Supplementary Information
Crustal structure and seismic data. The crustal models of the Encens experiment (Fig. S1)
presented in Fig. 2 are based on high-quality wide-angle seismic dataset analysed by different
groups using the same approach: the traveltime modelling method of [Zelt and Smith, 1992].
Manuscripts describing the analyses in detail are submitted or in preparation. In particular, we
note that [Watremez et al., subm.] conducted extensive tests for the Ashawq-Salalah segment
to fit for structure more rigorously than is strictly required. The Ashawq-Salalah model in Fig.
2 is the smoothest model that corresponds satisfactorily with the data. A key feature of this
model is the thickness of the crust and shallowing of the base of the crust with respect to the
other segments and margins. Double reflections in the lower part of the crust and the
associated high seismic velocity suggest some magmatic underplating after the lithospheric
rupture, and we believe this underplating associated with the post-rift volcanism expressed at
the surface by the volcano [Lucazeau et al., 2009; Autin et al., 2010]. Post-rift magmatism in
the two other eastern segments could hide the initial structure of the OCT of the AshawqSalalah segment and possibly exhumed continental mantle as suggested by Autin et al. [2010]
and support by our findings. Increased extension rates to the east explain the increasing width
of the OCT from 15 km to 50 km better than a westward propagation of the rift does. Eastwest spreading rate variability is also consistent with the magmatism-controlled
characteristics of the OCT [Behn and Ito, 2008]. Chron 6 could be recorded here in the
context of serpentinization process, which is able to produce magnetic lineations as proposed
in the West Iberia margin [Sibuet et al., 2007]. As the magnetic profile is not a deep-towed
one, the amplitude of these magnetic anomalies is very low (Fig. 1; 2; S1 and S2). Examples
of raytracing in the profile across the Ashawq-Salalah segment are shown in Figs. S2 and S3.
All of the instruments recorded data of comparable quality on this and the other segments.
The colored lines are the picks of phases observed on the records of the instruments. Some of
these records share reciprocity with the other instruments of the experiment. The reciprocity
relationships demonstrate the quality of the overall dataset especially on the Enc05 profile
shown by Watremez et al. [submitted]. We used the layer-stripping method of Zelt and Smith
[1992]. The Ashawq-Salalah segment is modelled by 7 layers (water, sediments 1, sediments
2, crust, basement, mafic body and mantle). The Taqah and Mirbat segments are modelled by
6 layers (Water, sediment1, sediment2, basement, crust, mantle). The sediments and acoustic
basement are constrained by the co-located multichannel seismic reflection profiles of the
Encens cruise [Leroy et al., 2006; Autin et al. 2010]. Once a seismic layer is well-constrained
(Chi2 ≈ 1), we model the underlying layer, to the bottom of the sequence. In Fig. 2 to 4, we
have represented the reflection points by a thick solid line and Figure S4 shows all the rays
used for the modelling.
Collection of the MCS data integrated the Ifremer/Genavir navigation system, which records
360 channels at a 4 ms sample rate. The seismic source used in the study area allows good
sediment resolution and deep reflector imaging. A tuned array of 14 air guns (first pulse
synchronized) totaling 42.5 l (2594 in3) was shot at 50 m intervals (about 20 s intervals). The
receiver array consisted of 360 groups of hydrophones spaced at 12.5 m intervals for a total
array length of 4500 m. It comprises 30 active sections, each 150 m in length. Geometry of
the shots and receivers are set according to GPS navigation data. This survey had a nominal
45-fold coverage. The processing sequence (Geocluster® 3100 and 4100, Linux Red Hat) is
detailed in the Autin et al. [2010].
In the Ashawq-Salalah segment, there is evidence of volcanism after continental break-up.
The seismic profile shown in Fig. S5 provides support for our interpretation of a volcano
located in the OCT. The seismic patterns such as the thick and strong reflection, diverging
reflectors in the southern flank and the conical shape, are characteristic of volcanic features.
Sills could be also imaged in other profiles near the area [Autin et al 2010].
The mussel-shaped bathymetric high has been recently faulted as evidenced by many
submarine landslides observed on the seafloor morphology. Other active sedimentary
instabilities are located in the continental domain (Figs. 1 and 3), revealing a persistent uplift
of this eastern margin segments (Taqah and Mirbat) corroborated by heat-flow modeling
[Lucazeau et al., 2008]. The seismic profile shown in Fig. S6 as well as the bathymetric map
support this interpretation of active submarine landslides related to a late uplift of the margin.
