Frauke Klingelhoefer

Jacques Déverchère

David Graindorge

Chafik Aïdi

Rabie Badji

Boualem Bouyahiaoui

Angélique Leprêtre

Abdelhafid Mihoubi

Marie-Odile Beslier

Philippe Charvis

Aïdi Chafik

Philippe Schnurle

Francoise Sage

Mourad Medaouri

Mohamed Arab

Rabah Bracene

Abdelkarim Yelles-Chaouche

Madjid Badsi

Audrey Galvé

Louis Géli

Formation, segmentation and deep crustal structure variations along the Algerian margin from the SPIRAL seismic experiment

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Introduction

Reactivation of passive margins and initiation of subduction are two of the presently still highly discussed problems in geosciences [START_REF] Lallemand | Subduction initiation from the earliest stages to self-susstained subduction: Insights from the analysis of 70 Cenozoic sites[END_REF][START_REF] Auzemery | Passive margin inversion controlled by stability of the mantle lithosphere[END_REF]. Although the movements of plates can today be measured with high precision using geodetic instruments, the processes of this transformation remain poorly understood, because the traces of subduction margin reactivation and the initiation of a subduction zone will often be obscured by the subduction.

Therefore, it is important to study examples of margin reactivation taking place at present times.

The Algerian margin situated in the western Mediterranean is one of the few examples of a rifted margin being taken into compression and tectonically reactivated (Déverchère et al., 2005;[START_REF] Arab | Tectonostratigraphic evolution of the eastern Algerian margin and basin from seismic data and onshore-offshore correlation[END_REF]. The deformation resulting from the convergence between Africa and Eurasia induces a shortening of 2 to 3 mm/y at the margin toe inducing moderate seismicity, including larger earthquakes, such as the Boumerdes 2003 (Mw = 6.9; [START_REF] Delouis | Slip distribution of the 2003 Boumerdes-Zemmouri earthquake, Algeria, from teleseismic, GPS, and coastal uplift data[END_REF] earthquake. The Algerian margin thus presents unique opportunities to study passive margin inversion and subduction initiation.

The Algerian basin formed as a back-arc basin behind a northward directed subduction during the Late Oligocene to Lower Miocene [START_REF] Réhault | The western Mediterranean basin geological evolution[END_REF] ending after the collision of the Kabylia blocks, which are parts of the « AlKaPeCa » domain (Alboran-Kabylides-Peloritani-Calabria) with the African continent [START_REF] Bouillin | Betic-Rifian and Tyrrhenian arcs: distinctive features, genesis and development stages[END_REF][START_REF] Frizon De Lamotte | The two main steps of the Atlas building and geodynamics of the western Mediterranean[END_REF]Abassene et al., 2016).

In the Tortonian, the marine gateway between the Atlantic ocean and the Mediterranean sea at the location of today's Strait of Gibraltar closed which led to the nearly complete desiccation of the basin and the deposition of thick layers of evaporites during the Messinian salinity crisis [START_REF] Le Pichon | La méditerranée occidentale depuis l'oligocène, schéma d'évolution[END_REF]. The crisis ended when the Strait of Gibraltar opened and the Atlantic water filled the Mediterranean basin during the Zanclean flood. Tethyan slab breakoff [START_REF] Maury | Post-collisional Neogene magmatism of the Mediterranean Maghreb margin: a consequence of slab breakoff[END_REF] and delamination of the edges of the African and Kabylian continental lithospheres (e. g. [START_REF] Roure | Alpine inversion of the North African margin and delamination of its continental lithosphere[END_REF]) generated an asthenospheric upwelling through the slab tear induced the thermal erosion of the Kabylian lithospheric mantle and post-collisional magmatism (Abassene et al., 2016). During the Plio-Quaternary, the ongoing convergence between the African and European plates led to an inversion of the north-Algerian margin (Frizon de [START_REF] Frizon De Lamotte | The two main steps of the Atlas building and geodynamics of the western Mediterranean[END_REF]Déverchère et al., 2005 and references therein).

Today, GPS measurements indicate a ~5 cm/yr shortening between the African and European plates, of which about 2-3 mm/yr occur offshore [START_REF] Calais | Evidence for a post-3.16-Ma change in Nubia-Eurasia-North America plate motions?[END_REF][START_REF] Nocquet | Geodetic measurements of crustal deformation in the Western Mediterranean and Europe[END_REF]Altamimi et al., 2017;[START_REF] Bougrine | Active deformation in Algeria from continuous GPS measurements[END_REF]. Although nowadays, the Algerian Margin is characterized by a moderate seismicity on-and offshore [START_REF] Buforn | Seismotectonics of the Ibero-Maghrebian region[END_REF][START_REF] Yelles-Chaouche | The Ain Temouchent (Algeria) Earthquake of December 22 nd, 1999[END_REF]) some larger earthquakes such as the Boumerdes 2003 (Mw = 6.9; [START_REF] Delouis | Slip distribution of the 2003 Boumerdes-Zemmouri earthquake, Algeria, from teleseismic, GPS, and coastal uplift data[END_REF] and the 1856 Jijel (Mercalli intensity IX; Harbi et al., 2011) earthquakes have happened in historic times. The reactivation leads to a downward bending of the oceanic plate against the continental plate (Hamai et al., 2015) and as proposed based on numerical modelling might lead to subduction in its central part (Hamai et al., 2018). This reactivation can also be recognized in seismic data imaging active faults, thrust faulting and ramps (Déverchère et al., 2005;[START_REF] Yelles-Chaouche | Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: the Khayr al Din bank[END_REF][START_REF] Arab | Tectonostratigraphic evolution of the eastern Algerian margin and basin from seismic data and onshore-offshore correlation[END_REF][START_REF] Leffondré | Ongoing inversion of a passive margin: Spatial variability of strain markers along the Algerian margin and basin (Mediterranean Sea) and seismotectonic implications[END_REF].

Rifting and oceanisation in back-arc basins are similar to those of most oceanic basins, however, some differences can be observed especially for the oceanic crust generated in these basins. In backarc basins the crust produced by spreading is affected by the hydrous melts from the subducting slab, therefore the crustal structure can depend from the distance to the slab [START_REF] Martinez | Mantle wedge control on back-arc crustal accretion[END_REF]. Oceanic crust emplaced close to an magmatic arc is proposed to show high lower crustal velocities [START_REF] Arai | Seismological study of Lau back arc crust: Mantle water, magmatic differentiation, and a compositionally zoned basin[END_REF]. In the Tyrrenian back-arc basin opening was accompanied by volcanism but at a later phase mantle material was exhumed directly to the seafloor without formation of igneous oceanic crust (Prada et al, 2015). Furthermore, studies from the Parece Vela Basin and the Mariana Trough Basin show that oceanic crust in these two back-arc basins is fundamentally different in crustal thickness and lower crust velocities [START_REF] Grevemeyer | Structure of oceanic crust in backarc basins modulated by mantle source heterogeneity[END_REF].

The authors therefore conclude that in back-arc basins an originally highly heterogeneous mantle can cause a large diversity of oceanic-type crust.

