Tectono-sedimentary evolution of the eastern Gulf of Aden conjugate passive margins: Narrowness and asymmetry in oblique rifting context

Accepted Manuscript Tectono-sedimentary evolution of the eastern Gulf of Aden conjugate passive margins: Narrowness and asymmetry in oblique rifting context

Nonn Chloé, Abdulhakim

Leroy

Sylvie,

Khanbari

Khaled,

Ahmed

PII: DOI: Reference:

S0040-1951(17)30400-6 doi:10.1016/j.tecto.2017.09.024 TECTO 127637

To appear in:

Tectonophysics

Received date: Accepted date:

30 June 2017 26 September 2017

Please cite this article as: Nonn Chloé, Leroy Sylvie, Khanbari Khaled, Ahmed Abdulhakim , Tectono-sedimentary evolution of the eastern Gulf of Aden conjugate passive margins: Narrowness and asymmetry in oblique rifting context. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tecto(2017), doi:10.1016/j.tecto.2017.09.024

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ACCEPTED MANUSCRIPT

Tectono-sedimentary evolution of the eastern Gulf of Aden conjugate passive margins: narrowness and asymmetry in oblique rifting context Nonn Chloé1, Leroy Sylvie1, Khanbari Khaled2, Ahmed Abdulhakim3,1 1 - Sorbonne Universités, UPMC, ISTEP UMR 7193, CNRS, 4 place Jussieu, 75252 Paris Cedex 05, France 2 - Department of Environmental and Earth Sciences, Sana’a University, Sana’a, Yemen 3 - Seismological and Volcanological Observatory Center, Herran Garden, 82187 Dhamar, Yemen. 


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Abstract

Here, we focus on the yet unexplored eastern Gulf of Aden, on Socotra island (Yemen), Southeastern Oman and offshore conjugate passive margins between the Socotra-Hadbeen (SHFZ) and the eastern Gulf of Aden fracture zones. Our interpretation leads to onshore-offshore stratigraphic correlation between the passive margins. We present a new map reflecting the boundaries between the crustal domains (proximal, necking, hyper-extended, exhumed mantle, proto-oceanic and oceanic domains) and structures

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using bathymetry, magnetic surveys and seismic reflection data. The most striking result is that the magma-poor conjugate margins exhibit asymmetrical architecture since the thinning phase (Upper Rupelian–Burdigalian). Their necking domains are sharp (~40-10 km wide) and their hyper-extended

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domains are narrow and asymmetric (~10-40 km wide on the Socotra margin and ~50-80 km wide on the Omani margin). We suggest that this asymmetry is related to the migration of the rift center producing significant lower crustal flow and sequential faulting in the hyper-extended domain. Throughout the

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Oligo-Miocene rifting, far-field forces dominate and the deformation is accommodated along EW to N110°E northward-dipping low angle normal faults. Convection in the mantle near the SHFZ may be

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responsible of change in fault dip polarity in the Omani hyper-extended domain. We show the existence of a northward-dipping detachment fault formed at the beginning of the exhumation phase (Burdigalien).

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It separates the northern upper plate (Oman) from southern lower plate (Socotra island) and may have generated rift-induced decompression melting and volcanism affecting the upper plate. We highlight

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multiple generations of detachment faults exhuming serpentinized subcontinental mantle in the oceancontinent transition. Associated to significant decompression melting, final detachment fault may have triggered the formation of a proto-oceanic crust at 17.6 Ma and induced late volcanism up to ~10 Ma. Finally, the setting up of a steady-state oceanic spreading center occurs at ~17 Ma.

Keywords (6 max): Gulf of Aden; oblique rifting; conjugate rifted margins; crustal domains; asymmetry; volcanism

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ACCEPTED MANUSCRIPT 1. Introduction At first order of observation, magma-poor divergent margins share a comparable large-scale architectural pattern of the ocean-continent transition (OCT) marking the gradual change from highly stretched and thinned unequivocal continental crust to steady state oceanic crust. The formation of the OCT takes place when the continental crust breaks apart and is dominated by lithospheric mantle dynamics. Its formation continues until the development of a spreading system associated with the setting up of a steady state spreading center dominated by asthenospheric mantle dynamics (e. g. Cannat et al. 2009; Leroy et al.,

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2010a, 2012). The OCT comprises part of the distal margin (the transitional domain and/or zone of exhumed continental mantle) and the outer domain (between the poorly defined basement of the distal domain and unambiguous oceanic crust, e.g. Péron-Pinvidic and Osmundsen, 2016). The presence of exhumed material is highlighted in current distal magma-poor margins (e.g. West Iberian and Newfoundland, Boillot et al., 1980; Jagoutz et al., 2007; Sibuet et al., 2007; Péron-Pinvidic and Manatschal, 2008; Dean et al.,

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2015; South Atlantic, Aslanian et al., 2009; Unternehr et al., 2010; Clerc et al., 2015; Australia and Antarctica, Beslier et al., 2004; Espurt et al., 2009; Gillard et al., 2015; Gulf of Lion, Jolivet et al., 2015; Moulin et al., 2015) and in analogues from mountain belts (e.g. Pyrenees, Lagabrielle and Bodinier, 2008;

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Lagabrielle et al., 2010; Clerc et al., 2012; Corsica and Alps, Müntener et al., 2000; Lagabrielle et al., 2015). The outer domain is defined as the limit between transitional domain and steady-state oceanic crust and is formed by tectonic and magmatic processes within the distal margin (e.g. Péron-Pinvidic and

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Osmundsen, 2016). Geological and geophysical characteristics of the outer domain differ strongly from one margin to another, as does the volume of magma supply (Péron-Pinvidic and Osmundsen, 2016; Stab

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et al., 2016).

In an oblique rifting context, the extension direction is oblique to the rift trend (Brune, 2014) (e. g. East

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African rift system, Corti, 2008; Gulf of California, Lizarralde et al., 2007; North Atlantic, Hosseinpour et al., 2013). The normal faulting that accommodates oblique rifting extension in conjugate margins is composed of three main fault populations, with rift-parallel, rift-perpendicular and intermediate-strike

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faults (Autin et al., 2010a; Brune, 2014). While magmatic processes weaken the lithosphere by efficient heating and mechanical strength reduction (Bialas et al., 2010), some studies have shown that oblique extension requires less energy than perpendicular extension (Brune et al., 2012; Heine and Brune, 2014). In this way, oblique rifting favors the localization of deformation and increases the rate of continental thinning (Bennett and Oskin, 2014), allowing the development of narrow rifts (Buck, 1991) as is the case in the Gulf of California (Bennett and Oskin, 2014); and the Gulf of Aden (Autin et al., 2010b). Despite these studies, we still require an explanation of how oblique rifting evolves in time and space to account for the formation of hyper-extended and exhumed domains. In the present state of knowledge, the transition between the hyper-extended domain and unambiguous oceanic crust remains gradual and also poorly constrained in terms of composition and processes. An important issue is to get

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ACCEPTED MANUSCRIPT more constraints on the formation of various domains as the deformation becomes localized toward the future area of breakup until the onset of oceanic crust accretion. The OCT is crucial for understanding how thinning processes are accommodated by tectonic structures that finally allow the mantle rocks to be uplifted toward the seafloor. Recent models consider perpendicular extension and tend to promote largescale detachment faults with continentward dip. Here, oblique extension is an additional parameter to take into account for the morphology, architecture, tectonic features, crustal domains mapping, which needs to be supported by more observations on present-day margins. The Gulf of Aden, formed by

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separation of the Arabian and Somalian plates (Fig. 1), is a natural laboratory to investigate conjugate margin structures and strain localization throughout the entire rift history in an oblique divergence context. This young oceanic basin (~17 My) is bordered by volcanic margins displaying seaward dipping reflectors typical of volcanic passive margins in the western part (Tard et al., 1991; Beydoun et al., 1998; Leroy et al., 2012; Ahmed et al., 2014; Stab et al., 2016) and magma-poor margins in the eastern part (Leroy et al., 2004; d’Acremont et al., 2005, 2006; Autin et al., 2010b; Leroy et al., 2010a; Watremez et al.,

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2011; Leroy et al., 2012). The oblique rifting causes a pronounced segmentation of the margins and various fault generations. The main Alula Fartak Fracture Zone (AFFZ) in the center of the Gulf of Aden basin limits and offsets the Aden (to the West) and the Sheba (to the East) spreading ridges (fig. 1). The

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eastern Gulf of Aden displays evidence of present-day magmatic and hydrothermal systems near the surface associated with partial melting beneath the rifted margins. From the early 2000s, the eastern Gulf of Aden has been the site of several geophysical and

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geological surveys that focused on the Encens-Sheba segment (Fig. 1) which lies between the AFFZ and the major Socotra-Hadbeen Fracture Zone (SHFZ) (Leroy et al., 2004, 2010b, 2012, d’Acremont et al.,

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2005, 2006, Lucazeau et al., 2008, 2009, 2010; Autin et al., 2010b; Bellahsen et al., 2013b; Basuyau et al., 2010; Korostelev et al., 2015, 2016). The present study focuses on the easternmost Socotra-Sharbithat

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segment between the SHFZ and the Eastern Gulf of Aden Fracture Zone (EGAFZ) (area framed in black, Fig. 1), a zone that is less studied and less well understood. We present an integrated compilation of geological and geophysical studies on the structural and stratigraphic architecture of the conjugate rifted

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continental margins on new seismic profiles combined with bathymetric, magnetic and gravimetric data. This study includes onshore field data from the outcropping conjugate margins of the Dhofar region (Sultanate of Oman) to the north and Socotra Island (Yemen) to the south (Fig. 2; Leroy et al., 2012; Bellahsen et al., 2013b; Robinet et al., 2013). The new dataset reveals the asymmetric style of these oblique magma-poor margins thanks to the mapping of their different crustal domains, the characterization of their stratigraphic and structural architectures and tectonic evolution. We discuss the nature and formation of the margins from the necking domain to OCT of the Socotra-Sharbithat segment. We give an explanation of how oblique extension and volcanic activity can control the 3D geometry and crustal architecture of the Gulf of Aden passive margins.

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ACCEPTED MANUSCRIPT 2. Geodynamic and geological settings 2.1 Tectonic setting Rifting in the Gulf of Aden started 34 Ma ago (Fig. 3) (Roger et al., 1989; Watchorn et al., 1998; Leroy et al., 2012; Pik et al., 2013; Robinet et al., 2013; Serra-Kiel et al., 2016), and 38 Ma ago in the easternmost part (Robinet et al., 2013; Pik et al., 2013). Kinematic studies highlight a N20°E-trending extension between the Arabian and Somalian plates, oblique to the main N70°E orientation of the Gulf of Aden basin (Fig. 1) (Jestin et al., 1994; Fournier et al., 2001). The extensional stress regime leads to oceanic

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spreading at 17.6 Ma between the western Shukra El Sheik Fracture Zone (SEFZ, Fig. 1) and the EGAFZ (Figs. 1, 3) (Leroy et al., 2010a). The present-day oceanic spreading rate increases from west (13 mm yr-1 along N35°E) to the eastern Sheba-Ridge (18 mm yr-1 along N25°E) (Jestin et al., 1994; Fournier et al., 2001).

2.2 Crustal thickness

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In the Socotra-Sharbithat segment, on the Dhofar margin, receiver function computations yield estimates of Moho depths ranging from 23 km near the coastline to 42 km on the outer part of the rift shoulders (Figs. 2.a, 4.a) (Tiberi et al., 2007; Leroy et al., 2012; Korostelev et al., 2015). On the southern margin; two

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segments can be distinguished on either side of the Hadibo transfer zone (HTZ) according to crustal thickness (Figs. 2.b, 4.b) (Ahmed et al., 2014): (i) the western segment is characterized by a crustal thickness varying from 27 km (in the South) to ~16 km (in the North) (Ahmed et al., 2014); (ii) the eastern

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segment displays a crustal thickness increasing from 23 km near the NE-SW-striking Momi Transfer Zone (MTZ) to 28.6 km at the location of the Haggier Mountains and along the HTZ (Figs. 2.b, 4.b) (Ahmed et

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al., 2014).

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2.3 Stratigraphic records

Previous field studies on sedimentary records and tectonic events of the eastern onshore conjugate margins (Fig. 3.a) (Roger et al., 1989; Fantozzi and Sgavetti, 1998; Razin et al., 2010; Leroy et al., 2012;

c).

