The Gulf of Lion Continental Margin (NW Mediterranean) Revisited by IBS: an Overview

The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview

MICHEL St~RANNE GOophysique Tectonique SOdimentologie, Universit( Montpellier 2, cc. 060, 34095 Montpellier cedex 05, France (e-mail." seranne@dstu, univ-montp2.fr)

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 Oligo- cene-Aquitanian rifted margin and associated Burdigalian oceanic basin have been used as case study for stretching models of 'Atlantic-type' margins. However, when the Inte- grated 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. Com- pilation 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.

'Atlantic-type' divergent passive margins are a formulation of extensional basins in convergent prime site for hydrocarbon accumulation, settings. The Gulf of Lion in the Western because of optimum geological conditions gener- Mediterranean was chosen as a natural labora- ated during their evolution (e.g. Edwards & San- tory where the structure and evolution of a fully togrossi 1989). As new models of basin formation developed rifted continental margin was formed and basin-fill architecture were developed, it during convergence of the African and Eurasian appeared that 'Atlantic-type' models did not plates. The Pannonian Basin is the other natural account for the evolution of many extensional laboratory chosen by IBS where continental margins. For example, a number of extensional extension occurred within the Alps (Horvfith & sedimentary basins are formed during or shortly Tari this volume). The Gulf of Lion (Fig. 1) is after mountain building (e.g. S6ranne & located between the Valencia Trough to the west Malavieille 1994; Cloetingh et al. 1995); in SE and the Provenqal margin of the Ligurian Sea to Asia and in the Mediterranean/Alpine region, the east. These basins represent the rifted conti- continental extension leads to oceanic crust nental margin of SW Eurasia, whose conjugate accretion, whilst the bordering lithospheric margins have drifted southward or southeast- plates converge at rate of several centimetres per wards, above the northwest-dipping subduction year (e.g. Dewey et al. 1989; Rangin et al. 1990). of the African and related plates (Apulia). The geodynamics of such extensional basins This contribution aims to give an overview of imply specific geological conditions controlling the structure and development of the Gulf of oil accumulation, which need to be understood in Lion within the tectonic framework of the order to produce successful exploration models. Western Mediterranean, in the light of the new One of the modules of the Integrated Basin results reached by IBS. The tectonics of the Gulf Studies program addressed the problems of the of Lion are then compared with examples of

SERANNE, M. 1999. The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: An overview. In: DURAND, B., JOLIVET, L., HoRvATN, E & SERANNE, M. (eds) The Mediterranean Basins: Tertiary Extension within the Alpine Orogen. Geological Society, London, Special Publications, 156, 15-36. 16 M. SI~RANNE

Fig. 1. Present-day seismic-tectonic map of the Mediterranean-Alpine region (modified from Reba'f et al. 1992), showing the location of the Oligocene-Recent extensional basins formed during N-S convergence of Africa and Eurasia. classic 'Atlantic-type' margins and of back-arc consequence of rifting and drifting of small basins. (100 km wide) continental blocks. Formation of such extensional basins occurred simultaneously with thrusting and mountain building in the sur- The western Mediterranean setting rounding areas, during continued northward The present-day structure of the Mediterranean motion of Africa with respect to Eurasia. The region (Fig. 1) results from the overall conver- exact kinematics and chronology of the events gence of Africa and Eurasia which involved resulting in the present-day western Mediter- several local and successive rifting and collision ranean is still intensely discussed; the number of episodes during the Tertiary (Le Pichon et al. unsolved questions increases as one goes back 1971; Auzende et al. 1973; Tapponnier 1977; further in time (for example see the contrasting Biju-Duval et al. 1978; R6hault et al. 1984; Der- Eocene reconstructions by Biju-Duval et al. court el at. 1985; Dewey et al. 1989). In the 1978; Boillot et al. 1984; Dercourt et al. 1985; eastern Mediterranean, part of the convergence Dewey et al. 1989; Bois 1993). However there is is still accommodated by northward subduction a consensus on the main features and on the suc- of Mesozoic Tethyan oceanic crust. On the other cession of tectonic stages which makes it poss- hand, the western Mediterranean is floored by ible to sketch out the Tertiary evolution of the oceanic crust formed during Neogene time as a western Mediterranean (Fig. 2).

