Géosciences Montpellier

Abstract This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia–Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj–Sirjan, Kermanshah and Urumieh–Dokhtar magmatic arcs), the P–T–t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW–NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115–85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene–Eocene (60–40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30–0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj–Sirjan Zone: 20–15 Ma, High Zagros: ~12–8 Ma; Simply Folded Belt: 5–0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.

The Variscan belt of western Europe is part of a large Palaeozoic mountain system, 1000 km broad and 8000 km long, which extended from the Caucasus to the Appalachian and Ouachita mountains of northern America at the end of the Carboniferous. This system, built between 480 and 250 Ma, resulted from the diachronic collision of two continents: Laurentia–Baltica to the NW and Gondwana to the SE. Between these two continents, small, intermediate continental plates separated by oceanic sutures mainly have been defined (based on palaeomagnetism) as Avalonia and Armorica. They are generally assumed to have been detached from Gondwana during the early Ordovician and docked to Laurentia and Baltica before the Carboniferous collision between Gondwana and Laurentia–Baltica. Palaeomagnetic and palaeobiostratigraphic methods allow two main oceanic basins to be distinguished: the Iapetus ocean between Avalonia and Laurentia and between Laurentia and Baltica, with a lateral branch (Tornquist ocean) between Avalonia and Baltica, and the Rheic ocean between Avalonia and the so‐called Armorica microplate. Closure of the Iapetus ocean led to the Caledonian orogeny: a belt resulting from collision between Laurentia and Baltica, and from softer collisions between Avalonia and Laurentia and between Avalonia and Baltica. Closure of the Rheic ocean led to the Variscan orogeny by collision of Avalonia plus Armorica with Gondwana. A tectonic approach allows this scenario to be further refined. Another important oceanic suture is defined: the Galicia–Southern Brittany suture, running through France and Iberia and separating the Armorica microplate into North Armorica and South Armorica. Its closure by northward (or/and westward?) oceanic and then continental subduction led to early Variscan (430–370 Ma) tectonism and metamorphism in the internal parts of the Variscan belt. As no Palaeozoic suture can be detected south of South Armorica, this latter microplate should be considered as part of Gondwana since early Palaeozoic times and during its Palaeozoic north‐westward drift. Thus, the name Armorica should be restricted to the microplate included between the Rheic and the Galicia–Southern Brittany sutures.

Detailed study of a 65 cm harzburgite section perpendicular to an amphibole pyroxenite vein from the Lherz massif reveals a strong mineralogical and chemical zonation with distance from the vein-host boundary. At less than ∽ 20 cm, the host peridotite is modally metasomatized and displays patterns of increasing Fe, Ti, Mn, Al, Ca, Na, and HREE, and decreasing Mg and Ni toward the vein contact This zone is also relatively impoverished in Cr but is enriched in K and Sr. It is characterized by relatively unfractionated, mainly convex-upward, chondrite-normalized REE patterns. At a distance over ∽ 20 cm, the host peridotite displays the typical feature of cryptic metasomatism, i.e., selective LREE enrichment in otherwise anhydrous mineralogy. The chondrite-normalized REE patterns vary from U-shaped in the range 15–25 cm to strongly fractionated in the range 25–65 cm. These variations encompass the whole range reported from metasomatized peridotite nodules in alkali basalts. They may be accounted for by a single, silicate-melt, metasomatic event associated with infiltration of the Pyrenean alkali basalts into the most refractory peridotites, during their ascent through the subcontinental lithosphere, ∽ 100 Ma ago. The proposed model involves a chemical evolution of the infiltrated melt with increasing distance in the host. At < 15–20 cm, the melt composition would be strongly influenced by the proximity of the vein conduit, because of the existence of advective chemical fluxes through grain boundaries (short-range porous flow; distance of percolation < 1 m) and into small branching cracks, and a possibly dominant diffusive flux within the infiltrated melt. This may explain the reactivity of the melt towards the anhydrous peridotite mineralogy, the existence of chemical gradients for most elements, and the lack of REE chromato-graphic fractionation. At a distance of > 20–25 cm, chemical exchange with the conduit would be negligible and the melt composition would be mainly controlled by re-equilibration of the peridotite matrix during long-range (> 1 m) porous flow percolation. Thus the melt would be buffered by the amphibole peridotite mineralogy, except for LREE. This may explain the lack of mineralogical reaction and chemical enrichments (except for REE) in this zone, and the chromatographic fractionation of REE. We propose a quantitative model of diffusion and percolation-controlled metasomatism associated with infiltration of alkali basalts into peridotites hosting vein-conduits. We also suggest that silicate-melt percolation may explain mineral disequilibrium features observed in mantle xenoliths.

