The Role of Active Crustal Faults in Geothermal Systems in Volcanic Areas: A Case Study of Southern Chile by Magnetotelluric Method

The Andean magmatic arc results from the on-going subduction of the Nazca and Antarctic beneath the South American Plates. The most active volcanoes in the Andes are concentrated in the Southern Volcanic Zone (SVZ) which extends from 33 to 46°S. This zone is characterized by the NNE-striking Liquiñe-Ofqui Fault Systems (LOFS), which extends over 1400 km and is offset by a group of NW-striking so-called Andean transverse faults (ATF).
Preferential fluid pathways along these regional fault systems are crucial for the spatial localization of surface manifestations in the Andean magmatic arc. Ranging from volcanoes to thermal springs, they represent the geothermal spectrum from high- to low temperature geothermal systems in this area.
Both, magmatic (melt) and geothermal fluids, are linked to comparatively high electrical conductivity and thus, prone to electromagnetic exploration. Given the depth of occurrence, here, we use magnetotelluric methods to develop flow concepts for both, low- and high temperature, fault-driven geothermal systems in volcanic areas. The low-and high-temperature systems of the Pucón/Villarrica area including 29 thermal springs (geochemical temperature estimations ranging from 84 to 184°C) and the Tolhuaca area including geothermal wells with temperature between 160 and 300°C, respectively, are considered representative examples.
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To identify the three-dimensional (3-D) effects generated by preferential fluid pathways in areas of interference between active cortical faults and volcanism surrounding the Pucón/Villarrica area, we used 31 broadband MT (BB-MT) stations deployed along E-W and N-S oriented profiles, intersecting the major fault systems. The 3-D inversion reveals eight electrical resistivity anomalies, which may be related to fluid pathways. Six of these anomalies are localized at a shallow-crustal level (<2 km) and are linked to volcanic or geothermal manifestations at the surface. Their connection to the surface is evident in the fault-related system, where the intersection of two damage zones may provide optimal vertical pathways. Highly conductive anomalies at mid-crustal level (>4 km and 2-5 km depth) coincide with or cross the eastern branch of the LOFS, as well as with the Villarrica volcanic chain and the Mocha-Villarrica Fault Zone (MVFZ). In such classical step-over and fault intersections, fluids are presumed to accumulate in related damage zones acting as a vertical conduits extending to the surface. A proven link between the Villarrica volcanic chain and a deep anomaly (>8 km) could not be established thus far.
To better understand the context of the deep low resistivity anomalies in the Villarrica volcanic chain and to investigate the implications of crustal faulting and volcanism and their consequence on crustal reservoirs, long-period (LP-MT) and broadband (BB-MT) magnetotelluric data were acquired surrounding of Villarrica volcano. The resistivity distribution shows the upper crust as highly resistive, but below and east of Villarrica volcano, the model suggests the presence of a magmatic reservoir at shallow crustal levels (between 1.5 and 3 km b.s.l.). This may well be a temporary magma storage zone. The middle crust contains several intermediate to low conductive features interpreted as fluid pathways and melt storage channels, revealing the important role of the fault systems. The lower crust contains zones of low resistivity suggesting the presence of partial melt and/or fluids, associated with deep reservoirs (8-20 km). They may represent a deep source; a significant proportion of this volume is likely to be distributed in non-eruptible parts of the reservoir. This would suggest that the melt is accumulated as highly crystallized mush or disconnected melt pockets. We concluded, that the fault core can act as a conduit for fluid flow during deformation and as a barrier when open pores are filled with precipitated minerals after deformation.
To investigate the role of faults in the other geothermal end-member, a high-temperature field, the Tolhuaca volcano has been studied along its northwest flank using 3-D inversion of MT data. The 3-D MT model depicts a conductive body at ∼2 km beneath Tolhuaca volcano, would correspond to the long-term residence of a subsurface magmatic intrusion associated with ATF structural control. Since we found no indications of a deep conductor in the study area, such as those observed in other high enthalpy geothermal systems, we conclude that the shallow magmatic deposit, which is cooling but still hot, is the heat source of the geothermal system. It is not located beneath the geothermal field but laterally offset. Therefore, the prevailing tectonic configuration in the Tolhuaca system, the interaction between LOFS and ATF, would promote the development of a long-lived shallow reservoir. On the other hand, we also detect a 300 m thick layer of high conductivity associated with argillic hydrothermal alteration. The MT model includes two resistive bodies in the upper crust beneath the laterally displaced argillic alteration layer to the west beneath the extinct Tolhuaca, which would correspond to a shallow reservoir (1000 m from the surface) and a deep reservoir (>1800 m from the surface) that previous resistivity models had not identified.