Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

Accepted Manuscript Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

María Clara Lamberti, Nicolás Vigide, Stefania Venturi, Mariano Agusto, Daniel Yagupsky, Diego Winocur, Hernán Barcelona, María Laura Velez, Carlo Cardellini, Franco Tassi PII: DOI: Reference:

S0377-0273(18)30535-3 https://doi.org/10.1016/j.jvolgeores.2019.02.004 VOLGEO 6551

To appear in:

Journal of Volcanology and Geothermal Research

Received date: Revised date: Accepted date:

20 November 2018 11 February 2019 12 February 2019

Please cite this article as: M.C. Lamberti, N. Vigide, S. Venturi, et al., Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex, Journal of Volcanology and Geothermal Research, https://doi.org/10.1016/j.jvolgeores.2019.02.004

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ACCEPTED MANUSCRIPT Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex María Clara Lamberti1, Nicolás Vigide1,2, Stefania Venturi3, Mariano Agusto1, Daniel Yagupsky2, Diego Winocur4, Hernán Barcelona2, María Laura Velez1, Carlo Cardellini5, Franco Tassi6. 1

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GESVA-IDEAN, Departamento de Ciencias Geológicas, FCEN, Universidad de Buenos Aires, Argentina. 2 LAMOGE-IDEAN, Departamento de Ciencias Geológicas, FCEN, Universidad de Buenos Aires, Argentina. 3 Geosciences and Earth Resources (IGG), National Research Council of Italy (CNR), Firenze, Italy. 4 Laboratorio de Tectónica Andina-IDEAN, Departamento de Ciencias Geológicas, FCEN, Universidad de Buenos Aires, Argentina. 5 Universita degli Studi di Perugia, Dipartimento di Fisica e Geologia, Perugia, Italy. 6 Universita degli Studi di Firenze, Dipartimento di Scienze della Terra, Firenze, Italy.

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* Correspondence: María Clara Lamberti [email protected]

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Keywords: Carbon dioxide, diffuse degassing, sources, extensional, Caviahue-Copahue Volcanic Complex Abstract

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The link between carbon dioxide diffuse degassing and its structural control was examined in the Caviahue – Copahue Volcanic Complex, over an area of approximately 10 km2 that encompasses five hydrothermal sites, namely, Anfiteatro, Copahue town, Las Maquinitas I, Las Maquinitas II and Las Máquinas. Both geochemical and structural approaches were applied in order to investigate the structural scenario releasing hydrothermal fluids in this area. Also, the first isotopic analysis of soil CO2 degassing in the Andean region is presented. The geochemical analysis shows well-defined CO2 diffuse degassing structures in Copahue town, Las Maquinitas I, Las Maquinitas II and Las Máquinas hydrothermal sites. Soil diffuse CO2 is fed by multiple sources: a deep, magmatic – hydrothermal, source, a biogenic source and a mixed source. The structural analysis indicates the presence of an extensional regime that gives place to three structural trends, consisting of NE-SW trending normal faults, NW-SE sinistral strike-slip faults and ENE-WSW right lateral strike-slip faults. The comparison between diffuse degassing structures directions and faults directions suggests that there is a structural control on diffuse degassing. Diffuse CO2 is released through an early extensional setting, and it is emitted through normal faults, strike-slip faults and through areas with higher structural damage, such as relay areas between fault segments and terminations of individual fault segments. Introduction It is now well established from a variety of studies that faults and fractures play a key role in the localization and evolution of hydrothermal systems (Sibson, 1996; Curewitz and Karson, 1997; Sibson 1

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex and Rowland, 2003; Fairley and Hinds, 2004; Faulkner et al., 2010; Dockrill and Shipton, 2010; Rowland and Simmons, 2012; Faulds et al., 2013; Fossen and Rotevatn, 2016; Tardani et al., 2016). Recent studies from Tamburello et al. (2018) have carried out a global point pattern analysis and have stated there is a positive spatial correlation between CO2-rich gas discharges and extensional tectonic regimes. These authors affirm that extensional scenarios play a key role in creating pathways for deepsourced CO2 at micro- and macro- scales. They also state that normal and strike-slip faults show a better correlation with CO2 discharges, compared to compressive faults, such as thrusts or transpressive faults.

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Among all kind of gas manifestations, soil diffuse degassing is the most appropriate one to study the structural setting that allows fluid flow. Diffuse degassing occurs from relatively restricted regions named diffuse degassing structures (Chiodini et al., 2001; Cardellini et al., 2003). The geometry of these structures can reveal plenty of information on the structural architecture releasing fluids from deep sources, both hydrothermal or magmatic.

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The Caviahue - Copahue Volcanic Complex is located in an arc setting, within a segment called Southern Volcanic Zone, in the Andean range. According to Tardani et al. (2016), fluid flow in the central part of this segment is controlled by the regional setting. The Liquiñe – Ofqui Fault System and first order structures, oblique to the volcanic arc, exert a primary role on the localization of hydrothermal emissions. In addition, these authors postulate that the northern termination of the Liquiñe – Ofqui Fault Zone, characterized by a horse-tail structural geometry, is a particularly efficient scenario for high vertical permeability, which promotes increased fluid flow in hydrothermal systems.

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The structural geology of the Caviahue - Copahue Volcanic Complex has been examined by several authors, such as Rojas Vera et al. (2010) or Melnick et al. (2006). Particularly, the study of the link between fluid flow and structures in this area was recently examined by Roulleau et al. (2017), using carbon dioxide diffuse flux, self-potential and helium isotopic data from fumarolic emissions. These authors describe the structural control of this system as a mesh of NE-striking faults, that constitute vertical permeable pathways for fluid circulation, and WNW striking faults, that represent lowpermeability pathways for hydrothermal fluid ascent, and which promote infiltration of meteoric water at shallow depths.

