Title: Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin
Abstract: TectonicsVolume 34, Issue 4 p. 662-679 Research ArticleFree Access Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin Laurent Jolivet, Corresponding Author Laurent Jolivet Université d'Orléans, ISTO, UMR, Orléans, France CNRS/INSU, ISTO,UMR, Orléans, France BRGM, ISTO, Orléans, France Correspondence to: L. Jolivet, [email protected] for more papers by this authorChristian Gorini, Christian Gorini Sorbonne Universités, ISTeP UPMC, Paris, France CNRS UMR 7193, Université Pierre et Marie Curie, Paris CEDEX, FranceSearch for more papers by this authorJeroen Smit, Jeroen Smit Sorbonne Universités, ISTeP UPMC, Paris, France Department of Earth Sciences, Utrecht University, Utrecht, Netherlands Now at TNO Geological Survey of the Netherlands, Utrecht, NetherlandsSearch for more papers by this authorSylvie Leroy, Sylvie Leroy Sorbonne Universités, ISTeP UPMC, Paris, France CNRS UMR 7193, Université Pierre et Marie Curie, Paris CEDEX, FranceSearch for more papers by this author Laurent Jolivet, Corresponding Author Laurent Jolivet Université d'Orléans, ISTO, UMR, Orléans, France CNRS/INSU, ISTO,UMR, Orléans, France BRGM, ISTO, Orléans, France Correspondence to: L. Jolivet, [email protected] for more papers by this authorChristian Gorini, Christian Gorini Sorbonne Universités, ISTeP UPMC, Paris, France CNRS UMR 7193, Université Pierre et Marie Curie, Paris CEDEX, FranceSearch for more papers by this authorJeroen Smit, Jeroen Smit Sorbonne Universités, ISTeP UPMC, Paris, France Department of Earth Sciences, Utrecht University, Utrecht, Netherlands Now at TNO Geological Survey of the Netherlands, Utrecht, NetherlandsSearch for more papers by this authorSylvie Leroy, Sylvie Leroy Sorbonne Universités, ISTeP UPMC, Paris, France CNRS UMR 7193, Université Pierre et Marie Curie, Paris CEDEX, FranceSearch for more papers by this author First published: 04 March 2015 https://doi.org/10.1002/2014TC003570Citations: 76AboutFiguresReferencesRelatedInformationPDFSectionsAbstractKey Points1 Introduction2 Geological and Geodynamic Setting3 Interpretation of the TGS-NOPEC Seismic Profile4 Discussion5 Conclusion AcknowledgmentsReferencesCiting LiteraturePDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessClose modalShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong postrift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multichannel seismic profile and other published geophysical data. We show that an 80 km wide domain of thin lower continental crust, the “Gulf of Lion metamorphic core complex,” is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios: the first one involving a simple coupling between mantle extension due to slab retreat and crustal extension and the second one involving extraction of the lower crust and mantle from below the margin by the southeastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc, and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow southeastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup. Key Points Exhumed lower crust and mantle make the distal Gulf of Lion margin Extraction of lower crust achieved by shallow-dipping detachments Slab retreat is the primary engine of extension and asthenospheric flow 1 Introduction The enigmatic nature of the ocean-continent transition (OCT) at magma-poor passive margins is a highly debated topic [Emiliani, 1965; Sibuet et al., 2006]. Recent advances show a transitional domain between clearly continental and clearly oceanic characteristics [Péron-Pinvidic and Manatschal, 2009; Leroy et al., 2010], and depending upon the chosen interpretation, the mechanisms explaining the final rupture of the continental lithosphere may strongly differ [Lavier and Manatschal, 2006; Ranero and Pérez-Gussinyé, 2010; Reston, 2010; Huismans and Beaumont, 2011; Reston and McDermott, 2011]. Several examples of Atlantic margins show a narrow zone of crustal thickness gradient (~50 km), followed seaward by a wide zone of strongly attenuated continental crust (~180 km wide) underlying a thick sag basin with an apparent lack of deformation that started to form in shallow water depth conditions [Contrucci et al., 2004; Moulin et al., 2005]. The presence of a localized weak lower crust may favor the early necking of the lithospheric mantle and spreading of the crust, leading to a final geometry close to that of Atlantic margins [Huismans and Beaumont, 2011]. Other examples show wide zones of exhumed mantle instead, and published scenarios involve the sequential development of several detachments, strongly controlled by inherited lithospheric structures [Whitmarsh et al., 2001; Manatschal, 2004]. Rifting mechanisms may also vary along strike, and passive