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Robinet et al., 2013; Robinet, 2013; Bellahsen et al., 2013b) allow onshore-offshore correlations (Figs. 3.b,

2.3.1 Pre-rift sequence: Cenozoic basement The Hadramaut Group (Figs. 2, 3.a) unconformably overlies the Neoproterozoic to Mesozoic pre-rift basement (Figs. 2, 4). It is widely preserved in the Arabian Peninsula and on Socotra Island (Beydoun and Bichan, 1969; Platel et al., 1992b; Roger et al., 1989) (Fig. 2). Two major transgressive phases are recorded by the Umm Er Radhuma (Thanetian – Cuisian) and Dammam carbonate platforms (Early Lutetian – Priabonian) (Fig. 3.a) (Robinet et al., 2013) separated by a period of emergence with gypsum and carbonate deposits of the Rus Fm (Cuisian – Early Lutetian) (Fig. 2.a, 3.a) (Robinet et al., 2013). The

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ACCEPTED MANUSCRIPT Eocene is characterized by a major uplift of the Arabian plate marked by widespread erosion at the top of the Dammam Fm (Fig. 3.a) corresponding to the premises of the Gulf of Aden rifting (Robinet et al 2013; Pik et al., 2013). It is associated with the development of local subsidence at the future location of the northeastern margin and with the accumulation marls and limestones belonging to the Aydim Fm (Barthonian-Priabonian, Fig. 2, 3.a) (Roger et al., 1989; Leroy et al., 2012; Robinet et al., 2013). Then, the top of the Aydim Fm is characterized by major erosion (Priabonian, Fig. 3.a).

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2.3.2 Syn-rift sequence: Dhofar Group The Dhofar Group was deposited in subsident grabens. It is divided into three formations bounded by two discontinuities related to subaerial emergences (Fig. 3.a): the Zalumah, Ashawq and Mughsayl Formations (Roger et al., 1989; Platel et al., 1992b; Robinet, 2013). The lacustrine carbonates belonging to the Zalumah Fm (Priabonian – Rupelian) are only recorded on the Dhofar margin (Figs. 2.a and 3.a) and are associated with a basinward shift of the depositional system followed by a phase of tectonic subsidence

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(Robinet et al., 2013). During the Early Rupelian (Fig. 3.a), subsequent local tilting of the deposits is recognized in onshore conjugate margins (Razin et al., 2010; Leroy et al., 2012) and is correlated with carbonate-siliciclastic deposits of the lower Ashawq Fm (Figs. 2 and 3.a). A main phase of subsidence is

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related to the deposition of shallow marine carbonates represented by the upper Ashawq Fm (Rupelian) (Figs. 2 and 3.a) (Roger et al., 1989; Robinet et al., 2013). Finally, the rifting reaches a climax leading to deposition of the deep marine Mughsayl Fm (Rupelian – Chattian) characterized by calci-turbidites and

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scarp breccias (Figs. 2 and 3.a) (Roger et al., 1989).

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2.3.3 Syn-OCT development sequence: Ayaft Formation During the transition between the deposition of syn-rift and post-rift sequences (Burdigalian) the entire

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margin was uplifted and affected by late tilting (Leroy et al., 2012). This event is recorded by major erosion of the proximal margin and sediment supply into the OCT zone on both conjugate margins (Autin et al., 2010b; Leroy et al., 2010b; Razin et al., 2010; Leroy et al., 2012). On the northern margin, the upper

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part of the Mughsayl Fm is defined by a regressive facies sequence overlain by a Gilbert-type delta succession, reflecting a rapid emergence induced by a general uplift of the margins (Leroy et al., 2012; Robinet, 2013). On the southern margin, the fan-delta and reefal platform deposits of the Ayaft Fm also record this event (Leroy et al., 2012) (Fig. 3.a). In addition, we consider that these two formations were deposited at the same time as the formation of the OCT on the distal margins (Fig. 3.a). The syn-OCT development sequence (called Syn-OCT sequence) is made up of conglomeratic fan-deltas evolving laterally to reefal carbonates that form the Ayaft Fm (Fig. 3.a). To simplify, we propose to call this sedimentary unit the Ayaft Fm on both the northern and southern margins (Fig. 3.a).

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ACCEPTED MANUSCRIPT 2.3.4 Post-rift sequence: Fars Group The post-rift sequence is poorly exposed (Fig. 2). On the northern margin, this unit is preserved in the Sharbithat and Ashawq-Salalah basins lying unconformably on the syn-rift sediments (Fig. 2.a) (Leroy et al., 2012). The post-rift sequence begins during the Langhian – Serravalian with the transgressive carbonates of the Adawnib Fm (also correlated with the Gubbara Fm (Leroy et al., 2012) indicating a shallow marine environment (Roger et al., 1989; Leroy et al., 2012; Robinet, 2013). This formation is overlain by the conglomeratic deposits of the Nar Fm (Miocene – Pliocene) in the Dhofar (Figs. 2.a and

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3.a) (Roger et al., 1989; Leroy et al., 2012). The Nar Fm is related to a regressive phase associated with the emergence of Dhofar during the Upper Miocene (Autin et al., 2010b; Bache et al., 2011). Uplifts of the margin occurred during the Quaternary as attested by the outcrops of stepped beachrocks along the coast (at ~2 m, ~11 m, ~23 m and ~60 m above the present-day sea level, Leroy et al., 2012).

2.4 Structural framework of the eastern Gulf of Aden

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In the Encens-Sheba segment (Figs. 1 and 2), the offshore margin is affected by second-order fracture zones delimiting three segments 20 to 30 km wide, arranged en echelon along the margin and displaying

Watremez et al., 2011; Leroy et al., 2012).

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contrasting lithospheric structures (d’Acremont et al., 2005; Leroy et al., 2010b; Autin et al., 2010b;

In the Socotra-Sharbithat segment, the main SHFZ (Figs. 1, 2) coincides with a major offset of the coastline along the Mirbat peninsula. The onshore part of the northern margin in Dhofar displays major Oligo-

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Miocene N110°E, E-W and N70°E-trending structures corresponding to the Hasik and Ashawq-Salalah Plain grabens (Platel et al., 1992b; Bellahsen et al., 2006) as well as the Sharbithat fault bounding the

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Sharbithat basin (Robinet et al., 2013) (Figs. 2). The Dhofar region is characterized by a slightly northwarddipping plateau (Fig. 2.a) where the Mirbat peninsula forms the higher relief (~1750 m of elevation, Fig.

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2.a). To the Southeast of the Dhofar margin, the Al Hallaniyah Islands culminate at 230 m (Platel et al., 1992a). The Socotra island (Fig. 2.b) is located ~650 km south of the Sharbithat Peninsula (Fig. 1) and is characterized by two main distinct structural domains separated by the NE-SW to N45°E-trending HTZ

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(Figs. 2.b and 5, Beydoun and Bichan, 1969; Razin et al., 2010a; Leroy et al., 2012; Bellahsen et al., 2013a; Pik et al., 2013). West of the HTZ, the western segment belongs to the necking zone (Ahmed et al., 2014) formed by tilted blocks bounding three main syn-rift basins (Fig. 2.b): the Allan Kadarma and the Sherubrub basins are controlled by a complex Low Angle Normal Fault (LANF, 18°) striking ~N110°E (Leroy et al., 2012; Bellahsen et al., 2013b). East of the HTZ, the eastern segment is controlled by a single large tilted-block forming the 1500-m-high Mount Haggier (Fig. 2.b).

2.5 OCT and volcanic events In the Encens-Sheba segment, magnetic anomalies, seismic reflection and refraction data highlight a narrow OCT (from ~15 km to ~50 km wide), which widens eastward (d’Acremont et al., 2005; Leroy et al.,

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ACCEPTED MANUSCRIPT 2010b; Autin et al., 2010b; Watremez et al., 2011; Leroy et al., 2012). The nature of the OCT is interpreted as either exhumed serpentinized mantle locally intruded by magmatic bodies or an ultra-slow spreading oceanic crust (Leroy et al., 2010b; Watremez et al., 2011). At the northern margin OCT, the OCT-ridge corresponds to the deepest basement high and is bounded to the south by the 5d magnetic anomaly (Autin et al., 2010b; Leroy et al., 2004). The OCT shows volcanic activity that formed the present-day domal morphology of the OCT-ridge (Autin et al., 2010b) associated with high heat flow interpreted as the result of post-rift volcanic activity (Lucazeau et al., 2009,

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2010). At least until 100 000 yr. ago, post-rift volcanism modified the OCT after its unroofing, influencing late-stage evolution of the margins as well as the newly formed oceanic domain (Autin et al., 2010b; d’Acremont et al., 2010; Leroy et al., 2010a).

During the post-rift period, a 5-km-thick underplated mafic body or partly intruded mafic material is emplaced at the crust-mantle interface (Autin et al., 2010b; d’Acremont et al., 2010; Leroy et al., 2010b). A significant uplift of the margin is probably induced by the channeling of Afar hotspot material

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along tectonic corridors (Leroy et al., 2010a). The seismic tomography images of the upper mantle suggests that transform faults may act as rheological barriers to the channeled flow of material from the Afar plume toward the east along the Aden and Sheba ridges systems (Georgen and Lin, 2003; Basuyau et

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al., 2010; Leroy et al., 2010a; Corbeau et al., 2014) (Fig. 1). Furthermore, the post-rift magmatic events are interpreted as the result of small-scale convection caused by an abrupt variation in temperature, nature and thickness of lithosphere at the OCT (Lucazeau et al., 2008) and also at the edge of the fracture zones

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(where continental and oceanic crusts are juxtaposed; Dumoulin et al., 2008; Korostelev et al., 2016).

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3. Data collection

We use seismic reflection and bathymetric data to propose a new interpretation of the offshore seismic

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stratigraphy and structure of the conjugate margins of the Socotra-Sharbithat segment and to outline the history of their formation and evolution. This contribution is the first attempt to gather data from the Encens-Sheba (Leroy et al., 2004), the Encens (Leroy et al., 2010a) cruises and the Marges-Aden new

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cruise (Fig. 4).

During the Encens-Sheba cruise (Fig. 4a) onboard the R/V Marion Dufresne in 2000 (Leroy et al., 2004), single-channel seismic reflection data were acquired using a source composed of two water guns totaling 1.3 l (80 in3), with a shot interval of 10.45 s. The receiver array consisted of a three-channel, 150m-long AMG 37/43 hydrophone streamer. The bathymetric data were processed to obtain a grid spacing resolution of 100 m. During the Encens cruise (Fig. 4.a) onboard the R/V L’Atalante in 2006 (Leroy et al., 2010a), multi-channel (360 traces spaced at 12.5 m) seismic reflection data were acquired using a single bubble type source of 14 air guns (Avedik et al., 1993). For seismic profiles Enc01 to Enc23, the air guns totaling 90.6 l and were shot at intervals of about 30 s. For the seismic profiles Enc24 to Enc67, the air guns totaling 42.5 l and were shot at intervals of 20 s. We combine a third new and unpublished dataset

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ACCEPTED MANUSCRIPT from the Marges-Aden (Mad, Fig. 4) cruise in the conjugate margins of the eastern Gulf of Aden carried out in 2012 onboard the Hydrographic & Oceanographic Survey Vessel Beautemps-Beaupré (Leroy et al., 2014) providing seismic, multibeam bathymetric, magnetic and Chirp data. The 24 channels seismic reflection data were acquired using an SN408 Laboratory system (SERCEL digital technology) operating at a speed of 8 to 10 knots. Two air guns (GI 105/105 and 45/45) at a pressure of 140 bars were fired at interval of about 5 to 10 s. The streamer length is of 300 m. The seismic data were processed with Sispeed® at a constant binning of 25 m. Multibeam bathymetry was collected using an EM120

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echosounder. The grids were obtained by interpolating the picked horizons from one horizon to another using Kingdom software®. Depth conversion for the depth to basement maps (Fig. 4) and for the depth sections (Figs. 6 to 12) were carried out according to velocities from available well data (Robinet, 2013) in the area and from OBS data(Leroy et al., 2010b; Watremez et al., 2011). We used P-waves velocities of 1 800 to 2 200 m.s-1 for post-rift units, 2 500 m.s-1 for syn-OCT and syn-rift sequences and 5 500 m.s-1 for the continental

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basement.

