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

1 Hydrothermal activity of the Lake Abhe geothermal field (Djibouti): Structural controls and paths for further exploration Bastien Walter1, Yves Géraud1, Alexiane Favier1, Nadjib Chibati1, Marc Diraison1 1 Université de Lorraine, UMR 7359 GeoRessources, École Nationale Supérieure de Géologie, 2 Rue du Doyen Marcel Roubault, Vandoeuvre-lès-Nancy 54518, France 5 Correspondence to: Bastien Walter ([email protected]); Yves Géraud ([email protected]) Abstract. Lake Abhe is sitting on the Gob Aad graben within the tectonic Afar triangle in the Republic of Djibouti. It is known for its exposures of massive hydrothermal chimneys on the lake’s eastern shore. The many hydrothermal surface manifestations on this side of the lake, including steam vents, hot 10 springs and carbonate chimney structures, reflect the geothermal field of this area. This study describes the structural settings of the Lake Abhe Geothermal Field (LAGF), using multiscale structural lineament distribution mapping. It also investigates the hydrothermal surface manifestations distribution in order to specify structural controls on local fluid flow and discuss its evolution. Structural features of the LAGF area are dominated by ESE-trending extensional faults that form a series of narrow elongated 15 horst, graben and half-graben structures. Fault interaction and accommodation zones, as well as fault intersections, relay ramps and possible breaching faults are also recognized and may represent interesting structural features in terms of fluid flow pathways. Hydrothermal chimneys and hot springs distribution over the LAGF area is controlled by the main structural trends, and show signs of higher hydrothermal activity located at intersecting structural traces. Field observations, in conjunction with 20 satellite images analysis, suggest a progressive lateral evolution of the LAGF hydrothermal fluid outflows over time. Therefore, this study provides new insights on the local tectonically driven fluid flow of the LAGF, that may support further exploration of this remarkable site and may promote its geothermal development. 1 Introduction 25 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). With Kenya leading a strong regional dynamic, being the fifth country worldwide having the most installed geothermal power generation capacity in 2020 (1.193 MWe; (Huttrer, 2021)), many countries of eastern Africa are now engaged in the exploration and development 30 of this renewable energy resource (ARGeo, 2022). Geothermal systems provide an opportunity for permanent and flexible power production in a large variety of environments. These systems can be distinguished based on their temperature, between high (>150°C), medium (between 150 and 100°C) and low (<100°C) enthalpy resources (Moeck, 2014; https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 2 Stober and Bucher, 2021). While the development of geothermal energy has long been focused on high 35 enthalpy systems producing a high energy yield, technological advances allowed a more efficient exploitation of the entire range of geothermal resources for electricity production as well as heat directuse (Rubio-Maya et al., 2015). 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 40 (World Bank, 2023a, 2023b), several projects are now aiming to develop adapted geothermal solutions. They correspond to autonomous geothermal small-scale systems designed for African communities considered as “off-grid” (i.e. not connected to a power grid). These solutions are based on the analysis of the needs of these local populations in terms of geothermal direct-use, and can include small-scale Organic Rankine Cycle technology for local electricity production. The ‘Geothermal Village’ (GV) 45 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). The Republic of Djibouti, located on the north-eastern end of the EARS, represents an interesting area for studying hydrothermalism in the context of intracontinental extension. Assessments of geothermal 50 systems worldwide have highlighted the critical role of fault and fracture systems on the near surface permeability (Jolie et al., 2021). The country is sitting on the tectonic Afar triangle and exhibits a complex fragmented relief, composed of high blocks and grabens, and a widespread geothermal activity (e.g. hot springs, fumaroles, hydrothermal alteration). These structures are mainly located in the western part of the country and along the Gulf of Tadjourah ridge (Gall et al., 2018). The Asal-Ghoubbet rift 55 area has been significantly studied in order to characterize the Lake Asal 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 GobAad graben is one of these area (Dekov et al., 2014; Awaleh et al., 2015). 