Ph.D. IPGS Abdulhakim Cecile Ahmed

Cecile Doubre

Sylvie Leroy

Derek Keir

Carolina Pagli

James O S Hammond

Atalay Ayele

Maxime Be De Berc

Marc Grunberg

Jerome Vergne

Dr Sylvie Doubre

Dr Derek Leroy

Dr Carolina Keir

Dr James O S Pagli

Dr Atalay Hammond

Dr Maxime Ayele

Mark Be De Berc

Dr Jerome Grunberg

Dr Romain Vergne

Daniel Pestourie

Birhanu Mamo

Nadaya Kibret

Dr Aude Cubas

Marianne Lavayssière

Olivier Janowski

Alessandro La Lengliné

Dr Emma L Rosa

Dr Finnigan Chambers

Dr Illsley-Kemp

Dr Khaled Aldamegh
email: kdamegh@kacst.edu.sa

Dr Alain-Pierre Tokam

Dr Patricia Persaud
email: ppersaud@lsu.edu

Maxime Be De Berc

Romain Pestourie

Daniel Mamo

Birhanu Kibret

Nadaya Cubas

Aude

Lavayssie Re 10

Marianne Janowski

Olivier Lengline

Alessandro La Rosa

Chambers

Finnigan Illsley-Kemp

Across and along-strike crustal structure variations of the western Afar margin and adjacent plateau: Insights from receiver functions analysis

Keywords: Africa, Continental margin, Afar margin, Southern Red Sea Rift, Receiver Functions, Crustal Africa, Continental margin, Afar margin, Southern Red Sea Rift, Receiver 37 Functions, Crustal Structure 38 39

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Introduction 40

The Afar Depression lies in northeast Africa where the Red Sea and Gulf of Aden 41 oceanic spreading ridges and the intra-continental East-African Rift meet, forming 42 a rift-rift-rift triple junction between the Nubian, Somalian, and Arabian plates 43 [START_REF] Mckenzie | Plate tectonics of the Red Sea 1353 and East Africa[END_REF][START_REF] Courtillot | Propagating rifts and continental breakup[END_REF][START_REF] Tesfaye | Early continental breakup 1461 boundary and migration of the Afar triple junction, Ethiopia[END_REF][START_REF] Doubre | Current deformation in 1127 Central Afar and triple junction kinematics deduced from GPS and InSAR 1128 measurements[END_REF]. 44

The Afar Depression is bound by the continental Danakil block to the northeast 45 (Fig. 1), and by the western and southern plateaus, to the west and south 46 respectively. The Afar Depression is thought to have formed shortly after the 47 impact of the Afar mantle plume and associated eruption of the bimodal Ethiopia-48

Yemen Trap Series of basalts and rhyolitic lavas [START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for 1219 plume birth and global change[END_REF]Pik et al., 49 1998Pik et al., 49 , 1999;;[START_REF] Pik | East-Africa on the rise[END_REF]. The extension of the proto-African lithosphere started in 50 the Oligo-Miocene (~29-26 Ma) shortly after the peak in Trap Series volcanism at 51 ~29-31 Ma [START_REF] Wolfenden | Evolution 1486 of the northern Main Ethiopian rift: Birth of a triple junction[END_REF][START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF][START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]. In the present-day, 52 several magmatic segments are the locus of extensional deformation with 53 networks of normal faults formed above narrow zones of episodic dike intrusion 54 (Fig. 1; [START_REF] Wright | Magma-maintained rift segmentation at continental rupture in the 1493 2005 Afar dyking episode[END_REF][START_REF] Grandin | September 2005 Manda Hararo-Dabbahu rifting event, 1186 Afar (Ethiopia): constraints provided by geodetic data[END_REF][START_REF] Smittarello | Transient deformation in the Asal-1434 Ghoubbet Rift (Djibouti) since the 1978 diking event: Is deformation 1435 controlled by magma supply rates?[END_REF]Barnie et 55 al., 2016). Therefore, the continental margin of the Afar region is a unique location 56 where the transition from the stable continental plateaus to incipient seafloor 57 spreading within the Afar rift axis is subaerially exposed [START_REF] Barberi | Volcanism in the Afar Depression: small-scale 1027 plate tectonic implications[END_REF]58 Medynski et al., Hayward and Ebinger, 1996;Beyene and Abdulsalam, 2005; 59

2016). 60

Despite reasonable consensus regarding the locus and mode of upper crustal 61 extension both at the stage of rift initiation and at the present day, few data allow 62 an informed interpretation of the structural evolution of the rifting between these 63 two periods. Continental margins underwater have been intensely investigated to 64 constrain the mechanisms leading to the continental break-up (e.g. Leroy et al., 65 2012). Particularly, seismic experiments help to draw the geometry of the margin 66 units and quantify, in terms of finite deformation, amount of stretching and 67 thinning due to tectonic processes and also the role of magmatism (e.g. Leroy et al., 68 2010;[START_REF] Clark | 1084 Tearing the lithosphere: Diachronous slab detachment along the 1085 Venezuelan margin from BOLIVAR seismic data[END_REF]d' Acremont et al., 2005[START_REF] Leroy | The Onshore-Offshore 1304 ENCENS Project: Imaging the Stretching/ Thinning of the Continental 1305 Lithosphere and Inception of Oceanic Spreading in the Eastern Gulf of 1306 Aden[END_REF]. However, due to the 69 subaerial exposure, passive seismic experiments and geological studies can be 70 more easily conducted on the Afar margin. 71

In addition to geologic studies [START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF][START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF], the Afar 72 crust has been imaged along rift with seismic refraction survey (Makris and 73 Ginzburg, 1987). The crustal thickness variations and internal seismic structure of 74 Afar margin have to date been imaged with low resolution using receiver function 75 studies from seismic networks with sparse station spacing (~40-km) (Dugda et al., 76 2005;[START_REF] Dugda | New contraints on crustal structure in 1137 eastern Afar from the analysis of receiver functions and surface wave dispersion 1138 in Djibouti. In: The Afar volcanic province within the East African Rift System[END_REF][START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Kibret | Crustal thickness estimates beneath four 1261 seismic stations in Ethiopia inferred from p-wave receiver function 1262 studies[END_REF]. None 77 of the previous studies offer a sufficiently good image of the crustal structure 78 across the margin to unravel the extensional history and any along-margin 79 variations in the role of tectonics and magmatism during the continental breakup. 80

A detailed image of the finite deformation will help to constrain the structure and 81 evolution of the margin prior and coeval with the continental rifting and answer 82 some of the remaining questions: Does the stretching and the thinning of the 83 continental lithosphere occur over a narrow or broad region? Is the rifting smooth 84 or stepped? What are interactions between mechanical stretching and thinning of 85 the lithosphere, and magma intrusions (e.g. [START_REF] Keir | The development of 1254 extension and magmatism in the Red Sea rift of Afar[END_REF]? 86

In order to resolve the high-resolution crustal structure of the western margin of 87 the Afar Depression for the first time we analyze data from 24 temporary 88 broadband stations distributed along two densely sampled profiles going from the 89 western plateau to central and northern Afar. We use P-to-S receiver functions 90 [START_REF] Burdick | Modeling crustal structure through the 1061 use of converted phases in teleseismic body waveforms[END_REF][START_REF] Langston | Corvallis, Oregon, crustal and upper mantle structure 1281 from teleseismic P and S waves[END_REF][START_REF] Ammon | The isolation of receiver effects from teleseismic P 1008 waveforms[END_REF] to image the crust-91 mantle transition and intracrustal discontinuities as well as to infer crustal 92 properties from Vp/Vs values. This allows us to understand the spatial 93 relationships between plate stretching, thinning and magma intrusion during 94 continental breakup. The comparison of the crustal structure along the two 95 profiles allows us to estimate the longitudinal variations of the rifting along the 96 margin and the respective role of tectonics and magmatism. 97 98 2. Background 99 100

Geological and Tectonic Settings 101

Rifting along the Gulf of Aden initiated ~34 Ma ago [START_REF] Leroy | From rifting to oceanic spreading in the Gulf of 1315 Aden: a synthesis[END_REF] followed by 102 the emplacement of voluminous Ethiopia-Yemen Trap Series on the Ethiopian and 103

Yemeni plateaus ~31-29 Ma ago (Baker et al., 1996a;[START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for 1219 plume birth and global change[END_REF]104 Ukstins et al., 2002). On the Ethiopian plateaus, they erupted and covered a thick 105 series of Mesozoic sedimentary rocks [START_REF] Bosworth | The Red Sea and Gulf of Aden 1053 Basins[END_REF]Beyene and 106 Abdelsalam, 2005) currently outcropping on the western plateau, overlying the 107 Neoproterozoic crystalline basement rocks (e.g. [START_REF] Alemu | 1002 The Paleozoic e Mesozoic Mekele Sedimentary Basin in Ethiopia: An 1003 example of an exhumed IntraCONtinental sag (ICONS) basin[END_REF]Le Gall et al., 108 2018;Fig. 1). They also likely covered parts of the region that would become the 109

Afar Depression. 110

The initiation of rifting within the Afar is thought to have begun at ~29-26 Ma 111 postdating the main flood basalt event. Geological studies from the central and 112 southern parts of the western margin of Afar propose three main stages of strain 113 migration from the border faults to the current magmatic segments (Wolfenden et 114 al., 2005;[START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]. In stage 1, starting just after the flood basalt deposits 115 of 29 to 26 Ma, narrow half-grabens initiate the rift, coeval with the emplacement 116 of rhyolites in these basins. Stage 2 (16-7 Ma) corresponds to the eastward 117 migration of extensional towards the rift axis, and accompanied by deformation 118 fissural basaltic volcanism. Finally, Stage 3 (<7 Ma) includes a further eastward 119 jump in the locus of extension and increase in localized magma intrusion and 120 voluminous basalts. Stage 3 is responsible for eruption of a ~1.5-km-thick Stratoid 121

Series lava pile of mostly basalts emplaced at 4.0-1.1 Ma that now covers 70% of 122

Afar [START_REF] Varet | Carte ge ologique de l' Afar central et me ridional[END_REF][START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]Fig. 1). After 1.1 Ma, the deformation and 123 magmatism became localized within discrete seismically and volcanically active 124 magmatic segments characterized by intense diking and volcanism (e.g. Barberi et 125 al., 1972;[START_REF] Abdallah | Relevance of Afar seismicity and volcanism to the 991 mechanics of accreting plate boundaries[END_REF][START_REF] Hayward | Variations in the along-axis segmentation of 1216 the Afar rift system[END_REF]Manighetti et al., 126 1998;[START_REF] Wright | Magma-maintained rift segmentation at continental rupture in the 1493 2005 Afar dyking episode[END_REF][START_REF] Grandin | September 2005 Manda Hararo-Dabbahu rifting event, 1186 Afar (Ethiopia): constraints provided by geodetic data[END_REF]. 127