Fig. 2. Sketch of the geodynamic evolution of the NW Mediterranean since Eocene. Compiled from different sources, including Bellon (1976); Coulon (1977); Girod & Girod (1977); Wildi (1983); Boillot et al. (1984); R6hault et al. (1984); Bergerat (1985); Dercourt et al. (1985); Bouillin (1986); Dewey et al. (1989); Sartori (1990); Bartrina et al. (1992); Mart/et al. (1992); Reba/et al. (1992); Roca & Guimer~t (1992); Ziegler (1992); Keller et al. (1994); Thompson (1994); Tricart et al. (1994); Mauffret et al. (1995); S6ranne et al. (1995); Olivet (1996); Saadallah & Caby (1996). GULF OF LION OVERVIEW 17 18 M. SI2RANNE

Until late Eocene time (Fig. 2a, b), the Iberian transect, the onset of oceanic accretion is dated plate was associated with several continental at latest Aquitanian-early Burdigalian (Fig. 2e) blocks characterized by Hercynian basement: (Burrus 1984)and it is marked in the strati- the Balearics, Corsica, Sardinia, Calabria and graphic record by a break-up unconformity the Kabylies blocks. To the SE, Iberia and the (Gorini et al. 1993). Oceanic accretion in the Hercynian blocks were bordered by a NW- basin allowed the anticlockwise rotation of dipping subduction, south of which extended the Corsica-Sardinia-Calabria. The pole and African plate and derived continental blocks amount of rotation are still under discussion such as Apulia (Dercourt et al. 1985). To the (see review in Vially & Tr6moli6res 1996). north, Iberia and the Hercynian blocks were Palaeomagnetic data suggest a rotation of some bounded by the North Pyrenean Fault Zone, 30 ~ which would have taken place between 21 along which they had been translated eastwards and 19 Ma (Montigny et al. 1981; Burrus 1984). during the late Cretaceous opening of the Bay of However, there is a growing body of evidence Biscay (Olivet 1996). The northward motion of for a longer period of rotation and oceanic Iberia (and associated blocks) resulted in colli- accretion lasting until latest Burdigalian-early sion with the European plate, and the formation Langhian (Fig. 2f) (Vigliotti & Kent 1990; of the Pyrenean thrust and fold belt (Olivet Vigliotti & Langenheim 1995; Chamot-Rooke 1996). The Languedoc-Provenqal segment of et al. this volume). Continental stretching on the the Pyrenees results from the collision of Sar- margins and accretion of some 200 km of dinia-Corsica with Provence. The kinematics oceanic crust is accommodated by equivalent east of this zone and the linkage with the southeastward retreat of the subduction zone Alps-Apennines are not clear, mostly due to the with respect to Europe, while between 35.5 Ma complex 3D geometry of the continental sub- and 19.5 Ma Africa (at the longitude of Algiers) ductions converging in the 'Ligurian knot' had a northwards motion of 160 km (Dewey et (Laubscher et al. 1992). During latest Eocene al. 1989). (Priabonian, Fig. 2b), shortening proceeded Southwest of the North Balearic Transform between Iberia and Europe while E-W exten- Zone, subduction occurred south of the sion initiated intra-continental rift basins in Kabylies blocks. Calc-alkaline volcanism started western Europe (e.g. Rhine Graben, Bergerat in the Valencia Trough in the Aquitanian (Fig. 1985). The intermediate zone of the future Gulf 2d), i.e. with the same delay as the onset of of Lion, was affected by left-lateral strike-slip extension with respect to the Gulf of Lion and tectonics, involving formation of oblique exten- lasted throughout the Burdigalian (Fig. 2e) sional basins in releasing bends. (Mart/et al. 1992; Verg6s & Sabat this volume). From Rupelian time, the Hercynian blocks Continental break-up occurred between the began to drift apart in several stages. During the Balearics and the Kabylies during Langhian first stage, in late Rupelian (Fig. 2c), continen- (Bartrina et al. 1992), leaving the Valencia tal rifting occurred between southern France Trough as an aborted intra-continental rift. The and Corsica-Sardinia-Calabria, while the Kabylies blocks drifted southwards and were Balearics and Kabylies, located SW of the thrust over northern Africa (Wildi 1983; North Balearic Transfer Zone (Mauffret et al. Bouillin 1986). The southward retreat of the 1995) or 'Accident Paul Fallot' (Durand-Delga subduction is confirmed by migration of the calc- & Fontbot6 1980) remained stable. In the alkaline volcanic arc from the Valencia Trough Valencia Trough, rifting was delayed until latest to the Kabylies around Langhian time. Late Oligocene-Aquitanian times (Fig. 2d) (Batrina Miocene to Recent magmatism in the Valencia et al. 1992). Subduction-related calc-alkaline Trough (Fig. 2f-h) is characterized by intraplate volcanism in Provence, Sardinia and the Valen- alkaline magmatism (Mart/et al. 1992). cia Trough (Bellon 1976; Coulon 1977; Girod & The second stage of evolution started during Girod 1977) indicate NW-dipping subduction of the mid-Miocene. After the end of oceanic the Apulian plate beneath Calabria-Sardinia- accretion in the Gulf of Lion and Ligurian Corsica during Chattian to Burdigalian. The basins, and cessation of the rotation of subduction system consisted of an accretionary Corsica-Sardinia, rifting occurred between the prism located in Calabria (e.g. Thompson 1994), latter and Calabria (Fig. 2g, h). Continental and a volcanic arc extending from Sardinia to extension gave way to oceanic accretion in the Provence through the western margin of Tyrrhenian during late Miocene, both extension Corsica (Bellon 1976; Girod & Girod 1977). and axis of accretion migrated southeastward Intra-continental rifting in the Gulf of during Messinian to Pliocene (Kastens et al. Lion-Ligurian basins was therefore in a back- 1988; Sartori 1990). By Tortonian, calc-alkaline arc setting (R6hault et al. 1984). On this volcanism in Sardinia had ceased and was GULF OF LION OVERVIEW 19 replaced by intraplate alkaline volcanism (Fig. were located on structural highs, thus 'missing' 2g) (Bellon 1976; Coulon 1977). Subduction was the syn-rift sedimentary record (Cravatte et aL still active, but the calc-alkaline volcanic arc had 1974); the refraction experiment covered the jumped to the Tyrrhenian Sea. The Calabrian Ligurian Sea and stopped along the eastern side accretionary wedge collided with the continental of the Gulf of Lion margin (Le Douaran et al. Apulian plate during this interval. In the north- 1984); ECORS deep seismic profile, located ern Tyrrhenian, rifting did not reach continental along these ESPs, left unexplored the deepest break-up. Extension migrated eastward, so did rift basins and the corresponding deep crustal compression in the accretionary wedge repre- structure; besides, the Italian equivalent CROP sented by the Apennines (Sartori 1990). profile off Sardinia is not across the exact conju- Outward migration of the subduction system or gate margin (de Voogd et aL 1991). As a result, subduction retreat is recognized throughout the the structure of the Gulf of Lion margin Mediterranean (Jolivet et al. 1994) and it is remained a poorly understood segment of the explained by sinking of the dense lithospheric NW Mediterranean compared to neighbouring footwall of the subduction in the asthenosphere Valencia Trough and Ligurian Sea, as illustrated (Malinverno & Ryan 1986). Doglioni (1991) has by the blanks in structural maps of Mauffret et al. suggested that such process is enhanced by west- (1995). ward-dipping subduction, due to the generalized eastward flow of the mantle with respect to lithospheric plates. Age of rifting: West European rifts versus back-arc basin The recent acquisitions: IBS-Gulf of Lion The Gulf of Lion is seen either as a southern The objectives of IBS-Gulf of Lion were to: (1) extension of the Tertiary West European Rift establish the 2D and 3D tectonics and kinemat- system (e.g. Bergerat 1985; Ziegler 1992) or as ics of extension, (2) define tectono-sedimentary one of the back-arc basins developed above a models of extensional basins, (3)investigate NW-dipping subduction in the western Mediter- thermal and mechanical processes controlling ranean (e.g. Auzende et al. 1973; Biju-Duval et al. lithospheric extension, in such complex settings. 1978; R6hault et al. 1984; Mauffret et al. 1995). Along with other independent projects (e.g. One way to distinguish between the two models Gorini 1993; Guennoc et al. 1994; Mauffret et al. is to document precisely the chronology of events. 1995) one of the aims of the IBS-Gulf of Lion Stratigraphic correlations of the different project, was to re-evaluate, develop and synthe- onshore basins of the southern margin of size existing data. No data acquisition was Eurasia from Valle Penedes to Northern Vat planned except selective field observations. half-grabens (Fig. 3) show remarkable correl- Instead, it was decided: (i) to compile and ation and contemporaneous unconformities homogenize available data (bathymetry, topog- related to otherwise identified geodynamic raphy, gravimetry, heat flow) in the Gulf of Lion events. The West European Rift system initiated and associated Alg6ro-Proven~al oceanic basin; in the Rhine Graben (Fig. 4) during late Eocene (ii) to use recent synthetic structural maps based time and extension migrated northward and on commercial seismic data to be published by southward (Ziegler 1992). The mid-Eocene tec- BRGM, (iii) to re-process the ECORS-Gulf of tonics of this area is characterized by left-lateral Lion deep seismic profile shot in 1988 which was motion along the NNW-striking bordering still largely unexploited, (iv) to update the struc- faults, consistent with N-S 'Pyrenean' contrac- tural study of the onshore part of the margin, tion (Bergerat 1985; Larroque 1987). East-west integrating the newly released seismic surveys. extension started during Priabonian time in the In spite of many studies undertaken during Rhine Graben (Larroque 1987; Maurin 1995), the 1970s and 80s, including thousands of kilo- the Limagne (Morange et al. 1971; Burg et al. metres of industrial and academic seismic profil- 1982; Bl6s et at,. 1989;Busson 1992), and the ing, industrial drillings, which turned the Gulf of Bresse and Valence grabens (Debrand-Passard Lion into a natural laboratory where subsidence & Courbouleix 1984; Busson 1992). The earliest (e.g. Steckler & Watts 1980), and basin models syn-rift sediments of the Camargue basin, only (e.g. Burrus & Audebert 1990) were tested, the known from industrial boreholes, are not dated. formation and evolution of this continental However, these evaporites and shales have been margin was still largely unknown at the begin- correlated with the evaporite series of the ning of the 1990s. Many of the operations under- Valence, Bresse and Rhine basins spanning the taken fell short of the expectations of the Priabonian-early Rupelian periods (Debrand- community. For example, all exploratory wells Passard & Courbouleix 1984; Busson 1992). In 20 M. SI~RANNE GULF OF LION OVERVIEW 21

