Magmatism at continental passive margins inferred from ambient-noise phase-velocity in the Gulf of Aden

Non-volcanic continental passive margins have traditionally been considered to be tectonically and magmatically inactive once continental breakup has occurred and seafloor spreading has commenced. We use ambient-noise tomography to constrain Rayleigh-wave phase-velocity maps beneath the eastern Gulf of Aden (eastern Yemen and southern Oman). In the crust, we image low velocities beneath the Jiza-Qamar (Yemen) and Ashawq-Salalah (Oman) basins, likely caused by the presence of partial melt associated with magmatic plumbing systems beneath the rifted margin. Our results provide strong evidence that magma intrusion persists after breakup, modifying the composition and thermal structure of the continental margin. The coincidence between zones of crustal intrusion and steep gradients in lithospheric thinning, as well as with transform faults, suggests that magmatism post-breakup may be driven by small-scale convection and enhanced by edge-driven flow at the juxtaposition of lithosphere of varying thickness and thermal age. Terra Nova, 28: 19 – 26, 2016

Magmatism at continental passive margins inferred from Ambient-Noise Phase-velocity in the Gulf of Aden Introduction Mechanical faulting and ductile stretching, as well as magma intrusion, accommodate extension of the lithosphere beneath rifts and can lead to continental breakup (McKenzie, 1978). During the early stages of rifting, crustal extension is commonly thought to occur mainly on border faults (e.g. Bellahsen et al., 2006). As the rift widens, extension commonly localizes in-rift to small offset fault networks, and in magma-rich settings to magma intrusion in axial volcanic segments (Ebinger and Casey, 2001). Once breakup occurs and a new midocean ridge forms, the conjugate rifted passive margins at the edges of the continents are commonly assumed to be tectonically and magmatically inactive. However, it is becoming increasingly recognized that the continental margins may remain active after breakup (Ebinger and Belachew, 2010;Pallister et al., 2010;Rooney et al., 2014) and continue to accommodate extension and modify the thermal structure and composition of the crust and upper mantle (Bellahsen et al., 2013a;Watremez et al., 2013). Despite its importance, we have few constraints on the distribution and time-scales over which magmatism persists at rifted margins. In order to constrain the magma plumbing system beneath a rifted margin, we use ambient-seismic-noise tomography to image the Rayleigh-wave phase-velocity structure of the crust and upper mantle beneath the young rifted margin on the northern side of the Gulf of Aden in Yemen and southern Oman (Fig. 1).
Fault-related sediment deposits in Oman and Yemen suggest rifting began at about 34 Ma along the whole Gulf (e.g. Leroy et al., 2012;Robinet et al., 2013; Fig. 1), approximately coeval with $ 25-31 Ma opening in the Afar region (e.g. Wolfenden et al., 2005;Ayalew et al., 2006;Stab et al., 2015). Rifting was associated with the development of 2-km deep grabens in the proximal part of the margin, with a present relief of 3000-4000 m between the submarine distal domain (at 2000 m deep) and the subaerial uplifted Yemen and Oman shoulders (Leroy et al., 2010a;Watremez et al., 2011;Figs 1 and 2). A prior stage of extension in the Cretaceous created the Jiza-Qamar and Gardafui grabens located in Yemen and in the south of Socotra Island (Fig. 1b); these were reactivated during the most recent extension between 34 and 18 Ma (Leroy et al., 2012). Extension occurs above a warm mantle of potential temperature (Tp) 1450°C in the west (Rooney et al., 2012;Ferguson et al., 2013) and likely above normal mantle of Tp 1350°C in the east (Lucazeau et al., 2010;Bellahsen et al., 2013a;Rolandone et al., 2013). Rifting above warm mantle in the west is associated with the production of voluminous flood basalts on the Yemeni plateaus synchronous with the onset of extension and the formation of volcanic margins with characteristic seaward-dipping reflectors (SDR) during breakup (Tard et al., 1991;Leroy et al., 2012;Ahmed et al., 2013;Korostelev et al., 2014;Fig. 1b). In Central Yemen, rifting  occurs above warm mantle without SDR formation but with lacustrine syn-rift sedimentation (a few meters). Rifting in the eastern part of the Gulf is associated with turbiditic synrift sedimentation (hundreds of meters) (Leroy et al., 2012;Robinet et al., 2013).
From $ 19 Ma to present, evidence for magmatism in the form of volcanism is recorded along most of the Gulf of Aden (Leroy et al., 2010b), with Quaternary-Recent vol-canic centers common in western Yemen (Manetti et al., 1991;Korostelev et al., 2014), localized to near the Masilah graben in central Yemen (Korostelev et al., 2015a) and observed at the foot of the margin offshore Oman in the eastern part of the Gulf of Aden (Lucazeau et al., 2009;Autin et al., 2010;Figs 1 and 2).
Both the continental rifted margin and the ocean ridge are segmented by major fracture zones. The Alula-Fartak fracture zone (AFFZ) has a maximum lateral offset of 180 km and divides the Jiza-Qamar Gardafui pre-existing basin (d' Acremont et al., 2005Acremont et al., , 2010Leroy et al., 2012;Bellahsen et al., 2013b); the active spreading ridge is localized at its northern edge in the east and at its southern edge in the west (Fig. 1b). The crustal thickness beneath the southern Arabia continental margin varies from 35-45 km beneath most of its relatively undeformed northern A s h a w q -S a la la h g .

