Félicie Korostelev
email: felicie.korostelev@gmail.com

Cornelis Weemstra

Sylvie Leroy

Lapo Boschi

Derek Keir

Yong Ren

Irene Molinari

Abdulhakim Ahmed

Graham W Stuart

Frédérique Rolandone

Khaled Khanbari

James O S Hammond

J M Kendall

Cécile Doubre

Ismail Al Ganad

Berhe Goitom

Atalay Ayele

Magmatism on rift flanks: insights from 1 Ambient-Noise Phase-velocity in Afar region 2

During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time towards the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phasevelocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems, but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process, and may persist through to early seafloor spreading. 06, UMR 7193, Institut des Sciences de la

Introduction

During the breakup of continents, stretching and thinning of the plate commonly causes decompression melting and volcanism. In the resultant magmatically active rift valleys it is widely thought that extension is initially accommodated mainly by border faulting, and progressively localizes to relatively narrow axial volcanic segments as the rift valley widens [e.g. [START_REF] Ebinger | Continental breakup in magmatic provinces: An Ethiopian example[END_REF]. However, it is becoming increasingly more recognized that magma intrusion and volcanism can occur on the rift flanks at an early stage of rifting [e.g. [START_REF] Maccaferri | Off-rift volcanism in rift zones determined by crustal unloading[END_REF]. These rift flank magmatic systems accommodate extension through diking [START_REF] Rooney | The protracted development of focused magmatic intrusion during continental rifting[END_REF], and thermally and compositionally modify the lithosphere [START_REF] Daniels | Thermal models of dyke intrusion during development of continent-ocean transition[END_REF]. Despite the importance of magmatic processes during continental extension, we have few constraints on their spatial and temporal variability. In order to address this issue we use ambient seismic noise tomography to image the Rayleigh wave phase-velocity structure of the crust in a region of late stage breakup at the conjugate margins of the southern Red Sea in Afar and Yemen.

Geochronological constraints in Ethiopia suggest rifting began 29-31 Ma on the western

Afar margin [e.g. [START_REF] Ayalew | Temporal compositional variation of syn-rift rhyolites along the western margin of the southern Red Sea and northern Main Ethiopian Rift[END_REF]Wolfenden et al., 2005, Fig. 1], approximately coeval with ∼35 Ma faulting along large portions of the Gulf of Aden to the east [START_REF] Leroy | Contrasted styles of rifting in the eastern Gulf of Aden: A combined wide-angle, multichannel seismic, and heat flow survey[END_REF]. Rifting was associated with the development of large offset border faults that currently define ∼2000-3000 m of relief between the submarine Red Sea and subaerial Afar depression with the uplifted Ethiopian and Yemeni plateaus [START_REF] Wolfenden | Evolution of the northern Main Ethiopian rift: birth of a triple junction[END_REF].

Extension is thought to have occurred above warm mantle with a potential temperature Terre Paris (iSTeP), F-75005 Paris, France.

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KOROSTELEV ET AL.: MAGMATISM ON RIFT FLANKS of ∼1450 degrees [START_REF] Rooney | Elevated mantle temperature beneath East Africa[END_REF], associated with voluminous flood basalts on the Ethiopian and Yemeni plateaus synchronous with the onset of extension [START_REF] Wolfenden | Evolution of the northern Main Ethiopian rift: birth of a triple junction[END_REF], and associated with ongoing magmatism [Ferguson et al., 2013]. At ∼21-23 Ma, magmatism occurred through dike intrusions along most of the eastern margin of the Red Sea [START_REF] Bosworth | The Red Sea and Gulf of Aden Basins[END_REF]. Magmatism on the rift flanks is ongoing, with the Quaternary to Recent volcanic centers of Sana'a, Dhamar and Marib located in Yemen [START_REF] Manetti | Magmatism of the eastern Red Sea margin in the northern part of Yemen from Oligocene to present[END_REF][START_REF] Korostelev | Crustal and upper mantle structure beneath south-western margin of the arabian peninsula from teleseismic tomography[END_REF]. In addition, thermal hotsprings are present along the conjugate southern Afar margin [START_REF] Keir | Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes[END_REF]. Magma intrusion and volcanism is also common within the rift valley. Since ∼10 Ma in Afar, extension via diking progressively localized to the rift axis [e.g. Plateaus [START_REF] Hammond | The nature of the crust beneath the Afar triple junction: evidence from receiver functions[END_REF]Ahmed et al., 2013, Fig.1].

