalline basement (Unit 13).

approximately perpendicular to the fault plane and parallel to the trench 3. 33 sampling points were carried out along the Cagno 1 profile (20 m west from the trench and 152 m long), while 29 points along the Cagno 2 profile (60 m west from the trench and 140 m long, Fig. 5), in Spring 2001 soon after the trench opening. Starting from the fault evidence on the surface, we performed a sampling with a mean interval of 2 m, increasing progressively the step up to 8 m, in function of the distance from the fault (Tables 1 and 2). The exposure to γ radiations and the radon concentrations were measured in situ, while soil gas were collected and successively analyzed in the laboratory. γ radiation was measured by means of Ludlum Instruments detector equipped with sodium iodide crystals NaI, doped with tallium (Tl) as inorganic scintillator. A scintillator is a material that exhibits scintillation, the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., re-emit the absorbed energy in the form of light. NaI(Tl) is by far the most widely used scintillator material for γ rays detection due to its fast response, excellent linearity, and a very stable light output over a wide range of temperatures. NaI(Tl) is very hygroscopic and needs to be housed in an air-tight enclosure to protect it from moisture. In the used detector, the inorganic scintillator is coupled to an electronic light sensor such as a photomultiplier tube (PMT), to obtain a scintillation detector or scintillation counter. PMT absorbs the light emitted by the scintillator and re-emits it in the form of electrons via the photoelectric effect. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator. Soil gas surveying consists in collecting and analyzing gas sample from the vadose zone to measure the concentrations of the gaseous species that permeate the soil pores. Sampling was accomplished in a period of stable and dry weather conditions, and in a short time to minimize any variations induced by different sampling periods. The gas samples were gathered with a probe constituted of a thin hollow stainless steel tube (8 mm of external diameter), approximately 120 cm long. The bottom of the probe is characterized by a series of small holes that allow the gas to pass into the probe from the soil. To avoid the major influence of meteorological variables, samples were collected to a depth of 0.6 m (e.g. Hinkle, 1994; Segovia et al., 1987). After a first sampling to purge the tube, a 60 ml sample was collected in order to analyze the radon concentration, followed by a further aliquot which was injected into a vacuum steel samplers/bottle to be analyzed in the laboratory. Radon analysis, executed by means of a portable scintillation counter of Lucas cells (EDA RD 200), is based on the principle of 222Rn to 218Po decay which results in the emission of α-particles. An α-sensitive ZnS(Ag) scintillation cell converts radiation to light energy, which in turn is converted to electrical impulse by a photomultiplier system and amplified by a logical circuit. A background (BG) measurement must be performed before each analysis to quantify the cell's residual activity; this value is considered in the total radon calculation. After the BG definition a gas sample, collected from the probe, is injected in the α-scintillometer cell and five successive, 1-minute counts, are made. For calculation purposes, only the fifth reading (Rn 5°) is considered. Total Rn activity can be evaluated with the following equations:
 Rntot ðpCi=LÞ ¼ Rn 5 −BG  10

ð1Þ

Rntot ðBq=LÞ ¼ Rntot ðpCi=LÞ  0:037

ð2Þ

In laboratory, the major (N2, O2, CO2) gases and light hydrocarbons (CH4, C2H4, C2H2, C2H6, C3H8) were analyzed by means of Carlo Erba

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

A. Sciarra et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

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Fig. 5. a) Measured profiles (Cagno 1 and Cagno 2) across Cagno Valley, 20 and 40 m west to the trench 3 (in the background). b) Picture of the trench 3. c) Detail of trench 3. In the dark brown sedimentary vein the ancient glass lantern was found. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

GC8000 gas-chromatograph, constituted by 4 columns. The measurement of sample concentrations is based on the comparison of gas reference standard, building calibration curves. Detectors used are: thermal conductivity detector (TCD) for N2, O2 and CO2 in order to achieve a sensitivity up to percentage and flame ionization detector (FID) for light hydrocarbons. The FID permits a sensitivity in the order of 100 ppb. 4. Results and discussion Generally, the abundance of soil gases in a fault zone is different from the atmospheric air composition (0.01 kBq/m3 for 222Rn, 0.038% for CO2, 1.4 ppm for CH4, 5.20 ppm for He, 78.08% for N2 and 19.4% for