basins were formed, and earlier Priabonian basins were reactivated (Fig. 4). From these correlations it can be gathered that the West European Rift system was initiated in Priabonian time, during a phase of E-W extension. The large basins (Al6s, Manosque- Forealquier, and probably Camargue) in South- ern France were formed during this time by oblique extension. A second phase of rifting due to NW-SE extension started in late Rupelian in the Gulf of Lion area (from southern France to Sardinia). This later phase of extension is contemporaneous with intraplate alkaline volcanism in Languedoc and onset of calc-alkaline Fig. 3. (a) Tectonostratigraphic correlations of major subduction-related volcanism in Sardinia onshore rift basins of the northern margin of the (Bellon & Brousse 1977). It is concluded that Alg6ro-Provenqal basin, compared with the Sardinia rifting of the Gulf of Lion margin occurred in conjugate margin, and the West European Rift late Rupelian time, in relation to back-arc exten- system. Location of the basins is indicated in Fig. 4. sion, as a geodynamical event distinct from the Time scale from Cande & Kent (1992). Data West European Rift. Spatial overlap of the two compiled from different sources, including Morange events in southern France resulted in the reacti- et al. (1971); Cherchi & Montadert (1982); Debrand- vation of Priabonian basins. Passard & Courbouleix (1984); Valette (1991); Bartrina et al. (1992); Biondi et al. (1992); Busson (1992); Roca & Guimerh (1992); Ziegler (1992); Structure of the margin and modes of Maerten & S6ranne (1995); Benedicto (1996). (b) crustal stretching Key. The Gulf of Lion margin extends SE of the C6vennes Fault (Fig. 5). Extensional structures the Manosque-Forcalquier and A16s basins are predominantly oriented NE, and overprint onset of syntectonic sedimentation along NNE- the Pyrenean thrust and fold belt. The interaction and NE-trending faults respectively, occurred in of inherited Pyrenean structures with extension late Priabonian time, following Pyrenean syn- has been analysed by S6ranne et al. (1995). The tectonic sedimentation (Bartonian) (Debrand- margin is segmented by several transfer fault Passard & Courbouleix 1984; Bergerat 1985; zones sub-parallel to extension. The most repre- Biondi et al. 1992). Finally, in the Campidano sentative segment which is analysed in the follow- basin of Sardinia, which represents the Gulf of ing sections is located between the Arl6sienne Lion conjugate margin, the earliest sediments and S6toise transfer zones (Fig. 5). date from the Priabonian (Cherchi & Montadert Moho depth data (wide-angle refraction and 1982). When Sardinia is replaced in a pre-rift the ECORS deep seismic reflection profiles) are position, Campidano basin-bounding faults mostly restricted to a dip section across the Gulf strike NNE, similarly to the rifts on the Euro- of Lion. The lateral crustal variability intro- pean margin. E-W extensional stress along NE- duced by the along-strike segmentation of the to NNE-trending faults involves oblique exten- margin could only be addressed by a 3D sion with a component of left-lateral strike-slip. approach. Gravity data can be used to obtain Late Rupelian continental syntectonic sedi- reliable geometry of the Moho. Compilations ments were deposited, after a hiatus, above an and homogenization of gravity data over the angular unconformity in small continental Liguro-Proven~al basin resulted in the prep- basins and in earlier basins formed during aration of maps, presented in a companion Eocene (Debrand-Passard & Courbouleix 1984) paper (Chamot-Rooke et al. this volume). A (Fig. 3). The late Rupelian unconformity can be totally new Moho depth map was derived by correlated from southern France to Sardinia; inversion of 3D gravity data. This map was suc- however it is not documented in SW France and cessfully tested against the few available Moho Catalunya, where syntectonic sedimentation depth measurements. related to extension started in late A section across the continental margin (Fig. Chattian-early Aquitanian times (Debrand- 6) allows to distinguish: (1) the poorly deformed Passard & Courbouleix 1984; Bartrina et al. upper-margin, located mostly onshore, (2)the 1992). As a result of NW-SE extension during continental shelf, (3) the slope and (4) the basin late Rupelian, northeast-trending fault-bounded floored by intermediate or oceanic crust. 22 M. SF,RANNE