60 This paper aims to shed light on one of these remote geothermal systems. Using remote sensing datasets and field observations, we present here a multiscale structural survey and a lineament distribution mapping, carried out in order to specify the structural settings of the LAGF. This paper also provides insights on the structural control on the LAGF fluid flow, based on the hydrothermal surface manifestations analysis observed in this area. 65 2 Geological setting Lake Abhe is located on the southwestern edge of the Republic of Djibouti, on the border with Ethiopia. The lake sits near the center of the triple rift junction of Red Sea and Gulf of Aden oceanic rifts with the continental East African rift system (EARS), also known as the Afar Triangle (Chorowicz, 2005) (Fig. 1). The lake occupies the western end of the Gob Aad closed tectonic basin, an ESE-WNW striking 70 asymmetrical graben that connects to the larger NNW-SSE striking Tendaho graben, in the vicinity of the Dama Ale volcano (Beyene and Abdelsalam, 2005; Polun et al., 2018). The Gob Aad graben shoulders found in the Lake Abhe area are composed of the 1 to 4 My basalt Stratoid Series, which https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 3 dominates the surface geology of the central Afar region (Deniel et al., 1994; Beyene and Abdelsalam, 2005, Michon et al., 2022). These volcanic units are affected by a pervasive ESE-WNW striking normal 75 faults system that forms a horst – graben network outcropping northeast to the lake (Demange et al., 1971). The basin 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 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 80 episodes of highstand 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, highly alkaline (pH=9.86) and hyper-saline (total dissolved solids >90,000 mg/L) (Awaleh et al., 2015). The Lake Abhe eastern shore is particularly rich in hydrothermal surface manifestations, including 85 steam vents, hot springs and hydrothermal chimney structures, spread over an area of about 100 km2 (Houssein et al., 2013; Dekov et al., 2014). The latter structures, exposed thanks to the recent lake shrinking over the unconsolidated sedimentary flats, make this area remarkable. These carbonate chimneys extend over about 5 km between the lake shoreline and the surrounding basalt hills to the east, as individual structures or as clusters. Inter-chimney materials consist of unconsolidated mixed 90 carbonate and siliciclastic mud, with localized weakly-lithified diatomites and mudstones (DeMott et al., 2021). These features are generally aligned in an ESE-trend, reflecting the regional fault network main direction. The chimney field is generally subdivided in the existing literature into two 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. Chimneys in the north show more lateral continuity 95 and are commonly a few meters high, whereas those in the south are more isolated and higher, up to a few tens of meter (Fig. 1). These hydrothermal chimneys have been extensively described at the macro-, mesoand micro-scale by DeMott et al. (2021) in terms of morphology, texture and fabric. They exhibit a variety of large-scale (meter to decameter) morphology, classified as massive, pinnacle, bulbous, barrel or frondose. Carbonate chimneys are commonly characterized by a high number of cmto dm100 scale tubular crystalline structures growing upward and/or outward from the chimney bases, and of cavities of similar scale, forming overall dendritic and/or honeycomb textures. At the micro-scale, these highly porous and friable structures are mostly composed of calcite with needle-like, sugar-like and / or dendritic fabrics, and with relatively little pore infilling except for minor amounts of evaporites. A 3-5 cm thick stromatolitic crust that coat the external surface of the chimneys is generally described. 105 Despite differences in terms of morphology and distribution between the two sub-fields, all the Lake Abhe chimneys are interpreted to have been formed during the last few tens of thousands of years as abiogenic products of mixing between hydrothermal sublacustrine springs and lake waters during lake highstand intervals (DeMott et al., 2021). Numerous hot springs (T: 70-100°C) found at the base of chimneys and hot steam vents at their apex reveal the current hydrothermalism of this area (Awaleh et 110 al., 2015). Hot springs generally form small ponds, partly irrigated, that are surrounded by vegetation that benefits the local fauna and pastoral communities. https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 4 Figure 1: 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. Inset map indicates the 115 study area location at the regional scale (adapted after Awaleh et al., 2018). https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 5 The LAGF is one of the geothermal prospect areas of the Republic of Djibouti that 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 geophysics surveys that led to the definition of a reservoir 120 model of the LAGF with the support of international organizations (Fahman et al., 2018; Samod et al., 2018). According to Awaleh et al. (2015), the LAGF reservoir is mainly fed by meteoric water that penetrates downwards through the fracture network in the basalt Stratoid Series and combine with deep regional groundwater in the thermal aquifer to a maximum depth of about 1 km. Reservoir temperatures estimated from geothermometers range between 120 and 160°C, with a mean temperature of about 125 135°C. Exploration work 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 & Souleiman, 2015; Varet et al., 2020). 