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# Structure and morphology of the Red Sea, from the mid-ocean ridge to the ocean-continent boundary

Antoine Delaunay, Guillaume Baby, Jakub Fedorik, Abdulkader M. Afifi, Paul

Tapponnier, Jérôme Dyment

► To cite this version:

Antoine Delaunay, Guillaume Baby, Jakub Fedorik, Abdulkader M. Afifi, Paul Tapponnier, et al. Structure and morphology of the Red Sea, from the mid-ocean ridge to the ocean-continent boundary. Tectonophysics, 2023, 849, 10.1016/j.tecto.2023.229728. insu-04155767

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Submitted on 7 Jul 2023

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# Structure and morphology of the Red Sea, from the mid-ocean ridge to the ocean-continent boundary

Antoine Delaunay<sup>a, *</sup>, Guillaume Baby<sup>a</sup>, Jakub Fedorik<sup>a</sup>, Abdulkader M. Afifi<sup>a</sup>, Paul Tapponnier<sup>b</sup>, Jérôme Dyment<sup>c</sup>

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## ARTICLE INFO

**Keywords:**

Red sea

Seafloor spreading

Mid ocean ridge

Transform faults

Oceanic crust

Ocean-continent boundary

Geodynamic

## ABSTRACT

The diversity and complexity of existing models on the structure of the Red Sea is the result of a mobile sedimentary cover that not only hides most of the basement and oceanic structures but also hamper the geophysical signals. In this study, we combined published with new unpublished geophysical data to propose a revised interpretation of the structure of the lithosphere between the Hanish Islands (ca. 13.5°N) and the Dead Sea Fracture Zone (ca. 27.5°N). The recognition of allochthonous salt and overburden allows to propose a structural model with simple geometries that are consistent with the present-day kinematic constraints (GPS velocities and Euler Poles) and with the rules of plate's motion on a sphere (Euler Theorem).

We mapped a continuous Mid Ocean Ridge segment parallel to the main rift orientation in the southern Red Sea that becomes highly discontinuous and more oblique towards the north as it gets closer to the Euler Pole. These segments all point towards the Euler pole and are consistently offset to the right by transform faults in the central and northern Red Sea cumulating an offset of 400 km. The transform faults are accommodating seafloor spreading in the Red Sea and cluster in three main fracture zones: (1) the Ikwan Fracture Zone near the Ikwan (or Brother) Islands, (2) the Zabargad Fracture Zone in the vicinity of the Zabargad Island and (3) the Jeddah Fracture Zone offshore the Jeddah/Port Sudan area (central Red Sea). Based on a new seismic reflection dataset and published data we propose an interpretation of the Ocean Continent Transition (OCT) in the Red Sea. We bring new evidence (magnetic lineaments, distance of the OCT to the ridge, basin stratigraphy) to support a Middle Miocene (ca. 14 Ma) age for the initiation of seafloor spreading throughout the Red Sea, and we discuss this event with the initiation of strike-slip motion in the Aqaba-Dead Sea transform zone. Finally, compiling all available data and literature, we propose a regionally coherent geodynamic reconstruction using two different poles of rotation for the rifting and seafloor spreading phases.

## 1. Introduction

The Red Sea and Gulf of Aden are young oceans formed by rifting and seafloor spreading that resulted from the northward drift of the Arabian Plate relative to the Nubian and Somalian plates. Spreading rates increase from 13 to 18 mm.yr⁻¹ along the Gulf of Aden (Jestin et al., 1994) and decrease northward along the Red Sea from 16 mm.yr⁻¹ at 18°N to 10 mm.yr⁻¹ at 27°N (Chu and Gordon, 1998). This motion is accommodated by transform plate boundaries along the Dead Sea Fault Zone to the west of the Arabian Plate, and the Owen Fracture Zone to its east. The northeast drift of the Arabian and Nubian plates is accommodating

the subduction of Tethyan oceanic lithosphere in the Makran trench and continental collision along the Bitlis-Zagros fold-and-thrust belt (Fig. 1).

Previous work on the regional kinematics show that the motion of Arabia relative to Nubia can be described by an Euler Pole located in northeast Libya/northwest Egypt, which was determined through seafloor spreading magnetic anomalies (Chu and Gordon, 1998) and GPS measurements (ArRajehi et al., 2010; Reilinger and McClusky, 2011; Schettino et al., 2016). In the southern part of the Red Sea, the separation between Arabia and Nubia is accommodated not only by seafloor spreading, but also by continental rifting in the Afar region which resulted in the anticlockwise rotation of the Danakil bloc (e.g. Eagles

<footnote><sup>a</sup> Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia</footnote>

<footnote><sup>b</sup> National Institute of Natural Hazards, Haidiam, Beijing, China</footnote>

<footnote><sup>c</sup> Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: antoine.delaunay@kaust.edu.sa (A. Delaunay).</footnote>
et al., 2002; McClusky et al., 2010).

Geologic data within the Red Sea are scarce because it extends through the territorial waters of five countries with overlapping economic interests, which hinders the acquisition and sharing of geological and geophysical data. The Deep Sea Drilling Project has established the presence of oceanic crust within the axial trough together with observation of axis-parallel magnetic anomalies (e.g., Drake and Girdler, 1964; Roeser, 1975; Hall, 1979) whereas most offshore exploration wells drilled along the margins penetrated continental crust (Bosworth, 1993). The oceanic basement is exposed only along the narrow axial trough and a few scattered deeps (Fig. 1). Elsewhere, the basement in the Red Sea Basin, whether continental or oceanic, is covered by an extensive blanket of Miocene and younger sedimentary rocks, including a > 1 km thick Middle to Late Miocene evaporites (e.g., Heaton et al., 1995; Mitchell et al., 2010; Mohriak, 2019). This sedimentary blanket hides the basement morphology, and leads to ambiguous interpretations of magnetic and gravity surveys (e.g., Girdler, 1985; Izzeldin, 1987; Augustin et al., 2021).

Despite the scarcity of data, there is no shortage of proposed hypotheses for the tectonic evolution of the Red Sea. These can be grouped in two categories: (1) mainly rift, and (2) mainly ocean. The “mainly rift” hypothesis (e.g., Girdler, 1968; Cochran, 1983; Bosworth, 1993; Bosworth and Burke, 2005; Ligi et al., 2018) assumes that the Tertiary sediments in the Red Sea basin are rift sediments that were split apart by seafloor spreading which created the relatively young (<5 Ma) oceanic

crust within the axial trough between 23°N and 14°N (Fig. 1b). This hypothesis implies that continental breakup has not propagated to the northern Red Sea, which remains a rift underlain by hyper-extended continental crust. The “mainly ocean” model (e.g., Girdler, 1968; Girdler, 1985; Girdler and Southren, 1987; Izzeldin, 1987; Dyment et al., 2013; Tapponnier et al., 2013; Augustin et al., 2021) propose that seafloor spreading started during the Middle Miocene throughout the Red Sea. Therefore, 2/3 of the Red Sea is underlain by Late Miocene and younger oceanic crust, which is hidden under a blanket of allochthonous sediments.