margins often show a segmentation such as in the Gulf of Aden [Gueguen et al., 1998; Leroy et al., 2010]. The amount of extension often varies with depth, and upper crustal extension can be much smaller than the extension required to explain tectonic subsidence, suggesting that the lower crust has accommodated much more extension [Driscoll and Karner, 1998]. This situation implies lateral migration of material within the crust and thus requires shear zones to accommodate this transfer. The transitional domain between the continental and oceanic crusts can then be composed of exhumed mantle, upper continental crust, or lower crust (see a discussion in Sibuet and Tucholke [2012]). The Gulf of Lion margin shows many of those features typical of Atlantic-type passive margins [Aslanian et al., 2009; Bache et al., 2010], but it evolved in an entirely different tectonic setting associated with back-arc extension [Gorini, 1993; Séranne, 1999]. The Gulf of Lion belongs to the Liguro-Provençal Basin (Figure 1) that formed during the Oligocene and Miocene in the back-arc region of the retreating Apennines subduction [Réhault et al., 1984; Gueguen et al., 1998; Barruol and Granet, 2002; Lucente et al., 2006]. We propose here an interpretation of a deep-penetration multichannel seismic (MCS) profile that shows the ocean-continent transition with unprecedented details, suggesting that most of the crustal thinning has been accommodated by a viscous outward flow of the lower crust and mantle below shallow-dipping detachments, leading to the formation of 80 km wide metamorphic core complex: the Gulf of Lion metamorphic core complex (GoL MCC). The upper and lower contacts of the GoL MCC are two NW dipping detachments that form reflectors visible on the MCS profile. We discuss several scenarios of extension and coupling between crustal and mantle deformation during back-arc extension. Figure 1Open in figure viewerPowerPoint Topographic and bathymetric map of the Mediterranean region showing the position of the main structures, thrust front, subduction zones, the main strike slip, and normal faults. The red arrows show the directions of ductile stretching and shear senses within metamorphic core complexes. The blue thick lines show the smoothed fast direction of SKS waves in the back-arc domain and the blue bars in the vicinity of the subducting slab [Jolivet et al., 2009]. The thin red line marks the location of the TGS-NOPEC MCS profile and the portion of the cross section of Gorini et al. [1994] used in this paper; the dashed black line marks the location of the ECORS profile used by Gorini et al. [1994]. The white dot along the profile marks the position of GLP2 drill hole. GoL: Gulf of Lion. This map shows that in back-arc domains, crustal stretching and asthenospheric flow due to slab retreat are partly coupled and that shear stresses can possibly be transmitted from the flowing asthenosphere to the lower crust. 2 Geological and Geodynamic Setting The Gulf of Lion margin belongs to the Liguro-Provençal Basin and its southern extension in the eastern Algerian Basin (Figures 1 and 2), the largest Mediterranean back-arc basin formed in the Oligocene and Miocene by the rotation of the Corsica-Sardinia block [Auzende et al., 1973; Dewey et al., 1973, 1989] as a consequence of the retreat of the Apennine subduction [Réhault et al., 1984; Gueguen et al., 1998]. After an Oligocene rifting episode (32–24 Ma), oceanic crust was formed in the early Miocene and the base of the middle Miocene until ~15 Ma [Westphal et al., 1976; Vigliotti and Kent, 1990; Gorini et al., 1993; Mauffret et al., 1995; Seranne et al., 1995; Zarki-Jakni et al., 2004; Chamot-Rooke et al., 1999; Séranne, 1999; Speranza et al., 2002; Bellahsen et al., 2012]. The inception of rifting in this region occurred contemporaneously with other Mediterranean back-arc basins [Jolivet and Faccenna, 2000], but in those other cases, back-arc extension only led to the collapse and thinning of a previously thickened crust (Aegean, Alboran, and Northern Tyrrhenian) or the formation of small oceanic basins with a very short oceanic ridge (southern Tyrrhenian) [Le Pichon and Angelier, 1979; Dewey, 1988; Kastens and Mascle, 1990; Nicolosi et al., 2006]. Figure 2Open in figure viewerPowerPoint Depth to prerift map, color map (depth shown in meter) compiled by Mauffret et al. [1995], grey scale map (depth in seconds, two-way travel time) from Rollet et al. [2002]. GLP2: Drill hole Golfe du Lion Profond 2. The red dotted line represents the seismic refraction profile of Gailler et al. [2009], the thin red line marks the location of the TGS-NOPEC MCS profile and the portion of the cross section of Gorini et al. [1994] used in this paper, and the dashed