Plate tectonics, involving a globally linked system of lateral motion of rigid surface plates, is a characteristic feature of our planet, but estimates of how long it has been the modus operandi of lithospheric formation and interactions range from the Hadean to the Neoproterozoic. In this paper, we review sedimentary, igneous and metamorphic proxies along with palaeomagnetic data to infer both the development of rigid lithospheric plates and their independent relative motion, and conclude that significant changes in Earth behaviour occurred in the mid- to late Archaean, between 3.2 Ga and 2.5 Ga. These data include: sedimentary rock associations inferred to have accumulated in passive continental margin settings, marking the onset of sea-floor spreading; the oldest foreland basin deposits associated with lithospheric convergence; a change from thin, new continental crust of mafic composition to thicker crust of intermediate composition, increased crustal reworking and the emplacement of potassic and peraluminous granites, indicating stabilization of the lithosphere; replacement of dome and keel structures in granite-greenstone terranes, which relate to vertical tectonics, by linear thrust imbricated belts; the commencement of temporally paired systems of intermediate and high dT/dP gradients, with the former interpreted to represent subduction to collisional settings and the latter representing possible hinterland back-arc settings or ocean plateau environments. Palaeomagnetic data from the Kaapvaal and Pilbara cratons for the interval 2780-2710 Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for 2700-2440 Ma suggest significant relative movements. We consider these changes in the behaviour and character of the lithosphere to be consistent with a gestational transition from a non-plate tectonic mode, arguably with localized subduction, to the onset of sustained plate tectonics.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

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Abstract The Early Palaeozoic Orogen of SE China consists of three litho‐tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite–granite domes, high‐temperature metamorphism (M2) and granitic plutons related to post‐orogenic crustal melting. We date here the syn‐M1 ductile shearing at 453 ± 7 Ma by U‐Th/Pb method on monazite. Previous ages and our new 40 Ar/ 39 Ar ages of biotites and muscovites show that the metamorphic rocks experienced syn‐M2 exhumation from 440 to 400 Ma. The Early Palaeozoic Orogen of SE China is an intracontinental belt in which decollements accommodated the north‐directed subduction of the Cathaysian continent. This orogen is an example of intracontinental subduction that was not preceded by oceanic subduction.

Pb isotopic studies of airborne particulate matter, incinerator ash, and gasoline have been carried out to determine sources of Pb pollution in urban areas from France and the southern United Kingdom. 206Pb/207Pb ratios in gasoline range from 1.061 to 1.094 (average values are 1.084 for France and 1.067 for the U.K.) while for industrially-derived Pb, 206Pb/207Pb ratios vary from 1.143 to 1.155. Natural Pb is more radiogenic and literature values for pre-industrial sedi ments give 206Pb/207Pb ratios of 1.19−1.20 in France and 1.17−1.19 in the U.K. The measured Pb isotopic signature of airborne particulate matter reflects the relative importance of each of these sources, and samples taken from urban areas close to traffic in France and the U.K. show 206Pb/207Pb ratios that vary widely from 1.085 to 1.158. While alkyl-lead additives in gasoline are typically still the dominant source of Pb in urban particulate matter, the relative importance of gasoline-derived Pb has decreased, and as a result other sources (industrial and natural) can be identified using isotopic studies. This is a consequence of recent EU environmental legislation that significantly limits concentrations of Pb in gasoline and the increased market penetration of unleaded gasoline. In addition, at a given location, the Pb isotopic composition of particulate matter can vary considerably due to temporal variations in sources (i.e., variations in traffic density) and with wind direction.

Abstract Gravimetry is a well-established technique for the determination of sub-surface mass distribution needed in several fields of geoscience, and various types of gravimeters have been developed over the last 50 years. Among them, quantum gravimeters based on atom interferometry have shown top-level performance in terms of sensitivity, long-term stability and accuracy. Nevertheless, they have remained confined to laboratories due to their complex operation and high sensitivity to the external environment. Here we report on a novel, transportable, quantum gravimeter that can be operated under real world conditions by non-specialists, and measure the absolute gravitational acceleration continuously with a long-term stability below 10 nm.s −2 (1 μ Gal). It features several technological innovations that allow for high-precision gravity measurements, while keeping the instrument light and small enough for field measurements. The instrument was characterized in detail and its stability was evaluated during a month-long measurement campaign.