Fig. 4. Structural map of Priabonian to Aquitanian syn-rift basins from the Rhine Graben to the Valencia Trough. The Gulf of Lion margin is located at the junction of the West European Rift system and the rifts of the NW Mediterranean. Although these basins are often associated in the literature, they differ by the age of rifting onset (Priabonian on one hand, and late Rupelian to Aquitanian on the other hand), structural trend (N-S and NE-SW), and orientation of extensional stress (E-W, and NW-SE). The two systems overlap in the onshore part of the Gulf of Lion, where the Camargue, Albs and Manosque-Forcalquier basins were initiated during Priabonian under E-W extensional stress, and were reactivated during Oligocene-Aquitanian times under NW-SE extensional stress. Compiled from R6hault et al. (1984); Bergerat (1985); Larroque (1987); Bartrina et al. (1992); S6ranne et al. (1995); Benedicto (1996). Bar, Barcelona; Mtp, Montpellier; Mar, Marseille. GULF OF LION OVERVIEW 23

Fig. 5. Structural map of the Gulf of Lion margin. The rift basins are controlled by NE-trending extensional faults overprinting the E-W Pyrenean thrusts in Provence and Languedoc. Thin-skinned extension (light shading) characterizes the onshore part of the margin, whereas the shelf and the slope area are deformed by basement-involved tectonics (dark shading). The margin is segmented by transfer zones parallel to extension. The ECORS seismic profile and ESP's corresponding to the section Fig. 6 are indicated. Ma, Marseille; Ni, Nimes; Mo, Montpellier; Na, Narbonne; Pc, Perpignan. NPFZ, North Pyrenean Fault Zone; MFB, Manosque-Forcalquier Basin; CB, Camargue Basin; AB, Albs Basin; LMB, Les Matelles Basin; HB, H6rault Basin. Modified after Sdranne et al. (1995).

Upper margin. Landward, extensional defor- the shoreline (de Voogd et al. 1991; Chamot- mation is bounded by the C6vennes Fault (Fig. Rooke et al. this volume), suggesting either 6). The Mesozoic-Eocene cover is affected by lower-crustal thinning during Oligocene rifting, NE-striking, SE-facing normal faults bounding or crustal stretching and thinning inherited from Oligocene or Early Miocene basins. The Juras- the Mesozoic rifting. Immediately east of the sic and Neocomian limestones cropping out in considered section, there is a good correlation Languedoc display numerous micro-faults between the isopaches of Mesozoic series which have yielded extensional stress tensors (Debrand-Passard & Courbouleix 1984) and the with a minimum stress axis oriented N120 ~ Moho (Chamot-Rooke et al. this volume) which (Arthaud et al. 1977), consistent with the basin- favours the second alternative. The onshore formation event. Interpretation of industrial Gulf of Lion margin is therefore characterized seismic reflection profiles indicates that the by thin-skinned extensional tectonics of the pre- basin-bounding faults detach above the Palaeo- rift sedimentary cover, which accommodated zoic basement, in Triassic evaporites and shales only several kilometres extension. (Maerten & S6ranne 1995; Benedicto 1996). The underlying continental crust is therefore not Continental shelf. Structurally, the continental affected by extension in this part of the margin. shelf extends mainly offshore, SE of the base- However, the crust is thinning towards the ment-ramp in which thin-skinned extension is margin from 30 km beneath the south Massif transferred. On the section presented (Fig. 6) Central (Sapin & Hirn 1974) to 20 km close to this ramp is a reactivated E-W-oriented 24 M. St~RANNE