black line marks the location of the ECORS profile used by Gorini et al. [1994]. Figure 3 shows a possible reconstruction in map view at the Oligocene-Miocene transition. The volcanic arc that was active in Sardinia in the Oligocene during the formation of the West Sardinia Graben results from the northward subduction of the Ionian lithosphere below the Corsica-Sardinia block and the future Apennines that form a thickening orogenic wedge. This arc runs obliquely to the Pyrenean-Provence mountain belt that results from the Eocene collision between Iberia and Europe. In the present situation, the Pyrenees end abruptly in the Mediterranean Sea where they are replaced by the Gulf of Lion margin. Remnants of the Eocene belt are found to the north of the Gulf of Lion in the Provençal foreland that was mostly preserved from extension. Figure 3Open in figure viewerPowerPoint Possible map view reconstruction of the western Mediterranean at the Oligocene-Miocene transition adapted from Lacombe and Jolivet [2005]. The GoL metamorphic core complex belongs to a set of metamorphic core complexes cropping out around the western Mediterranean Sea. Most of them correspond to the thinning of a thick crust during the Oligocene-Miocene postorogenic stage. The GoL metamorphic core complex instead results from the thinning of a crust that had not been strongly thickened before, and the extraction of the lower crust by the basal drag of asthenospheric flow led to extreme thinning and thus strong postrift subsidence once the volcanic arc had retreated southeastward following the Apennine slab. The study of SKS shear wave seismic anisotropy below the central and western Mediterranean [Barruol and Granet, 2002; Lucente et al., 2006] and the comparison of the obtained fast directions with the stretching directions observed in the metamorphic core complexes exhumed during the Oligocene and Miocene [Jolivet et al., 2009] suggest that the crust and the asthenospheric mantle were partly coupled during extension and slab retreat and that the kinematics of extension was controlled by asthenospheric flow toward the retreating slab (Figure 1). The mantle fabric cannot be directly dated of course, and it is the parallelism with the well-dated Oligocene-Miocene crustal fabric in all Mediterranean back-arc domains and the compatibility with the flow direction expected from the reconstructions involving slab retreat that led Barruol et al. [2004] and Jolivet et al. [2009] to attribute the mantle fabric to a Neogene phase of asthenospheric flow. The Liguro-Provençal Basin is bordered to the east by the Corsica-Sardinia block and to the north by the Provence margin. Both margins are segmented [Gueguen et al., 1998] with narrow portions (offshore Nice and Corsica) and wider portions (Gulf of Lion and Sardinia) portions. The Gulf of Lion margin is settled on the offshore extension of the Pyrenees and the Provence fold and thrust belt that predated the rifting episode. The continental crust, now extended, had thus been first thickened during the Late Cretaceous and the Eocene. Corsica has also recorded a crustal thickening event, as it was the southern extension of the Alps in the Eocene. The Gulf of Lion was thus in the vicinity of the triple junction between the Pyrenees and the Alps when extension started [Gorini et al., 1994; Vially and Tremolières, 1996; Chamot-Rooke et al., 1999; Séranne, 1999; Lacombe and Jolivet, 2005]. It is also on the southern extension of the West European Rift System. At the time of rifting, a calc-alkaline volcanic arc was active in Sardinia, southern Provence, and the Valencia Trough. It started some 30 Ma ago in Sardinia and Provence and slightly later in the Valencia trough. Magnetic anomalies and seismic velocities show the presence of oceanic crust in the central part of the basin [Le Douaran et al., 1984; De Voogd et al., 1991; Pascal et al., 1993]. The ocean-continent transition is characterized by an enigmatic crust with low-amplitude magnetic anomalies and abnormal velocities that suggest exhumed mantle or lower crustal material [Gailler et al., 2009]. The margin itself has accumulated a thick sedimentary cover, mostly during the postrift stage [Burrus, 1984]. The synrift basins are indeed rather small, and early studies of the Gulf of Lion passive margin have emphasized a paradox between an apparent low stretching of the crust, as indicated by studies of faulting in the upper crust, and a fast thermal subsidence, suggesting instead a strong crustal thinning [Burrus, 1984; Gorini, 1993; Séranne, 1999]. All subsequent works have shown the existence of an 80–110 km wide zone of anomalous thin crust forming the transition between the little extended Provençal continental crust and