Elie Guéraut. Territoires, culture et classes sociales. Espaces et sociétés (Paris, France), Erès, 2021, n°180-181, pp.262 -265. Elie Guéraut Ultime pièce d'un projet de réédition des travaux de Jean-Claude Chamboredon, à la suite d'un recueil de textes sur la question de la « jeunesse » sélectionnés par Paul Pasquali et d'un long texte sur la vie et l'oeuvre d'Emile Durkheim présenté par Dominique Schnapper1 , ce livre réunit certaines publications du sociologue ayant trait à la question des territoires, de leur production, de leur représentation et de leurs usages. Il propose ainsi de (re)découvrir tout un pan du travail empirique et théorique de Jean-Claude Chamboredon qui, sélectionné et introduit ici par Gilles Laferté et Florence Weber, pose le programme et les ambitions de ce que ces derniers se proposent d'appeler une « sociologie critique de l'espace ».

Livre recensé : Chamboredon, Jean-Claude. Territoires, culture et classes sociales (édition de Gilles Laferté et de Florence Weber). Editions Rue d'Ulm, collection « Sciences sociales », 394 p.

Lire et transmettre Jean-Claude Chamboredon « Editer un classique, c'est chercher le présent dans le passé pour qu'il advienne dans le futur » (p. 9), résument Gilles Laferté et Florence Weber, assumant ainsi pleinement l'entreprise de transmission du programme de recherche d'un sociologue qui fut, pour l'un et pour l'autre, au coeur de leur apprentissage des sciences sociales 2 . Ce livre a donc d'abord pour objectif de proposer une synthèse des travaux de Jean-Claude Chamboredon portant sur la question de l'espace, de sa production, de sa représentation et de son appropriation par les différentes classes sociales qui le pratiquent. Les textes sélectionnés ont été publiés entre 1977 et 1994, et se présentent de prime abord comme relativement éclatés thématiquement : la peinture de Jean-François Millet, la chasse, les politiques de peuplement ou encore les motifs iconographiques des manuels scolaires. Gilles Laferté et Florence Weber défendent pourtant la cumulativité des travaux de Chamboredon qui, malgré une dispersion temporelle, thématique et même disciplinaire, suivent en réalité plusieurs fils rouges, dont celui qui a présidé au travail de sélection des textes de ce livre, l'analyse du « rôle de l'espace dans la structure sociale et [du] rôle de la structure sociale dans la transformation de l'espace » (p. 9).

Derrière un parcours de recherche sinueux, la cohérence d'un projet intellectuel

Les douze textes réunis dans ce livre se succèdent par ordre chronologique de publication 3 , dont les grandes étapes sont restituées en cinq grandes parties. La première, « Les deux manières de Jean-François Millet », n'est constituée que d'un article consacré au peintre publié en 1977 dans les Actes de la recherche en sciences sociales. A travers le cas de l'invention de la figure de l'« éternel paysan », il s'agit pour Chamboredon de proposer une analyse de la production et de la circulation de biens symboliques qui engagent la représentation des groupes sociaux. La deuxième partie, « Les usages sociaux de l'espace rural », réunit trois textes proposant une analyse de la transformation des usages des espaces ruraux, qui perdent peu à peu leurs fonctions productives pour assurer des fonctions récréatives à destination des urbains. La troisième partie, « Enquêter sur les appartenances territoriales », est certainement la plus ambitieuse théoriquement, dans la mesure où celle-ci vise à défendre une méthode d'analyse biographique critique de la notion d'habitus (chapitre 5), à proposer une théorie de l'appartenance territoriale qui s'inscrirait dans les rapports de classe (chapitre 6), et enfin à mettre à l'épreuve empiriquement cette grille d'analyse à travers une réflexion sur la « naturalisation » de la campagne (chapitre 7). La quatrième partie, « Retour à la morphologie sociale », réunit deux chapitres à l'origine publiés dans le livre dirigé par Marcel Roncayolo Histoire de la France urbaine paru en 1985, ainsi que la réponse de Jean-Claude Chamboredon à un compte-rendu critique publié dans la revue Hérodote. Il est possible d'y voir, près de quinze années après l'enquête réalisée à Antony4 , une analyse actualisée des politiques de peuplement des grands-ensembles, nourrie par un détour de plusieurs années par l'analyse des territoires ruraux. Dans la dernière partie, « Pour une iconographie sociale », Chamboredon revient à la question des « icones » et des symboles à travers lesquels le monde social est dit et représenté. Après un texte théorique où Chamboredon propose une analyse des relations entre la « production symbolique et les formes sociales » (chapitre 11), cette grille de lecture est utilisée afin de rendre compte de « l'édification de la nation » (chapitre 12).

Cette organisation présente la vertu de donner à voir certaines des métamorphoses des intérêts de recherche du sociologue sur près de deux décennies. Il ne faut cependant pas voir dans cette organisation du livre, qui est tout à la fois historique et thématique, un principe de division fort : par exemple, lorsque Jean-Claude Chamboredon analyse la peinture de Jean-François Millet (chapitre 1), c'est pour rendre compte des logiques qui construisent la représentation des classes sociales et des territoires qu'elles investissent ; de même, l'étude de la chasse (chapitres 3 et 4) donne l'occasion d'une analyse des rapports de classe dans leur dimension résidentielle, qui questionne en filigrane les effets des transformations des activités productives des territoires. Bref, en dépit de ce parcours de recherche a priori sinueux, pour qui s'en tiendrait aux seuls énoncés thématiques, la lecture de ce livre fait apparaître toute la cohérence du projet intellectuel de Chamboredon : proposer une analyse relationnelle et spatialisée des classes sociales qui s'autorise à discuter des grandes transformations sociales et économiques du XXe siècle.

Une « sociologie critique de l'espace » ? Suivant Gilles Laferté et Florence Weber dans leur introduction, cette sélection de textes fait apparaître deux apports essentiels aux sciences sociales contemporaines. En premier lieu, la sociologie de Jean-Claude Chamboredon donne à penser la dimension spatiale de la stratification sociale. Qu'il s'agisse de la figure naturalisée et transhistorique du « paysan » (chapitre 1), des ruraux dépaysannés ou des bourgeois urbains approchés dans le cadre des enquêtes sur les pratiques cynégétiques (chapitres 3 et 4), les groupes sociaux sont non seulement analysés au prisme de leurs propriétés sociales mais aussi dans leur rapport à l'espace (chapitre 6). Cette attention particulière à la dimension géographique des faits sociaux concourt à apporter des connaissances empiriques de première importance à l'heure où l'étude des espaces ruraux est monopolisée par une ethnologie de la France qui privilégie l'étude du folklore, de même qu'elle vient éclairer un angle mort de la théorie des classes sociales telle que mobilisée par la tradition marxiste ou dans la sociologie générale que Pierre Bourdieu propose alors5 . Cette grille de lecture invite ainsi à prendre la mesure de l'autonomie relative des « espaces sociaux localisés »6 , en tant que système territorialisé de rapports sociaux, sans pour autant oublier que ces espaces et les populations qui y vivent sont construits à distance par des politiques publiques et des logiques de marché (chapitres 8, 9 et 10).

