Magnetoteluric Study in the Region De Los Lagos to Study Volcano-Tectonic Processes in the Southern Andes

In order to analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal prole of the Chilean continental margin. The data processing stage included the analysis of dimensional parameters, which as rst results showed a three-dimensional environment for periods less than 1s and two-dimensional for periods greater than 10s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identied in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identied as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80-100km, the ascending uids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range.


Introduction
The Southern Andes constitute an orogenic mountain belt developed on an active continental margin, with subduction between the Nazca and South American tectonic plates as the main cause of the magmatic and tectonic processes in the area (Hervé, 1994). The area of study is framed in the Southern Volcanic Zone of the Andes (SVZ), in this zone, the complex interaction between tectonic and magmatic processes is evident in the architecture and geochemical signature of the volcanic systems, Cembrano & Lara (2009) relate this spatial distribution of plutons, dikes and volcanic cones, with the lithospheric stress generated by the Liquiñe Ofqui Fault System. The Liquiñe Ofqui Fault Zone (LOFZ) corresponds to a strike-slip system of approximately 1200 km long, where most of the seismic activity of the continental crust in the South of Chile is concentrated (Lavenu & Cembrano, 1999). The South of Chile represents an attractive place for a volcano-tectonic study; since it is located in an orogenic margin, with an important system of faults dominating the terrestrial crust and active volcanic centers, it appears as a favorable environment for the development of natural disasters related to volcanoes and earthquakes. Arc volcanism and seismic activity are closely related to the distribution of uids from physical-chemical processes that occur in subduction (Saffer, 2017).
The process of generating magmatic uids that migrate through the continental crust depends on some physical parameters of the oceanic plate such as its composition, thickness, angle of inclination with which it subducts and its degree of hydration, which in turn depends on the age and porosity of the plate (Völker & Stipp, 2015). As the oceanic slab subduces the continental slab, due to the increase of temperature and pressure conditions, mineralogical changes are generated as a result of the dehydration of the Nazca plate. One of the most signi cant processes corresponds to the serpentinization process of the mantle, in which a metamorphic alteration is produced that affects the ultrama c rocks of the upper mantle. In seismic studies, the serpentinization zones can be observed as zones where the seismic velocity of S wave and the density of the mantle is drastically reduced (Hyndman & Peacock, 2003). Serpentinization can also generate seismic re ectivity, increased magnetization and a reduction in mechanical strength (Hyndman & Peacock, 2003). The other important process corresponds to eclogitization that occurs at greater depths where the temperature and pressure conditions are extreme, in this process all the remaining water present in the slab forming minerals is released (Leech, 2001). The As the oceanic slab dehydrates, through processes of serpentinization and eclogitization, uids are generated that rise and fall the point of function of the continental crustal rock, forming magmatic uids that migrate and control the creation of volcanoes in the arcs. These uids can be transported through fault networks to the surface of the Earth (Clemens & Mawer, 1992). Therefore, the stress of the lithosphere in uences the spatial distribution of plutons, dikes and volcanic cones.
The magnetotelluric method is useful in the study of the above processes because the conductivity of the crust and mantle rocks increases with the presence of uids, and it is one of the few geophysical methods that can penetrate to the depths at which these processes are generated.
The present work seeks to study the lithospheric structure of electrical resistivity in the Southern Andes to analyze processes of generation, migration and accumulation of uids related to the subduction between the Nazca and South American plates in the Chilean continental margin.

Geological Setting
The oceanic Nazca plate subducts beneath the continental South American plate along the Andes with a convergence rate of 6.6.cm/year and an azimuth of 77° (Angermann, et al., 1999). In the Lake District between 33° and 46° south latitude, the Nazca plate has a subduction inclination of ~ 20° (Tassara & Echaurren, 2012). The stretch of oceanic plate between 40°S and 41.5°S has an age of 17.3 Ma (Völker & Stipp, 2015).