the oceanic crust of the Liguro-Provençal Basin [Bache et al., 2010]. One cornerstone study was the acquisition of the MCS Etude Continentale et Océanique par Reflexion et Refraction Sismique (ECORS) profile [De Voogd et al., 1991; Pascal et al., 1993; Gorini et al., 1994] and associated refraction data that showed a thin crust with seismic velocities intermediate between continental and oceanic ones [Pascal et al., 1993]. Besides several tilted blocks, oceanward dipping normal faults, and thin synrift sediments, the ECORS profile and its later reprocessing by prestack depth migration show a distinct reflector shallowly dipping toward the continent, interpreted as an antithetic detachment plane [Mauffret et al., 1995; Seranne et al., 1995]. Recent seismic refraction experiments (ocean bottom seismometers (OBS)) suggest that this anomalous crust may be made of lower continental crust [Gailler et al., 2009], but its detailed geometry and exhumation mechanisms are still unknown because the lower part of the margin was so far poorly imaged by seismic profiles. 3 Interpretation of the TGS-NOPEC Seismic Profile Located close to the ECORS profile, the TGS-NOPEC seismic profile gives a new picture of the distal margin. This profile was acquired on the M/V Zephir 1 by DMNG for TGS-NOPEC with a Bold Airgun source and a 480-channel streamer and is here presented as a depth-converted version. First, a Stolt F-K time migration: velocities clipped (4000 m/s) and smoothed (480 common depth point reflections (CDPs)), velocity analysis interactively picked every 1.5 km, a multivelocity stack ±10 % picked function, then a Kirchoff time migration, velocities clipped (6000 m/s), and smoothed (480 CDPs). For more details about depth conversion, please contact TGS as the original acquirers and processors of the processed migration of the seismic line. Existing refraction seismic data acquired in 1990s (expanding spread profiles (ESP)) [Mauffret et al., 1995; Contrucci et al., 2001] are used to obtain propagation velocities in different layers and to identify the nature of crustal units. We have interpreted the whole TGS-NOPEC profile from the recent deposits down to the Moho, but this paper is mainly concerned with the deep portions of the distal margin (Figures 4-7). Once interpreted, the profile has been integrated in a complete section of the margin from the onshore Provençal section to the deep basin (Figure 8), using earlier works [Gorini et al., 1994; Mauffret et al., 1995; Seranne et al., 1995; Séranne, 1999]. Figure 4Open in figure viewerPowerPoint An interpretation of the TGS-NOPEC profile. (top) The seismic profile without interpretation. (middle) Line drawing. (bottom) Interpretation. UD: Upper Detachment, UCLCD: upper crust lower crust detachment, LCMD: lower crust-mantle detachment. Reflectors 1, 2, and 3: see text. A wide domain of lower crust has been exhumed within the continent-ocean transition zone below a series of shallow-dipping detachments forming the Gulf of Lion metamorphic core complex (GoL MCC). Once exhumed, it has then been cut by a series of steep normal faults dipping toward the continent. This geometry explains the contrasts, often noted, of a poorly extended upper crust and a strong finite stretching factor at crustal scale. The shape of the lower crustal body shows that it has been extracted from below the margin toward the ocean-continent transition. The presence of an (bottom) erosion surface at the base of the postrift sequence points to subaerial rather than submarine erosion during the rifting stage and shows that the entire subsidence occurred during the postrift episode as already described in detail by Bache et al. [2010]. Figure 5Open in figure viewerPowerPoint Details of a section of the distal margin located most up-slope and closest to well GLP2. Here the sediments rests directly on upper crustal basement (see details of more distal sections in Figures 6 and 7 and location in Figure 4). (left) The seismic profile without interpretation. (middle) Line drawing (1, 2, and 3 are the reflectors discussed in the text). (right) Interpretation. Figure 6Open in figure viewerPowerPoint Details of a section of the distal margin where the sediments rest directly on exhumed lower crust (see details of more proximal and distal sections in Figures 5 and 7 and location in Figure 4). (left) The seismic profile without interpretation. (middle) Line drawing (1, 2, and 3 are the reflectors discussed in the text). (right) Interpretation. Figure 7Open in figure viewerPowerPoint Details of a section of the distal margin where sediments rests directly on exhumed mantle (see details of more proximal sections in Figures 5 and 6 and location in Figure 4). (left) The seismic profile