# Volcanic passive margins and break-up processes in the Southern Red Sea

François Chauvet, Laurent Geoffroy, Bernard Le Gall, Marion Jaud

► To cite this version:

François Chauvet, Laurent Geoffroy, Bernard Le Gall, Marion Jaud. Volcanic passive margins and break-up processes in the Southern Red Sea. Gondwana Research, 2023, 117, pp.169-193. 10.1016/j.gr.2023.01.004. hal-05268835

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https://hal.univ-brest.fr/hal-05268835v1

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1 Volcanic passive margins and break-up processes in the Southern Red Sea

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3 François Chauvet¹, Laurent Geoffroy¹, Bernard Le Gall¹ and Marion Jaud¹

4 1 : Geo-Ocean, Univ Brest, CNRS, Ifremer, UMR6538, F-29280 Plouzané, France

5 francois.chauvet@univ-brest.fr (corresponding author) ; laurent.geoffroy@univ-brest.fr; blegall@univ-brest.fr; marion.jaud@univ-brest.fr

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9 Abstract

10 We have maped Oligocene to Quaternary volcanic sequences, emplaced across the Afar
11 magmatic province (AMP: Ethiopia, Eritrea, Djibouti, Yemen), by using 50 cm resolution Pleiades
12 satellite images. Systematic and precise measurements of flow sequences between Djibouti and SW
13 Yemen focus on variations in lava dips and dip directions. From these results, we are able to infer
14 the volcano-tectonic architecture of the upper crust across the southern part of the Afar magmatic
15 province and the evolution of crustal deformation over time. Our results provide evidence that post-
16 trap volcanism within the Afar area was emplaced during the stretching of continental crust, mostly
17 as seaward dipping reflector (SDR) -type volcanic wedges, commonly observed along volcanic
18 passive margins. Compilation of our results with published ages and structures of volcanic
19 sequences in western Afar and Djibouti allows us to map these SDR wedges along a 700 km-long
20 crustal cross-section between the Ethiopia and SW Yemen escarpments. During the Late Oligocene
21 to Lower Miocene, Inner SDR-type wedges developed during the formation of two crustal necking
22 zones on either sides of an uplifted and weakly deformed crustal Danakil C-Block. Following this
23 early extensional event, the C-Block was further stretched and thinned during the development of
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# 1. Introduction

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Magmatic breakup in Phanerozoic large igneous provinces (LIPs) generally occurs in a two-stage process (Courtillot et al., 1999; Geoffroy, 2005): a trap stage, that often follows an initial a-magmatic extension of the continental lithosphere (Guan et al., 2021), and a break-up stage, with the formation of volcanic passive margins (VPMs, Fig. 1a).

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Traps or plateau basalts correspond to a thick accumulation of flat-lying and low-viscosity mafic flows, emplaced within short time-lapses across wide areas (White and McKenzie, 1989). The sub-horizontal attitude of these lava piles attests that the continental crust recorded no significant extension during their emplacement, apart from the dilation associated with coeval emplacement of intrusive complexes through the crust (Klausen and Larsen, 2002; Geoffroy, 2005, Geoffroy et al., 2007).

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In contrast, the faulted structure, displayed by extrusive sequences at VPMs, implies that the crustal basement experiences marked tectonic stretching and thinning during magma emplacement (Fig. 1a). A common structural characteristic of VPMs is the presence of seaward dipping reflector (SDR) sequences that can form up to 20 km-thick wedges of dominantly mafic lava flows (Hinz, 1981; Planke et al., 2000; Stica et al., 2014; Geoffroy et al., 2015). In VPMs, the SDRs that form in an inner position record the early necking stage of the continental crust (Stica et al., 2014; Geoffroy et al.,
2015; Fig. 1). Though their internal architecture has been related to magma loading-related flexural mechanisms (Mutter et al., 1982; Klausen and Larsen, 2002; Buck, 2017; Morgan and Watts, 2018), the recognition of continentward-dipping listric detachment faults in onshore-exposed SDRs and in deep seismic profiles conversely demonstrates the importance of syn-magmatic tectonic extension during the formation of SDRs (Planke et al., 2000; Geoffroy, 2005; Clerc et al., 2015; Geoffroy et al., 2001; 2015; Chauvet et al., 2019; 2021). At the very least, Inner SDRs can be confidently considered as syn-rift units developed during pre-breakup crustal thinning and stretching stages of rifting, in stark contrast to the pre-rift nature of traps (Fig. 1). Outer SDRs have a distinct geometry (Franke et al., 2010; McDermott et al., 2018; Geoffroy et al., 2020, 2022; Chauvet et al., 2021, Fig. 1) and develop at a more mature stage in a distal transitional domain. Less deformed and flat-lying to gently dipping volcanic or volcano-sedimentary sequences may also be found above failed rifts and in the most distal parts of VPMs (Fig. 1; Franke et al., 2010; Soto et al., 2011; Geoffroy et al., 2022). The passive exhumation below a low-angle detachment of magma-intruded middle to lower continental crust, has been proposed below the Outer SDR and flat-lying flow sequences (Geoffroy et al., 2022; Fig. 1b). The subaerial origin of the SDR infilling sequences (Roberts et al., 1984; Eldholm et al., 1989; Planke et al., 2000), including the most distal types (Geoffroy et al., 2022), indicates that, during the development of VPMs, the sub-basaltic lithosphere rises buoyantly above a continuously melting upper mantle.

The various types of volcanic architectures mentioned above (traps, Inner and Outer SDRs, flat-lying flows) can therefore be used as diagnostical features for revealing the evolution of volcano-tectonic processes, leading up to the final continental break-up and onset of oceanic lithosphere (Geoffroy et al., 2022; Fig. 1b). We apply this approach to the ongoing break-up within the Afar Magmatic Province (AMP, Fig. 2a). In this study, the term AMP encompasses the flood volcanism that covers the Ethiopia and Yemen plateaus, as well as all younger sequences across
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Ethiopia, Eritrea, Somalia, Yemen and Djibouti. In the AMP, nascent volcanic passive margins are thought to have developed across an extensive crustal domain - the Afar Depression - superimposed on an initial trap volcanic province, and currently dissected by a triple system of converging accretionary axes (Figs. 2a, b; Tesfaye et al., 2003; Wolfenden et al., 2004; 2005; Stab et al., 2016).

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Past tectonic interpretations of the AMP have chiefly concerned its western and northern parts. Notably, the concept of a Western Afar Margin is based on the recognition of regional-scale SDR-like volcanic wedges in Ethiopia (Wolfenden et al., 2005). Initiation of incipient and symmetric VPMs in SE Afar (Djibouti) has also been assessed, but at a more local scale and from specific work arguing for crustal flexuring along the Makarassou structure (Le Gall et al., 2011) or from observations of SDR-type wedges along the northern edge of the Asal rift axis (Geoffroy et al., 2014). SDRs have been documented further east from offshore seismic imagery, recorded both along the western edge of the Gulf of Aden (Fig. 2c; Tard et al., 1991; Leroy et al., 2012) and along the southeastern edge of the Red Sea (N Tihama Plain in Fig. 2c; Davison et al., 1994; Ahmed et al., 2013), i.e., where successful continental break-up has indeed occured.

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It thus appears that marked syn-magmatic continental stretching and thinning occurred throughout the AMP in post-trap times, where, until now, no synthetic overview exists on the spatio-temporal development of continental rifting and incipient VPMs development across the entire Afar system. With regards to available exposures, this is chiefly due to a lack of accurate structural data on the internal architecture of volcanic sequences in SE Afar, i.e., in Djibouti and SW Yemen.

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The aim of the present paper is to fill this gap by investigating the upper crustal structure in SE Afar along a NE-SW-oriented transect, orthogonal to the regional structures, which extends from the SE Afar Depression in Djibouti, across the southern Danakil Block, and farther onto SW Yemen (Fig. 2c). This work is based on the interpretation of high-resolution Pleiades satellite imagery (with

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a 0.5 m resolution) showing in detail the 3D-structure of tilted lava sequences and related fault
populations in until now poorly investigated or currently inaccessible (Yemen) areas. Additionally,
these three study domains provide a unique opportunity to encompass the entire AMP volcanic
succession from the initial Oligocene traps to the Quaternary series. Emphasis has been placed on
lava tilt patterns and fault dip directions in order to identify eventual switches in polarities over
time. One major result of our work is to document indisputable SDR volcanic prisms, formed at
successive stages and locations, from the Miocene to the Quaternary period, and further regarded
as strong supportive evidence for a complex and unstable system of VPMs in SE Afar.

Once combined to partly revised published models on the western Afar, the new results
presented here allow us to elaborate on the first complete upper crustal transect through the entire
Afar system. By integrating available crustal thickness data, we propose a crustal-scale evolutionary
model, illustrating a new concept of break-up and plate separation within the Afar Triangle, via the
formation of paired VPMs and the coeval dismantling of a central continental block (Afar and Danakil
proto-crust).

## 2. Geological setting

We hereafter summarise the onshore geological context of the Afar system, with special
attention paid to the correlation of age and facies between the different post-trap magmatic
formations.

### 2.1. General overview

As mentioned, the AMP is located at the triple junction of the Red Sea, the Gulf of Aden and
the East African Rift (Fig. 2, McKenzie et al., 1970; Barberi and Varet, 1977; Tesfaye et al., 2003). The
three divergent axes have accommodated differential motions of the Nubia, Somalia and Arabia
plates since the Early Oligocene (Fig. 2b, e.g., Reilinger and McClusky, 2011). The AMP-related magmatic activity spans the past 31 Ma and should result from a regional sub-lithospheric thermal anomaly (e.g., Schilling et al., 1992; Burke, 1996; George et al., 1998; Furman et al., 2006; Rogers, 2006) associated with a regional dome-like uplift (Avni et al., 2012). The corresponding volcanic sequences currently cover the Ethiopia-Eritrea and Yemen plateaus, the Afar Depression, the Danakil Block and the southern Red Sea-western Gulf of Aden margins (Fig. 2a).

