# Hydrothermal activity of the Lake Abhe geothermal field (Djibouti): Structural controls and paths for further exploration

Bastien Walter, Yves Géraud, Alexiane Favier, Nadjib Chibati, Marc Diraison, Hassan Mohamed Magareh

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## To cite this version:

Bastien Walter, Yves Géraud, Alexiane Favier, Nadjib Chibati, Marc Diraison, et al.. Hydrothermal activity of the Lake Abhe geothermal field (Djibouti): Structural controls and paths for further exploration. Bulletin de la Société Géologique de France, 2025, pp.1-65. 10.1051/bsgf/2025026. hal-05355465

HAL Id: hal-05355465

https://hal.univ-lorraine.fr/hal-05355465v1

Submitted on 8 Nov 2025

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# Hydrothermal activity of the Lake Abhe geothermal field (Djibouti): Structural controls and paths for further exploration

## Caractérisation des chemins de circulation de fluides du système géothermique du lac

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### Abhé (Djibouti)

Bastien Walter¹, Yves Géraud¹, Alexiane Favier¹, Nadjib Chibati¹, Marc Diraison¹, Hassan Mohamed Magareh²

¹ Université de Lorraine, UMR-CNRS 7359, GeoRessources, F-54506 Vandœuvre-lès-Nancy,

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France

² Office Djiboutien de Développement de la Géothermie, Zone Industrielle, PK20, République de Djibouti

*Correspondence to:* Bastien Walter (bastien.walter@univ-lorraine.fr)

## Key words - Mots-clés

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Geothermal system ; Extensional tectonics ; Fluid pathways ; Hydrothermal chimneys ; Afar depression ; Djibouti
Système géothermique ; Tectonique extensive ; Chemins de circulation de fluides ; Cheminées
hydrothermales ; Dépression de l'Afar ; Djibouti

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**Abstract**

This study investigates the structurally-controlled fluid flow of the Lake Abhe Geothermal Field (LAGF), using multiscale structural lineament distribution mapping and field observations. The LAGF lies within the Gob Aad graben in the Afar depression, at the junction of three rifts, along the Djibouti–Ethiopia border. Numerous hydrothermal surface manifestations on the lake’s eastern shore, including steam vents, hot springs and carbonate chimney structures, reflect the geothermal activity of this area.

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Structural features of the LAGF area are dominated by ESE-extensional faults that form a series of narrow elongated horst, graben and half-graben structures. Fault interaction and accommodation zones, such as fault intersections and relay ramps, as well as possible breaching faults are also identified in the area. The control of the main ESE-structural direction over the distribution of hydrothermal chimneys and hot springs indicates these faults to be the primary permeability fluid pathways of the LAGF. Signs of enhanced hydrothermal activity at fault intersections further suggest that structural intersections locally increase fracture-related permeability. Field observations combined with satellite image analysis also reveal a lateral migration of the hydrothermal outflows over a short period of time (during the past several

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thousands to tens of thousands of years) from the SE to the NW. Finally, this study discusses the potential role of N-striking faults, which may either act as vertical drains channeling fluids from south to north or as barriers preventing eastward fluid migration.

Overall, this study provides new insights into the tectonically driven fluid flow dynamics of the LAGF, which may support further exploration of this remarkable site and promote its geothermal development.

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Résumé

Le lac Abhé est situé dans le graben de Gob Aad, au sein de la dépression de l'Afar, au niveau de la frontière entre la République de Djibouti et l'Éthiopie. Les nombreuses manifestations hydrothermales de surface présentes sur la rive orientale du lac, notamment les évents de vapeur, les sources chaudes et les cheminées carbonatées, reflètent l'activité géothermique de cette région. Cette étude décrit les circulations de fluides hydrothermaux du champ géothermique du lac Abhé (LAGF), contrôlé par la structuration tectonique de la zone, en s'appuyant sur une cartographie multi-échelle de la distribution des linéaments structuraux et des observations de terrain. Les caractéristiques structurales de la zone du LAGF sont dominées par des failles extensives orientées ESE, qui forment une série de blocs structuraux de type horst, graben et demi-graben. Le contrôle exercé par cette orientation structurale dominante sur la distribution des cheminées hydrothermales et des sources chaudes indique

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que ces failles constituent le principal chemin de circulation de fluides du LAGF. Des zones d'interaction de failles, telles que des intersections de failles et des rampes de relais ont également été identifiées dans la région. Ces structures représentent des chemins supplémentaires de percolation de fluides, améliorant ainsi la connectivité globale du réseau de failles du LAGF. Les observations de terrain, combinées à l'analyse d'images satellitaires, révèlent également une migration des circulations hydrothermales au fil du temps, migrant du sud-est vers le nord-ouest de la zone d'étude. De plus, cette étude discute du rôle potentiel que peuvent jouer les structures orientées N-S de cette zone, soit de drain canalisant les écoulements de fluides du sud vers le nord, soit de barrière limitant leur migration vers l'est. Ainsi, cette étude offre de nouvelles perspectives sur les circulations des fluides du LAGF, largement influencées par la tectonique. Ces résultats peuvent guider de futures explorations de ce site remarquable et favoriser son développement géothermique.

# 1 Introduction

Africa, in particular along the Eastern African Rift System (EARS), offers significant geothermal potential, providing great opportunities for these countries to reinforce their energy mix with renewable energy (IRENA, 2020). Kenya is leading a strong regional dynamic, being the fifth country worldwide having the most installed geothermal power generation capacity in 2020 with 1.193 MWe (Huttrer, 2021). In line with this trend, several

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other countries of EARS (Tanzania, Ethiopia, Djibouti) are now engaged in the exploration and development of this renewable energy resource (ARGeo, 2022), still largely underexploited (Lund and Toth, 2021). Geothermal systems provide an opportunity for permanent and flexible power production in a large variety of environments. Considering some of the region's socio-economic challenges, with 59% of the Sub-Saharan African population living in rural areas and only 28.5% of said population having access to electricity in 2020 (World Bank, 2023a, 2023b), several projects are now aiming to develop adapted geothermal solutions. The 'Geothermal Village' (GV) project, implemented within the framework of the LEAP-RE project and financed by the Horizon 2020 program for research and innovation of the European Union, is intended for such remote communities in the Republic of Djibouti (Varet et al., 2020).