3 Methods A field campaign focused on the SHCa and the basalt Stratoid Series outcropping east to these 130 chimneys was conducted in November 2021. The fieldwork on the volcanic rocks consisted in the petro-structural characterization of the surrounding basalt series in terms of analog reservoir and fluid flow key features (i.e. faults, fractures, hydrothermalized zones, etc.). A structural survey of plurimetric fractures affecting the chimneys of the SHCa was carried out, with tens of chimneys and hot springs observed in this area. An imaging and topographic survey of the SHCa was conducted with a DJI 135 Phantom 4 Pro drone and adjacent topographic datasets and images were merged using QGIS. As part of the structural survey of the studied area, structural lineaments were mapped using highresolution remote sensing data and interpreted with knowledge acquired from field geological observations. Detailed lineament mapping was carried out on two digital elevation model (DEM) displayed on QGIS Version 3.22.6, using two different fixed viewing scale depending on the DEM 140 resolution. The NASA Shuttle Radar Topography Mission (SRTM) Global 1 arc second DEM (~30m resolution) was used to map lineaments at the displaying scale of 1/60000 over a large area (NASA JPL, 2013). Focused on the main hydrothermal active area, a higher resolution DEM (0.5m resolution) was generated from PLEIADES tri-stereo multispectral (MS) satellite imagery. This DEM was used to map structural lineaments of the volcanic series and hydrothermal chimneys alignments separately at the 145 displaying scale of 1/10000. Rose diagrams representing lineament mapping results were generated with the Line Direction Histogram QGIS plugin (Tveite, 2015), with the lineament orientation distribution in intervals of 10°. The length of the single bins of these diagrams corresponds to the number of lineaments occurring in each of these intervals, weighted by the lineament’s length. Hot springs outflowing over the sedimentary flats of the LAGF area were mapped using color-saturated 150 GoogleEarth images (Ó Google Earth 2022). Hot springs were identified thanks to the green vegetation surrounding small ponds clearly visible on these images, based on field observations knowledge. https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 6 4 Results 4.1 Multi-scale lineament mapping 4.1.1 1:60,000 scale mapping of structural lineaments 155 90 structural lineaments were mapped using a 1/60000 viewing scale, over an area of about 361 km2 centered on the LAGF (Fig. 1). The average length of the lineaments is 3074 m, with a maximum length of 17.1 km, although 14 of these lineaments are cut by the mapping frame. The strongly dominant lineament direction is ESE-WNW (090 to 120°) (Fig. 2a). Minor sets of ENE-WSW (070 to 090°), SENW (120 to 140°) and N-S (170 to 180°) oriented lineaments are also observed. ENEand SE-trending 160 lineaments are rather highlighted north of the mapping area by the hydrographic network descending the Gob Aad graben shoulder. N-S oblique lineaments are observed through the Stratoid Series outcropping east of the LAGF. 165 Figure 2: Rose diagrams representing: a) 1/60,000 scale structural lineament mapping, b) 1/10,000 scale structural lineament mapping and c) 1/10,000 scale hydrothermal chimney alignment mapping. 4.1.2 1:10,000 scale mapping of structural lineaments and hydrothermal chimneys alignment 255 structural lineaments were mapped at a 1/10000 viewing scale within the Stratoid Series hills east to the LAGF (Fig. 3). The average length of the lineaments is 541 m, with a maximum length of 4.6 km, 170 although 17 of these lineaments are cut by the mapping frame. The strongly dominant lineament direction is ESE-WNW (090 to 120°) (Fig. 2b). Similar to this main direction, two secondary direction of ENE-WSW (070 to 090°) and SE-NW (120 to 140°) oriented lineaments are observed and form dihedral pattern well visible west nearby the contact with the sedimentary flats. Minor sets of oblique N-S (160 to 180°) and NE-SW (040 to 060°) trending structures are also identified. 