This study expounds on the “mainly ocean” model. Our understanding of the Red Sea is based on various data including stratigraphic, structural, bathymetric, seismic reflection, gravity, and magnetic surveys. First we provide a new tectonic map of the Red Sea’s mid-ocean spreading segments, faults and the ocean-continent boundary. We show that oceanic spreading has been synchronous throughout the Red Sea since the Middle Miocene (ca. 14 Ma). The resulting separation of the Nubian and Arabian plates can be defined around two Eulerian poles. Last, we propose a revised geodynamic history of the Red Sea Ocean from the initiation of a continental rift during the Late Oligocene to continental breakup during the Middle Miocene, and subsequent oceanic spreading and burial of oceanic crust by the advance of allochthonous salt and rafted minibasins.

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Tectonic and topographic setting of the Red Sea. a: Compilation of all available bathymetric data (see text for references). b: Our structural interpretation showing the seafloor spreading segments, major transverse structures, and exposed oceanic crust. Small orange circles show Euler poles for Arabia-Nubia (Je = Jestin et al. (1994); Ch = Chu and Gordon (1998); Ar = ArRajehi et al. (2010); Mc = McClusky et al. (2010); Re = Reilinger and McClusky (2011)). Green circle for Arabia-Danakil (Mc = McClusky et al., 2010). Blue circles for Nubia-Danakil (Vi = Viltres et al. (2020)). Spreading rates are from Chu and Gordon (1998) and GPS plate velocities from ArRajehi et al. (2010). Rectangles show the location of Figs. 3, 5, 6 and 7. c: Simplified tectonic map of the Arabian Plate and surroundings. DSFZ: Dead Sea Fracture Zone; OFZ: Owen Fracture Zone. d: Syn-rift to post-rift chronostratigraphic chart of the Red Sea (see text for references). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
## 2. Geological context

The Red Sea trends NNW, it is >1900 km long and its maximum width is 250 km. It consists of a narrow axial trough, 1000-2000 m deep, bounded in the southern Red Sea by a shallow (<200 m) shelf, which deepens in the northern Red Sea to ca. 800 m (Fig. 1a). Its margins are uplifted to 2–3 km (white areas in Fig. 1a), and expose exhumed Proterozoic basement in the Nubian and Arabian shields. The general orientation of the main Red Sea structures (spreading axis and rift related faults) are perpendicular or oblique to terrane boundaries and fault zones in the Proterozoic basement (Stern and Johnson, 2019). Pre-rift magmatism covered large areas in Ethiopia and Yemen with continental flood basalts (Ethiopian-Yemen traps; Hofmann et al., 1997). The onset of rifting is contemporaneous with Late Oligocene magmatism found onshore which is expressed as dykes swarms mainly located along the eastern margin (Coleman et al., 1979), as granite batholiths (e.g., Coleman et al., 1992) and as continental flood basalts.

Early research in the Red Sea included ship borne magnetic surveys, particularly covering the axial trough.

The linear magnetic anomalies characterized on the exposed oceanic crust in the southern Red Sea initially suggested that seafloor spreading initiated around 5 Ma (e.g., Vine, 1966). The extent of the oceanic crust under the Red Sea basin was questioned by the first tectonic models (McKenzie et al., 1970). However Girdler and Styles (1974) presented evidence of older linear magnetic anomalies under the shelf which they thought were Eocene. Later, Izzeldin (1987) presented evidences from aeromagnetic and seismic data of symmetrical magnetic anomalies extending under the shelf some 80 km from the spreading center. This was recently confirmed by high resolution multichannel seismic data which showed unambiguously that the oceanic crust was largely covered by allochthonous sediments and suggest that oceanization began in the Middle Miocene (Tapponnier et al., 2013) contemporaneously with the initiation of the Aqaba-Dead Sea strike-slip system.

The first stratigraphic evidence of rifting is represented by a late Oligocene continental formation known in the northern Red Sea as the Al-Wajh Formation in Saudi Arabia (e.g., Hughes and Johnson, 2005) and the Abu Zenima Formation in Egypt (e.g., Patton et al., 1994). They include a variety of red beds (conglomerates, sandstones, and siltstones) that were deposited within low subsiding half-grabens as alluvial fan, fluvial and lacustrine sediments. Their late Oligocene to early Miocene age is constrained by (1) basalt pebbles at the base of the red beds with radiometric dates of 25–26 Ma (Montenat et al., 1998) and (2) the appearance of early Miocene (Aquitanian) fauna in the overlying marine deposits that mark the first incursion of the sea in the northern Red Sea. These Aquitanian marine deposits are called the Musayr and Yanbu formations in Saudi Arabia (e.g., Dullo et al., 1983; Hughes and Johnson, 2005) and the Nukhul Formation in Egypt (e.g., Carr et al., 2003; Al-Husseini, 2012). In the southern Red Sea (south of Jeddah), the continental rift was filled by the Jizan Group made of a variety of basaltic lava flows, volcanoclastic sediments as well as the lacustrine rhyolitic tuff of the Baid Formation (Schmidt and Hadley, 1985). The Jizan Group is Oligocene (30–23 Ma) based on vertebrate fossils (Schmidt et al., 1983) and on $^{40}$Ar–$^{39}$Ar dating of the volcanics (Sebai et al., 1991).

Subsequently, the climax of rifting occurred during the Burdigalian with the deposition in open marine environments of debris flows, turbiditic sandstones and foraminiferal marls (Burqan Formation, equivalent to Lower Rudeis Formation in Egypt – e.g., Patton et al., 1994; Hughes and Johnson, 2005).

The continental and marine rift deposits of the Red Sea are unconformably overlain by a uniform sequence of interbedded marine clastics, carbonates and anhydrides sediments known as the Maqna Group in Saudi Arabia and the Upper Rudeis, Kareem and Belayim formations in Egypt (Patton et al., 1994; Hughes and Johnson, 2005). A diverse assemblage of marine fauna indicates a Langhian age. This formation is contemporaneous with a drastic decrease in tectonic subsidence in the Gulf of Suez (e.g., Moretti and Colletta, 1987) and along the Red Sea. It

also marks the initiation of the Aqaba-Dead Sea strike-slip system (e.g., Patton et al., 1994; Bosworth and Burke, 2005; Tubbs et al., 2014). Increasing restriction from the Tethys Ocean to the north due to this tectonic reorganization culminated in the deposition of a massive halite layer throughout the Red Sea and Gulf of Suez which corresponds to the Mansiyah Formation in Saudi Arabia, South Garib Formation in Egypt, Dunguna Formation in Sudan and Amber Formation in Eritrea (see Bosworth and Burke, 2005 and associated references). The age of this formation is very uncertain. By analogy with other large salt provinces such as the South Atlantic (e.g., Chaboureau et al., 2012), it is conceivable that this massive salt was deposited over a short period of time (×1 Myr) suggesting a Serravallian age. The original thickness of this massive salt is unknown due to its mobility, but it varies under the shelves from no salt (salt welds) to as much as 4 km thick (salt diapirs). The average original thickness might have been around 2 km as was estimated by Heaton et al. (1995) under the Yemen shelf. The massive salt is conformably overlain by a late Miocene layered overburden of interbedded clastics and evaporites known as the Ghawwas Formation in Saudi Arabia and Zeit Formation in Egypt (e.g., Hughes and Johnson, 2005). Along the margins, this formation consists of coarse- to fine-grained basement-derived siliciclastics. This middle-late Miocene evaporite sequence of massive halite and layered evaporites is truncated overall the Red Sea by the Messinian unconformity, or reflector “S” (Phillips and Ross, 1970).