The Algerian basin is bordered by the Alboran basin to the west and the Sardinia and Tunisia margins to the east, the Liguro-Provençal basin in the north and the north-African margin in the south. Our study area is located between -0.5 and 8.3°W along the Algerian margin from Mostaganem to Annaba. The North Tunisian Fracture Zone is located at the eastern boundary of the Algerian basin [START_REF] Auzende | Structure of the western Mediterranean Basin[END_REF][START_REF] Mauffret | E-W opening of the Algerian Basin (western Mediterranean)[END_REF]. The central and eastern continental margin the consist of the Kabylide basement, resulting from the Kabyle blocks collision with the African continent. A flexural basin filled with deep-water turbidites called Numidian flysch developed along the distal portion of the African margin [START_REF] Wildi | La chaîne tello-rifaine (Algérie, Maroc, Tunisie): structure, stratigraphie et évolution du Trias au Miocène[END_REF]. The sedimentary cover of the eastern Algerian margin is interpreted to be composed by syn-rift and post-rift sediments which are interpreted to be Middle to Upper Miocene [START_REF] Arab | Tectonostratigraphic evolution of the eastern Algerian margin and basin from seismic data and onshore-offshore correlation[END_REF].

The steep slope of the eastern Algerian margin is proposed to result from transcurrent motions during back-arc opening [START_REF] Mauffret | E-W opening of the Algerian Basin (western Mediterranean)[END_REF][START_REF] Schettino | Plate kinematics of the Western Mediterranean region during the Oligocene and Early Miocene[END_REF] followed by a strike-slip and thrust deformation [START_REF] Auzende | La marge nord-africaine considérée comme marge active[END_REF][START_REF] Dewey | Kinematics of the western Mediterranean[END_REF][START_REF] Kherroubi | Recent and active deformation pattern off the easternmost Algerian margin, Western Mediterranean Sea: New evidence for contractional tectonic reactivation[END_REF][START_REF] Yelles-Chaouche | Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: the Khayr al Din bank[END_REF].

From 30-35 Ma, northwards subduction of the Ligurian-Maghrebian Tethys between Eurasia and Africa started to retreat southward, forming the Liguro-Provençal basin [START_REF] Jolivet | Mediterranean extension and the Africa-Eurasia collision[END_REF][START_REF] Leprêtre | The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin[END_REF]. This slab retreat resulted into the collision of the AlKaPeca block (from Alboran, Kabylies, Peloritani, Calabria) with the north African margin, while former mountain belts were dismantled and new orogens and arcs formed at the thrust front (Jolivet et al., 2021, and references therein). At 17-15 Ma, slab tearing enhanced a new slab retreat westwards, forming the Algero-Balearic basin in the overriding plate (Haidar et al., 2021). At the scale of the Algerian margin, this evolution explains the formation of the Tell thrust-Atlas belt on land by closure of the Maghrebian Tethys, collision of the Kabylian blocks with Africa and strain propagation within Africa (Bracene and Frizon de Lamotte, 2002;[START_REF] Leprêtre | The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin[END_REF]. Offshore, the first stage of slab retreat resulted into the formation of typical extensional structures with tilted continental blocks, i.e., from east to west: Lesser Kabylia [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF], Greater Kabylia [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF] and Khayr-Al-Din bank (off Tipaza: Yelles et al., 2009). The second stage of slab retreat by westward tearing has partly reworked the previous structures (Khayr-Al-Din bank: [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF] and gave birth to (1) a transform-type margin (STEP-fault type) in the western part of Algeria [START_REF] Govers | Lithosphere tearing at STEP faults: response to edges of subduction zones[END_REF]Wortel, 2005, Badji et al., 2015), and (2) large magmatic and volcanic intrusions linked to asthenospheric upwellings (Collo and Cap de Fer massif, Hannibal High, oceanic spreading centers) within the continental margin and the oceanic domain [START_REF] Abbassene | A 17 Ma onset for the post-collisional K-rich calc-alkaline magmatism in the Maghrebides: Evidence from Bougaroun (northeastern Algeria) and geodynamic implications[END_REF][START_REF] Chazot | An overview on the origin of post-collisional Miocene magmatism in the Kabylies (northern Algeria): evidence for crustal stacking, delamination and slab detachment[END_REF][START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF][START_REF] Medaouri | The transition from Alboran to Algerian basins (Western Mediterranean Sea): chronostratigraphy, deep crustal structure and tectonic evolution at the rear of a narrow slab rollback system[END_REF] (for location see Figure 1).

In order to study questions about the geodynamic situation, the seismic hazard and hydrocarbon potential, five combined wide-angle (WAS) and multi-channel reflection (MCS) seismic profiles were acquired in 2009 onboard the R/V L'Atalante offshore Algeria. These profiles were modelled and published individually [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF][START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF][START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF][START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF], and this study aims to give a regional synthesis trying to reconstruct the geodynamic history of the Algerian continental margin.

Seismic data acquisition

During the SPIRAL (Sismique Profonde et Investigation Regionale du Nord de l'ALgerie) wideangle seismic cruise five profiles spanning the complete margin were acquired from east to west offshore Annaba (Profile GH), Jijel (Profile IJ), Greater Kabylia (Profile EF), Tipaza (Profile CD) and Mostaganem (Profile AB) (Figure 1). Between 40 and 45 marine seismometers (Auffret et al., 2005) were deployed along each transect and seismic shots were carried out using a 8909 in 3 (ca.

146 l) tuned airgun array. WAS shots along Profiles IJ, EF and CD were recorded by a 4600 m 360 trace digital seismic streamer from Ifremer, in addition, to the reflection seismic data along all profiles which were acquired during a dedicated previous leg. The shot interval for the WAS profiles was 60 seconds, translating to a distance of about 150 m. All profiles were extended on land by seismic stations, however mostly the stations closest to the coast recorded usable signals.

Magnetic data were acquired along all seismic profiles using a SeaSPY magnetometer towed either at the tailbuoy of the seismic streamer or at a distance of 150 m behind the ship. A Lockheed Martin BGM-5 gravimeter measured the gravity anomaly with a 25-m spacing at a vessel speed of 5 knots.

All five wide-angle seismic profiles image the margin from the oceanic crust in the basin, the ocean-continent transition zone up to continental crust of the Algerian margin (Figures 1 and2). To ensure an homogeneous interpretation, all five profiles were modeled using the « Rayinvr » software of Colin Zelt (Zelt and Smith, 1988). The software uses a combined forward and inversion approach to construct layered velocity models. While details of the modeling process are given by the local studies along each transect [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF][START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF][START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF][START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF], this study focuses on the comparison between profiles and a synthesis of the lateral changes of the deep structure of the northern Algerian margin. 1) [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF][START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF][START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF][START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF]. The velocities are contoured every 0.25 km/s and shaded zones are not constrained by seismic rays. Black lines mark the layer limits.