Extensional basins in southern and central Afar are filled with Pliocene-Recent 128 lacustrine and fluviatile sedimentary rocks interbedded with basaltic and felsic 129 lava flows [START_REF] Tesfaye | Early continental breakup 1461 boundary and migration of the Afar triple junction, Ethiopia[END_REF]. In northern Afar, the Danakil basin is known to 130 include at least 1 km of Holocene-Recent evaporites (e.g. [START_REF] Bastow | The development of late-stage continental breakup: Seismic 1041 reflection and borehole evidence from the Danakil Depression, Ethiopia[END_REF]Le 131 Gall et al., 2018). However, the sedimentary basin likely extends to 3-5 km depth 132 with Pliocene-Pleistocene-Recent age evaporites and basalt flows (Bastow and 133 Keir, 2011;[START_REF] Keir | The development of 1254 extension and magmatism in the Red Sea rift of Afar[END_REF]. 134

The western Afar margin preserves the rift morphology since the onset of the 135 flood basalt till the initiation of the seafloor spreading [START_REF] Tesfaye | Early continental breakup 1461 boundary and migration of the Afar triple junction, Ethiopia[END_REF]136 Wolfenden, et al., 2005;[START_REF] Keir | The development of 1254 extension and magmatism in the Red Sea rift of Afar[END_REF][START_REF] Ebinger | Crustal structure of active deformation zones in Africa: 1155 Implications for global crustal processes[END_REF][START_REF] Rooney | Constraining the magmatic plumbing system in a 1416 zonedcontinental flood basalt province[END_REF]. 137

This margin is characterized by stratigraphic and structural changes from the 138 south toward the north (e.g. [START_REF] Keir | The development of 1254 extension and magmatism in the Red Sea rift of Afar[END_REF]. For example, south of latitude 13°N, 139 the transition from the western plateau, covered here by a thick sequence of Trap series, 140 to the extended crust in the Afar Depression corresponds to a zone of antithetic faults 141 forming a wide zone of marginal basins [START_REF] Wolfenden | Evolution 1486 of the northern Main Ethiopian rift: Birth of a triple junction[END_REF][START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF]Stab et al., 142 2016) . North of latitude 13°N, where the Trap series does not outcrop on the western 143 plateau, the margin is characterized by normal faults forming a narrow zone of 144 marginal grabens [START_REF] Justin-Visentin | Dike swarms, volcanism and 1243 tectonics of the Western Afar margin along the Kombolcha-Eloa traverse 1244 (Ethiopia)[END_REF][START_REF] Kieffer | Flood and shield basalts from Ethiopia: Magmas from the African 1266 superswell[END_REF]: Wolfenden et 145 al., 2005;[START_REF] Rooney | Constraining the magmatic plumbing system in a 1416 zonedcontinental flood basalt province[END_REF][START_REF] Gall | The Red Beds series in the Erta Ale 1295 segment, North Afar. Evidence for a 6 Ma-old post-rift basin prior to 1296 continental rupturing[END_REF]. The seismic activity is more 146 intense in this area with several events of magnitude greater than 5.0 (Ayele et al., 147 2007;[START_REF] Illsley-Kemp | Seismicity during continental breakup in the Red Sea rift of 1229 Northern Afar[END_REF]. These changes along the Afar margin are spatially 148 associated with a northward thinning of the crust within the rift, and surface area of 149 Holocene volcanism [START_REF] Barberi | Volcanism in the Afar Depression: small-scale 1027 plate tectonic implications[END_REF][START_REF] Berckhemer | Deep seismic soundings of the Afar region and on 1044 the highland of Ethiopia, in Afar Depression of Ethiopia[END_REF]Makris and 150 Ginzburg, 1987;[START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Bastow | The protracted development of the continent 1037 ocean transition in Afar[END_REF]. 151 152

Previous constraints on crustal structure 153

Previous geophysical studies have contributed significantly to our knowledge of 154 the crustal structure of our study area. Estimates of the crustal thickness and 155 crustal composition beneath Afar and surrounding plateaus have been made using 156 controlled source seismic profiling (Makris and Ginzburg, 1987), inversion of 157 gravity data (e.g. [START_REF] Tiberi | In-verse models 1453 of gravity data from the Red Sea-Aden-East African rifts triple junction 1454 zone[END_REF]) and magnetotelluric imaging (e.g. Desissa et 158 al., 2013). Passive seismic techniques such as P-to-S and S-to-P receiver functions 159 (e.g. [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Lavayssiere | Imaging lithospheric discontinuities beneath the 1288 northern East African Rift using S-to-P receiver functions[END_REF], surface waves tomography 160 (e.g. [START_REF] Gallacher | The initiation of 1166 segmented buoyancy-driven melting during continental breakup[END_REF] and ambient noise tomography (e.g. Korostelev et al., 161 2015;[START_REF] Chambers | Using ambient 1070 noise to image the northern East African Rift[END_REF]. 162 Makris and Ginzburg (1987) used the data from an along strike seismic refraction 163 profiles to propose a four-layer crustal structure within the Afar Depression 164 underlain by an anomalous upper mantle (Pn velocity of 7.4-7.5 kms -1 ). This model 165 consists of a 3-5 km sedimentary cover (a P-wave velocity, Vp of 3.35-3.95 kms -1 ), 166 that overlies a basaltic layer (Vp=4.5 kms -1 ) on an abnormally thin upper crust (Vp 167 of 6.1-6.2 kms -1 ) and a lower crust (Vp of 6.7-7.0 kms -1 ). This study shows that the 168 crust thins from 26 km in the south and central Afar to 14 km in the north. Crustal 169 thickness inferred from gravity data [START_REF] Makris | The gravity field of the 1346 Red Sea and East Africa[END_REF][START_REF] Tiberi | In-verse models 1453 of gravity data from the Red Sea-Aden-East African rifts triple junction 1454 zone[END_REF], shows 170 thin crust beneath the Afar rift (14-23 km) and thick (~40 km) continental crust 171 beneath the western plateau that is partly intruded with high-density material. 172 Previous P-to-S receiver function studies [START_REF] Dugda | Crustal structure in Ethiopia and Kenya from receiver function analysis[END_REF][START_REF] Stuart | Crustal structure of the 1448 northern Main Ethiopian Rift from receiver function studies[END_REF]173 Hammond et al., 2011;[START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF][START_REF] Kibret | Crustal thickness estimates beneath four 1261 seismic stations in Ethiopia inferred from p-wave receiver function 1262 studies[END_REF][START_REF] Wang | Crustal structure beneath the 1479 Ethiopian Plateau and adjacent areas from receiver functions: 1480 Implications for partial melting and magmatic underplating[END_REF] and 174 a joint inversion of Rayleigh wave velocities and receiver functions (Dugda et al., 175 2007) confirm these variations, showing ~26-20 km thick crust beneath the Afar 176 Depression and rapidly thinning to ~16 km beneath the Danakil Depression of 177 northern Afar (e.g. [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]. The estimated crustal thickness 178 beneath the western and southern plateaus range from 36 to 45 km (Dugda et al., 179 2005;[START_REF] Stuart | Crustal structure of the 1448 northern Main Ethiopian Rift from receiver function studies[END_REF][START_REF] Cornwell | Imaging detailed crustal structure and magmatic intrusion across the 1089 Ethiopian rift using a dense linear broadband array[END_REF][START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]Kibret et al., 180 2019;[START_REF] Wang | Crustal structure beneath the 1479 Ethiopian Plateau and adjacent areas from receiver functions: 1480 Implications for partial melting and magmatic underplating[END_REF] . These studies also show a normal to elevated Vp/Vs 181 ratio for the plateaus and reaching very high values (Vp/Vs > 2.0) near the 182 magmatic segments where the crustal thickness is less than 26 km (e.g. Hammond 183 et al., 2011;[START_REF] Wang | Crustal structure beneath the 1479 Ethiopian Plateau and adjacent areas from receiver functions: 1480 Implications for partial melting and magmatic underplating[END_REF]. A seismic anisotropy study (Hammond, 2014) 184 shows that the melt beneath the Afar Depression is stored in interconnected 185 stacked sills within the lower crust consistent with high Vp/Vs ratios more than 186 1.95 obtained by receiver functions, an interpretation supported by slow lower crustal 187 S-wave velocities imaged using ambient noise tomography (e.g. [START_REF] Chambers | Using ambient 1070 noise to image the northern East African Rift[END_REF]188 Korostelev et al., 2015). In addition, S-to-P receiver functions identify a velocity 189 decrease with depth at 65-75 km beneath the western plateau interpreted as the 190 lithosphere-asthenosphere boundary (LAB;Rychert et al., 2012;Lavayssiere et al., 191 2018). The LAB is not imaged below Afar and interpreted as evidence for melt 192 percolation into the mantle lithosphere (Rychert et al., 2012;Lavayssiere et al., 2018). 193 However, the seismic stations used in the previous studies [START_REF] Dugda | Crustal structure in Ethiopia and Kenya from receiver function analysis[END_REF]194 Stuart et al., 2006;[START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]Lavayssie re et al., 2018;Chambers et al., 195 2019;[START_REF] Wang | Crustal structure beneath the 1479 Ethiopian Plateau and adjacent areas from receiver functions: 1480 Implications for partial melting and magmatic underplating[END_REF] were deployed at a spacing of several tens of kilometers 196 and those used by [START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF] the western Afar margin and the main axis of the Afar rift (Fig. 1). This network 210 has been configured to image the crustal structure across the transition from the 211 western plateau to the currently active rifts of the northern and central parts of 212 Afar (Fig. 1). For northern Afar we used 8 stations (N001 to N008) deployed along 213 an 86 km-long profile spaced by an average distance of 11 km [START_REF] Keir | Afar Margin Northern Profile[END_REF]214 Fig. 1). For central Afar we used 15 stations (S001, S003 to S009, S011 to S014, 215 S017, S018 and S020) deployed along ~225 km-long profile spaced by either ~20 216 km at the ends or ~10 km in the middle of the profile [START_REF] Doubre | Study of the structure 1130 of the lithosphere from the continental plateau of western Ethiopia to the 1131 active Afar rift (RESIF-SISMOB)[END_REF]Fig. 1). 217

For improved continuity of the southern profile, we added data from FINE station 218 installed from March 2007 to November 2009 [START_REF] Ebinger | AFAR. International Federation of Digital Seismograph 1152 Networks[END_REF]. All stations 219 recorded continuous data with a sampling rate of 100 Hz except FINE, which 220 recorded continuous data at 50 Hz. 221

For the receiver function analysis, the IRIS earthquake catalog was searched for 222 events with magnitude ≥ 5.5 Mb, occurring within an epicentral distance range of 223 30°-95° from the center of the network. To increase the azimuthal coverage, 224 regional events between 20° and 30° were also included (e.g. Park and Lavin, 2001;225 Salmon et al. 2011; Fig. 2d), with a specific treatment detailed in section 3.2.1. In 226 total, waveforms from 186 earthquakes including 11 regional events have been 227

selected. 228 SAC software [START_REF] Goldstein | SAC2000: Signal 1177 processing and analysis tools for seismologists and engineers[END_REF][START_REF] Goldstein | SAC availability for the IRIS community 1182 (online)[END_REF], TauP toolkit 229 [START_REF] Crotwell | The TauP Toolkit: Flexible 1094 seismic travel-time and ray-path utilities[END_REF]) and the 1D IASP91 velocity model (Kennett and Engdahl, 230 1991), were used to pick the P-wave arrival time and to prepare the raw seismic 231 data for receiver function analysis. The data were manually inspected and rated 232 based on the signal-to-noise ratio (S/N; the ratio between the amplitude of the signal 233 at P-arrival time to the maximum amplitude of the 5 s pre-arrival noise window), only 234 low-noise traces (S/N > 4) with clear P wave arrival recorded at each station were 235 kept for further analysis. This process resulted in the selection of ~126 236 earthquakes (including 11 regional events) for receiver function construction and 237 processing (Fig. 2d). After removing the mean and the first-order trends from the 238 selected waveforms, the data were filtered using a zero-phase Butterworth 239 bandpass filter with corner frequencies of 0.02-0.8 Hz and windowed 5 s before 240 and 35 s after the theoretical P arrival time. We then rotated the horizontal 241 seismograms into a great-circle path from the ZNE coordinate system to the ZRT 242 coordinate system. 243 244 3.2 Methods 245 246