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Fluid discharges at the Caviahue – Copahue Volcanic Complex are fed by a hydrothermal reservoir, mostly recharged by meteoric water. The hydrothermal reservoir is a vapor dominated field, with stratified layers, connected by fractures, with good vertical permeability within the reservoir. It is located predominantly in calc-alkaline lavas and pyroclastic rocks of Las Mellizas Formation. Water vapor in the fumaroles and in the geothermal wells constitutes over 96% of the fluids. The dry gas phase is dominated by CO2; acidic gases of uprising magmatic fluids are dissolved, whereas reduced gas species are generated and resealed (JICA-EPEN, 1992; Panarello, 2002; Agusto et al., 2013; Tassi et al., 2017). The purpose of this paper is to further study the above-mentioned link between fluid flow and structures in the Caviahue - Copahue Volcanic Complex, using an interdisciplinary approach. Carbon dioxide diffuse degassing data was examined together with a kinematic analysis of fault-slip data. A detailed diffuse degassing survey had already been done in the thermal areas of this volcanic complex and its results were published in Chiodini et al. (2015). The results of this paper shed new light on the efflux of CO2 over the surface not only of the thermal areas, but of a bigger zone that encompasses all

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex thermal sites and represents a high conductivity zone of hot fluid circulation (JICA-EPEN, 1992). Moreover, this study presents the first isotopic analysis of soil degassing of the Andean region. Overall, the present contribution generates fresh insight into the connection between the diffuse degassing structures and the faults and fractures generated by the local tectonic scenario. CO2 flux data and isotopic data are analysed together with structural data, applying a new interdisciplinary approach. Furthermore, a better description of the nature of the sources of soil CO2 is provided, through the analysis of new 13C-CO2 data.

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Geological and hydrothermal setting

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The Caviahue - Copahue Volcanic Complex (hereafter CCVC, 38°S – 71°W) is located in the Neuquén Province, in Northern Patagonia, Argentina. It is comprised on the Andean Southern Volcanic Zone (hereafter SVZ: 33°–46°S). Volcanism in the SVZ is related to the subduction of the Nazca Plate beneath the South American Plate between the Juan Fernández aseismic ridge in the north (32ºS), and the Chilean ridge in the south (46ºS) (Jordan et al., 1983) (Figure 1A).

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The SVZ overlaps with the Liquiñe – Ofqui fault zone (hereafter LOFZ). This major feature of the Southern Andes is a 1,200-km-long intra-arc strike-slip fault system, which is defined by a series of major NNE-striking, right lateral, strike-slip faults associated with NE-striking normal-dextral faults that splay off NNE-striking faults. This geometry forms duplexes and horsetail geometries at both ends of the fault system (Cembrano et al., 1996; Cembrano and Lara, 2009).

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The CCVC is emplaced in the northern termination of LOFZ (Figure 1B). This structural scenario consists of three fault systems: 1) NNE-striking sub-vertical master faults; 2) NE-striking steeply dipping splay faults and 3) ENE to EW-striking, steeply to moderately dipping local faults. These three fault sets constitute first, second and third order structures respectively. The scale of first order structures is regional, whereas second order structures constitute faults that connect and crosscut the master NNS-striking faults (Pérez-Flores et al., 2016). Finally, Pérez-Flores et al. (2016) identify centimeter-scale reverse faults as third order structures, southwards of the CCVC.

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

Figure 1. A. Location of the study area in the regional setting: the SVZ resulting from the subduction of the Nazca Plate beneath the South American Plate between the Juan Fernández aseismic ridge and the Chilean ridge. B. Location of the CCVC in the frame of the LOFZ: the caldera is developed at the termination of the horse-tail structure formed in the northern sector of this fault system. C. The surveyed area within the Caviahue Caldera, encompassing both the thermal sites and the area around them, here named HCZ (high conductivity zone). The black dots are the sampling points of diffuse CO2 and soil temperature. Map coordinates expressed in meters, UTM – WGS84, 19S.

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

Materials and methods

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This study focuses on an area within the CCVC, located inside the Caviahue Caldera, northeast of the Copahue volcano (Figure 1B). This area, according to gravity and electrical resistivity surveys, represents a high conductivity zone of hot fluids circulation (hereafter, HCZ; JICA – EPEN 1992), and it contains five hydrothermal sites: Anfiteatro, Copahue, Las Maquinitas I, Las Maquinitas II and Las Máquinas (Figure 1C). Fluid discharges in these sites are fed by a hydrothermal reservoir in the peripheries of Copahue volcano, mostly recharged by meteoric water. Acidic gases of uprising magmatic fluids (SO2, HCl, HF) are dissolved, whereas reduced gas species (H2, H2S, CO, CH4) are produced. Helium isotopes (R/Ra), in the majority of CCVC gas discharges, exhibit a MORB-like signature, while 13C-CO2 and 15N2-N2 values suggest that the magmatic source is significantly affected by contamination of marine sediments. Furthermore, gas discharges in the northwest sector of the caldera appear to be significantly contaminated by a shallow source, likely consisting of air saturated water, enriched in radiogenic helium (Agusto et al., 2013).

Carbon dioxide diffuse degassing

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A total amount of 1,819 measurements of CO2 flux (CO2) and soil temperature were collected in the HCZ, an area of approximately 10 km2. This area encompasses the five hydrothermal areas and the entire surface around them (Figure 1C). Data was collected in a time frame that encompasses the summers of 2014 to 2016.