Continental rifting along the Red Sea and Aden axes started at about 29-26 Ma (Menzies et al., 2001; Wolfenden et al., 2005), immediately following the main phase of trap emplacement at ~30 Ma (Hofmann et al., 1997) (Fig. 2c). Offshore surveys suggest that in the Gulf of Aden continental break-up and nascent oceanic spreading operated at ~17 Ma, east of the Shukra El Sheik fracture zone (Fig. 2a), and was delayed at ~10 Ma to the West (Leroy et al., 2012). Since 2-3 Ma, the Gulf of Aden break-up propagates westward into Afar through the Gulf of Tadjoura (Figs. 2b, c; Audin et al., 2004; Daoud et al., 2011; Leroy et al., 2012). According to Wolfenden et al. (2004), the extensional stage in Southern Afar would have started lately (~11 Ma), coeval to the northern Main Ethiopian Rift. This contrasts with Tesfaye et al. (2003) who suggested a simultaneous development of the three rifts during the initial phase of tectonic activity at ~25 Ma. The age of continental breakup and extent of oceanic crust along the Red Sea is more controversial, whereas its age is inferred to be either 5 Ma (Roeser, 1975; Cochran, 1983, 2005), or 13 Ma (Augustin et al., 2021; Fig. 2a).

The triangular Afar Depression, with a mean elevation of 450 m (Fig. 2b), is mostly floored by Plio-Quaternary flood basalts (Fig. 2a), with Late Oligocene-Miocene volcanics exposed along its borders (Fig. 2c; Barberi et al., 1975; Beyene and Abdelsalam, 2005; Wolfenden et al., 2005; Stab et al., 2016). To the East, the Danakil Block includes uplifted Precambrian basement rocks overlain by Mesozoic sediments to the north (Fig. 2c, Morton and Black, 1975) and by Miocene to Quaternary
144 volcanics to the south (Figs. 2c, 3; Gadalia and Varet, 1983; Le Gall et al., 2015). The birth of the Afar Depression is most generally linked to a counterclockwise rotation of the Danakil microplate during 145
146 Oligo-Miocene times (Burek, 1970; Sichler, 1980; Eagles et al., 2002; McClusky et al., 2010).

147 The nature of the crust within the Afar Depression is still an open issue. Crustal thicknesses 148 decrease from 45-35 km beneath the Ethiopia-Yemen bounding plateaus to 25-20 km inside the 149 depression, with minimum values of 18-14 km below the active rifts (Berckhemer et al., 1975; Tiberi 150 et al., 2005; Stuart et al., 2006; Dugda et al., 2006; Hammond et al., 2011; Reed et al., 2014, Ahmed 151 et al., 2022). While this lower crust must include a significant proportion of mafic intrusions to 152 generate its high Vp values (6.7 to 7.2 km.s⁻¹; Hammond et al., 2011), it is debatable how much 153 extended continental crust it may contain, if any. Similar velocities are frequent in the lower crusts 154 of volcanic continental rifts (Thybo and Nielsen, 2009), continental LIPs (Thybo and Artemieva, 2013; 155 Ridley and Richards, 2010), Icelandic-type lower crust (Foulger et al., 2003) as well as along VPMs 156 (e.g., Funck et al., 2016).

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158 **2.2. Geological formations**

159 **2.2.1. Pre-volcanic units**

160 Earliest AMP basalts were directly emplaced onto the Arabian-Nubian Precambrian 161 metamorphic basement (Windley et al., 1996) and locally on top of a thin sequence of Late Paleozoic 162 and a thick succession of Mesozoic to Paleogene sedimentary rocks (Fig. 2a; Davison et al., 1994; 163 Beydoun et al., 1996; Avni et al., 2012). Jurassic to Cretaceous sedimentary rocks infilled NW-SE to 164 W-E trending basins, which developed during two rifting stages (Upper Jurassic and Lower 165 Cretaceous; Beydoun et al., 1996; Ellis et al., 1996; Gani et al., 2009). Paleogene deposits consist of 166 thin shallow marine to fluviatile sandstones, lacustrine deposits and lateritic paleosols that are
conformably overlain by Early Oligocene volcanic traps (Menzies et al., 1990; Al-Subbary et al., 1998; Zanettin et al., 2006; Abbate et al., 2014).

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## 2.2.2. Volcanic traps

The 2 km-thick Early Oligocene traps, exposed on conjugate Ethiopia and Yemen plateaus, consist of basal fissural basalts, which are in turn overlain by an equal-thickness but more silicic member (massive rhyolitic ignimbrites, tuffs) and an uppermost bimodal basaltic/rhyolitic sequence (Mohr and Zanettin, 1988; Chiesa et al., 1989; Hofmann et al., 1997). The lower basaltic member yielded 31-30 Ma $^{40}$Ar/$^{39}$Ar ages (Baker et al., 1996; Hofmann et al., 1997; Ukstins et al., 2002), while the upper silicic sequence shows a net decrease in eruption rates that spanned a period of 30-27 Ma (Riisager et al., 2005; Zumbo et al., 1995a; Fig. 4).

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## 2.2.3. Post-trap magmatism in Yemen

Post-trap volcanic series are poorly preserved throughout Yemen (Fig. 2c, Manetti et al., 1991; Baker et al., 1997) except for Miocene and Plio-Quaternary shield volcanoes that locally cover Early Oligocene traps within the south-western part of the Yemen Plateau. This is in accordance with apatite fission track data that indicate rapid cooling due to erosional unroofing of the Yemen basement between 25 and 16 Ma (Menzies et al., 1992, 1997).

North of the plateau, a trachytic flow overlying 27 Ma-old tilted basalts yielded a $^{40}$Ar/$^{39}$Ar age of 19.9 Ma (Fig. 4, Baker et al., 1996). Along the Tihama Plain (Jabal An Nar; Fig. 2c), a fault-rotated rhyolitic succession, dated at 18.4 Ma (K/Ar age in Huchon et al., 1991), is unconformably overlain by a flat-lying and 50 m-thick Upper Miocene basaltic sequence that is 10 Ma-old (K/Ar method, Capaldi et al., 1987a; Fig. 4). Offshore S Yemen, a 16.7 ± 1.5 Ma-old (K/Ar age) basalt was drilled at the top of the SDR wedge (Fig. 2c; Tard et al., 1991) and is in agreement with a Lower to Middle
191 Miocene age of the earliest sedimentary rocks overlying the SDRs in the N Tihama Plain (Fig. 2c; El-
192 Anbaawy et al., 1992; Davison et al., 1994; Ahmed et al., 2013).

193 Despite of an apparent low volume of volcanic products preserved on SW Yemen, widespread
194 intrusive activity occurred between Late Oligocene to Lower Miocene (Fig. 4). In S Yemen, gabbro
195 and syenite plutons are dated at 22-21 Ma, whereas surrounding dyke swarms intruding both
196 Precambrian basement and the Mesozoic sedimentary basins yield $^{40}$Ar/$^{39}$Ar age clusters at ca. 25.5
197 Ma and 16-18.5 Ma (Zumbo et al., 1995a; Fig. 4). A 27-21 Ma-old dyking event is also documented
198 along the Tihama Plain and further north, along the Red Sea's eastern margin (Chiesa et al., 1983;
199 Capaldi et al., 1987b; Mohr, 1991; Sebai et al., 1991; Féraud et al., 1991).

200 The large granite intrusions along the Yemen Great Escarpment, intruding Mesozoic
201 sedimentary rocks and the base of the traps (Fig. 2c, Capaldi et al., 1987b; Chazot and Bertrand,
202 1995), are coeval with ~E-W crustal extension and crustal necking (Geoffroy et al., 1998). Their ages
203 range between 20-28 Ma (K/Ar method; Civetta et al., 1978; Capaldi et al., 1987b; Fig. 4). The 150
204 km-long Tihama Asir Complex, north of the Tihama Plain (Fig. 2b), consists of 24-20 Ma-old layered
205 gabbros and granophyres (K/Ar; Coleman et al., 1972), as well as dyke swarms emplaced in a NW-
206 SE direction (Stern and Johnson, 2019).

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2.2.4. Post-trap magmatism in Afar

209 Most post-trap volcanic activity concentrated inside the Afar Depression, on the Danakil Block
210 and along the southern Red Sea and western Gulf of Aden (Fig. 2c). In Ethiopia (W Afar), Late
211 Oligocene post-trap volcanic rocks (namely Dese basalts, Kemise and Dessie rhyolites, Fig. 4)
212 erupted between 26.5-24.6 Ma ($^{40}$Ar/$^{39}$Ar, Ukstins et al., 2002; in Wolfenden et al., 2005), are
213 confined within marginal grabens that run along the Great Escarpment's border faults and are
214 interpreted as syn-tectonic (Wolfenden et al., 2005; Tesfaye and Ghebreab, 2013; Fig. 2c). To the
east and south-east of that Great Escarpment, 20-10 Ma Miocene series (Burka, Aneno, and Bercha Formations) are dominated by ignimbrites and rhyolites with local basaltic intercalations (Wolfenden et al., 2005). These sequences display several angular unconformities and consistently dip towards the E and NE (Fig. 4; Wolfenden et al., 2005). According to mentioned authors, these Miocene sequences would constitute a first-order SDR-like volcanic wedge. The overlying Plio-Quaternary sequences, crosscut by numerous dikes rotated eastwards, likewise constitute riftward-dipping wedges, analogous to SDRs.

Further north and north-east, Stab et al. (2016) observed no internal unconformities within the Chifra and Finto Miocene series (cross-section G in Fig. 2c). However, the overlying Pliocene (5-4 Ma) Sullu Adu basalts were deposited as horizontal flows over tilted Miocene series (dip up to 30° NE) and underlying Early Oligocene traps (up to 60° NE; Figs. 1c, 3; Stab et al., 2016). Stab et al. (2016) suggest that both the Late Oligocene and Miocene volcanic formations were passively tilted during two main phases of stretching at 25 Ma and 7-5 Ma.

In the SE Afar, in Djibouti (Fig. 3a), the oldest volcanic activity is recorded by the Ali Sabieh complex that comprises of intrusive mafic bodies (laccolith, dykes and sills) and basaltic flows, which, between 28 and 20 Ma, were emplaced on top of a Mesozoic sedimentary basement (Figs. 1c, 2a, 3; Gasse et al., 1986; Audin et al., 2004; Le Gall et al., 2010). No evidence exists for any initial trap-like volcanism within this area (Barberi et al., 1975; Barberi and Varet, 1977). An overlying, acidic and Miocene Mabla Fm. is composed of massive rhyolitic domes and associated flows, as well as ignimbrites intercalated with basalts (Figs. 2a, 3). This volcanism was mainly fissural (Barberi and Santacroce, 1980; Gadalia and Varet, 1983). It is bracketed between 19 Ma and 9 Ma (Gasse et al., 1986; Barberi et al., 1975) and was thereby coeval with the Finto and Chifra Miocene series in W Afar (Stab et al., 2016; Fig. 4). The Mabla Fm. is more widely exposed across the southern Danakil Block, north of the Tadjoura Gulf (Fig. 3) and is disrupted by a sub-meridian network of extensional
239 faults that mostly pre-dated the overlying Dahla basaltic series (Gasse et al., 1985; Gaulier and Huchon, 1991; Vellutini et al., 1995a-b). Within this area, no lava flows older than 16 Ma have been documented and the early rift history of the southern Danakil Block therefore remains undocumented (e.g., Zumbo et al., 1995b).