O2) and their distribution is affected by geologic, biogenic, and meteorological factors. This is especially evident for gas leakage from deep faults (e.g. Toutain and Baubron, 1999). N2 and O2 were not reported in tables and discussed as their average values are very close to the atmospheric composition. To assess the anomalies and locally define the complex origins of the studied gas, the detection of a background value and its anomaly threshold constitute a fundamental step in the geostatistical approach (Ciotoli et al., 2007). Therefore, to define statistical populations for each parameter and to choose background values, collected data (Tables 1 and 2) were processed with a statistical approach, by means of normal probability plot (NPP) (Fig. 6). According to Sinclair (1974, 1991), the NPP

Table 1 Data gathered along the Profile 1, 20 m west to trench 3. Distance from the fault (m)

γ (mR/h)

Rn (Bq/l)

CH4 (ppmv/v)

C2H2 (ppmv/v)

C2H4 (ppmv/v)

C2H6 (ppmv/v)

C3H8 (ppmv/v)

CO2 (%v/v)

−120 −112 −104 −96 −88 −80 −72 −62 −56 −48 −40 −36 −32 −28 −24 −20 −16 −12 −8 −6 −4 −2 0 2 4 6 8 12 16 20 24 28 32

5.75 5.70 6.00 5.62 6.88 6.50 5.13 6.25 5.88 5.70 5.35 5.18 4.95 5.32 5.46 6.05 6.18 6.05 6.18 6.00 5.40 4.40 5.25 6.10 6.85 6.30 6.62 6.55 6.75 6.20 6.55 6.30 5.50

27.01 36.63 48.84 17.30 23.31 27.01 19.98 18.98 15.13 12.21 23.31 9.99 23.68 8.88 24.79 24.42 19.61 14.43 24.79 18.50 22.57 8.14 2.22 6.29 9.62 10.73 13.69 12.95 5.92 9.25 12.58 20.35 11.47

1.51 1.73 0.86 1.02 1.05 1.35 1.36 1.07 1.12 0.83 1.02 3.28 1.44 1.36 1.48 1.26 1.31 1.25 1.09 0.98 1.18 1.31 6.60 1.46 0.97 1.03 1.89 1.21 0.99 1.34 3.99 0.93 1.28

0.01 0.01 0.01 b.d.l. 0.07 0.01 0.01 0.01 0.01 0.01 b.d.l. 0.01 0.01 0.01 0.02 0.02 0.01 b.d.l. 0.01 0.01 0.05 0.01 0.02 0.01 0.01 0.02 0.03 0.01 0.02 0.01 0.03 0.05 0.01

0.01 0.07 b.d.l. b.d.l. b.d.l. b.d.l. 0.01 0.01 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.11 b.d.l. 0.12 0.02 0.08 0.11 b.d.l. 0.01 b.d.l. b.d.l.

0.01 b.d.l. 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.03 0.01 0.01 b.d.l. 0.01 0.01 0.01 0.01 0.04 0.01 0.01 0.02 0.04 0.01 0.03 0.01 0.04 0.01 0.01

0.12 0.19 0.23 0.11 0.11 0.13 0.19 0.13 0.09 0.11 0.11 0.19 0.12 0.17 0.23 0.19 0.18 0.18 0.13 0.21 0.06 0.17 0.40 0.19 0.18 0.17 0.18 0.06 0.15 0.15 0.36 0.17 0.16

0.96 0.68 0.85 0.65 0.48 0.40 0.51 0.57 0.47 0.77 0.65 0.04 0.74 0.89 0.65 0.82 0.47 0.28 0.34 0.55 0.49 0.24 1.14 0.22 0.32 0.35 0.72 0.42 0.40 0.31 0.09 1.05 0.38

Sampling distance ranges from 2 to 8 m increasing progressively in function of the distance from the fault. b.d.l.: below the detection limit (2 ppb for light hydrocarbons).

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

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A. Sciarra et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

Table 2 Data gathered along the Profile 2, 40 m west to the trench 3. Distance from the fault (m)

γ (mR/h)

Rn (Bq/l)

CH4 (ppmv/v)

C2H2 (ppmv/v)

C2H4 (ppmv/v)

C2H6 (ppmv/v)

C3H8 (ppmv/v)

CO2 (%v/v)

−134 −126 −118 −110 −102 −94 −86 −78 −70 −66 −62 −58 −54 −50 −46 −42 −38 −34 −30 −26 −22 −18 −14 −10 −6 −2 0 2 6

6.45 7.15 7.50 7.55 7.70 7.35 7.00 7.05 6.90 6.65 6.53 6.45 6.35 6.32 6.35 6.20 6.32 6.39 6.37 6.08 6.37 6.39 6.70 6.86 6.80 7.28 7.75 7.60 6.90