Un deuxième apport majeur de ce livre a trait à l'analyse des représentations des territoires et aux modalités de construction et de circulation de leur image (et donc de leur valeur). Les représentations associées aux espaces sont analysées au prisme des agents et des institutions qui les produisent et les diffusent, à l'instar des champs de production artistique (chapitres 1 et 11) et de l'école (chapitre 12). Ce faisant, si les faits sociaux apparaissent pour partie tributaires des territoires où ils se manifestent, l'espace, dans sa matérialité comme dans sa symbolique, procède d'une histoire sociale qu'il appartient au chercheur en sciences sociales de mettre au jour. En conséquence, l'image du territoire, loin de constituer un ensemble de valeurs et de pratiques sociales homogènes et stabilisées dans le temps, comme le donne à penser la notion d'identité régionale, est façonnée par des rapports sociaux qui opposent agents et institutions dans des luttes d'imposition symbolique. De ce fait, ces images ne s'imposent pas à tou-te-s selon les mêmes modalités ; leur inculcation est médiatisée par des expériences socialisatrices multiples à l'issue desquelles ces images sont appropriées par les individus, suscitant chez eux des « sentiments d'appartenance » à géométrie variable. Vingt à quarante années après l'écriture de ces textes, force est de constater que le succès de la notion d'identité n'a cessé de croître. Aussi, suivant Chamboredon, substituer à cette notion celle de sentiment d'appartenance s'avère aujourd'hui particulièrement utile à qui veut rendre compte des modalités de construction des représentations sociales et du processus à travers lequel les individus se les approprient (par leurs pratiques et leurs croyances), tout en mettant à distance la charge politique que présente la première.

Chamboredon J.-C.,

2015, Jeunesse et classes sociales, Paris, France, Éditions rue d'Ulm, 262 p. et Chamboredon J.-C., 2017, Émile Durkheim: le social, objet de science : du moral au politique ?, Paris, France, Éditions Rue d'Ulm, 111 p. 2 Jean-Claude Chamboredon fut en effet le maître de Florence Weber à son entrée à l'Ecole normale supérieure en 1977, avant de devenir elle-même professeure dans cette institution et d'encadrer la thèse de Gilles Laferté de 1997 à 2002.3 A l'exception du chapitre 10, une réponse de Jean-Claude Chamboredon à un compte-rendu de lecture de l'Histoire de la France urbaine dirigée (dir. Marcel Roncayolo) qui fait suite dans le livre aux chapitres que le sociologue y a rédigés (chapitres 8 et 9).

Dont est tiré l'article bien connu « Proximité spatiale et distance sociale » coécrit avec Madeleine Lemaire et publié en 1970 dans Actes de la recherche en sciences sociales.

Sur la question, voir Ripoll F., 2013, « Quelle dimension spatiale des structures sociales chez Bourdieu ? Localisations résidentielles et jeux d'échelle dans La Distinction », dans Coulangeon, P., Duval, J. (dirs.), Trente ans après « La Distinction » de Pierre Bourdieu, Paris, France, la Découverte, 2013, 423 p.

Laferté G., 2014, « Des études rurales à l'analyse des espaces sociaux localisés », Sociologie, N°4, vol. 5.


Daniel Sauter
email: daniel.sauter@unistra.fr

Philippe Werner

Georges Ceuleneer

Gianreto Manatschal

Mathieu Rospabé

Julie Tugend

Morgane Gillard

Julia Autin

Marc Ulrich

Sub-axial deformation in oceanic lower crust: Insights from seismic reflection profiles in the Enderby Basin and comparison with the Oman ophiolite

Keywords: Lower oceanic crust, oceanic accretionary processes, dipping reflectors, oceancontinent transition, Enderby Basin, Oman ophiolite

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Most of the oceanic crust created over the last 200 million years formed at intermediate-to fast-spreading mid-ocean ridges (50-200 km/Ma full rate) [START_REF] Karson | Geologic Structure of uppermost oceanic crust created at fast to intermediate-rate spreading centers[END_REF]. Despite the widespread occurrence of such magmatic crust, direct observations of its lower part made essentially of gabbroic cumulates are particularly scarce. There are only two locations (Hess Deep and Pito Deep in the Pacific Ocean) where in situ fast-spread plutonic crust is exposed in response to ridge propagation. Rocks from such deep horizons have been collected by submersible, remotely operated underwater vehicle and drilling, the section recovered by drilling being typically several tens of meter in length only (e.g. [START_REF] Brown | Textural Character of Gabbroic Rocks from Pito Deep: a Record of Magmatic Processes and the Genesis of the Upper Plutonic Crust at Fast-Spreading Mid-Ocean Ridges[END_REF][START_REF] Gillis | Primitive layered gabbros from fast-spreading lower oceanic crust[END_REF]. No in situ observations within the lowermost oceanic crust accreted at fast-spreading ridges have been made elsewhere, apart from some sites in large offset transform faults, where crustal structures might not be representative of standard accretion processes (cf. [START_REF] Constantin | Construction of the oceanic lithosphere by magmatic intrusions: Petrological evidence from plutonic rocks formed along the fast-spreading East Pacific Rise[END_REF].

Therefore, our knowledge of the deep crustal magmatic system along fast-spreading mid-oceanic ridges is mostly derived from seismic studies of the East Pacific Rise (EPR) and ancient analogues (e.g. ophiolites) although the tectonic setting of their genesis is still debated [START_REF] Macleod | Moist MORB" axial magmatism in the Oman ophiolite: The evidence against a mid-ocean ridge origin[END_REF]. Seismic reflection data have shown that there is a near static region of high melt proportions, the axial magma lens (AML), 1-3 km wide and a few hundred meters high, at a depth of 1-2 km beneath the ridge axis, for much of its length (e.g. [START_REF] Detrick | Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise[END_REF]. Beneath the AML, tomographic studies have indicated that there is a low velocity zone a few kilometers wide (e.g. [START_REF] Toomey | The threedimensional seismic velocity structure of the East Pacific Rise near latitude 9°30[END_REF], which is inferred to consist of a crystal mush, i.e. incompletely crystallized cumulates [START_REF] Sinton | Mid-ocean Ridge magma chambers[END_REF]. More recent seismic reflection studies have shown a series of melt lenses in the lower crust below the AML both on and off-axis (e.g. [START_REF] Marjanovic | A multi-sill magma plumbing system beneath the axis of the East Pacific Rise[END_REF]. In addition, melt in the lower crust and/or the crust-mantle transition zone was also inferred by seafloor compliance studies [START_REF] Crawford | Variations in the distribution of magma in the lower crust and at the Moho beneath the East Pacific Rise at 9°-10[END_REF]. Thus, there is an increasing set of arguments for a multilevel complex of melt lenses beneath the ridge axis [START_REF] Marjanovic | A multi-sill magma plumbing system beneath the axis of the East Pacific Rise[END_REF]. However, how these melt lenses contribute to the formation of both the upper and lower crust and what accretion processes are operating in the lower crust remain unclear. Two end-member models for crustal accretion have emerged from geophysical observations at fast-spreading ridges and more largely from field evidence in the Oman and other ophiolites [START_REF] Kelemen | Geochemistry of gabbro sills in the crust/mantle transition zone of the Oman ophiolite: Implications for the origin of the oceanic lower crust[END_REF][START_REF] Marjanovic | A multi-sill magma plumbing system beneath the axis of the East Pacific Rise[END_REF][START_REF] Nicolas | Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise[END_REF]. In the `gabbro glacier' model, most crystal growth occurs within the AML, which subsides by ductile flow to form the entire gabbro section (e.g. [START_REF] Quick | Ductile deformation and the origin of layered gabbro in ophiolites[END_REF]. The `sheeted sill' model, where gabbro formation occurs in situ in small magma bodies throughout the lower oceanic crust, better fits with the occurrence and structure of the interlayered mafic and ultramafic cumulates in ophiolites [START_REF] Kelemen | Geochemistry of gabbro sills in the crust/mantle transition zone of the Oman ophiolite: Implications for the origin of the oceanic lower crust[END_REF]. In the Oman ophiolite, recent studies of the crust-mantle transition zone have shown that faults, enabling water introduction at depth, may be a more common feature than previously expected in the formation of the lowermost oceanic crust in a context of high melt supply [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF][START_REF] Rospabé | Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone[END_REF]. Such faults have not been documented yet in seismic reflection profiles at fast-spreading ridges.