243 Post-Mabla volcanic series consist of late Miocene to Quaternary basaltic sequences (Dahla and Stratoïd Fms., Fig. 3a) that thicken dramatically westwards from the western edge of the South Danakil Block and into the Afar Depression, where lava piles reach thicknesses of up to 800 m and 1300 m, respectively (Le Gall et al., 2018a). Rhyolitic to trachytic intercalations within the upper part of the Stratoïd Fm. (Fig. 3a) are genetically linked to silicic volcanic centers (Barberi and Santacroce, 1980; Lahitte et al., 2003a). In the SE Afar area, a thin Upper Stratoïd Fm. discordantly overlies a Lower Stratoïd Fm., which is locally tilted toward the NE (Geoffroy et al., 2014). The age of the Dahla and Stratoïd basalts are bracketed within 9-4 Ma and 4-1 Ma, respectively (Fig. 4; Kidane et al., 2003; Lahitte et al., 2003b). Although coeval with the Dahla Fm., Sullu Adu basalts of Central Afar have also been correlated with the Stratoïd Fm. (Figs. 2c, 4; Stab et al., 2016).

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## 2.3. Structure and deformation of the SE Afar volcanic floor

255 The Afar Depression is floored by a discordant upper Stratoïd formation (> 1 Ma-old) that is dissected by a complex and dense fault network, which also shapes the landscape morphology (Figs. 2c, 3; Lahitte et al., 2003b; Geoffroy et al., 2014). This deformation has been assigned to the mechanical interaction between a pair of segmented Ghoubbet-Asal-Manda Inakir (GAMI) and Manda Hararo-Tendaho-Gobaad (MHTG) rift axes, which represent the on-land propagation of the Gulf of Aden and Red Sea spreading ridges, respectively (Fig. 2b, Barberi and Varet, 1977; Courtillot et al., 1984; Tapponnier et al., 1990; Manighetti et al., 1997, 2001; Doubre et al., 2017).
262 In Djibouti, the SE Afar rift architecture is (Fig. 3) dominated by a set of WNW-ESE to NW-SE trending tilted blocks (Gamarri-Dakka, Data-Yager and Garbi) that offset a ca. 1.1 Ma-old Upper Stratoïd Fm. (Kidane et al., 2003; Lahitte et al., 2003b) isolating three main basins mostly filled with late Quaternary sedimentary rocks (namely Gobaad, Hanlé, Der'Ela-Gaggade; Fig. 3b). The Asal-Ghoubbet rift (sensu stricto) is an active extensional structure that was the locus of post-stratoïd (< 0.8 Ma) volcanic activity (Fig. 3a; Manighetti et al., 1998; De Chabalier and Avouac, 1994).

268 Further north and north-east, upper Stratoïd basalts are spectacularly deformed along a faulted and sigmoidally-shaped monocline, facing SW to the south (i.e., toward the Ghoubbet-Asal axis), to the W at the western edge of the Danakil Block and again to the SW in the northern Manda Inakir rift (Fig. 3a). The central part of this monocline corresponds to the N-S trending 'Makarassou flexure', which was originally interpreted as an oblique transfer zone, linking the Asal-Ghoubbet and Manda-Inakir sub-rifts (Tapponnier et al., 1990). This interpretation was questioned by Le Gall et al. (2011), who emphasised the importance of recent vertical uplift within the Danakil Block.

275 More recently, Geoffroy et al. (2014) reinterpreted the Makarassou flexure as a rollover structure, developed within the hanging wall of a major NE-dipping post-Stratoïd detachment fault, the Doubié Detachment (Figs. 3a, b). According to them, the Doubié Detachment represents the recently (< 1 Ma) activated part of a much larger detachment fault system, which would extended till the border fault to the Gobaad half-graben (Geoffroy et al., 2014). The Doubié Detachment would thereby mark the present-day plate boundary between the Arabia and Somalia/Nubia plates, north of the Asal-Ghoubbet rift axis.

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283 **3. Methods**

284 In this study, we use high-resolution Pleiades imagery to calculate digital elevation models (DEMs), from which we were able to infer the dip and dip direction of individual flows within km-
thick sequences. Different Pleiades scenes were acquired (Fig. 2c): scenes 1 and 2 (SW Yemen),
scene 3 (southern Danakil Block) and scenes 4 and 5 (SE Afar Rift).

Pleiades satellites are designed for a range of very-high-resolution (VHR) remote sensing
applications. In particular, because of their high agility in roll, pitch and yaw, they offer along-track
stereo or tri-stereo acquisition capabilities. The images for this study were acquired in multi-view
stereo mode, with a base to height (B/H) ratio between 0.25 and 0.30 (suitable for mountainous
terrain). The stereo-pairs were processed using the Agisoft metashape® software. From the
obtained 3D point cloud, both a DEM and an orthoimage were extracted with a spatial resolution of
1 m and 0.5 m, respectively. This combination of Pleiades orthoimages and high-resolution DEMs
allows us to discriminate and orientate most of the planar geological structures, such as sedimentary
strata, volcanic flows, dykes and faults (Jaud et al., 2022).

The open-source 3D software CloudCompare® was used to visualise the 3D surface model and
to manually digitise boundaries between individual lava flows. We used a Matlab® routine to fit a
plane facet on the picked points list, using a least-square method (Jaud et al., 2022). The resulting
3D surface, hereafter named facet, was then characterised by its dip and dip direction, extracted for
the purpose of the study.

This method makes it possible to obtain a very large amount of structural data from
inaccessible outcrops covering large areas (Fig. 2c). An additional advantage is that the method
avoids errors related to direct compass measurement on ferromagnetic mineral-rich rocks.
Furthermore, field measurements are usually performed on small surfaces that may not be
representative of the averaged orientation of lava flows, whilst the facets determinations in this
study concerns are made from planar surfaces of up to several hundreds or even thousands of m²
(Supplementary Data Table S1). In this way, the natural roughness of lava flow surfaces is smoothed
out across an average and thereby present more reliable 3D planes. The accuracy of the method
(angular uncertainties less than 2°) is fully discussed in Jaud et al. (2022). A Geometric Quality Index (GQI) was defined as the ratio between the area (m²) of each facet against its “flatness index”, the latter being the base:height ratio of the triangular facet (Jaud et al., 2022). Among 2200 flow boundaries that were digitised during this work, 4.7% of these fell below this GQI threshold value (empirically defined at 100) and were therefore rejected (Supplementary Data Table S1). The only limitation of our method stems from the operator, who must carefully digitise one flow boundary at a time, while avoiding cross-cutting small-scale faults. Moreover, in the absence of field control, precise descriptions of lithologies are lacking, although image resolution is often sufficient to provide some clues.

In this paper, observations are shown in oblique 3D-views, as structural maps (Supplementary Data Figs. S1-S3) and along cross-sections (Figs. 5 to 11). Measured planes are plotted on lower-hemisphere, equal-area, stereonet projections (Stereonet program, Cardozo and Allmendinger, 2013; Allmendinger et al., 2012). In structural cross-sections, faults are chiefly recognized as stratigraphic discontinuities and unconstrained tilted normal faults are back-rotated to presumed pre-tilted dips of 65 ± 5°, as commonly applies to extensional settings elsewhere.

# 4. Structural interpretations from Pleiades imagery

## 4.1. SW Yemen

The two Pleiades scenes 1 and 2 (~50 km-long, 8-15 km-wide) in SW Yemen cover the SW edge of the Yemen Plateau, its Great Escarpment (GE), the GE's piedmont and the subsided Tihama coastal Plain (0 to 200m high; Fig. 5). On the Plateau, Early Oligocene traps and overlying Miocene to Quaternary volcanoes outcrop at altitudes between 1000 and 3660 m (Fig. 2c).

The elevated Oligocene traps to the east of the GE, are weakly tilted (< 7°) and variably block-faulted (Figs. 5b-d, 6a, Supplementary Data Figs. S1a, b). In contrast, within the coastal domain,
334 these traps define a series of tilted fault blocks, dipping from 30 to up to 40° SW, inside an array of
335 ~1-3 km-spaced and NE-dipping normal faults (Figs. 5b, c).

336 In scene 2 (Fig. 5c), the GE coincides with a SW-dipping upper crustal monocline of Mesozoic
337 sedimentary rocks and thin (10's m) patches of overlying volcanic traps. From the rim of this ca.
338 1400 m-high plateau and to the foot of the escarpment (at an elevation of ca. 500 m), the
339 sedimentary strata dips increase from horizontal to up to 40° SW, across less than 7 km
340 (corresponding to a ~5.5°.km⁻¹ rate of change; Fig. 5c, Supplementary Data Fig. S1b). This
341 monocline is disrupted by a dense network of mainly NE-dipping normal faults, with vertical throws
342 that do not exceed a few tens of meters (Fig. 5c). Along this monocline, both basal sedimentary
343 rocks and overlying traps have been tilted towards the SW (Figs. 4c, 5b).

344 Contrastingly, in scene 1, a set of SW- to W-dipping normal faults along the GE accommodated
345 an abrupt downthrown of volcanic Plateau sequences from an elevation of 1000 to < 500 m (Fig.
346 5b). The Late Jurassic-Cretaceous-Paleogene sedimentary rocks that form the immediate footwall
347 of this fault escarpment (Amran and Tawilah Groups, respectively; Grolier and Overstreet, 1978)
348 display an average dip direction at N036°E (Figs. 5d, 6a), but with dips decreasing systematically
349 upwards (Figs. 5b, 6a). These NE-thickening sedimentary wedges become up to 1000 m-thick and
350 likely resulted from the Mesozoic extensional events reported for this region (e.g., Beydoun et al.,
351 1996).

352 Farther seaward, both study areas exhibit highly tilted volcanic traps that are conformably
353 overlain by two different types of younger volcanic successions (of unknown ages), which both
354 display fan-like geometries. These younger volcanic wedges (depicted in oblique views in Fig. 6)
355 exhibit dips that decrease regularly upward from 30 to 8° (scene 2, Fig. 6c), 30 to 16° and 35 to 3°
356 (scene 1, Figs. 6d-e). Within the southern area (scene 2), bedding comprises of thin reddish and
357 erosionally resistant layers, alternating among ten meter thick and gradationally dark to whitish
layers (Fig. 6c). In the northern area (scene 1), wedges are soled by thick acidic flows that resemble the underlying tilted traps (Figs. 6d, e). Alternating reddish/whitish layers, quite similar to the volcanic wedges observed in scene 2, occur on top of this sequence (Figs. 6d, e).