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Soil CO2 fluxes were measured using two portable flowmeters developed and calibrated at the laboratories of Osservatorio Vesuviano and University of Perugia, and a third portable West Systems flowmeter, from GESVA – University of Buenos Aires, calibrated in the Argentinean National Commission of Atomic Energy (CNEA). The IR spectrometers consist of LICOR Li-800 and LICOR Li-820 detectors equipped with sensors operating in the range 0 – 20,000 ppm of CO2. The devices operate according to the static methodology described in Chiodini et al. (1998). The LICOR detectors measure CO2 concentrations in the range from 0 to 2 vol %. The reproducibility of the measurements was estimated to be around 10% for CO2 fluxes between 10 and 10,000 g m-2d-1 (Chiodini et al., 1998), although the uncertainty can be larger if fluxes are low (Carapezza and Granieri, 2004). During the first campaign, measurements with the three devices were performed at the same spots, in order to compare the obtained fluxes. The measured CO2-fluxes varied by less than 10% among the three instruments.

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Soil temperature was measured at the depth of 10 centimetres by means of a thermocouple TES 1300, equipped with a metallic probe. The thermocouple operates in the range of -50 – 1300 ºC and its resolution is of 0.1 ºC. CO2 flux data was processed in order to (i) investigate the origin of the CO2, (ii) map the spatial distribution of the diffuse emission and (iii) quantify the amount of CO2 daily released from the surveyed surface. One of the most well-known tools for assessing the origin of the CO2 is the Graphical Statistical Approach (hereafter GSA). As CO2 diffuse degassing is usually fed by several sources, which can consist of biologic, hydrothermal and/or volcanic sources, the CO2 values result in a multimodal distribution, which plots as a curve with one or more inflection points on logarithmic probability plots. The GSA method consists of the partition of these complex distributions into different log-normal

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex populations. These log-normal populations are defined by their proportion in the sample, the mean and the standard deviation (Sinclair, 1974, Chiodini et al., 1998, Cardellini et al., 2003). Although the GSA is a useful tool in the study of the diverse CO2 sources of the soil degassing, it is important to bear in mind the possible bias in the interpretations achieved with this methodology. This is because the partitioning procedure does not result into a unique solution and because the multimodal log-normal distribution may not be representing accurately the natural distribution of the CO2, which can be more complex than log-normal (Cardellini et al., 2003).

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The use of the Sequential Gaussian Simulation (hereafter SGS) is a well stablished approach in the mapping and CO2 output quantification. The SGS algorithm operates considering that the CO2 is an attribute that results from the realization of a stationary multivariate Gaussian random function. This method produces numerous equiprobable and alternative simulations of the spatial distribution of the attribute (Deutsch and Journel, 1998). The SGS produces a realistic representation of the spatial distribution of the CO2 fluxes reproducing the histogram and variogram of the original data (Cardellini et al., 2003).

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Six different data sets were here used for the SGS: the whole dataset, consisting of 1,819 measurements that cover the whole study area, and five datasets of detailed sampling of the thermal sites inside this “whole area”, already published in Chiodini et al. (2015). Experimental variograms of the normal scores were computed and modelled for each of the above-mentioned datasets. The models were used in the SGS procedure to create 100 simulations of the normal scores, using the program sgsim provided by Deutsch and Journel (1998). The simulated normal scores were then back-transformed into values expressed in original data units, applying the inverse of the normal score transform. The average of the values simulated at each cell of the grid in the 100 simulations was used to draw the maps of soil CO2 flux and soil temperature. Finally, for each simulation the total CO2 release was calculated by summing up the products of the simulated value of each grid cell by the cell surface.

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Carbon isotopic composition analysis

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Soil gas samples were collected adjusting the portable flowmeter for this purpose. This instrument was equipped with a valve in the gas line, placed just after the IR spectrometer, allowing gas samples for isotopic analysis to be collected during CO2 flux measurements. While the CO2 concentration was continuously measured, two samples were collected at each sampling location with a syringe and inserted in 10 ml vials, with a pierceable butyl rubber septum. Each sample was made at two different CO2 concentrations, namely, CCO2,A and CCO2,B. The carbon isotopic composition of CO2 (δ13C-CO2, expressed as ‰ vs. V-PDB) was analyzed by Cavity Ring-Down Spectroscopy (CRDS) using a Picarro G2201-i Analyzer (operating range: 3802,000 ppm). The Picarro gas inlet was equipped (i) with silicon connections ending with a needle inserted in the rubber septum of the vial and (ii) a copper trap for the removal of H2S to avoid potential interferences. The analytical error for δ13C-CO2 was 0.16 ‰ vs. V-PDB. The isotopic composition of 13CCO2 was computed using the mass balance equation used by Chiodini et al. (2008). 54 samples were collected, in order to obtain 27 isotopic composition values. The location of these samples is shown in Figure 1C. Samples were collected during February and March of 2018 and were analyzed in March 2018. 6

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex 1.2

Structural analysis

The structural study comprises the integration of published data (JICA – EPEN, 1992; Melnick et al., 2006) with two scales of analysis: (i) satellite images and digital elevation models interpretation (Palsar RTC, 12.5 meters), and (ii) a detailed outcrop-scale field survey.

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The remote structural mapping was carried out in order to characterize the main structural trends. The determination of the fault kinematics was accomplished measuring different types of kinematic indicators on meso-scale faults affecting Las Mellizas Formation, like iron-coated slickensides, crystal fibres on slip planes, and associated fractures, mainly Riedel fractures.