Data
Our dataset is based on continuous recordings from 142 seismic stations, mainly from temporary networks of broadband seismometers (Fig. 2). The data quality selection followed two steps: (i) selection of pairs of stations that recorded simultaneously for at least 6 months, and (ii) comparison of the measured pairs, keeping those that showed a good fit (Korostelev et al., 2015b). The duration of the cross-correlated signal thus varies by 6-18 months depending on the station pair. The method is described in the supplementary material (Figures S1 and S2 in Data S1).

Results
We computed phase-velocity maps for periods between 7 and 30 s, and present four examples at 10, 12.5, 17 and 20.5 s (Fig. 3). The images show Rayleigh-wave phase-velocity perturbations relative to the mean Rayleigh-wave phase-velocity. According to, for example, Lebedev and Van Der Hilst (2008) and Fry et al. (2010), 10 s Rayleigh waves are most sensitive to depths less than 20 km (upper and mid-crust), while 12.5-15.5 s periods are most sensitive to depths of 10-40 km (primarily the lower crust). The 17-20.5 s periods are most sensitive to depths of 20-25 km and have some sensitivity to depths of 50-60 km (uppermost mantle). Given that the lithosphere in the region is estimated to be 150 km thick (Rolandone et al., 2013), we are primarily imaging the uppermost mantle lithosphere in our longer-period maps. In the supplementary material, we provide some examples of surface-wave depth-sensitivity functions ( Figure S3 in Data S1).
The locus of major velocity anomalies is fairly constant from 10 to 12.5 s to the east of the Alula-Fartak fracture zone (AFFZ; Fig. 3). We image fast velocity perturbations beneath the eastern part of the Ashawq-Salalah graben beneath the Mirbat plain near the Socotra-Hadbeen fracture zone and in the Sharbithat area (Figs 1, 2 and 3). Fast velocity perturbations are also present beneath the Jiza-Qamar basin at 17 and 20.5 s in between the major faults.
At the 12.5 s period, the slow anomalies are concentrated in the center of the Jiza-Qamar basin and in the north of the Ashawq-Salalah graben (Fig. 3). At the 20 s period we also image slow anomalies in central Yemen at 49°E (Fig. 3).
The magnitudes of several of the distinct velocity perturbations vary with period. For example, beneath the Mirbat area (Oman, Fig. 2), the fast anomaly increases in magnitude from $ 3% at 12.5 s to 9% at 17-20.5 s (Fig. 3). The slow anomaly beneath the Jiza-Qamar basin is mostly more than À10% at 10 s, whereas at 12.5 s a larger proportion of the anomaly is -6% (Fig. 3).
The slow anomalies beneath the western part of the Ashawq-Salalah graben and beneath central Yemen correlate well with the areas of surface volcanism known offshore and onshore respectively (Figs 2 and 3). In addition, the shallower low-velocity anomaly located in the center of the eastern part of the graben corresponds well to the locus of maximum thickness of sediment infill of the Cretaceous basin in between the major faults (Figs 2 and 3). Sediment thickness is greatest in the east, where plate thinning is likely greatest (Brannan et al., 1997;Hakimi and Abdullah, 2014).
The values of the deep slow anomalies imaged using ambient noise are also in general agreement with the values of low crustal shear velocities (3.15-3.45 km s À1 from Pasyanos and Nyblade, 2007) in the crust, and their spatial extent correlates well with low velocities in the mantle constrained using P-wave teleseismic tomography (Korostelev et al., 2015a). The ambient noise provides an additional constraint on the depth of the anomaly in the upper mantle (20.5 s, Fig. 3).

