# 3D geological modelling of the Asal Ghoubbet Rift (Djibouti): New insights into fault geometry and stratigraphic formations for better geothermal resource assessment

Asma Abdi Ali, Damien Bonté, Adriana Lemgruber-Traby, Christine Souque, Fadi Henri Nader

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## ► To cite this version:

Asma Abdi Ali, Damien Bonté, Adriana Lemgruber-Traby, Christine Souque, Fadi Henri Nader. 3D geological modelling of the Asal Ghoubbet Rift (Djibouti): New insights into fault geometry and stratigraphic formations for better geothermal resource assessment. Journal of African Earth Sciences, 2026, 233, pp.105879. 10.1016/j.jafrearsci.2025.105879. hal-05491338

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Submitted on 3 Feb 2026

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# 3D geological modelling of the Asal Ghoubbet Rift (Djibouti): New insights into fault geometry and stratigraphic formations for better geothermal resource assessment☆

Asma Abdi Ali*, Damien Bonté<fnref n="1" />, Adriana Lemgruber-Traby, Christine Souque, Fadi Henri Nader

IFP Energies nouvelles, 1-4 Av. du Bois Préau, Rueil-Malmaison, 92852, France

## ARTICLE INFO

Handling Editor: DR Damien Delvaux

**Keywords:**

Asal-Ghoubbet Rift

Structural geology

3D geological modelling

Geothermal energy

## ABSTRACT

The Asal-Ghoubbet Rift (Djibouti), less than 1 million years old, is marked by intense tectonic and magmatic activity, a thinned crust, and higher thermal gradients. These characteristics make it a promising target for high-temperature geothermal development, but the lack of sustainable assess to fluids is an important challenge. In this context, building a reliable 3D geological model is an essential first step to characterize the subsurface geometry and support future geothermal assessment. This study integrates borehole data, surface observations, geophysical surveys, and structural interpretations to construct a representative 3D geological model. Due to the uneven spatial distribution of data, the model is also constrained by four representative geological cross-sections that define the geometry of stratigraphic units and faults. Geological features were interpolated using the Discrete Smooth Interpolation (DSI) method implemented in SKUA/GOCAD®. The model identifies 17 steeply dipping normal faults (~75°), oriented east or southeast, with near-pure extensional motion reaching depths of 3–5 km, consistent with seismological data. These faults divide the rift into structural blocks, which may influence hydrothermal fluid circulation. The modeled volcanic units, predominantly basaltic, generally have thicknesses below 600 m except for the Mabla complex and reflect continuous volcanism. The model also includes the brittle-ductile transition surface, interpreted as a major thermal boundary. Its reduced depth beneath the rift axis suggests a thermal anomaly likely related to crustal thinning and the presence of a shallow magmatic reservoir. This 3D model provides a valuable framework to guide future investigations and support more accurate assessments of the site geothermal potential.

## 1. Introduction

3D geological modelling is a crucial method to digitally visualize subsurface geological structures and understand geological events in space and time (Houlding, 2012; Calcagno et al., 2022). Recent developments in computer systems have enabled significant advances in 3D modelling software, such as Petrel, Leapfrog, Geomodeller, and SKUA-Gocad. This method has been applied to a wide range of domains such as engineering (Dong et al., 2015; Xiong et al., 2018), groundwater resource management (Lelliott et al., 2006; Hassen et al., 2016; Thornton et al., 2018), geothermal energy (Calcagno et al., 2022; Mas et al., 2022), and hydrocarbon exploration (Perrin et al., 2005). It is also

applied in mining resource exploitation (Wu and Xu, 2014), as well as in CO2 and gas storage (Kaufmann and Martin, 2009; Vo Thanh et al., 2019).

In the field of geothermal energy, 3D geological modelling not only allows for accurate representation of subsurface characteristics (Milicich et al., 2018), but also serves as a foundation for numerical simulations aimed at understanding fluid flow and heat transfer (Przybycin et al., 2017; Siler and Pepin, 2021), assessing the production potential of a site (O'Sullivan and O'Sullivan, 2016), and guiding decisions on the location and depth of production wells (Siler et al., 2019).

The geothermal exploration of the Asal-Ghoubbet Rift has been the subject of numerous studies during the 1970s and 1980s, conducted by

<footnote>* Article (Journal of African Earth Sciences).</footnote>

<footnote>* Corresponding author.
E-mail address: asmaabdiali93@gmail.com (A. Abdi Ali).</footnote>

<footnote n="1">¹ Now at BRGM, Orléans, France.</footnote>
various institutions such as the Bureau de Recherches Géologiques et Minières (BRGM) and the Centre National de la Recherche Scientifique (CNRS) (BRGM, 1984; Demange and Puvilland, 1993), and it continues today (Zan et al., 1990; Demange and Puvilland, 1993; GEOLOGICA, 2023). At the time this article is being written the area has eleven deep boreholes, three of which are deviated to intercept the main rift faults to better target permeable zones. All these boreholes had high temperatures at relatively shallow depth, approaching 300 °C at 2 km, making the area of particular interest with respect to electricity production (Zan et al., 1990; Carver et al., 2019). Data from the wells highlight the complexity of the geothermal system. Some boreholes situated in the external part of the rift indicate two convective zones, interpreted as reservoirs, and separated by a clay cap, with high-low permeability and highly saline fluids. The boreholes in the internal part of the rift are mainly dominated by a conductive regime in depth. At shallow depth a cooling perturbation is usually described as a seawater intrusion. This complexity is rather common in geothermal systems, especially in recent volcanic zones associated with tectonic extension, because of the heterogeneity of several interdependent factors: permeability, fluid origin, etc. (Jolie et al., 2021).