Main Results from the Spiral published studies

Transect AB offshore Mostaganem Situated at the western border of the Algerian margin offshore Mostaganem, close to the transition between the Algerian Basin and the Alboran Sea (Figure 1). The continental crust is imaged to be about 25 km thick, thinning to 5 km over a distance of only 20-30 km, leading to a slope of up to 45± 3° for the Moho interface (Figure 3) [START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF] and the continent-ocean transition zone (COT) is extremely narrow, less than the 10 km model resolution or non-existent. Both features, together with vertical faults observed in the corresponding MCS sections indicate that the margin here is marked by a major strike-slip fault [START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF]. Therefore the authors propose that this part of the margin from Oran to Ténès was influenced by a fossil subduction-transform edge propagator fault at the edge of the Gibraltar subduction zone as proposed earlier by [START_REF] Govers | Lithosphere tearing at STEP faults: response to edges of subduction zones[END_REF]. This implies a movement of the Alboran block along this part of the margin [START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF].

For better comparison a wide-angle forward model using the identical methods as for the other profiles was constructed along this profile (for exact methodology see [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF][START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF][START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF] for details). The software "rayinvr" from Colin Zelt was used to construct the model (Zelt and Smith, 1088). It was then additionally constraint by synthetic modeling and gravity modelling. The resulting models are very comparable, and the use of the same modelling strategie allows the comparison of layer geometry and velocities between all five profiles. A full presentation of the modeling procedure however, would be out of the scope of this manuscript.

The oceanic crust bordering the margin is about 3-4 km thick, thinner than normal oceanic crust with a mean of 7.1 km (as defined by [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] or 6.0 km (as defined by [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF] and characterised by elevated velocities for oceanic crust [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF], indicating that a moderate amount of serpentinised upper mantle material might have been included into the crust emplacement at slow spreading or at spreading located at a segment end [START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF].

). (A) Magnetic anomaly measured during the cruise (B) Final forward velocity model. Green region is interpreted to be of oceanic origin, the violet region the COT, the yellow region the thinned continental crust and orange unthinned continental crust. Bold red lines are for mean 1D velocity-depth (vz) profiles and thin red lines for vz-profiles every 10km along the model. (C) in the oceanic domain (D) in the COT (E) mean 1D velocity-depth profiles in the thinned continental crustal domain (F) in the continental domain. Grey line is mean velocity for thinned continental

crust [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF], grey area bounds of normal oceanic crust from [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] and light grey areas oceanic crust from [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF].

Transect CD offshore Tipasa

The Profile CD is located offshore Tipasa (west of Algiers) (location on Figure 1) and spans from oceanic crust of the Algerian basin over the Khayr-Al-Din Bank, a 20-30 km wide bathymetric high, onto the North Algerian margin [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF]; Figure 4). The oceanic crust here is ~5 km thick, about 1-2 km thicker than in the easternmost AB Mostaganem profile. The continental crust reaches 20-21 km thickness bordered oceanward by the Khayr-Al-Din Bank. The Khayr-al-Din Bank is of continental nature and is proposed to be a syn-rift tilted block made of Internal Zones terranes remobilized on the margin during rifting [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF]. The COT here is narrow (< 10 km) or absent indicating a highly oblique or shear movement during opening or that the oceanic accretion began just or shortly after the breakup of the continental crust. These two findings can be reconciled by a polyphased opening of the margin with a first phase corresponding to a N-S rifting and a second transcurrent episode during the westward migration of the Alboran block. The last phase then includes a compressional reactivation of the margin inducing a longwavelength flexure of the basin basement and a partial uplift of the Khayr-Al-Din Bank [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF]. [START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF].

(A) Magnetic anomaly measured during the cruise (B) Final forward velocity model. Green region is interpreted to be of oceanic origin, the violet region the COT, the yellow region the thinned continental crust and orange unthinned continental crust. Bold red lines are for mean 1D velocitydepth (vz) profiles and thin red lines for vz-profiles every 10km along the model (C) in the oceanic domain (D) in the COT (E) in the thinned continental crustal domain (F) in the continental domain.

Grey line is mean velocity for thinned continental crust [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF], grey area bounds of normal oceanic crust from [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] and light grey areas oceanic crust from [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF].

Transect EF offshore Greater Kabylia

The 210-km-long profile EF is located east of Algiers and reaches the Greater Kabylia region up to the southward limit of the internal zones on land [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF], and situated close to the region of the Boumerdes 2003 earthquake and its aftershocks. Along this profile the continental margin is wider than in the east, thinning over a distance of 70 km from 23 km to 8 km thickness (Figures 1 and5). The margin shows a peculiar asymmetrical shape of the crustal neck which has been proposed to result from an isostatic disequilibrium which might be characteristic for inverted rifted margins [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF]. At the foot of the margin a 4.5 km/s velocity body is imaged corresponding to either syn-rift sediments or fractured thin continental crust. The oceanic-type crust in the basin is of variable thickness between 4 km and 5.5 km and is characterized by high seismic velocities between 5.2 km/s and 6.8 km/s and can be divided into two domains, the Hannibal domain and a domain of very thin oceanic crust [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF]. In the northern Hannibal High zone, correlated to a sub-circular group of high amplitude magnetic anomalies (Figure 1 and Figure 5A in [START_REF] Schettino | Plate kinematics of the Western Mediterranean region during the Oligocene and Early Miocene[END_REF], the oceanic basement has an atypical three-layered structure interpreted to be volcanic products (lava flows, volcaniclastic and/or hyaloclastic products) overlying a thinner original oceanic crust and resulting from Miocene post-accretional magmatism. This rules thereby out the hypothesis that the Hannibal Ridge corresponds to a fossil N-S trending accretionary center as proposed earlier [START_REF] Mauffret | E-W opening of the Algerian Basin (western Mediterranean)[END_REF]. The structure of the margin corresponds to a magma-poor rifted margin, reshaped by a later transcurrent phase and reactivated by compression expressed by a blind thrust system under the margin rising toward the surface at the slope toe [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF]. [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF], grey area bounds of normal oceanic crust from [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] and light grey areas oceanic crust from [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF].

Transect IJ offshore Jijel

This 250-km-long profile spans from the Algerian Basin over the internal zones of the lesser Kabylies (Jijel and Mila) to the Tellian thrustbelt (Setif and Batna) margin offshore Jijel [START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF]; Figures 1 and6). The Lesser Kabylian continental crust reaches a thickness of 26 km thinning to about 5 km over a short distance of 70 km. Similar to the Greater Kabylie transect EF the necking zone is asymmetric with the upper crust thinning on a shorter distance than the lower crust. The authors propose that this could be explained by oceanward creep of ductile material in the lower crust [START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF]. The COT is existent but narrow, between 15 and 20 km. The oceanic crust northwards is homogeneous and has a thickness of ~5 km. It is located in a region where linear magnetic anomalies, located parallel to the profile can be identified (Figure 1), indicating that the oceanic crust here results from accretion at an oceanic spreading center. Masswasting processes were probably active after rifting and before the accretion of the Lesser Kabylia to Africa. Since the Messinian salinity crisis the margin has been dominated by gravity driven salt tectonics [START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF]. Salt tectonics started during the Pliocene, the thickening of the sediments at the foot of the margin pressed the salt into the offshore basin, as the salt is less dense than sand or clay. During the Messinian erosion and deposition of silico-clastic sediments on the margin prevailed. Grey line is mean velocity for thinned continental crust [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF], grey area bounds of normal oceanic crust from [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] and light grey areas oceanic crust from [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF].