However, lower crustal dipping reflectors have been observed on ridge flanks within both slow-and fast-spreading crust (see [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF] for a review) while they are not imaged along the ridge axis itself. Such lower crustal dipping reflections within fast-spread crust have been interpreted as arising from shear zones that form near the spreading center in the region with interstitial melt, and result from shear at the base of the crust [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF]. It has been further suggested that such differential motion at the base of the crust could result from plate reorganizations [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF].

While lower crustal events dipping toward the paleo-ridge axis have been predominantly described until now [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF][START_REF] Ding | Structures within the oceanic crust of the central South China Sea basin and their implications for oceanic accretionary processes[END_REF][START_REF] Reston | The structure of Cretaceous oceanic crust of the NW Pacific: Constraints on processes at fast spreading centers[END_REF], here we identify numerous both ridgeward and continentward dipping highamplitude reflectors in the lower oceanic crust of the Enderby Basin, between the Kerguelen Plateau and the Antarctic margin. Thanks to high-quality seismic reflection profiles, we are able to document the occurrence of such dipping reflections in two different but juxtaposed domains: typical oceanic crust and the ocean-continent transition. We relate these lower crustal dipping reflectors to syn-magmatic faulting described recently in the Oman ophiolite [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF][START_REF] Rospabé | Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone[END_REF].

These observations give some clues for the sub-axial tectono-magmatic processes that may occur within the oceanic crust.

Geological background

The Enderby Basin

Here, we focus on the oceanic domain and the ocean-continent transition within the central part of the Enderby Basin. It is bounded by the Enderby Land to the southwest, by the Elan Bank to the north and the southern Kerguelen Plateau to the northeast, by the Princess Elizabeth Trough to the east and by the Antarctic margin and the Mac Robertson Land to the south (Fig. 1). The Enderby Basin is thought to be formed during the Early Cretaceous as East Antarctica and India rifted apart (McElhinny, 1970). An intermediate spreading rate of ~60 km/Ma has been inferred from east-west trending magnetic anomalies M4 (126.7 Ma), M2 (124.1 Ma), and M0 (120.4 Ma) [START_REF] Gibbons | The breakup of East Gondwana: Assimilating constraints from Cretaceous ocean basins around India into a best-fit tectonic model[END_REF]. These anomalies have been identified north and south of an extinct ridge running parallel to the Elan Bank [START_REF] Gaina | Breakup and early seafloor spreading between India and Antarctica[END_REF] (Fig. 1). However, this extinct ridge is poorly constrained and, contrasting with many other extinct ridges, it is not linked to a gravity low [START_REF] Macleod | A global review and digital database of large-scale extinct spreading centers[END_REF]. Moreover, magnetic anomaly profiles in the Enderby Basin are scarce and, due to the absence of clear and continuous magnetic anomalies, their identification is debated (e.g. [START_REF] Golynsky | New continental margin magnetic anomalies of East Antarctica[END_REF]. It has been suggested that the lack of clear magnetic anomalies may result from seafloor spreading during the Cretaceous Normal Superchron [START_REF] Jokat | New aeromagnetic data from the western Enderby Basin and consequences for Antarctic-India break-up[END_REF]. Therefore, no consensus about the spreading history of the Enderby Basin has been reached yet (e.g. [START_REF] Davis | The crustal structure of the Enderby Basin, East Antarctica[END_REF]. By contrast, there is a consensus about the location of the inboard edge of unequivocal oceanic crust (LOC) (Fig. 1). It typically coincides with: (1) a prominent oceanward step-up in the basement level of 500-1000 m, (2) the continentward abrupt termination of a well-marked oceanic Moho in the seismic reflection profiles [START_REF] Stagg | Geology of the Continental Margin of Enderby and Mac[END_REF], and (3) the Enderby Basin Anomaly (EBA; [START_REF] Golynsky | New continental margin magnetic anomalies of East Antarctica[END_REF], a high-amplitude (350-500 nT) magnetic anomaly which is interpreted to mark the contact between strongly magnetized oceanic crust and less magnetized continental rocks [START_REF] Golynsky | New continental margin magnetic anomalies of East Antarctica[END_REF]. The domain to the north of the LOC is described as unambiguous oceanic crust [START_REF] Stagg | Geology of the Continental Margin of Enderby and Mac[END_REF]. The domain immediately to the south, within the oceancontinent transition, is referred to as transitional crust on the basis of seismic reflection profiles and limited refraction data (sonobuoys) [START_REF] Stagg | Geology of the Continental Margin of Enderby and Mac[END_REF]. It is thought to be made of stretched continental crust or exhumed mantle that was subsequently modified by magmatic intrusions during the formation of the initial-or proto-oceanic crust [START_REF] Gaina | Breakup and early seafloor spreading between India and Antarctica[END_REF][START_REF] Gillard | The role of serpentinization and magmatism in the formation of decoupling interfaces at magma-poor rifted margins[END_REF].

We use 10 high-resolution and deep-penetrating seismic reflection profiles from the GA-228 and GA-229 surveys collected by Geoscience Australia in 2000 and 2001, respectively [START_REF] Stagg | Geology of the Continental Margin of Enderby and Mac[END_REF]. During these surveys, 36-fold stacked and migrated deep seismic data (60 l airgun array source; 3600 m streamer; 288 channels; 16 s record length) were recorded between offshore western Enderby Land and the southern Kerguelen Plateau (Fig. 1) [START_REF] Stagg | Geology of the Continental Margin of Enderby and Mac[END_REF]. The seismic profiles are trending North-South, perpendicular to the LOC and the magnetic lineations of the Enderby Basin. They are thus thought to be collected along a flow line of the Enderby Basin spreading center, although the geometry and segmentation of this ridge is poorly known. This data set was used by [START_REF] Stagg | Geology of the Continental Margin of Enderby and Mac[END_REF] to map and describe distinct sectors along the Antarctic margin and the adjacent oceanic crust.