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In order to characterize the structural damage of the study area, over 370 lineaments were manually interpreted, over a SPOT 4 satellite image, with a panchromatic resolution of 10 meters. This highresolution satellite image resolved a great amount of detail on the Las Mellizas Formation lava roof, favored by the large outcrop surfaces. Interpreted lineaments were classified into two groups: (i) mayor faults and (ii) minor lineaments and faults. Mayor faults consist of features with along-strike lengths >200 meters, and with minimum fault throw of 10 meters. The kinematics of these mayor faults was confirmed by field observation and kinematic indicators found along their main fault plains. Minor lineaments and faults consist of mapable, simple or composite linear features of smaller dimensions, with along-strike lengths <200 meters. Given that no reliable kinematic indicators were found on faults belonging to this group of structures, they were mapped together with the lineaments. This approach was suitable for identifying the structural damage in the small area of Copahue town and Las Maquinitas hydrothermal sites.

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Principal strain axes have been computed using the moment tensor summation method as implemented in FaultKin® (Marrett and Allmendinger, 1990). The principal axes of shortening (λ3) and extension (λ1) for the incremental strain tensor and an average kinematic fault plane solution associated with each fault-slip population were obtained. The basic assumption applied is that the data represent a uniform deformation field. When non-clustered axes arise, kinematically heterogeneous faulting can be interpreted. Kinematic heterogeneity can arise from several causes, such as triaxial deformation, anisotropy reactivation, strain compatibility constraints and/or polyphasic deformations (Marrett and Allmendinger, 1990). Particularly, as defined by these authors, strain compatibility between two differently oriented faults or fault segments requires that they both must slip parallel to their line of intersection if no additional structures form. This would result in multiple sets of faults which have similar slip directions and needs to be considered in order to correctly interpret the present fault dataset. Diffuse degassing structure analysis

After the recognition of diffuse degassing areas, the diffuse degassing structures (hereafter DDS) were thoroughly examined. DDS are the regions through which diffuse degassing efflux from a deep source, hydrothermal or magmatic, takes place (Chiodini et al., 2001). The analysis of the DDS was done with the aim of comparing the directions of these geochemical anomalies with the directions of tectonic faults. Criteria for identifying DDS were as follows: a CO2 flux threshold value was selected, considering the partitioning of the data between anomalous and background data carried out with the GSA. Then, DDS were categorized into three classes: -

Class 1A: linear structures which connect diffuse degassing peaks; 7

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex -

Class 1B: features generated by the lateral coalescence of peaks;

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Class 2: linear features that draw the major axis of anomalies;

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Class 3: features that constitute linear boundaries of anomalies.

Recognized and classified DDS were mapped and plotted together in rose diagrams for comparison with the tectonic faults rose diagrams described in section 3.2.

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Results

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CO2 soil degassing

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The results of this research involve the efflux of CO2 over the surface not only of the thermal areas, but of the whole HCZ related to the circulation of hot fluids. This extensive area is characterized by a wide range of CO2 flux values, varying from <0.01 g m-2 d-1 to >18,200 g m-2 d-1. Soil temperature ranges from 8 to 92ºC. The whole data set is reported in the logarithmic probability plot of Figure 2. This graphic shows the results of the GSA analysis, which includes (i) the partitioned log-normal population (colored straight lines), (ii) their proportion, mean and standard deviation and (iii) the distribution of the original data on the probability plot (black dots).

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By means of this methodology, four log-normal populations were defined. The proportion, mean and standard deviation for each population are reported in Table 1.

Population

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Table 1. Estimated parameters of the partitioned populations of diffuse CO2 at the HCZ area at the CCVC.

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

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Figure 2. Probability plot of CO2 flux: the colored straight lines are the theoretical partitioned populations following the procedure of Sinclair (1974): populations A, B, C and D are shown with a red, green, purple and blue line, respectively. The proportion, the logarithm of the mean and the logarithm of the standard deviation are also shown for each population. The distribution of the original data is shown with black dots.

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Only 9% of the measured fluxes, in this wide area, belong to a high flux population. The CO2 flux mean value is 1,421 g m-2 d-1, and it is interpreted to represent a population fed by an endogenous, hydrothermal source. A large uncertainty is associated with this mean value, probably due to the relatively small number of samples available to define this high flux population (164 out of 1819).

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The other three populations are interpreted as “background populations”, meaning CO2 fluxes related to soil respiration processes. These three populations are characterized by low mean values, 54, 7 and 0.8 g m-2 day-1. A possible explanation for this may be the presence of diverse types of vegetation in the study area. The HCZ presents soils with peat, grasslands and poorly vegetated areas, where the biological production of CO2 is scarce. The mapping of the CO2 soil emission, of the soil temperature and the calculation of the total CO2 release were computed with the SGS method. One hundred simulations of the normal scores of the original data were performed, considering the computation and modelling of experimental variograms of the normal scores of the CO2 flux and of the soil temperature data. The relevant parameters for SGS application and estimation of the CO2 output are described in Table 2.

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex Variogram model,

Grid parameters: surface of

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CO2 flux

Spherical, 0.57, 180

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Table 2. Relevant parameters of SGS application for the mapping of CO2 flux and soil temperature, and estimation of the total CO2 output from the HCZ area at the CCVC.

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Figure 3 shows the resulting map of the calculation. As it can be seen from this map, CO 2 anomalies were only registered at the geothermal sites of Las Máquinas, Las Maquinitas I and II and Copahue town, where well defined DDS are exhibited. Soil temperature anomalies were also registered at the four former sites. The geothermal site Anfiteatro exhibits CO2 diffuse degassing, but with no clear DDS. The total amount of CO2 released by the whole area of the geothermal field is 208.5 t d-1.