In Fig. 1, from West to East, the main morphological units are given by The Coastal Range, south of 38°S, which is an accretionary prism metamorphosed in late Paleozoic to late Triassic times (Thomson & Hervé, 2002). The eastern ank of the Coastal Range is characterized by intrusive and volcanicsedimentary Mesozoic sequences suggesting the eastward migration of the volcanic arc from the Jurassic; The Depression or Central Valley, which constitutes the extension of the Osorno-Llanquihue basin towards the south, contains approximately 4 km of Cenozoic marine volcanic rocks and glacial sediments in a 70 km wide depocenter parallel to the trench (Jordan, et al., 2001); The eastern limit of the Central Depression is de ned by the North Patagonian Batolito and the current Volcanic Arc in the Northern Patagonian Andes, where the Liquiñe Ofqui Fault Zone is developed. The Batolito is 2000 km long and 200 km wide, it is composed mainly of plutonic alkaline rocks from the Middle Jurassic to the Miocene (Glodny, et al., 2008). The main water bodies in the study area are the lakes of glacial origin, Llanquihue Lake (L.LL) and Todos Los Santos Lake (L.TS).In the Southern Volcanic Zone of the Andes, cortical level structures related to the LOFZ and Andean Transverse Fault (ATF) have been identi ed. The oblique convergence between the Nazca and South American plates is the main cause of the transpressive dextral deformation in the Earth's crust, dominated by the sliding of regional to local scale by these fracture networks.
The LOFZ is associated with a dextral transpressive regime from at least the last 6 Ma (Cembrano, et al., 1996), extending from the town of Liquiñe (39°S) to the isthmus of Ofqui (47°S). At latitude 41°S, Lavenu & Cembrano (1999) attribute a vertical geometry to the fault line, generated by dextral transpressive systems of N-S orientation, with double vergence, developed between the Neogene and Quaternary. The LOFZ is characterized by sub-vertical NNE oriented master segments and transstensional faults from ENE to EW (Cembrano & Hervé, 1993).
Most, if not all, of the volcanic systems at the top of the LOFZ are monogenetic, suggesting that they were formed by geologically instantaneous events. Another important fault system to highlight in the Southern Volcanic Zone corresponds to the Andean Transverse Fault, these include a series of NW oriented faults, probably inherited from a pre-Andean architecture with inverse synestral kinematics and local reactivation of normal sliding (Radic, 2010). It is suggested that the ATF is older, dating from the pre-Andean, or equivalent to the LOFZ (Pankhurst, et al., 2006).
Fracture networks or faults in the continental crust have been considered as an e cient route through which uids are transported to the earth's surface (Clemens & Mawer, 1992). Therefore, the lithospheric stress in uences the spatial distribution of plutons, dikes and volcanic cones. However, crustal deformation not only plays an important role in magma migration. More importantly, it can exert fundamental control over magma-differentiation processes which, in turn, can determine the nature and composition of volcanism along continental margins.
The volcanic zone of the Southern Andes, is located between 33 ° S and 46 ° S. The complex interaction between tectonic and magmatic processes is evidenced by both the architecture and the geochemical signature of the volcanic buildings in this zone. Throughout the SVZ strike, variations in the composition of the products of the volcanoes and in their orientation are evident. The structures of NE-SW orientation and originating mostly from primitive magmas belong to stratovolcanoes and monogenetic cones associated to LOFZ guidelines while stratovolcanoes of WNW orientation and with compositional products from basaltic to rhyolithic rocks are associated to structures of the ATF (Cembrano & Lara, 2009). The main factors that explain the variations along the strike in the nature and composition of volcanism are the variation in crust thickness, the source of magma, the lithospheric nature and structure and the existence of a predominant intra-arc fault system (Cembrano & Lara, 2009). The thickness of the crust below the volcanic arc is approximately 50 km at latitude 33°S and decreases towards the South, reaching a thickness of 35 km at 46°S (Tassara & Yañez, 2003).