Jasmonates are plant signalling molecules that play key roles in defence against insects and certain pathogens, among others by controlling the biosynthesis of protective secondary metabolites. In Catharanthus roseus, the AP2/ERF-domain transcription factor ORCA3 controls the jasmonate-responsive expression of several genes encoding enzymes involved in terpenoid indole alkaloid biosynthesis. ORCA3 gene expression is itself induced by jasmonate. The ORCA3 promoter contains an autonomous jasmonate-responsive element (JRE) composed of a quantitative sequence responsible for the high level of expression and a qualitative sequence that acts as an on/off switch in response to methyl-jasmonate (MeJA). Here, we identify the basic helix-loop-helix (bHLH) transcription factor CrMYC2 as the major activator of MeJA-responsive ORCA3 gene expression. The CrMYC2 gene is an immediate-early jasmonate-responsive gene. CrMYC2 binds to the qualitative sequence in the ORCA3 JRE in vitro, and transactivates reporter gene expression via this sequence in transient assays. Knock-down of the CrMYC2 expression level via RNA interference caused a strong reduction in the level of MeJA-responsive ORCA3 mRNA accumulation. In addition, MeJA-responsive expression of the related transcription factor gene ORCA2 was significantly reduced. Our results show that MeJA-responsive expression of alkaloid biosynthesis genes in C. roseus is controlled by a transcription factor cascade consisting of the bHLH protein CrMYC2 regulating ORCA gene expression, and the AP2/ERF-domain transcription factors ORCA2 and ORCA3, which in turn regulate a subset of alkaloid biosynthesis genes.

[1] The present-day topography of the Tian Shan range is considered to result from crustal shortening related to the ongoing India-Asia collision that started in the early Tertiary. In this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event. Apatite fission track analysis and (U-Th)/He dating on apatite and zircon indicate that inherited Paleozoic structures were reactivated in the late Paleozoic-early Mesozoic during a Cimmerian orogenic episode and also in the Late Cretaceous-Paleogene (around 65–60 Ma). These reactivations could have resulted from the accretion of the Kohistan-Dras arc or lithospheric extension in the Siberia-Mongolia zone. Activity resumed in the late Mesozoic prior to the major Tertiary orogenic phase. Finally, the ongoing deformation, which again reactivates inherited tectonic structures, tends to propagate inside the endoreic basins that were preserved in the range, leading to their progressive closure. This study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas.

Abstract This paper presents the background for the calculation of physical properties of an aggregate from constituent crystal properties and the texture of the aggregate in a coherent manner. Emphasis is placed on the important tensor properties of 2nd and 4th rank with applications in rock deformation, structural geology, geodynamics and geophysics. We cover texture information that comes from pole figure diffraction and single orientation measurements (electron backscattered diffraction or EBSD, electron channelling pattern, Laue pattern, optical microscope universal-stage). In particular, we provide explicit formulae for the calculation of the averaged tensor from individual orientations or from an orientation distribution function (ODF). For the latter we consider numerical integration and an approach based on the expansion into spherical harmonics. This paper also serves as a reference paper for the mathematical tensor capabilities of the texture analysis software MTEX, which is a comprehensive, freely available MatLab toolbox that covers a wide range of problems in quantitative texture analysis, for example, ODF modelling, pole figure to ODF inversion, EBSD data analysis and grain detection. MTEX offers a programming interface which allows the processing of involved research problems as well as highly customizable visualization capabilities; MTEX is therefore ideal for presentations, publications and teaching demonstrations.

Based on observations from both modem convergent margins and sandbox modeling, we examine the possible conditions favoring frontal accretion and/or frontal and basal tectonic erosion. Mean characteristic parameters (μ, μ* b and λ) are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the Coulomb theory is applicable. Natural observations reveal that “typical accretionary wedges” are characterized by low tapers with smooth surface slope and subducting plate, low convergence rates and thick trench sediment, while “nonaccretionary wedges” display large tapers with irregular surface slopes and rough subducting plate, high convergence rates and almost no trench fill. Sandbox experiments were performed to illustrate the effects of seamounts/ridges in the subduction zone on the deformation of an accretionary wedge. These experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer. This is followed by a tunnelling effect of the subducting seamount through the frontal wedge material, which results in considerable sand reworking. At an advanced subduction stage, the décollement jumps back from a high level in the wedge to its former basal position. We conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion. In contrast, several conditions may favor tectonic erosion of the upper plate. First, oceanic features, such as grabens, seamounts or ridges, may trap upper plate material during their subduction process. Second, destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism.