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The Republic of Djibouti, located on the northeastern end of the EARS at the junction between three rift systems in the Afar depression (Fig. 1), is a remarkable place for studying hydrothermalism (Barberi and Varet 1977; Hirn et al., 1993; McClusky et al., 2003; Wolfenden et al., 2004). Heat flows in this area are very high and are associated with significant magmatic and hydrothermal events at each tectonic step, especially during the last phase since 3.5 Ma (Gaulier and Huchon, 1991; Wright et al., 2006; Hamling et al., 2009; Biggs et al., 2011; Chandrasekharam et al., 2018). Exploration studies for geothermal resources in the Afar depression were generally conducted in the vicinity of active volcanic edifices, such as the Lake Assal field in Djibouti (Houssein and Axelsson, 2010; Dekov et al.,

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2021) or the Tendaho site associated with intrusive dyke systems in Ethiopia (Tapponnier et al., 1990; Acocella et al., 2008, Temtime et al., 2018). The Asal-Ghoubbet rift area, located in the central part of Djibouti adjacent to the Gulf of Aden, has been significantly studied in order to characterize the Lake Assal geothermal field (D'Amore et al., 1998; Houssein and Axelsson, 2010; Abdillahi et al., 2016). However, due to remote and poorly accessible sites, other potential geothermal areas remain relatively unexplored. The Lake Abhe Geothermal Field (LAGF), located on the southwestern edge of the Republic of Djibouti along the Gob Aad graben is one of these areas (Dekov et al., 2014; Awaleh et al., 2015).

The Lac Abhe site features hydrothermal manifestations in the form of giant carbonate chimneys, steam vents and hot springs (Dekov et al., 2014, Demott et al., 2021). This site has the particularity of being located relatively far from recent volcanic edifices and manifestations (the nearest volcano Dama Ale is located more than 20 km away) and in an area of moderate seismic activity (see Fig. 1 in Ebinger et al., 2008). Fluid migration paths to feed this system are still poorly understood (Awaleh et al., 2015). A better understanding of fluid circulation zones, together with the identification of key elements of the geothermal system (e.g. heat and fluid sources, reservoir and caprock units) is essential for developing an effective geothermal exploration strategy. In this context, the objective of this study is to document the structural settings of the LAGF and its associated hydrothermal surface manifestations, in order to better constrain the functioning of this remote geothermal system. To achieve this, we combine remote sensing datasets with field observations to conduct a multiscale structural survey and a lineament distribution mapping. We discuss the role of the identified structural features in controlling fluid flow within this graben system. Finally, by analyzing the hydrothermal surface
manifestations, we also provide insights into the spatial evolution of fluid outflows at the LAGF over time.

## 2 Geological setting

The general tectonic setting corresponds to the Afar depression, made of the junction of three main rift systems, the Red Sea and the Gulf of Aden oceanic rifts and the continental Main Ethiopian Rift (MER) (Fig. 1). Tectonic activity was initiated by a phase of continental break-up of the Arabian-Nubian shield between 31 and 15 Ma followed by a short period of quiescence during 4 Ma (Schilling, 1973; White and McKenzie, 1989; Furman et al., 2006). Rifting activity resumed 11 Ma ago with extension rates of about 10 cm.y⁻¹ (Lahitte, 2003). This tectonic phase led to significant lithospheric thinning (Ruegg, 1975; Van Ngoc et al., 1981; Mlynarski and Zlotnicki, 2001), significant seismic activity (Hofstetter and Beyth, 2003; Keir et al., 2006; Ruch et al., 2021) and strong surface deformation (Doubre et al., 2017). For both Red Sea and Aden oceanic rifts, the mean spreading rates range between 1.1 and 2 cm.y⁻¹ and their spreading orientation is NE-SW (Gaulier and Huchon, 1991; Jestin et al., 1994; Cattin et al., 2005), while the MER spreading orientation is WNW-ESE at a rate ranging between 2.5 and 4 mm.y⁻¹ (Gaulier and Huchon, 1991; Wolfenden et al., 2004).

The lake Abhe is sitting on the western end of the Gob Aad graben (GG), and at the junction of several grabens and fault systems that compose the structural framework of area (Fig. 1, 2). The GG is located at the western end of the Aden rift, mainly composed of ESE-WNW
striking faults. Northwest to the lake, the NNW-SSE oriented Tendaho graben is the
southeastern end of the Manda Hararo rift, which corresponds to the southern part of the Red
Sea rift. Pointing towards the lake, the NNE-SSW striking Wonji fault belt (WFB) and its
eastern branch (E-WFB) with N-S oriented faults, represent the northernmost propagation of
the MER. These fault networks are found about 30-40 km south of the lake but shorter faults
with similar orientation are also present in the GG, east of the lake Abhe (Abatte et al., 1995;
Soliva and Schultz, 2008). The Tendaho-Gob Aad Discontinuity (TGD) (Fig. 1, 2) separates
the oceanic rifting domains in the north, which is accumulating most of the strain, from the
continental rift in the south. This structure is considered as a lithospheric feature with clear
geophysical signatures that could accommodate the change of orientation between both Red
Sea and Aden oceanic rifts, NW-SE to E-W from north to south (Hammond et al., 2013;
Korostelev et al., 2015). In its south-eastern part, the TGD is interpreted as the main
detachment fault dipping toward north, which controls the extension of the Aden rift, as well
as the development of several tilted half grabens: the Gob Aad, Hanle, Gaggade and Assal
grabens from south to north (Geoffroy et al., 2014; Awaleh et al., 2020, see maps in these
papers). Therefore, the lake Abhe site is located within the southernmost block, close to the
main detachment fault.

The Afar crust is a thinned continental crust, with an estimated thickness of about 20 km
(Makris and Ginzburg, 1987; Hammond et al., 2011; Geoffroy et al., 2014). The upper part of
the crust is composed of pre-Miocene formations, including (from bottom to top) Paleozoic
metasedimentary and magmatic series, overlain by Jurassic platform carbonates and Cretaceous detrital deposits. These units are well exposed in the Danakil horst, in the northern part of the Afar depression (Gasse et al., 1987; Le Gall et al., 2015). Since the Miocene, Central Afar has been floored by volcanic and sedimentary sequences emplaced through three successive stages of volcanism (Abbate et al., 1995). The Lower extrusive complex (Miocene-early Pliocene) consists of basaltic and rhyolitic successions (Varet, 1975; Gasse et al., 1987; Beyene and Abdelsalam, 2005). It is overlain by the Intermediary extrusive complex (Plio-Pleistocene), corresponding to the Afar Stratoid Series, a thick sequence of flood basalts with silicic interlayers and volcanic centres. With a thickness of about 1.5 km, the Stratoid Series extend over nearly two-thirds of the Afar depression, hide older structures and prevent establishing any clear reconstructions of the kinematics of Afar depression before 4 Ma (Varet, 1975; Gasse et al., 1987; Deniel et al., 1994; Doubre et al., 2017; Michon et al., 2022). The Upper extrusive complex (Pleistocene to present) comprises basaltic series emplaced in the Assal rift and around several volcanic centres, including the Dama Ale volcano on the western shore of Lake Abhe, which developed along the TGD fault system at its intersection with the MER (Mohr, 1967; Tazieff et al., 1972; Beyene and Abdelsalam, 2005; Ebinger et al., 2008; Polun et al., 2018). In parallel, young basins have been filled with alluvial, lacustrine, evaporitic, and aeolian deposits since the Pleistocene.