One of the most important volcanoes for its activity in the area corresponds to the Calbuco volcano (2,003 m.a.s.l.), it is built on a base of Miocene granite and meta-sedimentary ma c igneous rocks of the Upper Paleozoic and, unlike most of the volcanoes of the Southern Andes, it is located west of the main route of the Liquiñe-Ofqui Fault Zone, and is not clearly related to or controlled by regional structures. Its documented historical activity includes at least 12 eruptive cycles, 4 of which in the 20th century included effusive and explosive activity. The highly hydrated magmas that characterize the volcano's petrology increase its potential to generate explosive eruptions. The main hazards are associated with the volcano, however, there are the evidences of lahars and block and ash ows, mainly directed towards the fans to the Northeast, South and Southeast, in areas with growing population and infrastructure development (Selles & Moreno, 2011). In April 2015 the last volcanic eruption of the Calbuco was recorded. According to studies such as Castruccio, et al. (2016), the erupted products were mainly basaltic andesite. Another important volcanic structure to mention in our study area corresponds to the Osorno Volcano stratum belonging to the Middle Pleistocene (< ca. 200 ka). Together with the La Picada, Puntiagudo and Cordón Cenizos volcanoes, it forms a transversal volcanic chain of N60ºE orientation. The eruptions of the central crater, Osorno volcano, have generated basaltic pyroclastic ows and tephras of dacitic composition that date from the Holocene (< 10 ka) and are distributed in a wide sector around the volcano. The historical eruptive activity of the Osorno volcano has been essentially characterized by low explosive episodes among which the sural eruption of 1835 AD stands out. (Moreno, et al., 2010) Other volcanic centers in our study zone correspond to the Puntiagudo stratovolcano (2493 m.a.s.l.) that within its volcanic materials stand out the lava emissions and pyroclastic and laharic deposits of predominant basaltic composition and the Tronador volcano that corresponds to a mixed stratovolcano that houses a great glacier on its summit that plays a great role in the violent interaction between magma and water generating explosive reactions. (Mella, et al., 2005).

Magnetotelluric data
Magnetotellurics (MT) is an electromagnetic method of geophysical exploration used to deduce the electrical resistivity structure of the Earth at depths ranging from tens of meters to hundreds of kilometers. This method uses a source signal of external origin, natural variations of electric and magnetic elds of the Earth, generated by physical processes such as solar variations and magnetic storms. The spectra of these variations cover periods ranging from 10 -3 to 10 5 [s] and are generated by the interaction between the solar wind, the magnetosphere and the ionosphere of planet Earth (Vozoff, 1991).
The MT method is based on the simultaneous measurement of the total electromagnetic eld, i.e. the time variation of the magnetic eld B(t) and the induced electric eld E(t). The electrical properties of the underlying material can be determined from the relationship between the components of the measured variations of electric (E) and magnetic (B) elds, or transfer functions: the horizontal electric (Ex and Ey) and the horizontal magnetic (Bx and By) and vertical (Bz) eld components. The transfer functions consist of a complex impedance tensor which relates the measured electric eld components to the magnetic eld components and a geomagnetic transfer function which is often called a tipper and relates the horizontal magnetic eld components to the vertical magnetic eld component. In general the resistivity structure of the earth is three-dimensional obtaining an impedance tensor of four components, complex impedance tensor, however, if it is a two-dimensional structure where the resistivity does not vary along the geoelectric strike the diagonal components of the tensor are null as long as the coordinate system is parallel to the strike direction. In this case the two surviving tensor components are related to two independent modes, the magnetic transverse (TM) and the electric transverse (TE). Sometimes if the strike preference angle is known, the tensor can be rotated so that the diagonal components are minimized. Generally the elements of the impedance tensor can be plotted as apparent resistivity and phase as a function of period (reciprocal of frequency), these graphs can give us an idea of variations in the average resistivity of the medium that has been measured so that short periods are related to shallow depths and long periods to greater depths. Some components of the measured electric elds can be affected by a phenomenon called Static Shift that is generated due to shallow conductive structures, or due to the effect of topography and is observable in a delay at the beginning of the apparent resistivity curves that in ideal conditions should start at the same value. On the other hand, the geomagnetic transfer function serves to identify lateral resistivity contrasts and can be represented as arrows or induction vectors, under the convention of Wiese (1962) the vectors point in the opposite direction to the conducting body. When making long period measurements near the ocean it is expected to observe the so-called shore effect on the induction vectors for long periods, due to the large resistivity contrast between the ocean and the continent the orientation of the vectors over longer periods should be expected to be in the opposite direction and perpendicular to the shore.