The long-term isolation of South America during most of the Cenozoic produced a highly peculiar terrestrial vertebrate biota, with a wide array of mammal groups, among which caviomorph rodents and platyrrhine primates are Mid-Cenozoic immigrants. In the absence of indisputable pre-Oligocene South American rodents or primates, the mode, timing and biogeography of these extraordinary dispersals remained debated. Here, we describe South America's oldest known rodents, based on a new diverse caviomorph assemblage from the late Middle Eocene (approx. 41 Ma) of Peru, including five small rodents with three stem caviomorphs. Instead of being tied to the Eocene/Oligocene global cooling and drying episode (approx. 34 Ma), as previously considered, the arrival of caviomorphs and their initial radiation in South America probably occurred under much warmer and wetter conditions, around the Mid-Eocene Climatic Optimum. Our phylogenetic results reaffirm the African origin of South American rodents and support a trans-Atlantic dispersal of these mammals during Middle Eocene times. This discovery further extends the gap (approx. 15 Myr) between first appearances of rodents and primates in South America.

Precambrian terranes and units of the Dahomeyan (now Beninian belt) from Togo, Benin, and southwest Nigeria are described and compared with those from the northernmost part of the Brasiliano belt. The major phases of deformation and metamorphism occurred about 600 Ma in both continents. Foreland nappes derived from passive margin sedimentary beds of the West African craton display high-pressure metamorphism. Parautochthons include slices of an ophiolitic-type assemblage. The suture zone metabasic rocks are only recorded in the Beninian belt. They may represent the granulitized and eclogitized mafic root of ensimatic arc terranes, and have no equivalent in northeast Brazil. The Nigerian Province includes to the west a narrow elongated belt of high-pressure granulites, also found in the northernmost part of the Borborema Province of northeast Brazil. Two main lithostratigraphic units can be recognized in both provinces: (1) gray gneisses that derive from Archean plutonic rocks, thoroughly deformed, recrystallized, and remobilized during the Pan-African–Brasiliano thermo-tectonic events; and (2) Proterozoic monocyclic units displaying the same petrostructural evolution as reworked Archean, and in which we tentatively recognize a Lower Proterozoic group consisting of aluminous metaquartzites and pelitic schists, which were intruded by 2(?) to 1.8 Ga anorogenic granites, and a younger disconformable flysch-type unit of assumed Late Proterozoic age. Large-scale horizontal movements responsible for a flat-lying foliation and nappes affected several domains of the Nigerian and Borborema Provinces, whereas steep structures formed penecontemporaneously along synmetamorphic shear zones. Second-order late-metamorphic steep shear zones are considered as trans-continental lithospheric fractures that overprinted collisional structures. Among these, the 4°50′ Kandi fault and the Sobral fault represent a particularly good correlation of northeast Brazil and southwest Nigeria-Benin, and are used to propose a rigorous Precambrian fit across the south Atlantic Ocean.

IF=3.887

Abstract This paper presents the background for the calculation of various numbers that can be used to characterize crystal-preferred orientation (CPO), also known as texture in materials science, for large datasets using the combined scripting possibilities of MTEX and MatLab®. The paper is focused on three aspects in particular: the strength of CPO represented by orientation and misorientation distribution functions (ODFs, MDFs) or pole figures (PFs); symmetry of PFs and components of ODFs; and elastic tensors. The traditional measurements of texture strength of ODFs, MDFs and PFs are integral measurements of the distribution squared. The M-index is a partial measure of the MDF as the difference between uniform and measured misorientation angles. In addition there other parameters based on eigen analysis, but there are restrictions on their use. Eigen analysis does provide some shape factors for the distributions. The maxima of an ODF provides information on the modes. MTEX provides an estimate of the lower bound uniform fraction of an ODF. Finally, we illustrate the decomposition of arbitrary elastic tensor into symmetry components as an example of components in anisotropic physical properties. Ten examples scripts and their output are provided in the appendix.

[1] The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Béarn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm.