As a result, a reliable 3D geological model is needed to evaluate the resource characteristics (heat transfer and fluid flow) and could support the decision of the best exploitation strategy, including drilling of new wells. In this context, our study aims to define the 3D geometry of the Asal-Ghoubbet Rift. Due to the lack of possibility to use active seismic

reflection in volcanic areas, 3D geological modelling in such settings generally consists of using borehole data together with other type of geophysical methods (e.g., magnetotelluric) and geochemistry. In the Asal-Ghoubbet region our approach has been to use surface structural data, MT profiles, geological knowledge, and deep drilling data. As these data were unevenly distributed in the considered area (especially regarding the wells that are all located at the south of the area) we have constructed geological cross-sections that served as an input for the model. The resulting geological model is the first 3D model in the Asal-Ghoubbet area. The model covers an area of 16 × 11 km for a depth of 3 km with four stratigraphic units and 17 normal fault planes. This constitutes a first step toward a better understanding of the geothermal system that could lead to a more accurate evaluation of the geothermal potential in the region. The future of this model is evolutive, as it will be updated with new upcoming data. A better understanding would also come from the 3D restorations that would allow to provide a consistent model with the geological structure of the rift.

## 2. Overview of the study area

### 2.1. Geological context

The Asal-Ghoubbet Rift is one of the youngest rift segments (<1 Ma) within the Afar Depression, situated at the western end of the Aden Ridge (Fig. 1a). It represents the first subaerial segment of the ridge

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** a) Location of the Asal rift at regional scale. b) Simplified geological map (modified after (Stieltjes et al., 1976; Pasquet et al., 2022) showing the major stratigraphic units of the Asal-Ghoubbet Rift with the major faults (Manighetti et al., 1998). Black line shows the location of the geological cross-section.</figcaption></figure>
extending into the Afar region (Courtillot, 1982; Manighetti et al., 1997, 1998). This rift is often considered an analogue of a young slow-spreading ridge segment (Tapponnier et al., 1990; Manighetti et al., 1998). It opens at a rate of 16 ± 2 mm/year along a N40 ± 5°E direction (Stein et al., 1991; Chabalier and Avouac, 1994; Manighetti et al., 1998; Vigny et al., 2007; Doubre et al., 2017). It is a narrow graben-oriented NW-SE, extending between Lake Asal and the Ghoubbet Bay (Fig. 1b).

The onset of rifting in the Asal-Ghoubbet Rift is dated by effusive episodes that started approximately 900 ka ago (Manighetti et al., 1998; Doubre et al., 2007b). However, most of the structures older than approximately 300 ka year old have been covered by more recent lava flows (Manighetti et al., 1998). The last 300 ka period has been characterised by an intense magmatic activity, marked by large volumes of basaltic lava filling the rift floor and covering most of the earlier faults and tectonic activity. This has been accompanied by major fault formation (Stein et al., 1991; Chabalier and Avouac, 1994; Manighetti et al., 1998). During that time, the evolution of the rift has been marked by the activity of a central volcano, the Fieale volcanic system, whose products fill the rift floor (Pinzuti et al., 2010). The presence of a magma chamber at a depth of about 3 km has been suggested beneath the Fieale volcano (Demange et al., 1980; Ngoc et al., 1981). Magmatic activity decreased around 50 ± 20 ka, and since then, the successive basaltic lava flows that formed the Fieale volcano have been gradually dismantled by normal faults (Manighetti et al., 1998). It is this internal structure of the rift that started to develop between 40 and 30 ka with the development of major boundary faults (H and α on Fig. 1b) (Manighetti et al., 1998; Doubre et al., 2007b). Since then, a gradual collapse has been occurring on the Fieale volcano while volcanic activity has concentrated along the rift axis and on smaller volcanic edifices (Manighetti et al., 1998; Doubre et al., 2007b; Pinzuti et al., 2010). Additionally, a highly thinned crust (Ruegg, 1975; Pinzuti et al., 2013; Chauvet et al., 2023) and a shallow brittle-ductile transition zone, at about 5 km depth (Doubre et al., 2007b), have been identified beneath the rift axis.

The Asal-Ghoubbet rift is divided into three main zones: axial volcanic zone, internal margins, and external margins (Stieltjes et al., 1976; Manighetti et al., 1998). The magmatism in the rift is dominated by the so-called Asal basalt series (Fig. 1b), which exceeds more than 400 m thickness near the rift axis according to the data from the wells A5, F1, F2, and F3 (Zan et al., 1990; Turk et al., 2019). It is bounded by subvertical normal faults oriented NW-SE (N130 ± 10°) with displacements that exceed 150 m and a network of open fractures. Both faults and fractures are approximately parallel to the faults in the internal margin (Stein et al., 1991; Manighetti et al., 1998, 2001; Audin et al., 2001).