Data

Our dataset is based on continuous recordings from 89 seismic stations. Only a limited number of high-quality permanent seismic stations span the Afar-southern Red Sea mar- The ambient-noise cross-correlation technique relies on having simultaneous recordings of the noise field at two seismic stations so that the Green's function between them can be estimated [START_REF] Shapiro | Emergence of broadband Rayleigh waves from correlations of the ambient seismic noise[END_REF][START_REF] Wapenaar | Green's function representations for seismic interferometry[END_REF][START_REF] Halliday | Seismic interferometry, surface waves and source distribution[END_REF]. Because the different deployments of portable instruments occurred at different times, we are not able to estimate Green's functions for all receiver pairs. We partly compensate for this, however, by utilizing permanent stations from the IRIS and GEOSCOPE networks, providing data over a period during which several of the mentioned portable arrays were active.

Method

The ambient noise technique to study Earth structure is free of limitations imposed by the distribution of natural earthquakes. Extracting travel times from a multitude of station-station correlations therefore allows for relatively high-resolution tomographic inversions [e.g. [START_REF] Shapiro | High-resolution surface-wave tomography from ambient seismic noise[END_REF]. We follow the approach of [START_REF] Ekström | Determination of surface-wave phase velocities across USArray from noise and Aki's spectral formulation[END_REF],

discussed in further detail in section 3.2 of [START_REF] Boschi | On measuring surface wave phase velocity from station-station cross-correlation of ambient signal[END_REF], to estimate phase velocity from the ambient signal recorded at two stations.

The background seismic noise is to a large extent generated by the coupling of oceans with the solid Earth [e.g. [START_REF] Longuet-Higgins | A theory of the origin of microseisms[END_REF][START_REF] Hillers | Global oceanic microseism sources as seen by seismic arrays and predicted by wave action models[END_REF]. Because this area is almost surrounded by seas or oceans (Red Sea, Gulf of Aden, Indian Ocean), it is particularly suitable for ambient noise surface wave retrieval.

France.

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To maximize data quality, we (i) only used the pairs of stations that recorded simultaneously for at least 6 months, and (ii) compared measured and predicted Green's function for all station pairs, and discarded pairs that clearly showed a bad fit (see figure in supplementary material). The duration of cross-correlated signal varies by 6-36 months depending on the station pair. These long durations guarantee that all seasons and hence all possible azimuths of noise propagation are sampled [e.g. [START_REF] Stehly | A study of the seismic noise from its longrange correlation properties[END_REF]. Data processing was limited to whitening, as reasonable dispersion curves could be obtained without any filtering and/or "one-bit" amplitude compression.

Resolution

Station-to-station paths

To assess the resolving power of our inversion, we first show in Figure 2 the station-tostation paths corresponding to ambient Rayleigh-wave observations at each period. The solid gray line delimitates the area with good coverage, and therefore the zone of best resolution.

Random tests

We perform two random resolution tests [e.g. [START_REF] Verbeke | High-resolution Rayleigh-wave velocity maps of central Europe from a dense ambient-noise data set[END_REF] to assess the reliability of the tomographic inversion: one with structures smaller than 100 km (Fig. The tomography algorithm is that utilized e.g. by [START_REF] Verbeke | High-resolution Rayleigh-wave velocity maps of central Europe from a dense ambient-noise data set[END_REF]. Pixel-size is

0.1 • x0.1 • .
Our synthetic tests (Fig. 3) serve to validate both pixel size and select the values of regularization parameters that allow us to represent heterogeneities of scale-length such as in figure 3.a (left). The same parameterization and regularization is applied to real data in the following. Notice that resolution changes across the region of study, so that a unique resolution limit cannot be specified.

Results

We compute Rayleigh-wave phase-velocity maps for periods between 9 s and 25.5 s,

and present examples at 9, 15.5 and 20.5 s (Fig. 4; see supplementary material for other periods). According to e.g. [START_REF] Lebedev | Global upper-mantle tomography with the automated multimode inversion of surface and S-wave forms[END_REF] velocity anomalies is fairly constant from 9 s to 15.5 s (Fig. 4). We image positive velocity perturbations beneath the border faults of the eastern flank of the Red Sea in Yemen, beneath the Danakil Horst, and in central western Afar in the region between the rift margin and the axial volcanic segments (Sullu Adu area, Fig. 1). We also see positive velocity perturbations beneath the western Afar margin north of 12 • N. The main slow anomalies are located beneath Dabbahu Manda-Hararo axial volcanic system, beneath Durrie off-axis volcano and the southern axis extension to Kurub volcano (Fig. 1). We also find slow anomalies associated with the volcanic systems 150 to 200 km east of the rift margin in Yemen, and beneath the western Afar margin south of 12 • N (Fig. 4).