Within the southern area (scene 2), the fan-like volcanic sequence is crosscut by a set of three NE-dipping normal faults, rooted on what is likely a low-dipping extensional detachment fault; cf., NE-facing detachment in Figs. 5a, c, 6c. Its footwall block consists of a thick and highly deformed basaltic sequence, with dip directions varying from SW to ESE, immediately beneath the detachment (Unit 1 in Fig. 6c; pink unit in Figs. 5a, c and Supplementary Data Fig. S1b). This highly deformed footwall is cut by numerous veins from likely late-stage magmatic fluids and is tentatively identified as the lower trap basaltic member having been exhumed beneath the detachment.

Farther west, even younger volcanic sequences are exposed across a 5 km-wide area, which comprises of basal acidic flows, crosscut by a thick dyke (Scene 2, Figs. 5a, 6c) and overlain by alternating acidic/basaltic flows, interspersed among yellowish to whitish layers. These units locally include red layers with columnar jointing (likely rhyolites). Measurements performed within this distal sequence show a wide range of dip values (from > 30 to 8°), yet with relatively uniform dip directions towards the SW (Fig. 5c, Supplementary Data Fig. S1b). Here some locations again exhibit an upward decrease in lava dips, suggesting that the whole SW area consists of sequences that were emplaced during an extensional period of tilting. However, due to a low relief, these dip variations could not be fully quantified.

Inside the northern area (scene 1), a similar formation as in scene 2 forms a syn-extensional fan with flow dips that decrease upwards from ca. 35° to only 3° SW (Fig. 6e, Supplementary Data Fig. S1a). The width of this wedge is ca. 4 km, as measured from its bounding normal fault in the north-east and to recent onlapping sediments in the south-west (SW end of cross-section in Fig. 5b).
At the south-western end of the southern area (scene 2), i.e. inside the Tihama Plain (sensu stricto), the volcanic basement forms a set of low-relief cuestas (< 100 m) that prevent any precise location of faults and it therefore becomes impossible to accurately determine their dips variations with altitude (Figs. 5c, 6f, g). Throughout this distal area, however, measured dips reveal some scatter from 40 to 25° to the SW (Fig. 6g, Supplementary Data Fig. S1b).

386

## 4.2. The southern Danakil Block

Pleiades images for scene 3 cover a more than 55 km-long and 11 km-wide area of the central and eastern parts of the uplifted Southern Danakil Block (Figs. 2c, 3a, 7a). The area is mostly occupied by highly weathered acidic rocks of the 16-9 Ma-old Mabla Fm. and discrete inliers of 9-4 Ma Dahla basalts (Fig. 7a). Due to intense weathering and relatively smooth topography, the structure of the Mabla volcanics is poorly known (Vellutini et al., 1995a). This dominantly flow-and-dome pattern is cut by a N-S trending normal fault network that mostly pre-dates the Dalha event (Figs. 3a, 7a; Vellutini et al., 1995a). Our method proves robust enough in imaging the internal structural arrangement of lava and faults within this old rift series and able to clarify its relationship with overlying Dahla basalts (Figs. 7, 8, Supplementary Data Fig. S1c).

As detailed hereafter, the numerous measurements performed in this area allow us to identify opposite dip directions within both the Mabla and Dahla series to either side and away from an axial zone, located at the center of the analysed scene (Figs. 7a, b). Relatively shallowly-dipping (< 12°) Mabla volcanic rocks inside this ca. 5 km-wide axial zone are cut by normal faults that have similar strikes of N150-170°E (Fig. 7a) and also dip in opposite SW and NE directions (Fig. 7c).

Within the south-western zone of the southern Danakil Block, a series of ca. 1 km-wide SW-tilted fault blocks are primarily composed of Mabla rhyolites (and interbedded basalts), in addition to lower Dahla basalts. Oblique views of the internal architecture of these tilted blocks are provided
in Figs. 8a, b. Here, a 100 to 150m-thick succession of massive Mabla rhyolites exhibits < 15° dips that decrease upwards, while volcanic wedges with higher dips (< 26°) outcrop farther east (Fig. 7c).

Importantly, the oblique view in Fig. 8c. shows that overlying Dahla basalts are also slightly tilted 2-5° and decreasing upward in accordance with the underlying Mabla wedge. Despite difficulties in discriminating between thin basaltic flows at the base of the Dahla series, our structural measurements still suggest that there is no significant angular unconformity with the underlying Mabla volcanics. However, an extensive and nearly unfaulted Upper Dahla succession drapes the tilted fault block pattern (Figs. 7c, 8b, c).

The north-eastern zone of the southern Danakil Block, is dominated by a nearly symmetric tectono-magmatic pattern. Here, a 100-200m high topographic relief exposes massive Mabla rhyolitic lava flows and domes (Vellutini et al., 1995b) inside tilted fault blocks, with WSW-facing faults and lava dips that decrease from 25° to 5° NE (Figs. 7c, 8d, e; Supplementary Data Fig. S1c).

Most associated WSW-dipping faults, striking N140-160°E (Fig. 7a), terminate up against sealing Dahla basalts (Fig. 8e). Further east, Dahla basalts are confined to the lower coastal area, where these form a much larger and NE-dipping wedge with an exposed thickness close to 300 m at its edge (Fig. 8f). Again, lava dips decrease upwards from 10° to 2-3° (Fig. 8f, Supplementary Data Fig. S1c).

422

423 **4.3. The SE Afar floor in Djibouti**

The flip-flop tectonic model of Geoffroy et al. (2014), with switches in tectonic polarities through time, provide insights into the mechanisms of stretching recorded by the Asal upper crust during an early stage of break-up. The two-stage tectono-magmatic model by these authors implied that SDR-like volcanic wedges of pre-Upper Stratoïd basalts would dip to the E to NE, all the while that their predictions lacked extensive field control. Our structural measurements from the Gamarri
Block (SW) and to the Makarassou flexure (NE) aim to expand and improve the accuracy of this model (Pleiades scenes 4 & 5, Fig. 3a).

The eroded Hanlé and Eounda Dòbi fault escarpments (E and W of the Hanlé Basin, respectively, in Fig. 9a) were used to infer dip variations within Stratoïd basalts (Figs. 9b-c). The top of the sequence is on the Gamarri-Dakka (2-3°) and Data-Yager (3-6°) blocks slightly tilted towards the SW (Fig. 9a). Conversely, inside both of these blocks, underlying Stratoïd flows tend to dip NE and, in spite of a strong scatter, these dip measurements appear to increase down through the sequence (Figs. 9b-c). In order to restore the original architecture of the Lower Stratoïd volcanic sequences, it is thus necessary to correct for any post-Stratoïd tilt. Such restored extensional volcanic fans display a thickening direction towards the NE within both the Gamarri-Dakka (N032°E) and Data-Yager (N038°E) blocks (Figs. 9b, c).

Inside the south-eastern part of the Data-Yager Block, close to Yoboki (Fig. 3a), the upper part of the Stratoïd Fm. comprises a sequence of massive rhyolitic/trachytic flows inter-bedded by ignimbrites (unit 2 in Fig. 10a), which in turn are overlain by a 100 m-thick basaltic sequence (Barberi and Santacroce, 1980). Below the acidic flow sequence, dip measurements of Stratoïd basalts (unit 1) are somewhat scattered (Fig. 10b), around a dip direction towards the NE and dips up to 25-30°, which decrease both toward the north-east and up through in the stratigraphic pile (Figs. 10c, d; Supplementary Data Fig. S2).

Along the active Asal-Ghoubbet active rift axis (Fig. 3a), the Stratoïd Fm. is exposed at both the top of the Garbi Block and the foot of the Makarassou flexure, where it thins down to ~ 100 m and has been tilted by up to 20-25° (Figs. 3c, 9a; Le Gall et al., 2011). The underlying undated succession is commonly attributed to the Dahla basalts (Figs. 3, 9a; Vellutini et al., 1993). These are extensively exposed within the footwall of the Doubié Detachment, where Dahla basalts form most of a ca. 1600 m-tall eroded mountain range (Figs. 9a, 11a).
The strong tilt of the Dahla basalts enables us to accurately measure dip variations up through its flow sequence and to further discriminate bewteen two distinct structural units (#1 and #2 in Figs. 11a, b). Within the basal unit #1, flow dips progressively decrease upwards from 45° to 20°, whereas dip directions range between N200-240°E (Figs. 11a, c). One NE-facing normal fault within the Garbi Block exhibits a low dip of 36° (F1 in Fig. 11b). The angle of 61°, however measured between this fault and the surrounding tilted flows, suggests that the fault was also tilted together with the volcanic host-rock and thereby offer strong evidence for syn-magmatic growth faulting, contemporaneous with the emplacement of Dahla basalts along the SE Afar rift.

Farther north, the basal volcanic fan (unit 1) is overlain by a younger sequence (unit 2) that exhibits oblique and highly scattered dip directions between N200-360°E (Figs. 11a, d, Supplementary Data Fig. S3). The contact between these two units is unclear and may correspond either to an erosional surface or to a fault (dotted black line in Fig. 11b). Moreover, it is also unclear whether this unit belongs to the Lower Stratoïd or uppermost Dahla Fm. Regardless, along the SW flank of the Garbi Block, a thin sequence of upper Stratoïd flows rests on both Units 1 and 2 has an average dip and dip direction of 20° N220°E (Figs. 11a, b; Supplementary Data Fig. S3). Consequently, a similar 20° clockwise back-rotation around a N130°E axis was applied to every measurement from the Garbi Block (Figs. 11 b, c, d; Supplementary Data Fig. S3). While this correction does not change the average dip direction (~N230°E) of unit 1 (Fig. 11c), it strongly changes the average dip direction of unit 2 from N253°E to N004°E (Fig. 11d).

When analysing a continuously exposed vertical transect, such as along the hillside illustrated in Figure 11b, lava dips tend to increase down through unit 2 (Fig. 11e). This fan-like distribution of flows inside unit 2 thereby suggests that a wedge-shaped volcanic sequence was emplaced during the Upper Dahla-Lower Stratoïd magmatic event, dipping towards N, i.e. subsequent to and in an opposite direction to the wedge defined by unit 1 (Supplementary Data Fig. S3).
Our results thus confirm the assessment by Geoffroy et al. (2014) that NE-dipping volcanic wedges developed across SE Afar, prior to the Late Stratoïd event. Our results also outline the structural complexity of the Pliocene lava pile against the Danakil Block, where fans dip both toward the NE and SW.