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

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Figure 3. A. CO2 flux map of the HCZ, including the hydrothermal sites of Anfiteatro, Copahue town, Las Maquinitas I and II and Las Máquinas. B. Soil temperature map of the HCZ. For both maps, the coordinates are expressed in meters, UTM – WGS84, 19S.

The 13C-CO2 values of 27 soil gas samples around the thermal sites of the CCVC range from –18.86 to –6.06‰ vs. V-PDB. The results are shown in Table 3. The origin of CO2 in subduction zones can be explained in terms of three main components: (i) a mantle component (13C-CO2 ̴ -6‰ vs. V-PDB), (ii) marine limestone (13C-CO2 ̴ 0‰ vs. V-PDB) and (iii) organic sediments (13C-CO2 < -20‰ vs. V-PDB) (Sano and Marty, 1995; Schrag et al., 2013).

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

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Las Máquinas Las Máquinas Las Máquinas Las Máquinas Las Máquinas Las Máquinas Las Maquinitas I Las Maquinitas I Las Maquinitas I Las Maquinitas I Las Maquinitas I Las Maquinitas I Las Maquinitas I Copahue Copahue Copahue Anfiteatro Anfiteatro Anfiteatro Anfiteatro Anfiteatro Copahue Copahue Copahue Las Maquinitas II Las Maquinitas II Las Maquinitas II

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13C-CO2 vs. V-PDB -18.80 -11.02 -17.36 -17.27 -0.12 -6.12 -9.03 -7.51 -12.46 -21.23 -5.61 -1.96 -4.38 -8.21 -18.74 -11.38 -1.70 -0.94 -12.98 -7.55 -5.53 -9.25 -2.05 -18.03 -2.58 -7.46 -14.58

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COP 1 COP 2 COP 3 COP 4 COP 5 COP 6 COP 7 COP 8 COP 9 COP 10 COP 11 COP 12 COP 13 COP 14 COP 15 COP 16 COP 17 COP 18 COP 19 COP 20 COP 21 COP 22 COP 23 COP 24 COP 25 COP 26 COP 27

CO2 (gr/m-2/d-1) 17.91 226.77 201.02 34.84 193.52 34.28 1,177.26 74.01 15.50 35.78 80.63 3,163.70 49.82 327.76 303.09 83.91 1.65 1,809.19 12.51 46.18 37.42 50.82 366.27 7.04 93.00 473.20 18.58

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Table 3. Isotopical composition of diffuse CO2 sampled at the five hydrothermal sites of the CCVC. The carbon isotopic composition of CO2, δ13C-CO2, is expressed as ‰ vs. V-PDB. Structural analysis

Remote sensing analysis of lineaments and faults reveals a NE-SW main structural trend and a disperse NW-SE secondary one. The most striking structures in the Caviahue Caldera are NE-SW normal faults, resulting in a horst-and-graben setting with along-strike lengths ranging from a few meters to 1 km. The second most important set of faults is NW-SE; this set is interpreted as accommodation or transfer zones between the main extensional structures. They present strike-slip kinematics with minor normal components and usually develop shorter lengths than the previous set. Finally, a minor set, composed by ENE-WSW right-lateral strike-slip faults, is recognized. In the area within the Caviahue Caldera that presents CO2 diffuse degassing, two domains were defined: the Anfiteatro - Copahue domain and Las Maquinitas domain (hereafter, AC and LM, respectively). These domains are separated by a central horst structure. A detailed map of AC and LM domains was constructed combining the remote sensing interpretation with field survey (Figure 4). 12

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Figure 4. A. Main structural lineaments in the Caviahue Caldera. B. Rose diagram resulting of plotting the structural lineaments. C. Detailed structural map of Copahue town and Las Maquinitas thermal sites, with the fault plane solutions: average shortening axes are indicated with red dots, variation ranges are indicated with red circles. Average extension axes are indicated with blue dots, variation ranges are indicated with blue circles. AC: P-axis 66/069, alpha95 = 18.4º, T-axis 03/332, alpha95 = 42.6º; LM: P-axis 58/311, alpha95 = 23.0º, T-axis 31/138, alpha95 = 19.7º.

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Copahue town is located inside a graben with a general NE-WS trend (Mon, 1987). This graben is bordered by two groups of opposite-vergent faults. The northern border result of two discontinuous SE-dipping normal faults, whose trace forms a right-stepping morphology. The lack of overlap between these segments configures an approaching transfer zone according to Morley et al. (1990), pointing to an early extensional stage. Such transfer or accommodation zone may not generate a single structure, but it forms, instead, an association of minor structures. The southern border of the graben is formed by a group of NW-dipping nearly collinear minor fault segments. The termination of one of them is located at the southern end of the village, controlling the development of an alteration zone clearly observable on the image (alteration zone 1, in Figure 4C), probably associated with the related structural damage. A central horst separates the Copahue village graben from a minor graben, named Las Maquinitas graben, developed to the SE, which hosts the Maquinitas hydrothermal zone (alteration zone 2, in Figure 4C). The two antithetic boundary normal faults run parallel to each other, with NE-WS strike. The lower block is in turn affected by a couple of lower-scale normal faults, whose northeastern terminations appear to be controlling the development of the superficial alteration zone. Strain compatibility between fault populations was evaluated for each structural domain (Figure 5A, B). Although slightly scattered, slip direction maxima correspond well with intersections of mean fault sets (Figure 5 C, D). In AC, two conjugate pairs, one with NW-SE strike and the other with NE-SW strike, have mostly coherent slip which is subparallel to their intersection, suitably explaining the 13

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex kinematic heterogeneity found (Figure 5C). In LM, the striae density diagram shows a unimodal distribution, over which the average fault planes intersect each other. Therefore, strain compatibility between these faults can be inferred (Figure 5D).