Fig. 6. Section of the Gulf of Lion showing the distribution of tectonic style across the margin. Based on field observations and industrial seismic onshore data, and on the ECORS seismic profiled reprocessed during IBS project (Pascal et al. pers. comm.). Moho data from Pascal et al. (1993) and Chamot-Rooke et al. (this volume). Vertical = horizontal scale.

Pyrennean thrust; however the NE-trending have not exhumed deep levels of the crust in the N~mes Fault (Fig. 5) corresponds to this ramp for continental shelf. The hiatus of the Mesozoic most of the width of the studied area. Formation cover accounted by probably initial thin series of the onshore syn-rift Camargue basin was con- which were eroded during the Pyrenean orogeny trolled by the N~mes Fault. This basin comprises (Arthaud & S6guret 1981). Estimates of hori- two half-grabens: the Vistrenque and the Petit zontal extension across the 110 km wide conti- Rh6ne grabens, and presents the thickest (>4000 nental shelf area, computed by the chevron m) and most complete syn-rift succession of the method on the normal faults imaged by the margin (see below). Seismic reflection profiles ECORS profile, range between 15 and 20 km interpretation of the area undertaken during the (1.16 < [3 < 1.22). Taking into account possible course of IBS project has shown that the basin- footwall erosion and faults unresolved by bounding NJmes Fault was a ramp dipping seismic, such figures must be taken as minimal 25-30 ~ SE, into the upper crust; upwards, the values. Present-day mean crustal thickness fault cuts through the Mesozoic cover at a higher (from pre-rift to Moho) of the continental shelf angle (Fig. 6) (Benedicto et al. 1996). Offshore, is 18 km, assuming a beta factor of 1.16 to 1.22, the structures of the continental shelf are known leads to a pre-rift crustal thickness of 25.5 to 27 from the ECORS deep seismic profile, and from kin. Such low value is not in agreement with the industrial seismic reflection (Gorini 1993; Gorini presence of the Pyrenean orogen, constrained et al. 1994). Beneath the continental shelf the by numerous observations (Arthaud & S6guret Moho is 25 to 20 km deep. The major exten- 1981; S6ranne et al. 1995; Mauffret & Gorini sional faults dipping 25-30 ~ can be traced in the 1996; Vially & Tr6moli6res 1996). This discrep- upper crust and merge with the 5 km thick reflec- ancy could result from a phase of post-orogenic tive lower crust (S6ranne et al. 1995) (Fig. 6). extensional collapse (Gaulier et al. 1994) and They strike parallel to the N~mes Fault (Gorini crustal attenuation in late Eocene, prior to et al. 1993) and bound thin (<1 km) and discon- Oligocene rifting characterized by strike-slip tinuous syn-rift basins. The pre-rift basement deformation (Mauffret & Gennesseaux 1989). consists of Palaeozoic low-grade rocks or late Reflective lower crust within which merge the Variscan granite overlain by lower Miocene large extensional faults, as imaged on the sediments (Cravatte et al. 1974; Arthaud et al. ECORS may be compared to BIRPS profiles 1981). It is important to note that (i) high-grade around the British Isles (Klemperer & Hobbs rocks have never been found on the continental 1991), where upper crustal extension by normal shelf, and (ii) the Sirocco granite has not been faulting decouples in the lower crust subjected rejuvenated during either Pyrenean orogeny or to ductile flow (Reston 1990). In that case, a sig- Oligocene rifting (whole rock and mineral Rb/Sr nificant part of crustal thinning in the Gulf of age of 280 Ma) (Cravatte et al. 1974). This indi- Lion continental shelf could be taken up by cates that the major low-angle normal faults ductile stretching of the lower crust. GULF OF LION OVERVIEW 25