Autin, J., Leroy, S., Beslier, M. O., d'Acremont, E., Razin, P., Ribodetti, A., Bellahsen, N., Robin, C., and AlToubi, K. (2010), Continental break-up history of a deep magma-poor margin from seismic reflection
data (northeastern Gulf of Aden margin, offshore Oman), Geophys. J. Int., 180, 501-519.
Behn, M. D., and Ito, G. (2008), Magmatic and tectonic extension at mid-ocean ridges: 1. Controls on fault
characteristics, Geochem. Geophys. Geosyst., 9.
Leroy, S., Ebinger, C., d'Acremont, E., Stuart, G., Al-Lazki, A., Tiberi, C., Autin, J., Watremez, L., Beslier, M. O.,
Bellahsen, N., Lucazeau, F., Perrot, J., Mouthereau, F., Courrèges, E., Huchon, P., Rouzo, S., Bahalaf,
S., Sholan, J., Unternehr, P., Hello, Y., Anglade, A., Desprez, O., Beguery, L., Aouji, O., Daniel, R., AlToubi, K., Sage, F., and Khanbari, K. (2006), The onshore-offshore ENCENS project: Imaging the
stretching/thinning of the continental lithosphere and inception of oceanic spreading in the eastern Gulf
of Aden, in AGU Fall meeting, edited, AGU, San Francisco.
Lucazeau, F., Leroy, S., Autin, J., Bonneville, A., Goutorbe, B., Rolandone, F., d’Acremont, E., Watremez, L.,
Düsünur, D., and Huchon, P. (2009), Post-Rift Volcanism and high heat-flow at the Ocean-Continent
Transition of the Gulf of Aden, Terra Nova, 21, 285-292.
Lucazeau, F., Leroy, S., Bonneville, A., Goutorbe, B., Rolandone, F., d’Acremont, E., Watremez, L., Düsünur,
D., Tuchais, P., Huchon, P., Bellahsen, N., and Al-Toubi, K. (2008), Persistent thermal activity at the
Eastern Gulf of Aden after continental break-up, Nature Geosciences, 854-858.
Sibuet, J.-C., Srivastava, S., and Manatschal, G. (2007), Exhumed mantle-forming transitional crust in the
Newfoundland-Iberia rift and associated magnetic anomalies, J.Geophys. Res., 112,
10.1029/2005JB003856.
Watremez, L., Leroy, S., Rouzo, S., d'Acremont, E., Unternehr, P., Ebinger, C., Lucazeau, F., and Al Lazki, A.
(subm.), The crustal structure of the northeastern Gulf of Aden continental margin: insights from wideangle seismic data, Geophys. J. Int.
Zelt, C. A., and Smith, R. B. (1992), Seismic traveltime inversion for 2-D crustal velocity structure, Geophys. J.
Int., 108, 16-34.
Figure S1: Gravity map of the northeastern Gulf of Aden with the location of the profiles and
instruments (15 from IRD-Villefranche sur Mer and 20 from INSU Paris; 18 stations from
Encens-UK NERC project; 2 temporary and 4 permanent stations from Sultan Qaboos
University; 1 from ENS Paris).
Figure S2: Examples of wide-angle seismic profiles from the Ashawq-Salalah segment. The
colored lines are the picked phases of the two OBS presented (upper and middle panels).
Bottom panel: the dashed lines correspond to the layers of the seismic model, the red circles
to OBS presented among the OBS of the line (grey circles). The tracing rays have the same
color as the picked phases (only one ray is traced for every five rays received).
Figure S3: Example of a land station profile from the Ashawq-Salalah segment. Note the
good quality of the record. The picks of phases are reported on the record and seismic velocity
model.
Figure S4: Three profiles presented in this study with all the rays used for modeling.
Figure S5: Close-up of multi-channel seismic profile (Enc34) across the OCT of the Ashawq-Salalah
segment. Volcanic flows or sills are highlighted as well as diverging reflectors in the southern part of
the volcano [Autin et al. 2010].
Figure S6: A Bathymetric map of the margin from Encens and Encens-Sheba cruises data
and close-up of the spur-shaped bathymetric high. B Multi-channel seismic profile illustrating
the active submarine landslides (red arrows are sense of sliding along faults in yellow).