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From the paleo-stress analysis two similar stress tensors were obtained, evidencing an extensional NWSE stress regime produced by a sub-vertical shortening axis, and a sub-horizontal NW-SE extensional axis (Figure 5C).

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Figure 5. Structural results for the Anfiteatro – Copahue, AC, and Las Maquinitas, LM, sets. A-B, fault slip data stereogram (equal area, lower hemisphere projection). C-D, fault and slip media used for the analyses of strain compatibility for average planes with their respective striae: maxima fault in bold great circle, average slip sense in small yellow dots and arrows; contour diagram represents the slip direction density.

1.5

Diffuse degassing structures

Regarding the DDS analysis, Figure 6 B and C shows the lineaments interpreted after the criteria described on section 3.3. The selected threshold value was 250 g m-2 d-1, a value comfortably above the mean value of background population B, 54 g m-2 d-1 (section 4.1). Class 1A connects diffuse degassing peaks. The geometry of fault surfaces is usually irregular; in addition, fault sealing mechanisms induced by mineral precipitation take place along the fault trace (Frery et al., 2015). As a consequence, the permeability structure is usually highly heterogeneous 14

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex (Sibson, 1996). DDS classified as 1A structures were differentiated from the other classes by drawing their traces with dotted lines in figures 6 B and C. Classes 1B, 2 and 3 represent DDS generated by the lateral coalescence of peaks of diffuse flux, the major axis of a single anomaly and the linear boundary of an anomaly, respectively. Thus, four orders of DDS directions were identified, and a rose diagram was constructed considering diffuse degassing structures of Copahue town, Las Maquinitas I, Las Maquinitas II and Las Máquinas thermal sites (Figure 6 D).

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Although the rose diagram shows that DDS present multiple directions, one predominant strike direction NE-SW is identified. The second main direction would be a nearly orthogonal direction, NWSE. A third ENE-WSW direction can also be seen in the diagram.

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The three directions obtained from the DDS analysis are coincident with the structural data here presented and with the directions of the fault systems previously described in literature. This set of faults constitute the borders of a triangle-shaped horst structure that, according to gravity and electrical resistivity surveys, represent a high conductivity zone of hot fluids circulation (JICA-EPEN, 1992; Melnick et al., 2006). Chiodini et al. (2015) had stated that the geometries of the DDS in the CCVC were consistent with these three directions. The result of the analysis here presented supports this idea, but it recognizes the predominance of the NE-SW direction for fluid circulation over the other two directions.

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex Figure 6. A. Location of the detailed mapping of CO2 diffuse degassing. B. DDS marked in the CO2 soil degassing maps of Copahue town, Las Maquinitas I and II. C. DDS marked in the CO2 soil degassing maps of Las Máquinas thermal sites. Different colors were selected for the different orders of the DDS. D. Equal area-length weighted rose diagram of DDS lineaments, considering together the four classes of DDS.

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Soil CO2 origin

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The isotopic signature of CO2 indicates there are multiple sources feeding the diffuse CO2 efflux. The 13C values of soil CO2 were plotted versus their respective CO2 flux, following the methodology proposed by Chiodini et al. (2008). The plot also shows the 13C range expected for soil CO2 degassing from a pure deep source, from a pure biogenic source and from a mixed source (Figure 7).

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Figure 7. Soil CO2 flux vs 13C-CO2 composition. Samples of all the thermal sites, Anfiteatro, Copahue town, Las Maquinitas I and II and Las Máquinas, are shown together. δ13C-CO2 values are expressed as ‰ vs. V-PDB.

The deep 13C-CO2 range was constrained considering the isotopic data collected from the fumaroles of all the thermal sites of the Caviahue Caldera, during the last decade (Agusto et al., 2013; Tassi et al., 2017). The 13C-CO2 values of the fumarole samples range from -9.6 to -6.6‰ vs. V-PDB. This composition is considered as representative of the deep source that feeds all the thermal emissions in the CCVC, and it is indicated in Figure 7 as the field named “pure hydrothermal CO2 at CCVC”. Previous research has established that the hydrothermal system feeding the CCVC thermal manifestations consists of a boiling aquifer mainly fed by meteoric water, at depths of 800 – 1,000 m, 16

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex with temperatures from 200 to 215°C (JICA-EPEN, 1992). Geothermometric calculations in the H2OCO2-CH4-CO-H2 gas system indicate that fluids discharged by fumaroles in Copahue town, Las Máquinas and Las Maquinitas, equilibrate in a single vapor phase (JICA-EPEN, 1992; Agusto et al., 2013; Chiodini et al., 2015; Tassi et al., 2017). Additionally, hot acidic gases feeding this aquifer rise from a magmatic body, located at ~ 5 km depth (Velez et al., 2011).

