The isostatic evolution and bathymetry of rifted margins depends on thinning of continental crust, the volume of magmatic additions, lithosphere thermal perturbation during rifting and its post-rift re-equilibration, and sediment loading. Additionally, at some margins, bathymetric evolution may also be affected by basin isolation, where eustatic variations are not controlled by global sea-level changes, and mantle plume dynamic uplift and its collapse. The relative influence of these contributors to rifted margin bathymetric evolution varies from example to example.

Here we investigate the parameters controlling the palaeobathymetric evolution of the Nova Scotian rifted margin during the early stages of the opening of the Central Atlantic Ocean, following Triassic rifting, salt deposition and early Jurassic continental breakup. We use a 3D flexural backstripping technique which incorporates decompaction and post-breakup reverse thermal subsidence modelling to provide palaeobathymetric predictions through the Cretaceous down to the Late Triassic base salt.

Quantitative analysis of seismic reflection and gravity anomaly data together with residual depth anomaly analyses have also been used to determine variations of crustal thickness and crustal type as well as volumes of magmatic addition emplaced during rifting and continental breakup. We show the magma-rich to magma-poor transition of the Nova Scotian margin, characterized by seaward dipping reflectors (SDRs) in the SW, while in the NE mantle is possibly exhumed.

Comparison of our palaeobathymetric predictions with seismic observations and palaeoenvironments deduced from biostratigraphy of drill samples are in good agreement over the continental shelf. As expected, discrepancies exist more distally related to salt withdrawal and sediment gravity-driven sliding. Palaeobathymetries predicted seaward, on the first oceanic crust, range from 2 to 2.5 km; values in the range of those observed at young oceanic ridges.

The oceanic crust of the SW Nova Scotian margin shows well developed sequences of SDRs. Their morphology resembles that of inner SDRs of volcanic margins like the Norwegian and Greenland margins (North Atlantic), where drilling results indicate that they correspond to lava-flows emplaced near or above sea-level. Our predicted palaeobathymetry of top SDRs at breakup is nearly ~2km deeper than the expected near sea-level. This discrepancy suggests that the subsidence of this thick oceanic crust with SDRs requires an additional mechanism in addition to post-rift thermal subsidence.

Mantle plume uplift and collapse likely occurs at volcanic margins and has a long wavelength of the order of 500 km or more. However, the subsidence discrepancy we observe has a shorter wavelength and seems focused along the nascent spreading axis. Thinning of the thick oceanic crust after SDR emplacement by oceanward lateral flow of molten and ductile lower crust is an alternative possibility and may be a common occurrence at volcanic rifted margins after continental breakup.

How to cite: Tugend, J., Kusznir, N., Mohn, G., Deptuck, M., Kendell, K., Keppie, F. D., and Morrison, N.: Palaeobathymetry and anomalous subsidence at rifted margins: Observations from the magma-rich and magma-poor Nova Scotian margin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10023, https://doi.org/10.5194/egusphere-egu23-10023, 2023.

The binary magma-rich vs. magma-poor classification of rifted margins was introduced to distinguish between margins showing markedly different crustal architectures, in particular related to the occurrence of magmatic products: the “magma-poor” qualifier is attributed to margins that display a domain of exhumed mantle and whose crustal wedge is exclusively made of continental material, while margins whose continental crust is heavily intruded and overlain by extrusive magmatic flows (e.g., seaward dipping reflections (SDRs) in seismic sections) are regarded as “magma-rich”. Yet, distinguishing between inherited continental crust, newly created magmatic crust and serpentinized mantle in seismic data is challenging due to the comparable geophysical properties (density and seismic velocity). The only interfaces that can usually be identified with some confidence on seismic images are the top of the pre-rift basement and seismic Moho, which allow the determination of the first-order crustal shape of rifted margins. We investigate what the shape of rifted margins can tell us about the timing and volume of magma emplacement during rifting. We use a simple geometric/kinematic model to explore how the volume of magma and the timing of emplacement relative to crustal thinning impact the crustal shape and discuss how this approach may help us to better interpret and understand the tectono-magmatic processes at play during rifting.