4. Results from seismic stratigraphy and mapping of the Socotra-Sharbithat conjugate margins

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For the conjugate margins of the Socotra-Sharbithat segment, we first describe the offshore seismic stratigraphy. Here, we summarize the patterns of seismic units in Table 1 as well as the onshore-offshore correlations established along profiles Enc05 and Mad18 (Fig. 3.b, c) thanks to recent field studies (e. g.

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Leroy et al., 2012; Robinet et al., 2013). Due to the lack of well data, we propose a correlation based on seismic facies, geometries and stratigraphic level in the sedimentary sequence. In a second section, the

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bathymetric (Fig. 2) and depth to basement maps compiled from interpretations of the seismic data (Fig. 4) allow us to describe the morphological features of the conjugate margins. Finally, we identify and

margins (Fig. 5).

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describe the major crustal domains as well as the major structures and segmentation of the conjugate

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4.1 Sedimentary architecture 4.1.1 Pre-rift sequence These pre-rift units are representative of the upper crustal rocks and are rotated on top of the tilted blocks (Figs. 3.b, c, 6 to 12). Three main sequences are observed in the continental seismic basement (Figs. 3.c, d, Tab. 1): (i) the Neoproterozoic basement (NB), (ii) the Mesozoic basement (MB) and (iii) the pre-rift sedimentary sequence, also called the Cenozoic basement (CB1 to CB4). The CB1 seismic facies (Figs. 3.b, c; Tab. 1) may be correlated with the carbonates of the Umm Er Radhuma Fm onshore (Thanetian – Cuisian; Fig. 3.a) that lies unconformably over the MB and is overlain conformably by the Rus Fm. On the top of continental tilted-blocks, some small-offset listric normal faults cut and offset the brittle CB3 and CB4 units (see black small-offset normal faults, Figs. 3.b, c) and are

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ACCEPTED MANUSCRIPT rooted at depth roughly on a decollement layer at the top of CB2 unit (see pink horizon, Figs. 3.b, c; Tab. 1). We correlate the ~100 m-thick CB2 with the gypsum and marly limestone layers of the ~85-m-thick Rus Fm onshore (Robinet et al., 2013) (Fig. 3). Similar decollement layers in the Rus Fm have been highlighted in the adjoining Encens-Sheba segment (Autin et al., 2010b). CB3 is around 160 m thick and its seismic facies (Tab. 1) may be correlated with carbonates of the Dammam Fm onshore (which is 115-m-thick; Robinet et al., 2013; Serra-Kiel et al., 2016) affected by major erosion at the top (Late Barthonian; Leroy et al., 2012). The Late Barthonian – Priabonian is marked by the development of local subsidence at the

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future location of the Gulf of Aden and deposition of marls and limestones of the Aydim Fm in discordance on top of the Dammam Fm (Fig. 3). This suggests that CB4 may be correlated with the Aydim Fm (Fig. 3.a), insofar as CB4 displays laterally variable seismic facies and a thickness of ~250 m that are comparable with ~250 m of the Aydim Fm observed in the eastern Dhofar (Leroy et al., 2012; Robinet et al., 2013).

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4.1.2 Syn-rift sequence

The syn-rift sequence is deposited in continental basins with a thickness of less than 2 km (Fig. 7.b) and onlaps the surface of the tilted pre-rift sequence (Figs. 3 and 6.b, 7.b, 8 to 10; Tab. 1). The top of the syn-

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rift sequence on the conjugate margins corresponds to an erosive unconformity, onto which the syn-OCT sediments and post-rift sediments are deposited (Figs. 3.c, 7.b, Tab. 1). This sequence is located in continental basins containing fan-shaped growth sequences interpreted as coeval with normal faulting

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(Figs. 3, 6 to 11). The seismic horizons are mostly parallel to divergent, forming wedges that thicken and dip slightly towards the footwall of the main faults (Tab. 1). Two intra-syn-rift boundaries are clearly seen

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that delimit three distinct subunits called R1 to R3 from bottom to top (Figs. 3, 6.b, 7.b, Tab.1). In the offshore domain, there are no data allowing us to identify deposits coeval with the

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Zalumah Fm. R1 corresponds to the older syn-rift unit and appears to be tilted by the syn-rift fault. R1 is characterized by fan-shaped geometry and heterogeneous aspect of R1 which suggests variable lithologies. These observations are in favor of the coeval deposition of R1 offshore and the carbonates-

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siliciclastics of the lower Ashawq Fm onshore (Shizar member; Roger et al., 1989) deposited during the slight tilting of the onshore blocks during the Lower Rupelian (Fig. 3.a). R2 is characterized by well-expressed wedges (Fig. 7.b, Tab. 1), indicating a significant deepening during the deposition of R2 along the main faults. This unit may be correlated with the Rupelian carbonates of the upper part of the Ashawq Fm (Nakhlit Member, Fig. 3.a) deposited during the subsidence associated with rifting. The R2 unit presents heterogeneous lithologies as it shows variable amplitudes, alternating with chaotic units and strong reflections at its top. Mound-like geometries at the top of R2 strongly suggest reefal constructions (Fig. 7.b), as clearly seen onshore in the upper Nakhlit Fm (Fig. 3; Leroy et al., 2012; Robinet, 2013; Serra-Kiel et al., 2016).

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ACCEPTED MANUSCRIPT Finally, the syn-rift R3 unit is characterized by (i) pronounced fan-shaped reflectors that may be due to the significant tilting of blocks and (ii) by chaotic levels (Figs. 3, 6.b and 7.b) that may result from the destabilization of sediments created by newly formed landscape. This strongly suggest that R3 may be correlated with the deep marine Mughsayl Fm mapped onshore (Fig. 3.a; Roger et al., 1989; Robinet, 2013; Serra-Kiel et al., 2016) deposited during a period of major filling of the basin during the climax of rifting of the Gulf of Aden (Leroy et al., 2012).

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4.1.3 Syn-OCT sequence The syn-OCT sequence (called U2, Tab. 1) was deposited during a complex transition between the end of the syn-rift phase and the onset of oceanic spreading (Autin et al., 2010b). The well-stratified U2 unit reaches about 800 m in thickness on the northern margin but is thicker on the southern margin (~1300 m) (Fig. 8.a). The basal U2 boundary is characterized by onlaps that cover the pre-rift and syn-rift sediments in the continental domain (Figs. 3.b, c, 6.b, 7.b, 8, a et b, 9, 10.a, 11, 12). In the OCT, the U2 unit displays

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onlaps or downlaps that directly cover the OCT basement (Figs. 6.c, 7.a, c, 8.c, 9, 10, 11.b, 12, 13.a). The downlap terminations of the U2 sedimentary unit onto the northern part of the exhumed domain indicate that deposition is contemporaneous with the formation of the exhumed domain in the southern margin

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(between CMP 1080 and 1250, Figs. 7.a, c) and in the northern margin (Figs. 6, 10 to 13.a). The U2 unit is slightly disturbed by normal faults with no major offset and tilted by the activation of the main detachment fault exhuming the OCT acoustic basement (Fig. 6.c). The U2 unit is absent on the steady-

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state oceanic crust (Fig. 6 to 11 and 13.b). The deposition of U2 in the distal margin is coeval with the Ayaft Fm cropping out on the emerged margin of Socotra Island (Fig. 2.b, 3), even if the facies observed

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onshore (mainly fan delta evolving laterally to carbonates; Leroy et al., 2012) is evidently different from

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the offshore formation deposited in the more distal part of the margin.

4.1.4 Post-rift sequence

The post-rift sequence overlies the syn-OCT sediments, the syn-rift deposits sometimes onlaps the

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continental, transitional and oceanic acoustic basements surface (Figs. 6 to 13). The thickness of the postrift sequence reaches ~1 000 m on the southern margin, and ~2 200 m on the northern margin. We divide the post-rift sequence into four seismic units named U3 to U5’ from bottom to top of the sequence (Fig. 3,a, b). They are correlated in terms of seismic facies and probable timing with U3 to U5 as described in the Encens-Sheba segment (Bache et al., 2011; Baurion, 2012). U3 contains significant volumes of sediments deformed in Mass Transport Complexes (MTCs) (Figs. 6 to 11) deposited near the foot of the continental slope and on the basin floor. U4 shows an increased occurrence of MTC deposits in comparison with U3 (Figs. 6.a, 7.a, 8, 11). On the continental shelf break and in syn-rift basins, U4 displays canyon indentations or contourites, which also distort the D3 unconformity (Figs. 6.a, b, 7.b). U5 is also characterized by the occurrence of MTCs (Figs. 6.a, 7.a, 8.a, 10.a, and 11.a). We define a sub-unit called

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ACCEPTED MANUSCRIPT U5’ that is clearly expressed near the SHFZ and becomes laterally concordant with U5 (Figs. 3, 6 to 11). On the seafloor, some current-formed indentation features such as canyons or channels are recorded (Fig. 3.c, 7.b, c; CMP 200, Fig. 9).

4.2 Conjugate margins architecture 4.2.1 Margins morphology The depth to basement map (Fig. 4) represents the (i) the top of pre-rift sequences in the continental

oceanic crust in the oceanic domain (in blue, Figs. 7 to 11).

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domain (Fig. 3), (ii) the top of the intermediate crust in the OCT (Figs. 7 to 11) and (iii) the top of the

Between the Omani coastline and the Al Hallaniyah Island, the bathymetric map shows a ~5 000 km2 platform characterized by depths of less than 400 mbsl (Fig. 2.a). From the south of Al Hallaniyah Island to the southern prolongation of the Masirah Trough (Lat. 17°30’N, between Long. 55°30’E and 57°30’E, Fig. 2.a), the steep and narrow continental slope is straight and trends E-W, parallel to the

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coastline continuously over a distance of ~220 km. It deepens abruptly to reach a depth of 3200 mbsl at the foot of the slope (Fig. 2.a) while basement depths are between of ~4 000 to 5 500 mbsl at the bottom of the escarpment (Fig. 4.a).

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On the southern margin, to the North of Qalansiya, the depth of the seafloor drops to ~2500 mbsl over a distance of 10 km (Fig. 2.b) and the depth of the basement reaches about 3800 mbsl (Fig. 4.b). Near Hadibo city (Fig. 2.b), the HTZ is associated with a major offset in the Socotra coastline. In the offset

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zone of west of Hadibo city, an offshore platform with an area of ~400 km2 lies at less than 200 mbsl (Fig. 2.b). North of Socotra Island, two perched basins attain 3 500 mbsl of basement depth (Figs. 4.b, 7.b) and

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are bounded to the north by N115°E-trending major topographic highs (Figs. 2.b and 4.b). North of Hadibo city, the topographic highs culminate at 400 mbsl and seem to extend toward the east, parallel to the

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direction of the Socotra coastline (trending ~N120°E, Fig. 2.b). Finally, the bathymetry drops to ~3 000 – 3 500 mbsl to the North of these major highs (Lat. ~13°N, Fig. 2.b) and the basement deepens to 4 500 mbsl

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(Fig. 4.b).

4.2.2 Crustal domains Three major domains can be identified according to the geometry, bathymetry, sedimentary infill and seismic facies (Figs. 5 to 12): (i) the typical continental domain is marked by a succession of basins and basement highs, (ii) the unequivocal oceanic domain, where the basement reflectors are rough; (iii) between these domains, a complex transitional domain referred to here as the OCT. The OCT encompasses i) the exhumation domain and ii) the “outer-high” domain corresponding to the early onset of oceanic spreading, which represents proto-oceanic crust.

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ACCEPTED MANUSCRIPT Continental domain The top of the continental seismic basement exhibits a long-wavelength rough morphology generally found at depths from 160 to 5 500 mbsl (Fig. 4). The continental domain is defined by a series of basins and tilted blocks bounded by normal faults (Figs. 5 to 12). These blocks locally protrude at the surface through the sedimentary cover (Figs 6 to 12). On both margins, faults appear to be listric to low-angle (20° to 30°) in the proximal and distal zones (Figs. 6 to 12). This domain is characterized by magnetic anomalies (~-200 to 250 nT, Fig. 14.a) and gravimetric anomalies (~-110 to 100 mGal, Fig. 14.b) of variable

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amplitude. The Moho depths are obtained from receiver function computations extrapolated from the coastline of the southern margin (Fig. 4.a) (Ahmed et al., 2014) and seismic Moho locally observed on the northern margin (~7000 mbsl, Figs. 6, 10, 11). Two distinct sub-domains are identified (Figs. 6 to 11): the domain 1 with major dip-slip motion on normal faults and the domain 2 with minor dip-slip motion on normal faults.