P-to-S receiver functions 247

The receiver function technique is widely used to image seismic discontinuities 248 beneath a seismic station by deconvolving the vertical component from the radial 249 and/or transverse components. This aims to remove the instrument response, the 250 propagation path and the source mechanism effects from a broadband teleseismic 251 record [START_REF] Langston | Corvallis, Oregon, crustal and upper mantle structure 1281 from teleseismic P and S waves[END_REF]1979). We used the iterative time domain deconvolution 252 technique of [START_REF] Ligorria | Iterative deconvolution and receiver 1324 functions estimation[END_REF] to compute the receiver functions with 253 200 iterations. We used a Gaussian width factor of 2.5 in the deconvolution of the 254 vertical trace to predict the radial one [START_REF] Ligorria | Iterative deconvolution and receiver 1324 functions estimation[END_REF], except for 255 noisy stations for which we used a Gaussian width factor of 2.0 to excluded high 256 frequency noise above ~1 Hz. The percentage of recovery was evaluated from the 257 rms misfit between the original radial waveform and the radial receiver function 258 convolved with the vertical component. If the final deconvolution reproduces less 259 than 80 % of the signal, then the event is discarded from further analysis. From 260 this together with a visual inspection for coherence and stability, the number of 261 receiver functions included in the final analysis varies between 15 and 72 per 262 station (Table 1), depending on the background noise and the state of health of the 263

station. 264

Extra care is required if using regional events in receiver function analysis. They 265 are susceptible to complex P-wave arrivals due to upper mantle triplications (e.g., 266 [START_REF] Chu | Upper mantle P velocity 1081 structure beneath the Midwestern United States derived from triplicated 1082 waveforms[END_REF]. To check for any artifacts due to triplicated phases, we only kept 267 the events that will not change the H and/or Vp/Vs beyond the error limits 268 determined from the teleseismic events alone. For example, station S005 has the 269 maximum number of 10 regional events, whereas for most of the other stations 270 they are less than six. Figures S1b,c shows the RFs and H-k plots from station S005 271 with and without the regional events included, respectively. The estimated crustal 272 thickness and Vp/Vs are approximately the same. Similarly, figure S1d, shows 273 binned and stacked receiver functions in epicentral distance of 10° for three 274 stations that have six or more regional events. It is clear that the results from 275 regional events (20°-30°) are comparable to the results from teleseismic events 276 (30°-95°) for each station. 277 278

H-κ Stacking 279

To determine the average crustal properties, we analyzed receiver functions for 280 each station using the H-κ domain stacking technique of Zhu and Kanamori 281 (2000). This method enables the determination of Moho depth (H) and the 282 velocity ratio of crustal P and S phases (Vp/Vs, or κ) by considering the crust as a 283 homogeneous, horizontal, isotropic layer over a half-space. The inherited trade-off 284 in the receiver function analysis between the crustal thickness (H) and the average 285 crustal velocity properties can be partially resolved using the H-κ stacking 286 algorithm [START_REF] Ammon | On the nonuniqueness of 1006 receiver function inversions[END_REF], assuming a good assumption can be made about 287 the average P-wave velocity from nearby refraction profiles (e.g. Makris and 288 Ginzburg, 1987). With this information, we sum amplitudes of the receiver 289 functions at predicted arrival times for the Moho P-to-S conversion phase Pms and 290 its multiple converted phases, PpPs and PpSs+PsPs using different weights and a 291 range of H and Vp/Vs values. 292

The stacking amplitude in the H-k domain is given by:

293 294 s (Η, κ) = ∑ 𝑤 1 𝑛 𝑚=1 𝑟 𝑚 (𝑡 𝑝 𝑠 ) + 𝑤 2 𝑟 𝑚 (𝑡 𝑝 𝑝 𝑝 𝑠 ) + 𝑤 3 𝑟 𝑚 (𝑡 𝑝 𝑝 𝑆 𝑠 +𝑝 𝑠 𝑃 𝑠 ) (1) 295
296
where n is the number of the radial receiver functions at each station, Wj is a 297 weighting factor that represents the contribution of the direct conversion from the 298 Moho and its multiples according to signal-to-noise ratio (W1 + W2 +W3 = 1) and 299 rm(t) is the amplitude of the point on the receiver function at predicted arrival 300 time t (after the first P arrival) of the associated seismic phase corresponding to 301 the crustal thickness H and Vp/Vs. The stacking amplitude reaches its maximum as 302 the three phases stack coherently, and represents the best estimate for both H and 303

Vp/Vs (κ) beneath the station. Globally, we follow the same weighting factor 304 scheme W1=0.7, W2=0.2 and W3=0.1 of [START_REF] Zhu | Moho depth variation in Southern California 1501 from teleseimic receiver function[END_REF] that balance the 305 contribution of each phase proportional to low signal-to-noise ratio of multiples. 306

We modified the weighting factors in a few cases, by increasing those for multiples 307 when the Moho conversion phase amplitude is low (W1=0.5, W2=0.3 and W3=0.2: 308 N008; Figs. 2 and3) or when high-amplitude intra-crustal interface conversion 309 phases obscured the Moho Pms conversion (W1=0.4, W2=0.3 and W3=0.3: N002; 310 Fig. 2), or by decreasing them when the multiples are less clearly identifiable 311 (W1=0.8, W2=0.1 and W3=0.1: S018: Fig. 2). 312

The Vp/Vs ratio derived from the H-κ stacking method are related to the elastic 313 parameter Poisson's ratio (σ) through the simple relationship σ = 0.5(1-{1/[(VP 314 /VS) 2 -1]}) [START_REF] Zandt | Continental crust composition constrained 1496 by measurement of crustal Poisson's ratio[END_REF][START_REF] Ligorria | An investigation of the crust-mantle transition beneath 1326 North America and the bulk composition of the North American crust[END_REF], and is typically used to 315 provide important constraints regarding the bulk crustal composition (e.g. 316 [START_REF] Christensen | Poisson's ratio and crustal seismology[END_REF][START_REF] Chevrot | The Poisson ratio of the Australian 1073 crust: Geological and geophysical implications[END_REF]. Poisson's ratio variations 317 mainly depend on mineralogical composition of the crust (felsic, mafic) and/or the 318 presence of fluids rather than on pressure or temperature conditions (Christensen, 319 1996;[START_REF] Watanabe | Effects of water and melt on seismic velocities and their 1483 application to characterization of seismic reflectors[END_REF]. 320 321

Results stability and Uncertainty Estimates 322

Three important factors control the stability of the H-κ inversion results using the 323 receiver function method: 1) the average crustal P-wave velocity used to do the 324 inversion; 2) the back-azimuth coherency of the Moho converted phase Pms and 325 multiples PpPs and PpSs+PsPs and 3) the level of noise in the receiver function 326 waveform data. 327

Average crustal P-wave velocity (Vp) used in previous receiver function studies in 328

Ethiopia shows a large range of values (4.6 kms -1 -6.5 kms -1 ). For example, Dugda 329 et al. (2005) used constant average crustal velocity Vp = 6.5 kms -1 . Stuart et al. 330 (2006) used three different Vp values constrained from the EAGLE controlled-331 source profile of 6.1 kms -1 , 6.15 kms -1 and 6.25 kms -1 for the southern, central 332

Ethiopian rifts, and adjacent plateaus, respectively. [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF] used 333 values of 6.15 kms -1 and 6.25 kms -1 for the stations in Afar and on the Ethiopian 334 plateaus again based on nearby refraction profiles (Makris and Ginzburg, 1987). 335

Finally, the average crustal velocity used by [START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF] ranges from 4.8 336 kms -1 to 5.9 kms -1 based on the comparison of the stacking results using different 337 initial Vp values ranging from 4.5 kms -1 to 7.0 kms -1 and theoretical Vp versus κ 338 curve. In this study, we used Vp = 6.3 kms -1 for the stations located on the western 339 plateau and 6.0 kms -1 for the stations located within the Afar rift based on the 340 controlled-source experiments [START_REF] Berckhemer | Deep seismic soundings of the Afar region and on 1044 the highland of Ethiopia, in Afar Depression of Ethiopia[END_REF][START_REF] Makris | Gravity field and 1339 crustal structure of northern Ethiopia, in Afar Depression of Ethiopia[END_REF]341 Makris and Ginzburg, 1987;[START_REF] Mackenzie | Crustal velocity 1331 structure across the Main Ethiopian Rift: Results from two-dimensional 1332 wide-angle seismic modelling[END_REF][START_REF] Maguire | Crustal structure of the northern Main Ethiopian 1334 Rift from the EAGLE controlled-source survey: A snapshot of incipient 1335 lithospheric break-up, in The Afar Volcanic Province within the East 1336 African Rift System[END_REF], and the 342 average crustal velocity constraints from the previous receiver function studies 343 [START_REF] Dugda | Crustal structure in Ethiopia and Kenya from receiver function analysis[END_REF][START_REF] Stuart | Crustal structure of the 1448 northern Main Ethiopian Rift from receiver function studies[END_REF][START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF]. 344

To estimate the standard deviation for both crustal thickness H and Vp/Vs, we 345 employ the bootstrap resampling technique [START_REF] Efron | The Bootstrap Method for standard errors, 1158 confidence intervals, and other measures of statistical accuracy[END_REF]. The 346 bootstrap analysis was done by repeating the stacking procedure 200 times with 347 random data subsets that are resampled versions of the original data set for each 348 station. The 2 for each station is shown by error bars in the depth sections and 349 supporting information. The same technique was applied to estimate the errors 350 introduced to the results from the assumed average crustal P-wave velocity used 351 to determine the bulk crustal parameters (H and Vp/Vs; [START_REF] Tiberi | Crustal geometry of the northeastern Gulf of Aden 1458 passive margin: localization of the deformation inferred from receiver 1459 function analysis[END_REF]. We 352 recalculate H and Vp/Vs values using different average crustal Vp in the range 5.8-353 6.8 kms -1 to test the errors introduced by assuming average crustal Vp (Fig. S5). 354

As suggested by several authors (e.g. [START_REF] Mohsen | A receiver function study across the 1369 Dead Sea Transform[END_REF][START_REF] Nair | Southern African crustal 1378 evolution and composition: constraints from receiver function studies[END_REF]Ogden et 355 al., 2019), for crustal thickness of less than 50 km, a Vp variation of 0.1 kms -1 will 356 affect the Moho depth by a value less than 1 km. 357 358