The slope. Definition of modes of crustal exten- & Barbier 1987) or in the Galicia margin (Beslier sion in the slope area relies heavily on interpre- et al. 1993). Geometry of upper crustal faults sug- tation of the ECORS-Gulf of Lion profiles; gests extension of 12 km to 17 km, depending on however, the profiles had not been processed the age of the graben in the North Pyrenean further than the initial commercial processing Fault zone (see discussion in Gorini et al. 1993; provided by the operator CGG, which greatly S6ranne et al. 1995; Benedicto et aL 1996) dis- limited the reliability of the interpretations (de tributed over the 50 km wide slope area. Com- Voogd et al. 1991). In the first stage, stack and parison of such moderate extension ratio (1.32 < post-stack FK migrations were performed (Truf- [3 < 1.51) with the important crustal stretching (12 fert et al. 1993) using velocity model derived from km mean crustal thickness over the slope area) ESP data reprocessed by (Pascal et al. 1993). A points to either very thin pre-rift crustal thick- second set of processing consisted in pre-stack ness (16-18 km), or more likely to lower crustal depth migration, that focused on the slope area thinning processes. At the base of the slope, a of the margin (Pascal et al. 1994), which gave rise landward dipping detachment ('T' reflector) to new interpretations and geodynamic impli- (Pascal et al. 1993)cuts across the lower crust. cations, presented in S6ranne et al. (1995). The This detachment exhumes either lower crustal slope area is characterised by a rapid rise of the material from beneath the continental margin Moho between ESP 202 and 203 (Fig. 6), the lack (S6ranne et al. 1995) or mantle material and of highly reflective lower crust above the Moho, exposes it in the deep basin, similarly to models and tilted blocks separated by low-angle faults of lithospheric mantle denudation (Boillot et al. merging into a sub-horizontal reflector. The 1988). The former hypothesis accounts better for latter is interpreted as a detachment similar to the extreme crustal stretching in the slope area the 'S' reflector in the Bay of Biscay (Le Pichon than the latter.

Fig. 7. Tectonic models of rift basins observed on the Gulf of Lion margin. Their relation with the tectonic style allows to predict their position on the margin. See text for comments. 26 M. SI~RANNE

The basin extends basinwards of the emergence basement-cover interface (e.g. H6rault Basin, of the 'T' reflector. The <5 km thick crust pre- or Manosque-Forcalquier Basin, Benedicto sents seismologic features intermediate between 1996). In these basins, syntectonic clastic sedi- continental and oceanic or mantelic material mentation rapidly passes to lacustrine organic- (Pascal et al. 1993; Pascal pers. comm.). The con- rich limestones and marls which have good tinent-ocean boundary has been placed at hydrocarbon potential. However, clastic sedi- different locations (Le Douaran et al. 1984; mentation derived from erosion of Mesozoic Mauffret et al. 1995). According to velocity carbonates provides rather poor quality reser- analyses associated with IBS reprocessing of an voirs (Vially & Tr6moli~res 1996; Mascle & ECORS profile, oceanic crust is identified Vially this volume). Thin-skinned extensional beyond ESP 205. This thin (3-4 km) early tectonics favoured the formation of shallow oceanic crust suggests low ratio of partial sedimentary basins (depth limited by depth of melting in the mantle related to a low potential d6collement) with probable low geothermal temperature, as discussed by Chamot-Rooke et potential (the continental crust was not aL (this volume). The basin basement is there- stretched). The thickest syntectonic deposits are fore an exhumed lower crust or lithospheric expected where the Mesozoic cover was the mantle north of ESP 205 and accreted oceanic thickest, and/or where d6collement levels are crust beyond. into the late Palaeozoic sediments. For example, the A16s depocentre, where the Mesozoic section is over 3 km thick and where d6colle- Extensional basins models ment occurs in the Triassic or late Palaeozoic Reappraisal of existing geological data, backed sediments (Roure et al. 1992), is presently over 2 by newly released industrial seismic reflection km thick and an extra 1 km has been eroded, profiles in the landward end of the margin shows following inversion (Roy & Tr6molibres 1992). that thin-skinned extensional tectonics dominates As a result, organic matter in that basin has the onshore margin (S6ranne et al. 1995; Vially & reached the oil window (Mascle & Vially this Tr6moliSres 1996). Detailed structural and sedi- volume). mentological analyses of the onshore basins (Philibert 1992; Maerten 1994; Benedicto 1996; Hanging-wall syncline. Most of the small basins Sanchis 1997) allowed the definition of several located north of Montpellier (Fig. 5) display a types of thin-skinned extensional basins (Fig. 7). similar architecture (Benedicto 1996). They consist of an asymmetric syncline with the Meso- D~collement basin. An original d6collement zoic-Eocene sequence on the SE limb and a model for the formation of extensional sedi- reduced Neocomian or Eocene steeply dipping mentary basins has been proposed for the NW limb, against a footwall ramp cutting across H6rault Basin (Maerten 1994) (Fig. 7). A synrift Jurassic limestones. In the core of the syncline basin is formed by extensional faulting of the syn-rift sediments present growth structures pre-rift cover above a d6collement level located (Fig. 7). Accommodation results from extension in the Triassic shales and evaporites. The exten- across a ramp in the Mesozoic sequence associ- sional d6collement emerges at surface level ated with a shallow flat in the Neocomian or above inherited high-angle faults down-faulting Eocene sequence, that forms the hanging-wall the basement. These faults were active during syncline. The basin-fill of Les Matelles Basin Mesozoic extension, but remained inactive (Fig. 5) offers the opportunity to validate the during Oligocene rifting. Unlike other interpre- kinematics of hanging-wall syncline formation tations (Route et al. 1992, 1994; Mascle et al. (Ellis & McClay 1988) by reconstructing the 1995), accommodation and depocentre growth structures in association with the evol- migration does not result from folding of a ution of the drainage (Benedicto et al. this hanging-wall flat stripped off the footwall, into a volume). The basin fill is very thin and filled with hanging-wall syncline, but from out-of-sequence proximal clastics derived from the hanging-wall extensional faulting of the pre-rift cover. Conse- flat (Mesozoic and Eocene carbonate) and lacus- quently, offset along the basement-cover inter- trine limestones (Benedicto et al. this volume). face is reduced (2.7 km for the H6rault Basin instead of 6.5 km in a hanging-wall syncline Half-graben on basement ramps. In the domain model), in better agreement with regional sec- of basement involved extension, half-grabens tions (Maerten & S6ranne 1995). These d6colle- are formed along the major normal faults, and ment basins are usually located along the the Camargue basin (Fig. 5) is the most repre- margins of the Mesozoic extensional basin, sentative example. It was formed by extension characterized by important offset of the along the landwardmost extensional basement GULF OF LION OVERVIEW 27