The Oman ophiolite

The Oman ophiolite is the largest coherent remaining fragment of the Tethyan oceanic lithosphere. It is interpreted as formed along a highly productive [START_REF] Ceuleneer | Thermal structure of a fossil mantle diapir inferred from the distribution of mafic cumulates[END_REF][START_REF] Pallister | Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber[END_REF] and possibly fast-spreading center ∼95-97 million years ago [START_REF] Boudier | Kinematics of oceanic thrusting in the Oman ophiolite: model of plate convergence[END_REF][START_REF] Rioux | Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust[END_REF]. Major ductile shear zones were recognized as common features in the mantle and crustal sections of the Oman ophiolite since pioneer structural studies [START_REF] Amri | Tectonic setting for the genesis of oceanic plagiogranites: evidence from a paleo-spreading structure in the Oman ophiolite[END_REF][START_REF] Boudier | Kinematics of oceanic thrusting in the Oman ophiolite: model of plate convergence[END_REF][START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF][START_REF] Jousselin | The Moho transition in the Oman ophioliterelation with wherlites in the crust and dunites in the mantle[END_REF][START_REF] Jousselin | Detailed mapping of a mantle diapir below a paleo-spreading center in the Oman ophiolite[END_REF]. They are characterized by intense deformation with the development of peridotite mylonites and flaser gabbros on thicknesses ranging from a few meters to several hundred meters.

The main ones can be followed along strike over distances of several tens of kilometres. Shearing initiated at very high temperature, close or above the solidus of peridotites and gabbros, frequently in the presence of melt, and usually stopped before the rock cooled down significantly, although most of these shear zones were still active at greenschist facies metamorphic conditions.

The shear zones revealed by the early structural mapping of the Oman ophiolite show a clear continuity with the metamorphic sole and, hence, have been attributed to emplacement tectonics (intra-oceanic thrusting close to the ridge axis) [START_REF] Boudier | Kinematics of oceanic thrusting in the Oman ophiolite: model of plate convergence[END_REF][START_REF] Boudier | Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge[END_REF]. Until recently, this interpretation was extended by most geologists to all kinds of ductile and brittle fault zones affecting the mantle and the deep crustal section of the Oman ophiolite, apart from minor ones interpreted in terms of oceanic spreading (e.g. [START_REF] Dijkstra | Microstructures and lattices in the Hilti mantle section (Oman Ophiolite): Evidence for shear localization and melt weakening in the crust mantle transition zone?[END_REF]. Circularity in the way of reasoning maintained the confusion: Oman ophiolite being supposed to derive from a fastspreading centre where faulting (at least down to deep levels of the crust) was supposed to play an anecdotic role relative to igneous accretion, faults observed in Oman were a priori attributed to later events, among other the intra-oceanic thrusting.

However, more detailed studies have revealed the existence of high temperature shear zones, which did not record strike slip or inverse kinematics calling for convergent tectonics. A normal shear sense is actually recorded by many faults among those which have an azimuth subparallel to the one of paleo-ridge axis, including high temperature ductile faults [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF][START_REF] Rospabé | Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone[END_REF].

Moreover, our structural observations and petrological data from the Maqsad area highlight that faults were active early at Moho level, since the magmatic stage, and that they contributed to the petrological and geochemical organization of the lower crust.

Due to the deep introduction of water they triggered, these faults exerted a strong control on the reaction leading to the transformation of mantle harzburgite into dunite at the Moho and on melts migration and crystallization paths, and continued to serve as major avenues for hydrothermal fluids down to lower temperatures [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF][START_REF] Rospabé | Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids[END_REF]. These faults are either parallel to the strike of the sheeted dike complex (i.e. parallel to the regional orientation of the inferred paleo-ridge axis) or slightly oblique to it [START_REF] Rospabé | Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone[END_REF]. They mostly have a ridgeward dip ranging from 65° to 10° and some are clearly listric, with dips progressively shallowing as the crust-mantle boundary is approached, inducing early tilting of blocks of hardly consolidated layered cumulates (Fig. 2). Normal syn-accretion faults highly altered in a wide temperature range have been identified in other massifs of the Oman ophiolite (e.g. [START_REF] Zihlmann | Hydrothermal fault zones in the lower oceanic crust: An example from Wadi Gideah, Samail ophiolite[END_REF] and seem to be the rule rather than the exception. In the case of the fault studied in detail by [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF], the rotation of these blocks of layered cumulates, at high angles relative to the Moho, was accommodated by an anastomosing fault network connected to the main fault plane. It is locally underlined by thin screens of gabbroic micropegmatites, which represent former hydrated evolved melts crystallizing as amphibole-bearing gabbros and by ptygmatic folds, pointing to viscous deformation of a sheared and compacting crystal mush [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF] (Fig. 2). Flat-lying undeformed cumulate layers, in the same differentiation stage as the deformed ones, settled directly over the tilted blocks, which is the main evidence for syn-magmatic block rotation (Fig. 2). In this example, outcrops witnessing synmagmatic deformation exceptionally escaped lower temperature deformation and alteration. This evidences a remarkable lateral evolution in the deformation style on a distance of a few hundred meters. Moving away from the zone where syn-magmatic structures are preserved, i.e. moving toward the main fault that was active down to greenschist facies conditions, shearing continued at sub-solidus temperatures along some fault planes, and brittle deformation structures become more and more prevalent [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF].

Lower crustal gabbros from Oman generally present a quite regular igneous (modal) layering sub-parallel to the crust-mantle boundary ("paleo-Moho"). This monotonous orientation is totally disturbed within several hundred meters from these syn-magmatic normal faults. Minor faults are spaced by a characteristic distance of ~250 m on average, but the major ones may be several kilometers apart. Moving from the top to the base of the crust-mantle transition zone, the deformation conditions evolved from brittle to ductile. Most of these faults were active in a broad temperature interval, with the development of serpentine and carbonate breccias that can be intruded by gabbroic dikes altered in greenschist facies conditions. These brecciated zones reach up to 10 m in thickness in the few major faults and have been zones of intense fluid circulation [START_REF] Rospabé | Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone[END_REF]. The fact that water penetration occurred early during the development of these faults is attested by geochemical gradients continuous on distances reaching a few dozen of meters away from the faults [START_REF] Rospabé | Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone[END_REF].

Concerning the lower temperature water circulation and greenschist facies alteration, the connection with former seafloor hydrothermal vents is attested by the occurrence of Fe, Ni and Cu sulfide mineralizations within these faults [START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF].

A recent survey showed that the Moho steepens in the massifs located close to the front of the Oman ophiolite. This unusual dipping is attributed to a ductile shear zone that was active in very high temperature conditions, close to the gabbro solidus [START_REF] Ceuleneer | Crustal accretion along a giant hightemperature normal fault at the front of the Oman ophiolite: evidence from the Bahla and some other massifs[END_REF]. The lower crustal cumulates are affected by plastic deformation on a thickness reaching 2 km and the along strike extent of the fault reaches 100 km subparallel to the paleo-ridge axis. The curvature of this mega-fault was continentward at the time of accretion. Away from the Moho, the decrease in the deformation intensity is correlated with a progressive flattening of the layering and foliation of the gabbros (listric fault kinematics). This major feature, of regional extent, is interpreted in terms of syn-accretion tectonics [START_REF] Ceuleneer | Crustal accretion along a giant hightemperature normal fault at the front of the Oman ophiolite: evidence from the Bahla and some other massifs[END_REF].