In order to obtain a lithospheric model of the electrical resistivity structure of the study area, several eld campaigns were carried out between December 2018 and April 2019, where 14 long-period and 10 broadband stations were installed along the pro le transverse to the coast ( g1). Three types of stations were used to carry out the measurements, ADU (Analog/Digital Signal Conditioning Unid), NIMS (Narod Intelligent Magnetotelluric Systems) and Lemi417. All stations were oriented with a geomagnetic coordinate system. The data processing included the application of lters to the obtained time signals to eliminate noise signals where possible, remote reference application in some cases to improve the data quality of the stations that were measuring simultaneously. The signals obtained with ADU stations were processed with the robust process developed by Egbert & Booker (1986), those obtained from NIMS stations were processed with the code developed by Egbert & Livelybrooks (1996) and the signal obtained with Lemi stations were processed with the robust code developed by Egbert (1997).
We proceeded to calculate the strike in period intervals for all stations with the code developed by Smith (1995) in order to know if there was any preferential direction of the structure and to diagnose a twodimensional space. In gure 2 it can be seen that for the range of periods between 10 -3 s and 10 0 s the structure does not have a preferential orientation, evidencing a three-dimensional environment, while for periods greater than 1s the structure begins to behave two-dimensionally. In gure 3 the induction vectors are plotted in a length vs period pro le where we can observe characteristic behaviors of areas where we expect to nd conductive bodies in the inversion models. The small magnitude of the arrows enclosed by the dotted celestial line and the W-E direction of the arrows enclosed by the purple line suggests a conductive zone towards the west of the Llanquihue lake, the direction of the vectors enclosed by the pink dotted line could be indicating the presence of a conductive body around the Osorno Volcano, while the magnitude of the arrows enclosed by the green dotted line indicates the presence of a body of low resistivity in the Andes mountain range.
For long periods (>10 1 s), the arrows were expected to have a regional behaviour in W-E direction due to the coast effect, however, it can be observed in the vectors delimited by the yellow dotted line that have a SW-NE direction. This behavior of the vectors is studied by Brasse, et al. (2009) in the Chilean continental margin, between latitudes 38°-41°S and attributes it to the resultant between the effect of the ocean and the presence of a layer with structural electrical anisotropy in the crust.
An electrically anisotropic medium can be, for example, a space in uenced by micro-scale hydrated faults that can become a macro-scale structure if these structures have a preferential general orientation. From the above, it can be inferred that one way to mitigate the effect of anisotropy on the data is to work with the periods between 10 1 s and 10 3 s, since this range has a general preferential strike (Martí, 2014)

Magnetotelluric data inversion
We carried out 2D and 3D inversions in order to obtain a realistic model that is close to the geology of the region. In addition, Martí (2014) recommends testing different inversion software, different models and altering different parameters in the inversions until we obtain models without artifacts resulting from structural electrical anisotropy.
For the two-dimensional inversion we used the WinGlink software that uses the 2D inversion code from Rodi & Mackie (2001) , this algorithm minimizes an objective function, by means of the non-linear conjugate gradient method (NLCG), which penalizes the differences between the data and the model response, also condemning the second spatial drifts of the resistivity model, in order to obtain a contrast of resistivities in a smooth model. Taking into account the dimensional analysis performed previously, the data set was chosen between the range of periods 10 1 s and 10 3 s, taking into account that anisotropy is mostly re ected in the induction vectors, it was chosen to model only the TE and TM mode data. Background resistivities of 100 and 500 .m were tested, the minimum frequency used was 0.00008hz, error oors was set to 20% for apparent resistivities and 5% for phases. A ne grid with growth factor 1.2 was used in depth and to the sides of each station (see g 4 a). Although, the topography option was activated, it is not in uential in the inversion when working with long period data.
To the inversion a priori known information of the zone was delivered, an ocean was drawn with a resistivity of 0.3 m and a mean depth of 5km according to the bathymetry registered by the National Oceanic and Atmospheric Administration (www.ngdc.noaa.gov/mgg/topo/topo.html).
From the analysis of results of synthetic data modeling and the comparison between 3D and 2D inversion results, Chang-Hong, et al. (2011) the possibility of obtaining a true 3D resistivity structure from 2D pro le data was demonstrated. Furthermore, our results show that not only a reasonable threedimensional image can be obtained under the 2D pro le but also 3D structures close to the pro le that cannot be obtained with a two-dimensional inversion are achievable, provided that the four elements of the impedance tensor and the tipper are modeled, since the elements of the diagonal have a special effect on the recovery of the distribution of three-dimensional anomalies close to the pro le..