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Domain 1 — On the Socotra margin, the size of crustal tilted blocks (10 – 17 km wide) decreases oceanward in domain 1 (Figs. 7.a, 8.a and 12). Tilted crustal blocks are individualized by large-offset normal faults dipping oceanward (from ~5 to 14 km of dip-slip motion, Fig. 5.b, 7.a, 8.a, 9.a and 12) and

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covered by syn-tectonic sedimentary wedges (Figs. 3.c and 7.b)). The internal layering at the tops of the crustal blocks suggests the presence of tilted pre-tectonic strata (Figs 3.c and 7.b) that are intensely deformed by a series of oceanward-dipping small-offset normal faults (Figs. 3.c and 7.a, b, 10.a). The

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domain 1 is delimited towards the center of the basin by a final major fault (F1) dipping oceanward (~ 7 to 14 km of dip-slip motion and a fault heave of 5 to 13 km, Figs. 7.a, 8.a., 9.a, 12). The offshore part of

Island (~30 km, Fig. 5).

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domain 1 is wider in the western segment (~20 - 60 km, Fig. 5) than in the eastern segment of Socotra

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On the Oman margin, the seismic survey does not cover the entire domain 1 (Figs. 6.a, 10.a, 11.a). However, by combining bathymetric (Fig. 2.a) and seismic reflection data (Fig. 4.a), we observe a steep slope oriented E-W to the south of the Al Hallaniyah Islands (200 m above sea level, Lat. 17°30’N, Figs. 2.a,

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5.a). To the NNE of profiles Enc21 (Figs. 6.a and 6.b) and Enc26 (Fig. 10.a), this escarpment corresponds to a large-offset normal fault (F1’) that thins the crust by about 4 to 6 km over a distance of 12 – 25 km (up to ~4 000 – 6 000 mbsl at Lat. 17°20’N, Fig. 4.a) (Figs. 4.a, 6.a, b, 10.a and 12). The fault heave is estimated at around 11 km associated with a dip-slip motion of 13 km (Figs. 6.a, 10). Point 1 corresponds to a peculiar point localized at the foot of the F1’ fault (Figs. 6.a, 7.a, 8.a, 9.a, 10, 12) and represents the oceanward limit of domain 1, separating it from domain 2. The F1’ fault crosscuts the entire crust as shown by high-amplitude, moderate-frequency reflectors that deepen oceanward, defining the southern end of the fault in the continental crust (profile Enc21, Fig. 6.b; profiles Mad14, Mad16-1 and Enc19, Fig. 11.a and 12). F1’ tends toward some strong chaotic reflectors between 7.5 and 8 s TWTT at another

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ACCEPTED MANUSCRIPT peculiar point, we call point 2 (Figs. 6.a, b, 11.a, 12) that is located ~15 km to the south of the point 1 on profile Enc21 (Figs. 6.c, 10.a). Domain 2 — Domain 2 is characterized by negative gravity anomalies (mean value -50 mGal, Fig. 14.b). This domain displays numerous normal faults of ~1 to 8 km of dip-slip motion affecting the basement (Figs. 6 to 12). The continental blocks (~ 2 – 10 km wide) are generally angular and are cut and rotated by listric normal faults (Figs. 6 to 12). The blocks display syn-tectonic sedimentary wedges and internal layering indicating pre-tectonic strata (Figs. 6.b, 10.a and 13). Most of the faults dip toward the north on

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both margins (Fig. 5). On the southern margin, the normal faults that delineate the small tilted blocks are about N70°E to N120°E-trending (Fig. 5.b) and are all oceanward-dipping (Figs. 6.a, 7.a, 8.a and 11.a). The outer edge of the continental domain is bounded by a sub-horizontal LANF which dips oceanward and shows a major offset of more than 10 km (fault in green, Figs. 7.c, 8, 9 and 12).

On the northern margin, domain 2 is variable in structural fabric from on either side of the

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Qalansiya Transfer Zone (QTZ; Fig. 5). West of the QTZ, three main extensive structural directions are clearly seen (N30°E, N70°E and N110°E) along the main southward-dipping trend (area in pink, Figs. 5.a). East of the QTZ, the domain 2 is characterized by 2 – 10 km wide tilted blocks bounded by continentward-

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dipping faults (Figs. 11 and 12). On the offshore Oman margin, crustal blocks are to be decoupled at the base at ~7000 mbsl by a horizontal large-offset normal fault which is clearly picked out on the line due to a significant impedance contrast (green line, Figs. 5.a, 10.a and 11.a). This layer seems to correspond to

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the seismic Moho.

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Ocean-Continent Transition

On the conjugate margins, the OCT widens eastward from 10 km to 40 km (Fig. 5). This domain is located

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on the distal margins where: (i) the geological and geophysical characteristics are typical of neither continental nor oceanic crust, (ii) we observe the presence of a syn-OCT sedimentary unit deposited during the OCT formation (e. g. Autin et al., 2010a) and (iii) where the magnetic signal has low amplitude

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and no clear oceanic magnetic anomalies can be identified (Fig 14.a) (Leroy et al., 2010b). On the southern margin, the OCT basement forms a basin (at 1 to 2 km depth) along the margin at the foot of the continental slope, completely buried beneath the syn-OCT and post-rift sediment cover (Figs. 7.c, 8, 9, 12). The OCT basin (Figs. 5.b, 12) widens from the west (~16 km, Fig. 7) to the east (~27 km, Fig. 8). The southern boundary of this basin corresponds to an oceanward-dipping normal fault with large offset at the distal termination of the continental domain (in green, Figs. 7 to 9, 12). The northern boundary of the OCT basement corresponds to the top of a positive relief, which represents the southern boundary of the unequivocal oceanic crust domain (Figs. 7 to 9, 12). In the OCT, the internal structure of the exhumed domain is characterized by high-frequency, low-amplitude to chaotic reflectors (Figs. 7.c, 8, 9).

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ACCEPTED MANUSCRIPT On the northern margin, the OCT basement presents a smooth morphology forming a ridge along the margin (Figs. 6, 10 to 13.a) that globally increases in width eastward (from ~25 to 50 km, Fig. 5.a). The exhumed domain (Figs. 6.c, 10 to 13.a) corresponds to the dome-shaped footwall of the fault (OCT-ridge) on the edge of the continental domain (Figs. 6.c, 10.b, 11.a, 12, 13.a) defined by high-amplitude reflectors (between CMP 1000 and 1200, Fig. 6.a) (Fig. 12). The top of the exhumed basement is more reflective (Figs. 6.c, 13.a) than the top of the continental tilted blocks (Figs. 6.a, 10.a, 11.a). The U2 unit displays restricted syn-depositional displacements on the flanks of the OCT-ridge or in the vicinity of the proto-

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oceanic crust (Figs. 10.b, 13.a). Near the SHFZ (Lat. 16°45N, Long. 55°30E, Fig. 5), the morphology of the acoustic basement is characterized by a ~25 km wide basin in the distal part of the margin that is completely buried beneath the syn-OCT and post-rift sediment cover (Fig. 10). The OCT faults are flat (<30°) and change their orientation along the margin (Fig. 12).

Finally, in the OCT basement, the crustal reflections on both margins are dipping similarly toward the North on both margins (planar fault in green, Figs. 6 to 8 and 12; domal fault in green, Figs. 6, 10 to

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13) and the faults dip towards the ocean on the southern margin and towards the continent on the northern margin.

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Oceanic domain

The oceanic domain is defined by the rough topography of the top of the seismic basement (Fig. 13.b), mostly found at depths of ~2 500 to 5 000 mbsl (Figs. 4), which locally protrudes through the post-rift

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sedimentary cover (Fig. 2, 8, 9, 11.a and profiles AA’, FF’, JJ, LL’ in Fig. 12). The top of the oceanic substratum shows a highly reflective uppermost reflector picked out by low frequency, high amplitude

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and significant impedance contrast with the overlying post-rift sediments (Fig. 6 to 12 and 13.b). As a consequence, the base of the post-rift sequence overlying the oceanic crust is marked by the presence of

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a transparent seismic unit (Fig. 13.b). The U3 and U4 post-rift units are affected by numerous minor normal faults with offsets of a few meters (Fig. 13.b). The oceanic domain displays characteristic magnetic

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anomalies (from -100 to 150 nT; area in blue, Fig. 14.a) and positive free-air gravity anomalies (Fig. 14.b).

4.2.3 Structural pattern: detachment faults and transfer zones On the conjugate margins, the apparent dip of the normal faults ranges from 20° to 30° (Figs. 6 to 12). On the southern margin, profiles Mad02 and Mad04 display faults of about 20° dip in domain 1, and about 30° dip in domain 2 (Figs. 7.a and 12), similar to the LANFS on the Socotra Island (Fig. 2). On the northern margin, the apparent dip of normal faults is about 20°, except for profile ENC19 which displays a dip of ~30° (Fig. 6, 10 to 12). The exhumation fault at the northern edge of the OCT-ridge shows an apparent dip of 10° to 25° (10° on Profiles ENC26 and MAD 14, 15° on profile ENC58, 20° on Profiles ENC23 and Mad16, 25° on profile ENC19, Figs. 10 and 12). In domain 2, such LANFs may result from late tilting of pre-rift units and flattening of the fault dip at depth. Indeed, most of the faults form an

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ACCEPTED MANUSCRIPT angle of 50° to 30° with the pre-rift units on both margins (Figs.6 to 11). This is the case for the major structures located in domain 1 of the southern margin, which form an angle of 50° with the tilted pre-rift sequence (see migrated cross-sections, Figs. 7 to 9). On Socotra Island, the size of the Kadarma titled block onshore is similar to that of the first blocks observed offshore (12 km wide; Fig. 7.a) in this area. Conversely, in the eastern part, a single large tilted block forming the 1 500-m-high Mount Haggier belongs to the proximal domain (crustal thickness of 25 km; Ahmed et al., 2014) (Fig. 2.b). However, the offshore part of the eastern Socotra margin is

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characterized by 15 – 25 km wide tilted blocks delimited by N110°E-N120°E oceanward-dipping normal faults similar to those of the western segment of Socotra (Figs. 5, 7, 8). All extensional structures (N100°E and N70°E-trending faults) dip oceanward in the emerged and submerged areas farther south (Fig. 5). The predominant N70°E trend of the onshore Oman margin is not observed in the offshore margin (Fig. 5). The western part of the margin is structured mostly by oceanward-dipping normal faults trending N20° to N45°E that define the SHFZ and the N45°E-trending coastline in the Sadah area (Fig. 6.a).

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The N20°E elongate SHFZ is the only structure that can be correlated onland with the N30°E Qarabiyan fault (Fig. 5.a). South of the Al Hallaniyah Islands, faults affecting the basement are mainly oriented N90°E to N120°E (Fig. 6.a). These normal faults dip toward the ocean west of the QTZ, with most of them dipping

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continentward east of the QTZ (Fig. 6.a). E-W to N°110E structures are not observed in the emerged area (Fournier et al., 2004; Robinet, 2013; Robinet et al., 2013) (Fig. 2.a), while there is no significant segmentation illustrated by N30°E accommodation zones as expressed in domain 2 (Fig. 5.a). Both

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transfer zones cutting Socotra island (HTZ and MTZ) and trending NE-SW to N70°E may be correlated at sea to a transfer zone (Figs 5.b) which affects the continental domain 2 and OCT domains and extends as

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far as the oceanic crust (Fig. 5.b). Only one secondary right lateral Balan-Kadarma transfer zone has been mapped onland (Bellahsen et al., 2013b) (BKTZ; Fig. 2.b, 5.b), that could be correlated with the N30°E-

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trending QTZ (Fig. 5.b).