We show that crustal shape and inflection points at distal margins can be used to identify magma-poor rifted margins and the occurrence of exhumed mantle. Moreover, the crustal shape and inflection points of magma-poor rifted margins provide direct insights into the dominant processes controlling crustal thinning (e.g., pure-shear stretching, viscoplastic necking, and Coulomb controlled hyperextension) and also the delay of magma emplacement with respect to crustal thinning (e.g., inherited depleted subcontinental mantle, extension rate).

In contrast, shapes of magma-rich margins are more challenging to interpret due to the difficulty to distinguish between continental and magmatic material. We show that different factors may impact the budget and/or timing of magma emplacement and control their distinctive shape, including: (1) the initial conditions from inheritance (e.g., mantle temperature, fertility, and water content); (2) the mode of lithosphere extension (e.g., pure shear vs. depth-dependent lithosphere thinning); and (3) external rift-independent factors (e.g., elevated temperature from mantle plumes).

Crustal shapes allow us to define modes and conditions of crustal thinning at so-called magma-poor rifted margins. In contrast, to interpret crustal shapes of so-called magma-rich rifted margins and understand their tectono-magmatic evolution requires additional information such as timing and budget of magma-emplacement in the crustal wedge, paleo-bathymetry and subsidence history.

How to cite: Manatschal, G., Tomasi, S., Chenin, P., and Kusznir, N.: Linking rifted margin crustal shapes with the timing and volume of magma emplacement, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4140, https://doi.org/10.5194/egusphere-egu23-4140, 2023.

Rifted margins are often associated with widespread and thick evaporite (salt) deposits, typically formed during the latest stages of rifting, immediately prior to continental breakup. These margins are also characterized by pronounced salt tectonics, which is commonly attributed to gravity-driven salt flow and characterized by kinematically-linked domains of updip extension, translation and downdip shortening. The precise spatial and temporal links between these processes, their relative contributions and the role of rifting and rifted margin architecture on salt deposition and tectonics are still a topic of debate on many margins. We apply 2D thermo-mechanically coupled finite-element modelling of lithospheric extension to investigate the evolution of salt basins along wide rifted margins and the interplay between rifting and salt basin geometry with syn- to post-rift salt tectonics. The models use a geodynamically self-consistent approach where the geometries of the lithosphere and salt basins are not prescribed. They show that late syn-rift salt basins form as a single large basin across both conjugate margins that are later separated by continental breakup and oceanic spreading. This produces syn-depositional salt flow and stretching of the distal salt over an outer margin trough with emplacement of a syn-breakup allochthonous salt nappe over newly-formed seafloor (i.e., oceanic crust and/or exhumed mantle). The post-rift evolution is characterized by updip extension that is balanced by downdip diapir shortening, and pressure-driven nappe advance, which is largely independent of the other two processes. The results are comparable to examples from various salt-bearing rifted margins, including the South Atlantic and Gulf of Mexico, and help us understand their genesis and evolution.

How to cite: Pichel, L., Huismans, R., Gawthorpe, R., Faleide, J. I., and Theunissen, T.: The Inception and evolution of wide salt-bearing rifted margins – insights from numerical modelling and natural systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3123, https://doi.org/10.5194/egusphere-egu23-3123, 2023.

Continental rifting is often oblique to the rift axis or plate boundary, comprising many active rifts and mature rifted margins on Earth. Previous research has identified the role of vertical strike-slip and transform structures in oblique extension but has also shown that the initiation of long-distance syn-rift vertical strike-slip motion requires preexisting weaknesses. The Southern part of the Equatorial passive rifted conjugate margins is a typical example that exhibits orthogonal rift segments separating with transform faults with different lengths and orientation. We aim in this study to 1) understand the influence of these inherited weaknesses on the pattern of faulting, 2) to evaluate the consequences of oblique margin formation for rift related topography, and 3) to explore the interaction between tectonic and surface processes in the context of oblique rifting. We use most recent advances in 3-D forward geodynamic modeling coupled with surface processes. Preliminary results support the importance of inherited weak zones in shaping segmented oblique continental margins, with highly contrasting tectonic and subsidence histories in the orthogonal and transform segments. These results compare well with observations from the Equatorial passive rifted conjugate margins and provide insight into the factors that may drive the timing and magnitude of vertical motions and associated sediment flux.