Description of the seismic reflection profiles in the Enderby Basin

Here we gather several examples from different seismic profiles to get an allencompassing view of the typical seismic structures from the unambiguous oceanic crust in the northern part of the survey area to the ocean-continent transition zone to the south.

The northernmost part of the survey area

In the northernmost end of the seismic sections we observe a well-defined oceanic crust characterized by: (1) a smooth flat and reflective top basement, (2) some short reflections in the shallowest part of the crust, (3) a transparent unit in the upper crust located above, (4) a more reflective lower crust, and (5) a set of horizontal reflectors at the base of the crust. We interpret this latter as the oceanic Moho, although various geological structures at the base of the crust may generate high-amplitude reflections (e.g. [START_REF] Collins | Two-dimensional seismic reflection modeling of the inferred fossil oceanic crust/mantle transition in the Bay of Islands Ophiolite[END_REF]. The thickness of the oceanic crust is usually 2-2.2 s TWTT corresponding to ~6.4-7 km (using [START_REF] Canales | Segment-scale variations in crustal structure of 150-to 300-k.y.-Old fast spreading oceanic crust (East Pacific Rise, 8°15'N-10°15[END_REF] relationship between crustal thickness and crustal reflection travel times), which is within the range of normal oceanic crust thicknesses (6 ± 1 km; e.g. [START_REF] Christeson | Synthesis of Oceanic Crustal Structure from Two-Dimensional Seismic Profiles[END_REF]. In some places the crust is thicker than the normal oceanic crustal thickness. This is the case south of the Kerguelen plateau, where the crustal thickness reaches 3.2 s TWTT (~10 km) but also at the western edge of the survey area where the crust reaches 2.5 s TWTT thickness (7.9 km at 60°E) (Supplementary Fig. S1). Both the flat top basement and the thickness of the oceanic crust argue for a formation at a magma-rich spreading center.

Oblique reflectors in the oceanic crust away of the LOC (area 1)

294

To the south, in area 1 (see Fig. 3), the most striking and intriguing features of the 295 seismic reflection profiles are the numerous high-amplitude dipping reflectors in the 296 lower oceanic crust. These deep reflectors occur along sections more than 150 km-297 long. They define a layer of almost constant thickness at the base of the oceanic crust

298

(1.3 s TWTT on average that is ~4.6 km using a 7 km/s for the lower crust) (Electronic 299 Supplement 1). A discontinuous upper horizontal reflector marks locally the top of this 300 lower crustal layer (at 0.7-0.8 s TWTT depth below top basement that is ~1.9 km using 301 a 5 km/s for the upper crust). At the base, the reflection Moho is well marked and 302 defines an oceanic crust of normal thickness (~2-2.1 s TWTT on average) (Fig. 3).

303

The spacing between the deep reflectors is variable, reaching several kilometers.

304

However, in sections where their distribution is more regular, the spacing is typically This dense network of ODRs close to the LOC shows some variability from one profile to the other: e.g. profile GA-228/07 displays triangular-shaped areas with groups of deep reflectors dipping preferentially oceanwards or continentwards (Figs. 3 and4b).

The top basement remains flat in area 2 except in profile GA-229/30. There, it is associated with a wedge of shallow reflectors in the uppermost crust beneath a small asymmetric basin at the seafloor (Supplementary Fig. S4). The ODRs systematically sole out onto the Moho, which is particularly well marked by a single continuous bright reflector (Figs. 3 and7). In this area the Moho is nearly horizontal or slightly dipping continentwards, defining a thicker oceanic crust (~2.4 s TWTT) than in area 1 (Fig. 7). Above the termination of the reflection Moho, an oceanward step-up in the top basement is systematically observed (Fig. 7). It is 0.5 TWTT high (~1000m at 4 km/s for the uppermost crust) on average. Locally, within this basement high, 2-3 shallow reflectors are slightly dipping oceanwards and are recovered progressively by flatter lying reflectors in a fan shape structure, similarly to the OSDRs observed in area 1 (Supplementary Fig. S5).

The transitional domain to the south of the LOC (area 3)

The area 3 (see Fig. 3) to the south of the LOC shows a progressively deeper and rougher top basement reaching >8 s TWTT. Some parts of the top basement still show high reflectivity, but the top basement becomes less reflective continentwards as intense normal faulting results in numerous rotated blocks (see Electronic Supplement 1). These normal faults, mainly dipping oceanwards, are rooting at a discontinuous horizontal reflector at ~1 s TWTT depth beneath the top basement. This reflector has been called UR (for upper reflector) by [START_REF] Gillard | The role of serpentinization and magmatism in the formation of decoupling interfaces at magma-poor rifted margins[END_REF] in contrast to a discontinuous weaker lower reflector (LR) that is observed locally deeper at about 10 s TWTT. Some of these normal faults are sealed by horizontal shallow reflectors in the uppermost basement. Deep reflectors are also observed in area 3 (see Figs. 3 and Electronic Supplement 1). Close to the LOC, one of these reflectors is observed dipping continentwards as deep as 11 s TWTT (Fig. 3).

Discussion

Synthesis of observations and first order interpretation away of the LOC

Both the alternating series of ODRs and CDRs and the local crosscutting relationships between these reflectors contrast with earlier observations of lower crustal reflectors dipping predominantly toward the paleo-ridge axis (e.g. [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF][START_REF] Ding | Structures within the oceanic crust of the central South China Sea basin and their implications for oceanic accretionary processes[END_REF][START_REF] Reston | The structure of Cretaceous oceanic crust of the NW Pacific: Constraints on processes at fast spreading centers[END_REF]. These new observations indicate that the ODRs and CDRs may locally be formed contemporaneously. We describe hereafter, from top to bottom of the oceanic crust, the various reflectors and infer their nature.

We interpret the CDRs that are locally reaching the upper crust as rotated dikes as they are distributed below, perpendicular to the OSDRs that we interpret as lava flows (Fig. 6). This systematic dipping of the shallowest lava flows toward the ridge and the rotation of the sheeted dike complex were described at fast-spreading centers by [START_REF] Karson | Geologic Structure of uppermost oceanic crust created at fast to intermediate-rate spreading centers[END_REF] on the basis of geological investigations of major fault scarps and DSDP/ODP Drill Holes. Evidence of shallow block rotation pre-dating the cessation of the magmatic activity along ridge parallel normal faults (i.e. with the same orientation as the deep faults) were also reported in Oman [START_REF] Macleod | Ridge axial segmentation in the Oman ophiolite: evidence from along-strike variations in the sheeted dyke complex[END_REF].

The flat top basement above the wedges of OSDRs indicates that their formation occurred close enough to the paleo-ridge axis so that the latest lava flows within the neo-volcanic zone at the paleo-ridge axis sealed these wedges that were not deformed subsequently on the ridge flank. The neo-volcanic zone at fast-spreading ridges (zone of lava accumulation and dike intrusion at the ridge axis) is usually very narrow, few kilometers wide but the extent of these wedges within our study area shows that it may reach up to 10 km off-axis.