The magnitude of several of the distinct velocity perturbations varies subtly with period.

For example, beneath the eastern rift margin (Tihama Plain, Yemen, Fig. 1), the positive anomaly increases in magnitude from 5% at 9 s to 7% at 20.5 s (Fig. 4). The slow anomaly beneath the western Afar margin flank south of 12N is mostly more than -3% at 9 s, whereas at 15.5 and 20.5 s a larger proportion of the anomaly is -4 to -6% (Fig. 4).

The slow anomalies beneath Yemen and beneath the axial volcanic segment of Dabbahu-Manda-Hararo in Afar correlate well with the locus of surface volcanism (Fig. 4). In addition, figure 5 shows the surface distribution of known thermal springs in the region [START_REF] Keir | Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes[END_REF]. The slow anomaly beneath the western Afar margin is beneath the locus of thermal springs on the western Afar margin, whereas north of 12 using ambient noise also correlates well with the spatial extent of high Vp/Vs ratios in the crust constrained using P-S receiver functions [START_REF] Hammond | The nature of the crust beneath the Afar triple junction: evidence from receiver functions[END_REF].

Discussion

Seismic wave velocity is known to be affected by the temperature and chemical composition of the medium of propagation (crustal rocks), as well as by the concentration of fluids, such as partial melt that might be present within crustal rocks [e.g. [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF][START_REF] Karato | The dynamic structure of the deep earth: an interdisciplinary approach[END_REF]. We image slow velocities beneath axial regions of localized magma intrusion, consistent with the hypothesis that major surface-wave slow anomalies are associated with magmatism (Fig. 4). In addition, the lowest velocities in our images are beneath zones of active volcanism and geothermal activity near the flanks of the southern Red Sea conjugate margins (Fig. 4 and5). The magnitude of the anomalies and spatial association with regions where either partial melt or fluids released from cooling magmatic systems are present suggests the crust beneath the flanks of the rift is currently being modified by magmatic processes. Ongoing magmatism occurs at the rift flanks in spite of the majority of strain having shifted to the rift axis since the onset of rifting at 11 Ma [START_REF] Wolfenden | Evolution of the northern Main Ethiopian rift: birth of a triple junction[END_REF]. Beneath the western Afar margin, where slow anomalies are associated with geothermal systems rather than known volcanoes, geological studies suggest early border faulting at 30 Ma was associated with spatially localized volcanism in the marginal graben systems [START_REF] Ayalew | Temporal compositional variation of syn-rift rhyolites along the western margin of the southern Red Sea and northern Main Ethiopian Rift[END_REF]. Our velocity maps suggest that the magmatic systems beneath the rift flanks that were active during the onset of rifting, remain magmatically active throughout the breakup process either through con- tinued minor accumulation of partial melt in reservoirs, dike intrusion, and / or ongoing conductive cooling leading to release of fluids such as water [START_REF] Keir | Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes[END_REF][START_REF] Holtzman | Organized melt, seismic anisotropy, and plate boundary lubrication[END_REF].

The low-velocity anomalies in our phase-velocity maps under the rift axis are observed with higher amplitude in the upper crust (Fig. 4, period = 9 seconds), whereas the lowvelocity anomalies located beneath the rift flanks are observed both in the upper and lower crust, but with higher amplitude in the lower crust (Fig. 4, period = 15.5 seconds).