481

## 5. Discussion

The structural data acquired from this study of the Yemen, Danakil and SE Afar Depression supplement those acquired by other authors for Western (Wolfenden et al., 2005; Stab et al., 2016) and SE Afar (Le Gall et al., 2011; 2015; Geoffroy et al., 2014). Combined, these data enable us to propose an evolutionary model for AMP-related volcanic successions over the past 30 Myrs. To this end, we firstly discuss the polarities of various syn-tectonic volcanic wedges identified in this work, as a function of their ages of emplacement. The volcanic architecture of the AMP is then related to a crustal-scale thickness profile.

490

### 5.1. Structure and timing of identified SDRs

#### 5.1.1. Late Oligocene to Lower Miocene: the early syn-rift period

We have shown that immediately postdating trap volcanic sequences, onshore SW Yemen volcanic rocks to the east of the Tihama Plain displayed the fault-controlled wedge geometry that characterize Inner SDRs (Fig. 6a). Inside scene 2, this inner wedge is overlain with a thick and highly tilted sequence (25-40° to the SW, Fig. 6g), which suggests that another hidden SDR developed in a more distal (i.e. seaward) position within the Tihama Plain (Figs. 5c, 6f, g).

Approximatively 200 km north from this studied area, available seismic data also show how the acoustic basement beneath the Tihama Plain is made up of an Inner SDR-like sequence that forms a > 50 km wide and up to a 4 km-thick seaward-dipping wedge (Fig. 2c; Davison et al., 1994;
Ahmed et al., 2013). An analogous 6-km thick and 40-50 km-wide Inner SDR is also seismically imaged offshore of the S Yemen margin, albeit related to the opening of the Gulf of Aden (Fig. 2c; Tard et al., 1991). All of these Inner SDRs rest upon a high-velocity lower crust (Ahmed et al., 2013). The more proximal wedges identified by our study thus represent the eroded tip of a wide Inner SDRs system, currently hidden by sediments along the Tihama Plain. This highly tilted sequence, identified beneath the Tihama Plain in scene 2 (Figs. 5c, 6f, g), could either represent a second Inner SDR, or the eroded top of an Outer SDRs system, characterized by an extensive set of strongly arcuate and diverging reflector sequences, whereas Inner SDR are characterized by poorly arcuate reflector sequences (Fig. 1a). Compared to these two types of wedge architectures, flow attitudes across the Tihama Plain (Fig. 5c) conform better to that of Outer SDRs (Fig. 1a).

Unfortunately, no radiometric age dating exists for any of those Yemen's SDRs, which prompts a debate on when they were formed during the rifting and break-up of the southern Red Sea. Large volume of magmatic intrusions was emplaced onshore, along the Upper Oligocene to the Lower Miocene Great Escarpment of Yemen. These consist of dyke swarms, as well as gabbroic and granitic plutons, which mostly span 27-20 Ma and up to 18.5-16 Ma (Fig. 4 and references therein). The granitic plutons along the GE are coeval to the upper crustal and seaward-dipping flexure and thereby, to the main necking stage of the crust, during the rifting of the southern Red Sea (Geoffroy et al., 1998). The 24-20 Ma Tihama Asir Complex (Fig. 2b) was also emplaced along this zone of thinning (from 40 to 20 km) of the continental crust (Mooney et al., 1985; Stern and Johnson, 2019). From these observations, we infer that the post-trap Inner SDR system in SW Yemen developed during the late Oligocene-Lower Miocene necking of Yemen continental crust, together with related mafic dyke and granitic pluton intrusion.

It remains difficult to propose an age for the end of the development of these SDRs. Two major stratigraphic unconformities are documented within the area. The younger one is located inside the
Jabal an Nar area, where a sub-horizontal and 10 Ma-old basaltic sequence overlies a seaward-tilted sequence of Lower Miocene rhyolites (ca. 18 Ma, Fig. 2c, Capaldi et al., 1987a; Huchon et al., 1991).

Farther north, Lower to Middle Miocene carbonate and siliciclastic sediments, followed by massive salt deposits, overlie and thus postdate the SDR inside the N Tihama Plain (Hughes and Beydoun, 1992; El-Anbaawy et al., 1992; Davison et al. 1994). K/Ar dating of the upper sequence of the S Yemen SDR system provides a solitary late Lower Miocene age (16.7 +/- 1.5 Ma; Tard et al., 1991).

Combined, these data suggest that Inner SDR development along the SW Yemen margin was most likely bracketed between ~27 Ma and ~16 Ma or 10 Ma.

The SDRs identified in SW Yemen show systematic changes in dip directions from N239°E (northern area, scene 1, Fig. 5f) to N198°E (southern area, scene 2, Fig. 5f). This southward rotation of SDR dip directions might result from the location of these studied areas on an arcuate segment that connects the rift axes of the Red Sea and Gulf of Aden (e.g., Fig. 2c).

### 5.1.2. Miocene to Pliocene evolution

The opposite dip directions displayed by the Mabla (16-9 Ma) and Dahla (9-4 Ma) SDRs on either sides of the southern Danakil Block requires distinct fault dynamics during their formation (Fig. 7c). While the original thicknesses of these partially hidden SDRs are unknown, it is worth noting that the Dahla Fm. sequence exposed on top of Mabla wedges, is very thin. Moreover, the Mabla Fm. is dated to 19-9 Ma in Djibouti, while no lavas older than 16 Ma have been documented on the Southern Danakil Block.

We note that exposures of the Dahla Fm. are much more scattered and indeed scarcer on the east flank of the Danakil Block than on its western side (Fig. 3a). In contrast, it was previously observed that the Dahla sequence thickens strongly from the Danakil Block and towards the Afar Depression (Le Gall et al., 2011). During a recent field survey, we observed SW-dipping thick
successions of Dahla Fm. bounded by NE-dipping normal faults, which conform to a SDR-like wedge geometry (Fig. 12). An overall south-westward thickening of Dahla volcanics, along the base of the Garbi Block, together with associated SW-tilted normal faults (F1 in Fig. 11), also demonstrate that the Dahla Fm. is coeval to syn-magmatic crustal stretching to the west of the Danakil Block.

At the scale of the analyzed Pleiades scenes (Fig. 7), the crustal extension, related to the development of Mabla SDRs, was oriented at ~N070E (average dip direction of N257°E) and at N064°E in the SW and NE zones of the Danakil Block, respectively, Fig. 7b), whereas the extension direction during the Dahla basalts was ~N045E (average dip directions of N048°E and N221°E, Fig. 7c). This records a ~25° apparent counterclockwise rotation of the direction of extension, from EW to NE-SW, during Pliocene.

The lava wedge defined by the uppermost Dahla sequences (unit 2, Fig. 11), along the Doubié-Asal rift axis, thickens towards the north, i.e., in an opposite dip direction to those documented in the SW-dipping lower Dahla wedge (unit 1, Fig. 11). This implies that a complex reorganisation of the strain field occurred from Late Miocene to Pliocene, with regional extension switching from ENE-WSW during Mabla volcanism, to NE-SW and finally N-S during Upper Dahla volcanism. Most of all, as described below, the dramatic reversal of SDR dip directions (from SW to NNE) during Upper Miocene to Pliocene along the Djibouti continental break-up axis is a striking event.

### 5.1.3. Pliocene to Quaternary evolution as recorded by the Stratoïd Formation

We have demonstrated how 4-1 Ma-old Stratoïd sequences in Djibouti form a first-order SDR that continually dips NE, consistent with the emplacement of Dahla formation along the Doubié-Asal rift axis (N030-060°E, Figs. 9b, c, 10b). The abrupt thinning of Stratoïd sequences towards the Danakil Block (Fig. 9a) is consistent with having formed along a major SW-dipping growth fault, located south-west of the uplifted Garbi Block. This SDR terminates against unconformably overlying Upper Stratoïd basalts, which were tilted towards the SW by a NE-dipping normal fault network and
573 as part of a flip-flop system, of detachment faults with opposite senses of displacements (Figs. 3b, 574 9a; Geoffroy et al., 2014).

575 The western and southern edges of the Danakil Block thereby developed an initial crustal 576 flexure during the development of both Mabla and Lower Dahla SDRs and contemporaneous with 577 anticlockwise block rotation (in map view). During a second generation of SDRs made of Upper Dahla 578 and Lower Stratoïd lavas, the Danakil Block became an uplifted footwall and an erosive domain. A 579 N-S trending fault system developed along the western side of the Danakil Block, which post-dates 580 the Upper Stratoïd Fm. (Fig. 9a). Such a young relative age links this fault system to the most recent 581 (< 1 Ma) NE-directed crustal extension across SE Afar and the development of the Doubié 582 Detachment during onset of early continental break-up along the Doubié-Asal and Manda Harraro 583 rift segments (Geoffroy et al., 2014).

584

585 **5.2. Conjugate margin architecture through time**

586 The main structural units identified in this work form part of a 700 km-long crustal-scale cross-587 section between the Ethiopia and Yemen conjugate plateaus (Fig. 13a). This profile is nearly 588 orthogonal to most regional structures and thereby parallel to kinematic trends during Neogene.

589 **5.2.1. Crustal thickness pattern and necking zones**

590 While available seismic receiver functions dataset constrain the crustal thickness of the Afar 591 Depression and the Ethiopia-Yemen plateaus (Fig. 13b; Dugda and Nyblade, 2006; Hammond et al., 592 2011; Ahmed et al., 2013; Reed et al., 2014; Ahmed et al., 2022), there are unfortunately no 593 geophysical records for the Danakil Block or Bab al Mandab Strait. Crustal thicknesses below the 594 Danakil Block were therefore extrapolated from those obtained south of the Tadjoura Gulf (Fig. 13b; 595 Dugda and Nyblade, 2006). This may be acceptable, since these 23 km are in agreement with
thicknesses obtained from seismic refraction measurements across the northern (Eritrean) part of
the Danakil Block (Makris and Ginzburg, 1987).