How to cite: Theunissen, T., Huismans, R. S., Rouby, D., Wolf, S., and May, D.: Oblique continental rifting. Insights from 3-D forward coupled geodynamic-surface process modelling and application to the Equatorial passive margins formation., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13192, https://doi.org/10.5194/egusphere-egu23-13192, 2023.

Continental rifting is one of the four fundamental geological processes of the Wilson cycle. Rifting results from the continuous stretching of a continental mass and involves mechanical, thermodynamic, and rheological processes. It may last several tens Myrs and be followed by a catastrophic breakup stage (drifting), which determines cessation of continuous deformation and the final separation of a continent into two distinct tectonic plates that grow by accretion of oceanic lithosphere. To date, the transition to sea-floor spreading and the conditions for the development of a new ocean have not been fully understood. We present numerical experiments showing that a nonlinear viscoelastic model of the cratonic lithosphere, allowing accumulation of elastic strain over several Myrs, may explain the major features of the rift-drift transition. The model incorporates thermodynamic effects associated with viscous shearing, showing how thermal anomalies generated in the lithosphere during rifting play a major role in the break-up style. A fundamental result of the experiments is that extension is always accompanied by transverse material waves in the lithosphere, with wavelengths of the order of thousands km and periods of several tens kyrs. These waves induce an oscillating topography and could be responsible for high−frequency transgressive–regressive cycles in rift lakes. At sufficiently high extension rates, deformation localizes and these ultra-slow waves determine cyclic shear failure, with formation of X-shaped cross structures through the lithosphere that prelude to the final rupture. A comparison with the Red Sea evolution shows that onset of extension could be older than the widely accepted age of 27-30 Ma and that an older phase of uniform stretching without localization could have preceded the formation of a rift valley.

How to cite: Schettino, A. and Ranalli, G.: Ultra-slow transverse waves during continental extension: A numerical model of the rift-drift transition, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8326, https://doi.org/10.5194/egusphere-egu23-8326, 2023.

Detachment faults are developed in different tectonic settings and can record several important tectonic events, such as the rifting and breakup of continents and the spreading of mid-ocean ridges. Megacorrugation is a special structural feature of the detachment fault, characterized by gently domed, overall turtleback shape and prominent undulations of the fault surface that parallel the fault slip direction, corresponds to specific formation conditions. However, the formation mechanism of megacorrugation is still controversial.

To date, there are many controversies regarding the formation mechanism of megacorrugations. Most of these existing models come from the analysis of submarine geomorphic data and onshore field outcrops, lacking direct observation of three-dimensional structures. Therefore, the limitation of adequate datasets might be the main reason for the controversial understanding of the genesis of megacorrugations.

In this study, we finely image the detachment fault in the northern continental margin of the South China Sea using 3D seismic data. Typical megacorrugations are identified on the detachment fault surface. We find that megacorrugations are the result of the superposition of extension-parallel and extension-perpendicular uplifts, and these uplifts are successively controlled by two stages of magma during detachment fault activity. Meanwhile, several accommodation faults, as the key factor controlling the formation of megacorrugations, are discovered on the detachment fault surface for the first time. These accommodation faults control the distribution of early magma and determine the style of megacorrugations. Consequently, the megacorrugations have a formation mechanism dominated by both tectonism and multistage magmatism. This formation mechanism is consistent with the characteristics of the intermediate-type margin. The megacorrugations are the structural features of intermediate-type margins, which are different from the type of magma-poor and magma-rich margins, providing a new constraint for the classification of passive continental margins. Furthermore, we infer that accommodation faults may be widespread in the megacorrugations of mid-ocean ridges; thus, the formation mechanism proposed in this paper is likely common in megacorrugations.

References

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Gao, J. et al. The continent–ocean transition at the mid-northern margin of the South China Sea. Tectonophysics 654, 1–19 (2015).

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Zhang, C. et al. Syn-rift magmatic characteristics and evolution at a sediment-rich margin: Insights from high-resolution seismic data from the South China Sea. Gondwana Res. 91, 81–96 (2021).