Receiver Function Depth Migration 359

The common-conversion point (CCP) technique is a robust method to transfer the 360 receiver functions from the time-domain to the space-domain (e.g. Kind et al., 361 2002) and image lateral variations of the subsurface discontinuities along a 362 profile. In this study, we perform a common conversion point (CCP) stacking using 363 the [START_REF] Zhu | Crustal structure across the San Andreas Fault, Southern 1498 California from teleseismic converted waves[END_REF] technique, based on the back projection of the amplitude vector of 364 the receiver functions along the ray paths using a given 1-D velocity model. The 3-365 D space beneath the profile is binned and the amplitudes of all the radial receiver 366 functions with rays passing through each bin are averaged to get the amplitude for 367 that bin. To create our depth migrated cross-sections along both the northern and 368 southern profiles, we use a 1-D Vp model based on the nearby refraction profiles 369 [START_REF] Berckhemer | Deep seismic soundings of the Afar region and on 1044 the highland of Ethiopia, in Afar Depression of Ethiopia[END_REF]Makris and Ginzburg, 1987;[START_REF] Mackenzie | Crustal velocity 1331 structure across the Main Ethiopian Rift: Results from two-dimensional 1332 wide-angle seismic modelling[END_REF]370 Maguire et al., 2006) and the IASP91 velocity model [START_REF] Kennett | Traveltimes for global earthquake 1259 location and phase identification[END_REF]. 371

The Vs model at each station location is deduced from the Vp/Vs results of the 372 current study, constrained by the previous RF studies [START_REF] Stuart | Crustal structure of the 1448 northern Main Ethiopian Rift from receiver function studies[END_REF]373 Hammond et al., 2011;[START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF] and Vs models of [START_REF] Chambers | Using ambient 1070 noise to image the northern East African Rift[END_REF]. 374

Therefore, the velocity model varies along both profiles and is constrained to 375 maintain the average crustal velocity Vp comparable to that used for H and Vp/Vs 376 calculations. To optimize for the resolution, the CCP depth images are obtained 377 with a bin width of 10 km (equivalent to the estimated width of the first Fresnel 378 zone for an interface at 30 km depth, assuming a signal frequency of 1.2 Hz and S-379 wave velocity of 3.4 kms -1 ; e.g. [START_REF] Salmon | A major step in the continental 1423 Moho and its geodynamic consequences: the Taranaki-Ruapehu line, New 1424 Zealand[END_REF]. The separation between the 380 centers of the bins is set to 2 km to preserve the spatial resolution and the vertical 381 phase and/or its multiples may be disturbed by reverberations from near surface, 391 low velocity sediment layers and intracrustal and/or upper mantle interfaces at 392 some stations (Figs. 3 and S1). In this study, we present the results ordered and 393 grouped depending on them being on the northern or southern profiles (Fig. 1) 394 and in light of different geological and tectonic features (e.g. [START_REF] Keir | The development of 1254 extension and magmatism in the Red Sea rift of Afar[END_REF]. 395 396

Receiver functions observations 397 398

Northern profile 399

For the stations located on the western plateau (N001 to N005), the Pms phase is 400 observed 6 s after the direct P-wave arrival time. Before the Pms phase, i.e. within 401 1.0-4.0 s after the direct P-wave arrival time, we observe a strong and coherent 402 negative intra-crustal phase and two positive intra-crustal phases on the majority 403 of the individual (Figs. 3 andS1) and stacked radial receiver functions (Figs. 2 and 404 S2e). The arrival time and amplitude of the first positive phase depends on the 405 back-azimuth (Figs. 3 andS1), and could be a P-to-S conversion at the bottom of a 406 low velocity shallow layer or could suggests anisotropy or dipping layers. The 407 second strong positive phase is interpreted as a conversion from a mid-crustal 408 interface at ~20 km depth. A clear positive and coherent conversion phase (Phs) 409 arrives behind the Moho P-to-S conversion phase (Pms) at ~9.5 s delay time (Fig. 410 S1e), that [START_REF] Levin | Shear zones in the Proterozoic lithosphere of the 1321 Arabian Shield and the nature of the Hales discontinuity[END_REF] associated with the Hales discontinuity and was 411 reported by previous RFs studies beneath Nubian and Arabian continental shields 412 [START_REF] Sandvol | Grid search 1427 modeling of receiver functions: implications for crustal structure in the 1428 Middle East and North Africa[END_REF][START_REF] Park | Receiver functions from regional P waves[END_REF][START_REF] Ayele | Insights into rifting from shear wave 1014 splitting and receiver functions: An example from Ethiopia[END_REF]. 413

The amplitudes of the transverse components are much smaller than the 414 corresponding radial amplitudes (Fig. 2). The transverse receiver functions show 415 an intracrustal negative and positive energy that is varying over the back-azimuth, 416 which may suggest the presence of anisotropy (Levin and Park, 1997a). 417

As shown in Fig. 3b, at station N006 the results show a variation in the amplitude 418 and arrival time of the Pms-converted phase on the radial RFs as well as the 419 polarity reversal of the direct P and Pms-converted phases on the transverse RF as 420 a function of the back-azimuth. This suggests the presence of a dipping Moho. The 421 azimuthal RF variations of the reverberated latter phases show a complex pattern 422 which may indicate the presence of 3-D features such as dipping interfaces and 423 seismic anisotropy within the crust. Such complex pattern and the lack of a 424 homogeneous earthquake distribution with back-azimuth, means it is not possible 425 to do a detailed crustal anisotropy analysis. 426

The data from the stations located within the marginal graben at the eastern end 427 of the northern profile (N007 and N008) show a positive intra-crustal phase in the 428 radial receiver functions at ~ 1.3-1.5 s after the direct P-wave arrival time (Figs. 2,429 3 and S1). The most prominent positive phase visible in the majority of both 430 individual and stacked radial receiver functions (Figs. 2, 3 and S1) arrives within a 431 delay time of 3.0-5.0 s after the direct P-wave arrival and is interpreted as the P-to-432 S conversion phase (Pms) at the Moho discontinuity. At the easternmost station 433 (N008), we note a clear and strong negative phase arrives following the Pms phase 434 at ~4.0 s delay time after the P-wave arrival (Fig. 2). 435 436

Southern profile 437

The two westernmost stations (S001 and S003) on the western plateau are 438 characterized by a clear Pms Moho conversion phase arriving with a 6.0 s delay 439 time after the first P-wave arrival and multiples within 18-22 s delay time. A clear 440 pre-Moho conversion phase arrives with a 2.9 s delay time after the first P-wave 441 arrival likely originating from an intra-crustal discontinuity between the upper 442 and lower crusts at ~20 km depth. This phase is particularly coherent and 443 stronger in the S003 individual receiver functions and on the stacked receiver 444 functions of the two stations (Figs 2 and3). The transverse RFs of the stations 445 S001 and S003 show a significant energy associated with a group of positive and 446 negative pulses preceding the Pms Moho conversion phase. At the expected arrival 447 time of the Pms conversion phase the transverse RFs display small positive 448 amplitudes for the events approaching the station from easterly back-azimuths, 449 (Fig. S2a). 450

The four stations (S004, S005, S006 and S007) positioned within the basin of the 451 foothill area between the western plateau and the marginal graben are 452 with the radial RFs (Fig. S2c). 469

At the five stations (S014, FINE, S017, S018 and S020), which are located near and 470 within the Afar rift axis, the radial receiver functions have two positive phases 471 within 4 s after the P-wave arrival time (Figs. 2, 3 and S1). The first low amplitude 472 phase arrival has a delay time of 1-1.5 s with respect to the direct P-wave, and is 473 more likely a conversion at the bottom of low velocity shallow layer. We interpret 474 the second strong and coherent phase arrival at a delay time of 3.5-3.8 s after the 475 direct P-wave with corresponding multiples at 10-10.5 s and 13.5-14.5 s, as the 476 conversion from the Moho (e.g. Figs. 2 andS1a andS2d). The transverse RFs 477 characterized by a clear negative and positive energy arrive between the onset of 478 the P-wave and the Pms Moho conversion phase. In addition, we observe a 479 significant positive energy at the expected arrival time of the Pms Moho 480 conversion phase for events approaching from east-southeast directions (Fig. 481 S2d). 482 483

Crustal thickness and Vp/Vs ratio 484

The H-κ stacking results show a well-defined peak on the H-k plots (Figs 4 and485 S2). The crustal thickness H and Vp/Vs estimated from receiver function analysis 486 are shown on the topographic maps of Afar in Fig. 5a and Fig. 5b, respectively. The 487 estimated crustal thickness (H) and Vp/Vs ratio using the H-κ stacking technique 488 and associated errors from bootstrap method are shown in Table 1. 489 490

Northern Profile 491

Our crustal thickness estimates for the western plateau range from 37.6±2.3 to 492 38.8±1.9 km with an average crustal thickness of ~38.0 km. For station N006 493 located at the eastern edge of the western plateau results show two peaks on the 494 H-κ plot that correspond to discontinuities at ~28.0 km and 37.0 km, in addition to 495 another peak corresponding to an intra-crustal interface at ~20.0 km depth (Fig. 496 S3). By comparing the Pms piercing points of N006 (H ~28.0 and 37.0 km) with 497 the piercing points of N005 (H ~38 km) and N007 (H ~24 km) as shown by figure 498 S4, it is clear that a crustal thickness of ~37.0 km could be attributed to the 499 plateau side to the west of this station and ~28.0 km for the foothill area to the 500 east. We obtain 24.2 ± 1.5 km (N007) located at the foothill area and 19.4 ± 1.3 km 501 (N008) at the eastern edge of the marginal graben (Figs. 5a andS5). 502

The Vp/Vs ratio is relatively normal (1.73±0.01-1.79±0.05) for the stations on the 503 western plateau (N001-N005) and for the station (N007) at the western edge of 504 the marginal graben. At the eastern end of the northern profile at the riftward 505 edge of the marginal graben the N008 station shows high Vp/Vs ratio of 1.93 ± 506 0.02. 507

508

Southern Profile 509

The crustal thickness along the southern profile decreases from the W to E in 510 three steps, such that four different zones of uniform crustal thickness and Vp/Vs 511 ratio could be identified along the profile (Figs. 6 andS7). These zones broadly 512 correlate with the pre-rift crust of the western plateau, the foothill area between 513 the western plateau and the marginal graben, the stretched crust of the marginal 514 graben and Awra plain, and finally the thinned crust within and in the vicinity of 515 the Afar Manda-Harraro rift segment (MHRS) (Figs. 1,S7). 516

We estimate an average crustal thickness of 42.0 ± 1.0 km for (S001 and S003; first 517 zone) on the western plateau (Figs. 2, 5a and S7). Except for the S004 station 518 located on/or near the steep border faults with a crustal thickness estimated to 519 37.2 ± 4.0 km, the crustal thickness for (S005-S007; second zone) within the 520 foothill area is 34.0-34.6 km, with an average crustal thickness of 34.2 ± 1.0 km 521 (Figs. 5a, S7 andTable 1). 522

In the third zone of extended crust beneath the marginal graben and the Awra 523 plain, five stations (S008, S009, S011, S012 and S013) with a spacing of 5-12 km 524 were used to constrain the crustal thickness (Figs. S1,S7). The associated crustal 525 thickness ranges from 27.6 to 28.2 km (Table 1), with an average value of 28.0 km. 526