This is consistent with the proposed plumbing systems of axial and flank volcanic systems of the nearby Main Ethiopian Rift, where petrological constraints on flank volcanism are good. The volcanic products observed on the flanks and at the axis of the MER are not identical: they consist mainly of trachytes for the flanks, and mainly of rhyolites and basalts for the axis [START_REF] Peccerillo | Petrogenesis of silicic peralkaline rocks in the Ethiopian rift: geochemical evidence and volcanological implications[END_REF]. Petrological models indicate that the origin of the off-axis trachytes is probably high-pressure fractional crystallization of asthenospherederived basalts, with this fractionation occurring at the base of the crust [START_REF] Peccerillo | Petrogenesis of silicic peralkaline rocks in the Ethiopian rift: geochemical evidence and volcanological implications[END_REF]. [START_REF] Rooney | Structure of the Ethiopian lithosphere: Xenolith evidence in the Main Ethiopian Rift[END_REF][START_REF] Rooney | Lithospheric modification during crustal extension in the Main Ethiopian Rift[END_REF] suggest that these off-axis volcanic products result of moderate-degree partial melting at 50-90 km depth and undergo fractional crystallization in complex plumbing systems spanning depths throughout the crust [START_REF] Rooney | Insights into extensional processes during magma assisted rifting: Evidence from aligned scoria cones[END_REF]. The volcanic rocks at the axis are asthenospheric basalts produced by rift-related decompressional melting, rather than other potential sources such as melting in the crust.

The axial basalts undergo fractional crystallization mostly in the upper crust [Peccerillo et al., 2007]. Thus, the axial magmatic chamber is shallow (in the upper crust), and the melt ascension from the asthenosphere is probably rapid. At the flanks, however, there is a complex plumbing system with stacked-reservoirs both in the upper and lower crust [START_REF] Rooney | Insights into extensional processes during magma assisted rifting: Evidence from aligned scoria cones[END_REF]. The geothermal systems of the flanks are probably fed or heated by such a complex plumbing system (Fig. 5). Our surface-wave velocity maps are consistent with this model and therefore suggest similar magmatic plumbing systems for the southernmost Red Sea.

According to [START_REF] Medynski | MAGMATISM ON RIFT FLANKS X -21 magma reservoirs: Spatial and temporal evolution of magma supply in the dabbahu rift segment (afar, ethiopia) over the past 30 kyr[END_REF], the magma supply has decreased in the Dabbahu-Manda-Hararo axis reservoir since 15 kyr. An off-axis reservoir, located 15 km to the west of the Dabbahu-Manda-Hararo rift beneath Durrie volcano has been actively fed since 15 kyr, and is currently imaged using magneto-telluric techniques (Fig. 1) [START_REF] Desissa | A mantle magma reservoir beneath an incipient mid-ocean ridge in Afar, Ethiopia[END_REF]. It is consistent with our phase-velocity maps, where the maximum amplitude for the northern Dabbahu-Manda-Hararo seems to be slightly to the west of the rift, beneath Durrie volcano (Fig. 4).

In the past, geodynamic models of breakup ignored the presence and impact of maintained magmatism at rift flanks on the thermal and subsidence history of the rift during late stage breakup and early seafloor spreading. At the southern Red Sea, where seafloor spreading is young, our new crustal-velocity maps coupled with surface expression of volcanism (Fig. 4) show clear evidence for ongoing magmatism beneath the rift flanks in Afar and Yemen (Sana'a, Dhamar and Marib volcanic fields, Fig. 1) [START_REF] Korostelev | Crustal and upper mantle structure beneath south-western margin of the arabian peninsula from teleseismic tomography[END_REF][START_REF] Corbeau | Uppermost mantle velocity from Pn tomography in the Gulf of Aden[END_REF]. Similarly, there is evidence for ongoing dike intrusion further north

Conclusions

Our study provides new high-resolution phase-velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal evolution during breakup. Low-velocity anomalies are imaged in the crust beneath the axial volcanic systems, but also in the upper and lower crust beneath rift flanks where hydrothermal activity and ongoing volcanism are observed at the surface. Our results show that the crust beneath the southern Red sea rift flanks is currently being modified by magmatic processes, and that this activity is continuous from the onset of rifting. We therefore demonstrate that rift flank magmatism after breakup may be more common than it was previously thought in context of margins with excess magmatism.

  gins and so temporary experiments using portable broad-band equipment are our major source of information on the structure of the area. A seismic deployment was conducted between March 2009 and March 2010 as part of the YOCMAL (Young Conjugate Margins Laboratory) project, with 23 stations covering western Yemen during one year[START_REF] Korostelev | Crustal and upper mantle structure beneath south-western margin of the arabian peninsula from teleseismic tomography[END_REF] Corbeau et al., 2014, Fig.2]. We also use data from 41 stations in the Afar Consortium network (UK and US, from March 2007 to November 2009) [e.g. Keir et al., 2011], five stations of the Horn of Africa network in Yemen and Ethiopia [from June 1999 to December 2002, e.g. Sicilia et al., 2008] and six temporary stations recording from May 2011 to September 2012 in Eritrea [e.g. Hammond et al., 2013, Fig.2]. One station de la Terre Paris (iSTeP), F-75005 Paris, D R A F T February 27, 2015, 9:20am D R A F T X -6 KOROSTELEV ET AL.: MAGMATISM ON RIFT FLANKS from the Djibouti temporary network was added to our dataset, together with permanent seismic stations in Djibouti, Yemen and Ethiopia.