The overlying Plio-Pleistocene section (Lisan Group), consists of open marine clastic sediments and carbonates deposited above the Messinian unconformity. It marks the return to normal marine conditions that allowed reef growth along the margins (e.g., PLAZIAT et al., 2008).

## 3. Data and methods

### 3.1. Bathymetric data

The compilation of the bathymetry of the Red Sea (Fig. 1a) is based on gridded bathymetric data of the General bathymetric Chart of the Oceans (GEBCO) with a 300 m resolution, merged with higher resolution multibeam data acquired over the axial segments and deeps in the central and northern Red Sea (Ehrhardt et al., 2005b; Mitchell et al., 2010; Augustin et al., 2014). We also utilized multibeam bathymetric surveys with a 30 m resolution for interpretation of the mid-ocean ridge extending from 19°N to 16.2°N, including the Saso Deep (Fig. 1).

### 3.2. Gravity data

Free air gravity anomaly data are from Sandwell et al. (2014) that combines new radar altimeter measurements from CryoSat-2 and Jason-1 satellites with existing data. See Sandwell et al. (2014) for details about this data.

### 3.3. Aeromagnetic data

The magnetic anomaly maps used in this study are a compilation of several datasets, including Cochran (2005a) in the northern part of the Red Sea, Izzeldin (1987) in the middle part, and Almalki et al. (2014) in the southern part. Our interpretation is based on the published images of these datasets.

### 3.4. Seismic data

Two seismic reflection datasets were used to construct the depth to basement map of the central Red Sea area. The first 2D survey was acquired with a set of 8 to 10 km long streamers and processed through prestack depth migration. It has a spacing of ca. 10 × 10 km and extends from the shelf depth of ca. 300 m to the mid oceanic ridge in ca. 2300 m of water with a total length of about 4380 km (Fig. 2a). The time depth
conversion of this dataset was done using a multi-layered velocity model in order to illuminate subsalt targets. The second survey was acquired by CGG in 1976 and their interpretation was published by Izzeldin (1987). It consists of 16 cross-profiles, oriented N60 and spaced ca. 40 km apart, covering the central part of the Red Sea (Fig. 2a) from the Sudanese shelf (depth of ca. 100–500 m) to the Saudi shelf (depth of ca. 200). The interpretation of these profiles was converted to depth by Mitchell et al. (2020) using a 3-layer velocity model.

### 3.5. Structural mapping

The structural map of the Red Sea presented here is consistent with a regional tectonic model. Indeed, the motion of a rigid plate with respect to another is described by a pole and an angle of rotation, as is the motion of a point on a sphere (Euler Theorem: e.g., Joseph, 2020). Therefore the motion along transform faults, hence the direction of these transform faults, is along small circles around that pole of rotation (Le Pichon, 1968). This absolute constraint will not tolerate any exception if the plates are rigid and properly defined. Conversely, the direction of spreading centers are generally perpendicular to that of the transform faults, but it is not an absolute constraint as oblique ridge segments are not unusual. These rules were applied in mapping Red Sea ridge and structures.

Mapping was done on ArcMap 10.8.1 using the data previously described. In order to preserve angles and distances, we used the Lambert Conformal Conic projection system with a central meridian at 48°E and standard parallels 1 and 2 at 17°N and 33°N respectively. This projection's datum is Ain El Abd 1970. Figs. 1 and 8 were projected using

UTM37N with WGS84 datum.

### 3.6. Seismic interpretation

The seismic surveys are tied to exploration and DSDP wells throughout the eastern part of the Red Sea, (e.g., Ahmed, 1972; Whitmarsh, 1974; Heaton et al., 1995; Hughes and Johnson, 2005).

Stratigraphic mega-sequences were mapped based on well ties and the principles of seismic stratigraphy (Vail and Mitchum Jr, 1977). Regional discontinuities were identified on the basis of reflection termination patterns, reflection amplitude, wavelength and continuity (Fig. 2). The offshore sequences from oldest are:

1. The acoustic basement which is a transparent to chaotic seismic unit, lacking coherent reflections. Its base is not imaged and its top is a high amplitude, long wavelength discontinuous reflector.

2. The salt bodies which are made of a transparent seismic facies, very inhomogeneous in thickness (diapirs and salt welds), limited at the bottom by the acoustic basement.

3. The layered evaporites unit is made of parallel to subparallel undulated reflectors, onlapping the salt bodies. The reflectors of this unit are truncated at the top by the reflector "S" (i.e., reflector "S"; Phillips and Ross, 1970), marking the upper limit of the layered evaporites at the Mio-Pliocene boundary.

4. Finally, the uppermost unit, limited at the base by the reflector "S" and at the top by the seafloor, displays flat, parallel, high amplitude small wavelength reflectors, onlapping all other observed features. This unit is made of all the Plio-Pleistocene and most recent

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** a. Localization of the seismic dataset. b. Typical seismic facies and reflectors found across the Red Sea Basin. c. Schematic section from Red Sea axis towards the arabian margins with location of typical seismic facies shown in (a.). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
hemipelagic sediments. Note that in the central trough of the Red Sea, where oceanic crust is observed, the top of the acoustic basement is the seafloor (Fig. 3b).

## 3.7. Basement depth map

Depth to basement was mapped throughout the Red Sea using published (Izzeldin, 1987) and unpublished bathymetric, seismic, gravity and magnetic data, together with previous interpretations (Izzeldin, 1987; Izzeldin, 1989; Shi et al., 2018). The seismic horizon corresponding to the top basement was georeferenced and digitized within

the Move software and interpolated using an ordinary kriging method with a grid spacing of 5 km (Fig. 3).

## 3.8. Kinematic reconstructions

The reconstructions presented in this study were done using GPlates (2.3.0) software (Müller et al., 2018). Arabia was restored to its pre-rift position relative to Africa in two steps, corresponding to the seafloor spreading and rift stages of continental separation. First, Arabia was rotated in a clockwise direction around the Euler Pole of ArRajehi et al. (2010) back to the initiation of seafloor spreading ca. 14 Ma ago,

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Right lateral discontinuities within central Red Sea basement. a: Map showing the depth of the basement across the central Red Sea based on interpretation of reflection seismic data. Inset shows sources of data: Blue: a grid of unpublished 2D seismic data on the Arabian side, line spacing x10 km. Grey: Saudi-Sudanese survey (Izzeldin, 1987). b: Line drawing of a 2-D seismic line (A-B) revealing a major fault offsetting the basement. Oceanic crustal ages were estimated using a spreading rate of 6.2 km.Myr⁻¹ at 21.8°N (Chu and Gordon, 1998). (AB located in (a)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
bringing the Red Sea into alignment with the abandoned Gulf of Suez rift and honouring the onshore geological constraints compiled by Stern and Johnson (2019). Second, we derived an Euler pole for the initial rift stage (30–14 Ma) by projecting the rift border faults and rift-parallel dikes which we mapped on both sides of the Red Sea. This rift pole was then used to restore Arabia to its pre-rift position.