The H-κ estimates for the crustal thickness of (S014,FINE, S017, S018 and S020; 527 fourth zone) within and in the vicinity of the rift axis of central Afar range from 528 23.2 km to 19.6 km (Fig. S7, Table 1), with an average value of 21.0 ± 1.0 km. 529

Our Vp/Vs ratios for the southern profile could be grouped into two main groups 530 of the western marginal area and central rift area. Eleven stations located within 531 the western marginal area, eight of them associated with intermediate crustal 532 composition show elevated Vp/Vs ratios in the range from 1.79 to 1.87 and three 533 stations located within the second zone of the foothill area, show low to normal 534

Vp/Vs ratios in the range 1.72±0.03 to 1.76±0.02. On the eastern end of the 535 southern profile, data from S014, FINE, S017, S018 and S020 stations located to 536 the west and within the MHRS show high Vp/Vs ratio of 1.95±0.05, 1.99±0.02, 537 1.96±0.02, 1.92± 0.01 and 2.02±0.01, respectively (Figs. 5b andS7). 538 539

Depth Migrated Profiles 540

Figure 6 shows the migrated receiver functions along the northern and southern 541 profiles. The Moho depth obtained from the H-k stacking method of Zhu and 542 Kanamori (2000) and corrected for the station elevation is plotted with error bars 543 from the bootstrap technique of [START_REF] Efron | The Bootstrap Method for standard errors, 1158 confidence intervals, and other measures of statistical accuracy[END_REF]. 544 545

Northern profile 546

For the northern profile, the Moho interface appears as a strong positive phase 547 (indicated by the red color, Fig. 6a) and can be easily traced. It is almost horizontal 548 beneath the western plateau (N001-N006) at an average depth of ~36 km, and 549 then rises in two gradual changes to a depth of ~18 km below the eastern end of 550 the profile. The first crustal thinning (∼14 km vertically over ~10 km horizontally) 551 occurs at the edge of the plateau between N006 station located on the western 552 plateau and N007 station located at the foot of the escarpment within the marginal 553 graben zone. The second crustal thinning occurs within the marginal graben zone 554 between the stations N007 and N008, (∼5 km vertically over 15 km horizontally). 555

The crust continues to thin eastward to <15 km towards Dallol within the Danakil 556 Depression (Makris and Ginzburg, 1987;[START_REF] Tiberi | In-verse models 1453 of gravity data from the Red Sea-Aden-East African rifts triple junction 1454 zone[END_REF][START_REF] Bastow | The protracted development of the continent 1037 ocean transition in Afar[END_REF]557 Hammond et al., 2011;[START_REF] Illsley-Kemp | Extension and stress during continental breakup: Seismic 1225 anisotropy of the crust in northern afar[END_REF]. 558 559

Southern profile 560

The SW-NE-migrated image along the southern profile (Fig. 6b) shows a clear 561 conversion from the Moho at a depth range from ~39-40 km at the western end to 562 ~19-20 km beneath the eastern limit of the profile. The first locus of the crustal 563 thinning occurs at the edge of the escarpment from 42 km beneath the western 564 plateau to ~34 km beneath the foothills zone (8 km vertically over 20 km 565 laterally). The second locus occurs between S007 and S008 stations located at the 566 foothills zone and the marginal graben zone, respectively, reaching an average 567 crustal thickness of 28 km (6 km vertically over 10 km horizontal distance). The 568 last locus of the crustal thinning is at the western edge of the Sullu-Adu range 569 located at the eastern half of Awra basin reaching an average crustal thickness of 570 ~21 km beneath the MHRS at the latitude ~12°N. 571 572 5 Discussion 573 574

Comparison with previous estimates of the crustal structure 575

Northern Profile: 576

Our crustal thickness estimate of ~38 km for the western plateau of the northern 577 profile is in a good agreement with the previous receiver functions (~39.0 km; 578 [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Ogden | A reappraisal of 1390 the H-κ stacking technique: implications for global crustal structure[END_REF]; Fig. 7), and from gravity inversion 579 (~35.0-38.0 km; [START_REF] Makris | The gravity field of the 1346 Red Sea and East Africa[END_REF]Tiberi et al., 2001). Similarly, our Vp/Vs 580 values ~1.70-1.79 are consistent with the results of ~1.71-1.77 for the stations 581 ADYE and SMRE [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Ogden | A reappraisal of 1390 the H-κ stacking technique: implications for global crustal structure[END_REF]Fig. 8). The crustal 582 thickness obtained from S-to-P receiver functions [START_REF] Lavayssiere | Imaging lithospheric discontinuities beneath the 1288 northern East African Rift using S-to-P receiver functions[END_REF], and 583 ambient noise tomography [START_REF] Chambers | Using ambient 1070 noise to image the northern East African Rift[END_REF] is thinner than the crustal 584 thickness estimated by the current study, and likely the crustal thickness is 585 underestimated in their studies (e.g. see [START_REF] Lavayssiere | Imaging lithospheric discontinuities beneath the 1288 northern East African Rift using S-to-P receiver functions[END_REF] for a discussion 586 on this). At the eastern end of the northern profile, the crustal thickness and Vp/Vs 587 ratio of 19.4±1.3 km and 1.93±0.02 for N008 compare favorably with the values of 588 20.0±2.0 km and 1.98±0.1 for the nearby station HALE [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]589 Figs. 7, 8). 590

Southern Profile: 591

Our estimated average crustal thickness of 42.0 km for the elevated western 592 plateau is in good agreement with the previous P-to-S receiver functions results 593 (38-44 km; [START_REF] Dugda | Crustal structure in Ethiopia and Kenya from receiver function analysis[END_REF][START_REF] Stuart | Crustal structure of the 1448 northern Main Ethiopian Rift from receiver function studies[END_REF][START_REF] Cornwell | Imaging detailed crustal structure and magmatic intrusion across the 1089 Ethiopian rift using a dense linear broadband array[END_REF]Hammond 594 et al., 2011;[START_REF] Kibret | Crustal thickness estimates beneath four 1261 seismic stations in Ethiopia inferred from p-wave receiver function 1262 studies[END_REF][START_REF] Wang | Crustal structure beneath the 1479 Ethiopian Plateau and adjacent areas from receiver functions: 1480 Implications for partial melting and magmatic underplating[END_REF], with the crustal thickness 595 results from joint inversion of surface waves and Pms receiver functions (~40 km; 596 [START_REF] Dugda | Thin lithosphere beneath the 1141 Ethiopian plateau revealed by a joint inversion of Rayleigh wave group 1142 velocities and receiver functions[END_REF], with the estimates from seismic refraction profiles (40-45 km; 597 Makris and Ginzburg, 1987;[START_REF] Mackenzie | Crustal velocity 1331 structure across the Main Ethiopian Rift: Results from two-dimensional 1332 wide-angle seismic modelling[END_REF][START_REF] Maguire | Crustal structure of the northern Main Ethiopian 1334 Rift from the EAGLE controlled-source survey: A snapshot of incipient 1335 lithospheric break-up, in The Afar Volcanic Province within the East 1336 African Rift System[END_REF] and 598 crustal thickness from gravity inversion (~40 km; [START_REF] Tiberi | In-verse models 1453 of gravity data from the Red Sea-Aden-East African rifts triple junction 1454 zone[END_REF]. For the 599 marginal area (marginal graben and Awra plain), our estimates of the crustal 600 thickness of ~28.0±1.4 km are consistent with the previous P-to-S receiver 601 function results (~25.0-29.0±1.3 km; [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]. 602

Our Vp/Vs results of ~1.79-1.87±0.03 for most of the stations on the western 603 2014) (Fig. 7). Similarly, our high Vp/Vs values above 1.9 are comparable to the 615 results for the nearby stations obtained by [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF] and less than 616 the values estimated by [START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF] for the region to southeast of our 617 stations (Fig. 8). The higher values of [START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF] may be the result of the 618 assumed average crustal velocity used in the receiver function analysis of ~5.1 619 kms -1 . 620 621

Crustal thicknesses and architecture 622

Western Plateau 623

For the stations located on the western plateau, observations of the high 624 amplitude coherent Moho converted arrivals and consistent estimated crustal 625 thickness beneath the western plateau stations of each profile indicate that the 626 Moho interface is mostly flat beneath the western plateau, while the intra-crustal 627 conversions are laterally variable in amplitude and depth at some stations. A 628 recent study of [START_REF] Ogden | A reappraisal of 1390 the H-κ stacking technique: implications for global crustal structure[END_REF] show that the reliability of the high frequencies 629

RFs results are very low at the center of the flood-basalt province where the 630 gradational Moho thickness is >13.0 km and such reliability increases toward the 631 outside of the flood-basalt. However, they concluded that H-κ stacking maintains 632 reliability at low frequencies when the gradational Moho thickness is less than 633 13.0 km. Their study did not include results from the stations located nearby to 634 our stations. At the southern part of the western plateau, the wide-angle 635 reflection/refraction results at the latitude of the Main Ethiopian Rift (MER) show 636 a ~47.0 km thick crust. This study constrained the thickness of the high-velocity 637 layer at the bottom of the lower crust to be ~15.0 km with clear reflections from 638 the top and bottom of this layer and an apparent velocity of (7.4 kms -1 ). Cornwell 639 et al. (2010), show that the high-velocity layer thins northeastward from 18.0 to 640 6.0 km with clear Pms conversions from the top and the bottom of the layer. 641

Our receiver function (calculated with both a maximum frequency of ~0.6 and 642 ~1.2 Hz; e.g. [START_REF] Cassidy | Numerical experiments in broad-band receiver function 1068 analysis[END_REF][START_REF] Julia | Evidence for mafic lower crust 1240 in Tanzania, East Africa, from joint inversion of receiver functions and 1241 Rayleigh wave dispersion velocities[END_REF] from the station located at the 643 eastern edge of the western plateau along the southern profile does not show 644 double peak Pms phase (Figs. 3a, d andS2a). Instead, it shows a large amplitude 645 intracrustal phase and Pms converted phase with a lag time of (6s), the latter 646 consistent with the Pms phase from the bottom of the high-velocity layer of 647 [START_REF] Cornwell | Imaging detailed crustal structure and magmatic intrusion across the 1089 Ethiopian rift using a dense linear broadband array[END_REF], as well as a strong crustal reverberation (Fig. 3d). Our 648 results are consistent with a recent receiver function study, which yielded the 649 highest stacking amplitude values for the western plateau nearby to our stations 650 [START_REF] Wang | Crustal structure beneath the 1479 Ethiopian Plateau and adjacent areas from receiver functions: 1480 Implications for partial melting and magmatic underplating[END_REF]. This indicates that the Moho beneath the stations on the 651 western plateau is flat with a large change in seismic velocity contrast between the 652 crust and the mantle and does not allow us to constrain the thickness of the 653 intruded high-velocity layer. This could be due to a thin intruded layer (< 4.0 km) 654 below the vertical resolution owing to the wavelength of the incoming plane waves 655 and/or to a low velocity contrast between this layer and the overlying part of the 656 lower crust. However, if the crust-mantle transition is not a sharp boundary, our 657 estimated crustal thickness depends on the long wavelength of the teleseismic 658 waves we use, and more likely to select the center of the crust-mantle transition 659 gradient (e.g. [START_REF] Ogden | A reappraisal of 1390 the H-κ stacking technique: implications for global crustal structure[END_REF]. In that case, we estimate the crustal thickness 660 (include the magmatic additions) to be ~45.0-46.0 km in comparison with the 661 average crustal thickness of ~38.0 km for the plateau beneath the northern profile 662 (off-flood-basalt stations). In fact, the Jurassic sediments (pre-flood basalts 663 surface) lies at an average altitude of ~2200 m [START_REF] Gani | Blue Nile incision on the 1169 Ethiopian Plateau: Pulsed plateau growth, Pliocene uplift, and hominin 1170 evolution[END_REF], and the higher 664 topography (~3700 m) of the western plateau at the latitude of the southern 665 profile is due to the emplacement of ~1500-1800 m-thick flood-basalts deposits at 666 the top of the Jurassic sediments. Hence, if the crust-mantle transition is a sharp 667 boundary, then this has the implication that the western plateau still under 668 dynamic support process as proposed by [START_REF] Sembroni | Longterm, deep-mantle support of the Ethiopia-Yemen Plateau[END_REF]. On the other 669 hand, if the crust-mantle is a gradational boundary of ~8.0 km thickness, the 670 implications are that the western plateau is locally compensated with magmatic 671 additions at the base of the lower crust as [START_REF] Tiberi | In-verse models 1453 of gravity data from the Red Sea-Aden-East African rifts triple junction 1454 zone[END_REF] concluded from 672 interpretation of gravity and seismic data. 673