In Vietnam, the Triassic Indosinian collision affected coevally the Truong Son belt and the Kontum Massif,which were not independent tectonic units, but parts of the same Gondwana-derived Indochina continental block. This thermotectonic event took place synchronously throughout Vietnam, during the Lower Triassic 250–240-Ma time interval, as demonstrated by numerous geochronological data, combining Ar–Ar and U–Pb dating methods. Structural and kinematic investigations, in the Indosinian metamorphic rocks, reveal that the collisional process resulted from a consistent northwest-striking convergence of Indochina with respect to the adjacent blocks. It is suggested that this motion was taken up by a pair of opposite subduction zones: to the north, beneath South China, and to the west, beneath western Indochina, along the Song Ma and Po Ko sutures, respectively. Tectonic markers, calc-alkaline subduction-related volcanism and granitic intrusions and the generation of high-pressure rocks that have been recently discovered support this geodynamic setting, at least along Po Ko. Along the northwest-trending Song Ma zone, the obliquity of the convergence during subduction and subsequent collision resulted in the development, within the Truong Son Belt, of a set of subparallel dextral mylonitic shear zones, under amphibolite-facies metamorphism. The intermediate segments remained weakly metamorphic or even almost devoid of metamorphism. Along Po Ko, the convergence was near-orthogonal, with a left-lateral strike-slip component; the ongoing continental subduction resulted in the reworking of the Kontum granulitic basement and the development of Indosinian HP granulitic conditions; the subsequent extension-related exhumation operated approximately in the same northwestwards direction. This Indosinian evolution, applied on a continental crust that had been probably affected, as in South China, by a Caledonian-related event, as judged by the general unconformity of the Lower Devonian sediments, the widespread occurrence of magmatic crystallisation ages of ca 450 Ma (Ordovician-Silurian), and by the approximately similar age of the primary granulitic episode in the Kontum Massif. The similarities of the Devonian facies over central, northern Vietnam and South China imply a land connection, possibly as a consequence of a Caledonian collision along Song Ma, but this does not preclude a further oceanic opening and a closure during the Indosinian.

Sandbox experiments, using a two‐dimensional and a three‐dimensional approach, are used to study the deformation of margins in response to seamount subduction. Successive mechanisms of deformation are activated during the subduction of conical seamounts. First, reactivation of the frontal thrusts and compaction of the accretionary wedge is observed. Then, back thrusting and, conjugate strike‐slip faulting develops above the leading slope of the subducted seamount. The basal decollement is deflected upward in the wake of the subducting high, and a large shadow zone develops behind the seamount trailing slope. Consequently, frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone. When the main decollement returns to its basal level in the wake of the seamount, the margin records a rapid subsidence and a new accretionary wedge develops, closing the margin reentrant. The sediments underthrusted in the wake of the seamount into the shadow zone, are underplated beneath the rear part of the accretionary wedge. Substantial shortening and thickening of the deformable seaward termination of the upper plate basement, associated with basal erosion is observed. Seamount subduction induces significant material transfer within the accretionary wedge, favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin. The subduction and underplating of relatively undeformed, water‐ladden sediments, associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal decollement. Consequently, significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount.

Abstract The Gulf of Lion margin is one of the Tertiary extensional basins of the western Mediterranean that opened during convergence of Africa and Europe. This Oligocene-Aquitanian rifted margin and associated Burdigalian oceanic basin have been used as case study for stretching models of ‘Atlantic-type’ margins. However, when the Integrated Basin Study (IBS) project was initiated, several outstanding questions remained about the present structure and the geodynamic setting of the margin within the Western Mediterranean. IBS-Gulf of Lion research was based on the existing onshore and offshore, industrial and academic data, which were heterogeneous and unevenly distributed. Compilation of the stratigraphic correlations on a regional scale allowed precise calculation of the timing of rifting, and clarification of the relationships with Alpine and Mediterranean geodynamics. Reprocessing of the existing ECORS deep seismic reflection profiles shed new light on the extensional structure and mechanisms of extension of the continental margin. Structural and sedimentological studies onshore led to the definition of new tectonostratigraphic models for extensional basins. Results of structural analyses showed a partitioning of the extensional deformation processes across the continental margin. 3D gravity modelling of the margin and basin area led to the production of a new map of the Moho depth by inversion, and testing several hypotheses for the origin of the present day subsidence. Although the Gulf of Lion margin displays structural and stratigraphic features similar to ‘Atlantic-type’ margins, its structure and evolution corresponds to that of a rifted margin of a large continent formed during the opening of a marginal basin. Integration of the new results of IBS-Gulf of Lion within the geodynamic evolution of the western Mediterranean suggests that the Oligocene rifting of the Gulf of Lion represents the initial stage of a succession of rifting events and back-arc basin formation, due to continuously retreating subduction during convergence of Africa and Europe.