Transect GH offshore Annaba

The 240-km-long profile is located on the eastern edge of the Algerian basin near the Sardinia Channel (Figure 1 and7) spanning over the Lesser Kabylian block close to Annaba up to the Tellian thrustbelt [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF] (Figure 1). Here the margin is wider than along the western profiles and the 27 km continental crust thins over a distance of about 50 km to about 15 km and symmetrically between the upper and lower crust. The COT is about 30 km wide, therefore about 10-20 km wider than in the West. The oceanic crust is about 5.5 km thick and thereby thinner than normal ocean crust. At model distance 50km in the oceanic domain a small ridge is imaged proposed to be of volcanic origin active during the Miocene [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF] (Figure 7). S-wave modelling performed in the oceanic domain shows Poisson's coefficients more characteristic for gabbro than for peridotites, which confirms an oceanic origin of the crust rather than from upper mantle material exhumed and serpentinised at opening of the basin [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF](Figure 7). Based on the crustal structure of this profile the authors propose that the eastern Algerian basin opened during the southeastward migration of the European forearc before the collision, along a NW-SE trending spreading center. This geometry atypical for back-arc basins can be explained by the differing directions of the subduction rollback eastward for the Corsica-Sardinia block, and southward for the Kabylian blocks [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF] (Figure 1). velocity-depth profiles in the continental domain. Grey line is mean velocity for thinned continental crust [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF], grey area bounds of normal oceanic crust from [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] and light grey areas oceanic crust from [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF].

Synthesis and discussion the SPIRAL wide angle seismic profiles

The comparison of the interpretation of the five wide-angle seismic profiles shows laterally varying differences in the structure of the oceanic crust, the width of the COT and the steepness of the continental slope (Figure 8). Profile IJ offshore Jijel (E) Profile GH offshore Annaba [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF][START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF][START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF][START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF].

Synthesis and discussion of the structure of the oceanic crust in the Algerian basin.

Along the five transects the crust imaged in the ocean basin is between 3.5 km and 5.5 km thick and divided into two layers. Comparison of the 1D velocity-depth profiles of the 5 transects shows that the crust is not of thinned continental origin [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF] but consistent with crust of oceanic origin, thinner than average oceanic crust of 7.2 km thickness [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] (Figures 9 and10). However, a more recent compilation using modern wide-angle seismic data from 2D profiles finds that magmatic crust from fast-and intermediate spreading centers results in a crustal thickness of 6±1 km [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF] (Figure 9A). Compared to these data, the oceanic lower crustal velocities offshore Algeria remain lower than this magmatic crust compilation, a fact that can be explained by slow accretionary rates leading to lower crustal velocities [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF] (Figure 9 B,C, andD). The mean vz-profiles in our study region fit best with compilations of velocity data from segments ends, with only upper crustal velocities being higher than the corresponding bounds, with an increasing discrepancy from west to underlay is bound from normal oceanic crust [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF], gray line is continental crust [START_REF] Christensen | Seismic velocity structure and composition of the continental crust: A global view[END_REF] and dark blue underlay is magmatic crust from Grevemeyer et al., 2018 (B) dark blue underlay is crust from slow accretion from [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF] dark blue underlay is crust from slow accretion segment ends from Grevemeyer et al., 2018 (D) dark blue underlay is crust from slow accretion in segment centers from [START_REF] Grevemeyer | Structure of oceanic crust and serpentinization at subduction trenches[END_REF] Such thin oceanic crust is produced by slow to ultra-slow accretionary ridges [START_REF] Bown | Variation with spreading rate of oceanic crustal thickness and geochemistry[END_REF]) (Figure 10 B), such as the Southwest Indian Ridge [START_REF] Muller | Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge?[END_REF], or the Mohns Ridge in the North Atlantic (Klingelhoefer et al., 2000 a and b) (Figure 10 A). Although the crust produced at Mohns Ridge appears to be divided into two layers corresponding to basalts (Layer 2) and gabbros (Layer 3), the accreted crust at the Southwest Indian Ridge consists of a basaltic layer directly overlying a layer of serpentinised upper mantle, due to a lack of melt production (Mueller et al., 2000) (Figure 10 B). Comparison with the Algerian Basin shows that the basaltic layer (Layer 2) in the Algerian Basin is characterized by higher velocities than those of very slow ridges. This can be explained by the decrease in porosity in the upper crustal layers due to the weight of the sedimentary layers along the Algerian margin leading to the closure of pore space. )

Although the velocity-depth profiles from offshore Algeria fit relatively well magmatic oceanic crust they might also fit exhumed and partly serpentinised mantle material, displaying velocities similar to those of oceanic crust. Such zones composed of mantle exhumed and serpentinized by seawater have been sampled and imaged along magma-poor passive continental margins (e.g. Boillot et al., 1995;[START_REF] Beslier | Kinematics of peridotite emplacement during North Atlantic continental rifting, Galicia, northwestern Spain[END_REF][START_REF] Dean | Deep structure of the ocean-continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: The IAM-9 transect at 40 20′ N[END_REF][START_REF] Funck | Crustal structure of the northern Nova Scotia rifted continental margin (eastern Canada)[END_REF][START_REF] Van Avendonk | Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada[END_REF][START_REF] Afilhado | From unthinned continent to ocean: The deep structure of the West Iberia passive continental margin at 38 N[END_REF] (Figure 10 C andD). These zones can be up to 100-150 km wide (e.g. [START_REF] Funck | Crustal structure of the northern Nova Scotia rifted continental margin (eastern Canada)[END_REF] and are characterized by thicknesses between 4-6 km. The Moho then corresponds to the serpentinization front in the mantle, and no or few reflections from this boundary are identified in the ocean-bottom seismometer data. The crust of the Algerian Basin shows a twolayer distribution that is generally not identified in these zones of exhumed and serpentinized mantle. After the phase of mantle exhumation during the opening of a margin and before the establishment of a stable oceanic accretion a thin proto-oceanic crust is often produced at spreading centers [START_REF] Jagoutz | The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?[END_REF][START_REF] Klingelhoefer | Imaging proto-oceanic crust off the Brazilian Continental Margin[END_REF]. The first embryonic crust may also be composed of a mixture of gabbros and serpentinites and in this case will be characterized by high crustal velocities that are higher than the velocities of the gabbros, a fact that is not observed off Algeria. However, s-wave modeling offshore Annaba indicates an absence or very low rate of serpentinisation in the basin's crust [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF], and we propose therefore the oceanic crust offshore Algeria is mainly of magmatic origin, without large intrusions of upper mantle material.

In comparison between the profiles, the profile IJ offshore Jijel presents the most homogeneous oceanic crust in terms of velocities and crustal thickness (Figure 8). While the eastern profiles, offshore AB offshore Annaba, IJ offshore Jijel and CD offshore Greater Kabylia have lower upper crustal velocities than the western profiles offshore GH offshore Mostaganem and CD offshore Tipasa. These slightly elevated velocities in the lower crust (Figure 8). This lateral variation might be due to the mantle temperature or position of the slab at the time of accretion or other local factorsas it might also be explained by differences in age of several blocks in the Alboran Sea.