175 74 hydrothermal chimney alignments were picked based on the topography they constitute over the sedimentary flats (Fig. 3). The two main alignment directions are ESE-WNW (090 to 120°) and ENEWSW (060 to 090°) (Fig. 2c). Massive chimney structures of the GHCa south of the mapping area tends https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 7 to be located at the intersection of these two alignment directions. N-trending alignments are also observed in the densest part in terms of chimneys (i.e. SHCa), but not in the southern part of the 180 mapping area. Figure 3: 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. Hydrothermal chimney alignments observed over the basin sedimentary flats are 185 shown as green lines. Map location is shown in Figure 1. 4.2 Structural geometry of the LAGF surrounding hills Based on field observations and fault scarp morphology on satellite images, we added an interpretation of structural covering to the 1:60,000 scale lineament mapping results. This structural mapping is focused on a 40 km² area east to the LAGF. This densely faulted area displays variable topography that 190 range 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 westward plunge of the Stratoid Series towards the basin (Fig. 4a). https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 8 A significant number of ESE-trending 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 northsouth axis covering the area, as exhibited on topographic profiles (Fig. 4b). In the northern part of the 195 area, both northand 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. 5a). The southern part of the area exhibits mostly synthetic south-dipping normal faults, with a slightly denser fault spacing, that forms a tilted block geometry (Fig. 5b). Across this series of narrow horsts and grabens, overlapping synthetic normal faults interact and produce localized zones dipping steeper to 200 the west, interpreted as relay ramps. The most prominent ramp is being identified in a graben of the northern part of the area (Fig. 5c). The dominant ESE-trending fault set is affected by a few N-trending 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 2 adjacent normal faults. 205 https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 9 Figure 4: a) Topographic map of the LAGF area generated from SRTM Global DEM. Elevation ranges from ~240 m (pale green) to ~550 m (white). White dashed lines are locations of topographic profiles represented in Figure 4b. Normal faults, interpreted from field and topography observations, and unresolved lineaments from 1/60,000 scale structural lineament mapping are shown as black lines. Viewpoints location of Figure 5 is shown on the map. b) Topographic profiles extracted SRTM Global DEM, with 210 uniform vertical exaggeration along the profiles. Normal faults location is pointed out on the profiles. https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 16 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 to tilted blocks is observed in the southern part. A possible explanation for such geometry evolution is that the extension rate was higher north closer to the Gob Aad graben north boundary faults. With respect to this bimodal fault geometry, 310 hydrothermal manifestations are found more numerous in the northern part of the LAGF in the continuation of the horst and graben structures. 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. 5c). Relay zones were extensively 315 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 Gob Aad graben, in the Dobe and Hanle grabens, including ramp development, normal faulting of the ramp, complex block 320 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 crosscutting faults developed near the hinge zones and form elongate fault blocks that can rotate along 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 Nand NWN-trending structural lineaments are 325 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-trending extensional faults. No block rotation is however recognized in the LAGF area. This can be due to the fact that these breaching faults that precede rotation are not sufficiently developed to separate in depth fault blocks from the ramp (i.e. the hanging 330 wall). A significant number of these N-trending breaching faults tends to be aligned (Fig. 4). With a significant extension rate and displacement transfer, 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 alignment instead of a commonly described en335 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). Such breaching fault linkage could contribute to explain the overall N-trending 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 340 is observed about 10 km east from the LAGF within the volcanic series (Fig. 1, 9). This assumption of a significant influence of N-trending structures on this area’s morphology can be supported by multimethods geophysics surveys carried out in the LAGF. Resistivity model indicates a low resistivity layer, approximately 300-400 m thick in the SHCa and GHCa vicinity, interpreted as the thickness of conductive sediments (Fahman et al., 2018). In comparison to the slight dip of the Stratoid Series hills 345 plunging westward within the basin, the thickness of these sediments may appear relatively important. This may suggest the existence of such N-trending extensional structures that favor the Stratoid Series https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 17 burial. In addition to the main ESE-trending normal faults, ramps and associated breaching faults may as well be buried and masked by the sedimentary cover. 