Most of the western plateau stations of the northern and southern profiles show a 674 clear intra-crustal conversion phase with a lag time of ~2.9-3.0 s from a mid-675 crustal discontinuity at ~20.0-21.0 km depth, likely showing the upper crust-676 lower crust boundary beneath the western plateau, similar to the discontinuity 677 mentioned by [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]. Toward the southern part of the western 678 plateau at the latitude of the Main Ethiopian Rift (MER), a clear intracrustal 679 interface is modeled by the active seismic profile. This interface separates the 680 upper and lower crust with Vp of 6.1-6.4 kms -1 and 6.6-7.1 kms -1 , respectively 681 (Makris and Ginzburg, 1987;[START_REF] Mackenzie | Crustal velocity 1331 structure across the Main Ethiopian Rift: Results from two-dimensional 1332 wide-angle seismic modelling[END_REF][START_REF] Maguire | Crustal structure of the northern Main Ethiopian 1334 Rift from the EAGLE controlled-source survey: A snapshot of incipient 1335 lithospheric break-up, in The Afar Volcanic Province within the East 1336 African Rift System[END_REF]. A 682 similar interface has been reported and interpreted as the Conrad discontinuity in 683 previous receiver function studies of the eastern Red Sea conjugate margin of 684

Yemen and southeastern Gulf of Aden margin [START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF][START_REF] Ahmed | Crustal structure of the Gulf of Aden southern 998 margin: Evidence from receiver functions on Socotra Island (Yemen)[END_REF]. It was 685 estimated at the same depth of ~21.0 km beneath the Yemeni high plateau covered 686 by the Cenozoic continental flood basalts [START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF]. Previous seismic 687 refraction profiles across the Arabian shield confirm the existence of this interface 688 between the upper crust with an average Vp of 6.3 kms -1 and the lower crust with 689 an average Vp of 7.0 kms -1 [START_REF] Mooney | Saudi Arabian 1371 seismic refraction profile: a traveltime interpretation of crustal and upper 1372 mantle structure[END_REF][START_REF] Egloff | Contrasting structural styles of the eastern 1162 and western marginsof the southern Red Sea: The 1988 SONNE 1163 experiment[END_REF]. This 690 discontinuity suggests a mafic composition for the lower crust, likely altered by 691 magmatic intrusions or partial melt as a result of Miocene-Oligocene flood basalt 692 and the ongoing magmatic activity [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF]. 693

Interestingly, this interface is almost imaged at the same depth of 20-21 km 694 beneath the Arabian shield, beneath Jordan [START_REF] Mooney | Saudi Arabian 1371 seismic refraction profile: a traveltime interpretation of crustal and upper 1372 mantle structure[END_REF]Stern and 695 Johnson, 2010;and references therein), beneath Yemen plateau (Ahmed et al., 696 2013) and beneath the western plateau in this study. Although the exact physical 697 meaning of this discontinuity is not clear, it is most likely separates the slower 698 brittle upper crust from the faster ductile lower crust [START_REF] Stern | Continental lithosphere of the Arabian Plate: a 1442 geologic, petrologic, and geophysical synthesis[END_REF]. 699

Our Vp/Vs values for the undeformed northern part of the western plateau with 700 1000-2000 m sedimentary cover are below 1.80 with an average value of ~1.74. 701 This is lower than the global average of 1.77 and best explained by Precambrian 702 crust of felsic to intermediate compositions [START_REF] Zandt | Continental crust composition constrained 1496 by measurement of crustal Poisson's ratio[END_REF]703 Christensen, 1996), which is lacking significant magmatic modification before or 704 during the breakup [START_REF] Christensen | Seismic velocity structure and 1078 composition of the continental crust: a global view[END_REF][START_REF] Thompson | Precambrian Crustal Evolution: Seismic 1468 constraints from the Canadian Shield, Earth[END_REF]Ogden 705 et al., 2019). 706

The western plateau at the latitude of the southern profile is characterized by 707 elevated Vp/Vs values which may indicate that the lower crust composition has 708 been modified due to mafic intrusions associated with the Cenozoic flood basalts 709 [START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for 1219 plume birth and global change[END_REF][START_REF] Pik | Isotopic and trace 1405 element signatures of Ethiopian flood basalts; evidence for plume 1406 lithosphere interactions[END_REF][START_REF] Cornwell | Imaging detailed crustal structure and magmatic intrusion across the 1089 Ethiopian rift using a dense linear broadband array[END_REF]Hammond et al., 710 2011), partial melt in the lower crust [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF], 711 or be fluids circulation within the upper crust [START_REF] Keir | Lower-crustal earthquakes near the Ethiopian rift induced by magma 1247 injection[END_REF]Korostelev et al., 712 2015;[START_REF] Chambers | Using ambient 1070 noise to image the northern East African Rift[END_REF]. 713 714

Crustal thinning and composition across western Afar margin 715

Northern Afar 716

For the first time we create a high resolution profile of the crustal structure across 717 the northern Afar margin into the Danakil Depression that offers an opportunity to 718 interpret the rifting evolution leading to the establishment of the currently highly 719 active segment of a divergent plate boundary. Taking into account the previous 720 constraints on the crustal thickness near Erta' Ale rift zone and the Dallol 721 Depression deduced by Makris and Ginzburg (1987) and [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF], 722 the variations of the Moho depth estimated in this study can be extended from the 723 margin to the active rift axis. 724

The onset of the crustal extension occurs at the ~1400 m high escarpment 725 between N006 and N007 stations, (i.e. between 39.8°E-39.9°E; Figs. 1,S6) with 726 ~14.0 km vertical thinning over ~10.0 km horizontal distance. The crustal 727 thinning continues eastward in two more gradient changes before the crust 728 reaches a thickness of ~14.0 km at the rift axis [START_REF] Berckhemer | Deep seismic soundings of the Afar region and on 1044 the highland of Ethiopia, in Afar Depression of Ethiopia[END_REF]Makris 729 and Ginzburg, 1987). We note that, the lateral change of the crustal thickness 730 across the western margin of northern Afar shows comparable crustal thickness 731 variations to that beneath Yemen which is the eastern Red Sea conjugate margin of 732 Ethiopia, with ~35.0 km beneath the Yemen plateau, ~22.0 km for the Yemen 733 coastal plain and a lowest value of ~14.0 km on the Yemen Red Sea coast (Ahmed 734 et al., 2013;Fig. 7). 735

Our close station spacing allows us to constrain the locus of the crustal extension 736 more precisely than previous studies. Assuming a pre-rift crustal thickness of 38.0 737 km beneath the western plateau, a crustal thickness of ~19.0 km beneath the 738 marginal graben and reaching the final thickness of ~14.0 km at the rift axis, we 739 estimate a stretching factor β of ~2.0 for the marginal graben and β of ~2.7 for the 740 northern Afar area between the western plateau and the rift axis. This factor 741 obtained for northern Afar is slightly higher than the maximum value obtained for 742 the Yemen Red Sea conjugate margin (β ~ 2.5, [START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF]. 743

There is a clear eastward change of the Vp/Vs ratio from low-to-intermediate 744 values, below the western plateau and the area at the foot of the escarpment (< 745 1.80) to high values for the area near the eastern border faults of the marginal 746 graben (> 1.90), and reaching very high values (>2.0) at the current rift axis (e.g. 747 [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]. The high Vp/Vs ratio we found for the station N008 748 indicates an important change in the bulk composition of the crust due to mafic 749 intrusions along the eastern boundary antithetic faults of the marginal graben 750 [START_REF] Illsley-Kemp | Seismicity during continental breakup in the Red Sea rift of 1229 Northern Afar[END_REF]Zwaan et al., 2020b) or could be due to the existence of 751 crustal fluids. The reported eastward increase in the Vp/Vs ratios toward the rift 752 axis [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF] suggests more mafic/ultramafic material and/or may 753 indicate a partial melting within the crust increasing with proximity of the rift axis. 754 This is consistent with observations of recent magma intrusion related ground 755 deformation at the axis of the Danakil Depression (e.g. [START_REF] Nobile | Dike-fault interaction during the 2004 Dallol intrusion at the 1383 northern edge of the Erta Ale Ridge (Afar, Ethiopia)[END_REF]Pagli, et 756 al., 2012). 757

The structure of the northern Afar margin has been primarily controlled by a 758 stretching phase in the early stages of continental rifting c.a ~26-20 Ma (Ukstins et 759 al., 2002;[START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF], most likely related to the extension induced by 760 the divergence between Arabian and Nubian plates. It should be noted that during 761 this tectonic event the thickness of both the upper crust and lower crust decreases 762 by a factor of two. This is evident from the depth of the mid-crustal discontinuity 763 dropping from ~20.0 km beneath the western plateau to ~9.0-10.0 km beneath 764 the eastern edge of the marginal graben (station N008; Figs. 6a,S2e). In the late 765 stages of continental rifting after 11 Ma the extension between the marginal 766 grabens and the rift axis is accommodated by thinning of the upper crust by a 767 factor of 2, while the lower crustal thickness remains constant. This is also evident 768 from the mid-crustal discontinuity at ~5.0 km beneath the Danakil Depression 769 (Makris and Ginzburg, 1987). This significant thinning of the upper crust over 770 short length scales is consistent with geological constraints suggesting that rifting 771 along the marginal area of northern Afar associated with upper crustal extension 772 and dominant tectonic deformation [START_REF] Bastow | The protracted development of the continent 1037 ocean transition in Afar[END_REF][START_REF] Keir | The development of 1254 extension and magmatism in the Red Sea rift of Afar[END_REF]Bastow 773 et al., 2018). In contrary, an increase in rift magmatism associated with dike 774 intrusions confined to the rift axis has initiated relatively recently (Quaternary) in 775 the evolution of the rifting of northern Afar (e.g. [START_REF] Bastow | The protracted development of the continent 1037 ocean transition in Afar[END_REF]Nobile et 776 al., 2012;[START_REF] Pagli | Shallow axial magma chamber at the slow-spreading Erta Ale 1394 Ridge[END_REF]. 777 778