without interpretation. (middle) Line drawing (1, 2, and 3 are the reflectors discussed in the text). (right) Interpretation. Figure 8Open in figure viewerPowerPoint A cross section of the Gulf of Lion margin including the TGS-NOPEC profile (B), and comparison with the seismic velocities along the ECORS profile (A) [Gorini et al., 1994]. The numbers in the profile are P wave seismic velocities are shown in km/s. Difference in interpretation stems from the spacing of ESP and the lesser quality of the ECORS profile compared to the new profile studied here. Nature and Age of Sedimentary Cover of the Distal Margin The basement is covered with some 8 km of Oligocene to Quaternary sediments. The structure of sedimentary deposits can be summarized as follows [Gorini et al., 1993; Bache et al., 2010]. The synrift (Oligocene-Aquitanian) sequences are covered by the postrift (from Burdigalian to Messinian), and the Messinian evaporites with their transparent facies are easily identified. The Pliocene-Quaternary sequence shows a uniform thickness in the deep margin with evidence for gravitational sliding above the salt layer, including synsedimentary normal faults and rollover structures upslope, as well as folds and diapirs downslope [dos Reis et al., 2008]. Key observations to interpret the deep stratigraphy come from the Golfe du Lion Profond 2(GLP2) hole (location in Figures 1 and 2) located on a basement high, higher up on the margin [Guennoc et al., 2000]: above a metamorphic basement, a first breccia cemented in continental conditions is overlain by Stampian (Rupelian) marine deposits followed by Burdigalian marines clays. Figure 5 shows the details of the deep margin section (see also Figures 6 and 7). Several reflectors separating domains with different seismic facies can be identified. The deepest sediments (yellow) show discontinuous high-amplitude, low-frequency reflections with a fan-shaped suggesting tilting of a crustal block. In the distal part of the profile, several tilted blocks are clearly imaged with synrift sediments in-between. The deepest sediments (yellow) are imaged by discontinuous high-amplitude, low-frequency reflections. These synrift deposits are fan-shaped and restricted to a series of basins, most of which are half-grabens above tilted basement blocks. The tilting of these basement blocks resulted in the apparent seaward onlaps of the synrift sequences. The exact base of the synrift sediments is unclear, and part of the yellow sequence could therefore correspond to reflections from the basement. The thickness of synrift sediments shown in Figures 4 and 5 should thus be taken as a maximum estimate. They are overlain by horizontal sediments (blue) with discontinuous low-amplitude reflections, interlayered with more transparent sediments. The sediments are laterally continuous and do not show any tilting. The basal contact with the synrift breccia (reflector 1) can be followed across a large portion of the profile all the way to the GLP2 drill hole. Clear truncations of the lower series by the upper series across reflector 1 indicate a phase of erosion, probably in subaerial conditions. This erosional surface can also be deduced from truncations higher up on the margin (Figure 6, right side). This phase of subaerial erosion has already been recognized higher up on the margin by Bache et al. [2010]. The blue sequence is then covered by a sequence of more continuous reflections (violet), with high amplitude near the base (reflector 2), present all over the lower part of the profile, grading into more transparent sediments toward the northwest when approaching GLP2, in which these sediments correspond to the Burdigalian marine clays that rest directly on top of the drilled breccia. The blue interval sandwiched between reflectors 1 and 2 is not present in GLP2, and the Burdigalian sediments rest directly on top of the erosion surface and the Stampian marine deposits. One can thus attribute the lower sedimentary sequences seen on the seismic profile to the following intervals, from base to top. The lower sequence, below the erosion surface (reflector 1), corresponds to synrift continental deposits, similar to the continental breccia drilled at GLP2. After a phase of erosion, a first postrift marine flooding has deposited shallow water marine deposits (blue interval), contemporaneous with the Stampian (Rupelian) marine deposits of GLP2. This sequence lasts until the Lower Burdigalian; it corresponds to the initiation of the deep basin. Then, a second flooding event deposited well-stratified sediments, probably starting with shallow water limestones seen as the prominent reflections of reflector 2 and passing progressively upward into deeper marine deposits, regularly