The rotation of the dikes and lava flows in the uppermost part of the crust must be accommodated below. The apparent sigmoid shape of the CDRs and ODRs and the local occurrence of a horizontal reflector marking the top of the layer of these dipping reflectors suggest a decoupling layer there that could serve both for the CDRs and ODRs in the lower crust and for the rotated dikes above (Figs. 4 and S2). It is located at 0.7-0.8 s TWTT below the top basement (~1.9 km at 5 km/s), which corresponds to the depth of the magma chamber at fast-spreading ridges and therefore to the base of the dike complex. A decoupling level at the base of the dike complex was already suggested by [START_REF] Varga | Paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the Hess Deep Rift: Implications for axial processes at the East Pacific Rise[END_REF].

At the base of the crust, the rooting of the dipping reflectors onto the Moho indicates that this latter corresponds to another decoupling level (Figs. 3 and7). The nearly horizontal reflection Moho together with the smooth top basement suggest that the processes forming these dipping reflectors are restricted to the lower crust as neither the base nor the top of the crust are affected. Considering that water is unlikely to penetrate down to 11.5 s TWTT deep within the mantle beneath a relatively thick magmatic crust, we suggest that the local occurrence of dipping reflectors in the mantle (Fig. 5) is best explained at depth by melt intrusions.

Synthesis of observations and first order interpretation close to the LOC

The dense network of ODRs close to the LOC is not observed elsewhere in the Enderby Basin, suggesting that it is a specific feature at the onset of spreading. The thicker oceanic crust (~0.3 s TWTT more that is ~1 km thicker than normal) closer to the LOC than away shows that the magma supply was up to 15% larger at the onset of spreading than afterwards. This increase of the magma supply was sudden as suggested by the 1 km-high step-up in the top basement above the termination of the reflection Moho. We interpret the pile of shallow reflectors within this step-up as a thick wedge of lavas flowing toward the continent (Supplementary Fig. S5). The base of this wedge could then correspond to the paleo-top basement of a pre-oceanic domain. The loading of volcanic material may have produced the flexure of this existing lithosphere.

The crustal thickness returned normal (2-2.1 s TWTT) after the onset of spreading. It decreased progressively as testified by a flat top basement and a ridgewards rising reflection Moho (see Fig. 3 and Electronic Supplement 1). There, starting from the LOC area, the ODRs become shorter, are progressively replaced by CDRs and finally terminate when the crustal thickness returns to normal. This suggests therefore a control of the occurrence ODRs vs. CDRs by the magma supply at the initial ridge.

This magma supply at the initial ridge may also display some variations. We have shown that locally the set of ODRs is replaced by CDRs just beneath a small asymmetric basin at the top basement and a wedge of lava flows underneath (Supplementary Fig. S4), which suggests a syn-tectonic volcanic building and thus a tectonic control. We therefore propose that there is a tradeoff between magma supply and extensional tectonics within the LOC area and that this tradeoff controlled the changes from ODRs to CDRs. For example, a basin at the top basement with a syntectonic wedge of lava flows underneath would indicate either less magma supply or a higher extensional rate that controls the termination of the set of ODRs. Alternating triangular-shaped areas with preferentially ODRs or CDRs (Fig. 3) would then show that this tradeoff could change over a short time scale. However, the nearly flat top basement above these dipping reflectors, shows that the proximity to the ridge axis may allow the equalization of the overall magma supply on a longer time scale by leveling the relief by the latest lava flows. Without well constrained spreading rates for the Enderby Basin, we cannot estimate these time scales.

Close to the LOC, the dense network of deep oceanward dipping reflectors that systematically sole out onto a sharp and highly reflective Moho suggests that the Moho behaved as a decoupling layer. Moreover, the branching out of the reflection Moho at its continentward termination, following either ODRs or CDRs that are also observed in the transitional domain to the south of the LOC (Supplementary Fig. S5), also suggests a tectonic control of the deepest structures of the lower oceanic crust at the onset of spreading. Therefore we propose that, although the initial spreading center can be largely seen as a magmatic system like the EPR (where plate separation is accommodated by ~2% tectonic strain; [START_REF] Escartín | Interplay between faults and lava flows in construction of the upper oceanic crust: The East Pacific Rise crest 9°25′-9°58′N[END_REF] relative to the adjacent hyper-extended and intruded continental lithosphere [START_REF] Gillard | The role of serpentinization and magmatism in the formation of decoupling interfaces at magma-poor rifted margins[END_REF], the newly formed oceanic crust may be deformed. The denser network of dipping reflectors close to the LOC would then suggest that this tectonic deformation is slightly more intense at the onset of spreading than later, once the oceanic ridge is more mature.

The progressively deeper and rougher top basement to the south of the LOC (Fig. 3)

was interpreted as exhumed mantle with increasing magmatic addition toward the LOC [START_REF] Gillard | The role of serpentinization and magmatism in the formation of decoupling interfaces at magma-poor rifted margins[END_REF]. This is also shown by normal faults that are sealed by lava flows close to the LOC but that offset the top basement far away from it. Such increasing magma volume from the continent toward the ocean with magma addition both on the top and within the basement together with complex magma-fault relationships have already been described in other magma-poor rifted margins [START_REF] Gillard | Birth of an oceanic spreading center at a magma-poor rift system[END_REF].

How do observations in the

Oman ophiolite help to understand the nature and the origin of the lower crustal dipping reflectors? [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF] proposed that the lower crustal dipping reflections within the fast spread crust offshore Alaska arise from shear zones that form near the spreading center. They extensively discuss the physical properties of tectonic and magmatic features that could ultimately be imaged in the lower oceanic crust. They conclude that either solidified melt that was segregated within the shear structures, mylonitization along shear zones, crystal alignment and/or a combination of these processes may result in the bright dipping reflections [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF]. These authors noted, however, that shear zones with scales similar to the lower crustal reflections they imaged have not been observed to date in ophiolites, with special reference to the Oman ophiolite supposed to have formed in a similar tectonic setting than their study area. As seen above (section 2.2), this assertion is contradicted by recent observations in Oman that led to revisiting previous interpretations.

Accordingly, we suggest that syn-magmatic normal faulting revealed by new observations in the Oman ophiolite may correspond to the lower crustal dipping reflectors described in this study. We note similarities with the geometry of the dipping reflectors in the oceanic crust. They both dip either ridgeward or continentward within the lower crust and root at the crust-mantle transition zone. They also both occur within the hot ridge axial zone as evidenced by the latest lava flows sealing the top basement that remains almost flat and undeformed. It is important to note that the overall internal deformation is quite variable in Oman, where we observed faults with small normal displacement and major shear zones with displacement reaching likely one or more kilometers. The thickness of these major shear zones may reach tens of meters and could thus be wide enough to be imaged by seismic reflection techniques. In Oman, the faults appear to be closer spaced than on the seismic reflection profiles but this is likely a question of spatial resolution.