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Table 3 shows soil δ13C-CO2 values isotopically heavier than those corresponding to the “pure hydrothermal CO2 at CCVC” field, ranging between -6.6 and -0.12 ‰ vs. V-PDB. The occurrence of secondary processes affecting the uprising deep-sourced fluids at shallow depths might explain the positive shift in δ13C-CO2 values. In particular, both (i) gas diffusion through the soil (e.g. Cerling and Quade, 1993 ; Capasso et al., 1997 , 2001 ; Di Martino et al., 2016 ; Etiope et al., 2009 and references therein; Federico et al., 2010 ; Kayler et al., 2010 ) and (ii) microbial CO2 uptake processes (Tassi et al., 2015 and references therein; e.g. Freude and Blaser, 2016 and references therein) are able to produce an isotope fractionation effect resulting in an enrichment in 13C in residual CO2. Accordingly, 13 C enrichments in soil gas CO2 with respect to fumarolic fluids were observed in diffuse degassing areas fed by hydrothermal sources (e.g. Notsu et al., 2005; Capasso et al., 2017). Nevertheless, an alternative explanation might be related to the methanogenesis process that takes place in the hydrothermal reducing environment. The reduction of CO2 to CH4 gives place to a kinetic isotope effect for carbon, leading to the enrichment of 13C in CO2 (Whiticar, 1999). The formation of CH4 from CO2 gives place to the continued preferential removal of the isotopically lighter molecules, resulting in a progressive shift in the CO2 towards heavier, 13C-enriched values (Whiticar, 1999).

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Therefore, values of 13C-CO2 ranging from -6.6‰ and -0.12‰ are included in the graphic of figure 7, in a field named “hydrothermal CO2”.

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In order to delimit a field for the background values of 13C-CO2, the isotopic composition of soilrespired CO2 analyzed by Amundson et al. (1998) was considered. Biogenic CO2 refers to the carbon dioxide produced in soils, as a result of biological processes: organic matter decomposition and root respiration. The isotopic composition of the biogenic CO2 that diffuses across the soil – atmosphere interface reflects the isotopic composition of the flora. Thus, the biogenic field, indicated in blue in Figure 7, ranges from -27‰ to -14‰ vs. V-PDB (Amundson et al., 1998).

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Samples in the range between the “pure hydrothermal CO2 at CCVC” and the “biogenic” fields are representative of a mixed isotopic composition, named “mixture between biological and hydrothermal”. Soil CO2 values plotting in this field indicate that emissions are fed by both sources, the deep and the biogenic one. The GSA (section 4.1) suggests that the CO2 fluxes measured in the HCZ are fed by one hydrothermal source, which represents a minor proportion of the data, and by three biogenic sources. This is probably due to the fact that the GSA was performed analyzing 1,819 CO2 flux measurements over the whole HCZ of 10 km2, whereas the isotopic analysis was carried out with 27 samples collected in the five hydrothermal sites. The hydrothermal log-normal population A, which represents 9% of the whole dataset, encompasses the “hydrothermal CO2”, “pure hydrothermal CO2 at CCVC” and “mixture between biological and hydrothermal” fields, interpreted after the isotopic analysis. Diffuse degassing structures and their link with faults and fractures The geochemical and structural data analyzed in this work reveal that: (i) large soil CO2 diffuse degassing anomalies occur at the CCVC, mainly through Copahue town, Las Maquinitas I, Las 17

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex Maquinitas II and Las Máquinas thermal sites; (ii) there are multiple sources feeding soil CO2; and (iii) results from fault-slip analysis indicate the occurrence of an extensional regime, with main NE-SW normal faults, secondary NW-SE strike-slip faults and a minor set, composed by ENE-WSW rightlateral strike-slip faults.

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In order to analyze the link between the structural architecture and diffuse degassing, length-weighted rose diagrams constructed for faults and DDS were plot together in Figure 8. Both diagrams exhibit a multimodal circular distribution, with a predominant NE-SW direction and secondary NW-SE and ENE-WSW populations.

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Figure 8. Length-weighted rose diagrams for lineaments and faults orientations and for DDS lineaments.

The similarity of the shape of both diagrams can be interpreted as evidence of the structural control that faults exert on diffuse degassing. Multiple structural trends allow the up-rise of hydrothermal CO2. The NE-SW normal faults constitute the most important pathway for diffuse CO2, giving place to the most conspicuous DDS. The secondary NW-SE and ENE-WSW fault sets would also give place to the up-rise of CO2, allowing minor DDS to develop. Figure 9 shows an integration of the diffuse degassing data and the structural mapping, in the area of Copahue town, Las Maquinitas I and Las Maquinitas II thermal sites. The CO2 flux map in this area was overlaid with the main normal faults that constitute the horst and graben structure. The fault plane solutions generated for AC and LM domains are also plotted on the map. The first observation to emerge from Figure 9 is that DDS locate within the graben structures, both in the Copahue village graben and in Las Maquinitas graben (Figure 4). Although the map in Figure 9 suggests that DDS would be associated to the main NE-SW normal faults, the comparison between faults and DDS 18

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

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directions presented in Figure 8 supports the idea that diffuse CO2 reaches the surfaces through all the fault sets present in the area.

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Figure 9. Integration of the CO2 diffuse degassing data with the structural data in Copahue town and Las Maquinitas I and II thermal sites. Map coordinates expressed in meters, UTM – WGS84, 19S.

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The integration of the diffuse degassing data and the structural data was not carried out in the Las Máquinas thermal site, since this area does not present conspicuous fault traces, and the fault-slip analysis was not possible.