The most recent major rifting episode in the Asal-Ghoubbet Rift occurred in 1978. It was characterized by two significant earthquakes (mb = 5 and 5.3) in the Ghoubbet Bay (Abdallah et al., 1979; Lepine et al., 1980). This phase was followed by a basaltic fissural eruption at the northwestern end, giving rise to the Ardoukoba volcanic cone (Demange et al., 1980). The effects of this event included an extension of about 2 m in a N40° direction and a subsidence of up to 70 cm in the rift floor (Ruegg et al., 1979). Presently, the most active deformations are localized around the Fieale edifice and the northeastern part of the rift (Doubre et al., 2007b; Doubre et al., 2017).

## 2.2. Geothermal context

The Asal-Ghoubbet Rift has been the subject of various investigations regarding geological, geochemical, and geophysical studies to identify the geothermal resources (Ruegg, 1975; Stieltjes et al., 1976; Barberi and Santacroce, 1980; Lepine et al., 1980; Sanjuan et al., 1990; Zan et al., 1990; Houssein and Axelsson, 2010; Aden et al., 2021; Ragueh et al., 2024). Spatially, the Asal-Ghoubbet rift has had two sub-sites identify as favourable for the development of geothermal energy: Gale-Le-Koma and Fieale (Fig. 1b).

Several drilling campaigns have been conducted in the Asal-Ghoubbet rift since the 1970s to confirm the geothermal potential. In 1975, the first wells, A1 and A2, were drilled at Gale-Le-Koma by the two French institutions: BRGM and CNRS (Zan et al., 1990). Following this first stage, an Italian Company, Aquater, drilled from 1987 to 1988 four additional wells: A3, A4, A6, and A5. The first three (A3, A4, and A6) are located on the southwest margin of Gale-Le-Koma, showing a maximum temperature of 265 °C, 344 °C and 281 °C respectively. A5 was drilled in the internal rift not far from the Fieale caldera and gave a maximum temperature of 359 °C (Zan et al., 1990). In 2016, the Djiboutian Office for the Development of Geothermal Energy (ODDEG) drilled a well, Gale-Le-Koma 1 (Glc1), which had a maximum temperature of 138 °C during drilling (Ahmed, 2018). In the Fieale caldera, three new wells (F1, F2, and F3) have recently been drilled by Electricity of Djibouti (EDD) in 2019 with maximum temperatures ranging from 352 °C to 363 °C (Turk et al., 2019). Finally, in the region of Gale-Le-Koma, ODDEG drilled, P1, in 2022. Limited information has been acquired in the wells but the thermal profiles available show a very low thermal gradient in some part of the wells, symptomatic of a convection effect at Gale-Le-Koma. In the Fieale caldera, however, the gradient indicate a predominant conductive regime (Carver et al., 2019). This purely conductive regime contrasts with the convective systems observed in similar geodynamic contexts such as Iceland (Arnórsson, 1995).

# 3. Data and methods

## 3.1. Available data

To build our geological model, we have used existing data from the literature such as structural data, lithostratigraphic well data, and magnetotelluric (MT). The detail of the data and their origin is shown in Table 1.

### 3.1.1. Structural data

The structural map of Manighetti et al. (1998) provides a representation of the faults in the rift mapped at the surface (Fig. 1b). These faults, labelled with letters (J, H, G, F, D, B, α, δ, β, γ, ε), were digitized to provide their manifestation at the surface. All faults are normal faults with an orientation northwest to southeast and a dips between 60 and 80° (Stein et al., 1991; Manighetti et al., 1998). Faults K, J, and ε have been dated to approximately 500, 400, and 300 ka, respectively. In contrast, Faults H and α are more recently formed, having developed within the last 50 ka. The faults on the outer margin (J and ε) are inactive and the most recent tectonic and seismic activity is concentrated between the faults on the inner margin, H and α (Manighetti et al., 1998; Doubre et al., 2007b; Cattin, 2005).

The depth extent of the faults cannot be accurately determined from surface observations alone. However, the analysis of relocated hypocenters by Doubre et al. (2007a) provides valuable insights into seismic activity at depth. These data indicate that the majority of earthquakes are concentrated between 2 and 3 km depth in the central part of the rift (Fiale area), with local extension down to 5 km at the rift margins. These observations suggest that the brittle-ductile transition occurs at approximately 3 km depth in the rift center and between 5 and 6 km on its margins. The relocation precision is high, with an average uncertainty of about 40 m both horizontally and vertically, and a mean distance of 420 m between initial and relocated positions (95 % of events exhibiting a deviation of less than 1.5 km) (Doubre et al., 2007a). The hypocenters align in narrow clusters parallel to the main normal faults (α, ε, γ, D), highlighting a strong structural control on the seismicity distribution (Fig. 2).