The synthetic crustal profile in Figure 13(a) shows that the 35-45 km thick crust beneath both
Yemen and Ethiopia plateaus, decreases to 17-30 km, below both the Afar Depression and the
Danakil Block (Fig. 13b). Three crustal thin zones can be distinguished, i) beneath the Bab Al Mandab
Strait, separating the SW Yemen coast and the Danakil Block (Fig. 13a), ii) along the edge of the
Ethiopian GE, and iii) beneath the Asal-Ghoubbet active rift. The geochemical signatures of recent
volcanism in the Asal rift suggests a sharp necking of the lithosphere from 85 +/- 15 km beneath the
rift shoulders to about 40 km beneath the rift axis (Pinzutti et al., 2013). A shallow brittle/ductile
transition zone (~600 °C isotherm) at ca. 5 km depth is suggested beneath the rift axis (Doubre et
al., 2007).

To synthetise, two crustal necking zones are inferred beneath the Ethiopian and Yemen Great
Escarpments where the continental crust is thinned by up to 50 % (Fig. 13a). In between necking
zones, the Afar Depression and the Danakil Block both also correspond to moderately thinned
crustal domains (about 50 %, without considering the addition of magma), which that extends for
at least ~ 370 km along strike (Fig. 13a).

The conjugate necking zones are associated with both continentward-dipping Moho slopes
and riftward-dipping upper crustal flexures that pinch out towards the Afar Depression in Ethiopia
and the Eastern Danakil Block in Yemen. The upper crustal flexures are accommodated by a network
of dominantly continentward- and subordinate riftward-dipping normal faults, which
accommodated a riftward tilting of Early Oligocene volcanic traps (Fig. 13a; Wolfenden et al., 2005;
Tesfaye and Ghebreab, 2013; Stab et al., 2016; Zwaan et al., 2020). The high dips of these traps (30°
- 45°) together with an absence of thickness variations within flexured lava piles, indicate that these
trap lavas predate crustal thinning (Davison et al., 1994; Wolfenden et al., 2005; Stab et al., 2016).
620 Wolfenden et al. (2005) additionally showed that Late Oligocene to Pliocene volcanic sequences in western and south-western Afar dip E-NE within syn-rift volcanic fans, interpreted as 621 SDR-like volcanic wedges (Fig. 13a). Therefore, E-facing normal faults within proximal marginal 622 grabens likely constitute antithetic structures, formed during the development of an inner-most SDR 623 roll-over anticline, as correctly drawn by Wolfenden et al. (2005), whereas W-facing faults are part 624 of a continentward-dipping pattern that developed inside this SDR wedge (Fig. 13a; see Geoffroy, 625 2005; Chauvet et al., 2019). 626

627 Within large age uncertainties, the Inner SDR system along the SW Yemen margin developed 628 from the end of trap volcanism, at ca. 27 Ma, and to sometime in between 16 and 10 Ma (e.g. § 629 5.1.1). The synchronicity and symmetrical distribution of Inner SDR-like volcanic sequences along 630 both Ethiopia and Yemen necking zones is striking (Fig. 13a). Apatite fission track data from SW 631 Yemen further indicate that most exhumation of these rift shoulders occurred between 25 to 16 Ma 632 (Menzies et al., 1992, 1997) i.e. sub-contemporaneous with Inner SDRs-formation and crustal 633 necking along the Yemen margin. 634

635
5.2.3. Danakil Block and Afar Depression

636 We have shown that Miocene to Pliocene volcanism inside both the Danakil Block and the SE 637 Afar Depression was emplaced during successive syn-magmatic crustal stretching episodes. Our 638 structural analysis of the Danakil Block, together with Stab et al.'s (2016) observations in Central 639 Afar, show that the Miocene extension did not strictly focus along the Ethiopian margin of the Afar 640 Depression, as proposed by Wolfenden et al. (2005), but was more widely distributed and affected 641 the crust of the entire Afar and Danakil Block. 642

642
The structural arrangement of volcanic wedges depicted in this paper and Wolfenden et al.
643
(2005), formed during Mabla and Dahla volcanism suggest that "conjugate SDRs" developed with
opposite facing directions, between the Ethiopian GE and the western flank of the Danakil Block (Fig. 13a). These conjugate Mio-Pliocene SDRs are, however, mostly hidden by an extensive cover of Stratoïd basalts.

East of the Danakil Block, a counterpart pair of 'conjugate' SDRs is assumed to have developed, along the Bab al Mandab rift axis (BaM), before 10 Ma, and during parts of the Mabla volcanism (Fig. 13a). In addition, < 10 Ma-old Dahla basalts also formed a set of SDRs that thicken toward the BaM-axis (Figs. 7c, 8f, 13a). In such a scenario, it becomes possible for flat-lying and discordant basaltic succession within the Jabal An Nar of Yemen (Fig. 2c) to represent the eroded edge of a SDR that grew westward, and conjugate to eastward SDR-growth from the Danakil Block. In our Pleiades satellites scene 2, we also observe a highly tilted volcanic succession that underlies the Tihama Plain as a whole yet overlies the Inner SDR (Figs. 5c, 6f, g). In our synthetic cross-section (Fig. 13a), we tentatively relate this extreme tilting of a distal volcanic sequence to the eroded top of an Outer-type SDR. The latter may tentatively be correlated to the Dahla SDRs, which would have developed, concomitantly, along both the Central Afar and BaM rift axes.

A distinct evolution occurred during the onset of Upper Dahla and Lower Stratoïd volcanism, where our results demonstrate that, from Pliocene to ~1 My, mafic SDRs developed extensively, dipping N to NE and covering the entire Afar Depression (Fig. 13a). This reorganisation likely started during the emplacement of uppermost Dahla flow sequences (N-dipping wedge of unit 2, in Fig. 11e). NE-dipping Early Oligocene traps and Oligo-Miocene series, locally exposed within the Finto and Affara Dara areas in NW Afar (Stab et al., 2016; Fig. 2c, 13a), are tentatively re-interpreted as pre-extensional and syn-extensional volcanic units, respectively, constituting the basement to Pliocene to Quaternary SDRs that grew toward the NE.

# 6. Summary and concluding remarks
We propose that the Ethiopia-Afar-Yemen system, since the Oligocene, developed as illustrated by cross-sections (Fig. 14a) and paleogeographic maps (Fig. 14b), during six stages, which also conform to kinematic constraints imposed by Arabian-Nubian-Somalian relative motions (compilation from Müller et al., 2016).

The first-order tectono-magmatic segmentation of the Afar area is interpreted as being related to three transform-like faults (Fig. 14b, stage 2). The Shukra El Sheik-Bia Anot fault-zone (Fig. 2a, Fig. 14b, stage 2) borders the southern part of the Ali Sabieh Block and separates an area where extension was syn-magmatic and consecutive VPMs developed (southern Red Sea and western Gulf of Aden) from an area east of the Gulf of Aden, wherein extension and breakup has been mostly amagmatic (Audin et al., 2004; Leroy et al., 2012). The Holhol fault zone in Djibouti (Fig. 14b, stage 2) bounds northward the Ali Sabieh Block (Audin et al., 2004). Also, the highly magmatic Central Afar area is distinctly isolated from the Erta Ale segment to the North, dominated by Mio-Pliocene sedimentary infilling, by the ill-defined Alyata tranverse fault zone (Stab et al., 2016; Le Gall et al., 2018b).

Stage 1. As observed in the evolution of LIPs worldwide (e.g., Geoffroy, 2005), the Early Oligocene trap event was not accompanied by any significant crustal extension within the AMP, apart from minor magmatic dilatation of the continental crust. The present-day absence of trap volcanics on both Danakil and a more southerly located Ali Sabieh Block (Fig. 2c), suggests that these blocks were uplifted and eroded towards the end of trap emplacement and during the beginning of stage 2 (e.g., Garfunkel and Beyth, 2006).

Stage 2. Immediately after the trap event, the continental lithosphere recorded significant stretching and thinning. Inner SDRs, controlled by continent-ward dipping syn-magmatic growth faults, developed from 27 to ~16 Ma and along both sides of an uplifted and eroded
basement domain, corresponding to the initial Danakil Block (Fig. 14, stage 2). During this time period, this initial wide Danakil Block acted as a weakly deformed continental uplifted area, corresponding to the common footwall of the antithetic upper-crustal normal faults bounding the Ethiopian and Yemen Inner SDR system. The development of this initially wide horst-type structure, corresponds exactly to the early development of VPMs as modelled and observed by Geoffroy et al. (2015, 2020) (Fig. 1b). In this model, antithetic continentward-dipping detachment faults rooting in the lower crust early isolate a central continental block, namely the C-Block, that controls both the flexuring of the rift shoulders and the development of Inner SDRs (Fig. 1b). Similar C-block have been imaged west of India, in the aborted Laxmi basin (Nemčok and Rybár, 2017; Geoffroy et al., 2020) and more recently in the Artic Ocean (Nikishin et al., 2022).

Stage 3. From 16 to 10 Ma, the Afar area as a whole underwent more distributed crustal stretching and thinning. This was associated with a progressive dismantlement of the Initial Danakil C-Block (especially to the west), together with the formation of the early Afar Depression. This extension stretched the C-Block by as much as 200% to 400% (Redfield et al., 2003). A similar tectonic stretching and thinning of an initial C-Block leading to its fragmentation was also predicted by Geoffroy et al. (2015). Two pairs of conjugate and Inner-type SDRs, made up of Mabla acid and basic volcanic rocks, formed on both sides of the present-day Danakil Block (Fig. 14a, stage 3). At this stage, two mature volcanic rift axes were active, along both the BaM Strait and in Central Afar (Fig. 14b). The subaerial volcanism along the BaM axis was associated with very low subsidence of the rift, as evidenced by evaporitic deposition during Middle Miocene (El-Anbaawy et al., 1992; Heaton et al., 1995; Fig. 14b).
716 Stage 4. Between 10 to ~5 Ma, the continued tectono-magmatic activity centered along the BaM
717 and W-Afar axes led to the development of a new pair of conjugate SDRs, on both sides of
718 the present-day Danakil Block, made up of Dahla basalts (Fig. 14a, stage 4). We suggest this
719 extension may have reached the level of mechanical breakup (Geoffroy et al., 2022; Fig.
720 1b), considering the striking resemblance of highly dipping and prograding flow sequences
721 in the Tihama Plain SW Yemen with Outer SDRs (Figs. 1, 13a).
722