The historic seismic activity of the GG is very low compared to the other parts of the Afar depression, except a cluster of seismicity around the Dama Ale Volcano (Hofstetter and Beyth, 2003; Ruch et al., 2021). The few seismic mechanisms described in this area show a normal activity of the ESE-WNW striking faults (Sigmundson, 1992; Keir et al., 2006; Ayele et al., 2016). The current kinematics of the blocks of the Lake Abhe area, deduced from GPS and InSar measurements, is compatible with this local setting of relative low strain (Doubre et
al., 2017; Moore et al., 2021). With this strain field in the Lake Abhe area, all the ESE-WNW striking faults acted as normal faults as the result of the Quaternary NNE-SSW extension, whereas the other faults with a different orientation could have been reactivated as strike-slip faults (Gaulier and Huchon, 1991; Abatte et al., 1995; Soliva and Schultz, 2008).

The basin of the God Aad, where the lake Abhe is located, is filled by Lower Pleistocene - Holocene lacustrine and detrital sediments, slightly deformed and post-dating the normal fault system. These sediments marked the successive periods of climate changes and their significant lake-level variations. The lake-level evolution from the past 70 ky underwent successive transgressions with high-water level fluctuating by almost 200 m, with Holocene episodes of high stand identified at about 10-8 ky and 7.5-4 ky (Gasse, 1977; Gasse and Street, 1978; Khalidi et al., 2020). The modern lake surface has an elevation of about 240 m above sea level but varies seasonally by a few meters depending on river water input. This lake is currently a closed basin, and water is highly alkaline (pH=9.86) and hyper-saline (total dissolved solids >90,000 mg/L) (Awaleh et al., 2015; 2018).

The eastern shore of Lake Abhe hosts a high concentration of hydrothermal surface features, including steam vents, hot springs, and carbonate chimney structures, distributed across an area of approximately 100 km² (Houssein et al., 2013; Dekov et al., 2014). These carbonate chimneys extend over about 5 km between the lake shoreline and the surrounding basalt hills to the east, occurring either as individual structures or in clustered formations. They emerge through unconsolidated sediments composed of mixed carbonate and siliciclastic mud, with
localized weakly-lithified diatomites and mudstones (DeMott et al., 2021). The chimneys are generally aligned in an ESE-trend. In the existing literature, the chimney field is generally subdivided into two sub-fields separated by a small wadi: the Small Hydrothermal Chimneys area (SHCa) in the north and the Great Hydrothermal Chimneys area (GHCa) in the south (Fig. 3). Chimneys in the north show more lateral continuity and are commonly a few meters high, whereas those in the south are more isolated and higher, up to a few tens of meters.

These hydrothermal chimneys have been extensively described at the macro-, meso- and micro-scale by DeMott et al. (2021) in terms of morphology, texture and fabric. They display a wide range of large-scale morphologies (from meters to decameters), classified as massive, pinnacle, bulbous, barrel or frondose. While morphological and spatial variations exist between the two sub-fields, all chimneys are interpreted to have formed over the past several tens of thousands of years. Their development is mainly attributed to physico-chemical interactions and abiogenic mixing between hydrothermal fluids from sublacustrine springs and lake waters during lake highstands. Geochemical analyses, particularly Ca isotopes from a chimney in the southern part of the LAGF, indicate that internal layers formed by non-equilibrium calcite precipitation with a predominantly hydrothermal Ca source, whereas the external layer incorporated Ca mainly from lake water (Dekov et al., 2014). A minor input from stromatolitic processes during lowstand phases has also been identified. (DeMott et al., 2021). Some of these chimneys, now exposed thanks to the current relative low stand lake level, remain active with hot steam vents observed at their apex. Many hot springs are also
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described across the area, often found at the base of the active chimneys, revealing the current hydrothermalism of this area (Awaleh et al., 2015). The near-surface plumbing system of one of these hydrothermal structures was imaged by geophysics (Piolat et al., 2023). The temperatures of the geothermal water samples measured by these authors from 16 hot springs scattered across the LAGF ranged from 71 to 99.7°C (Awaleh et al., 2015). Hot springs generally form small ponds, partly irrigated, that are surrounded by vegetation that benefits the local fauna and pastoral communities. Geothermal waters from these springs are moderately alkaline (pH=7.61–8.80) (Awaleh et al., 2015). The continuous flow of these waters in this rather dry area allows the development of vegetation around these sources. This vegetation facilitates identification of the hot springs in the LAGF landscape as well as on the satellite images.

The LAGF benefited from surface exploration surveys (Demange et al., 1971; Moussa and Souleiman, 2015; Abdillahi et al., 2016). As part of geothermal energy development programs, Djiboutian national institutions (ODDEG, CERD) initiated geological, geochemical and geophysical surveys that led to the definition of a reservoir model of the LAGF with the support of international organizations (Fahman et al., 2018; Samod et al., 2018). Exploration works of the LAGF concluded to a medium-enthalpy geothermal system that could benefit from small-scale stand-alone electric production systems and/or in cascade thermal direct-use (Moussa and Souleiman, 2015; Varet et al., 2020).
## 3 Methods

The structural analysis on the field was conducted in November 2021 and it was focused on the LAGF hydrothermal chimney structures and the volcanic rocks of the Stratoid Series outcropping east to these chimneys. Alongside field observations of the different hydrothermal manifestations (chimneys, hot springs, steam vents), a structural survey of plurimetric fractures affecting the hydrothermal chimneys of the SHCa was carried out. The structural study of the Stratoid Series surrounding the LAGF consisted, in the field, in the identification and mapping of the major fault structures, based on their appearance in the landscape geomorphology and their kinematics (from microtectonic analysis). To complete the structural survey of the studied area, structural lineaments were manually mapped using high-resolution remote sensing data. Field data and remote sensing data were combined to produce a structural map of these volcanic units. Structural lineaments are defined as mappable rectilinear or slightly curvilinear coherent structures, characterized by distinct patterns from adjacent features and caused by tectonic activities (O'Leary et al., 1976; Ahmadi and Pekkan, 2021).