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The combination of findings of this study supports the conceptual premise that hydrothermal fluids in the CCVC find favorable pathways through normal fault traces and strike-slip fault traces. Furthermore, the position of diffuse degassing anomalies with respect to the fault traces suggests that diffuse degassing occurs in several structural scenarios. Diffuse CO2 rises: (i) through fault traces, (ii) through relay areas between fault segments and (iii) at the terminations of individual faults. According to Fossen and Rotevatn (2016), relay and transfer structures are locations of fault interaction where strain is transferred from one structure to another. These authors state that these relay regions typically represent pathways for vertical migration of fluids, given that they promote structural damage. Relay and transfer structures usually present an increased structural complexity, with a large number of faults and fractures and a wider range of orientations than that of single, isolated faults. At the termination of individual faults, these propagate and interact, concentrate stress, which causes active fracturing and continual re-opening of fluid-flow conduits, permitting long-lived hydrothermal flow despite potential clogging of fractures due to mineral precipitation. Consequently, relay zones represent an important control on fluid transport in the crust (Curewitz and Karson 1997; Fossen and Rotevatn, 2016). Figure 10 exhibits a conceptual model for the structural scenarios of deep-sourced CO2 diffuse degassing, observed in the northern sector of the study area, which comprises Copahue town, Las 19

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex

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Maquinitas I and Las Maquinitas II thermal sites. This model considers that CO2 reaches the surface through normal and strike-slip fault traces, and through areas with higher structural damage: at relay areas between fault segments and at the termination of fault segments.

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Figure 10. Conceptual model of the structural scenario releasing deep CO2 at Copahue town and Las Maquinitas thermal sites.

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As explained in the introduction, Tamburello et al. (2018) have stated that extensional tectonics and, in particular, normal and strike-slip faults constitute the most suitable scenario for CO2 to rise to the surface. The findings of this study are in good agreement with this premise, and they also suggest that areas with higher structural damage play a key role on the localization of CO2 diffuse anomalies. Conclusions The aim of the present research was to examine the relationship between deep carbon dioxide diffuse degassing and the structural scenario at the CCVC. This work contributes to existing knowledge of structural control on fluid flow by providing an innovative approach, analyzing geochemical data together with structural data. It also provides the first isotopic analysis of soil diffuse degassing emissions of the entire Andean region. The following conclusions can be drawn: 20

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Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex Carbon dioxide soil diffuse degassing is emitted through all the thermal sites comprised in the HCZ within the CCVC, in Anfiteatro, Copahue town, Las Maquinitas I and II and Las Máquinas. However well-defined DDS are developed at the four sites. The isotopic data and the GSA analysis revealed that multiple sources feed CO2 diffuse emissions. The 13CCO2 suggest that CO2 is fed by a deep source, by a biogenic source and by a mixture of these two sources. CO2 released from the hydrothermal reservoir undergoes, in the hydrothermal reducing conditions, an isotopic fractionation process, probably related to the methanogenesis process.

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The results of the structural analysis indicate that there are three structural trends: the main set consists of NE-SW-trending normal faults, and two secondary sets consist of NW-SE sinistral strikeslip faults and ENE-WSW right lateral strike-slip faults. Overall, the structural architecture responds to an early extensional stage.

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The DDS analysis showed that diffuse CO2 is emitted to the surface through structures of several directions. The rose diagram plotted considering CO2 anomalies of all the hydrothermal sites show a main NE-SW direction, and secondary NE-SW and ENE-WSW directions.

Faults with normal and with strike-slip kinematics both allow the up-rise of diffuse CO2.

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The similarity of the DDS and structural rose diagrams suggests that diffuse degassing has a clear structural control. The NE-SW normal faults constitute the most important pathway for diffuse CO2. The secondary NW-SE and ENE-WSW fault sets would also give place to the up-rise of CO2, allowing minor DDS to develop.

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The localization of diffuse degassing anomalies suggests that diffuse CO2 finds favorable pathways in areas with higher structural damage: at relay areas between fault segments and at the termination of individual fault segments.

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Funding

Acknowledgments

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The research leading to these results has received funding from the projects UBACyT 20020150200230BA, UBACyT 20020170200221BA, PICT-2015-3110 and PICT-2016-2624.

References

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We are grateful for the reviewers, and for the careful and constructive review by Donaldo M. Bran.

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Tassi, F., Agusto, M., Lamberti, C., Caselli, A.T., Pecoraino, G., Caponi, C., Szentiványi, J., Venturi, S., Vaselli, O., 2017. The 2012 - 2016 eruptive cycle at Copahue volcano (Argentina) versus the peripheral gas manifestations: hints from the chemical and isotopic features of fumarolic fluids. Bulletin of Volcanology, 79 (10), 1-14.

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Tassi, F., Venturi, S., Cabassi, J., Gelli, I., Cinti, D., Capecchiacci, F., 2015. Biodegradation of CO 2, CH4 and volatile organic compounds (VOCs) in soil gases from the Vicano-Cimino hydrothermal system (central Italy). Org. Geochem. 86, 81-93, doi:10.1016/j.orggeochem.2015.06.004.

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Velez, M.L., Euillades, P., Caselli, A., Blanco, M., Martinez Diaz, J., 2011. Deformation of Copahue volcano: inversion of InSAR data using a genetic algorithm. Journal of Volcanology and Geothermal Research 202, 117-126.

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ACCEPTED MANUSCRIPT

Structural architecture releasing deep-sourced carbon dioxide diffuse degassing at the Caviahue – Copahue Volcanic Complex Highlights

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Soil diffuse CO2 at the CCVC is fed by multiple sources: magmatic, hydrothermal and biogenic. Over 200 tons per day of diffuse CO2 are released by the CCVC. The structural scenario releasing deep CO2 consists of an extensional regime. Faults of normal and strike-slip kinematics act as pathways for diffuse CO2 up-rise. Areas of high structural damage also constitute favorable pathways for fluid flow.

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