To further constrain the fault geometry at depth, we also analyzed well-constrained earthquake focal mechanisms available in the region (Lépine and Hirn, 1992; Doubre et al., 2007a, 2007b). Although some structures are associated with recent seismicity, the majority of major faults in the rift have remained seismically quiet over the last two
<table><caption>**Table 1**
Dataset used to build the geological model of the Asal Rift.</caption><thead><tr><th>Type</th><th>Name</th><th>Author</th><th>Key points</th></tr></thead><tbody><tr><td>Structural map</td><td>Tectonic map of Asal-Ghoubbet Rift</td><td>Manighetti et al. (1998)</td><td>Fault trace at the surface</td></tr><tr><td rowspan="2">Geological maps</td><td>Asal Rift</td><td>Stieltjes et al. (1976)</td><td>1:50,000 Geological Map of the Asal Rift.</td></tr><tr><td>Geological map of the Asal Rift and Transform Zones</td><td>Khodayar (2008)</td><td>The geological Map of the local and regional strikes and tilt of the series.</td></tr><tr><td rowspan="10">Well data</td><td>Asal 1 (A1)</td><td>Demange and Puvilland (1993)</td><td>Reached the top of the Dalha series (TVD = 1146 m)</td></tr><tr><td>Asal 2 (A2)</td><td></td><td>Reached the top of the Dalha series (TVD = 1554 m)</td></tr><tr><td>Asal 3 (A3)</td><td>(Zan et al., 1990)</td><td>Reached the top of the Dalha series (TVD = 1316 m)</td></tr><tr><td>Asal 4 (A4)</td><td></td><td>Reached the Mabla series (TVD = 2013 m)</td></tr><tr><td>Asal 5 (A5)</td><td></td><td>Reached a depth of the Mabla series (TVD = 2105 m)</td></tr><tr><td>Asal 6 (A6)</td><td></td><td>Reached the top of the Dalha series (TVD = 1761 m)</td></tr><tr><td>Gale-Le-Koma 1 (GLC_1)</td><td>(Ahmed, 2018; Pasquet et al., 2023)</td><td>Reached the Stratoide series (TVD = 600 m)</td></tr><tr><td>Fieale (F1)</td><td>(Turk et al., 2019; GEOLOGICA, 2023)</td><td>Reached the Mabla series (TVD = 2482m), deviated from 600m</td></tr><tr><td>Fieale (F2)</td><td></td><td>Reached the Mabla series (TVD = 2246 m), deviated from 250m</td></tr><tr><td>Fieale (F3)</td><td></td><td>Reached the Mabla series (TVD = 2409 m), deviated from 600m</td></tr><tr><td>Magnetotelluric profiles</td><td>2D MT profiles</td><td>(Aden et al., 2021, 2022; Aden, 2023)</td><td>2D conceptual model interpreted along 2D MT profile perpendicular and parallel to the Asal Rift.</td></tr></tbody></table>

decades. Approximately 70 % of the studied mechanisms indicate steeply dipping normal faults (~75° ± 10° for the more inclined nodal planes), striking east to southeast, with nearly pure extensional motion (rakes between -85° and -100°). Based on these seismological and structural constraints, we assumed in our model that all faults extend to a depth of 3 km.

### 3.1.2. Lithostratigraphic well data

In total, eleven wells were drilled in the Asal-Ghoubbet system, ranging from 600 to 2400 m in depth. As shown on Fig. 1b, seven are in the outer part of the rift to the south at Gale-Le-Koma, while four are in the central part of the rift also in the south at Fieale. The lithostratigraphic framework of the Asal Rift is derived from the lithological logs of geothermal wells (Zan et al., 1990; Demange and Puvilland, 1993; Turk et al., 2019; Guelleh et al., 2022; GEOLOGICA, 2023). The stratigraphic sequence of the Lac Asal site consists of four distinct units (Zan et al., 1990; Ragueh et al., 2025). The Asal Series comprises basalts, clay layers, and hyaloclastites in the Gale-Le-Koma wells, while the Fieale wells, which are located more in the inner part of the rift, mostly show basalts. At the surface, lacustrine deposits intercalate with or lie on top of the volcanic formations (Stieltjes et al., 1976). The maximum age of the Asal series is estimated at 1.05 Ma (Manighetti et al., 1998). This

series extends from the surface to a depth of 380 m at Gale-Le-Koma and up to 470 m at Fieale. The Stratoid Series is mainly basaltic with intercalations of trachyte and rhyolites at Gale-Le-Koma and predominantly trachytic in the Fieale wells. The depth of that series goes up to a 1000 m and dates between 4 and 1 Ma (Barbari, 1975; Barberi and Santacroce, 1980). The Dalha Series, dating from 8.9 to 3.8 Ma (Gaulier and Huchon, 1991), consists mainly of basaltic lava flows with intercalations of rhyolites and trachytes. Finally, the Mabla Series, dated between 19 and 9 million years old (Varet, 2018), is characterized by a significant rhyolitic formation overlying the basalts, interbedded within this series.

### 3.1.3. Magnetotelluric profiles

Magnetotelluric profiles (MT) surveys were carried out in the Asal-Ghoubbet rift area by the BRGM in 1988 and by the Iceland Geo-Survey (ISOR) in 2008 (ISOR, 2017). Two profiles from these surveys were reprocessed and reinterpreted by ODDEG in 2020. These two profiles, oriented SE-NW (P2) and SW-NE (P1), respectively cross the Gale-le-Koma and Fieale areas (Fig. 3). The advantage of this passive electromagnetic method lies in its ability to image conductive structures in three dimensions. The same anomalies appear on both profiles, with four horizons. The first layer, C1, is a very thin, highly resistive layer that could consist of recent unaltered basaltic lava flows; this layer corresponds to the first few hundred meters of the boreholes from 0 to 400 m depth. The second layer, C2, represents a highly conductive horizon, corresponding to the Stratoid series where several influxes of water have been identified in the wells between 400 and 900 m depth. Underlying this horizon is the third layer, C3, that represents a very high resistivity zone identified between 900 and 1500 m depths. At greater depths, the fourth layer (C4) consists of two high-resistivity anomalies below 1500 m, which could be interpreted as geothermal reservoirs. These anomalies correspond to the zones where the Mabla series has been described and were intercepted by boreholes drilled on the site (F1, F2, and F3). Recently, conceptual interpretative models parallel and perpendicular to the Asal-Ghoubbet rift were developed from MT profiles (Aden et al., 2021, 2022). Such models support the interpretation of the four layers described above.

