Shallow anatomy of hydrothermal systems controlled by the Liquiñe-Ofqui Fault System and the Andean Transverse Faults: Geophysical imaging of fluid pathways and practical implications for geothermal exploration

Highlights

• We combined ERT, nakamura's method, and geological mapping to localize thermal water.

• Hotspring's thermal water is fed by vertical conduits aligned with local faults.

• Significant amount of hot water is dispersed in horizontal bodies within sediments.

• The hydrothermal systems are twice longer within sediments than the hotspring area.

• We present a model of the shallow anatomy of fault-controlled hydrothermal systems.

Abstract

We combined geoelectric and seismic ambient noise methods to image the shallow depth (<30 m) distribution of thermal waters in two fault-controlled hydrothermal systems located in southern Chile. The bedrock depth was constrained with seismics, while hotsprings and mapped faults were imaged by low-electrical-resistivity domains (<160 Ωm) defined with electrical resistivity tomographies (ERT). The distribution and shape of low-resistivity-domains suggest that thermal fluids follow complex pathways, including deep vertical conduits hosted in fractured rock and shallow horizontal bodies hosted in sediments. These results indicate that the studied hydrothermal systems are at least twice longer within the sediments than the superficial area covered by hotsprings.

Introduction

Thermal water hosted in hydrothermal systems controlled by faults represents an important alternative source of energy and water resources for humanity. Understanding these hydrothermal systems is essential for developing low-enthalpy geothermal process as a renewable energy solution (e.g. Thayer et al., 1989; Lund and Falls, 1997; Jubaedah et al., 2015), to achieve the sustainable management of geothermal reservoirs (DiPipo, 2012, and references therein), and also to model the complete hydrological balance in basins close to mountain ranges (e.g., Taucare et al., 2020). Hydrothermal systems controlled by faults do not necessarily require a heat source from a magmatic body, as volcanic geothermal systems demand, since the deep circulation of meteoric water through faults is enough to promote the existence of thermal springs at the surface (e.g., Jolie et al., 2021). These systems have been widely studied; for example, the Têt fault in France (Taillefer et al., 2018), around Lake Tiberias (Magri et al., 2016), the Basin and Range Province in the USA (Blackwell et al., 2012; Siler et al., 2019), the Grimsel Pass in the Swiss Alps (Diamond et al., 2018), the foothills of the Andes in Northern Chile (Hoffmann-Rothe et al., 2004) and Central Chile (Taucare et al., 2020), and around the Liquiñe area in the Chilean Southern Andes (Daniele et al., 2020). Hydrothermal systems controlled by faults are frequently recorded around the world because faults represent high-permeability domains where water is canalized, promoting the upflow of thermal water and the occurrence of hot springs (e.g., Magri et al., 2016; Taillefer et al., 2018; Üner and Dusunur Dogan, 2020), and crustal faults are recorded in all active tectonic setting. These hydrothermal systems have commonly been studied from a regional point of view; thus, the shallower expression of fault-controlled hydrothermal systems (first few tens of meters) remains poorly understood in many places, as is the case for the Southern Andes. However, the shallow part of these hydrothermal systems is extremely important since many man-made facilities involving low-enthalpy direct-use geothermal processes are built at these depths.

In hydrogeological exploration, geophysical methods have been used to infer the distribution of groundwater (Kirsch, 2006; Viguier et al., 2018; Levy et al., 2019) and image the internal structure of faults (Tripp et al., 1978; Storz et al., 2000; Richard et al., 2010; Giocoli et al., 2011; Estay et al., 2016; Yañez et al., 2020). The main non-invasive survey methods for such applications are electromagnetic since the electrical properties of saturated rocks and sediments strongly depend on the electrical properties, the amount, and the interconnectivity of interstitial water (Archie, 1942). One of the most widely used geophysical methods in hydrothermal environments is electrical resistivity tomography (ERT), which can be used to image the electrical resistivity distribution of the subsurface by injecting electrical currents and measuring electrical potentials along a profile (Telford et al., 1990). Several studies have used ERT at thermal sites worldwide (Spichak and Manzella, 2009; Richards et al., 2010; Fikos et al., 2012; Chabaane et al., 2017; Levy et al., 2019). Moreover, several mathematical relationships have been proposed to relate the bulk electrical conductivity of rocks and sediments with their fluid content, for example, Archie's law (Archie, 1942) and the Waxman and Smith equation (Waxman and Smith, 1968). Archie's law has been used to localize petroleum in sandstones (Archie, 1942) and localize cold aquifers in sedimentary environments (Kirsch, 2006 and examples therein). Although geophysical methods and Archie's law have been widely used in hydrogeological research, only a few studies (Levy et al., 2018; 2019; Hermans et al., 2014, 2015) have applied both tools together to investigate thermal water circulation.