The time constraints used in our reconstruction are based on the spreading rates of Chu and Gordon (1998), on the magnetic anomalies of the southern and central Red Sea (Dyment et al., 2013; Okwokwo et al., 2022), and on the tectono-stratigraphy along the Arabian margin (see Section 2). This deformation timeframe will be further discussed.

# 4. Results

## 4.1. General morphology of the mid-ocean ridge

The mid-ocean ridge (MOR) in the Red Sea is situated along its deepest parts (Fig. 1a). The term MOR is used sensu lato to refer to the seafloor spreading axis. While it does not necessarily imply the presence of a topographic ridge on the seafloor, it is clearly a ridge as displayed on the basement depth map in the central Red Sea (Fig. 3a).

Fig. 1 shows the narrow and deep axial trough where the oceanic basement is exposed, as opposed to the flanking shelves which are underlain by a 1–6 km thick blanket of sediments (e.g., Fig. 3; Izzeldin, 1987; Heaton et al., 1995; Tubbs et al., 2014). The depth of the axial trough ranges from 1000 to 2800 m, whereas the broad shelves are shallower, and their depth increases from 200 m in the south to ca. 1000 m in the central and northern parts of the Red Sea. Bathymetric surveys compiled by Augustin et al. (2014) in the axial trough of the central Red

Sea show that it has the morphology of slow spreading ridges, with short ridge segments nested within symmetrical grabens which are offset by right-stepping discontinuities. The axial valley is deepest in the central Red Sea, and becomes shallower towards the North and South (Fig. 1a). Between Yemen and Eritrea, there are no detailed bathymetric data covering the MOR segment south of the Saso Deep. Here, the MOR is straddled by a chain of basaltic volcanoes that form the Farasan and Dahlak islands whose petrology was described by Gass et al. (1973). The MOR terminates at the transverse Hanish volcano chain (Fig. 1b), which is a transfer zone between the Arabian Plate and Danakil bloc (Viltres et al., 2020).

The largest exposure of oceanic crust occurs in the South between 14°N and 20.5°N, where the axial trough is bisected by a long nested axial graben floored by volcanic features (Fig. 4a). Away from the spreading axis (between 5 and 20 km), these features become progressively covered by flat lying, thickening pelagic sediment wedges as described from DSDP Sites 225–229 (Stoffers and Ross, 1974). At distances >20–25 km from the axis, the oceanic crust is overridden by the advancing compressional front of the allochthonous Middle Miocene halite and its rafted overburden of Late Miocene interbedded evaporites and shales whose thickness exceeds 1–2 km (Fig. 4).

Between 20.5°N and 25.4°N, the window of exposed oceanic crust along the axial trough becomes narrow and discontinuous, reflecting the competition between seafloor spreading and the advancing salt fronts from both the Arabian and Nubian margins. Eventually, when salt flow velocities exceed the half-spreading velocities, the salt fronts meet along a salt suture and the mid-ocean ridge is completely covered (Augustin et al., 2014). These relatively shallow, smooth-bottomed areas where a salt suture is observed were defined by Tramontini and Davies (1969), as

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Line drawing of 2 seismic lines orthogonal to the spreading center of the southern Red Sea (approximate location is shown on Fig. 7). Both lines display a compressional front of Middle Miocene and younger allochthonous sediments overthrusting the younger oceanic crust. a: Axial volcanoes formed during dominantly magmatic phases are split in two and drifted in opposite directions during dominantly tectonic phases. b: Wide and deep axial graben (Saso deep) was formed during a dominantly tectonic phase. A small axial volcanic ridge may be marking a shift towards a dominantly magmatic phase. Spreading rates based on Chu and Gordon (1998). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
“inter-trough zones”.

In the northern axial trough, the ultra-slow spreading axis becomes progressively more oblique and spreading velocities decrease towards the Euler Pole. It is mostly buried under the allochthonous sedimentary rocks which conceal the underlying basement topography, except in a few windows at the Mabahiss, Shaban, and Conrad deeps (Fig. 1). The Mabahiss and Shaban deeps both expose volcanic features which indicate the presence of oceanic crust in the north (Haase et al., 2000).

## 4.2. Discontinuities across the oceanic basement in the Red Sea

The presence of extensive oceanic basement in the central Red Sea was initially proposed by Izzeldin (1987) based on seismic, gravity, and magnetic surveys acquired by the Saudi Sudanese Commission. The latest models relying on satellite gravity and magnetic data confirm this interpretation (Shi et al., 2018; Augustin et al., 2021; Okwokwo et al., 2022). Fig. 3a shows the depth to the basement in the central Red Sea. It forms an axial high along the seafloor spreading axis that progressively deepens towards the margins in the direction of plate separation. This geometry is quite typical of oceanic crust worldwide and is consistent with the “mostly oceanic” model for the Red Sea. The deepest basement (ca. 6000 m) is located ca. 100 km from the mid-ocean ridge on both sides. Beyond this limit, rifted continental basement rises towards both margins in a staircase of tilt-blocs/half grabens (not shown on Fig. 3b) until it outcrops along the coastal plains. The axial ridge segments (highs) are regularly offset to the right (up to 30 km), which amounts to a total right lateral offset between 20°N and 22°N of ca. 150 km (Fig. 3).

Fig. 3b shows the interpretation of a 2D seismic line across one of these major transverse discontinuities. The top of the acoustic basement is a prominent reflector which was loop-tied throughout the seismic grid and tied to oceanic basement penetration in DSDP site 226. The

basement is downdropped by ca. 1.2 km, which juxtaposes 3.5 Ma Pliocene oceanic crust against deeper, Upper Miocene (> 5.5 Ma) oceanic crust. The Late Miocene oceanic crust is allochthonously covered by the mobile salt of the Middle Miocene Mansiyah Formation and its overburden of Late Miocene minibasin fill of the Ghawwas Formation. The down stepping of the acoustic basement on the section (Fig. 3b) corresponds to one of the major discontinuities observed on the basement depth map (Fig. 3a). These discontinuities have a consistent orientation as they all fit segments of small circles centered on the Euler Pole of ArRajehi et al. (2010) for rotation of the Arabian Plate relative to the Nubian Plate.

Figs. 5 to 8 show our new interpretation of the structures and discontinuities in the Red Sea, divided into three major segments that are the northern, the central and the southern Red Sea. The MOR is rift-parallel in the south, but becomes rift oblique towards the north. The overall right lateral offset (ca. 400 km) of the Red Sea axis is accommodated by ridge-ridge transverse structures clustering in three main fracture zones that are from north to south, the Ikwan Fracture Zone, the Zabargad Fracture Zone and the Jeddah Fracture Zone in the central Red Sea (Figs. 1 and 3).