How to cite: Guan, W., Huang, L., Liu, C., Wang, X., Zhang, L., and Wu, Z.: New insight for genesis of megacorrugations in detachment fault: combined control of accommodation fault and magmatism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2548, https://doi.org/10.5194/egusphere-egu23-2548, 2023.

During rifting, continental crust necks, leading to significant thickness reduction in a few tens of kilometres. However, deformations associated with the necking process remain elusive due to few outcrop examples and a lack of seismic data coverage that clearly images crustal architecture at depth. Here we use deep, high-resolution seismic data across a well-developed necking zone in the northeastern South China Sea passive margin to show the structural style associated with the crustal necking. Seismic stratigraphy in the necking domain can be divided into pre-, syn- and post-rift sequences based on the nature of sequence-bounding unconformities and their relation with faults. Seismic expression of continental crust exhibits two types of reflection characteristics – homogeneous upper crust and layered lower crust. The necking domain shows significant thinning that reduced its thickness from ~30 km to less than over 10 km in a distance of about ~50 km and is characterised by seaward removal of layered lower crust, while the homogeneous upper crust thickness remains largely unchanged in thickness. The necking domain is bounded by inner and outer breakaway complexes that define a portion of flexed crust. Crustal flexure is evidenced by progressive tilting of the necking domain that gradually increases the pre-rift sequence dip from 0° to 10°. Within the tilted necking domain, densely-spaced, landward-dipping minor faults and fractures are organised in a domino configuration, implying a top-to-the-continent movement and a simple shear deformation of the whole continental crust. We suggest that the flexed necking domain could be home to fractured reservoir providing that it is effectively sealed by post-rift sequences.

How to cite: Deng, H.: Crust necking of the northeastern South China Sea: Insights from deep seismic data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11694, https://doi.org/10.5194/egusphere-egu23-11694, 2023.

The Arabian Margin experienced intense volcanism over the last 10 Ma, including volcanic eruptions as recent as 600 years ago. What is more, two earthquakes with magnitude > 5 have been recently reported with normal faulting along the Arabian Margin, suggesting that the Arabian Margin is undergoing active deformation. Due to the limited number of GPS stations within the Arabian plate, investigating the intraplate deformation was challenging. A new set of GPS data with 87 stations is used in this work to investigate the Arabian margin rigidity and intraplate deformation (Aldaajani et al., 2021). This new GPS velocities show higher residuals along the Arabian margin that produces dilatational strain rate pattern within the Arabian margin, in the vicinity of the Makkah-Madinah Transtensional zone. The causes of these GPS residuals along the Arabian Margin are unknown. In this work, we use the finite element modeling approach to highlight the mechanical deformation processes along the Arabian margin and test their driving forces. These candidate forces are related either to the edge forces as introduced by the Red Sea rift, the Arabian Margin interior forces as introduced by calculating the Gravitational Potential Energy, or the basal tractions as driven by sub-lithospheric topography and mantle flow. Our results indicate that the GPS residuals are not likely linked with the Gravitational Potential Energy forces. Instead, the basal tractions along an asthenospheric channel, which aligns geographically with the Makkah Madinah Volcanic Line, is the potential driving force for the observed deformation along the Arabian margin.

How to cite: Aldaajani, T. and Furlong, K.: On the driving forces of the rifting processes along the Makkah-Madinah Transform Zone, Western Arabia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17406, https://doi.org/10.5194/egusphere-egu23-17406, 2023.

A consensus has emerged over the past two decades that significant extension at crustal depths in the northern East African Rift is achieved not by ductile stretching but by magma intrusion. The implications of this for crustal structure and Moho architecture have all been the focus of intense study. East Africa's deep convecting mantle has also been the focus of intense research, with most workers now accepting of the super-plume model over traditional 'Morgan' plumes (albeit with some ongoing discussion concerning the precise internal architecture of the superplume).  In contrast, our understanding of East Africa's lithospheric mantle and, in particular, the depth to the lithosphere-asthenosphere boundary (LAB), remains remarkably poor.  For example, some studies have postulated that no lithospheric mantle exists below large parts of Afar and the Ethiopian rift where magma-assisted rifting is now underway; others have argued to the contrary, asserting that a melt-rich lithospheric mantle is essential to explain first order observations including mantle seismic anisotropy, and the depth at which melts last re-equilibrated with the mantle prior to eruption. Here we will review some of the seismological and petrological evidence that has featured in this debate, including critically assessing the efficacy of different seismological techniques for determining LAB depth in magmatic versus non-magmatic sectors of the EAR.  We show that petrology contributes strongly to the EAR LAB debate, with the added benefit that it allows the assessment of plate thickness through time.  Finally, we look to recent observations from the Turkana Depression, where a lithosphere thinned during multiple, superposed episodes of rifting, offers the chance to assess lithosphere-asthenosphere interactions in more detail than can be achieved elsewhere along the rift.