17.39 16.65 13.69 16.65 24.05 10.73 10.73 14.80 1.48 7.40 7.77 10.73 19.61 2.59 16.28 14.06 15.54 28.86 20.72 17.39 26.27 17.39 12.95 20.35 19.61 19.61 7.40 3.70 15.91

1.75 1.39 1.94 1.09 1.19 0.91 1.12 1.15 0.14 1.14 0.66 1.32 1.46 1.13 0.86 1.22 1.43 1.10 0.92 1.08 0.78 1.10 1.51 1.21 1.19 1.28 2.09 1.44 1.70

0.01 0.01 0.01 0.07 0.01 b.d.l. 0.01 0.01 0.01 0.01 0.01 b.d.l 0.01 0.01 0.01 0.01 0.01 b.d.l 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.07 0.01 0.04

0.06 b.d.l b.d.l 0.10 b.d.l. b.d.l. b.d.l b.d.l. b.d.l b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.13 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.03 0.10 0.01 b.d.l.

0.01 0.01 0.01 0.07 b.d.l. 0.01 0.01 0.01 0.01 b.d.l 0.01 0.01 0.01 0.01 0.01 b.d.l 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.04 0.07 0.01 0.03

0.17 0.15 0.12 0.40 0.09 0.43 0.07 0.19 0.12 0.24 0.32 0.18 0.13 0.12 0.16 0.2 0.14 0.13 0.15 0.13 0.19 0.15 0.18 0.14 0.22 0.15 0.40 0.18 0.32

0.66 0.69 0.79 0.70 1.04 0.74 0.71 0.69 0.29 0.42 0.36 0.50 0.73 0.70 0.60 0.30 0.64 0.81 0.81 1.08 1.14 1.13 1.24 0.91 1.11 0.69 0.76 0.68 0.67

Sampling distance ranges from 2 to 8 m increasing progressively in function of the distance from the fault. b.d.l.: below the detection limit (2 ppb for light hydrocarbons).

Fig. 6. Normal Probability Plot (NPP) referred to the Cagno 1 and Cagno 2 profiles, to define statistical populations for each parameter and to choose contour thresholds of: a) Rn; b) CO2; c) CH4; d) C3H8.

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

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provides a good method to distinguish different populations and a more objective approach to statistical anomaly threshold estimation (Ciotoli et al., 2007). Considering only the values higher than background (Fig. 6), trends of geo-gas concentrations related to a geological sketch of the Cagno 1 and Cagno 2 profiles are shown in Figs. 7 and 8. Among light hydrocarbons we selected C3H8 as the most representative. To better appreciate the spatial distribution of the measured data, γ radiation profiles include all data, considering also the background values. 4.1. Cagno 1 profile The radiometric exposure of substratum shows a discrete uniformity in the Cagno 1 profile, ranging from 4.40 to 6.88 μR/h. These values are significantly lower than the average generally measured in the Sila massif. Indeed, average exposure values measured by Seismological Laboratory of UNICAL during several surveys in 1995–2000 period range from 13 to 14.5 μR/h (unpublished UNICAL data). Maximum values (N 80 μR/h) were found in the Fossiata locality, close to the Cecita lake, often associated to small springs located at the bottom of granitic rocks. Also Calcara et al. (1996) found the same geochemical distribution: in the Fossiata sector they measured very high γ values emitted from outcropping rocks and soils, while in the Cagno area exposure was lower than in other sectors of the Sila plateau. As already mentioned, the γ radiation originates from uraniumbearing minerals. This element preferentially concentrates in the Earth's crust and a close relationship between the type of igneous rock and its uranium content is largely recognized (e.g. Larsen and Gottfried, 1960; O'Neil, 1998; Plant et al., 2003); acid igneous or granitic rocks have the highest concentrations (2–10 mg kg−1), basaltic rocks contain lower concentration (0.3–0.8 mg kg−1) while the concentrations in sedimentary rocks show a wide range (Buccianti et al., 2009). The low γ radiation values measured in the Cagno area are possibly due to the uranium ions leaching phenomena from weathered granitic rocks of basement (e.g. Taboada et al., 2006; Fig. 7). The measured exposure values result lower in the central part of the studied area, where low-permeability clay deposits are present. Clay deposits of the Sila plateau are derived from lacustrine deposits (Galli and Bosi, 2003) and not from soil developed on granitic rocks. Therefore we can reasonably hypothesize that uranium contents are lower in the Sila lacustrine clays than in the altered granitic deposits. Unfortunately mineralogical composition of lacustrine clays is not available to confirm or reject our inferring. In addition, at time of sampling, a cold shallow aquifer (2 m below ground level as measured in a well located in the western part of the valley) was recognized in the Cagno 1 profile (Fig. 7a). In the presence of water and oxidizing conditions, uranium can form the uranyl ion (UO2+ 2 ) and water-soluble complexes (depending on pH; e.g. Taboada et al., 2006) which can be easily mobilized from vadose waters, creating local anomalies (e.g. Calcara et al., 1996; Hem, 1989). Unfortunately, at time of the survey, the water table was not chemically analyzed, preventing the chance to confirm this hypothesis. Then, the presence of a water table may have created a sealing effect for radioactivity uprising. Radon values increase progressively on the footwall side of the fault. The maximum radon value (48.84 Bq/L; 3 times higher than the average values of the study area) was measured 100 m far from the LF, in soils originated from altered granodiorites (locally defined “sabbioni”). “Sabbioni” are characterized by higher specific surface area than granite Fig. 7. a) SW-NE geological sketch of Cagno 1 profile (20 m west to the trench 3 and 150 m long). Pink: crystalline basement (Unit 13); cyan: lacustrine clay (Unit 12); yellow: colluvial deposits (Unit 2). A shallow well and its water table are represented in the south-western sector of the sketch. b) Spatial distribution of γ radiation (μR/h) considering all measured data; c–f) spatial distribution of soil gas concentrations (Rn, CO2, CH4 and C3H8) considering only the values higher than background. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