4.2.4 Volcanic features

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No volcanic features have been previously identified in the Socotra-Sharbithat segment. Evidence for the presence of volcanoes, as suggested by the bathymetric map, is supported by 3D morphology, magnetic, and seismic data interpretations (Figs. 2, 7.b to 11.b). On the conjugate margin OCTs (Fig. 5.b, 7.c, 10.b) and northern margin domain 2 (Fig. 5.a, 10.b, 11.b), volcanoes are defined by series of circular domes (Figs. 2, 7.b to 11.b) displaying high-amplitude low-frequency reflections with a rather disrupted top and a high-impedance boundary. The horizon marking the top of the volcanoes (see violet horizon, Figs. 7.c, 10.b, 11.b) is overlain by distinct syn-OCT and post-rift onlaps. The base of the dome is seldom well defined and the internal reflections are chaotic (Figs. 7.c, 10.b, 11.b). At the location of volcanism, the magnetic anomalies (from 50 to 125 nT, Fig. 14.a) seems to be positive and the gravity anomalies increase

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ACCEPTED MANUSCRIPT from ~-50 mGal up to 70 mGal in the northern margin hyper-extended domain and OCT (Fig. 14.b) and from ~-20 mGal up to 10 mGal in the southern margin OCT. Locally, the top of the OCT-ridge (Figs. 6.c, 13.a) shows comparable seismic patterns (highamplitude reflections) to the adjoining western Encens-Sheba segment, suggesting a partly volcanic origin for this ridge (Lucazeau et al., 2009; Autin et al., 2010b). Minor faults affect the syn-OCT and post-rift units U2 to U4 and are sealed by U5 at the top of domes (Figs. 7.c, 10.b). The upward curvature of reflectors seems to decrease from U2 to U4 and younger units show onlaps onto the older and more

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tilted reflectors (Figs. 7.c, 10.b).

5. Discussion

In the Socotra-Sharbithat segment, the structural and stratigraphic analysis of seismic profiles (Fig. 12) allows us to propose a nature of the seismic basement, from necking domain to OCT, to discuss the localization of deformation, the related asymmetry (upper/lower plate and brittle/ductile layers) and the

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associated volcanism. We summarize these observations on 3D morpho-tectonic sketches showing a reconstruction of the margins at late syn-rift (Fig. 15.a, b) and late syn-OCT stages (Fig. 15.c, d).

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5.1 Nature of the seismic basement

5.1.1. Domain 1: Necking zone of continental crust In agreement with previous studies we propose that domain 1 corresponds to the necking domain. For

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example, in the Iberia-Newfoundland or Brazil-Angola conjugate margins, the necking domain has been defined as a zone where seismic Moho defines an inflection point associated with intense crustal thinning

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from ~30 to less than 10 km (Péron-Pinvidic et al., 2009; Mohn et al., 2010; Péron-Pinvidic et al., 2013). The top basement is also marked by a basinward increase in accommodation space (Sutra and

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Manatschal, 2012), and the top basement and the Moho converge leading to a specific wedge shape of the continental crust (Péron-Pinvidic et al., 2013). In the Socotra-Sharbithat segment, the necking domain corresponds to an area of a drastic crustal

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thinning, from ~40 km onland (Korostelev et al., 2015) to 10 km offshore (Fig. 6). It exhibits a slope morphology (Figs. 4, 5) and is characterized by tilted continental blocks (10 to 17 km-wide) individualized by oceanward-dipping large-offset normal faults (~5 to 14 km of dip-slip motion, Figs. 7 to 9). A final major fault (F1’ in Figs. 6.a, 10.a; F1 in Figs. 7.a, 9.a) delimits the necking domain on the conjugate margins. The transition from the R1 Fm, characterized by slightly divergent reflectors, to the R2 Fm, characterized by well-defined fan-shaped reflectors, shows the basinward increase in accommodation space during the Rupelian.

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ACCEPTED MANUSCRIPT 5.1.2. Domain 2: Hyper-extended continental crust We propose that domain 2 represents a hyper-extended continental crust, based on the following observations: (1) the upper crust is significantly thinned to less than 10 km (Moho visible in the Enc21 and 26 profiles, Figs. 6, 10.b); (2) a succession of tilted blocks, far smaller than the tilted blocks in the necking domain, is bounded by LANFs spaced between 2 and 14 km; (3) the chaotic R3 Fm (Aquitanian) is deposited and may result from the destabilization of sediments during the climax of rifting. Such extremely thinned continental crust has been recognized locally in the adjoining Encens-

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Sheba segment (Autin et al., 2010b; Leroy et al., 2010b) and in many continental margins (the Iberian margin, Labrador-West Greenland conjugate margins (Whitmarsh et al., 2001; Reston and Pérez-Gussinyé, 2007), in the South China Sea (Savva et al., 2013), in the Atlantic margins (Péron-Pinvidic et al., 2013); Bay of Biscay-Pyrenees, (Lagabrielle et al., 2010; Roca et al., 2011; Teixell et al., 2016) or in the mid-Norwegian margin (Osmundsen and Ebbing, 2008)).

Point 1: Point 1 is located at the foot of large-offset normal faults that thin the crust by about 4 to

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6 km (see blue point, Figs. 6.a, 7.a, 8.a, 9.a, 10, 12) and limit the necking domain from the hyper-extended domain. This morphological boundary is observed on both margins and is used to delimit the necking zone. It may be defined as the nearest point to the coast where the crustal thickness is reduced to 10 km

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or less (Osmundsen and Redfield, 2011). In this case, point 1 corresponds to the Taper Break (TB) point according to the definition of Osmundsen and Redfield (2011). Point 2: The major conjugate thinning faults are comparable with the two large normal faults

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identified on the Socotra-Sharbithat conjugate margins (F1 on Figs. 5.b, 6, 8, 9, 12; F1’ on Figs. 5.a, 10, 12). Bounding the necking and the hyper-extended domains (Figs. 5, 6.a, b, 7.a, 8.a, 9, 10.a, 12, 15), dipping

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seaward and forming the 3 000 to 4 000 m high rift flanks, these faults penetrate deep into the continental crust and reach the mantle at point 2 in our studied zone (see red point, Figs. 6.a, b, 10.a,

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11.a, 12). On the Omani margin, the point 2 is interpreted as the Coupling Point (CP) which corresponds to the first point where the brittle fault which crosscuts the entire crust and penetrates into the mantle (Péron-Pinvidic et al., 2013) and is located near the TB (Figs. 6.a, b, 10.a, 11.a, 12).

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In geodynamical models, during rifting, brittle deformation propagates from the brittle crust into the brittle lithospheric mantle (Van Avendonk et al., 2009) and may lead to the formation of H blocks (H for hanging, Lavier and Manatschal, 2006; also called keystone grabens, Huismans and Beaumont, 2011), bounded by major large-offset normal faults. Based on the well-known example of the IberiaNewfoundland conjugate margins, Svartman Dias et al. (2015) propose a numerical model of narrow and asymmetrical margins which show that after about 3 Ma of extension, the H block reaches a width of ~50 km before it breaks apart and becomes delimited by major normal faults forming two rift flanks ranging in height from 3 000 to 4 000 m (Svartman Dias et al., 2015). Our reconstruction of the Gulf of Aden eastern margins fits the size deduced from these models. Indeed, in our study, the present-day hyper-extended domain is narrow (less than ~90 km cumulated on

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ACCEPTED MANUSCRIPT both margins, Fig. 12) and asymmetric (~10 - 40 km on the southern margin, Fig. 5.b; ~50 - 80 km on the northern margin, Fig. 5.a). Taking into account a stretching factor on fault heaves (βf) of ~1.6 in the hyperextended domains of both margins, the initial size of the H-block in the studied area is consistent with a width of 50 – 60 km (βf = βff/ βf0, where βf is the stretching factor on fault heaves, βf0 is the initial length of the block and βff is the final length after the extension along the fault).

5.1.3. OCT Domain: exhumed domain and proto-oceanic crust

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Two main sub-zones are identified in the OCTs: (i) the exhumed domain (between the green and the light blue boundaries, Figs. 5 to 13.a) and (ii) the proto-oceanic crust defined as a gradual transition from exhumed to a steady-state oceanic crust (between the light and dark blue boundaries, Figs. 5 to 13.a). The proto-oceanic crust displays internal chaotic reflections in continuity with that of the oceanic crust but the top of the acoustic basement is less reflective (Figs. 6 to 13.a). It is not overlain by a transparent seismic unit as observed in the case of steady-state oceanic crust (Fig. 13.b) and is directly in contact with syn-

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OCT sequence (U2, Figs. 6 to 13). The U2 sequence displays onlaps or downlaps over the oceanwarddipping termination of the exhumed basement surface (Figs. 6.c, 7.c, 8.a, 9, 10.b, 11.a). In the exhumed domain, profile ENC26 displays multiple generations of detachment faults in the

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OCT domain (Fig. 10). We identified two northward-dipping detachments that could successively exhume deeper units (Fig. 10.a). Detachment fault 2 is formed at the footwall of detachment 1, providing evidence of a rejuvenation of the footwall by the second detachment fault. This kind of successive detachment

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faulting is observed in field such as on Tinos island continental crust (Brichau et al., 2007). It could lead to the unroofing of a broad expanse of mantle such as observed on the conjugate margins of Iberia-

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Newfoundland (Reston and McDermott, 2011) or Australia-Antarctic (Gillard et al., 2015). If we identify the presence of a detachment fault, this raises the question of the nature of the material exhumed by this

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major structure. In the adjacent Encens-Sheba segment, previous studies suggest that the OCT-ridge is composed, at least partially, of exhumed serpentinized mantle (Autin et al., 2010b; d’Acremont et al., 2006; Leroy et al., 2010b; Watremez et al., 2011). Geophysical studies at the deep Galicia margin have

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also documented the presence of such a ridge composed of peridotites (Peridotite Ridge, Sibuet et al., 2007, Davy et al., 2016). In the Socotra-Sharbithat segment, the exhumed basement of the southern margin is non-reflective and forms a basin filled by syn-OCT and post-rift sequences (Figs. 7 to 9). This basin widens eastward on the Socotra margin (from 3 to 18 km, Fig. 12). The exhumed basement of the northern margin is defined by a dome-shaped footwall at the fault that bounds the hyper extended domain (OCT-ridge, Figs. 6.c, 10.b, 12 and 13.a). On the Newfoundland-Iberia margins, the transitional basement shows many smooth-shaped basement highs formed of serpentinized peridotites (Beslier et al., 1996; Boillot et al., 1989; Sibuet and Tucholke, 2013) strongly similar to the OCT-ridge on the Omani margin in the Socotra-Sharbithat segment (Figs. 5, 10, 12). Moreover, the unreflective character of the upper part of the basement is thought to be the result of extensive serpentinization enhanced by vigorous

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ACCEPTED MANUSCRIPT seawater circulation (Pickup et al., 1996). This significant serpentinization may explain, at least in part, the non-reflective top of basement in the southern exhumed domain of our studied zone, where fluid circulation in the high-permeability basement could occur in the starved margins as it has been corroborated by heat-flow measurements in the north (Lucazeau et al., 2010). Another explanation for the southern OCT domain could be found in the high thermal regime model which shows strong coupling between the mantle and the crust that favors the extraction of the lower crust (Gulf of Lion: Jolivet et al. 2015). The southern margin transitional basement is unclear and may

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correspond to very thin lower continental crust as also proposed on the wider (200 km) Angola passive margins (Aslanian et al., 2009; Sibuet and Tucholke, 2013), the margins of Alpine Tethys (Mohn et al., 2012) or in the Cantabrian margin (Roca et al., 2011). We are unable to show that the structure of the southern OCT domain conforms to one of these models due to a lack of deep seismic data in the area. The thermal regime of the OCT increases with the onset of decompression mantle melting, the localization of plate divergence/segmentation and establishment of a ridge-type thermal regime during

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the onset of seafloor spreading (Bronner et al., 2011; Cannat et al., 2009; Jagoutz et al., 2007; PéronPinvidic et al., 2013). Just before the emplacement of the spreading ridge, a proto-oceanic domain (or outer domain, e.g. Péron-Pinvidic and Osmundsen, 2016) is formed. In the Socotra-Sharbithat segment,

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volcanic units downlap or onlap onto the exhumed domain (Figs. 6.a, 10.b, 13.c), and evolves to a steadystate oceanic crust similar to that proposed for the Australian-Antarctica margins (Gillard et al., 2015). The proto-oceanic domain corresponds to a gradual transition between exhumation and steady-state oceanic

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crust accretion, triggered by the final breakup that could be associated with the development of largescale low-angle detachment fault exhuming a footwall characterized by serpentinized mantle, gabbroic

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bodies and basalt flows. The final breakup corresponding to the establishment of a steady- state oceanic

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crust occurs within the proto-oceanic domain (e. g. Cannat et al., 2009).