How to cite: Bastow, I. and Rooney, T.: East Africa's elusive LAB, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10156, https://doi.org/10.5194/egusphere-egu23-10156, 2023.

How to cite: Michon, L., Famin, V., and Quidelleur, X.: Evolution of the East African Rift System from trap-scale to plate-scale rifting, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-961, https://doi.org/10.5194/egusphere-egu23-961, 2023.

Continental rifting is currently active in East Africa, where breakup of the African continent is generally occurring in relatively focused rift zones within two uplifted plateaus, with magma intrusions the primary mechanism for strain accommodation throughout the crust and mantle lithosphere. Linking the two narrow rift valleys is the low-lying, and as-yet poorly studied Turkana Depression - an unusually broad 300km-wide region of diffuse faulting, seismicity and magmatism. How the East African Rift has developed here remains elusive and is complicated by the fact the Depression was variably stretched by several superposed episodes of failed rifting since the Mesozoic.

Utilising data from the NSF-NERC-funded TRAILS seismic network, we produce the first detailed crustal and uppermost-mantle shear-wave velocity model below the Turkana Depression, illuminating Moho and lithosphere-asthenosphere boundary topography that ultimately shed light on rift development in a multiply-rifted region. We find Turkana’s lithosphere is relatively melt-poor, unlike the Ethiopian rift and Plateau further north, which have undergone extensive lithospheric modification by voluminous Cenozoic flood-basalt magmatism and magma-assisted rifting. The lower crust below rift zones in Turkana is not associated with markedly slow (melt) or fast (cooled gabbroic intrusions) wavespeeds suggesting magmatic extension has not dominated rift development in Turkana. Throughout the Depression, the thinnest crust resides within failed Mesozoic rift zones which the present-day East African Rift appears to circumnavigate, not exploit. Fast uppermost mantle wavespeeds below the thinnest crustal regions indicate post-Mesozoic rifting, re-equilibrated and possibly melt-depleted mantle lithosphere, which now renders the plate stronger and more refractory than regions not previously rifted. Refractory Proterozoic lithosphere also present in southern Ethiopia may have influenced strain localisation and the broad, complex rift zone between Ethiopia and Kenya.

How to cite: Kounoudis, R., Bastow, I., Ebinger, C., Darbyshire, F., Musila, M., Ogden, C., Ayele, A., Bendick, R., Sullivan, G., Ugo, F., Mariita, N., and Kianji, G.: Seismic Imaging of Heterogeneous Lithosphere Beneath the Unusually Broad Turkana Depression, East Africa, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12655, https://doi.org/10.5194/egusphere-egu23-12655, 2023.

Traditionally interpretations assume that as magma-rich rift settings mature, the magmatism accommodates greater amounts of extension at the expense of mechanical deformation. However, the importance of faulting in the final stages of magma-rich rifting remains poorly constrained, with the data (e.g. structural geological mapping, seismic reflection and borehole data) from rifts near to break-up a rarity. The Danakil Depression (Northern Afar), is undergoing the final stages of continental break-up, thus providing the ideal natural laboratory to conduct high resolution, quantitative analysis on the architecture, extension and subsidence facilitated by faulting in an active rift setting before seafloor spreading initiates. >500 rift axis faults were identified using remote sensing data (satellite imagery, DEMs), with quantitative analysis showing an increase in fault density, length and connectivity away from magmatic segments. Kinematic and earthquake focal mechanism data demonstrate a transition from transtensional opening in the northern and central sub-regions of the rift to oblique opening in the southern Giulietti Plain and Tat-Ali sub-regions of the Danakil Depression. The oblique opening is attributed to the along-axis step between the Erta-Ale and Harak sub-regions. Integration of seismic reflection and borehole data with the mapped faults shows that extension is primarily accommodated by magmatism within the rift center, with faulting more significant towards the ends of the rift. ~30% of crustal extension is accommodated by axial faulting in areas of low magmatism, highlighting the importance of faulting even in the final stages of magma-rich rifting. Comparing our findings with spreading ridge morphology and structure, which is relevant due to the rift maturity and extensive magmatism present, we conclude that the Danakil Depression is in a transitional stage between continental rifting and seafloor spreading. Spatial changes in the importance of faulting and magmatism in accommodating extension, alongside rift morphology, resemble the relationships observed along spreading ridges. From our observations we have shown that axial faulting still plays a vital role in the final stages of break-up despite the increased importance of magmatism.