Following [START_REF] Bécel | Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data[END_REF], we hypothesize that the bright reflections observed in our seismic data set might have a hybrid tectonic and magmatic origin. In Oman, both late magmatic intrusions and low-temperature water circulation occur within the synmagmatic fault zones. The water introduction produced mineralogical reactions that were active down to greenschist facies and that are absent from gabbroic cumulates located away of the fault zone. We therefore suggest that these lithological modifications, restricted to the vicinity of syn-magmatic fault zones, may result in velocity and density contrasts sufficient enough to create an impedance contrast at the location of these faults. We further speculate that these processes could thus explain the brightness of the dipping reflectors observed in our seismic reflection profiles.

Another possible interpretation might be looked for in the anisotropy/polarizing effects related to the igneous layering. Away from faults the bedding is quite regular but this regularity is totally lost in the vicinity (hundreds of meters) of syn-magmatic faults, as illustrated by our observations in Oman. This sudden lack of regularity (which goes along with an increase in alteration) might induce contrasts in the seismic wave propagation and be imaged as a reflector. More work is needed to reinforce the comparison between lower crustal dipping reflectors and syn-magmatic faults.

Toward a more complex model of magmatic accretion

Summing up our observations of lower crustal dipping reflectors in the Enderby Basin and their comparison with syn-magmatic and high temperature ductile faults observed in the Oman ophiolite suggest that internal sub-axial deformation processes are more complicated than those shown in most current models of mid-ocean ridges. As a matter of fact, one of the main point revealed by studies in the Oman ophiolite is that very high temperature (up to syn-magmatic), high temperature (sub-solidus plastic deformation) and low temperature (brittle) deformation may coexist along the same fault over distances of several hundred meters at Moho depth. This huge temperature gradient likely results from the juxtaposition and interaction between crystallizing magmas and hydrothermal fluids induced by the episodic nucleation of faults in a context of continuous spreading [START_REF] Rospabé | Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone[END_REF].

A clear effect of crustal deformation is the rotation of originally horizontal lava flows and originally steeply dipping dikes to ridgeward and outward-dipping orientations, respectively (Fig. 8). These rotations are evident in outcrops of the upper crust at fastspreading centers and in the Oman ophiolite [START_REF] Macleod | Ridge axial segmentation in the Oman ophiolite: evidence from along-strike variations in the sheeted dyke complex[END_REF][START_REF] Pallister | Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber[END_REF]. Axial loading of lavas is generally considered to be the driving mechanism for this asymmetrical, subsidence of upper crustal units [START_REF] Dewey | Geometry of plate accretion[END_REF]. This sub-axial subsidence is rapid beneath the ridge axis and decreases up to the edge of the active volcanic zone, a few kilometers for fast-spreading ridges [START_REF] Karson | Geologic Structure of uppermost oceanic crust created at fast to intermediate-rate spreading centers[END_REF]. However, our observations show that, within this axial zone, deformation may affect the underlying deeper gabbroic units as well. Flexure of the upper crustal units in the brittle regime creates accommodation space.

At deeper levels, we suggest that early, syn-to sub-magmatic deformation as well as later brittle deformation is accommodated by syn-magmatic faulting as evidenced by the network of the dipping reflectors in the lower oceanic crust (Fig. 8). We infer a decoupling level at the base of the dikes, as already suggested by [START_REF] Karson | Structure of uppermost fast-spreading oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise: Geochemistry[END_REF] and [START_REF] Varga | Paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the Hess Deep Rift: Implications for axial processes at the East Pacific Rise[END_REF] for the fast-spreading East Pacific Rise, which results in a partitioning of the deformation between the upper and lower crust. Gabbroic sills may have been emplaced beneath the dike complex and not have fully crystallized when the subsidence and deformation occurred in overlying units [START_REF] Karson | Discovering the Deep: A Photographic Atlas of the Seafloor and Ocean Crust[END_REF][START_REF] Yoshinobu | Hypersolidus deformation in the lower crust of the Josephine ophiolite: evidence for kinematic decoupling between the upper and lower oceanic crust[END_REF].

Differences in melt supply along the axis of the fast-spreading East Pacific Rise have been mapped by seismic reflection experiments with pure melt zones, inferred to correspond to regions of fresh magma supply from the mantle asthenosphere, and mush zones, inferred to have undergone cooling and crystallization and to be more evolved [START_REF] Singh | Melt to mush variations in crustal magma properties along the ridge crest at the southern East Pacific Rise[END_REF]. Significant temporal heterogeneities in the magma plumbing system beneath this ridge were also suggested to account for the observed differences between the geochemistry and petrology of the plutonic rocks recovered from Hess Deep and Pito Deep [START_REF] Perk | Petrology and geochemistry of primitive lower oceanic crust from Pito Deep: implications for the accretion of the lower crust at the Southern East Pacific Rise[END_REF]. We suggest that such short timescale changes of magma supply and subsequent thermal structure may influence the lower crustal sub-axial deformation. The predominance of ODRs in the lower crust during high magma delivery at the onset of seafloor spreading suggests that synmagmatic faults rooting at the Moho are a particularly efficient way for accommodating space at the base of the crust in the hot axial zone in addition to new melt intrusions (Fig. 8). The occurrence of CDRs on top of the ODRs and their progressive lengthening and deepening up to the complete replacement of the ODRs while the magma supply turns back to normal would then suggest that CDRs rather result from the compaction and subsidence of the lower crust as it cools. This mode of spreading with somewhat less magma supply is more likely to trigger vertical mass transport. Some of the CDRs that reach locally the upper crust may thus indicate an episodic coupling of the upper and lower crust when the magma budget/pressure is lower. Both ODRs and CDRs are thus contemporaneous and participate to the hydrothermal cooling of the crust but their extent could be related to the tradeoff between magma supply and tectonic deformation. This latter is probably relatively constant at the mature spreading center but may be variable at the onset of spreading.

Conclusions

The main results of our interpretation of high-quality seismic reflection profiles across the ocean-continent transition in the Enderby Basin, between the Kerguelen Plateau and the Antarctic margin, are as follows:

1. While lower crustal events dipping toward the paleo-ridge axis were predominantly described until now, we identified numerous both ridgeward and continentward dipping high-amplitude dipping reflectors in the lower oceanic crust;

2. We suggest that these lower crustal dipping reflectors correspond to synmagmatic faults previously described in the Oman ophiolite; Copyright Commonwealth of Australia (Geoscience Australia).

Fig. 1 :

 1 Fig. 1: Map of the eastern Enderby Basin. The background image is the free air gravity anomaly grid derived from satellite altimetry data (Sandwell et al., 2014). The black dashed line indicates the landward edge of the oceanic crust (LOC) after Gaina et al. (2007). The white dashed line indicates the presumed extinct ridge axis after Gibbons et al. (2013). The squares show the magnetic anomaly picks from Gibbons et al. (2013). The black lines indicate the seismic profiles from Geoscience Australia used in this paper. Their thickened and dashed parts correspond to the sections shown in Fig. 3. The white star indicates the location of Fig. 6. Ker., Kerguelen Island; CIR, Central Indian Ridge; SEIR, Southeast Indian Ridge; SWIR, Southwest Indian Ridge.

