R1 has been identified 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 Chile, for example, Brasse, et al. (2009), Kapinos, et al. (2016) and Cordell, et al. (2019). On the other hand, Worzewski, et al. (2011) with a study of MT in Central America where the Nazca, Cocos and Caribbean plates interact observe this important decrease in resistivity in the same zone where DeShon, et al. (2006) they observe a decrease in the S wave velocities, both studies propose that this is a zone of serpentinization of the mantle.
Below the Central Valley, conductive anomalies formed by bodies C1 and C2 are located superficially in the crust, just above where the Nazca plate discontinuity appears. In the three-dimensional model (see central profile), 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 profile located a few meters from our magnetotelluric profile in the zone of the Central Valley, studied by Jordan, et al. (2001), shows some seismic reflectors in the zone where bodies C1 and C2 are observed that relate to faults at a depth not greater than 10 km, we could affirm that these faults are facilitating the migration and distribution of fluids in the central valley.
The C3 anomaly can be interpreted as a zone of fluid 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 fluids 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 profile. 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 1𝛺m 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 clarified 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 sufficient either to analyze the focal mechanisms and to confirm that the events are own of the volcano, we can affirm 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, finding that the inversions create artifacts related to a static displacement in the electric field 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 fluid 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) confirm that transverse strike-slip faults are oriented favorably with respect to the stress field 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 fluids 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.
According to Selles & Moreno (2011) the Calbuco volcano, does not present evidence of being located on an important regional structure, however, Perez-Florez, et al. (2017) outline some guidelines of the Andean Transverse Fault system, which present orientation NOT located in the southern volcanic zone of the Andes.
Morgado, et al. (2019) through a petrochemical study of samples from the Calbuco eruption in 2015, propose the existence of a mush type deposit, located in the upper crust up to 5.5–9.5 km deep. Delgado, et al. (2017) suggest a pressurized flattened spheroid shape for the reservoir below the Calbuco volcano through modeling of the co-eruptive subsidence signal. Considering the 2015 lava flows, Morgado, et al. (2019) restricted the pre-eruptive conditions to: temperature 900–1060 °C, pressure 2–6 kbar, 2–4% H2O and a variable amount of SiO2 between 55.6 and 56.9%. Castruccio, et al. (2016) calculated SiO2 between 54–55 wt% and Na2O between 3.6–3.8%, 4–5% H2O and pressures of 180–240 MPa.
Using the SIGMELTS web application (Pommier & Le-Trong, 2011) a model is built with the above parameters, estimating an electrical resistivity of the variable magmatic reservoir between 2–10 𝛺m, which could be an indicator that, by model equivalence, our C7 anomaly is less deep and more conductive.
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 𝛺m according to Palacky (1987).