How to cite: Hurman, G., Keir, D., Bull, J., McNeill, L., Booth, A., and Bastow, I.: The role of rift axis faulting in the final stages of magma-rich rifting: the Danakil Depression, Afar, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14948, https://doi.org/10.5194/egusphere-egu23-14948, 2023.

The Afar depression at the northern end of the East African Rift system is the only analog on earth where magmatic continental rifting and associated ongoing break-up processes are exposed onshore. This unique active system presents the key advantage to expose extensional structures related to ocean-continent transition, with magmatic rift segments characterized by contrasted morphologies, and magmato-tectonic styles. The main goal of this study is to identify the location and investigate the functioning and persistence of magma reservoirs at the active magmatic segments in the central Afar depression (Manda Hararo, northern Tendaho grabben), in order to (i) highlight their relationships and potential control with the first- and second-order local segmentation, and (ii) understand the interplay between magmatic and tectonic processes during the generation of such magmatic crust. We combine remote sensing, field investigations, precise and comprehensive mapping of volcanic and tectonic structures, cosmogenic (³⁶Cl) exposure dating of lava surfaces, and geochemical analysis to constrain the temporal frame and the dynamics of magmatic and tectonic processes. The first result of remote sensing analysis allows us to identify two active and self-consistent axial rift subsegments within this extensional system, map detailed lava flow fields which form these segment surfaces and investigate their relationships with caldera formation and focussed fissural activity. Geochemical analysis and dating of lava flows from this Manda Hararo rift system will be conducted to test the integrity of this model of contiguous subsegments.

How to cite: Birhane, Y. G., Pik, R., Bellahsen, N., France, L., Flahaut, J., Schimmelpfennig, I., Ayalew, D., and Yirgu, G.: Detailed Architecture of the Manda Hararo Magmatic Segment in Afar, Ethiopia:, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3935, https://doi.org/10.5194/egusphere-egu23-3935, 2023.

The East African Rift (EAR) is a large opening system that allows the observation of all stages of rift evolution from continental opening in the south to oceanization in the north (Ethiopia-Djibouti). Also, the Asal–Ghoubbet active rift, in the Republic of Djibouti, is composed of a magmatic crust and tends to evolve into an oceanic crust. It’s a site of interest for geothermal energy and natural hydrogen. Previous studies have indicated that dihydrogen (H₂) emanates from this rift. However, the well-known serpentinization reaction is not the mechanism generating H₂ at this site. Rather, the H₂ is generated as follows: (1) by alteration of basaltic lava at depth via reaction with seawater flowing from Ghoubbet Bay towards Lake Asal; (2) by simple degassing of the volcanic chamber located a few kilometers below the Fiale Caldera in the rift axis; or (3) as a result of pyritization processes via the oxidation of H₂S.

Drill cuttings from the Fiale 1 (F1) and Gale le Goma 1 (Glc1) geothermal wells (located on the inner and outer rift margins, respectively) were analyzed to determine where H₂ is generated. Total rock analyses indicated distinct zones at depths of 464 m and 280 m for F1 and Glc1, respectively, representing the boundary between the Asal and Stratoïd Basalts. ⁵⁷Fe Mössbauer analyses show a decrease in the percentage of Fe³⁺ at depth, indicating that Fe²⁺-rich material, particularly in the Stratoïd Basalts, may be a source of H₂.