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from which they originate, allowing a more efficient release of radon through the soil (e.g. Calcara et al., 1996). Radon minimum value (2.22 Bq/L, below the background value and then not shown in Fig. 7c) was detected in the silt-rich deposits in the central part of the hanging valley (Table 1). The observed distribution seems to be linked to the substratum permeability. High radon values were found in zones characterized by fractured and altered uraniumrich rocks, while low values were measured close to the fault plane, sealed by cataclasis and clay formation through feldspar weathering. CO2 concentration values show two positive anomalous zones (Fig. 7d). The first zone is located close to the fault, where the colluvial soils are present, with maximum values equal to 1.14%v/v. The second anomalous area is placed about 100–120 m west to the fault, where the crystalline substrate (Unit 13) directly outcrops at surface, with maximum values of 0.96%v/v. Moreover, a further positive peak is present 30 m east of the fault (1.05%v/v). Methane and propane distribution (Fig. 7 e–f) and other minor light hydrocarbons as ethylene, acetylene and ethane (Table 1), show a good correlation each other and also with the above-considered species. The highest light hydrocarbon values are found above the fault (6.60 ppmv/v for CH4, 0.02 ppmv/v for C2H2, 0.11 ppmv/v for C2H4, 0.04 ppmv/v for C3H6 and 0.40 ppmv/v for C3H8) and at few meters eastward (3.99 ppmv/v for CH4, 0.03 ppmv/v for C2H2, 0.11 ppmv/v for C2H4, 0.04 ppmv/v for C2H6 and 0.36 ppmv/v for C3H8). Moreover, a small peak (1.73 ppmv/v for CH4, 0.07 ppmv/v for C2H4 and 0.23 ppmv/v for C3H8) was measured 100–120 m west of the fault, showing a positive correlation with the maximum radon values. 4.2. Cagno 2 profile The Cagno 2 profile was carried out in the western part of the studied area, 40 m from the LF toward the mountain, at higher elevation than Cagno 1 (Fig. 5a). Spatial evolution of analyzed species is shown in Fig. 8. γ radiation trend (Fig. 8b) shows values generally higher than Cagno 1, ranging from 6.08 to 7.75 μR/h. The lowest values (below 6.5 μR/h) are present only in the middle of the valley, characterized by colluvial soils (Unit 2). Radon concentration highlights a positive peak of 24.05 Bq/L 100 m west of the fault, at the same distance of high values of Cagno 1 profile (Fig. 7c). The highest radon concentration (28.86 Bq/L) was found in the hanging valley, where permeable colluvial deposits allow the flow toward surface. The difference of radon behavior between the two profiles is related to the different lithology (clayey and colluvial deposits) present in the hanging valley and, probably, to a different distribution of the U-bearing minerals inside the rock matrix. Unfortunately, petrographic and mineralogical data of considered formations are not available to confirm or reject our inferring. Differently from the Cagno 1, the presence of a local aquifer was not recognized in the Cagno 2 profile, minimizing the buffer effect of the water table. As in the Cagno profile 1, two positive anomalous zones in CO2 concentrations were highlighted (Fig. 8d) in Cagno 2. Maximum values of 1.24%v/v and 1.04%v/v were found close to the fault (colluvial soil) and about 100–120 m westward (crystalline substrate, Unit 13), respectively. The highest hydrocarbon values (Fig. 8e–f; Table 1) follow the CO2, Rn and γ radiation distribution with anomalies on the fault (2.09 ppmv/v for CH4, 0.07 ppmv/v for C2H2, 0.10 ppmv/v for C2H4, 0.07 ppmv/v for C2H6 and 0.40 ppmv/v for C3H8) and 100–120 m Fig. 8. a) SW-NE geological sketch of Cagno 2 profile (40 m west to the trench 3 and 142 m long). Pink: crystalline basement (Unit 13); yellow: colluvial deposits (Unit 2). b) Spatial distribution of γ radiation (μR/h) considering all measured data; c–f) spatial distribution of soil gas concentrations (Rn, CO2, CH4 and C3H8) considering only the values higher than background. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015