There are several MT studies, which deliver 3D models from data collected along a 2D pro le, for example, Meqbel For the three-dimensional model, a logarithmic grid was designed according to the order of periods and the maximum depth expected to be reached with each set of data. The growth factor in x and y direction was set at 1.3 while 1.2 was chosen for the depth.
A total of 52 frequencies were used for the impedance tensor and 50 frequencies for the tipper. The smoothing factor was set at 0.7. The oor error for the diagonal components was 7%, while for the components outside the diagonal and the tipper 5%. For this model the bathymetry of the Paci c Ocean was taken into account, this was downloaded from the National Oceanic and Atmospheric Administration and with an electrical resistivity de ned at 0.3 m.
Tests were performed including and excluding the Nazca plate in the 2D and 3D grids of the a priori models, the Nazca plate was drawn as a layer of 9.000 m resistivity with the depths indicated in the study by Tassara & Echaurren (2012) that presents a model of the depth of the Slab, the continental moho and the contact between lithosphere and asthenosphere in our study zone. We observed that the bodies generated in each case were similar in location, magnitude and shape, however we obtained better RMS in the models that did not include the Nazca plate.
Finally, we performed sensitivity analyses in both models obtained in order to validate the need of each body within the model for the nal t of the curves of each station.
The sensitivity analysis consists of eliminating the structure and studying the response of the model through forward modeling. The response of the new model is compared with the response of the original model to analyze how it is modi ed when a body is removed. If the variation in uences different stations and the size and distance to the location of the body is consistent with the perceived uctuation in the curves of each site, it can be concluded that the body is necessary for the t of the observed data.

Result
The two-dimensional model (Fig. 5) was tted with an RMS of 1.88% after 200 iterations. In it, several conductive and resistive bodies can be observed that can be roughly related to the known geology and tectonics of the zone. In Fig. 5, the resistive body R1, with values between 1,000 and 8.000 m, is intrinsically related to the subducting Nazca plate; it must be taken into account that the plate has an important resistivity decrease at 50 km depth. R2, on the other hand, is a highly resistive body, located under the Osorno Volcano and between the Llanquihue and Todos Los Santos lakes. It is located within the continental crust, from a shallow depth to ~ 20 km. Conductive "C" bodies are often related to uids, hot zones, salt -water bodies, etc. Towards the west of the pro le, in the central depression, two conductive bodies appear, shallow C1 and C2 and with resistivity of 1 and 7 m respectively. Towards the east of the pro le, near the Tronador volcano, a body "C3" is observed that crosses the earth's crust, from the mantle, with a resistivity of between 13 and 50 m. In magnetotellurics, broadband data has better resolution for shallow depths while long period data has better resolution for greater depths. Although the present work inverted models with both broadband and long period data, it should be noted that the periods used for broadband stations were ≥ 10 1 s, so it is not expected to obtain high resolution of the surface bodies. For the interpretation of results, the existence of equivalent models that will deliver the same data t, but with a completely different body distribution, should also be considered. For example, a small conductive body could be equivalent to a body with lower conductivity, but larger size.
It is believed that the seismicity in the South of Chile is quite lower, compared to the seismicity rate of the rest of the country, several authors such as Agurto-Detzel, et al. (2014) and Lange et al., (2008), a rm that the seismicity of this zone is dominated mainly by the Liquiñe Ofqui fault system. By plotting the seismicity registered since 2013 in our electrical resistivity image, it is easy to notice the accommodation of the earthquakes in the subduction zone and in the zone where the fault system is believed to cross the latitude of the pro le. Figure 5a shows three depth cuts parallel to the station pro le (located in Fig. 5b). In this model different conductive and resistive bodies can be seen below and outside the pro le. There is a good correlation with the bodies "C" and "R" generated by the 2D inversion with the bodies generated by the 3D inversion being C5, C6 and C7 three conductive bodies that only appear in the 3D, this could be due to the fact that in the two-dimensional model the value of the resistive R2 is very high and fades the resistivity value of these bodies. The reason why some bodies are shown in the 2D inversion, even when they are outside the pro le, according to the 3D model, may be due to a high resistivity contrast between these bodies and the resistivity below the stations, for example, C1 and C2.