5.2 Hydrothermal surface manifestations distribution and evolution 350 In geothermal areas, hydrothermal manifestations such as thermal springs and travertine 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 ridgevs. chimney-type deposits) produced 355 from hydrothermal fluids discharged from thermal springs, many studies have reinforced the evidences of travertine deposition along the traces of brittle structures (De Filippis et al., 2013; Brogi et al., 2021, and references therein). The distribution of the LAGF hydrothermal manifestations following these structural orientations described in the area support the idea of tectonically driven morpho-tectonic features. 360 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 case, fault-related permeability is described as strongly compartmentalized and fluids emerge only in isolated spots along the fault trace (Brogi et al., 2021). Among several parameters (hydrothermal fluid and CO2 fluxes, lake 365 water level, etc.), fault and fracture-induced permeability controls chimneys localization and development. This permeability can be enhanced at structural intersections, marked by hints of enhanced hydrothermal activity (e.g. chimney size, density). Structural intersections with “x-shaped” fault and fracture networks can indeed act as dilatational zones and fluid preferential pathways under various tectonic stress regimes, producing significant relative vertical fracture permeability (e.g. Sibson, 370 1996, 2000; Person et al., 2012). In the GHCa, the massive and currently active chimneys are distributed at the intersection between ESEand ENE-trending hydrothermal traces (Fig. 3). In the SHCa, intersecting patterns localizing higher hydrothermal activity are more complex to observe as this area has an overall higher chimney density. This density may reflect a more diffused hydrothermal fluid flow thanks to the numerous intersecting structural traces (ESE-, ENE-, NNE-trending), underlined by 375 the chimney alignments that composed this area. Chimney distribution over these 2 areas therefore suggests that fault and fracture-induced permeability in the LAGF is favored locally by structural intersections. Hydrothermal chimney and travertine deposition testify of the high encrusting capacity of hydrothermal fluids along the conduits (Brogi et al., 2021). Long-lived hydrothermal flow and renewed fault and 380 fracture-induced permeability is guaranteed by tectonic activity (Curewitz and Karson, 1997). Active faulting and fracturing enable continual re-opening of fluid flow conduits despite potential clogging of fractures due to mineral precipitation. Age of the LAGF chimneys is poorly constrained. These structures, formed subaqueously during the successive lake level rises of the Lake Abhe throughout the Late Pleistocene and Holocene, have been deposited sequentially over multiple cycles of lake 385 transgression and regression (DeMott et al., 2021). The current hot springs, as well as the different phases of hydrothermal chimney formation of the LAGF therefore indicate this area to have undergone https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 18 regular tectonic pulses over the last few tens of thousands of years, renewing fracture permeability and triggering (re-)activation of thermal springs. Macro-scale syn-tectonic fractures that shaped some of the chimneys’ morphology during their formation over time in the SHCa support this idea (Fig. 6e, f). 