5.2 Localization of deformation and asymmetry of the margins Data in the Socotra-Sharbithat segment show asymmetric margins, with a narrow and steep southern

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margin (100 – 160 km, Fig. 5.b) and a wide and smooth northern margin (200 – 400 km, Fig.5.a). On the contrary, the adjoining Encens-Sheba segment exhibits asymmetric margins characterized by a narrower and steeper (125 – 180 km) margin off Dhofar compared to the southern margin (300 km) (d’Acremont et al., 2005). The distribution of faults related to the oblique rifting, ductile vs. brittle layers, the thermal regime, may be invoked as key factors to explain the extension and/or the asymmetry of the continental domain (e.g. Brun, 1999; Bialas et al., 2010; Huismans and Beaumont, 2011; Bellahsen et al., 2013a, 2013b).

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ACCEPTED MANUSCRIPT 5.2.1. Tectonic style of margins On the Socotra-Sharbithat conjugate margins, listric normal faults have a gentle apparent dip of 20°- 30° on the Socotra-Sharbithat conjugate margins (Figs. 6 to 12). However, the angle between the pre-rift series and the normal fault is 50° (Fig. 12). This is consistent with an initial fault dip of around 50° - 60° taking into account the oblique component related to the intersections of seismic lines that may lower the apparent dip of structures (see dataset in Fig. 4). Firstly, as extension proceeds, the development of LANF dips may be evidenced by the major rotation of pre-rift units on listric faults. Secondly, the

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occurrence of polyphase faulting can account for the formation of LANF dips: in the rolling hinge model, the detachment fault is associated with the flexural rotation of steep faults of the hanging-wall, which become gently dipping on the hanging-wall during the unroofing and then finally inactive (Reston, 2007; Reston and McDermott, 2011). This is consistent with our results showing 2 – 10 km wide blocks in the hyper-extended domain bounded by LANFs, which imply the presence of a shallow detachment fault at less than 10 km of depth as observed for example in the Porcupine basin or in the Flemish Cap margin

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(Reston and McDermott, 2011). Thirdly, a higher thermal regime of the hyper-extended domain may also provoke the development of LANFs in the upper crust rooted at shallow level on the ductilely-tinned lower crust as suggested for the Pyrenean paleo-margin (Clerc et al., 2017; Clerc and Lagabrielle, 2014).

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This assumption involves High Temperature-Low Pressure (HT-LP) conditions that are consistent with the presence of volcanism in the distal margins of the Socotra-Sharbithat segment and may explain the localized thinning of the crust.

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On Socotra island (Fig. 2.b), E-W to N120°E trending LANFs, dipping around 10-20°, have been highlighted (Leroy et al., 2012; Bellahsen et al., 2013b) in the necking domain (thickness of the crust less

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than 20 km; Ahmed et al., 2014). The early syn-rift sedimentary rocks are lying in these faults hangingwall and were deposited during their activity (Leroy et al., 2012; Bellahsen et al., 2013b). This sedimentary

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structure indicates low-angle fault activity since the onset of faulting (see cross section on Fig. 2.b; Fig. 15.a, b). As in the case of the Basin and Range province (e. g. Longwell, 1945; Wernicke, 1981), the Gulf of Corinth (e. g. Sorel, 2000), the Pyrenees (Masini et al., 2011) or the Aegean Sea (e. g. Lister et al., 1984;

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Lecomte et al., 2010), the presence of LANFs on Socotra implies the activity of a major shallow decollement layer in the crust (Brun and Choukroune, 1983; Lagabrielle et al., 2005). In the necking domain, a low-angle extensional detachment fault may involve the initiation and the rotation of faults located beneath the décollement surface at low-angle (Lagabrielle et al., 2005). Onshore investigations have identified two main phases of deformation during the Oligo-Miocene in Oman and on Socotra Island related to the rifting obliquity: a N150°E to N160°E extension and a N-S to N30°E extension (Lepvrier et al., 2002; Fournier et al., 2004; Leroy et al., 2012; Bellahsen et al., 2013b). Kinematic studies indicate that the HTZ was reactivated during Tertiary rifting as a normal fault zone trending N45°E to N70°E in relation with a N160°E extension (Leroy et al., 2012, 2012; Bellahsen et al., 2013b). Furthermore, a N20°E extension is compatible with ENE-WSW right-lateral strike-slip motion

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ACCEPTED MANUSCRIPT along the HTZ (Fig. 2.b). Some authors interpret these two main extensional trends in terms of stress permutation within a unique stress field in the eastern Gulf of Aden (Fournier et al., 2004). For other authors, the early rifting stage is characterized by N20°E – trending extension associated with WNW-ESE faults. During a second rifting stage, increased extension may have led to counter-clockwise rotation from N20°E to N160°E on the eastern margin (Lepvrier et al., 2002; Bellahsen et al., 2013b) as elsewhere in the Gulf of Aden (Lepvrier et al., 2002; Huchon and Khanbari, 2003; Bellahsen et al., 2006). A third stage of N°20E extension (Autin et al., 2010b; Bellahsen et al., 2013b) may be related to the migration of

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deformation and its localization in distal parts of the margin. On conjugate margins, the necking domains are delineated by significant E-W and N110°E trending faults, respectively in the north and in the south (Figs. 4, 5): (i) the E-W intermediate trend mainly affects the necking domains (Fig. 5), which seems to correspond to the intermediate faults identified in the Encens-Sheba segment developed during the early rifting phase in an oblique rifting setting (Bellahsen et al., 2013a); (ii) the N110°E trend is the most represented in the hyper-extended

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domains and in the OCT (Fig. 5), as in the case of the exhumation fault (in green; Fig. 5). These N110°Etrending structures on the distal margin and in the OCT have also been recorded in the Encens-Sheba segment (Leroy et al., 2012; Bellahsen et al., 2013a). This strongly suggests that strain was localized into a

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narrow zone along N110°E faults as the deformation increased perpendicularly to the direction of divergence (N20°E). This deformation may be linked primarily to the far-field forces that dominated throughout the rifting of the eastern Gulf of Aden, thus imposing the N110°E-trending faults as suggested

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by Bellahsen et al. (2013b). N120°E-trending faults are observed in the southern margin and form an angle of 10° with the N110°E-trending faults of the northern margin, due to the clockwise rotation of ~10°

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of the Somalian plate relative to the Arabian plate during steady-state oceanic spreading as suggested by

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the previous kinematic studies (e. g. Fournier et al., 2010).

5.2.2. Brittle vs. Ductile layers Based on the narrow and asymmetrical Iberian-Newfoundland margins (Hopper et al., 2006), the thermo-

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mechanical numerical models of Brune et al. (2014) suggest that steady-state rift migration is maintained by sequential faulting in the brittle crust and is controlled by lower crustal flow. It would not be a concave-downward detachment that generates the asymmetry but an array of short-lived oceanward dipping normal faults that would act sequentially in time (Brune et al., 2014). When lithospheric thinning occurs along slip of the dominant initial normal faults (crust-mantle coupling), the process results in upwelling of hot asthenosphere which is more pronounced near the down-dip end of the dominant thinning fault of the rift system. In the lower crust, ductile shear zones develop that are enhanced by the 600°C isotherm in relation to the upwelling of hot mantle (Brune et al., 2014). If we try to apply this model to the eastern Gulf of Aden, we can consider an initial 40 km-thick continental crust divided into the (brittle) upper crust and the (ductile) lower crust as proposed by previous geophysics studies (Basuyau et

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ACCEPTED MANUSCRIPT al., 2010; Watremez et al., 2011; Korostelev et al., 2015). Then, mantle upwelling under the narrow hyperextended domain (i.e. on the Socotra margin) may account for the significant flow in the lower crust and explain why it is almost never observed on the Socotra margin (with the exception of a bright and subhorizontal reflection which is located at a depth of 1 km on profile Mad16-3 (Figs. 8.b, 12)). Conversely, the lower crust can be identified at the base of the continentward-tilted blocks in the northern hyper-extended domain (CMP 7400 on Fig. 6) and at the foot of the F1’ thinning fault, which delimits the necking domain (Figs. 6.b). Hence, the dominant thinning fault seems to correspond to the

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southward-dipping major F1’ normal fault (south of the Al Hallaniyah Islands in Fig. 5.a). Then, the cooling and hardening of mantle material takes place at the footwall side of the exhumation channel, allowing steady-state migration of the rift. During the migration, the lower crust flows within the exhumation channel toward the downward tip of the thinning fault F1’ and the faulting becomes sequential (Brune et al., 2014). The crust becomes progressively brittle and faults penetrate into the mantle, eventually leading to break up of the crust until the mantle is exhumed with very little magmatic activity. Crustal thinning

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induces the formation of an asymmetric hyper-extended domain and the development of a wider margin on one side of the rift with crust being thinned down to 10 km as, for instance, in Iberia-Newfoundland (Lister et al., 1991; Huismans and Beaumont, 2003; Hopper et al., 2006; Shillington et al., 2006; Brune et

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al., 2014). In this way, large parts of the hyper-extended margin originate from the formerly opposite side of the rift zone, as seen in the case of the Iberia-Newfoundland margins (Brune et al., 2014). Hence, on the Socotra-Sharbithat conjugate margins, some parts of the wider northern hyper-extended domain may

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originate from the former southern margin before localization of the final lithospheric break-up. To conclude, the model of Brune et al. (2014) may be consistent with our tectonic evolution of the eastern

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Gulf of Aden margins but cannot be fully demonstrated. Deep data are necessary to properly image the deep structure of the southern margin.

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In this scenario, the significant flow of the lower crust imply HT-LP conditions during the exhumation in the Socotra-Sharbithat segment in a similar way than that proposed for the Cretaceous paleomargins of the Northwestern Pyrenean zone (Clerc and Lagabrielle, 2014). As in the western

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Pyrenean paleomargin (Clerc and Lagabrielle, 2014), the rifting of the Socotra-Sharbithat segment margins behaves in a brittle mode and inducing the development of large-offsets normal faults. The Mauléon basin presents low grade of HT-LP metamorphism (with temperature lower than 350°; Clerc and Lagabrielle, 2014) that may probably also affect distal margins of the eastern Gulf of Aden segment. Drilling and dredging in distal margins are necessary to confirm or reject this assumption.

5.2.3 Upper-plate / Lower plate and role of transfer zones The asymmetry of both margins supports the existence of a lithospheric low-angle detachment fault (Fig. 15.c, d). The role of detachment faulting during rifting is still debated and causes some discrepancies. Extension may actually involve multiple generations of detachment faults (e. g. (Hayward and Ebinger,

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ACCEPTED MANUSCRIPT 1996; d’Acremont et al., 2005), thus making it difficult to distinguish the upper plate (hangingwall) from the lower plate (footwall) (Driscoll and Karner, 1998; Reston and Pérez-Gussinyé, 2007; Rosenbaum et al., 2008). In the Gulf of Aden, previous authors have attempted to distinguish the upper and the lower margins using sigmoidal N75°E structures and linear N110°E-N120°E structures, respectively (Fournier et al., 2007). Moreover, the HTZ has been interpreted on the southern margin as an upper-plate/lower-plate transfer zone (Fournier et al., 2007). However, several other detailed studies indicate that all fault trends are clearly expressed on both margins (Autin et al., 2013; Bellahsen et al., 2013a). Our study also shows

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that the offshore structures of the Socotra margin are dominated by oceanward-dipping normal faults (Figs. 12 and 5.a) all along the Socotra-Sharbithat segment without any changes across the HTZ (Fig.13.b). On the northern margin, a northward-dipping subhorizontal detachment fault (Figs. 6.c and 12) is identified beneath the oceanward edge of the continental domain (profile Enc26, Fig. 10.b). East of the QTZ, this detachment fault is capped by continentward-dipping 2-10 km wide continental blocks (Figs. 5.a, 6.a, 11.a, 12, 13). This normal fault is defined by a strong impedance contrast indicating the juxtaposition

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of materials of distinct nature (Fig. 8.g), which argues strongly in favor of a northward dip of the final detachment fault. Hence, the southern margin (characterized by oceanward-dipping structures) would correspond to the lower plate and the northern margin (defined by continentward-dipping structures) to

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the upper plate, as proposed by Lister et al. (1986).