The Liquiñe-Ofqui fault system (LOFS) and the Andean Transverse Faults (ATF) are two major fault families that can be observed along the Southern Andes (Fig. 1a). Previous studies have documented that the LOFS and the ATF have played key roles in the development of fossil and active hydrothermal systems (Sánchez et al., 2013; Tardani et al., 2016; Pérez-Flores et al., 2016; Wrage et al., 2017; Veloso et al., 2019; Yañez Carrizo and Rivera Herrera, 2019; Piquer et al., 2019; 2020; Daniele et al., 2020; Pearce et al., 2020). Distinct geochemical signatures have been documented for thermal sites hosted in the LOFS and ATF. Hydrothermal systems hosted in the ATF have being related to greater crustal isotopic contaminations and higher-enthalpy geothermal reservoirs (Sánchez et al., 2013; Tardani et al., 2016; Wrage et al., 2017; Daniele et al., 2020). Most of these hydrothermal systems have a δ_D - δ¹⁸O signature that matches with the regional meteoric water line, suggesting that the primary water source is rain infiltration (Wrage et al., 2017; Negri et al., 2018; Daniele et al., 2020). The volume and strike-orientation of faults in the LOFS and ATF with respect to the regional stress field (σ₁∼N60°E; Cembrano and Lara, 2009) has been recognized as the key feature to understand the regional circulation of thermal fluids along these hydrothermal systems (Sánchez et al., 2013; Tardani et al., 2016; Pérez-Flores et al., 2016; Roquer et al., 2017). Moreover, the intersection of branches of the LOFS and ATF apparently plays a fundamental role in the occurrence of hydrothermal systems in this part of the Andes (Roquer et al., 2017). Although the hydrothermal systems of the Southern Andes have been recently studied at the regional scale, little is known about the shallow anatomy and thermal water circulation at the local scale of these systems.

In this study, we combined ERT, ambient seismic noise (Nakamura, 2019), geological studies, and Archie's law with the following aims: first, to illustrate how this geophysical and geological information can be combined to reveal the shallow (first 30 m) distribution of thermal waters in fault-controlled hydrothermal systems; and second, to show how the observation of the shallow anatomy of these hydrothermal systems can be used to infer the fluid pathways and the size of the systems, information that is useful for geothermal exploration. Specifically, we apply this methodology in the Fucha thermal site hosted in the Liquiñe-Ofqui fault system and the Hipólito Muñoz thermal site hosted in an Andean Transverse Fault (Fig. 1b). The Fucha thermal site was selected since it represents an excellent site to calibrate and understand shallow thermal water circulation, considering its well-exposed outcrops, which clearly show the geological units involved in the hydrothermal system. Besides, the Fucha thermal site is spatially related to an NS-striking dextral-reverse fault which is representative of the major regional faults of the LOFS; thus, this thermal site could reveal features of many similar typical thermal sites distributed around the entire Southern Andes (e.g., the Palguin, Chihiuo, Rayencó, and Trafipan thermal sites; Sánchez et al., 2013; Daniele et al., 2020). Meanwhile, the Hipólito Muñoz thermal site was selected since it is an example of a thermal spring emplaced on one fault of the ATF, which is representative of the other fault families located in the Southern Andes and has been associated with several thermal sites, for example, the Nevados de Chillán, Coñaripe, or Rincón sites (Sánchez et al., 2013; Tardani et al., 2016; Wrage et al., 2017). The two studied hydrothermal sites can be used as models for the understanding of geothermal fluid flow within fractured rocks and shallow sediments, and of how these systems can be explored for direct-use implementations.