The structural lineament interpretation in this area was completed by a geomorphological analysis which consisted of tracing lineaments based on the observation of linear or slightly curvilinear shapes, elevation offsets highlighted by color contrasts or streams / wadis radical course changes. In line with field observations, all selected lineaments are considered as faults
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that shape the morphology of the area. Detailed lineament mapping was carried out on two
digital elevation models (DEM) displayed on QGIS Version 3.22.6, at two different scales.

The NASA Shuttle Radar Topography Mission (SRTM) Global 1 arc second DEM (~30 m resolution) was used to map lineaments at the displaying scale of 1:60,000 over a large area (NASA JPL, 2013). Focused on the main hydrothermal active area, a higher resolution DEM (0.5 m resolution) was generated from PLEIADES tri-stereo multispectral (MS) satellite imagery.

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This DEM was used to map structural lineaments of the volcanic series and hydrothermal
chimneys alignments separately at the displaying scale of 1:10,000. The chimney alignments
are clearly recognizable as they represent the only relief above the sedimentary flats in this
area. Because fault trace directions can vary and be curvilinear, each continuous lineament is
composed of several segments in order to capture the precise shape of each structure. Rose
diagrams representing lineament mapping results were generated with the Line Direction
Histogram QGIS plugin (Tveite, 2015). This tool represents the distribution of the directions
of line segments for a lineament dataset. The accumulated length of the line segments for each
direction bin (intervals of 10°) determines the shape of the Rose diagrams. Segments of faults
cut by the map borders are included in the rose diagrams without distinction. Hot springs
outflowing over the sedimentary flats of the LAGF area were mapped using color-saturated
GoogleEarth images (©2022 Google Earth). Hot springs were recognized thanks to the green
vegetation surrounding small ponds, observed in the field and clearly visible on these images.

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# 4 Results

## 4.1 Multi-scale lineament mapping

### 4.1.1 1:60,000 scale mapping of structural lineaments

Ninety structural lineaments were mapped using a 1:60,000 viewing scale, over an area of about 361 km² centered on the LAGF (Fig. 3a). The average length of the lineaments is 3074 m, with a maximum length of 17.1 km, although fourteen of these lineaments are cut by the map borders. The most important lineament direction is ESE-WNW (N100-N120°; Fig. 3b) and corresponds to the regional trend, with secondary sets of E-W (N080-N100°) and SE-NW (N120-N140°). These sets represent the curvilinear nature of the segments that compose such major structures. Minor sets of ENE-WSW (N050-N070°) and N-S (N160-N180°) oriented lineaments are also observed. N-S lineaments are observed through the Stratoid Series outcropping east of the LAGF.

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### 4.1.2 1:10,000 scale mapping of structural lineaments and hydrothermal chimneys alignment

A total of 255 structural lineaments was mapped at a 1:10,000 viewing scale within the Stratoid Series hills east to the LAGF (Fig. 4a). The average length of the lineaments is 541 m, with a maximum length of 4.6 km, although seventeen of these lineaments are cut by the

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mapping frame. The most important lineament direction is ESE-WNW (N090-N120°) (Fig. 4b). Two secondary directions of ENE-WSW (N070-N090°) and SE-NW (N120-N140°) oriented lineaments are observed and form a dihedral pattern well visible west near the contact with the sedimentary flats. Minor sets of oblique N-S (N160-N180°) and NE-SW (N040-N060°) striking structures are also identified.

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Seventy-four hydrothermal chimney alignments were picked based on the topography they constitute over the sedimentary flats (Fig. 4a). A main alignment direction is ESE-WNW (N110-N120°; Fig. 4c) and two secondary orientations are E-W (N090-N110°) and ENE-WSW (N060-N080°E). Massive chimney structures of the GHCa south of the mapping area tend to be located at the intersection of these two alignment directions. N-striking alignments are also observed in the densest part in terms of chimneys (i.e., SHCa), but not in the southern part of the mapping area.

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## 4.2 Structural geometry of the LAGF surrounding hills

Based on field observations and fault scarp morphology (i.e., elevation offsets) analysis on the DEM, we have mapped the normal fault network recognized in a 40 km² area east to the LAGF. This structural mapping is based on the 1:60,000 scale lineament mapping results, from which some of the major lineaments could be interpreted as normal faults. This densely faulted area displays variable topography that ranges from about 550 m (above sea level) in the east of the map to 240 m in the floor of the graben nearby the lake shore, with a general

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westward plunge of the Stratoid Series towards the basin (Fig. 5a). A significant number of ESE-striking lineaments produces scarps and topographic variations that can be attributed to normal faults. These normal faults can be further divided into two subsets along a north-south axis covering the area, as exhibited on topographic profiles (Fig. 5b). In the northern part of the area, both north- and south-dipping normal faults are observed, forming horst and graben structures with apparent fault throw at the surface up to a hundred meters and fault spacing of about 1 km (Fig. 6a). The southern part of the area exhibits mostly synthetic south-dipping normal faults, with a slightly denser fault spacing (less than a kilometer), that forms a tilted block geometry (Fig. 6b). Across this series of narrow horsts and grabens, overlapping synthetic normal faults interact and produce localized zones dipping steeper to the west, interpreted as relay ramps. The most prominent ramp is being identified in a graben of the northern part of the area (Fig. 6c). The dominant ESE-striking fault set is affected by a few N-striking structures, with no clear dip direction. These structures do not seem to crosscut faults, although some of these tend to be aligned, and rather seem to link two adjacent normal faults.

## 4.3 Hydrothermal chimney fractures field analysis

Field observations of more than 100 hydrothermal chimneys located in the SHCa were carried out in order to support remote sensing data and images interpretation and investigate structural features within this chimney field. On the field, chimney alignments described above (Fig. 4) consist in the alignment of several individual and/or clustered structures. Such
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aligned structures can exhibit lateral continuity with the base of chimneys touching each other or can be separated (meter to decameter) with unconsolidated sediments (Fig. 7a, b).

Numerous separated chimneys belonging to the same axis appear to be connected with inter-chimney materials consisting of localized spots of weakly-lithified carbonate and siliciclastic mixed materials outcropping on the surface according to the axis direction (Fig. 7c). Chimney clusters are generally elongated following the direction of the alignment they are connected to, with chimney summits lined up to the same direction (Fig. 7a). No relationship between chimney morphology type and alignments was identified as different types were observed along the same alignment. No particular change in chimney size or morphology was recognized at the intersection of alignment axes.