723 Stage 5. At ~5 Ma, tectono-magmatic activity ceased along the BaM rift axis and entirely focused on
724 the Central Afar axis. From 5 to 1 Ma (i.e., prior to the end of the Dahla Fm. and during the
725 Lower Stratoïd Fm.), a set of NE-dipping SDR wedges developed along SW-facing growth
726 faults and throughout the whole S-Afar Depression (Fig. 14a). At the scale of the entire
727 magmatic province, this break-up stage became fundamentally asymmetric. The present-
728 day Danakil Block and the Garbi area both acted as the footwall to the most distal SDR
729 wedges within this cross-section. The end of Stratoïd volcanism is marked by a remarkable
730 flat topography across the lowlands of the Southern Afar area, in association with a sudden
731 decline in magma-flux and the end of syn-magmatic extension. The very top of the upper
732 Stratoïd Fm. (representing less than 100 m in thickness) is frequently discordant to
733 underlying wedges, reflecting a sudden cessation of syn-magmatic extension (Geoffroy et
734 al., 2014).
735

735 Stage 6. The most recent volcanism (< 1Ma) is mostly confined within ~30 km-wide and ~60 km-
736 long en-echelon tectono-magmatic segments, distributed along the Manda Hararo-
737 Tendaho and GAMI axes (Figs. 1c, d; Barberi et al., 1970; Beutel et al., 2010; Keir et al.,
738 2013; Ebinger et al., 2013). Continental break-up, with the earliest onset of oceanic-type
739 accretion centers, occurred within Northern Afar and Djibouti (Asal), in connections with
the propagation of the oceanic Red Sea and Gulf of Aden rift systems, respectively. In Djibouti, the post-Stratoid deformation, predating this break-up, was associated with a reworking of ancient VPMs, by a system of NE-dipping detachment faults that propagated ahead of the earliest Tadjoura Gulf and Asal-Ghoubbet oceanic accretionary systems (Geoffroy et al., 2014). The most recent strain field in the southern Afar depression has been controlled by such detachment-type tectonics and also by the extension along the Ethiopian rift.

To conclude, the remote mapping from stereo-photogrammetry of pairs of high-resolution satellite images (Pleiades data set) drastically improve our prior understanding of the structural architecture of volcanic sequences which were successively emplaced across the AMP. In particular, the recognition of successive volcanic wedges enable us to constrain the complex development of conjugate VPMs in Afar. Our detailed study not only adds many new observations at the scale of the entire AMP but also provides a comprehensive framework for the area, from the initial onset of volcanism to the final rupturing of lithosphere. We propose that the evolution of this area mimics the structure and evolution of magmatic continental breakups in general (Figure 1) with, notably, the development of an initial C-Block, similar to that observed or suggested for other systems (Quirk et al., 2014; Foulger et al., 2019; Geoffroy et al., 2020; Nikishin et al., 2022).

However, some issues still remain to be clarified. The coexistence from 16 to 5 Ma of two volcanic rift axes that develop on either side of the Danakil block is a peculiar evolution which imposes to take into consideration the 3D development of propagating rift systems which could mechanically interact. As shown in the N-Atlantic, breakup processes may be strongly perturbed by external forcings such as kinematic reorganization as well as internal forcings such as structural
763 inheritance, rheological heterogeneities or evolution with time and mantle melting of in-extension
764 continental lithosphere (e.g. Schiffer et al., ESR, 2019).

765

766 Acknowledgments

767 This work was funded by the Dynamarges think tank. The Pleiades images used in this study were
768 ordered as part of the ISIS program of the CNES (French National Centre for Space Studies) and
769 provided by DataTerra via the DINAMIS platform. Dr. Martin B. Klausen and another anonymous
770 reviewer are warmly thanked for their constructive and thorough comments and suggestions that
771 significantly improved this contribution.

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1181

## Figure captions

1183

1184 Fig. 1. A simplified sketch (a) and development model (b) for crustal domains and volcanic architectures, as observed across volcanic passive margins (modified from Geoffroy et al., 2022).

1186 Pre-extension volcanic traps are overlain by Inner SDR wedges that develop during a crustal necking stage and along each side of an upper crustal central block (C-Block, stage #1 in b). These reflectors tend to pinch out landward forming wedge or fan-shaped sequences that are bound by continentward-dipping listric normal faults, which flatten toward the top of a ductile middle to lower crust. Where observed, Outer SDRs develop above the distal domain of the margin (stage #2 in b). Outer SDR are characterised by a set of strongly arcuate-diverging reflector sequences, which migrate seaward and are deflected above a sub-horizontal to slightly continentward-dipping basal surface (e.g., Chauvet et al., 2021). Flat-lying flow sequences may develop farther seaward and coeval with likely passive exhumation of continental lower crust (Geoffroy et al., 2022; stage #3 in
b). The high Vp observed in the lower crustal part of VPMs (High velocity lower crust in a) is thought to be derived from the injection of mafic magma into the lower continental crust (Geoffroy et al., 2022).

Fig. 2. Maps of the studied areas. All maps are projected using UTM 38N WGS84. a: Simplified geological map of the Afar magmatic province (AMP, in red and orange), which encompasses the Oligo-Miocene to Quaternary magmatism in Ethiopia, Eritrea, Yemen, Djibouti and Somalia. Areas floored with oceanic crust are from Leroy et al. (2012) in the Gulf of Aden and from Augustin et al. (2021) in the Red Sea. b: Tectonic map of the Afar triple junction and surrounding Ethiopia and Yemen Plateaus (topography from the 30m resolution ALOS digital elevation model). Large white arrows represent current kinematic vectors relative to a fixed Nubian Plate (ArRajehi et al., 2010). Active rift axes within the Afar depression (thin white arrows indicating direction of propagation) are from Manighetti et al. (2001). AG, Asal-Ghoubbet; AL, Alayta; EA, Erta'Ale; MER, Main Ethiopian Rift; MHTG, Manda Hararo-Tendaho-Gobaad; MI, Manda Inakir; TA, Tat'Ali. c: Geological map of the Ethiopian Great Escarpment, Afar Depression and SW Yemen, modified from Audin et al. (2004), Wolfenden et al. (2005), Le Gall et al. (2015) and Stab et al. (2016) including cross-sections from Wolfenden et al. (2005, A to F) and Stab et al. (2016, G). Areas of Pleiades data for this study are outlined and labeled 1-5. Extend of SDRs, as determined from seismic reflections along the Yemen coastal plain and the Aden Aden Gulf margin are shown by cross-hatching (Tard et al., 1991; Davison et al., 1994; Ahmed et al., 2013). Dotted orange unit within the Erta'Ale segment represents the sediments of the Danakil and Red Beds series, Miocene in age (~20 to ~6 Ma; Le Gall et al., 2018b).

Fig. 3. a: A Geological map of Djibouti, modified from Le Gall et al. (2015). Note the fault pattern that dissects the Upper Stratoïd Fm. (dated < 1 Ma), which controls the development of four successive sedimentary half-basins (Gobaad, Hanlé, Der'Ela-Gaggadé, Asal-Ghoubbet). Oblique views of Figs. 10 and 11 are located on the map with white arrows that also show the approximate direction of each camera. b. Oblique view of the topography (ALOS digital elevation model) and morphology of the recent SE Afar rift system in Djibouti.

Fig. 4. Volcano-stratigraphic chart used in this work. Compilation from Wolfenden et al. (2005) and Stab et al. (2016) for Ethiopia/W Afar. Other references, used to establish the lower and upper
bound of volcanic formations, are quoted on the figure. Colour scheme is the same as used in Figs. 2 and 3.

Fig. 5. a: Geological maps of study areas inside SW Yemen (Pleiades scenes 1 and 2, Fig. 2c). Structural measurements are reported in Supplementary Data Figs. S1a, b. The basal trap basalt $^{40}$Ar/$^{39}$Ar age (white box) is from Baker et al. (1996). The oblique views in Fig. 6 are localised by white arrows that also show the approximate direction of each camera. b-c: Cross-sections along profiles located in a., and where local dips are reported as min and max dip values, measured within identified syn-tectonic wedges. d-f: Poles of measured planes and their contoured density distributions, listed according to their location along the profile (Plateau, Great Escarpment, Grand Escarpment piedmont and Tihama Plain, respectively). Measured planes with the lowest- and highest- dips are plotted as thick dashed great circles. Arrows are not projections but correspond to mean dip directions measured within syn-tectonic sequences.

Fig. 6. Oblique views of SW Yemen Pleiades orthoimages (scenes 1 and 2, see Fig. 5a for locations) draped onto photogrammetric DEM (Digital Elevation Model). A selection of structural measurements (dip/dip direction) is plotted directly onto the oblique view, where colours of these label boxes refer to the colour scheme for stratigraphic units.

a: Oblique ESE view of the Great Escarpment (scene 1; Figs. 5b), with main stratigraphic boundaries between the Precambrian basement (Ba), Jurassic to Tertiary sediments (S) and Early Oligocene volcanic traps. The lower basaltic #1 and upper acidic #2 members are distinguishable in this area. All structural data are plotted in the stereonet (poles and planes). Green arrow corresponds to the mean dip direction measured within syn-tectonic sedimentary sequences. White dotted lines illustrate fan-like disposition of sedimentary strata. b: Oblique NW view of sediment monocline and overlying volcanic traps at the foot of the Great Escarpment (scene 2; Fig. 5c). Stratigraphic marker (located by white arrow) corresponds to a greenish to whitish (tuffaceous or volcano-detrital) layer located between the basaltic (#1) and acidic (#2) members of the traps. c: Oblique SSW view of syn-extensional volcanic piles identified in scene 2 (Fig. 5c). Volcanic units are tentatively correlated to basaltic and acidic members of the traps (#1 and #2, dip values in white) and to a younger syn-extensional unit (#3, dip values in orange). Note progressive decrease in the dip values up through unit #3. White arrow locates a thick dyke (50m in thickness) that cross-cuts the syn-extensional sequences. d, e: Oblique SE views of syn-extensional volcanic successions identified in scene 1 (Fig.
5b). In d, the boundary between pre- and syn-extension volcanic units is outlined by an orange line.
In e, the distinction between pre- (dip values in white) and syn-extension (dip values in orange)
volcanic units is arbitrary, the pre-extension traps dipping ~ 35° NE (Fig. 5b).

Fig. 6. (continued). f, g: Overview and zoomed views, toward WNW and NW respectively, of volcanic sequences outcropping across the Tihama Plain (scene 2; Figs. 5c). Note relatively steep dips ranging from 25 to 40° SW. Here, structural measures do not indicate a clear fan-like architecture but, rather, a succession of sub-parallel seaward dipping lavas.