  3.a) and a second one with structures larger than 100 km (Fig.3.b). The input synthetic random 3 Department of Geoscience and D R A F T February 27, 2015, 9:20am D R A F T X -8 KOROSTELEV ET AL.: MAGMATISM ON RIFT FLANKS velocity model consists of alternating random structures of opposite sign with a maximum velocity variation of 1.5% relative to the reference velocity. Synthetic phase velocities were computed between the same station pairs as in the observed database. Figures 3.a and b show the input velocity models and the retrieved velocity models from these tests for periods of 9, 15.5 and 20.5 s. These synthetic tests indicate that our inversion can resolve most of the Afar-southern Red Sea margins region, with some degradation of the recovered solutions near the edges of the illuminated area.

  and Fry et al. [2010], 9-s Rayleigh waves are most sensitive to depths < 20 km (upper and mid crust), while 15.5s are most sensitive to 10 -40 km depth (primarily the lower crust). 20.5-s Rayleigh waves can sample down to 70 km, at the top of the upper mantle. The locus of major Engineering, Delft University of Technology, D R A F T February 27, 2015, 9:20am D R A F T X -9

Southampton

  Red Sea flank from InSAR (interferometric synthetic aperture radar) and seismicity studies at Harrat Lunayyir volcanic system in Saudi Arabia [e.g.[START_REF] Pallister | Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia[END_REF][START_REF] Ebinger | Length and timescales of rift faulting and magma intrusion: the Afar rifting cycle from 2005 to present[END_REF]. There, localized subsidence, horizontal opening and earthquakes in April to May 2009 are best modeled by intrusion of a dike and induced normal faulting. These studies, combined with the evidence presented by our new surfacewave velocity maps, demonstrate that rift flank magmatism during late stage breakup may be more common than previously assumed.

Acknowledgments.Figure 1 .

 1 Figure 1. Structure of the Afar and southern Red Sea region. The crustal thicknesses are

  [START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF] Rooney et al., North of ∼16 • N, all the extension is accommodated in the main Red Sea rift, spreading at ∼15 mm/yr. Moving south of 16 • N, the extension is progressively accommodated in the Afar Depression reaching ∼20 mm/yr at 13• N [McClusky et al., 2010; Vigny et al.,

			X -5
	2006]. The crust beneath Afar varies from 25 km thick beneath most of Afar, to 15 km
	thick beneath the Danakil depression (Afdera-Erta'Ale segment) in the north [Makris and
	Ginzburg, 1987; Bastow and Keir , 2011, Fig.1]. The crustal thickness is ∼25 km thick
	beneath the Danakil block and increases to 40-45 km beneath the Ethiopian and Yemeni
	2011], with the current locus of strain being ∼70-km-long, ∼20-km-wide axial volcanic
	segments such as the Dabbahu-Manda-Hararo segment in central-west Afar [e.g. Hayward
	and Ebinger , 1996]. Here, episodic intrusion of dikes fed from crustal magma chambers
	at both the segments centers and tips accommodates the majority of extension [e.g. Keir
	et al., 2009; Grandin et al., 2010, 2011].
	Current opening across the kinematically complex southern Red Sea rift is constrained
	with relatively high-density GPS [e.g. ArRajehi et al., 2010; McClusky et al., 2010] and
	InSAR measurements[e.g. Pagli et al., 2014]. These data show that south of ∼16 • N, the
	rift bifurcates into two branches: the main Red Sea and the subaerial Red Sea rift in Afar
	(Danakil Depression). Partitioning of extension between rift branches varies along-strike.
	2 CNRS, UMR 7193, Institut des Sciences
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Ferguson, D. J., J. Maclennan, I. Bastow, D. Pyle, S. Jones, D. Keir 12 Yemen Geological Survey and mineral Resources Board, Sana'a, Yemen.

13 School of Earth Sciences, University of Bristol, Bristol, UK.