Other back-arc basins show thin oceanic crust probably due to low lithospheric temperatures due to the presence of a subducting slab leading to hydration of the mantle, e.g. the Philippine Sea Basin [START_REF] Louden | The crustal and lithospheric thicknesses of the Philippine Sea as compared to the Pacific[END_REF], the Tyrrenian sea [START_REF] Contrucci | Deep structure of the North Tyrrhenian Sea from multi-channel seismic profiles and on land wide angle reflection/refraction seismic recording (LISA cruise): Geodynamical implications[END_REF] (Figure 10 E) or the Liguro-Provencal Basin (Figure 10 F). These basins are entirely underlain by thin oceanic crust. The Liguro-Provencal Basin, a Mediterranean back-arc basin created from the rotation of the block Corso-Sarde in the Oligocene, shows very similar thickness and velocities of the crust underlying the basin to those in this study. The fact that both basins are narrow implies that the slab was close to the accretionary spreading center. Therefore, we propose that the Algerian and Liguro-Provencal basin were created in similar circumstances of a cold mantle leading to reduced magma supply at the accretionary centre and most probably at slow spreading rates [START_REF] Van Hinsbergen | Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation[END_REF], located close to the downgoing slab (Facenna et al., 2004) leading in both cases to is thin but nevertheless oceanic crust. (Klingelhoefer et al., 2000) (B) Green underlay is from South-West Indian Ridge (C) Green underlay is bound from southern Iberia Abyssal plain [START_REF] Dean | Deep structure of the ocean-continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: The IAM-9 transect at 40 20′ N[END_REF] (D) Green underlay is bound from west Iberia margin [START_REF] Afilhado | From unthinned continent to ocean: The deep structure of the West Iberia passive continental margin at 38 N[END_REF]) (E) Blue underlays are bounds from Tyrrenian back-arc basin [START_REF] Contrucci | Deep structure of the North Tyrrhenian Sea from multi-channel seismic profiles and on land wide angle reflection/refraction seismic recording (LISA cruise): Geodynamical implications[END_REF] (F) Blue underlay is bound from Liguro-Provencal back-arc basin. Blue underlays correspond to crust interpreted to be magmatic and green undelays to regions interpreted to result from serpentinisation of exhumed upper mantle material.

Synthesis and discussion of the ocean-continent transition zone

The ocean-continent transition zone (COT) of the north Algerian margin is extremely narrow, between < 10 km and 30 km wide (Figure 8). This is about 70 km more narrow than the COT at for example the the NW Moroccan margin, which opened during perpendicular rifting of the central Atlantic [START_REF] Biari | Deep crustal structure of the North-West African margin from combined wide-angle and reflection seismic data (MIRROR seismic survey)[END_REF]. To the west along the profiles AB offshore Mostaganem and CD offshore Tipasa the resolution of our dataset does not allow to rule out a complete absence of the COT locally. (Figure 8). This absence can be explained by the genesis of the margin as a STEP (Subduction-Transform Edge Propagator Fault) fault [START_REF] Govers | Lithosphere tearing at STEP faults: response to edges of subduction zones[END_REF]Wortel, 2005, Gallais et al., 2013;[START_REF] Badji | Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator[END_REF]. A STEP fault is formed near the termination of a subduction related to slab rollback. [START_REF] Govers | Lithosphere tearing at STEP faults: response to edges of subduction zones[END_REF] proposed the existence of a STEP fault behind the subduction of the Gibraltar arc (Figure 12).

Along the profile CD offshore Tipasa the COT is also very narrow or absent (Figure 4) indicating shear opening. On the other hand, the continental nature of the Kayr-Al-Din Bank indicates a first phase of rifting in a North-South direction. In this region, the extremely reduced width of the transition zone implies a phase of transform movement setting in after the creation of the Khayr-Al-Din Bank. This phase might be linked to the passage of the Alboran block after the initial opening [START_REF] Rosenbaum | Reconstruction of the tectonic evolution of the western Mediterranean since the Oligocene[END_REF][START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Medaouri | Structural styles and neogene petroleum system around the Yusuf-Habibas ridge (Alboran basin, Mediterranean sea)[END_REF].

The COT is located further from the coast in the Greater Kabylia region, and it is as narrow as along the Tipasa profile (Figure 5). It is close to the position of the Hannibal Ridge, proposed to be an extinct ridge producing an E-W accretion [START_REF] Mauffret | E-W opening of the Algerian Basin (western Mediterranean)[END_REF]. This hypothesis was not confirmed by the wide-angle seismic profile EF, although the orientation of the profile, also N-S is not ideally situated to answer this question [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF]. The authors propose that it is composed of rounded magnetic anomalies from later volcanism. On the other hand, off Jijel the COT is very close to the coast, also with a very reduced width (Figure 6). In this region its width might be related to an E-W opening of the basin [START_REF] Mauffret | E-W opening of the Algerian Basin (western Mediterranean)[END_REF]. In the east of the Algerian basin offshore Annaba the COT is 20-40 km wide (Figure 7). This profile is located in the transition between the Algerian Basin and the Sardinia Passage which itself is underlain by highly thinned continental crust [START_REF] Peirce | Southern segment of the European Geotraverse-a wide-angle seismic refraction experiment in the Sardinia Channel[END_REF] and outside the zone of identified magnetic anomalies in the southeast of the basin [START_REF] Schettino | Plate kinematics of the Western Mediterranean region during the Oligocene and Early Miocene[END_REF]) (Figure 11).

Synthesis and discussion of the zone of crustal thinning

The thinning zone of the continental crust of the Algerian margin is narrow compared to the Atlantic passive continental margins. To the west along the profile AB offshore Mostaganem it is 20-30 km wide and to the east along the profile GH offshore Annaba about 50 km wide. The largest width is off the coast of Greater Kabylia where it is about 60 km wide (See summary in Table 1). In comparison on the Moroccan Atlantic margin it has a width of about 100-120 km. This can be partly explained by the greater thickness of the pre-rift continental crust.

Profile

Moho

Synthesis and discussion of the unthinned continental crust

The thickness of the unthinned Algerian continental crust is not entirely constrained along the margin, due to the lack of reverse shots along the wide-angle seismic profiles. However it seems to be reduced to a thickness of about at least 25 km (Figure 2), about 10 km thinner than the Atlantic unthinned continental crust in Morocco [START_REF] Contrucci | The crustal structure of the NW Moroccan continental margin from wide-angle and reflection seismic data[END_REF]. This difference is in good agreement with recent gravimetric modeling [START_REF] Fullea | The structure and evolution of the lithosphere-asthenosphere boundary beneath the Atlantic-Mediterranean Transition Region[END_REF] indicating a Moho depth of about 25km at the Algerian Margin. The yellow line is interpreted as the southern limit of the oceanic crust, the orange as the southern limit of the COT and the red as the southern limit of the thinning zone. The Hannibal region is marked black outlines [START_REF] Mauffret | E-W opening of the Algerian Basin (western Mediterranean)[END_REF], in brown the Greater and yellow the Lesser Kabylia block and red outlines the correlated magnetic anomalies [START_REF] Schettino | Plate kinematics of the Western Mediterranean region during the Oligocene and Early Miocene[END_REF]. Dashed lines mark STEP 1 and STEP 2 faults from [START_REF] Medaouri | The transition from Alboran to Algerian basins (Western Mediterranean Sea): chronostratigraphy, deep crustal structure and tectonic evolution at the rear of a narrow slab rollback system[END_REF].