## 4.3. Northern Red Sea

Fig. 5 shows the major structures in the northern Red Sea. The continental shelves are generally 600–1000 m deep, except for a 30 km wide belt of shallow reef platforms along the coast. The shelves deepen gradually towards a broad oblique axial trough at a depth exceeding 1200 m (Fig. 5a). The basement is fully blanketed by allochthonous salt and sediments. However, there are windows exposing oceanic crust in the Conrad, Shaban and Mabahiss deeps at depth of 1550, 1600 and 2300 m respectively. There are also windows exposing the continental

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** New tectonic interpretation of the northern Red Sea showing spreading center and transform faults. For location see Fig. 1. a: Combined bathymetric data (see references in text). White dots represent earthquake locations between 1922 and 2022 (USGS earthquake catalog). NWB: Northwest Bounding fault. SEB: Southeast Bounding fault. VR: Volcanic Ridge. b: Total intensity magnetic anomaly map contoured at 50nT intervals by Cochran (2005b). c: Free Air satellite Gravity anomaly data by Sandwell et al. (2014). Isoline spacing 20mGal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
basement in the Ikhwan (Brothers) Islands, and Zabargad Island (Fig. 6a).

The spreading axis extends for 600 km from the Sinai triple junction to the Nereus Deep following a general orientation of ca. N26°W, aligned with the Gulf of Suez (Fig. 5a).

Even though sediments are entirely covering the basement, the few morphologic features observed on the seafloor may be controlled by deeper structures imaged on the total intensity magnetic anomaly map. These features include: (1) the Conrad Deep which is a 2 by 10 km elongated deep with an overall strike of N20°E, parallel to the spreading direction, which was described by Ehrhardt et al. (2005a) as a pull-apart basin witnessing transtension along a fault structure in the basement; (2) the Shaban Deep which is a 6 by 5 km wide deep made of a central volcanic ridge striking along the MOR and witnessing seafloor spreading (Ehrhardt and Hübscher, 2015); (3) around 27°N, a normal fault scarp, parallel to the rift orientation that is offset to the right (Fig. 5a); and (4) offshore Umluj, the northwest edge of the shelf which is aligned with the northernmost discontinuity in the Zabargad Fracture Zone (Fig. 6a). Onshore on the Zabargad Island itself, Marshak et al. (1992) and Bosworth et al. (1996) identified a set of faults displaying fault planes and striations parallel to the spreading direction that they interpreted as the extremity of the Zabargad Fracture Zone (Fig. 6a).

There are no available aeromagnetic surveys that cover the northern Red Sea, and we rely on less accurate shipborne surveys compiled by Cochran (2005b), shown in Fig. 5b. These surveys are low resolution, not reduced to pole, and do not resolve the linear magnetic anomalies that we used to map the spreading axis in the central and southern Red Sea. Nevertheless, the magnetic dipole anomalies in Fig. 5b correspond to oceanic basement exposed in the Mabahiss and Shaban deeps (Ehrhardt and Hübscher, 2015). Moreover, some anomalies are aligned NE-SW, parallel to our inferred faults following the trend highlighted and described in the new magnetic anomaly map compiled by Issachar et al. (2022). These transverse structures are oblique to the onshore rift normal faults, and are parallel to the small circles centered on the Euler

Pole of rotation calculated by ArRajehi et al. (2010) for Nubia-Arabia plate motion. These features are similar to the structures observed in the basement depth map of the central Red Sea (Fig. 3). Finally, we plotted the earthquakes from the USGS earthquake catalog covering the last 100 years. Although the area is seismically quiet, there are several low magnitude (<3.9) earthquakes aligned along the transverse faults observed in the basement (Fig. 5). In particular, an earthquake of magnitude 5.5 in June 2020 is located on one of our mapped faults, and his moment tensor displays clear strike-slip component (Fig. 5a).

We count 15 right-lateral structures cumulating an offset of ca. 260 km in the northern Red Sea, half of which occurs within the Zabargad Fracture Zone (Figs. 1, 5 and 6), marking the transition from the northern to the central Red Sea.

4.4. Central Red Sea

The 400 km highly segmented central Red Sea extends from the Zabargad Fracture Zone to the Suakin Deep following a general orientation of N163°E. The narrow shelves drop by ca. 500 m to a wide, flat and smooth area, symmetric on both sides of the Red Sea. The bathymetry then drops dramatically to the axial valley by ca. 1000 m in the Inter Trough Zones (Tramontini and Davies, 1969) where salt namakiers blanket the spreading axis Where the salt doesn't fully cover the seafloor, the bathymetry can drop by as much as 2000 m (Figs. 6a and 7d).

The gravity and particularly the magnetic reversal anomalies are highly segmented in the central Red Sea which makes their interpretation difficult (Searle and Ross, 1975; Okwokwo et al., 2022). It seems for most of them that they are parallel to the spreading axis orientation, but some are so short that it is impossible to distinguish their strike direction. Moreover, the lack of such anomalies along the spreading axis within the Inter Trough Zones increased the ambiguity on how to interpret them (Searle and Ross, 1975). Despite all of this, detailed studies of the magnetic anomaly pattern in the central Red Sea combined with numerical modelling (e.g. Searle and Ross, 1975; Okwokwo et al.,

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** New tectonic interpretation of the central Red Sea. White dots represent earthquake locations between 1922 and 2022 (USGS earthquake catalog). Same as Fig. 5 except inset in a. Fault planes and striations for strike-slip faults measured on Zabargad Island by Bosworth (1995). Red curve is the mean fault plane direction measured, and b: Reduced to Pole magnetic anomaly map from the Saudi-Sudanese aeromagnetic survey (Zahran et al., 2003). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
2022), proved that they could be interpreted as oceanic crust magnetic anomalies (e.g. Girdler and Styles, 1976; Izzeldin, 1987; Dyment et al., 2013; Okwokwo et al., 2022; Fig. 6b). The gravity anomaly map by Sandwell et al. (2014) shows discontinuous structures offsetting a gravity high along the ridge axis (Fig. 6c). Despite the apparent chaos, these discontinuities follow a consistent pattern which allowed for their mapping throughout. They offset to the right the magnetic anomalies and follow a trend parallel to the relative spreading direction (Fig. 6). These discontinuities observed on the gravity and magnetic anomaly maps reflect the deep elongated structures cross-cutting the basement (Fig. 3a). From the Nereus to the Suakin deeps, the spreading axis accumulates an offset of ca. 200 km (Figs. 1 and 6).