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Because of the high porosity and friability of chimney material and of their complex morphology, planar structures as fractures are difficult to be distinguished at the meso- and micro-scale within these features. In the field, macro-scale observations recognized two types of plurimetric long planar structures. The first type is composed of irregular cracks, generally sub-vertical with an aperture up to several centimeters. They were identified on many chimneys with no consistent azimuth direction between them (Fig. 7d). Such cracks, commonly propagating from the top to the base of the chimneys are considered as collapsing cracks with no tectonic origin. The second type are moderately to highly dipping (between 30 to 90°) pluri-metric long planes that cut through the chimneys. A set of parallel planes or antithetic planes sharing the same azimuth is often found through the same chimney (Fig. 7b,

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e, f). Ninety-one planes, identified through several tens of different chimney structures located in the SHCa, were measured in the field in order to analyze their orientation distribution (Fig. 8). These planes were either parallel or oblique to the alignment along which they were measured. The overall orientation distribution shows three dominant sets N- (N350-N020°), ENE- (N040-N080°) and ESE- (N090-N120°) striking planes. The ESE-striking set is consistent with the ESE dominant structural direction observed in the LAGF area through structural lineaments and chimney alignments. The two other N- and ENE-striking sets of planes also show consistent directions with the chimney alignments, especially in this specific part of the SHCa that is the only one where N-S alignment was recognized. A number of chimneys shaped by these fractures in their lower part exhibit a preserved bulbous form at their top (Fig. 7e, f). This morphology suggests these bulbous upper growths were formed after fracturing of the lower existing parts of the chimneys, the hydrothermal activity being maintained beyond. The formation of these chimneys is therefore considered as syn-tectonic.

## 4.4 Hot springs mapping

Hot springs represent another important hydrothermal feature of the LAGF. They generally form decimeter to meter-wide small ponds and flow towards the lake along small incisions through the sedimentary flats (Fig. 9a, b). Based on Awaleh et al. (2015) water sample measurements and our field observations across the LAGF, all the springs correspond to geothermal hot springs (i.e., water T > 45°C). No warm or cold springs (i.e., water T < 45°C)
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have been observed in the SHCa and GHCa. These hot springs are commonly associated with active hydrothermal chimneys. They are found at their base and surround some of these chimneys, especially in the GHCa. However, many hot springs are also found in the LAGF area with no chimney in close proximity.

With the knowledge from field observations of several hot springs located in the SHCa and the GHCa, hot springs of the whole LAGF area were mapped using satellite images (Fig. 10).

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Most of them are located in the northern part of the LAGF, with a distribution following the overall ESE-WNW structural trend. In this part of the LAGF, a few hot springs are dissociated from any hydrothermal chimneys, especially on the northern and western ends of the area where no such structures are observed near the sources. On the other hand, no hot springs were observed in the field or in the satellite images in the eastern part of the SHCa, which corresponds to the area with the highest relative density of chimneys of the LAGF. No steam vents were also observed through the chimneys of this part of the SHCa during field work. This absence of hydrothermal manifestations (i.e. springs or steam) suggests these hydrothermal chimneys to be currently inactive. In the GHCa in the south, hot springs are associated with great chimneys of several tens of meters high. However, smaller sized chimneys in this area show no signs of geothermal water flow. A few springs were also identified at the contact of the Stratoid Series hills with the basin sediments where structural lineaments were mapped.

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# 5 Discussion

## 5.1 Structural features of the LAGF

Global assessments of geothermal systems have highlighted the critical role of fault and fracture networks in controlling the near-surface permeability (Jolie et al., 2021). In rift settings, fault and fracture networks formed or reactivated by crustal strain play a key role in fluid circulation, acting as conduits or barriers depending on their orientation to the stress field (Bense et al., 2013). Fluid migration over horizontal distances of several tens of kilometers can occur through structural features, such as fault damage zones, fault relay or intersection zones, which together form the main pathways of hydrothermal percolation networks (Fossen and Rotevatn, 2016; Walter et al., 2019).

The structural features of the LAGF area are dominated by ESE-striking extensional faults. Field measurements by Gaulier and Huchon (1991) identified these faults as normal, with a principal extension direction oriented N20. This extensional regime remains active today, as evidenced by GPS and InSAR analyses (Doubre et al., 2017; Moore et al., 2021). These faults form a series of narrow elongated horst, graben, and half-graben structures visible east of the LAGF in the Stratoid Series. This structural pattern is recognizable over a wide area, tens of kilometers north through the northern GG shoulder, up to the southwest boundary of the adjacent Hanle graben (Fig.1, 2) (Tesfaye et al., 2008). Considering the general length (several to tens of kilometers) of these structures, the faults observed east of the LAGF likely
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extend westward within the volcanic series buried beneath the sedimentary basin and the active geothermal area. Satellite images and field investigations indicate an evolution in the dip of these ESE-striking faults between those observed in the northern and the southern parts through the Stratoid Series east to the LAGF. A set of synthetic and antithetic normal faults, forming horst and graben structures, is observed in the northern part while a denser network of south-dipping synthetic faults associated with tilted blocks is observed in the southern part. This spatial variation in fault geometry may reflect a higher amount of extension in the north, closer to the GG north boundary faults.

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Accommodation zones between adjacent normal faults with complex arrays of smaller-scale structures and with at least one significant relay ramp are recognized nearby the LAGF. The latter structure type is marked in the area by significant topographic variations (Fig. 6c).

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Relay zones were extensively documented in continental rift systems to result in the interaction between adjacent and/or overlapping fault segments, by accommodating displacement transfer through complex deformation structures (e.g. Trudgill and Cartwright, 1994; Peacock, 2002; Hemelsdaël and Ford, 2016). Variety of typical accommodation zone structures were described north to the LAGF and the GG, in the Dobe and Hanle grabens, including ramp development, normal faulting of the ramp, complex block rotation and development of breaching faults that link the interacting graben-bounding faults (Tesfaye et al., 2008). As displacement increases, ramps steepen and become broken up by cross cutting faults developed near the hinge zones and form elongate fault blocks that can rotate along

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antithetic faults. These breaching faults are considered in part as open (mode 1) fractures.