Comparison to other narrow continental margins in the Mediterranean

Comparison to a STEP impacted margin in the Ionian sea

The existence of three STEP faults has been proposed in the Mediterranean basin, in association with the slab edges at the Gibraltar arc, the Calabria trench and the Hellenic trench [START_REF] Govers | Lithosphere tearing at STEP faults: response to edges of subduction zones[END_REF]. Slab rollback can lead to the formation of slab tear faults which then leave STEP faults as a surface expression [START_REF] Govers | Lithosphere tearing at STEP faults: response to edges of subduction zones[END_REF]. The Calabrian subduction zone is narrow (250 km wide) and characterized by a steep dipping slab (Selvaggi and Chiarabba, 1995). Slab rollback of more than 800km caused by lateral tearing of the slab led to the formation of a STEP fault in the upper plate [START_REF] Gallais | Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)[END_REF][START_REF] Gutscher | Tectonic expression of an active slab tear from high-resolution seismic and bathymetric data offshore Sicily (Ionian Sea)[END_REF]. Imaging the precise location of this major STEP fault in the Ionian sea was one of the objectives of the DYONISUS deep seismic cruise [START_REF] Dellong | Crustal structure of the Ionian basin and eastern Sicily margin: Results from a wide-angle seismic survey[END_REF][START_REF] Dellong | Reply to comment by A. Argnani on "Geometry of the deep Calabrian subduction from wide-angle seismic data and 3-D gravity modeling[END_REF][START_REF] Dannowski | Ionian Abyssal Plain: a window into the Tethys oceanic lithosphere[END_REF]. One seismic profile crossing the basin shows an abrupt thinning margin at the Malta escarpment, probably the remnant of a transform margin from the Early Mesozoic [START_REF] Dellong | Crustal structure of the Ionian basin and eastern Sicily margin: Results from a wide-angle seismic survey[END_REF]. The STEP fault is proposed to follow the trend of the Alpheo fault system rather than the Malta escarpment [START_REF] Dellong | Crustal structure of the Ionian basin and eastern Sicily margin: Results from a wide-angle seismic survey[END_REF] (Figure 12).

In a comparison, the deep structure of the Calabrian margin resembles the Algerian margin. The zone of crustal thinning is narrow, only 30-50 km as compared to several hundreds of kilometers e.

g. along the Atlantic continental margins, leading to a steep Moho inclination and narrow continental shelf. The transition zone between continental and oceanic crust is narrow, in agreement with a transform opening rather than a purely orthogonally rifted margin. The oceanic basin of the Ionian basin is underlain by crust which is thinner than normal (5-6 km), but divided into two separate layers and likely of magmatic origin (Figure 13). This is similar to the structure imaged along the western zone of the Algerian oceanic basin. These similarities are in good agreement with our hypothesis that the northwestern Algerian margin formed under the influence of a STEP fault formed by a retreating slab.

Comparison to the Liguro-Provencal basin

The Liguro-Provencal basin formed by the rotation of the Corso-Sardinian block with respect to the Ibero-European plate from the Oligocene in a back-arc basin position [START_REF] Le Pichon | La méditerranée occidentale depuis l'oligocène, schéma d'évolution[END_REF][START_REF] Réhault | The western Mediterranean basin geological evolution[END_REF]. The basin was the focus of several deep seismic surveys. During the cruise CROC 2 in 1988 37 expanding spread profiles were acquired throughout the Liguro Provencal basin. These measurements helped to cartography oceanic and continental regions throughout the basin [START_REF] Le Douaran | La structure profonde d'une partie du bassin de Mediterranée nord-occidentale les resultats de la campagne de sismique à 2 navires CROC2. Rapports et procès verbaux des réunions-Commission internationale pour l'exploration scientifique de la Mer Méditerranée[END_REF][START_REF] Pascal | The ocean-continent boundary in the Gulf of Lion from analysis of expanding spread profiles and gravity modelling[END_REF][START_REF] Contrucci | A Ligurian (western Mediterranean Sea) geophysical transect revisited[END_REF][START_REF] Rollet | Back arc extension, tectonic inheritance, and volcanism in the Ligurian Sea, Western Mediterranean[END_REF]. More modern wide-angle seismic data were acquired during the SARDINIA cruise (R/V L' Atalante, 2006) in the Provencal basin with the principal objective to determine the nature of the crust in the basin and along its margins. During the cruise six combined wide-angle and reflection seismic profiles were acquired. Two of the profiles were extended on land and present a conjugate pair of the Gulf of Lions-Sardinian magins (Figure 13) [START_REF] Gailler | Crustal structure of a young margin pair: new results across the Liguro-Provencal Basin from wideangle seismic tomography[END_REF][START_REF] Moulin | Deep crustal structure across a young passive margin from wide-angle and reflection seismic data (The SARDINIA Experiment)-I. Gulf of Lion's margin[END_REF][START_REF] Afilhado | Deep crustal structure across a young passive margin from wide-angle and reflection seismic data (The SARDINIA Experiment)-II. Sardinia's margin[END_REF]. Along these profiles, the margin at the Sardinia side is characterised by a thinner continental crust, probably due to the presence of the slab at this side eroding the margin crust during subduction. Tilted fault blocks present along the Gulf of Lions margin lead to a broader transition zone of 150 km as compared to only 70 km at the Sardinian side. The sediment layer in the basin is up to 8 km thick marked by strong salt tectonics. High lower crustal velocities up to 7.6 km/s are observed at the COT along both margins and might be related to the presence of exhumed mantle material or volcanic underplate [START_REF] Gailler | Crustal structure of a young margin pair: new results across the Liguro-Provencal Basin from wideangle seismic tomography[END_REF]. The asymmetry of the margin can be explained by its formation as a back-arc basin, as the Sardinia side which was close to the subducting slab during the rollback of the téthysian slab shows a more abrupt thinning and thinner continental crustal thickness. The oceanic crust flooring the basin has a thickness of 5 km [START_REF] Moulin | Deep crustal structure across a young passive margin from wide-angle and reflection seismic data (The SARDINIA Experiment)-I. Gulf of Lion's margin[END_REF][START_REF] Afilhado | Deep crustal structure across a young passive margin from wide-angle and reflection seismic data (The SARDINIA Experiment)-II. Sardinia's margin[END_REF].

Eastward of these profiles a recent deep seismic study proposes the existence of hyper-extended continental crust thinned of only 2.5 km thickness [START_REF] Dannowski | Seismic evidence for failed rifting in the Ligurian Basin, Western Alpine domain[END_REF]. Alternatively, the velocities could be explained by a lateral isolated magmatic intrusion in the sediments [START_REF] Dannowski | Seismic evidence for failed rifting in the Ligurian Basin, Western Alpine domain[END_REF]. The crustal thickness increases toward the NE where stretching was less due to the rotational opening of the basin. Mantle velocities are slightly lower than normal (7.8 km/s) which is in good agreement with a serpentinisation of the mantle of about 20% [START_REF] Carlson | Mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites[END_REF].