## 3.2. 3D modelling approach

The common approach of 3D geological modelling in volcanic systems is the use of surface knowledge combined with geophysical data and well knowledge. Active seismic methods cannot be applied in volcanic regions due to recent basaltic cover (Pujol and Smithson, 1991; Planke et al., 2000; White et al., 2003). In this study, the rift is exclusively constrained at depth by the data from eleven boreholes (Zan et al., 1990; Turk et al., 2019; GEOLOGICA, 2023), surface structural data (Stein et al., 1991; Manighetti et al., 1998; Doubre et al., 2007b), and MT profiles. Associated with surface geology knowledge, the continuation of surface-observed structures at depth have been used to build cross-sectional views.

We used the 3D software SKUA/GOCAD® (https://www.aspentech.com/en/products/sse/aspen-skua) in which the geometry and properties of the object are interpolated using the Discrete Smooth Interpolation (Mallet, 1992; Frank et al., 2007; Caumon et al., 2009; Collon et al., 2015). The 3D interpolation is performed on a tetrahedral mesh by applying an approach based on optimizing the spacing between node locations, considering various geological and structural constraints. These constraints include the identified fault contacts at the surface, interpretations made from vertical geological sections, as well as geological unit contacts identified in boreholes. By using the Discrete Smooth Interpolation approach, complex geometric and structural relationships are better integrated into the model, ensuring optimization of the interpolated surfaces based on these different data while maintaining the continuity and consistency of the structural model.
<figure><img src="image_6.png" /><figcaption>Fig. 2. Earthquake hypocenters along profile lines A (modified from Doubre et al., 2007b), showing the brittle-ductile transition zone.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 3. Two 2D magnetotelluric profiles (P1 and P2) from ODDEG (2020) showing the location of geothermal wells.</figcaption></figure>
### 3.3. Workflow for 3D geological model

After collecting the available data, several steps were undertaken to develop the geological model (Fig. 4). In summary, the information from existing data, combined with the geological knowledge of the region, facilitated the creation of geological sections that illustrate the extent of the structures at depth. These sections were used as input data for constructing the 3D model and guiding the interpolation process. The workflow followed to build the 3D geological model included the following steps.

**Data selection and Information extraction:** The selection of data is primarily based on a thorough understanding of the available sources and a detailed bibliographic review. For our area of interest, the entire set of data shown on Table 1 was identified and extracted from the relevant sources. As these point and planar datasets provide only partial geological information, cross-sections were created to better integrate the vertical dimension and interpret subsurface structures.

**Data Inputs to the Model (Construction Geological Cross-Sections):** Four geological cross-sections were created (Fig. 5), based on a combination of borehole data (that define stratigraphic boundaries), structural data at the surface, and geological and structural knowledge of the region. A key feature of the Asal-Ghoubbet rift is the presence of normal faults, approximately 10 km in length, oriented NW-SE, with dips ranging from 60° to 80°, based on fault scarp observations (Stein et al., 1991; Manighetti et al., 1998; Doubre et al., 2007b). Although there is a lack of data regarding the geometry and extent of the faults at depth, we assumed that these faults extend to a depth of 3 km, consistent with the brittle-ductile transition zone, which is located at a depth of 5 km in the Asal-Ghoubbet rift (Doubre et al., 2007b). The cross-sections were constructed perpendicular to these main faults and extend from the NE to the SW, reaching a depth of 3000 m below sea level. Their orientation and extension were chosen to intersect the rift's faults, with depths selected based on the available boreholes, the deepest being at 2700 m. The eleven deep boreholes at this site are all located along a single cross-section, cross-section A (Fig. 5). When examining the magnetotelluric (MT) profiles discussed in the previous section and the

results from earlier studies (Aden et al., 2021, 2022), it was found that the structure remains consistent. This allowed the stratigraphic units identified in cross-section A, where the boreholes are located, to be extrapolated to cross-sections B, C, and D.

**Import, Georeference, and Digitize the Vertical Cross-Sections:**

The geological cross-sections were imported into the modelling framework to be integrated into the 3D geological model. They are initially loaded in TIF format and require to be georeferenced. The coordinates of the cross-sections are then assigned on the basis of the known vertical scale of the image and subsequently visualized in the GOcad 3D interface. Once these sections are visualized, a thorough manual task is required to digitize the interfaces of the subsurface formations (faults, peaks, lower layers, etc.) and their associated orientations.

**Add Chronological Information:** Chronological information was integrated into the geological model, including the ages of the different layers as well as the identification of fault ages. This helps to better understand the sequence of geological events, such as tectonic deformations, as well as erosion or sedimentation processes that occurred over time (even though, in our case, there are no erosion or sedimentation processes). Clay layers were identified in the boreholes of the outer part of the rift (Fig. 5a), but these were not included in the model calculation to avoid interpolation issues. These layers are very localized, and their extent remains uncertain. Only the different volcanic series described in the borehole logs namely the Asal series, the Stratoid series, the Dalha series, and the Mabla series were modeled. The integration of this temporal information enhances the model's accuracy and will facilitate a better understanding of the site's geological history for future model restoration.