With relay ramps identified in the LAGF surrounding hills, the minor sets of N- and NNW-
striking structural lineaments are interpreted, at least in some cases, as breaching faults. These
structures indeed exhibit similar shape and characteristics to the breaching faults described in
the Dole and Hanle grabens located about 50/60 km north, oblique to the dominant ESE-
striking extensional faults. No block rotation is however recognized in the LAGF area. This
can be because these breaching faults that precede rotation are not sufficiently developed to
separate in depth fault blocks from the ramp (i.e. the hanging wall).

A significant number of these N-striking breaching faults tend to be aligned (Fig. 4). When extension and displacement become sufficiently large, crosscutting breaching faults are described to link together, forming a continuous fault connecting originally separated fault segments (Tesfaye et al., 2008; Fossen and Rotevatn, 2016). With such an observed alignment instead of a commonly described en-echelon distribution, the development of these breaching faults may be influenced by preexisting structures underlying the Stratoid Series that control in part the segmentation of this extensional fault system (e.g. Morley et al., 2004; Tong et al., 2014; Whipp et al., 2014). These preexisting structures could correspond to the northern extension of the eastern branch of the WFB (E-WFB), observed about 40 km south to the study area (Fig. 2). The WFB structures are considered to have acted as normal faults prior to the GG formation and to have been subsequently reactivated as shear structures following the change of the stress orientation alongside with the GG formation (Soliva and Schultz, 2008).
Such breaching fault linkage influenced by such preexisting structures could contribute to explain the overall N-striking boundary between the Stratoid Series hills and the basin sediments of the LAGF area, as well as the fault-controlled depression filled with sediments that is observed about 10 km east from the LAGF within the volcanic series (Fig. 2). This assumption of a significant influence of N-striking structures on this area's morphology is supported by multi-methods geophysics surveys carried out in the LAGF. The resistivity model presented in Fahman et al. (2018) indicates a low resistivity layer, approximately 300-400 m thick in the SHCa and GHCa vicinity, interpreted as conductive sediments. In comparison to the slight dip of the Stratoid Series hills plunging westward within the basin, the thickness of these sediments may appear relatively important. This may therefore suggest the existence of such buried N-striking structures in the area, reactivated as shear structures with an extensional component that favor the Stratoid Series burial.

## 5.2 Hydrothermal surface manifestations distribution and evolution

In geothermal areas, hydrothermal manifestations such as thermal springs and hydrothermal carbonate depositions have been often associated with active faulting and tectonics (Bense et al., 2013; Frery et al., 2015; Brogi et al., 2021). The concept of “travitonics”, introduced by Hancock et al. (1999) describes the inseparable processes between brittle deformation at shallow crustal levels and travertine deposition. Among the different travertine deposit types (i.e., fissure ridge- vs. chimney-type deposits) produced from hydrothermal fluids discharged
from thermal springs, many studies have reinforced the evidence of travertine deposition along the traces of brittle structures (De Filippis et al., 2013; Brogi et al., 2021). The distribution of the LAGF hydrothermal manifestations following these structural orientations described in the area supports the idea of tectonically driven morpho-tectonic features.

LAGF hydrothermal chimneys correspond to mound-type deposits, which are end-members in comparison to fissure-ridge deposits (i.e. continuous massive travertine over fault trace).

Fissure-ridge development is inhibited by faults affecting unconsolidated sediments. In such cases, fault-related permeability is described as strongly compartmentalized and fluids emerge only in isolated spots along the fault trace (Brogi et al., 2021; Piolat et al., 2023). Among several parameters (source of Cations, CO₂ fluxes, hydrothermal fluid composition, lake water level, etc.), fault and fracture-induced permeability controls chimneys localization and development. This permeability is enhanced at structural intersections, marked by hints of enhanced hydrothermal activity (e.g. chimney size, density). Structural intersections of fault and fracture networks can indeed act as dilatational zones and fluid preferential pathways under various tectonic stress regimes. This produces significant relative vertical fracture permeability and localizes fluid flow (e.g., Sibson, 1996, 2000; Person et al., 2012). In the GHCa, the massive and currently active chimney structures (i.e. associated with hot springs and steam vents) are distributed at the intersection between ESE- and ENE-striking hydrothermal traces (Fig. 4, 10). In the SHCa, intersecting patterns localizing higher hydrothermal activity are more complex to observe as this area has an overall relative higher
chimney density than the GHCa. This density may reflect a more diffused hydrothermal fluid flow thanks to the numerous intersecting structural traces (ESE-, ENE-, NNE-striking), underlined by all the chimney alignments. These traces may correspond to conjugated structures formed in the sediment by activation of the faults in underlying basalts. Therefore, the chimney distribution of these two areas validates that fault and fracture-induced permeability in the LAGF is favored locally by structural intersections.

Long-lived hydrothermal flow and the renewal of fault- and fracture-induced permeability are sustained by tectonic activity (Curewitz and Karson, 1997). According to the synthesis proposed by Brogi et al. (2021), based on dozens of case studies, the development of mound-type hydrothermal features is driven by discontinuous fluid circulation through fault and fracture networks, modulated by seismic cycles. Active faulting and fracturing enable regular re-opening of fluid flow conduits, alternating with temporary closure due to carbonate precipitation. All the LAGF hydrothermal chimneys are interpreted to have formed within the past few tens of thousands of years at most (DeMott et al., 2021). U–Th dating performed on a single chimney located in the SHCa indicates that these structures formed no earlier than 0.82 kyr BP (0.28 ± 0.54), supporting a recent formation of the carbonate chimneys (Dekov et al., 2014). From the observations of this study, the SHCa exhibits in its eastern part a significant number of inactive hydrothermal chimneys (i.e., no steam vent or springs), whereas in the northern and western parts, many hot springs occur without associated chimneys. Since the chimneys originally developed underwater (DeMott et al., 2021), the current low lake level
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may explain the absence of recently formed chimneys near active springs. Taken together, the extinction of chimneys in the east and the occurrence of recent springs in the north and west suggest a westward lateral migration of hydrothermal outflows over a short period of time. Chimney formation, the macro-scale syn-tectonic fractures that shaped the morphology of some of them during their development (Fig. 7e, f), and the present hot springs all indicate that the LAGF has experienced repeated tectonic pulses over the past several thousands to tens of thousands of years. This tectonic activity has maintained permeability levels sufficient to supply the remarkable hydrothermal surface manifestations of the LAGF. However, the observed lateral migration of hydrothermal outflows likely reflects localized fracturing over time.