The Ligurian basin margin north of this profile was studied during a 3D seismic experiment and no E-W change of crustal type is observed [START_REF] Dessa | The GROSMarin experiment: three dimensional crustal structure of the North Ligurian margin from refraction tomography and preliminary analysis of microseismic measurements[END_REF] A comparison of these profiles from the Liguro Provencal basin with the Algerian margin shows that the continental crustal thickness and width of the tinning zone of the Algerian margin are similar to the Sardinian margin (Figure 14). However, the Gulf of Lions side of the Liguro-Provencal basin is characterized by a 50-80 km wider thinning zone. The specific context of opening related to a continental block is similar for the margins of West-Sardinia and Algeria, as they both were located at the side of the slab rollback. This special geodynamic position might explain on one hand the thin continental crust and on the other hand the formation of an abrupt margin with a narrow COT.

The continent-ocean transition zone in the Liguro Provencal basin is characterised by the presence of a layer of unusual high (7.5 km/s) velocities in the lower crust. These velocities are proposed to correspond to serpentinised upper mantle material [START_REF] Gailler | Crustal structure of a young margin pair: new results across the Liguro-Provencal Basin from wideangle seismic tomography[END_REF], and are not imaged at the neighbouring Ligurian margin, probably due to the fact that full oceanisation has not been reached along these profiles [START_REF] Dessa | The GROSMarin experiment: three dimensional crustal structure of the North Ligurian margin from refraction tomography and preliminary analysis of microseismic measurements[END_REF][START_REF] Dannowski | Seismic evidence for failed rifting in the Ligurian Basin, Western Alpine domain[END_REF]. Although some high lower crustal velocities were identified at the Algerian margins oceanic domain, these do not exceed 7.2 km/s and are not spatially related to the COT. So, while the Liguro-Provencal basin structure display characteristics highly similar to magma-poor Atlantic volcanic margins (eg. Iberia : [START_REF] Minshull | The peridotite ridge province in the southern Iberia Abyssal Plain: Seismic constraints revisited[END_REF]Goban Spur : Bullock and Minshull, 2005) the Algerian margin shows a more narrow transition between continental and oceanic domains atypical for large ocean basins.

The oceanic crust in both basins is thin, between 4-5 km, with an irregular magnetic anomaly pattern, except a region of regular fan-shaped anomalies offshore Jijel in the Algerian basin [START_REF] Schettino | Plate kinematics of the Western Mediterranean region during the Oligocene and Early Miocene[END_REF]Haidar et al., 2021). No structures corresponding to an extinct spreading center have been imaged in the Liguro Provencal basin and although the existence of an extinct spreading center has been suggested in the Algerian basin [START_REF] Mauffret | Tectonics and deep structure of the north-western Mediterranean Basin[END_REF], later work

proposes the magnetic anomalies to be associated with Miocene postrift volcanism [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF]. This thin oceanic crust without regular magnetic anomalies might be representative for backarc spreading basins, where spreading might occur along short ridge widely spaced segments [START_REF] Medaouri | The transition from Alboran to Algerian basins (Western Mediterranean Sea): chronostratigraphy, deep crustal structure and tectonic evolution at the rear of a narrow slab rollback system[END_REF] or due to a partial melting during the second phase in a region of serpentinised mantle [START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF]. Grand Banks -Van Avendonk et al., 2006;Iberia Abyssal Plain -Dean et al., 2008) (F) Mean vz-profiles from the oceanic regions extracted underneath the basement underlain by velocity envelopes from typical oceanic crust from the Atlantic and Pacific [START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] . [START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF][START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF][START_REF] Aïdi | Deep structure of the continental margin and basin off Greater Kabylia, Algeria-New insights from wide-angle seismic data modeling and multichannel seismic interpretation[END_REF][START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF]Badji et al., 2013;Liguro Provencal Basin : Pascal et al., 1993;[START_REF] Contrucci | A Ligurian (western Mediterranean Sea) geophysical transect revisited[END_REF][START_REF] Gailler | Crustal structure of a young margin pair: new results across the Liguro-Provencal Basin from wideangle seismic tomography[END_REF]Moulin et al., 2014;[START_REF] Klingelhoefer | Imaging proto-oceanic crust off the Brazilian Continental Margin[END_REF][START_REF] Dannowski | Seismic evidence for failed rifting in the Ligurian Basin, Western Alpine domain[END_REF].

Conclusions

Comparison of five onshore-offshore wide-angle seismic transects at the north Algerian margin allowed to constrain crustal structure variations and influence of the opening of the basin on its deep structure:

1) The Algerian Basin is underlain by a ~5 km thin oceanic crust. This kind of crust is typical for back-arc basins generated in an environement of reduced mantle temperature due to the closeness of the cold subducting slab. Upper crustal velocities increase from the East to the West along the Algerian offshore and the westernmost profiles show slightly elevated lower crustal velocities, probably linked to local varying parameters during accretion.

2) A very narrow or absent COT is observed along all the Spiral profiles with the exception of the easternmost Annaba (GH) profile. This might be due to a second phase of opening in an east-west direction, with the Alboran Block passing along the profile Tipasa (CD) and Mostaganem (AB). The Mostaganem profil (AB) shows the most narrow margin, steepest slope and Moho inclination bearing good resemblance with other STEP impacted margins.

Offshore Tipasa (profile CD) the margin is impacted by two phases, an initial phase of rifting creating the Khayr-Al-Din bank and a second phase of transform deformation due to the STEP fault.

3) The Hannibal ridge earlier proposed to be a N-S oriented spreading center in the middle of the Algerian basin [START_REF] Mauffret | E-W opening of the Algerian Basin (western Mediterranean)[END_REF] corresponds probably to several submarine volcanic centers formed after initial opening of the basin itself. 6) No high velocity zone in the lower crust corresponding to serpentinised mantle rocks has been imaged at the Algerian margin. This kind of high velocity zone has previously been detected in the Liguro-Provencal Basin. However, the oceanic crust underlying both basins is comparable in thickness and velocities. Spreading on both basins likely occurred along short segments of accretionary ridge segments in a more diffuse way than along steady midocean ridges in full sized oceanic basins.

Figure 1 :

 1 Figure 1 : (A) Tectonic history of the Mediterranean region. Simplified from Hinsbergen et al., 2013. Red frame shows extent of lower panels. (B) Location map of the wide-angle and reflection seismic profiles of the SPIRAL project. Dotted black lines mark the MCS profiles of the first leg and red lines WAS profiles from the second leg. Red dots represent positions of the marine seismometers and orange inverted triangles those of seismic landstations. (C) Magnetic anomaly from Schettino and Turco, 2006, with High = 700 and Low = -250 nT. HH = Hanibal High, LMA = Linear magnetic anomalies, GKB = Greater Kabylia block, LKB = Lesser Kabylia block, KAD = Khair-Al-Din bank, CdF = Cap de Fer massif.