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Fig. 9. SW-NE geological sketch of Cagno 2 profile, summarizing the good correlation between the geophysical and geochemical measurements.

westward (1.94 ppmv/v for CH4, 0.07 ppmv/v for C2H2, 0.10 ppmv/v for C2H4, 0.04 ppmv/v for C2H6 and 0.43 ppmv/v for C3H8). In both profiles spatial distribution of soil gas survey highlighted a general association among the considered species in two specific zones: the former close to the fault and the latter about 100 m to the west (Fig. 9). This phenomenon is less marked in the profile 1 due to the presence of clayey soils and cold shallow waters, both causing a possible sealing effect. Association of geo-gas (i.e. CO2, CH4 and light hydrocarbon concentrations), measured at the center of the studied valley, constituted by marsh and peat deposits, could be related to some mechanisms affecting their distribution in the shallow environment such as organic matter decomposition, rock weathering, root respiration and redox processes. On the other hand, the positive correlation among 222Rn, CO2, CH4 and C3H8 high values, in the area placed about 100 m west of the LF (Figs. 7E–F and 8E,F) could suggest the presence of a previously unknown blind fault, only affecting the shallowest strata (Fig. 9). This tectonic structure could be responsible for the observed spatial distribution of the gases, promoting their advective transport toward the surface. Indeed, anomalous gas concentrations over active faults have been widely recognized over active fault systems (e.g. Baubron et al., 2002; Ciotoli et al., 1999; Klusman, 1993; Zhang and Sanderson, 1996). Moreover, CO2 and CH4 could have played a dominant role as carrier gas for nondiffusive transport and redistribution of trace gases such as radon. Finally, the overlapping between geo-gas concentration and γ radiation encourages the hypothesis that analyzed species behavior is mainly controlled by substratum permeability. 5. Conclusions To investigate the fluid migration mechanisms through preferential leakage pathways in tectonically active areas, a multidisciplinary study, coupling geochemistry and geophysics measurements, was carried out across the Lakes Fault. Soil gas and γ exposure patterns allowed us to identify two sectors characterized by anomalous values of γ radiation, radon, CO2, CH4 and light hydrocarbons: close to the LF and 100 m westward. The association of several gas species, which may have different origin, allowed to distinguish from anomalies due to local conditions (e.g. lithology, U-bearing minerals distribution, permeability), and gas upraising along preferential pathways. Local anomalies are probably due to the presence, in the central part of the profiles, of both colluvial and peaty silt deposits. In particular, CO2, CH4 and light hydrocarbons probably stem from the organic matter present in the marsh and peat deposits at the center of the hanging valley. Moreover, the local water table might act as a barrier affecting the sub-surficial fluid circulation as well as radioactivity uprising.

Geo-gas and radioactivity anomalies, possibly linked to the presence of a buried fault, were recognized 100 m from the LF, where soils originated from altered granodiorites are present. Finally, the positive correlation between geo-gas concentration and γ radiation suggests that substratum permeability is the main controlling factor of analyzed species distribution. The combined geochemical and geophysical investigation used in this work should be extended to the remnant sectors of the Sila massif with the aim to define areas characterized by both high levels of radon from soils and sub-surficial gas upraise possibly linked to the structural setting of the area.

Acknowledgment This research was financially supported by Italian MURST (now MIUR—Ministero dell'Istruzione, dell'Universita' e della Ricerca Scientifica e Tecnologica) through project “Structural setting and fluid circulation: development of an integrated system of geophysical and geochemical methods for the study of the relations between active the geodynamic processes and for the seismic risk assessment (Calabria) (RSV 1) - Cluster C11-B, plan 1”.

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Please cite this article as: Sciarra, A., et al., Geochemical and radiometric profiles through an active fault in the Sila Massif (Calabria, Italy), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.015