### 4.5. Southern Red Sea

The continental shelves in the southern Red Sea are broad and shallow (generally <100 m) and form reef-studded banks including the Farasan and Dahlak islands (Figs. 1 and 7). They are underlain by the allochthonous sequence moving towards the Red Sea axis (Fig. 7d). Even though the shelves cover >80% of the seafloor, the compression front is

marked by a sudden bathymetry drop of >1000 m located about 20 km from the spreading axis exposing the rugged oceanic crust morphology (Figs. 1, 4 and 7). The northwest-southeast spreading axis lies within a well-defined and continuous axial graben from the Suakin to the Saso deeps (Fig. 7a). Our interpretation of the seismic lines (Fig. 7d), shows a classic “rift-in-rift” geometry with the axial graben defining the axial trough. Very thin layers of hemipelagic sediments drape the graben shoulders. The salt front displays evidence for thrusting and folding over the acoustic basement which appears as a strongly reflective, eastward deepening interface, broken up by minor faults (Fig. 7d). On the Fig. 7b, clear, continuous magnetic lineaments parallel to the ridge axis were described as clear oceanic seafloor magnetic anomalies (e.g. Izzeldin, 1987; Chu and Gordon, 1998; Dyment et al., 2013; Okwokwo et al., 2022). They are of decreasing amplitudes and increasing wavelengths going further away from the axis as a result of alteration (Klitgord et al., 1974) combined with deepening of the oceanic crust (Dyment et al., 2013).

While the northern and central Red Sea are highly segmented, the striking difference here is the lack of such discontinuities offsetting the spreading axis between 19.5°N and 16.3°N. Instead, a long wavelength

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** New tectonic interpretation of the southern Red Sea. White dots represent earthquake locations between 1922 and 2022 (USGS earthquake catalog). Bathymetry, magnetic and gravity anomalies as Fig. 6, b: dashed white lines numbered from 1 to 5 represent the symmetric magnetic anomalies. Chrons after Dyment et al., 2013, as follow: 1: 1n, 2: 2n, 3: 3n, 4: 4n, 5: 5n. Blow-up view shows the detailed bathymetry of an overlapping spreading center where the MOR (red) is offset. d. Dip cross section in the Red Sea showing the deepening of the volcanic oceanic basement towards the shoreline up to 95 km where is inferred the OCB. Half spreading rate at 19.4°N ca. 7.3 km.Myr⁻¹ (Chu and Gordon, 1998). (AB located on (a)). Above the section, gravity (blue) and magnetic (red) profiles along seismic section. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
bend in the ridge orientation is observed with very small offsets of the MOR without discontinuity observed in the magnetic or gravity anomalies (Fig. 7b). South of the Saso Deep, 4 small left-lateral discontinuities offset the ridge by no more than a total of 30 km. These are the only offsets to the left described in the Red Sea.

# 5. Discussion

## 5.1. Seafloor spreading

Given the amount of sediment and salt flooring the Red Sea, the interpretation of bathymetry or other geophysical data is challenging. This led to multiple interpretations and two end member models for the structure of the Red Sea. The first limits the extent of oceanic crust in the Southern Red Sea where high amplitude and short wavelength lineated magnetic anomalies younger than 5 Ma were clearly identified (e.g., Girdler, 1968; Cochran, 1983; Bosworth and Burke, 2005). The second one suggests that the Red Sea is entirely floored by oceanic crust (Girdler and Southren, 1987; Izzeldin, 1987; Tapponnier et al., 2013) since ca. 13–14 Myr (Izzeldin, 1987; Augustin et al., 2021). The different data sets presented in this study support the latter.

### 5.1.1. Magnetic anomalies

At the origin of the controversy about the nature of the crust in the Red Sea is that the only clear high amplitude, short wavelength anomalies symmetric with respect to the ridge that are characteristic of oceanic crust are restricted to the southern Red Sea, along a 40 km wide zone around the MOR. Beyond this zone, the anomalies are of low amplitude and long wavelength. This can be accounted for by low-pass filtering due to the thick sediment cover (Levi and Riddihough, 1986; Hall, 1989; Dyment et al., 2013; Tapponnier et al., 2013; Okwokwo et al., 2022) as seen in the Andaman Basin (e.g., Kamesh Raju et al., 2004), in the Gulf of Mexico (e.g., García-Reyes and Dyment, 2021), and along the Juan de Fuca Ridge (e.g., Levi and Riddihough, 1986; Currie and Davis, 1994). It could also reflect the absence of a well-defined layer of pillow lavas whose emplacement was hampered by rapid, abundant sedimentation (Dyment et al., 2013; Tapponnier et al., 2013). Instead, the formation of slow cooling dikes and sills would lead to larger crystals that result in weaker, less coherent magnetization, generating longer-wavelength anomalies (Irving, 1970; Dyment et al., 1997).

In the central and northern Red Sea, the magnetic anomalies are highly discontinuous (Figs. 5b and 6b), almost chaotic (Fig. 5b). Nonetheless, a detailed interpretation of the magnetic data in the Mabahiss Deep (Guennoc et al., 1988; Cochran, 2005b), or in the central Red Sea (Searle and Ross, 1975; Hall, 1979; Izzeldin, 1987; Okwokwo et al., 2022), show that they are partly organized like seafloor spreading anomalies. The rather discontinuous pattern of the anomalies was explained a long time ago by the amount of transverse faults and fractures in these areas (Searle and Ross, 1975; Izzeldin, 1989). North of the Mabahiss Deep, Augustin et al. (2021) established that seafloor spreading occurred since ca. 13 Ma, but the nature of the basement remains unclear. We cannot rule out the possibility of altered mantle-derived rocks exhumed by major detachment faults to accommodate plate divergence such as the South West Indian Ridge (SWIR) in an amagmatic setting (Sauter et al., 2013).

### 5.1.2. Ocean continent transition

Studies of continental passive margins show huge structure variations correlated with the amount of extruded volcanic material. It goes from margins with a very low volcanic activity displaying thin continental crust with tilted blocs and exhumed mantle (e.g., Lavier and Manatschal, 2006) to the so-called volcanic margins where great amount of magmatic products such as Seaward Dipping Reflectors (SDRs) emitted during extension are found (e.g., White and McKenzie, 1989). The transitional domain between purely continental crust and oceanic crust is difficult to define and is subject to interpretation, hence

the difficulties to precisely constrain the OCT (Eagles et al., 2015). Some characteristics of the OCT domain described by Leroy et al. (2010) can be imaged by bathymetric surveys, seismic reflection (e.g., geometry of the acoustic basement, syn-OCT sediment infill), seismic refraction (e.g., seismic velocities) and gravity and magnetic anomalies (e.g., no clear magnetic anomaly should be observed beyond the OCT). Finally, petrological observations within drill holes locally provide direct information but are rarely available throughout the margin. Unfortunately in the Red Sea, either the data is not available, or it is old and its quality is largely hampered by the thick sediment and salt cover.