390 The SHCa exhibits in its eastern part a significant number of inactive hydrothermal chimneys (i.e. no steam vent or springs). On the other hand, many hot springs with no associated chimney features are recognized in the northern and western part of the SHCa. The relative current lowstand lake level can explain the absence of chimneys formed alongside recent hotsprings. Therefore, the lack of current hydrothermal manifestations within extinguished chimneys in the east, as well as the recent springs in 395 the north and west may suggest a westward progressive lateral evolution of the hydrothermal fluid outflows over time. Given the sequential depositional model of the chimneys during successive transgressive intervals described by DeMott et al. (2021), some chimneys are likely to have been buried during a transgressive interval if they weren’t active at this moment. Offshore seismic reflection data show such inactive chimneys buried in the northern part of the lake (DeMott et al., 2021). Other buried 400 chimneys are therefore likely to be present in the LAGF area, depending on the lateral evolution of the fluid outflows over time. Consequently, continuous tectonic activity over at least tens of thousands of years maintained an overall sufficient permeability to develop these remarkable hydrothermal surface manifestations all over the LAGF area. However, spatial outflow evolution appears to have occurred depending on localized tectonic pulses, fracturing and thermal springs (re-)activation. 405 5.3 Implications for the LAGF development 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 410 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 & Rotevatn, 2016, and references therein). Similarly, intersection of multiple faults represents a high structural complexity zone that can act as fluid 415 preferential pathway with significant vertical fracture permeability, especially in low-porosity rocks (Curewitz & 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; 420 Taillefer et al., 2017). An inventory of more than 400 geothermal actives 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. The hydrothermal chimneys described in this area represent a useful surface proxy to decipher key 425 structural features distribution in depth and help identifying of the current most active upflow zones. Complementary geophysics surveys would however be needed to confirm this and to precise potential https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 19 drilling targets. Awaleh et al. (2015) indicated that the LAGF reservoir is fed by meteoric water that penetrates downwards through the fracture network in the basalt Stratoid Series and combine with deep regional groundwater in the thermal aquifer to a maximum depth of about 1 km. Numerous unresolved 430 questions remain about the geothermal reservoir parameters (depth, extent, host rock nature, etc.), the transition and localized flow processes from basalt series to poorly consolidated sediments, or the LAGF specific location in the Gob Aad basin. The LAGF appears indeed as the only significant geothermal occurrence for exploitation in the entire basin. These unraveled parameters therefore represent critical information to start geothermal development in this area. 435 6 Conclusion The LAGF Stratoid Series surrounding hills, as well as the hydrothermal surface manifestations of this area, were characterized through a multiscale structural survey and a lineament distribution mapping. Structural features of the LAGF area are dominated by ESE-trending extensional faults. The overall geometry of the structural blocks delimited by these faults evolves between the north and the south of 440 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 this studied area. Some of the structural lineaments identified with a Nand NWN trend may represent possible breaching faults of 445 such accommodation features. 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 in depth and therefore suggest that structural intersections can locally enhance fault and fracture-induced 450 permeability. Plurimetric long tectonic fractures that cut through the hydrothermal chimneys were also recognized, orientation distribution being consistent with chimney alignment directions and the main structural trends of the area. Field observations, in conjunction with satellite images analysis, suggested a progressive lateral evolution of the LAGF hydrothermal fluid outflows over time, with regular tectonic pulses renewing fracture permeability through chimneys and triggering (re-)activation of 455 thermal springs. This study finally demonstrates the strong connection between LAGF structural features and the hydrothermal manifestations distribution. It provides new insights on the local tectonically driven fluid flow evolution. This work may therefore support further exploration of this remarkable site and promote geothermal development in the area. 7 Data availability 460 All raw data can be provided by the corresponding authors upon request. https://doi.org/10.5194/egusphere-2023-397 Preprint. Discussion started: 14 March 2023 c Author(s) 2023. CC BY 4.0 License. 20 8 Author contributions YG acquired the funding; all authors contributed to collect data and conceptualize the study; BW wrote the manuscript draft and produced the figures; YG, AF, NC and MD reviewed and edited the manuscript. 465 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 LEAP470 RE partnership, all the partners are gratefully acknowledged. 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