Many models reproduce the asymmetry of the Iberia-Newfoundland margins (Huismans and Beaumont, 2011) based on the tectonic-evolution models of Péron-Pinvidic and Manatschal (2009). Conversely to the

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previous model described from Brune et al (2014; previous section), the asymmetry of conjugate margins is related to a large detachment fault cross-cutting the entire crust of lithosphere (e.g. Lavier and

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Manatschal, 2006; Huismans and Beaumont, 2011). However in these models, both sides of the Newfoundland-Iberia margins appear to be the upper plate (upper plate paradox, Driscoll and Karner,

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1998). Huismans and Beaumont (2011) propose that depth-dependant extension results in crustal necking breakup before the lithospheric breakup in magma poor margins. The subsidence of the H bloc is accompanied of the necking of the lower lithosphere. However, this model does not show how the crustal

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breakup occurs asymmetrically. In the eastern Gulf of Aden, the present-day margins are asymmetric and the northern margin clearly appears to be the upper plate at the beginning of the exhumation phase, as the detachment fault remains observable (Fig. 6). However, we have no evidence that the northwarddipping detachment fault described on Figures 6.c and 12 was active during the entire rifting history. This is also the case for the majority of other passive margins (e. g. Iberian-Newfoundland margins, Rosenbaum et al., 2008). Furthermore, we can observe change in fault polarity on either side of a transfer zone in an oblique rifted margin setting that have also been recorded on the Norwegian margin (Tsikalas et al., 2001; Wilson et al., 2006), in the Gulf of Suez (Bosworth, 2015) or in the case of the Bay of Biscay-Pyrenees system (Roca et al., 2011). Lister et al. (1986) and Roca et al. (2011) proposed that dip changes of the underlying

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ACCEPTED MANUSCRIPT detachment fault across a transfer fault could induce a change of the sense of rotation of the overlying tilted blocks. Indeed, in the northern Omani margin, the normal faults are southward dipping in the hyper-extended domain only between the SHFZ and the QTZ (see cross-section GG’, HH’ in Fig. 12; Fig. 5.a). The extensional faults are northward dipping between the QTZ and EGAFZ (see cross-sections II’ to LL’ in Fig. 12; Fig. 5.a). Whereas tilting of continental blocks on the entire southern margin is toward the south, with a northward dipping of the faults (Figs. 6, 7, 8, 12 and 5.b). We suggest that the main fracture zones affect the sense of shear of the margins. Small-scale convection in the mantle created by localized

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discontinuities in the age and thickness of the lithosphere (Dumoulin et al., 2008; Korostelev et al., 2015; Ballmer et al., 2015) could control a higher crustal geotherm and lead to this change of fault dip polarity. This assumption is supported by the presence of a low-velocity anomaly from ~50 to 160 km depth in the SHFZ continuation (Lat. 17,5°N, Long. 55°5E, between the Hasik and Shuwaymiyah cities, Fig. 5.a) (Basuyau et al., 2010; Korostelev et al., 2015) and by the existence of present-day heat flow anomalies

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located near the SHFZ (Lucazeau et al., 2010).

5.3 Late magmatism

The major difference between magma-rich and magma-poor rifted margins is related to the timing and

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amount of magmatic activity involved in relation to lithospheric extension, breakup and plate separation (Courtillot et al., 1999; Sengör and Burke, 1978; White and McKenzie, 1989; Coffin and Eldholm, 1994). The margins of the Socotra-Sharbithat segment are magma-poor, just like for the adjacent segment of

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Encens-Sheba (d’Acremont et al., 2005; Autin et al., 2010b). We do not observe any record of significant melting during lithospheric extension until the development of the proto-oceanic crust. Yet minor

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volcanic activity occurs in the hyper-extended and the exhumed domains of the northern margin (Figs. 5.a, 10.c, 11.a, 12) and only in the exhumed domain of the southern margin (Fig. 5.a, 7 to 9, 12). In distal

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margins, the local mismatches between the gravity anomalies and the margin domain mapping (Fig. 14.b) are correlated to the mapped volcanism. This volcanism (potentially associated with other rock-alteration processes as tectonic activity, hydratation, metamorphism), may have alterated physical properties of the

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basement and induce change in the geophysical signals as observed in the Norwegian margin (PéronPinvidic et al., 2016).

On the northern margin, volcanoes appear during the localization of deformation (along coupled structures and nearby the minor transfer zones), hence during the formation of the hyper-extended domain. Our dataset shows that the presence of volcanoes affects the development of the syn-OCT (U2) unit, and post-rift units from U3 to U4, with a decrease in deformation of the deposits from base to top (Figs. 7.c, 8.a, 9, 12). Thus, we consider that continuous volcanic activity intrudes the sediments from OCT formation (Middle Burdigalian, U2; Leroy et al., 2012) up to unit U4, which is dated in the adjoining segment at 10 Ma (Bache et al., 2011). Accordingly, the study of mantle buoyancy variations in the Gulf of Aden indicates a density contrast of -20 kg/m3, which may have controlled the onset of partial melting

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ACCEPTED MANUSCRIPT associated with crustal breakup during deposition of the syn-OCT sediments (Watremez et al., 2013). Analogous late volcanic activity has already been highlighted in the Encens-Sheba segment (Lucazeau et al., 2009; Autin et al., 2010b; Leroy et al., 2010b; Watremez et al., 2011) and in the Socotra-Sharbithat segment, based on Pn and teleseismic tomography (Corbeau et al., 2014; Korostelev et al., 2015, 2016). Our observations concerning the asymmetry and development of magmatic activity suggest that the Omani margin may have undergone a hotter thermal regime than the southern margin before and during OCT formation. Usually in rifted margins (e.g. Australian margin, Driscoll and Karner, 1998; Iberia-

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Newfoundland margins, Jeanniot et al., 2016), brittle deformation is focused in a narrow zone close to the ocean-continent boundary during activity of the detachment fault. The extremely thinned continental crust close to the low-angle fault is highly intruded and overprinted by volcanism associated with riftinduced decompression melting (White and McKenzie, 1989; Bown and White, 1995) affecting the upper plate. Mapping of the volcanoes in the Socotra-Sharbithat segment shows that volcanism could also occur during exhumation of the deep mantle rocks along a northward-dipping detachment fault owing to

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decompression melting. As exhumation of the subcontinental mantle along detachment faults occurs, the magma supply could increase gradually on the upper plate (Espurt et al., 2009). Flip-flop type detachment systems could develop during exhumation of the mantle as suggested in the Australian margin (Gillard et

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al., 2015) and first proposed for the southwestern Indian ridge (Sauter et al., 2013). This process may have triggered the development of volcanism in the newly created OCT basement of the southern margin (Fig.

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5.b, 7.c, 8.a, 9, 12).

5.4 Tectono-sedimentary evolution

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The stratigraphic observations and interpretation from the seismic lines are summarized here to propose

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a tectono-sedimentary evolution of the eastern Gulf of Aden in three main stages (Fig. 15).

5.4.1 Syn-rift stage

Three main syn-rift units identified in the offshore basins are correlated with onshore units (Fig. 3.a): the

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lower Ashawq Fm (Shizar member, Lower Rupelian), the upper Ashawq Fm (Nakhlit member, Rupelian Chattian) and the Mughsayl Fm (Chattian – Aquitanian) correlate, respectively, with the R1, R2 and R3 units offshore.

On Socotra Island, rifting is marked by LANFs possibly rooted in the upper-lower crust boundary. In the necking domain, the 10 – 17 km wide tilted crustal blocks of pre-rift sequence and basement (Fig. 3) are characterized by tilting inside the pre-rift sequence. Faults affecting the pre-rift sequence are in the hangingwall of rifted blocks rooted in a common layer corresponding to the evaporitic Rus Fm (Fig. 3.b and 3.c). The necking domains of conjugate margins are delimited oceanward by major faults (F1 and F1’, Fig. 5, 12, 15.a, c) with a heave of 5 to 13 km (Figs. 7, 9, 12), which are asymmetric (e. g. east of the HTZ, ~30 km wide in the eastern Socotra margin and ~80 km in the Omani margin Fig. 5). The hyper-extended

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ACCEPTED MANUSCRIPT domain is wider on the southern margin (~10 - 40 km, Fig. 5.b) than in the north (~50 - 90 km, Fig. 5.a). The Socotra margin displays oceanward-dipping faults and the Omani margin shows continentwarddipping faults decoupled from the substratum that we interpret as the result of a northward dipping lowangle detachment fault (Fig. 15).

5.4.2 Syn-OCT stage As extension proceeds and uplift occurs onshore, late tilting and steepening of the slope margins affect

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the conjugate margins. During the Burdigalian, this event controls the deposition of the Ayaft Fm on the proximal margins (Robinet et al., 2013) and the development of the flat syn-OCT development unit (U2) in the OCT basins on both margins (Platel and Roger, 1989; Autin et al., 2010b; Razin et al., 2010; Leroy et al., 2012). Some volcanoes at the top basement are located in the hyper-extended domain of upper plate and in the OCT of the southern margin. This volcanism affects syn-OCT (U2) and post-rift units from U3 to

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U4.

5.4.3 Post-rift stage

The post-rift sequence (U3 to U5, Figs. 3 to 12) reflects a slight northward shift of the deposition profile at

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least at the beginning of spreading. Two main post-rift phases may be distinguished: (i) the first phase is characterized by the intensification of MTCs (in yellow, Figs. 6 to 11), taking place in the interval from U3 (well-lithified) to U4 (defined by the sudden deposition of MTCs). (ii) the second phase is characterized by

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an uplift leading to the steepening of the margin slope and the increased abundance of MTCs in the Encens-Sheba segment (Bache et al., 2011). In the Socotra-Sharbithat segment, this uplift is recorded by

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major erosion and MTC deposition and marks also the end of the post-rift volcanism.

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6. Conclusions

In this study, we defined the seismic stratigraphy and the crustal domains along the eastern Gulf of Aden margins between the major SHFZ and EGAFZ. This enables us to propose a map showing the limits

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between the continental (proximal, necking, hyper-extended) domains and the OCT domains. Our observations show that continental margins are asymmetric since the thinning stage. In immerged margins, the crustal attenuation is accommodated along low angle normal faults dipping toward the north implying a shallow decollement layer in the necking domain. Faults trends evolve from E-W to N110°E in the necking zone to the hyper-extended and OCT domains respectively. This observation suggests that the strain localization occurs in a narrow zone as the deformation increases perpendicularly to the direction of divergence, under far-field stress. The present-day conjugate margins present sharp necking domains (~40 to 10 km wide) as well as asymmetric and narrow hyper-extended domains (10 - 40 km wide on the southern margin and ~50 - 80 km wide on the northern margin). The asymmetry of the distal margins could be produced by the steady-state rift migration toward the north accomplished by sequential

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ACCEPTED MANUSCRIPT faulting in the upper crust and significant lower crustal flow as suggested by the model of Brune et al. (2014). In this way, large part of the southern hyper-extended domain originates from the formerly northern margin. Our observations point the existence of a clearly expressed northward-dipping detachment fault that initiate at the beginning of the exhumation phase (Burdigalian). The upper plate (Omani margin) and lower plate (Socotra margin) can be mapped with a change near the main fracture zones where thermal effect could be proposed. Minor volcanism occurs in the hyper-extended domain of the Omani margin, probably due to rift-induced decompression melting during the unroofing of

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serpentinized subcontinental mantle along the northward-dipping exhumation fault. Our results suggest that multiple generations of detachment faults affect the transitional basement as the deformation localizes in the OCT. These structures are associated to a gradual increase in magma supply that could have triggered the formation of a proto-oceanic crust until the setting up of a steady-state oceanic spreading center (at ~17 Ma).

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Acknowledgements

Partenariat – TOTAL - UNIVERSITÉ PIERRE ET MARIE CURIE – CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE. This study is a contribution from Actions Marges (Total, CNRS-INSU, IFREMER, BRGM). We

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are indebted to François Le Corre, Jean-Claude Le Gac, Captain Rémi de Monteville, the officers, and the crew members of the BHO Beautemps‐Beaupré, and to the French Navy hydrographers and the hydrographic team of the Mission Océanographique de l’Atlantique for their assistance in data acquisition

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in 2012. We deeply thank the Yemeni colleagues Dr I. Al Ganad and the authorities for authorisations of work in the waters of Yemen. We deeply thank the Omani colleagues, Dr Issa El Hussein, Dr Ali Al-Lazki, Dr

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English style and grammar.