It is also possible that the location of the bodies outside the pro le in the 3D model is related to the in uence of the tipper that was discarded in the 2D inversion due to structural electrical anisotropy. However, there are studies such as the one by Slezak, et al. (2016), located in the Northwest of Poland where, without having evidence of structural electrical anisotropy in the crust, they obtain in the 3D model conductive bodies outside the pro le that are not evident in the 2D model.
The model we chose to interpret and discuss corresponds to the 3D model, with a RMS 1.95%, because it covers all the bodies shown in the 2D model and additionally provides other bodies that converge well with the geology of the zone. The three-dimensional model was obtained with all the available information from the impedance tensor and tipper.

Discussion
R1 has been identi ed as a regional body at great depths related to the Nazca plate that subduces under the South American plate and its decrease in resistivity at 50 km depth is related to the process of serpentinization by an early dehydration of the Slab, this serpentinization of the mantle produces an increase in porosity that leads to a decrease in electrical resistivity, which can be further reduced by the production of electrically conductive magnetite networks (Stesky & Brace, 1973). The Nazca plate has been determined by several long term magnetotelluric studies, in the South Central and Southern zone of Below the Central Valley, conductive anomalies formed by bodies C1 and C2 are located super cially in the crust, just above where the Nazca plate discontinuity appears. In the three-dimensional model (see central pro le), body C2 seems to migrate from the mantle. Geologically, we relate these bodies to the early dehydration of the Slab, in this process, water bound to the pores is expelled from the subducting slab to shallow depths by compaction and lithitization (Hyndman, et al., 1997).
The result of a seismic pro le located a few meters from our magnetotelluric pro le in the zone of the Central Valley, studied by Jordan, et al. (2001), shows some seismic re ectors in the zone where bodies C1 and C2 are observed that relate to faults at a depth not greater than 10 km, we could a rm that these faults are facilitating the migration and distribution of uids in the central valley.
The C3 anomaly can be interpreted as a zone of uid accumulation resulting from a partial fusion related to the Nazca plate serpentinization or eclogitization process and high pressure and temperature conditions at depth. Numerical models by Völker & Stipp (2015) suggest that approximately 50% of the water intake uids from the crust and mantle are released in the front arc or rear arc, this could explain why the C3 conductor is much larger than the other conductors and why it is located more to the east of the pro le. This important conductivity anomaly is comparable to anomalies found in studies conducted in other zones that have developed typical surface expressions such as volcanic arcs around the world, e.g., magnetotelluric studies in British Columbia (Soyer & Unsworth, 2006), in Oregon (Cascadia) (Jiracek & Curtis, 1989), in Mexico (Jödicke, et al, 2006), in Northern Chile (Schwalenberg, et al., 2002) and (Brasse, et al., 2002), in Greece (Galanopoulos, et al., 2005), in Japan (Nankai) (Ichiki, et al, 2000) and (Ryukyu Trench) (Shimakawa & Honkura, 1991), in Costa Rica (Worzewski, et al., 2011), in Central and Southern Chile (Cordell, et al., 2019), (Brasse, et al., 2009) and (Kapinos, et al., 2016) The C5 body with resistivities around 1 m located north of the Osorno volcano at a depth less than 10 km could be related to an active magmatic and/or hydrothermal system associated with the volcano, its such low resistivity values correlate well with a conductive zone of molten mass accumulation. The report of volcanic activity in the Los Lagos region (SERNAGEOMIN, n.d. ) has recorded multiple seismic activity between 2017 and 2019, related to the Osorno volcano; Most of the seismic events have been located towards the Northwest of the volcano at depths of less than 10 km, coinciding with the location of the anomaly, however, it must be clari ed that the location of the events it provides is not exact, also, taking into account that the last episode related to a volcanic eruption of the Osorno dates from the year 1835 and that the information of the reports is not su cient either to analyze the focal mechanisms and to con rm that the events are own of the volcano, we can a rm then that these events are attributed to the System of Fallas Liquiñe Ofqui. An important consideration relates to the effect of including or excluding topography in inversion algorithms. For the Merapi volcano, located in Indonesia, Müller & Haak (2004) they show that the induction vectors suggest a conductive structure centered in the volcano that they attribute to the high topography of its cone, that is, in these cases special care must be taken to differentiate how much of the response of the induction vector is due to the topography and how much to the conductivity of the rock. Brasse & Eydam (2008) also analyze the topographic effect on volcanic buildings with a high slope, nding that the inversions create artifacts related to a static displacement in the electric eld resulting from the effect of the topography. According to the above and knowing that in the 2D and 3D models of our study the topography was not included, it is very likely that C5 is simply an artifact created by the inversion software as an effect of the topography of the cone of the Osorno volcano.