stratified, from the Burdigalian upward. The “blue” sequence is deposited on the basement all the way to the southernmost part of the profile, while the “yellow” synrift sequence is not seen in the southernmost half-grabens (Figure 7). The blue interval (Aquitanian to Burdigalian) corresponds to a transition from the synrift to the postrift (syn-ocean-continent transition, syn-OCT sediments hereafter). A similar transition can be seen onland with the same timing on the Provençal margin [Oudet et al., 2010]. On the Sardinian side of the rift, Oligocene (Rupelian-Chattian) subaerial clastics (Ussana formation), contemporaneous with the activity of the volcanic arc, are covered with transgressive early Aquitanian marine deposits [Casula et al., 2001], coeval with the blue interval on the deep Gulf of Lion margin. The seismic basement seen on the distal Gulf of Lion margin was thus covered with synextension continental deposits filling half-grabens before a marine transgression at the end of Oligocene or earliest Miocene. A phase of subaerial erosion is also registered in the distal margin. These observations are in favor of a crustal nature for the basement of the distal margin. Various interpretations have been discussed so far in the literature. Deep Reflections and Nature of Basement Two prominent series of reflectors can be seen in the deep part of the profile below the sediments. The first series of discontinuous and high-amplitude reflections is located below the half-grabens and synrift deposits of the distal margin (lower crust-mantle detachment; Figures 4 and 7). This reflector shows a shallow dip toward the continent and ramps up to the base of the syn-OCT sediments (blue color in Figures 4-7). Farther south, two half-grabens that are filled with the same marine transgressive sequence have an opposite polarity. The point where the lower crust-mantle detachment reflector is unconformably covered with the marine deposits corresponds approximately with the end of a basement block of abnormally high seismic velocities [Pascal et al., 1993] (Figure 8) in the deep part of the distal margin (see discussion below on these abnormal velocities). Typical mantle velocities are reported, and the velocity contrast matches the lower crust-mantle detachment high-amplitude reflections, suggesting that the lower crust-mantle detachment is the local Moho, thus implying that the tilted blocks further to the south are made of upper mantle lithologies. The second series of reflections (Figures 4-7) is observed farther north with a similar low northward dip. Several distinct shallow-dipping discontinuous reflectors with a ramp and flat geometry are similar to a continuous reflector seen on the ECORS 1 and 4 profiles [De Voogd et al., 1991]. This series of reflection is well known as the T reflector, commonly observed on passive margins, sometimes also named the S reflector and interpreted either as the contact between the upper and lower crusts or as an interface within the lower crust [Le Pichon and Barbier, 1987; Gorini et al., 1994; Mauffret et al., 1995; Gernigon et al., 2004]. In the case of the Gulf of Lion, the T reflector is interpreted as the top of an abnormally high-velocity layer that is diversely interpreted as lower crust or a mixture of lower crustal material and serpentinized mantle [Gailler et al., 2009; Bache et al., 2010; Aslanian et al., 2012]. This contact merges toward the northwest with a series of more or less continuous reflectors (upper detachment (UD)) connected to a major shallow-dipping normal fault controlling a syn-ocean-continent transition half-graben. Upslope, the margin is cut by several normal faults bounding upper crustal blocks and synrift sediments (Figures 4 and 7). Previously acquired data on the P wave seismic velocity structure give indications on the nature of the basement in the distal part of the margin, in the transitional domain between the continental margin (domain I) and the characteristic oceanic crust (domain III) (Figures 4). A comparison (Figure 8) between the velocity structure of Pascal et al. [1993] and our interpretation of the Petroceltic International PLC seismic profile shows that the domain of lower continental crust corresponds to a portion of the crust, in which the upper part has quite high P wave velocities (6.6 km s−1), higher than expected for the upper crust, while the lower part has velocities similar to those of the lower crust imaged farther inland. The nature of the crust in this transitional domain between continental and oceanic crusts has been more recently studied and discussed in details by Gailler et al. [2009] (domain II). Data from the expanding spread profiles [Contrucci et al., 2001] and the more recent Sar