Central Afar 779

Unlike the northern profile, in central Afar the number of stations along our 780 southern profile allows us to continuously image the Moho depth and to constrain 781 the crustal composition variations from the western plateau to the rift axis. The 782 crustal thickness decreases eastward by ~8.0 km over ~20.0 km horizontal 783 distance between the western plateau and the second zone located at the foot of 784 the ~1700 m high escarpment. Then it decreases in two more steps by ~6.0 km 785 and ~7.0 km over ~10.0 km horizontal distance from the second to the third and 786 to the fourth zones, respectively (Fig. S7). The estimated β factors between these 787 zones are 1.24, 1.21 and 1.30 respectively. We estimate an overall β of 2.1 for 788 central Afar assuming an initial crustal thickness of ~42.0 km beneath the western 789 plateau and a final crustal thickness of ~20.0 km (S020) at the eastern end of the 790 profile. Our estimated stretching factor β for central Afar is comparable with the 791 estimate from the previous receiver function studies [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF] but 792 remains smaller than the β value of 3.0 and 2.7 predicted from plate 793 reconstructions [START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF] and from cross section balancing (Stab et 794 al., 2016), respectively. The discrepancy between our estimated β and the 795 predicted β is best explained by magmatic addition to the crust during extension 796 (e.g. [START_REF] Thybo | Magma-compensated crustal thinning in 1471 continental rift zones[END_REF][START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]. This implies 4.0 or 5.0 km of 797 magmatic addition depending on the predicted β value used. 798

Most of the stations located within the western marginal area of central Afar 799 Ma; [START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF][START_REF] Ayalew | Differential 1010 fractionation of rhyolites during the course of crustal extension, Western 1011 Afar (Ethiopian rift)[END_REF], and/or that the crust has been 806

intruded by granitic plutons (fast S-wave mantle material) such as along the 807

Yemen conjugate margin [START_REF] Hughes | The Red Sea-Gulf of Aden: 1221 biostratigraphy, lithostratigraphy and palaeoenvironments[END_REF][START_REF] Davison | Geological evolution of the southeastern Red Sea Rift 1117 margin, Republic of Yemen[END_REF]808 Geoffroy et al., 1998;[START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF]. The slightly elevated values for the other 809 stations that are distributed in the marginal graben and the Awra plain may 810 indicate that the lower crust composition has been modified due to mafic 811 intrusions associated with the Cenozoic flood basalts [START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for 1219 plume birth and global change[END_REF]Pik et 812 al., 1999;[START_REF] Cornwell | Imaging detailed crustal structure and magmatic intrusion across the 1089 Ethiopian rift using a dense linear broadband array[END_REF][START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF], partial melt in the lower 813 crust [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF][START_REF] Wang | Crustal structure beneath the 1479 Ethiopian Plateau and adjacent areas from receiver functions: 1480 Implications for partial melting and magmatic underplating[END_REF], fluids 814 circulation within the upper crust [START_REF] Keir | Lower-crustal earthquakes near the Ethiopian rift induced by magma 1247 injection[END_REF][START_REF] Korostelev | Magmatism on rift flanks: Insights from ambient 1272 noise phase velocity in Afar region[END_REF]815 Chambers et al., 2019), or thick sedimentary deposits within the basin (Stab et al., 816 2016). 817

Finally, at the vicinity and within the MHRS, the significant increase in the Vp/Vs 818 ratio is coincident with the thin crust (20.0 km) toward the central axis of the rift. 819 Such high values of Vp/Vs ratio above 1.90 are very rare (except for few minerals 820 like serpentine) and likely indicate a small amount of partial melt and/or fluids in 821 the crust [START_REF] Watanabe | Effects of water and melt on seismic velocities and their 1483 application to characterization of seismic reflectors[END_REF][START_REF] Christensen | Poisson's ratio and crustal seismology[END_REF][START_REF] Zandt | Continental crust composition constrained 1496 by measurement of crustal Poisson's ratio[END_REF]. 822 [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF] used receiver function forward modeling for three stations 823 located west and northwest of MHRS (to the north of our stations) to show that 824 partial melt in the lower crust may exist beneath regions of thinner crust of central 825 Afar. [START_REF] Reed | Receiver function constraints on 1411 crustal seismic velocities and partial melting beneath the Red Sea Rift and 1412 adjacent regions, Afar Depression[END_REF] used the theoretical velocity-melt relationship developed 826 by [START_REF] Watanabe | Effects of water and melt on seismic velocities and their 1483 application to characterization of seismic reflectors[END_REF] for granitic rocks with rhyolitic melt, to estimate 4.2-11.4% 827 melt fraction at the location of their stations located in central Afar to the 828 southeast of our stations. 829

Additional observations supporting the existence of partial melt within the crust 830 at the vicinity and within the MHRS come from imaged slow shear wave velocities 831 within the crust and the uppermost mantle beneath the magmatic segments 832 [START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF][START_REF] Stork | 1445 Uppermost mantle (Pn) velocity model for the Afar region, Ethiopia: An 1446 insight into rifting processes[END_REF][START_REF] Korostelev | Magmatism on rift flanks: Insights from ambient 1272 noise phase velocity in Afar region[END_REF]Chambers et al., 833 2019), magnetotelluric results that also point to a wide conductive zones within 834 the crust beneath MHRS interpreted as a large magma volumes that contains 835 ~13% melt fraction [START_REF] Desissa | A mantle magma reservoir beneath an incipient mid-ocean ridge 1120 in Afar, Ethiopia[END_REF][START_REF] Didana | Magnetotelluric imaging of upper 1122 crustal partial melt at Tendaho graben in Afar, Ethiopia[END_REF]Johnson et al., 2016) 836 and recent series of dike intrusions at MHRS [START_REF] Keir | The development of 1254 extension and magmatism in the Red Sea rift of Afar[END_REF]. Finally, Hammond 837 (2014) invert H-κ data at three stations within and in the vicinity of MHRS for melt 838 fraction and anisotropy and show that the melt is likely stored in the form of 839 stacked sills within the lower crust with a maximum thickness of 2.8 km and 840 connected by some melt inclusions in preferential vertical alignment. Hammond 841 (2014) shows that the anisotropy, and resultant amplitude dominance of the slow 842 shear wave in H-k stacking is a likely cause of the high Vp/Vs ratios for the average 843 crust of the MHRS. 844

The crustal structure variations across the western margin of central Afar are 845 consistent with the distinct geological and structural domains of this region. From 846 the west, the early stretching stage of the continental rifting (c.a ~26-23 Ma; 847 correspond to first step of the Moho interface between S003 and S004; second 848 zone; Fig. S7) occurs below the main escarpment. This stretching phase is 849 characterized by large offset normal fault, distributed over a wide area (Stab et al., 850 2016) and accompanied with the formation of the half graben basin at the foot of 851 the escarpment and the emplacement of felsic magmatism (Wolfenden et al., 852 2005). The following extension event occurs ~30.0 km eastward below the 853 western bounding faults of the marginal graben and corresponding to the second 854 step of the Moho interface from the second to the third zones (between S007 and 855 S008; Fig. S7). This extension event occurs after the emplacement of many 856

Miocene giant dykes within and in the vicinity of the marginal graben (Chorowicz 857 et al., 1999;[START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF][START_REF] Ayalew | Differential 1010 fractionation of rhyolites during the course of crustal extension, Western 1011 Afar (Ethiopian rift)[END_REF]Zwaan et al., 2020b). Zwaan et al. 858 (2020a, b) suggest that this event is related to the rotation of the Danakil block 859 and dated from geological studies as between 16-7 Ma [START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF]860 Zwaan et al., 2020a, b). During this extension event the stretching was distributed 861 over a wide area, most likely accompanied with the formation of antithetic normal 862 faults and NE tilted blocks were created [START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]. A third extension event 863 occurs below the western edge of Sullu-Adu range (between S013 and S014;Figs.,864 1,S7). This event was accompanied with uplift of the Sullu-Adu area, subsidence of 865 the Awra plain and extensive magmatism [START_REF] Bosworth | The Red Sea and Gulf of Aden 1053 Basins[END_REF][START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]. 866

However, the results deduced from our dense network of stations indicate a 867 differential extension between the brittle and ductile layers of the crust, such that 868 the upper crust thins from ~21.0 beneath the western plateau to ~7.0 km beneath 869 the fourth zone, whereas the lower crust thins from ~21.0 km to ~12.0 km. Hence, 870 these observations requires a de collement or ductile fault surfaces to 871 accommodate the upper crust lower crust differential extension and distribute the 872 upper crust extension over a wide area. 873 874

Implications for the evolution of the rifting 875

Continental rifting leading to continental breakup and oceanic spreading 876 encompasses a complex interplay of extensional faulting, thinning of the crust and 877 lithosphere and intrusive-extrusive magmatism [START_REF] Buck | Consequences of asthenospheric variability on continental 1057 rifting[END_REF][START_REF] Geoffroy | Volcanic passive margins[END_REF]878 Lavier and Manatschal, 2006;[START_REF] Cannat | Assessing 1064 the conditions of continental breakup at magma-poor rifted margins: 1065 What can we learn from slow spreading mid-ocean ridges?[END_REF][START_REF] Leroy | Contrasted styles of rifting in the eastern Gulf of Aden: 1308 A combined wide-angle, multichannel seismic, and heat flow survey, 1309[END_REF]Nonn et al., 879 2019). Because of the geological observations that have been preserved in Afar 880 due to the subaerial context of the region and because the rifting is not complete 881 yet, the results on the crustal structure and the Vp/Vs ratios along our two profiles 882 help to discriminate between the possible mechanisms of extension involved 883 during the formation of the margin, over the specific period from the onset of the 884 rifting, i.e. ca. 26 Ma, to the time of the setting up the Stratoï d series, i.e. ca. 4-2 Ma. 885

Separated by only ~150.0 km with a major transfer zone (EATZ; Fig. 1), the two 886 sections along the western Afar margin show strong dissimilarities in their deep 887 structure, particularly regarding the geometry of the Moho depth across the 888 margin. The two sections differ primarily by the amount of the stretching and the 889 width of the extended area from the western plateau to the rift axis. In northern 890