However, as aging oceanic crust gets deeper towards the shoreline up to the OCT, and because it is flexing the thinned continental crust (Parsons and Sclater, 1977), we infer that the OCT is located in a zone where the acoustic basement is the deepest (Fig. 7d). On the seismic line from the southern Red Sea (Fig. 7d), the possible transition from a purely oceanic domain to a continental or transitional crust, lies roughly 100 km east of the ridge axis at 19.5°N (Fig. 7). Bearing in mind that a combination of approach is required to infer the precise location of the OCT, we constrained this limit along the Arabian margin on seven seismic lines combined with the gravity data as a marker of the deepest part of the basin. We also used gravity data on the Nubian margin (Figs. 5c, 6c and 7c). This limit respects the constraints given by the different wells displayed in Fig. 8 and honors the position of the Ikwan and Zabargad Islands where continental crust was described (Lancelot and Bosch, 1991).

### 5.1.3. Age of seafloor spreading

The exact timing of seafloor spreading initiation in the Red Sea and hence the age of the transition from rifting to spreading is still debated. It can be inferred from the distance of the OCT with the ridge axis and the spreading rates calculated by Chu and Gordon (1998), as recently proposed by Augustin et al. (2021) with an age of 13 Ma for the initiation of seafloor spreading. In our study, the distance of the OCT to the axis is ca. 100–110 km in the southern Red Sea, to ca. 70 km in the northernmost Red Sea, implying a minimum age for the initiation of seafloor spreading around ca. 14 Ma (Fig. 7d), slightly older but consistent with the ages of Augustin et al. (2021). However, since the location of the OCB is not well defined, this age has to be compared with other methods. The oldest oceanic crust related magnetic anomaly was identified by Dyment et al. (2013) as a long wavelength magnetic anomaly spanning an age between 13.5 and 15 Ma (i.e., anomalies 5AB to 5B) (Fig. 7b), within range of the ages calculated with spreading rates. Another method is to look at the tectono-stratigraphic framework of the Arabian margin which shows that the Red Sea rift related faults were sealed by the carbonates of the Wadi Waqb Formation and the massive salt of the Mansiyah Formation that were dated using biostratigraphy around 15 Ma (Hughes and Johnson, 2005; Hughes, 2014). Finally, the tectono-stratigraphic interpretation of seismic data in the Midyan peninsula shows that strike-slip motion along the Gulf of Aqaba, southern termination of the Dead Sea Fracture Zone, occurred during the deposition of the Middle Miocene Mansiyah salt (ca. 15 Ma; Tubbs et al., 2014). These observations are coherent with a coeval initiation of seafloor spreading in the Red Sea and the activation of the Aqaba/Levant faults around 14 Ma. The best way to reconcile magnetics, gravity, seismic lines, basin stratigraphy and regional tectonics, is therefore to consider that seafloor spreading in the Red Sea has been active for at least 14 Myr. However, further studies are required to better understand the relationship between rifting, seafloor spreading and transform motion along the DSFZ.

### 5.1.4. Transform faults

Transform faults are very common plate boundary features from fast to slow spreading ridges (e.g. Wilson, 1965). At slow and ultra-slow spreading ridges such as the Mid-Atlantic Ridge and the SWIR, second-order ridge segments are often separated by small-offset non-transform discontinuities, whereas the primary large-offset segmentation is
<figure><img src="image_12.png" /></figure>

**Fig. 8.** Tectonic and geodynamic setting of the Red Sea with our interpretation of the spreading axis, transform faults and oceanic-continental crust boundary. Euler Poles are after Reilinger and McClusky (2011), McClusky et al. (2010) and Viltres et al. (2020). Da. Danakil; Da D. Danakil Depression; EA. Erta'Ale; AATZ. Alayta-Afdera Transfer Zone; DMH. Dabbahu-Manda-Hararo; AG. Asal-Ghoubbet; Nu/Da 1. Nubia Danakil Euler pole between 11 and 5 Ma (Reilinger and McClusky, 2011); Nu/Da 2. Nubia Danakil Euler pole since 5 Myr (McClusky et al., 2010); Ar/Da. Arabia Danakil Euler pole (McClusky et al., 2010). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

accommodated by transform faults (e.g. Patriat et al., 1997).

With our new dataset and reinterpretation of available published data, we showed the major discontinuities observed in the gravity and magnetic anomalies map of the Red Sea reflect deeply rooted structures affecting the oceanic basement in the central Red Sea which are associated with systematic offsets of the MOR (Fig. 3).

The strike of onshore rift faults of the Arabian Margin as well as the present trend of the coastlines give a first approximation to the initial rift-strike orientation. In the southern Red Sea, it was NW-SE which was orthogonal to the spreading orientation (NE-SW). This is witnessed by parallel, continuous magnetic anomalies that are symmetric about the

axis. In such setting, no discontinuity accommodate the deformation. Along the Jeddah-Port Sudan segment between 20°N and 23°N, the rift orientation would have been almost north-south. Therefore, spreading (NE-SW) oblique to the rift orientation would ultimately lead to the segmentation and break-up of the axis into smaller sections joined by orthogonal transform faults (Searle and Ross, 1975). This intense fracturing of the oceanic basement is what we observe in the basement depth map (Fig. 3). Moreover, some of these features seem to be aligned with older structures observed onshore in Arabia and Nubia (Stern and Johnson, 2019), striking in the same direction and still seismically active (Mitchell, 2015; Samkari et al., 2022). These lineaments and
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Reconstruction of the Red Sea opening since initial fit. a: (28 Ma) Early rifting phase. b: (14 Ma) Transition from rifting phase to drifting phase and seafloor spreading in the Red Sea. Spreading orientation shifted dramatically and is then aligned with DSFZ orientation. c: (11 Ma) Drifting phase and initiation of Danakil Microplate rotation. d: (0 Ma) Present setting, all structures north of 18°N are constrained by a unique Euler pole of rotation (31.7°N; 24.62°E) while structures south of 18°N are constrained by a pole in the strait of Bab el Mandeb (13.4°N; 42.9°E; McClusky et al., 2010). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
discontinuities were recently highlighted by Issachar et al. (2022) as magnetic trends in their new magnetic anomaly map. Segmentation of the rift into closely spaced spreading segments could therefore the result of continental transfer faults influenced by NNE-trending Proterozoic shear zones and sutures. We argue that these discontinuities are steady features in time. Satellite altimetry by Sandwell et al. (2014) shows that elongated zones of strong Vertical Gradient of Gravity align along the transverse structures highlighted from the basement depth map (Fig. 3). They may reflect contrasting basement densities resulting from older, denser oceanic crust in contact with younger, less dense oceanic crust along inferred transform discontinuities, or, more likely, the bathymetric low associated to these structures. The Conrad Deep is a good example as it is aligned along one of these faults and was described by Ehrhardt et al. (2005b) as a pull-apart structure due to transtension along a transform fault. More evidence of these transform structures are brought by Marshak et al. (1992) who explained the uplift of the Zabargad Island as transpression within the Zabargad Fracture Zone which is coherent with our interpretation putting the island at the tip of one of our mapped transform fault. Furthermore, the sinistral (left lateral) kinematic expressed in the focal mechanism of one of the earthquakes displayed on Fig. 5a is accommodating the global displacement of Arabia relative to Nubia along these structures. Finally and most importantly, these structures are all parallel to the relative plate motion direction and are aligned with segments of the small circle centered on the Euler pole of Arabia-Nubia rotation. This is characteristic of transform faults (Le Pichon, 1968) that follow the rules of rigid deformation on a sphere (Euler theorem).