The anomaly named C6, located to the east of the Osorno volcano, coincides with the location of the Liquiñe Ofqui fault zone at 72.28°W in length. The elongated shape of this body supports well the idea that it is a uid ascent facilitation conduit due to a fractured weakness zone related to the fault system.
The papers by Cembrano & Lara (2009) and Perez-Florez, et al. (2017) con rm that transverse strike-slip faults are oriented favorably with respect to the stress eld and promote vertical migration of magma. It should be noted that the C6 body, in our model, does not present resolution for depths greater than 20 km. This trace of the fault is also observed in the study of Zuñiga Armijo (2019), by means of broadband magnetotelluric measurements, Zuñiga Armijo (2019) proposes that this fault is facilitating the ascent of uids towards a mush type reservoir of the Osorno volcano.
The C7 anomaly shown in the three-dimensional inversion is a conductive body, located under Calbuco volcano that could be interpreted as a magmatic reservoir of depth between 10 and 20 km that is connected to a conduit or structure of the Andean transverse faults and that is directed towards the trace of the Liquiñe Ofqui fault. There is a difference in the composition of the products in the two volcanoes, the Osorno is predominantly basaltic while the Calbuco is andesitic. However, ground, petrographic and geochemical observations suggest that the andesitic composition of the Calbuco is the result of a contamination of subcortical basaltic magmas with magmas generated at the cortical level (Lopez-Escobar, et al., 1992). Thus, the difference between the composition of the Osorno and Calbuco products could be explained by an apparent differentiation of the magma that has to travel a greater distance to reach the volcanic reservoir, or perhaps it could also be due to the fact that the magmatic material of this volcano has a longer residence time in the crust than the rest of the volcanic centers in the southern zone of the Andes.
Finally, the highly resistive R2 is associated with the granitic base Batolito Nor-Patagónico, located in the southeastern part of the Osorno volcano, the Cretaceous base and in the western foot, the Miocene base.
The North Patagonian Batolith is mainly formed by granodiorites, diorites, tonalites, granites and tonalites of hornblende and biotite, mainly igneous rocks that are in the range of resistivity of 1000-100000 m according to Palacky (1987  The position of the large conductor (C3) in the main mountain range is similar to that found by various magnetotelluric studies around the world in subduction zones and which they interpret as a zone of uid accumulation, generated by a partial fusion related to the phenomenon of eclogitization of the plate that can occur at different depths depending on the degree of hydration or age of the slab.
Other processes of migration and accumulation of uids through the continental crust were identi ed in our resistivity image, such as the C5, C6 and C7 anomalies, associated with the Osorno Calbuco volcanoes and the fault systems that dominate the zone. The C5 anomaly that was initially associated with a magmatic chamber located 10 km deep northwest of the Osorno volcano was nally discarded and interpreted as an artifact created by the inversion of the volcanic cone topography. Availability of data and materials: In the following link reviewers can view information supporting the research. Only they can access the data for information purposes. This information is private and cannot be reproduced or modi ed. This information can only be seen by the reviewers of the paper for information purposes. The information will be published once the article is accepted and published in the journal. https://osf.io/zsqjc/?view_only=c606c5b2ba7a4991a539630d7e876d5f Competing interests: We have no con ict of interest in this investigation.        Comparison between measured data (dots) and model response (continuous lines) at four exemplary sites. 2D and 3D model t curves Best and worst tting curves.