Figure 2 :

 2 Figure 2 : Final velocity profiles from forward modelling of the five wide-angle seismic transects from west (top) to east (bottom)(location see Figure1)[START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Mihoubi | Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration[END_REF]

  (A) Profile AB offshore Mostaganem (B) Profile CD offshore Tipasa (C) Profile EF offshore Greater Kabylia (D) Profile IJ offshore Jijel (E) Profile GH offshore Annaba. Colour scale in (E) is valid for all profiles.

Figure 3 :

 3 Figure 3 : Final forward velocity model of the Profile AB offshore Mostaganem (location on Figure 1). (A) Magnetic anomaly measured during the cruise (B) Final forward velocity model. Green

Figure 4 :

 4 Figure 4 : Final forward velocity model of the Profile CD offshore Tipasa[START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF].

Figure 5 :

 5 Figure 5 : Final forward velocity model of the Profile EF offshore Greater Kabylia (Aïdi et al., 2018). (A) Magnetic anomaly measured during the cruise (B) Final forward velocity model. Green region is interpreted to be of oceanic origin, the violet region the COT, the yellow region the thinned continental crust and orange unthinned continental crust. Bold red lines are for mean 1D velocity-depth (vz) profiles and thin red lines for vz-profiles every 10 km along the model (C) and (D) in the oceanic domain (E) in the COT (F) in the thinned continental crustal domain (G) in the continental domain. Grey line is mean velocity for thinned continental crust(Christensen and

Figure 6 :

 6 Figure 6 : Final forward velocity model of the Profile IJ offshore Jijel (Mihoubi et al., 2014). (A) Magnetic anomaly measured during the cruise (B) Final forward velocity model. Green region is interpreted to be of oceanic origin, the violet region the COT, the yellow region the thinned continental crust and orange unthinned continental crust. Bold red lines are for mean 1D velocitydepth (vz) profiles and thin red lines for vz-profiles every 10km along the model (C) in the oceanic domain (D) in the COT (E) in the thinned continental crustal domain (F) in the continental domain.

Figure 7 :

 7 Figure 7 : Final forward velocity model of the Profile GH offshore Annaba (Bouyahiaoui et al., 2015). (A) Magnetic anomaly measured during the cruise (B) Final forward velocity model. Green region is interpreted to be of oceanic origin, the violet region the COT, the yellow region the thinned continental crust and orange unthinned continental crust. Bold red lines are for mean 1D velocity-depth (vz) profiles and thin red lines for vz-profiles every 10km along the model (C) mean 1D velocity-depth profiles in the oceanic domain (D) mean 1D velocity-depth profiles in the COT

Figure 8 :

 8 Figure 8 : Interpretation of the velocity profiles based on the original work . The velocities are contoured every 0.25 km/s. Brown areas depict continental crust and blue areas oceanic crust. High velocity zones in the lower crust are marked in pink and volcanic material in red. Approximate extent of the COT is shown by a transparent cross striped area. (A) Profile AB offshore Mostaganem (B) Profile CD offshore Tipasa (C) Profile EF offshore Greater Kabylia (D)

  east. Only profile AB falls out of both the[START_REF] White | Oceanic crustal thickness from seismic measurements and rare earth element inversions[END_REF] the Grevemeyer et al., 2018 ranges for upper crustal velocities in oceanic crust (Figure9A).

Figure 9 :

 9 Figure 9 : Comparison of 1D velocity-depth profiles underneath the basement in the oceanic domain. Red line : profils AB Mostaganem, Orange line : profile CD Tipasa; Green line : profile EF Greater Kabylia; dark blue line : profile IJ Jijel, blue line : profile GH Annaba. (A) Light

Figure 10 :

 10 Figure 10 : Comparison of 1D velocity-depth profiles underneath the top of the basement in the oceanic domain. Red line : profile AB Mostaganem, Orange line : profile CD Tipasa; Green line : profile DE Greater Kabylia; dark blue line : profile IJ Jijel, blue line : profile GH Annaba. (A) Blue underlay is bound from Mohns Ridge(Klingelhoefer et al., 2000) (B) Green underlay is from South-

Figure 11 :

 11 Figure 11 : Domain boundaries indicated by the five transects. The yellow line is interpreted as the

Figure 12 :

 12 Figure 12 : Comparison of the Algerien margin offshore Mostaganem with the Dyonisus 1 profile located in the Ionian Sea. (A) Dyonisus profile 1 (Dellong et al., 2018) (B) SPIRAL AB (C) Vzprofiles extracted underneath the basement at the continental part of Ionian sea profile (D) Vzprofiles extracted underneath the basement at the oceanic part of Ionian sea profile (E) Vz-profiles extracted underneath the basement at the oceanic part of the Algerian profile (F) Vz-profiles extracted underneath the basement at the continental part of the Algerian profile.

Figure 13 :

 13 Figure 13 : Interpreted crustal structure from wide-angle seismic profiles from the Liguro-Provencal Basin and the Algerian Basin. (A) Gulf of Lions (Moulin et al., 2015) (B) Sardinia (Afilhado et al., 2015) (C) Profile GH offshore Anaba from the SPIRAL cruise (D) Location map (E) Mean vz-profiles from the oceanic regions extracted underneath the basement underlain by velocity envelopes from serpentinized upper mantle(Grand Banks -Van Avendonk et al., 2006;

Figure 14 :

 14 Figure 14 : (A) Moho depth in the Liguro Provencal and Algerian basins (B) Lower crustal velocity directly above the Moho in the Liguro Provencal and Algerian basins (Algerian Basin :

4)

  The thickest crust detected during the Spiral survey corresponds to the African continental crust or the Kabylides blocks and is about 22-25 km thick. This reduced thickness in comparison with unthinned continental crust might be due to the influence of subduction at the margin, similar to the reduced thickness of the Sardinian basement compared to the Gulf of Lions basement. 5) Comparison to the deep structure of a STEP fault impacted margin located in the Ionian Sea shows that the Algerian margin at Mostaganem is caracterised by several features typically arising from the influence of a STEP fault such as a very narrow COT and a steep Moho inclination at the continental margin.

Table 1 :

 1 Comparison of Moho slope and width of the necking zone along all profiles andcomparison to the Gulf of Lions and Ionian sea.

	slope	Width of necking zone (km)

Acknowledgements

The authors would like to thank the captain and the crew of R/V L'Atalante and the technical seismic teams of Genavir for their excellent work during the data acquisition. The GMT (Wessel and Smith, 1995) software package was used in the preparation of this paper. The location maps were made using the Qgis software (QGIS Development Team, 2021).

The SPIRAL project is a collaboration between and funded by : CRAAG (Centre de Recherche en Astronomie, Astrophysique et Géophysique), Sonatrach, and DGRST (Directorate-General for Scientific Research and Technological Development) in Algeria, and the CNRS (Centre National de la Recherche Scientifique), Ifremer (Institut Français de Recherche pour l'Exploitation de la Mer), IRD (Institut de Recherche pour le Développement), and the Universities of Brest and Nice in France.