**Defining the Volume of Interest:** This involves defining the space in which geological processes will be modeled. The volume was determined based on the horizontal and vertical boundaries of the four geological cross-sections in the study area, considering the depth of the main geological formations and the structures to be analyzed (faults, strata, etc.).

**Build the Fault Network:** The fault network constitutes the main structural framework of the Asal-Ghoubbet rift. The main faults were

<figure><img src="image_8.png" /><figcaption>Fig. 4. Workflow used in the modelling process.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 5. The four cross-sections used as input data for 3D geological model construction.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 6. A 3D view of the faults in the study area.</figcaption></figure>
first digitized from the surface structural map and the geological cross-sections. To ensure the geometry of the faults, precise adjustments were made, and additional constraints were applied if necessary to extrapolate or project the boundaries of neighboring horizons, thus avoiding artificial or unrealistic discontinuities. In total, ten fault planes were constructed, all of which are parallel normal faults.

**Build the Horizon Framework:** The horizons of the cross-sections were digitized in the same way as the fault planes and then interpo-lated block by block, initially disregarding the faults. Each horizon was consistently assigned across all the cross-sections. Four different geological units were integrated into the model presented.

## 4. Results

The 3D fault model of the Asal-Ghoubbet Rift reveals a network exclusively composed of NW-SE trending normal faults, with relatively uniform dip angles (Fig. 6). These orientations are consistent with the regional extension direction (Manighetti et al., 2001).

The modeled stratigraphic horizons include the Asal, Stratoid, Dalha, and Mabla units, spanning from the Miocene-Pliocene to the Quaternary. These horizons are continuous across most of the rift, with no major discontinuities or unconformities observed (Fig. 7).

Depth maps illustrate the vertical variation of the horizons and allow visualization of the rift's structural compartmentalization (Fig. 8). They highlight central grabens and peripheral horsts. The Asal unit ranges from -150 m below sea level to +400 m, reflecting relatively minor recent subsidence. The Stratoid unit ranges from 0 to -600 m, Dalha from -600 to -1200 m, and Mabla from -1200 to -3000 m. Tight contour spacing observed for Dalha and Mabla indicates steeper slopes and more pronounced deformation, consistent with an active rifting environment. The depths and thicknesses of the units are summarized in Fig. 9. The Asal unit is relatively thin, reflecting moderate recent subsidence in the central graben. Stratoid and Dalha have intermediate thicknesses, corresponding to older deposits and steeper slopes due to

normal fault activity. The Mabla unit is the deepest and thickest, reflecting prolonged accumulation of volcanic and sedimentary deposits as well as intense deformation associated with rift extension. This progressive increase in thickness with depth illustrates the combined effect of subsidence and structural compartmentalization on the internal architecture of the rift.

The model incorporates the brittle-ductile transition, located at approximately 3 km depth beneath the inner rift and between 5 and 6 km in the outer rift areas (Fig. 10). This transition represents a major mechanical boundary within the crust, separating the upper brittle regime, dominated by normal faulting, from the deeper ductile layer, where rock deforms continuously. Its reduced depth at the center of the rift reflects crustal thinning and is likely associated with the presence of a shallow magmatic reservoir (Doubre et al., 2007a, 2007b). The model grid demonstrates how the fault network organizes the rift into well-defined structural compartments (Fig. 11). These blocks display variations in stratigraphic thickness and orientation, indicating that fault geometry strongly controls the internal architecture of the rift. The spatial arrangement of faults and blocks also highlights potential zones of enhanced permeability along fault corridors and localized structural highs or depressions.

The model's fit to input data was assessed using an error histogram for all stratigraphic surfaces (Fig. 12). Most errors are centered around zero, indicating good overall agreement between the modeled surfaces and well data. Over 80 % of measurements show deviations within ±50 m. The error distribution also reflects local uncertainties related to limited control points in some areas and the inherent complexity of faulted volcanic sequences.

## 5. Discussion

The 3D fault model derived in this study shows that faults within the Asal Rift exhibit consistently uniform dip angles (Fig. 6). This geometry likely reflects a relatively homogeneous extensional stress field,

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** 3D view of the different horizons displayed with the faults in the study area.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 8. Depth structure map of the Asal Rift showing: (a) Asal Unit, (b) Stratoid Unit, (c) Dalha Unit, and (d) Mabla Unit.</figcaption></figure>

consistent with the NW-SE regional extension direction reported in previous studies (Lépine and Hirn, 1992; Doubre et al., 2007a; Delvaux and Barth, 2010). In addition to tectonic control, this fault geometry may also be influenced by magmatic processes, such as the presence of shallow magma reservoirs (Hayward and Ebinger, 1996; Wright et al., 2006), or a deeper ductile zone beneath the rift that governs the orientation of near-surface normal faults (Doubre et al., 2007a, 2007b).

Similar structural configurations have been reported in other young and active rift systems, where fault dip uniformity appears to be strongly controlled by magmatic activity (e.g., (Forslund and Gudmundsson, 1992; Autin et al., 2013)).