Fig. 7. a: Geological map of an analysed Pleiades scene of the SW Danakil Block (scene #3 in Figs. 2c, 3a), modified from Le Gall et al. (2015). Normal faults are from this study. Structural measurements are displayed in Supplementary Data Fig. S1c. Thick dark grey line corresponds to the cross-section in c. A more southerly located thick dotted line is a complementary cross-section that has been extrapolated northward in c. The $^{40}$Ar/$^{39}$Ar age (within white box) of the Mabla Fm. is from Zumbo et al. (1995b). The oblique views of the Fig. 8 are located by white arrows that also show the approximate direction of the camera. b: Interpolation map of dip directions across the same area as in a. The rose diagram on top of the interpolation map shows all the pole of planes together with the color-code of the dip directions. The three stereonets show the poles and planes measured in the three identified zones (southwestern, axial and northeastern). Steepest dips are plotted as thick dashed great circles, with associated dips being reported in white boxes. Coloured arrows correspond to the mean dip direction measured within the corresponding sequences. Note that the axial zone is characterised by poorly organised dip directions and low dips. c: Composite cross-section compiled from profiles located in a.

Fig. 8. Oblique view extracts (see Fig. 7 for locations) of Pleiades orthoimages draped onto corresponding photogrammetric DEM. Da: Dahla Fm.; Ma: Mabla Fm. a-b: Two illustrations of upward decreasing dips of massive rhyolites and basalts within the Mabla Fm. c-f: Structural relationships between Dahla basalts and Mabla rhyolites. Note that the throw of most of normal faults attenuates within the Dahla Fm. Dashed white lines highlight the attitude of some volcanic flows. Note how upper sequence of Dahla basalts seals underlying normal faults in c and e.
Fig. 9. a: Oblique map NW view across the eastern Afar rift in central Djibouti (ALOS DEM) with cross-section illustrating the structure of Stratoïd and Dahla Fms., beneath a NE-dipping Upper Stratoïd Fm., which forms a regional flat-lying surface in the DEM. The top of the Stratoïd Fm. is dissected by < 1 Ma extensional faults. The Doubié Detachment (Geoffroy et al., 2014) occurs at the tip of an active Asal-Ghoubbet rift axis. b: Dip variations within the Stratoïd Fm. (n = 63) measured along the Hanlé Fault. c: Dip variations within the Stratoïd Fm. (n = 62) measured along the Eounda Dôbi Fault. In b and c, the black dots correspond to present-day poles of planes, while blue dots represent tilt-corrected poles of planes. The steepest planes within the Stratoïd Fm. are plotted as thick dashed great circles, with associated dip being reported in white boxes. Blue arrow shows the mean dip direction.

Fig. 10. a: Oblique NW view across the Data-Yager Block (see location in Fig. 3) on top of which a selection of dip/dip dir. measurements have been added. Cross-section illustrates how the basaltic section (unit 1) underlies the rhyolites (unit 2). The Upper Stratoïd basaltic sequence has been tilted to the SW, whereas underlying sequences form a set of NE-ward dipping lavas. Note the fan of the NE-dipping Stratoïd series (unit 1), as underlined by the white dashed lines. b: Stereonets of the poles to lava flows measured within the rhyolites (unit 2), the underlying basaltic section (unit 1) and inside the syn-extensional fans depicted in c and d. The steepest planes within these two volcanic fans are plotted as thick dashed great circles, with corresponding dip value reported within white boxes. Arrows show the mean dip direction. c: Fan-like disposition of NE-dipping Stratoïd basaltic flows, with an upward decrease of dips (see location in a). d: Flexure developed towards the NE within a syn-extensional basaltic sequence located beneath the rhyolites. Note again the upward decrease of the flows dip and the horizontal attitude of uppermost stratoïd flows.

Fig. 11. a-b: Oblique NW views of the Garbi Block, at different scales (see location in Fig. 3). The Stratoïd Fm. is reduced here to its upper member, as exposed on the SW flank of the tilted block (flows with brownish patina). The underlying, blue-grey and whitish volcanic succession, albeit not dated, likely represents the Dahla Fm (e.g., Le Gall et al., 2015) and is made up of transitional basalts, within two differently oriented structural units (#1 and #2). In a, note upward decrease of dips within unit 1. A NE-dipping normal fault ("F1") has been tilted together with the Dahla series. In b, a dashed black line represents the apparent boundary between these two units, inferred from dip measurements. c-d: Stereonets of the poles to lava flows, measured within units 1 (purple poles)
and 2 (blue poles), as well as before (top) and after (bottom) tilt correction. Rose diagrams highlight a scatter of flow plane dip directions measured across the entire Garbi Block (Supplementary Data Fig. S3). Arrows show mean dip direction of planes. e: Plot of tilt-corrected dips (°) vs altitude (m) along the hillside located in b. Corresponding tilt-corrected measurements are also reported on the stereonet.

Fig. 12. a, b: Field photographs illustrating a SW-dipping thick succession of Dahla Fm. basalts along the SW flank of the Danakil Block. Note the apparent decrease in dips of Dahla Fm. basalts, toward the SW, in b. c: locations of photographs a and b.

Fig. 13. a: Regional synthetic cross-section at a crustal scale (profiles drawn in b and in Fig. 2c). The Moho profile is based on the published results quoted in b. No assumption is done onto the precise nature of the crust beneath the SDRs which remains unknown. The ages of different volcanic wedges are reported above the topographic profile. The Finto and Affara Dara areas are covered by Stab et al.'s (2016) cross-section in N Afar (profile B in b). Note that Quaternary sediments and recent volcanism are omitted. b: Available crustal thickness data (in km) plotted on the map of the Fig. 2c. Crustal thickness measurements are projected normally onto the profile from distances less than 50km.

Fig. 14. a: Cross sectional evolution for the Ethiopia-Afar-Yemen rift system, from Early Oligocene traps emplacement (31-27Ma) to the present-day, at a crustal scale (initially, ~35km thick). b: Kinematic reconstructions from GPlates based on the recent Euler's poles estimations of Müller et al.'s (2016) for stages 2 to 6. Because tectonic motions before 20 Ma are not considered in the Muller et al.'s (2016) model, a simplified sketch is used for stage 1, to account for the possible initial fit between the Arabian and Nubia-Somalian plates. AS: Ali Sabieh Block. Plate boundaries in the final sketch (bottom) are modified from Viltres et al. (2020). Transfer/transform zones are from Audin et al. (2004), Leroy et al. (2012) and Stab et al. (2016). Ar: Arabia, BaM: Bab al Mandab; Da: Danakil, GAMI: Ghoubbet Asal-Manda Inakir; OC: oceanic crust, Nu: Nubia, So: Somalia.
a

VOLCANIC PASSIVE MARGIN (VPM)

Proximal

Necking

Distal

Traps

Inner SDRs

Outer SDRs

Flat-lying flows

Upper Crust

Lower Crust

High velocity lower crust
(7.2 - 7.3 km.s⁻¹)

Oceanic upper
and lower crust

b

1. Necking stage: C-Block

<figure><img src="image_2.png" /></figure>

2. C-Block dismantlement

<figure><img src="image_3.png" /></figure>

3. Lower crust exhumation

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /></figure>

b Scene 1

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

c Scene 2

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>
<figure><img src="image_23.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>
a

<figure><img src="image_28.png" /></figure>

b

<figure><img src="image_29.png" /></figure>

Rhyolitic flows (n = 61)

<figure><img src="image_30.png" /></figure>

Tilted sequences
beneath rhyolites (n = 44)

<figure><img src="image_31.png" /></figure>

Extensional fan in c (n = 5)

<figure><img src="image_32.png" /></figure>

Extensional fan in d (n = 7)

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>
<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

Profile of
measures
plotted in e

<figure><img src="image_37.png" /></figure>

N = 88
S
N229

<figure><img src="image_38.png" /></figure>

N = 88
S
N229

<figure><img src="image_39.png" /></figure>

Tilt correction
(20° clockwise
around the axis
striking N130°E)

N = 88
S
N229

<figure><img src="image_40.png" /></figure>

n = 88

<figure><img src="image_41.png" /></figure>

<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /></figure>

n = 88
<figure><img src="image_45.png" /></figure>
a

NECKING ZONE

SW

Wolfenden et al.'s (2005)
cross-section (A)

Southward extrapolation
and modification from Stab
et al.'s (2016) cross-section (B)

SE Afar rift
(this study)

Danakil
(this study)

Bab Al Mandab
Strait

SW Yemen
(this study)

Not studied area NE
covered with traps

<figure><img src="image_46.png" /></figure>

NECKING ZONE

Faults

Syn-magmatic growth faults (inferred)

Recent normal faults (< 1 Ma)

<figure><img src="image_47.png" /></figure>
a

Stage 1.

Traps

31-27 Ma

Upper crust

Lower crust

Moho

Traps emplacement

Stage 2.

<figure><img src="image_48.png" /></figure>

27 - 16 Ma

<figure><img src="image_49.png" /></figure>

<figure><img src="image_50.png" /></figure>

<figure><img src="image_51.png" /></figure>

Syn-magmatic necking

Stage 3.

16 - 10 Ma

<figure><img src="image_52.png" /></figure>

Central Afar

BaM

Dismantlement of the C-block

> 4 - 1 Ma (Upper Dahia + Stratoid)

<figure><img src="image_53.png" /></figure>

Rift axis

Active fault

<figure><img src="image_54.png" /></figure>

Upift

Inactive fault

Stage 4.

10 - 5 Ma

<figure><img src="image_55.png" /></figure>

Central Afar

BaM

Stage 5.

~ 5 - 1 Ma

<figure><img src="image_56.png" /></figure>

GAMI

Abandon of the Bab al Mandab rift axis (BaM) and asymmetric VPM development in Afar

Stage 6.

< 1 Ma

<figure><img src="image_57.png" /></figure>

GAMI

Pre-oceanization tectonic

<figure><img src="image_58.png" /></figure>

Syn magmatic growth faults

Transfer zone

Main rift axis

Oceanic crust

b

<figure><img src="image_59.png" /></figure>

<figure><img src="image_60.png" /></figure>

<figure><img src="image_61.png" /></figure>

<figure><img src="image_62.png" /></figure>

<figure><img src="image_63.png" /></figure>

<figure><img src="image_64.png" /></figure>
# Volcanic passive margins and break-up processes in Afar - Yemen

## Structure of volcanic sequences and crustal architecture

<figure><img src="image_65.png" /></figure>

<figure><img src="image_66.png" /></figure>

<figure><img src="image_67.png" /></figure>

<figure><img src="image_68.png" /><figcaption>Stereo-photogrammetric DEM of Pleiades satellite images</figcaption></figure>

<caption>High frequency measurements of dip and dip direction within volcanic sequences</caption>

<caption>Crustal-scale margin architecture and reconstitution of the rift dynamic</caption>