Afar, the stretching factor β of the brittle crust and ductile lower crust is ~4.0 and 891 ~2.0, respectively, whereas in central Afar β is ~3.0 for the brittle crust and ~1.6 892 for the ductile lower crust. The sharp decrease in the crustal thickness between 893 the western plateau and the marginal graben during the early stages of extension 894 (β ~ 2.0 and 1.5 for the northern and central Afar, respectively) suggests different 895 rate of the mechanical extension that has been accommodated by large border 896 faults. During the latter stages of extension between the marginal graben and rift 897 axis, the differential thinning between the brittle and ductile crustal layers 898 suggests that the two layers were decoupled and associated with mechanical 899 extension in the brittle crust and ductile flow in the lower crust (e.g. Jolivet et al., 900 2015). In northern Afar, the extension during these latter stages is accommodated 901 by the thinning of the upper crust only and distributed over a narrow extended 902 area (~40 km). This is consistent with the high Vp/Vs ratio, the thin seismogenic 903 layer (~5 km; [START_REF] Craig | Earthquake 1101 distribution patterns in Africa: Their relationship to variations in 1102 lithospheric and geological structure, and their rheological implications[END_REF][START_REF] Nobile | Dike-fault interaction during the 2004 Dallol intrusion at the 1383 northern edge of the Erta Ale Ridge (Afar, Ethiopia)[END_REF] and the thin effective elastic 904 thickness (Te ~6 km; [START_REF] Ebinger | Soft plates and hot spots: views from Afar[END_REF][START_REF] Pe Rez-Gussinye | Effective elastic thickness of Africa and its relationship to other prozie for 1399 lithospheric structure and surface tectonics[END_REF]905 Ebinger et al., 2017). The stretching and thinning across the western margin of 906 central Afar occurs on a wide distance (>100 km), accommodated by thinning of 907 both the upper and lower crust and associated with decrease in crustal thickness 908 by three successive steps from the stable continental western plateau to the active 909 rift axis. Whereas the Vp/Vs ratios do not show any correlation with the crustal 910 thickness and/or age, but well correlate to the mafic intrusions and increased melt 911 within the crust. These observations are consistent with a rifting history at the 912 latitude of central Afar dominated by a discontinuous riftward migration of the 913 extensional strain over time associated with intense magmatism (Wolfenden et al., 914 2005;[START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]. 915

Geological evidences across the margin of central Afar show that stretching and 916 thinning phases took place between extensive magmatic pulses, during the 917 Oligocene pre-rift period (main continental flood basalt; ~31-29 Ma), late 918 Oligocene-early Miocene (Miocene Basalts and Rhyolites; ~25-22 Ma), late 919 Miocene (Miocene Basalts and Rhyolites; ~9-7 Ma) and the Pliocene (Stratoid 920 series; ~5-2 Ma; [START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF]. Our results provide compositional and spatial 921 constraints with multiple crustal thinning steps consistent with the hypothesis of 922 episodic thinning and related magmatic cycles from the western plateau down to the 923 active central Afar magmatic segment. 924

The along-strike variations of the structure of the margin evidenced at depth by 925 our results but also at surface from several geological studies (Wolfenden et al., 926 2005;[START_REF] Bastow | The protracted development of the continent 1037 ocean transition in Afar[END_REF][START_REF] Stab | Modes of rifting in magma-rich settings: Tectono-magmatic evolution of 1439 Central Afar[END_REF][START_REF] Gall | The Red Beds series in the Erta Ale 1295 segment, North Afar. Evidence for a 6 Ma-old post-rift basin prior to 1296 continental rupturing[END_REF] (1) The crustal thickness beneath the western plateaus is 37-43 km, and decreases 943 eastward to its minimum ranges from 14-23 km beneath the central rift axis. The 944 decrease in the crustal thickness is more pronounced beneath the foothills and 945 marginal grabens, but the crust also thins elsewhere in discrete steps. This is 946 consistent with jumps in strain localization during rifting, rather than progressive 947 strain migration. The crustal thinning of northern Afar points towards a β of 2.7, 948 and in central Afar of 2.2. Both values are lower than a β of 3.0 predicted by plate 949 reconstructions, with the discrepancy best explained by 2-5 km of magmatic 950 addition to the crust. The amount of crustal thinning is higher in northern Afar, 951 pointing towards increased plate stretching in the north. 952

(2) The Vp/Vs ratios suggest that the crustal composition along the western 953 margin of Afar could be classified with unmodified crust with felsic to normal 954 crustal composition, (western plateau next to northern Afar), altered crust due to 955 magmatic intrusion and/or additions (western plateau and marginal area of 956 central Afar) and highly modified crustal composition associated with the 957 presence of partial melt at the central rift axis. 958

(3) There is a significant difference between the western margin characteristics of 959 current study (white circles) and previous RF studies in Ethiopia (red triangles, 1580[START_REF] Hammond | The nature of the crust beneath the Afar triple 1203 junction: Evidence from receiver functions[END_REF]; inverted white triangles, [START_REF] Stuart | Crustal structure of the 1448 northern Main Ethiopian Rift from receiver function studies[END_REF]magenta 1581magenta pentagon, Dugda et al., 2005)), RF in Yemen (white stars, [START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF]. Active 1582 seismic profiles (+ sign), in Afar [START_REF] Berckhemer | Deep seismic soundings of the Afar region and on 1044 the highland of Ethiopia, in Afar Depression of Ethiopia[END_REF]Makris andGinzburg, 1583 1987), in the Red Sea [START_REF] Egloff | Contrasting structural styles of the eastern 1162 and western marginsof the southern Red Sea: The 1988 SONNE 1163 experiment[END_REF], in the Western Gulf of Aden (Laughton 1584 andTramontini, 1969) and in Djibouti [START_REF] Ruegg | Main results about the crustal and upper mantle structure 1419 of the Djibouti region (T.F.A.I)[END_REF]. 1585 1586 Figure 8. Regional map showing Vp/Vs ratio based on the results of the current 1587 study (white circles) and previous RF studies in Ethiopia (red triangles, Hammond 1588et al. 2011;inverted white triangles, Stuart et al., 2006;magenta pentagon, Dugda 1589magenta pentagon, Dugda et al., 2005)), RF in Yemen (white stars, [START_REF] Ahmed | Crustal structure of the rifted volcanic 994 margins and uplifted plateau of Western Yemen from receiver function 995 analysis[END_REF]

From

  were located along a profile across the south-197 eastern part of central Afar. Thus, these studies are limited in providing high-198 resolution images of the crustal structure from the continental stable plateaus to 199 the active rift axes. May 2017 to September 2018, a network including 9 SEIS-UK broadband 205 seismic stations (7 Guralp CMG-3ESPD-60s and 2 CMG-40T-30s with Guralp CMG-206 DCM datalogger) and 20 French Sismob-RESIF broadband seismic stations (Guralp 207 CMG-40T-30s with Nanometrics Taurus digitizer) were deployed along two SW-208 NE oriented profiles, more or less perpendicular to the large fault scarps forming 209

  bin width is set to 0.5 km less than the expected vertical resolution equivalent to 382 the half of the minimum wavelength. 383 384 4. Results 385 Most of the stations show a strong and easily identifiable Pms Moho conversion 386 phase on individual and stacked receiver functions (Figs. 2, 3, S1 and S2). The 387 corresponding multiple PpPs, is also identifiable in most cases, but not as 388 consistently visible as Pms, while the latter arrival multiples PpSs+PsPs are often 389 ambiguous and only clearly observed at a few stations. The Moho conversion 390

  characterized by a clear Moho conversion phase (Figs. 1, 2 and S2b). At station 453 S004, the Pms conversion phase arrives with a delay of 5.0 s after the arrival of the 454 direct P phase with PpPs and PsPs multiples arrives at 16.5 s and 21.0 s, 455 respectively. At stations (S005, S006 and S007) the Moho conversion phase Pms 456 and its multiples (PpPs and PsPs) arrive at ~4.0-4.5 s, ~14.8-15.2 s and ~18.5-457 19.2 s delay time after the first arrival (Figs. 3, S1a and S2b). The transverse RFs 458 show significant energy from intracrustal interfaces and at the expected arrival 459 time of the Pms conversion phase, with the latter characterized by a polarity 460 change for the events approaching the stations from eastern (positive) and 461 western (negative) directions (Fig. S2b). 462 Stations sampling the marginal graben and Awra plain (S008, S009, S011, S012 463 and S013) show, a clear Pms conversion phase arrive within a time window of 3.8 464 to 4.0 s and its multiples PpPs and PsPs arrive at 12.0-12.5 s and 15-16.0 s after 465 the P-wave arrival time, respectively (Figs. 2, S1a and S2c). The transverse RFs 466 show a significant negative and positive energy between the direct P wave arrival 467 and the Moho converted phase characterized by reversed polarity in comparison 468

  plateau and within the marginal area of central Afar (southern profile) are similar 604 to the results from the previous receiver function studies(~1.81-1.85±0.05; 605 Hammond et al., 2011;[START_REF] Dugda | Crustal structure in Ethiopia and Kenya from receiver function analysis[END_REF] Figs. 8, S7). Regarding the stations at the 606 foot of the escarpment (half graben basin; southern profile), our Vp/Vs values of 607 ~1.72-1.76±0.03 are consistent with the results obtained by Dugda et al. (2005) 608 and Hammond et al. (2011) for the nearby station DIYA. 609 In the easternmost zone within and in the vicinity of the rift axis of central Afar, 610 the crustal thickness and Vp/Vs results (~21.4±1.7 and 1.99±0.02) are similar to 611 the results obtained by Hammond et al. (2011) using the same data at the station 612 FINE. For the other stations our average crustal thickness value of 21.0±1.0 km is 613 in a good agreement with result obtained by Hammond et al. (2011) and Reed et 614 al. (

(

  southern profile) show elevated Vp/Vs ratios which suggest intermediate to mafic 800 composition except three stations that show low to normal Vp/Vs ratios. The latter 801 with low to normal values are located within the foothill area and reflect that the 802 Precambrian felsic crust has been poorly contaminated or intruded by mafic rocks. 803 These low Vp/Vs values could also result from the emplacement of felsic volcanic 804 strata coincidence with the onset of faulting along the central Afar margin (26-22 805

  can be 927 interpreted as the result of a difference of stage of the continental breakup 928 evolution associated with a distinct conditions in which the continental extension 929 occurs, such as the local strain rate and/or the depth and the temperature of the 930 upper mantle which change the rheology of the crust and of the lithospheric 931 mantle. This different rheological behavior is marked by the major transfer zone 932 (EATZ) and on either sides of this transfer zone, we note the important variability 933 of the magma supply. receiver function technique and teleseismic data recorded by a 938 temporary broadband seismic array deployed along two profiles crossing the 939 western margin of Afar to constrain the tectonic processes that are involved in the 940 evolution of the rift. Our results provide constraints on the following 941 characteristics of the crustal thickness and composition: 942

Figure 4 .Figure 5 .Figure 6 .Figure 7 .

 4567 Figure 4. Thickness (H) versus Vp/Vs ratio diagrams from the H-k stacking 1561 method for the 6 stations presented in Figure 4. The white point indicates the 1562 maximum stacking corresponds to the value indicated in Table 1. The scale bar is 1563 the amplitude of stacking function. 1564 1565 Figure 5. Maps of crustal thicknesses (H) and Vp/Vs ratios calculated using the H-1566 k method; a) Topographical map with the crustal thickness variations across 1567 western Afar margin and b) Topographical map with the Vp/Vs ratios across the 1568 western Afar margin. MHRS: Manda Harraro Rift Segment. 1569 1570 Figure 6. Topography (vertically exaggerated) and migrated cross-sections along 1571 the two profiles. Red colour indicates velocity increase with depth, and blue colour 1572 velocity decrease with depth. Scale bar shows the amplitude of positive (red) and 1573 negative (blue) polarities of arrivals. The Moho depth estimated from H-k stacking 1574 method and corrected for the station altitude is plotted with the small circles and 1575 the vertical bars represent the errors estimated from the bootstrap method. a) 1576 Northern profile -Northern Afar and b) Southern profile -central Afar. 1577 1578 Figure 7. Regional map showing crustal thickness (H) based on the results of the 1579

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