Fig. 2 :

 2 Fig. 2: Evidence of syn-magmatic normal faulting and block rotation at Moho level in the Oman ophiolite, Maqsad area (see also Abily et al. (2011)). (a) Simplistic sketches illustrating the possible context of acquisition of the structures observed in the field (left) and the evolution in the conditions of deformation along the fault (right). (b) Main structural elements observed on this outcrop: magmatic layering (thin black lines); ductile faults (thick black lines and arrows), deformed crystal mush layer (red symbols). (c) Closer view on the zone of former crystal mush with complex modal layering induced by crystal sorting and melt injection during viscous deformation. (d) Detail on a ptygmatic fold and on the upper horizon of pegmatite (former melt layer that made possible the mechanical decoupling between the tilted blocks of layered cumulates and the overlying cumulates).

Fig. 3 :

 3 Fig. 3: Line drawing of 4 seismic profiles across the landward edge of the oceanic crust (LOC indicated by the green band). Blue lines indicate oceanward dipping reflectors (ODRs); red ones indicate continentward dipping reflectors (CDRs); yellow ones show nearly horizontal reflectors and orange ones at the top basement indicate oceanic seawards dipping reflectors (OSDRs). The thick orange lines at the bottom of the crust show the reflection Moho and the black lines beneath it show where the Moho is continuous and highly reflective. The blue band indicates the ridgeward end of the dense network of ODRs occurring close to the LOC. Vertical exaggeration is approximately x2. Uninterpreted and interpreted versions of profiles 229/32 with no vertical exaggeration are shown in the Electronic Supplement 1.

305 2 Fig. 4 :

 24 Fig. 4: Examples of dipping reflectors crossing each

Fig. 5 :

 5 Fig. 5: Example of reflectors that are observed both

  At the LOC the Moho branches out in a set of deep reflectors that shallow or deepen 384 parallel to the ODRs and CDRs, respectively (Figs.7 and S5).

385Fig

  Fig. 6: Example of oceanic seawards

Fig. 8 :

 8 Fig. 8: Cartoon of the structure of the oceanic ridge showing the formation of dipping reflectors by subaxial deformation (CDR, continentwards dipping reflectors; ODR, oceanwards dipping reflectors; OSDR, oceanic seaward dipping reflectors). Oblique reflectors in the lower crust are syn-magmatic faults. Isotherms (dotted blue lines) and hydrothermal circulation paths (green line) are schematic. AML: axial melt lens. No vertical exaggeration.

3 .

 3 Evidence for such syn-accretion tectonism at depth together with the systematic rotation of originally horizontal lava flows pre-dating the cessation of the magmatic activity argue for deformation within the axial zone of magmatic spreading centers.

Fig. 1 :Fig. 3 .

 13 Figure captions:Fig.1: Map of the eastern Enderby Basin. The background image is the free air gravity anomaly grid derived from satellite altimetry data[START_REF] Sandwell | New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure[END_REF]. The black dashed line indicates the landward edge of the oceanic crust (LOC) after[START_REF] Gaina | Breakup and early seafloor spreading between India and Antarctica[END_REF]. The white dashed line indicates the presumed extinct ridge axis after[START_REF] Gibbons | The breakup of East Gondwana: Assimilating constraints from Cretaceous ocean basins around India into a best-fit tectonic model[END_REF]. The squares show the magnetic anomaly picks from[START_REF] Gibbons | The breakup of East Gondwana: Assimilating constraints from Cretaceous ocean basins around India into a best-fit tectonic model[END_REF]. The black lines indicate the seismic profiles from Geoscience Australia used in this paper. Their thickened and dashed parts correspond to the sections shown in Fig.3. The white star indicates the location of Fig.6. Ker., Kerguelen Island; CIR, Central Indian Ridge; SEIR, Southeast Indian Ridge; SWIR, Southwest Indian Ridge.

Fig. 2 :

 2 Fig.2: Evidence of syn-magmatic normal faulting and block rotation at Moho level in the Oman ophiolite, Maqsad area (see also[START_REF] Abily | Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite[END_REF]). (a) Simplistic sketches

Fig. 3 :Fig. 4 :

 34 Fig. 3: Line drawing of 4 seismic profiles across the landward edge of the oceanic crust (LOC indicated by the green band). Blue lines indicate oceanward dipping reflectors (ODRs); red ones indicate continentward dipping reflectors (CDRs); yellow ones show nearly horizontal reflectors and orange ones at the top basement indicate oceanic seawards dipping reflectors (OSDRs). The thick orange lines at the bottom of the crust show the reflection Moho and the black lines beneath it show where the Moho is continuous and highly reflective. The blue band indicates the ridgeward end of the dense network of ODRs occurring close to the LOC. Vertical exaggeration is approximately x2. Uninterpreted and interpreted versions of profiles 229/32 with no vertical exaggeration are shown in the Electronic Supplement 1.Fig. 4: Examples of dipping reflectors crossing each other (a: profile 229/32 and b: profile 228/07) defining a layer of almost constant thickness at the base of the oceanic crust. The reflection Moho is well marked almost all along the bottom of the layer. See figure caption and location in Fig. 3. Approximately no vertical exaggeration. Copyright Commonwealth of Australia (Geoscience Australia).

Fig. 5 :

 5 Fig. 5: Example of reflectors that are observed both in the crust and in the mantle as deep as 11.5 s TWTT (line 229/33). Note some rare examples of dipping reflectors that are shallowing but do not reach the top basement as they are covered by long and continuous reflectors. See figure caption and location in Fig. 3. Approximately no vertical exaggeration. Copyright Commonwealth of Australia (Geoscience Australia).

Fig. 6 :

 6 Fig. 6: Example of oceanic seawards dipping reflectors (OSDRs) forming up to ~10 km-long wedges in the uppermost part of the crust (profile 229/31). Same figure caption as Fig. 3. See location in Fig. 1. Approximately no vertical exaggeration.

Fig. 7 :

 7 Fig. 7: Denser network of oceanward dipping reflectors (ODRs) and continentward dipping reflectors (CDRs) (1 reflector every ~500 m) close to the landward edge of the oceanic crust (profile 229/32). Vertical exaggeration is approximately 2x. See figure caption and location in Fig. 3. Copyright Commonwealth of Australia (Geoscience Australia).

Fig. 8 :

 8 Fig. 8: Cartoon of the structure of the oceanic ridge showing the formation of dipping reflectors by sub-axial deformation (CDR, continentwards dipping reflectors; ODR, oceanwards dipping reflectors; OSDR, oceanic seaward dipping reflectors). Oblique reflectors in the lower crust are syn-magmatic faults. Isotherms (dotted blue lines) and hydrothermal circulation paths (green line) are schematic. AML: axial melt lens. No vertical exaggeration.

Acknowledgements

Seismic sections of the AGSO surveys GA-228 and GA-229 have been provided by Geoscience Australia and are published with the permission of Geoscience Australia.

These seismic data are freely available from Geoscience Australia upon request. We would like to acknowledge the support and kindness of Aaron Rockey and the other employees of Geoscience Australia. Gravity grid of Sandwell et al. (2014) is freely available from the website of the UCSD (topex.ucsd.edu/cgi-bin/get_data.cgi). We thank Alexander Golynsky for providing the magnetic grid of the Antarctic margin. We also gratefully acknowledge Jeffrey Karson and the Editor, Jean-Philippe Avouac, for their review and helpful comments. Support was provided by the French Institut National des Sciences de l'Univers INSU-CNRS.