**5.3 Implications for the LAGF development**

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The structural analysis of the LAGF surrounding hills and the distribution of hydrothermal manifestations indicate the presence in the area of fault interaction and accommodation zones, as relay ramps and fault intersections, generally described as favorable pathways for fluid flow. Relay ramps, breached or not, typically exhibit increased structural complexity compared to a single fault zone, with enhanced density and connectivity of faults and fractures and a wider range of orientations (e.g. Peacock and Sanderson, 1994; Peacock, 2002; Conneally et al., 2014). These characteristics make relay ramps a prime pathway for vertical fluid flow in the crust, affecting all kinds of fluids (e.g., hydrothermal, CO2, hydrocarbons,
etc.) (Fossen and Rotevatn, 2016). Similarly, intersection of multiple faults represents a high structural complexity zone that can act as a fluid preferential pathway with significant vertical fracture permeability, especially in low-porosity rocks (Curewitz and Karson, 1997; Sibson, 2000). Structural intersections are described as well as key features for outflow and exploitation of different kinds of fluids (e.g. Gartrell et al., 2004; Walter et al., 2019). More specifically regarding geothermal systems, outflows occurring predominantly at fault intersections have been extensively described for fault-controlled plays (e.g., Craw, 2000; Rowland and Sibson, 2004; Taillefer et al., 2017). An inventory of more than 400 geothermal active sites in the Great Basin region (USA) highlighted that more than half of these sites are hosted by fault interaction zones, as fault intersections and relay ramps (Faulds et al., 2011).

These latter structural features therefore appear as reliable prime targets for the LAGF development.

This development also requires the identification of several key elements of the geothermal system: the sources of fluids and heat, the pathways they follow, and the reservoir and its caprock. According to Awaleh et al. (2015), the LAGF reservoir is mainly fed by meteoric water that penetrates downwards through the ESE-striking fault and fracture network in the Stratoid Series and combines with deep regional groundwater in the thermal aquifer to a maximum depth of about 1 km. The clayey sediments forming the GG infill could act as a trap for the geothermal fluids. However, this low-permeability cover is locally disrupted by fractures, enabling fluids to rise to the surface, interact with lake water, and precipitate calcite
that forms the chimneys. In addition to the dominant ESE-striking faults, the N- and NNW-
striking structures observed east of the lake could also play a crucial role at depth. They can
be considered as continuations of the N-striking E-WFB (Fig. 2). Like the E-WFB, these
structures may act as shear zones with respect to the extensional axis of the deformation
tensor, with a NNE-SSW to NE-SW extension direction observed in the central Afar area
(Abbate et al., 1991, Gaulier and Huchon, 1991, Soliva and Schultz, 2008; Doubre et al.,
2017). In terms of hydraulic functioning, shear zones are often described as having a fault
core that acts as a barrier and damaged zones that act as drains (Bense et al., 2013; Walter et
al., 2019).

Considering all these features together, different scenarios may explain the present-day
hydrothermal activity of the LAGF (Fig. 11). These scenarios should not be seen as mutually
exclusive, and intermediate configurations are also possible. In one case, the main heat source
could be the magma chamber of Dama Ale volcano, west of Lake Abhe (Fig. 2, 11). Meteoric
water infiltrates through the fault network of the near-volcano environment and mixes with
deeper fluids located in aquifers in the Stratoid Series volcanic bedrock. Heat is transported
eastward convectively along the ESE-striking fault corridors by the flowing fluids. However,
the N- and NNW-striking faults in the LAGF area may locally limit this migration. Shearing
motion along these structures, consistent with the regional deformation tensor, reduces their
transverse permeability. Under such conditions, fluids preferentially rise to the surface along
structural features with relatively high vertical fracture permeability, such as fault
565 intersections and relay zones. These N- and NNW-striking structures would thus tend to limit lateral fluid migration along the ESE-striking fault corridors at depth, while facilitating vertical upflow. In an alternative scenario, meteoric water infiltrates south of the GG, through the TGD detachment fault zone, the associated ESE-striking faults, as well as the E-WFB structures (Fig. 2, 11). These latter faults extending north in the GG below the sedimentary cover allow fluids to migrate northwards at depth. In this case, the heat source would mainly correspond to the conductive heat flow induced by the crustal thinning in the Afar depression.

Fluids would then rise locally in the LAGF area through features of high vertical permeability, such as intersections and relay ramps, in a way similar to the first case.

Numerous unresolved questions remain about the geothermal reservoir parameters (depth, extent, etc.), the transition and localized flow processes from the basalt series to the poorly consolidated sediments, or the specific location of the LAGF in the Gob Aad basin. The LAGF appears indeed as the only significant geothermal occurrence for exploitation in the entire basin, which makes these unresolved aspects critical for initiating geothermal development. Nevertheless, the hydrothermal chimneys described in this study provide valuable surface proxies to decipher key structural features distribution in depth and help identify the current most active upflow zones.
**6 Conclusion**

The multi-scale lineament survey, supported by field observations, reveals a strong structural control on the distribution of key surface hydrothermal features at the LAGF, including high-temperature hot springs (up to 100°C), steam vents and carbonate chimney structures. To better understand this relationship, a detailed structural and lineament analysis was conducted across the LAGF and its basaltic substratum exposed east of the active hydrothermal zone. Structural features of the LAGF area are dominated by ESE-striking extensional faults. Their overall geometry evolves between the north and the south of the studied area. Synthetic and antithetic faults form a series of narrow elongated horsts and grabens in the northern part whereas a denser network of south-dipping synthetic faults form half-grabens associated with tilted blocks in the southern part. Accommodation zones between adjacent normal faults, with at least one significant relay ramp were also identified in the area. Some of the structural lineaments identified with a N- and NNW-strike may represent possible breaching faults of such accommodation features, developed under the influence of underlying N-striking structures observed about 40 km south of the study area <u>and affecting the Stratoid Series bedrock.</u>

The hydrothermal surface manifestations mapping reveals the control of structural directions on chimneys and hot springs distribution, following the main lineament trends. Hints of
higher hydrothermal activity localized at intersecting structural traces are also observed.

Hydrothermal chimneys represent proxies of the LAGF structural features distribution at depth and therefore suggest that structural intersections can locally enhance fault and fracture-induced permeability. Field observations, combined with satellite image analysis, suggest a lateral migration of the LAGF hydrothermal outflows over a short period of time (during the past several thousands to tens of thousands of years). This migration is likely driven by episodic tectonic pulses renewing fracture permeability through chimneys and triggering (re-)activation of thermal springs.