For all these reasons, we interpret these long lived, deeply rooted structures oriented parallel to the spreading direction as transform fault discontinuities. Because of the thick salt and sediment cover, these transform features are not directly visible on the bathymetry as opposed to the Western Gulf of Aden to which it could be compared. Nonetheless, between these main transforms, smaller offset discontinuities not affecting the basement could be interpreted as Non-Transform Discontinuities (NTD; Mitchell et al., 2015). As opposed to the transform faults, they are not steady features (Schouten et al., 1985) and can be oriented in any direction relative to plate motion (Gràcia et al., 2000).

## 5.2. Opening history of the Red Sea

Our new structural interpretation allows us to propose the following reconstruction model from continental rifting to seafloor spreading, constrained by the rules of plate motions on a sphere (Fig. 9). Red Sea opening started as a continental rift ca. 28–25 Myr ago that was extending from the North of the Gulf of Suez to the Somali-Ethiopian border (Bosworth and Burke, 2005). As a first order approximation we used the projection of the main rift structures on the Arabian and Nubian sides as well as the main Red Sea parallel dikes that intersect north of the Gulf of Suez in order to define an Euler Pole of rotation for the rifting phase (38.45°N; 25.95°E). During the early rifting stage, the DSFZ did not exist and Sinai and Arabia moved together as one rigid plate relative to Nubia. At the initiation of phase, the width of the rift zone was ca. 50 km in the north and ca. 80 km in the south (Fig. 9a), while by the end of the rifting phase and initiation of seafloor spreading it was ca. 85 km wide in the north and ca. 110 km in the south (Fig. 9b). The resulting displacement during rifting was orthogonal to the orientation of the main faults, and the total extension in the southern Gulf of Suez was of ca. 35 ± 1 km (Bosworth, 1995).

A shift in the opening orientation from rift orthogonal to oblique is recorded by the onset of seafloor spreading overall the Red Sea around 14 Ma, but might have started earlier, following the trend of the Gulf of Aqaba / Dead Sea Fracture Zone (Fig. 9b). Since then, the movement of Arabia relative to Nubia is a counterclockwise rotation around an Euler Pole located at 31.7°N; 24.62°E which falls within the 95% confidence pole of ArRajehi et al. (2010). At 11 ± 2 Ma (Reilinger and McClusky, 2011), in the southern Red Sea (Fig. 9c), the Danakil Microplate

individualized from Nubia and started rotating in a counter clockwise direction around an Euler Pole located at the tip of the plate in the Gulf of Zula (Reilinger and McClusky, 2011), partitioning the extension in the southern Red Sea between the Red Sea itself and the Danakil Depression.

Today, the motion of Arabia relative to Africa is still a rotation around the pole located in northern Libya, which is accommodated in the north by strike-slip motion along the DSFZ. In the south, the morphology of the Red Sea is strongly influenced by the rotation of the Danakil Bloc (Fig. 9d).

## 6. Conclusions

We re-interpreted the Red Sea lithosphere structure and morphology using bathymetry, potential field (magnetic and gravity anomaly) and seismic data to confirm the existence of oceanic crust throughout the Red Sea since ca. 14 Myr. These data show the MOR segments are mostly aligned towards the Euler pole, are oblique to the overall trend of the Red Sea, and their offset from each other (5–30 km) is accommodated by transform faults. The main difference between the southern and northern Red Sea lies in the direction and rate of plate motion as a result of the geodetic constraints given by the Euler pole of rotation to the north. Consequently, the ca. 30° counterclockwise strike change and halving of the spreading rate (ca. 16 to ca. 10 mm·yr⁻¹) between 18°N and the Suez triple junction result in a shift from slow, orthogonal (South Mid-Atlantic Ridge) to oblique, ultra-slow (SWIR) modes of seafloor spreading. The obliquity of spreading in the central and northern Red Sea is accommodated by oceanic transform faults stopping ca. 40 km short of the coastline on the Ocean Continent Boundary. Three large transform fault systems were consequently identified (Jeddah, Zabargad, El Ikhwan, from south to north).

From Sinai to 18.5°N, our structural model is resolved by a unique Euler Pole that falls within the 95% confidence of ArRajehi et al. (2010) for rotation of Arabia with respect to Nubia. In the southern Red Sea, from 18.5°N southward, the transform faults direction do no longer align with the pole to the north but to a pole of rotation between Arabia and Danakil, which marks partitioned deformation between the Red Sea and the Danakil Depression.

## CRediT authorship contribution statement

**Antoine Delaunay:** Conceptualization, Methodology, Investigation, Validation, Writing – original draft, Visualization. **Guillaume Baby:** Resources, Writing – review & editing, Visualization. **Jakub Fedorik:** Writing – review & editing. **Abdulkader M. Afifi:** Conceptualization, Resources, Validation, Writing – review & editing, Supervision, Funding acquisition. **Paul Tapponnier:** Conceptualization, Methodology, Investigation. **Jérôme Dyment:** Conceptualization, Methodology, Investigation, Writing – review & editing.

## Declaration of Competing Interest

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

## Data availability

- DEM used in all figures: Shuttle Radar Topography Mission (SRTM) Void Filled 3 arc-seconds. Link: https://usgs.gov/centers/eros/science/ usgs-eros-archive-digital-elevation-shuttle-radar-topography-mission- srtm-void?qt-science_center_objects=0#qt-science_center_objects. - Shading superimposed on DEM was performed using ArcGIS10.2 – Shading tool. - USGS earthquake catalog and moment tensors were compiled from https://earthquake.usgs.gov/earthquakes/map/ accessed on February 1, 2022. - Magnetic anomaly map in northern Red Sea: Cochran, 2005. - Magnetic anomaly map central Red Sea: Saudi
Sudanese survey, Zahran et al., 2003. - Free Air gravity anomaly: Sandwell and Smith (2014) link: https://topex.ucsd.edu/grav_outreach/#grid. - Red Sea opening Reconstruction: This paper with the use of software GPlates 2.3 link: https://www.earthbyte.org/download-gplates-2-3/. - Basement depth map in the Red Sea: Published data (Izzeldin, 1987) and interpretation of proprietary seismic data for this paper. - Seismic line drawings: proprietary seismic data interpreted for scientific purposes. - Structural framework: this paper, the vector data of our interpretation is available as .kml file in the supplementary data.

## Acknowledgements

This study was completed under KAUST's Competitive Research Grant 4082 on "Geologic Evolution of the Red Sea and Gulf of Aqaba". We thank oil companies which operated in the Red Sea for the opportunity to interpret their data for scientific purposes. We also thank numerous colleagues in academia and the oil industry for stimulating discussions. We thank Petroleum Experts (Petex) for providing an academic licence of the Move software for structural modelling. Finally, we would like to thank Neil Mitchell and two anonymous reviewers for their constructive reviews that helped significantly improve the quality of our manuscript.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.tecto.2023.229728.

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