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Samir Sobhi, A. Al Rajhi, Dr Hilal Al-Azri, M. Al-Araimi and K. Al Toubi. Dr M.S.N. Carpenter post-edited the

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ACCEPTED MANUSCRIPT Péron-Pinvidic, G., Huchon, P., Manatschal, G., 2009. Ocean-continent transition. Comptes Rendus Geosci., Transition Océan-Continent 341, 357–362. doi:10.1016/j.crte.2008.12.007 Péron-Pinvidic, G., Manatschal, G., 2009. The final rifting evolution at deep magma-poor passive margins from Iberia-Newfoundland: a new point of view. Int. J. Earth Sci. 98, 1581–1597. doi:10.1007/s00531008-0337-9 Péron-Pinvidic, G., Manatschal, G., Osmundsen, P.T., 2013. Structural comparison of archetypal Atlantic rifted margins: A review of observations and concepts. Mar. Pet. Geol. 43, 21–47. doi:10.1016/j.marpetgeo.2013.02.002

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ACCEPTED MANUSCRIPT Figure captions Figure 1 : Topographic and bathymetric map of the Gulf of Aden area highlighting first-order segmentation. Black arrows: absolute plate motion directions. Boxes outlined in black show the areas studied on the eastern Gulf of Aden conjugate margins, between the SHFZ (Socotra-Hadbeen Fracture Zone) and the EGAFZ (Eastern Gulf of Aden Fracture Zone). The Jurassic and cretaceous basins correspond to: (i) the Balhaf graben, the Masilah and the Jeza-Qamar basins in Yemen; (ii) the Berbera, the Darror basins in Somalia. SESFZ: Shukra El Sheik Fracture Zone. KAIFZ: Khanshir Al Irqah Fracture Zone. AFFZ: Alula Fartak fracture zone. AMFZ: Al Mukalla Fracture Zone. OFZ: Owen Fault Zone.

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Figure 2 : Simplified geological maps of the conjugate margins in the studied area, between the SocotraHadbeen Fracture Zone (SHFZ) and the Eastern Gulf of Aden Fracture Zone (EGAFZ). On background, swath bathymetric map of the offshore conjugate margins with a resolution of 100 to 50 m (data from several cruises: Encens-Sheba (2000 on R/V Marion Dufresne, DOI 10.17600/200020), Encens (2006 on R/V L’Atalante, DOI 10.17600/6010030) and Marges-Aden (2012 on BHO Beautemps-Beaupré, DOI 10.17600/12090040). Below the maps, geological cross-sections show major structures of the studied areas, with no vertical exaggeration. (a) Northern margin, geological map of Southeastern Oman (Dhofar area) modified after Robinet et al. (2013) from Roger et al. (1989), Le Métour et al. (1992), Dubreuilh et al. (1992), Platel et al. (1992a, b, c, d), Roger et al. (1992), with geological cross-section AB trending N-S. The Neoproterozoic crystalline basement (in red) is composed of various metamorphic, plutonic and volcanic rocks (Mercolli et al., 2006; Leroy et al., 2012). The Huqf Group corresponds to siliciclastics, carbonates and evaporites (Le Métour, 1995; Forbes et al., 2010). The Mesozoic pre-rift sedimentary sequence, comprised of Cretaceous carbonates, unconformably overlies the Neoproterozoic unit (Beydoun and Bichan, 1969; Leroy et al., 2012). (b) Southern margin, geological map of Socotra modified after Leroy et al. (2012) and Bellahsen et al. (2013) showing the island divided into two main structural domains separated by the Hadibo transfer zone (HTZ). The geology of the eastern domain consists of a major basement high without a syn-rift basin, while the western part is represented by the geological cross-section C-D modified after Leroy et al. (2012) with three main syn-rift basins. J: Jebel. F.Z: Fault Zone. T.Z: Transfer Zone.

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Figure 3 : Synthesis of onshore stratigraphy and correlation with various units proposed for the stratigraphy of the offshore profiles. (a) Synthesis of onland stratigraphy for Southeastern Oman and the Socotra Island units, based on Roger et al. (1989), Fantozzi and Sgavetti (1998), Fournier et al. (2004), Razin et al. (2010), Bellahsen et al. (2006), Leroy et al. (2012), Bellahsen et al. (2013), Robinet (2013) and Robinet et al. (2013) and summary of different units proposed for the offshore stratigraphy. The working correlation is based on seismic facies, sediment geometries and location of sedimentary units in the stratigraphic column in accordance with the tectono-sedimentary evolution of the Gulf of Aden. See text for explanation. (b) Northern margin, identification of units and seismic facies as defined in the necking zone of the margin offshore on the multichannel seismic reflection profile ENC05. (c) Southern margin, identification of units and seismic facies as defined in the necking zone of the margin offshore on the high-speed seismic reflection profile MAD18. UER: Umm Er Radhuma. CB: Cenozoic basement. MB: Mesozoic basement. NB: Neoproterozoïc basement. Figure 4 :Track map of seismic cruises and depth to basement map based on the dataset developed from surveys (seismic reflection data and swath bathymetry). Onshore first-order structures after (Leroy et al., 2012; Robinet et al., 2013) and other references therein. SHFZ: Socotra Hadbeen Fracture Zone. EGAFZ: Eastern Gulf of Aden Fracture Zone. Table 1 : Summary of key observations of the seismic units, with their reflection characteristics and ages.

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ACCEPTED MANUSCRIPT Figure 5 : Structural sketch of the conjugate margins with major structures, different crustal domains and the interpreted segmentation. In background, bathymetry (obtained from the Encens-Sheba, Encens and Marges-Aden cruises; Fig. 4) and shaded topographic maps (SRTM) (Jarvis et al., 2008). Note that red structures are drawn at the foot of the fault. ED: Exhumed Domain. POD: Proto-Oceanic Domain. SHFZ: Socotra Hadbeen Fracture Zone. EGAFZ: Eastern Gulf of Aden Fracture Zone. HTZ: Hadibo Transfer Zone. QTZ: Qalansiya Transfer Zone. BKTZ: Balan-Kadarma Transfer Zone. MTZ: Momi Transfer Zone. H. Mt: Haggier Mt. H: Hadibo city. Q: Qalansiya city.

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Figure 6 : (a) Detail of the northern margin (seismic profile ENC21) with the different sedimentary sequences identified on the profiles of both conjugate margins (Fig. 3) and interpretation of five crustal domains. At bottom: migrated seismic line and its interpretation. (b) Interpretation of basement structures, location of the Taper Break (TB) and Coupling Point (CP) (see text for definition). (c) OCT ridge and proto-oceanic crust on profile ENC21. HED: Hyper-extended domain. MTC: Masstransport Complex deposits. OCT: Ocean-continent transition.

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Figure 7 : (a) Detail of the southern margin (seismic profile MAD04) with the different sedimentary sequences identified on the profiles of both conjugate margins (Fig. 3) and identification of five crustal domains (necking, hyper-extended continental, exhumed, proto-oceanic and stable oceanic domains). At bottom: migrated seismic line MAD04 and its interpretation. The Moho depth projected onto the interpreted profile is from Ahmed et al., 2014. (b) Interpretation of the sedimentary sequences in a syn-rift perched basin on the MAD04 line. (c) Interpretation of basement structures and sedimentary units within the ocean-continent transition on line MAD04. MTC: Mass-transport deposits. OCT: Ocean-continent transition. TB: Taper Break. For northern conjugate profile, see profile ENC26 (Fig. 6).

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Figure 8 : (a) Detail of the southern margin (seismic profile MAD13) with the different sedimentary sequences identified on the profiles of conjugate margins (Fig. 3) and identification of five crustal domains (necking, hyper-extended continental, exhumed, proto-oceanic and stable oceanic domains). At bottom: migrated seismic line MAD13 and its interpretation. The Moho depth projected onto the interpreted profile is from Ahmed et al., 2014. The Taper break (TB) is located between the tilted blocs 1 (B1) and 2 (B2). B2 is located in the hyper-extended domain. (b) Zoom on seismic sections MAD16-3 presented without (right) and with interpretation (left) showing the eastern prolongation of tilted block B2 (of 4 km-wide) in the hyper-extended domain and the inferred lower crust. (c) 3D representation of bathymetric map at the location of B1 and B2 and location of section CD on the profile MAD16-3. Schematic section along the margin of B1 and B2 showing lateral changes in the width and geometry of the tilted block B2 along the margin. On profile Mad13 (Fig. 8), from CMP 850 to 1 400, the TB forms the southern boundary of an 8-km-wide rifted block (B2) comparable to those observed in the necking domain (as B1, Fig. 8.a). Nevertheless, we exclude this tilted block from the necking domain because its eastern prolongation on Mad16-3 (see B1, Fig. 8.b, c) seems to be entirely decoupled by a fault of 5 km of dip-slip motion. This fault is followed by a northward sub-horizontal high-amplitude reflector located at 1 km depth in the basement (Fig. 8.c) suggesting that B2 corresponds to an extensional allochthon and supporting the attribution of B2 to the hyper-extended domain. MTC: Mass-transport deposits. OCT: Ocean-continent transition. Figure 9 : Detail of the southern margin (seismic profile MAD18) with the different sedimentary sequences identified on the profiles of both conjugate margins (Fig. 3) and identification of five crustal domains (necking, hyper-extended continental, exhumed, proto-oceanic and stable oceanic domains). At bottom: migrated seismic line MAD18 and its interpretation. The Moho depth projected onto the interpreted profile is from Ahmed et al., 2014. MTC: Mass-transport deposits. OCT: Ocean-continent transition. TB: Taper Break. The proto-oceanic crust shows internal chaotic reflections like the oceanic crust. However, this domain (i) is not overlain by the transparent unit recorded on top of the oceanic

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Figure 11 : Detail of the northern margin (seismic profile ENC19) with the different sedimentary sequences identified on the profiles of both conjugate margins (Fig. 3) and interpretation of five crustal domains. At bottom: migrated seismic line and its interpretation. (b) Interpretation of key site (ENC19): a major volcano (around 30 km in extent) in the hyper-extended continental domain. ED: Exhumed domain. MTC: Mass-transport deposits. OCT: Ocean-continent transition.

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Figure 12 : Sketch showing evolution of the conjugate margins from the Socotra-Hadbeen fracture zone (SHFZ) in the west to the Eastern Gulf of Aden fracture zone (EGAFZ) in the east. See structural sketch for location. The map (inset) shows the location of the different profiles. ED: Exhumed domain. POC: Proto-oceanic crust. HTZ: Hadibo Transfer Zone. MTZ: Momi Transfer Zone. H. Mt: Mount Haggier. H: Hadibo.

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Figure 13 : Seismic sections with zooms on key sites Identified by letters (A to C), presented without (top) and with interpretation (bottom). See maps on Fig. 11 for location. (a) Zoom of the steady-state oceanic crust. (b) Zoom of the OCT-ridge and its relation with the proto-oceanic crust.

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Figure 14 : (a) Magnetic profiles on conjugate margins, over the OCT, in northern margin (top) and southern margin (bottom). (b) Free-air gravimetric profiles on conjugate margins, over the OCT, in northern margin (top) and southern margin (bottom). Black dashed lines: volcanism identified on seismic profiles. F1 OD: Oceanic Domain. HTZ: Hadibo Transfer Zone. MTZ: Momi Transfer Zone. H. Mt: Mount Haggier. H: Hadibo. QTZ: Qalansiya Transfer Zone.

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Figure 15 : 3D morpho-tectonic evolution of the eastern Gulf of Aden conjugate margins: during the late syn-rift phase (~Aquitanian–Lower Burdigalian), presented as a cutaway drawing of (a) the basement and (b) with sedimentary infill; during the late syn-OCT phase (Middle–Upper Burdigalian), presented as a cutaway drawing of (c) the basement and (d) with sedimentary infill. This reconstruction takes into account the 3D morphology of the conjugate margins inferred from seismic interpretation of actual structures, volcanism and sedimentary infill.

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