This study finally demonstrates the strong correlation between the structural framework of the LAGF and the spatial distribution of hydrothermal phenomena. While surface N-striking faults are scarce, they may play a significant role at depth. Two potential roles are proposed for these structures: either acting as vertical drains channeling fluids from south to north, or as barriers preventing eastward fluid migration. Between these two assumptions, a range of intermediate structural scenarios may exist. To refine the understanding of the LAGF subsurface fluid pathways, new geophysical surveys, particularly deep electrical resistivity tomography, would be valuable. Such data could support the development of autonomous, small-scale geothermal systems designed for “off-grid” communities (i.e., not connected to a centralized power grid). These systems, tailored to local energy needs, may include small-scale Organic Rankine Cycle (ORC) technology for electricity production, as well as direct-use heating.
**7 Data availability**

All raw data can be provided by the corresponding authors upon request.

**8 Author contributions**

YG acquired the funding; all authors contributed to collect data and conceptualize the study;

BW wrote the manuscript and produced the figures; YG, AF and MD reviewed and edited the manuscript and the figures; HMM provided support for the field work.

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**9 Competing interests**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**10 Acknowledgments**

This study was carried out within the framework of the "Geothermal Village" program under the LEAP-RE partnership, all the partners are gratefully acknowledged. We are greatly thankful to Jacques Varet and ODDEG partners for field insights and logistical support during fieldwork. We are grateful to Jérôme Ammann for providing drone photographs, and Carolina Dantas Cardoso for providing hot spring photographs used in this paper. We thank the

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reviewers and the journal editor for their thoughtful and thorough review and editorial
suggestions.

**11 Financial support**

This work benefited from the financial support from the European Commission Horizon 2020 funded project LEAP-RE (Long-Term Joint EU-AU Research and Innovation Partnership on Renewable Energy), registered under the grant agreement ID 963530 (DOI: 10.3030/963530).

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13 Figure captions

Figure 1: Overview of the Afar depression and its main magmatic segments, modified after Varet (1975) and Wolfenden et al. (2004). The white dashed line represents the Tendaho–Gob Aad discontinuity (TGD), which marks the transition from Red Sea–oriented to Main Ethiopian Rift (MER)–oriented structures. Major grabens and fault belts in the vicinity of the study area (white rectangle) are indicated: TG – Tendaho Graben; AR – Asal Rift; HG – Hanle Graben; GG – Gob Aad Graben; WFB – Wonji Fault Belt; E-WFB – East Wonji Fault Belt.

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Figure 2: Satellite image of the Lake Abhe area with nearby grabens (©2023 Google, TerraMetrics). The purple area locates the recent flows from the Dama Ale Volcano. The black lines correspond to the regional structural pattern (modified after Barberi and Varet, 1977 and Polun et al., 2018). WFB: Wonji Fault Belt, E-WFB: East Wonji Fault Belt, TGD: Tendaho-Gob Aad Discontinuity.

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Figure 3: a) Location map of the LAGF and the two Small Hydrothermal Chimneys area (SHCa) and Great Hydrothermal Chimneys area (GHCa). 1/60,000 scale structural lineament mapping results are represented as red lines. b) Rose diagrams representing the 1/60,000 scale structural lineament mapping (the whole dataset is presented on the half rose diagram on the right, and a zoom of its central part (orange rectangle) is shown on the left to highlight minor sets).

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Figure 4: a) Lineament mapping of the LAGF area on a Pléiades dataset-based DEM hillshade image. Structural lineaments within the Stratoid Series hills are shown as blue lines.

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Hydrothermal chimney alignments observed over the basin sedimentary flats are shown as green lines. b) 1/10,000 scale structural lineament mapping and c) 1/10,000 scale hydrothermal chimney alignment mapping. For each sub-figure, the whole dataset is presented on the half rose diagram on the right, and a zoom of its central part (orange rectangle) is shown on the left to highlight minor sets.

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Figure 5: a) Topographic map of the LAGF area generated from SRTM Global DEM. White dashed lines are locations of topographic profiles represented in Figure 5b. Normal faults, interpreted from field and topography observations, and unresolved lineaments from 1/60,000 scale structural lineament mapping are shown as black lines. Viewpoint location of Figure 6 is shown on the map. b) Topographic profiles extracted SRTM Global DEM. Normal fault location is pointed out on the profiles.

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Figure 6: a) Graben geometry in the northern part of the LAGF area. b) Tilted blocks geometry in the southern part of the LAGF area. c) Perspective view of a relay ramp, generated from Google Earth (©2022 Google Earth). Location of these three viewpoints is represented on Figure 5.

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Figure 7: Large-scale hydrothermal chimney distribution and characteristics. a) Drone aerial photograph of a part of the SHCa. b) Adjacent massive chimneys, exhibiting crosscutting subparallel planar structures with orientation. c) Sublinear weakly-lithified carbonate and siliciclastic mixed materials outcropping on the surface between hydrothermal chimneys. d) Irregular-shaped collapsing crack observed across a hydrothermal chimney. e) & f) Hydrothermal chimneys exhibiting bulbous-shaped tops bounded by crosscutting subparallel planar structures, with orientation.

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Figure 8: a) Location map of hydrothermal chimney crosscutting planar structures measured in the SHCa, represented as oriented orange symbols (satellite image from ©2022 Google Earth). Green lines correspond to 1/10,000 scale hydrothermal chimney alignment mapped

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over the LAGF area. Map location is shown in Figure 4. b) Rose diagram and stereographic projection represent overall measurements of the hydrothermal chimney crosscutting planar structures. The different sized green arrows indicate the main directions of chimney alignments by relative importance (cf. Figure 4c).

Figure 9: a) Drone aerial photograph showing aligned hot springs observed in the SHCa, flowing northward with associated vegetation. b) Example of a hot spring found in the sedimentary flats of the LAGF area (geochemistry sampling device for scale).

Figure 10: Location map of hot springs in the LAGF area, with structural lineament mapping results (satellite image from ©2022 Google Earth).

Figure 11: Schematic representation of two possible configurations for hydrothermal activity within the LAGF: 1) Meteoric water infiltrates and is heated in the vicinity of the Dama Ale volcano. Fluids migrate eastward along ESE-striking fault corridors until they intersect N-striking faults acting as barriers that prevent further eastward migration. Upflow to the surface occurs along structurally favorable zones with enhanced vertical permeability, such as fault intersections and relay zones. 2) Meteoric water infiltrates south of the GG through major structural features. It is heated by conductive heat flow related to crustal thinning, and then channeled northwards along the continuation of the E-WFB, acting as vertical drains. Fluids rise to the surface where these structures intersect the ESE-striking fault corridors in the LAGF area.
14 Figures

<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>1050 Figure 7</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>1055 Figure 9</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 10</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 11</figcaption></figure>