Based on well data from the rift center and the outer rift margin, it is evident that H₂ is present at the surface in the rift axis and that this area offers good remnant potential because of the presence of Fe-rich chlorite. Conversely, few H₂ emissions were measured at the surface on the outer rift margins, although well data showed some H₂ (~0.25%) at depth. The presence of a cap rock in the rift axis has not yet been proven; however, the high loss on ignition and the mineralogy in well Glc1 may indicate that the rocks are sufficiently altered to offer potential as a seal. If so, the rift margins would offer greater exploration potential than the rift center.

How to cite: Pasquet, G., Duttine, M., and Moretti, I.: Early onshore basaltic alteration and its natural hydrogen potential in the Asal–Ghoubbet rift, Republic of Djibouti., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2867, https://doi.org/10.5194/egusphere-egu23-2867, 2023.

Active continental rifting in Ethiopia has led to formation of numerous volcanoes and geothermal systems with associated socio-economic potential for generating clean energy.

Aluto and Corbetti are two silicic volcanoes in the Central Main Ethiopian Rift (CMER) that have been closely examined. Past studies provided insights into their formation in the extensional magma-tectonic context of the CMER, into causes of volcanic unrest and surface deformation and seismic activity, as well as their geothermal systems. However, many aspects about the structure of the volcanoes’ underlying transcrustal magmatic system remained unanswered.

Here, we present new 3-D electrical conductivity models of these volcanoes, obtained from inversions of magnetotelluric (MT) data, providing the most detailed images of the associated magmatic and geothermal systems across multiple scales so far.

The models from Aluto and Corbetti provide evidence for several hypothesized properties of the associated magmatic systems. The cross-rift model, enclosing Aluto, shows that the volcano’s lower crustal melt source, west of the rift axis, also feeds volcanos in the western part of the rift, which has been debated in the past.  Our Corbetti model confirms the existence of a shallow magmatic intrusion, as it has been modelled from InSAR and gravimetry studies.

We estimate thermodynamically constrained melt fractions and interpret geothermal flow structures. The inferred melt fractions indicate crystalline magmatic mush systems in rheological lock-up, where melt is extracted slowly through buoyancy processes, while mechanical trapping explains the observed compositional gaps.

How to cite: Dambly, L., Samrock, F., Grayver, A., and Saar, M.: Unraveling the Transcrustal Magmatic Mush and Geothermal Systems of Aluto and Corbetti Volcano in the Main Ethiopian Rift using Magnetotellurics , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7681, https://doi.org/10.5194/egusphere-egu23-7681, 2023.

In rift basins, the spatial arrangement of extensional faults can influence the facies and the thickness distribution of the syn- and post-sedimentary infill, which can harbour good potential for geothermal systems. In this framework, unravelling the tectono-stratigraphic evolution of a rift basin is decisive, as it can influence one of the key parameters for planning geothermal doublets: aquifer thickness.

In our study, the West Netherlands Basin, located in one of the Netherlands most densely populated areas, is used as a case study. Up to 2022, 14 geothermal doublets were realized in the area, with the main target being the syn-rift deposits of the Late Jurassic Nieuwerkerk Formation. As a NW-SE  oriented transtensional basin, the West Netherlands Basin developed as consequence of Mesozoic extensional tectonics, after which it became inverted during the Late Cretaceous and Cenozoic. Using publicly available seismic 3D and well data, our renewed interpretation of the study area shows two important rift events. The first one during the Early-Mid Jurassic and the second one, partly controlled by structures of the former, during the Late Jurassic, coinciding with the deposition of the Nieuwerkerk Formation.

Our study adds to the understanding of a multiple stage rifting history in the West Netherlands Basin. This is important, as the process influences reservoir thicknesses and with that, the amount of MW that can be extracted from geothermal aquifers. Therefore, this study forms a bridge between providing an integrated picture of the West Netherlands Basin and how the basins geological history affects its geothermal resources.

How to cite: Weert, A., Vinci, F., Ogata, K., Amory, J., and Tavani, S.: How multiple stage rifting influences the planning of geothermal systems: a case study from the West Netherlands Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-291, https://doi.org/10.5194/egusphere-egu23-291, 2023.