While the hydraulic behavior of fault zones in the Asal-Ghoubbet Rift is not well constrained, some inferences can be drawn from fault slip rates. Several studies suggest that high slip rate faults are typically
<table><caption>Fig. 9. Lithostratigraphy and thickness of the different units (1)(Manighetti et al., 1998) (2)(Barbari, 1975) (3)(Gaulier and Huchon, 1991) (4)(Varet, 2018).</caption><thead><tr><th>Age</th><th>Stratigraphy units</th><th>Lithology</th><th>Thickness Max (m)</th></tr></thead><tbody><tr><td>Holocene</td><td>Asal Series</td><td>lacustrine deposits, basalt and hyaloclastites</td><td>400</td></tr><tr><td>Pleistocene</td><td>1,05 Ma<sup>(1)</sup></td><td>Stratoïd Series</td><td>600</td></tr><tr><td>Pliocene</td><td>4 Ma<sup>(2)</sup></td><td>Dalha Series</td><td>600</td></tr><tr><td rowspan="3">Miocene</td><td>8.9 Ma<sup>(3)</sup></td><td>Mabla Series</td><td>rhyolites interstratified with basalts</td><td>> 1000</td></tr><tr><td>Middle</td><td></td><td></td><td></td></tr><tr><td>Lower</td><td>19 Ma<sup>(4)</sup></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>Fig. 10. 3D view of the different horizons displayed with the faults in the study area with an additional horizon representing the brittle-ductile transition (BDT) zone.</figcaption></figure>

associated with injection-type faults characterized by higher permeability (Tanikawa et al., 2010). Evidence from post-seismic drilling in Iceland, following a magnitude 5.8 earthquake, shows that fault zone permeability can change rapidly over short timescales (Claesson et al.,

2004). In the Asal-Ghoubbet Rift, slip rates vary significantly across the fault network (Stein et al., 1991; Doubre et al., 2007a, 2017). The faults located in the internal part of the rift (faults A, α, β, and γ) exhibit the highest slip rates, suggesting they play a major role in accommodating
<figure><img src="image_14.png" /><figcaption>Fig. 11. The 3D structural model grid of the Asal-Ghoubbet rift for the upper 3 km.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Distribution of mismatches between input data and modeled surfaces for the geological formations and the brittle-ductile transition (BDT) surface.</figcaption></figure>

regional extensional strain, in addition to deformation related to dike intrusions along the rift axis, while faults located in the external part of the rift (e.g., faults F and H) show much lower slip rates. These variations imply that faults near the rift axis may act as preferential pathways for fluid flow, although permeability is also modulated by lithology, hydrothermal alteration, and magmatic intrusions.

Hydrogen anomalies observed through fumaroles in the inner part of the rift, especially near faults F and D, indicate active hydrothermal circulation (Pasquet et al., 2022). Self-potential surveys support this interpretation and suggest that NW-SE oriented faults facilitate the upward migration of shallow hydrothermal fluids (Mlynarski and Zlotnicki, 2001). Additionally, Numerical models further demonstrate that fault permeability significantly influences deep fluid migration (Aden et al., 2021), highlighting the importance of integrating fault geometry in future 3D hydrothermal simulations.

The continuous nature of the volcanic horizons (Fig. 7) indicates prolonged volcanic activity across the Miocene-Pliocene to Quaternary (Rooney et al., 2023; Tortelli et al., 2025). This sustained magmatic history reflects a long-lived thermal and extensional regime, likely maintained by repeated intrusions of magma from the mantle and lower crust (Cattin, 2005). Geothermal drilling data support this hypothesis of continuous volcanism. Basalts dominate the stratigraphy, and

hyaloclastite deposits within the Asal unit suggest lava-water interactions (Zan et al., 1990). A clay-rich horizon capping the Stratoid series has been interpreted as a hydrothermal caprock, formed by alteration due to ascending geothermal fluids (Demange and Puvilland, 1993), likely facilitating the trapping of deep fluids and sustaining convective circulation.

Recharge zones were identified in the Stratoid unit, especially in A3 and A6 on the outer part of the rift, and high losses were encountered while drilling below depths of 1000 m. These observations suggest local permeability variations, although the relative contributions of lithologic contrasts versus fracturing remain uncertain (Zan et al., 1990). The Dalha unit, and within it, which is located on the inner part of the rift suggests very little permeability. Well tests from the Fiale geothermal field note vertical permeabilities of between 2.1 and 4.6 darcy-meters in this unit (Guelleh et al., 2022; GEOLOGICA, 2023). These spatial contrasts underscore the heterogeneous nature of the volcanic formations and the need for detailed petrophysical characterization using outcrop analogues.

The brittle-ductile transition represents a major thermal boundary within the rift, occurring at approximately 3.5 km depth in the inner rift and 5–6 km in the outer rift (Fig. 10; Doubre et al., 2007b). This transition zone coincides with the ‘hot pipe’ zone of magmatic upwelling,
where temperatures reach up to ~600 °C. This estimate aligns with borehole temperature measurements of ~360 °C at 2.1 km depth (Zan et al., 1990; Ragueh et al., 2025). Assuming a thermal gradient of ~170 °C/km, temperatures of ~600 °C are expected near 3.5 km depth. Higher temperatures (1100–1250 °C), consistent with basaltic magma melting, are anticipated at greater depths (Lee et al., 2009; Persikov et al., 2020). Petrological and geochemical analyses (Aden et al., 2025) suggest the presence of sill-like shallow magmatic reservoirs (<5 km) that are hydraulically connected to deeper crustal magma bodies between 7 and 18 km depth. Similar magmatic architectures have been documented in the Tendaho Graben and Manda-Hararo segments of the Afar Rift, as well as in Icelandic rift zones (Grandin et al., 2010; Didana et al., 2014; Caracciolo et al., 2023) including this thermal boundary as a lower boundary condition in future numerical simulations will help constrain geothermal gradients, simulate fluid circulation more realistically, and better predict reservoir locations in this magmatically active context.