Discussion
Seismic-wave velocity is known to be affected by the temperature and chemical composition of the medium of propagation (crustal and mantle rocks) as well as by the concentration of fluids, such as partial melt and geothermal fluids (e.g. Christensen and Mooney, 1995; Karato et al., 2003). We image slow velocities beneath zones of known active volcanism in southern Yemen ( Fig. 1  and 2, see the locations of volcanoes), in agreement with the hypothesis that the major surface-wave slow anomalies are associated with magmatism. The magnitudes of the anomalies and their spatial distribution in regions where extension has occurred suggest that magmatic processes currently modify the crust beneath the flanks of continental rifted margins. Beneath the proximal margin, geological studies suggest early border faults were active at 34 Ma but not associated with much volcanism; yet our velocity maps suggest that magmatic systems are currently active in these regions $ 20 Ma post-breakup.
The slow anomalies in our phasevelocity maps beneath the Jiza-Qamar graben are observed in the lower crust, with higher amplitude in the north (Fig. 3; period = 10 and 12.5 s). These velocities could be indicative of fluids (especially partial melt) in the crust beneath the proximal grabens of the Gulf of Aden northern margin (Jiza-Qamar, Ashawq-Salalah and Sharbithat grabens) where surface volcanism is lacking (Fig. 2). Basuyau et al. (2010) identified localized slow velocities at around 60 km depth, interpreted as zones of partial melt, which supports the inference that our overlying crustal slow anomalies may be caused by regions of intrusion lacking eruption. The highly segmented continental margin of the Gulf of Aden, and especially the 180 km long offset of the AFFZ, could localize the transport of fluids through the lithosphere. Magmatism and particularly dike intrusion is common in transform fault zones (e.g. Gudmundsson, 1987Gudmundsson, , 1995. In addition, it has been demonstrated that uplift lessens the potential energy and may favour the  (Gudmundsson, 2000). Dike migration could be arrested and deflected into sills by sharp horizontal layering such as the base of the crust, the basement-sediment interface, and sedimentary layering within the basin (Gudmundsson, 2011). The 4-kmthick sediments of the Jiza-Qamar basin located along a major transform fault zone and uplifted after the breakup are thus likely to be a favourable location for diking and magmatism. Figure 4 shows an interpreted cross-section through the Jiza-Qamar basin based on our phase-velocity models. The present-day crustal thickness and basin fill, with 4-kmthick Cretaceous sediments, are inferred from computed receiver functions and upper-crustal borehole data respectively (Hakimi and Abdullah, 2014;Korostelev et al., 2015a). The crust was thinned during the last rifting episode during the Oligo-Miocene (Leroy et al., 2012, and references therein).
In the past, magmatism at nonvolcanic continental passive margins has commonly been ignored, and the impact of magmatism not expressed at the surface has not been considered when estimating the thermal subsidence history of the margins after breakup. Post-breakup crustal intrusion with limited volcanism after breakup along the highly segmented Gulf of Aden rifted margin is likely maintained by small-scale convection created by the large steps in temperature and lithospheric thickness at the  edge of the Arabian plate (Dumoulin et al., 2008;Lucazeau et al., 2008). Large steps in the topography of the base of the lithosphere are also enhanced in the vicinity of major transform faults, where the juxtaposition of lithosphere of different thermal ages can also trigger small-scale convection (Korostelev et al., 2015a).
Channelization of mantle flow from the Afar hotspot along the oceanic ridge and the fracture zones could also be invoked to explain these anomalies (Leroy et al., 2010b;Corbeau et al., 2014). On a local scale, upward magma migration at the rift margin may be aided by pre-existing border faults.

Conclusions
Our study provides new high-resolution phase-velocity maps of the crust and uppermost mantle of the northern margin of the Gulf of Aden using ambient-noise tomography to constrain present-day crustal structure, 20 Ma after continental breakup.  Fig. 4 (a) Location map with simplified tectonic structures, boreholes and cross-section. (b) Schematic cross-section of the Jiza-Qamar basin modified from Brannan et al. (1997) and Hakimi and Abdullah (2014). Black dots show Moho depths inferred from receiver functions (Korostelev et al., 2015b). XMFZ, Xiis-Mukalla fracture zone; BMFZ, Bosaso-Masilah fracture zone; AFFZ, Alula-Fartak fracture zone; SHFZ, Socotra-Hadbeen fracture zone. Low-velocity anomalies are located beneath the Jiza-Qamar (Yemen) and Ashawq-Salalah (Oman) basins and could be caused by ongoing magmatism with no surface expression. This study suggests that magmatism can persist beneath continental passive margins after breakup without being expressed at the surface.
At a lithospheric scale, the ongoing magmatism may be related to small-scale convection at the step in lithospheric thickness beneath the rifted margin, and enhanced by contrasting thicknesses and thermal ages of the lithosphere near fracture zones and transform faults (Korostelev et al., 2015a).