Faults segment the rift into discrete structural blocks (Fig. 11), consistent with gravity and magnetotelluric data revealing abrupt density variations (Ragueh et al., 2024). This geometry aligns with orthogonal rifting processes, which generate long border faults oriented perpendicular to the extension direction and significant subsidence (Agostini et al., 2009; Corti, 2012). Beyond structural control, fault-bounded compartments strongly influence hydrothermal fluid circulation. Depending on fault geometry, continuity, and mineralized fillings, these structures can act as either conduits or barriers to flow (Curewitz and Karson, 1997; Faulds et al., 2010). In the Asal-Ghoubbet Rift, such compartmentalization appears to influence thermal regimes. Wells at Gale-le-Koma (A3, A6) show signs of active hydrothermal convection, while wells at Fiale (A5, F1, F2, F3) show mostly conductive heat transfer (Ragueh et al., 2025). These contrasts suggest that fault connectivity and structural heterogeneity govern heat and fluid dynamics within the reservoir.

The close match between modeled surfaces and input data, with most errors centered around zero (Fig. 12), supports the reliability of the 3D structural framework. Variability in errors reflects local uncertainties and limited control point density. Overall, the Discrete Smooth Interpolation (DSI) algorithm effectively represents both stratigraphic surfaces and fault planes. However, lithological information remains limited at the scale of the rift due to data scarcity. Future improvements could involve outcrop-based analogue studies (Jablonska et al., 2021) or gravimetric interpretations (Guglielmetti et al., 2013) to incorporate lithologies into an enhanced model.

This 3D structural model, built from available data, offers a robust foundation for further geothermal studies, including dynamic simulations and sensitivity analyses involving fault permeability and reservoir connectivity.

## 6. Conclusion

In this study, we developed a 3D geometric model of the Asal-Ghoubbet Rift. The principal findings are as follows.

* This is the first integrated 3D model of the Asal Rift, combining geological, geophysical, and borehole data to provide a coherent representation of stratigraphic horizons and major fault structures.

* The model reveals a consistent geometry of normal faults, suggesting a homogeneous extensional stress regime, potentially influenced by shallow magmatic processes and regional tectonics.

* The complex fault architecture and structural compartmentalization appear to influence the heterogeneous distribution of permeability and heat transfer. This is consistent with observed variations in thermal regimes within wells, likely due to local changes in conduction and convection processes.

* The continuous accumulation of volcanic units from the Miocene to the Quaternary, without major stratigraphic breaks,

indicates sustained volcanic activity that maintains the thermal regime of the system.

* The integration of the brittle-ductile transition as a thermal boundary condition allows for the estimation of the depth and temperature of the magmatic reservoir roof, enhancing the model's relevance for deep geothermal applications.

* The spatial fit between modeled surfaces and input data supports the overall consistency and reliability of the model.

* This 3D model provides a robust framework for future thermal and hydrodynamic simulations to improve understanding of fluid circulation mechanisms and support more accurate geothermal resource assessments in the rift.

* Moreover, the possibility of using this model for 3D kinematic restoration opens important prospects for reconstructing the tectonic evolution of the rift and improving the understanding of associated thermal and hydraulic processes.

## CRediT authorship contribution statement

**Asma Abdi Ali:** Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Data curation, Conceptualization. **Damien Bonté:** Writing – review & editing, Validation, Supervision, Funding acquisition. **Adriana Lemgruber-Traby:** Validation, Supervision, Software. **Christine Souque:** Supervision, Funding acquisition. **Fadi Henri Nader:** Project administration, Funding acquisition.

## Funding

This research was funded by IFP Energies Nouvelles as part of a doctoral research project within the Earth Sciences and Environmental Technologies Division, carried out by the first author.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Asma Abdi Ali reports financial support was provided by IFP Energies Nouvelles. Damien Bonte reports financial support was provided by IFP Energies Nouvelles. Adriana TRABY reports financial support was provided by IFP Energies Nouvelles. Christine Souque reports financial support was provided by IFP Energies Nouvelles. Fadi Henri Nader reports financial support was provided by IFP Energies Nouvelles. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

We would like to thank IFP Energies Nouvelles for funding this doctoral research, as well as for its contribution and support. We also express our gratitude to the Director General of ODDEG, Dr. Kayad Moussa Ahmed (Djiboutian Office for the Development of Geothermal Energy), for providing us with the data and all the internal reports necessary for the completion of this PhD. We would also like to pay tribute to Christine SOUQUE, one of the co-authors and co-supervisors of this research, who sadly passed away during the revision process of this paper. Finally, we sincerely thank the reviewers for their constructive and detailed comments, which have significantly helped to improve the clarity, quality, and scientific rigor of the manuscript.

## Data availability

The 3D geological model of the Asal-Ghoubbet Rift, deposited in the Zenodo repository as supplementary data https://zenodo.org/records/16923255?preview=1&token=..., is provided in .zmap (XYZ) format for both faults and stratigraphic horizons. These files can
be directly imported into geological modeling and visualization software.

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