o v~ rm ,..-.,,

o &z

~.~

#~ ~ ~ o~ o~ o ~ ~ ='

= ~- .~

Z 28 M. Sl~RANNE fault of the margin, whose dip-slip offset is sections across the margin (Bessis 1986; Burrus increased by the motion along the d6collement & Foucher 1986; Burrus et al. 1987). They all of the thin-skinned extension. The Camargue found that the amount of post-rift subsidence basin represents the thickest syn-rift depocentre was large with respect to the moderate syn-rift of the margin, comprising 4000 m of Early Oligo- subsidence on one hand, and with respect to the cene and Aquitanian syn-rift deposits. Detailed young age of the rifting on the other hand. In analyses of industrial seismic reflection profiles addition, these studies revealed that uniform and of borehole data (Benedicto et al. 1996) stretching calibrated on present-day crustal show that the N~mes fault, which bounds the thickness adequately accounts for subsidence of graben to the NW, forms a low-angle basement the shelf, whereas the slope and deep basin ramp at depth. The basin fill consists of a trans- records 1 km excess subsidence (Burrus & gressive series ranging from continental lacus- Audebert 1990). Such different patterns of sub- trine silts and lagoonal evaporites to shallow sidence for the shelf and the slope cannot be marine clays (Valette 1991), which presents poor accounted for by a single set of initial pre-rift reservoir qualities (Vially & Trdmoli~res 1996). conditions across the margin (Gaulier et al. The varying mode of deformation and geometry 1994). This is in agreement with the recognition of the hanging wall controlled the development that the slope of the margin extends SE of the of gravity-driven thrusts within the evaporitic North Pyrenean Fault (S6ranne et al. 1995) formation which increased the thickness of which is a terrane boundary separating the halite, presently extracted by Elf-Atochem European and the Sardinia-Corsica litho- (Valette & Benedicto 1995). spheres, giving different rheologies. Basinwards of the Camargue basin, syn-rift New constraints brought by the reprocessing basins also display diverging sedimentary fill in of the ECORS profile (Pascal et al. 1994) and fault-bounded grabens. However, they are much gravity modelling of the Alg6ro-Provenqal basin thinner than the Camargue basin, probably as a (Chamot-Rooke et al. this volume) show that the result of elevated topography at the onset of initial oceanic crust observed at the conti- rifting (Sdranne et al. 1995). Although the syn- nent-ocean boundary was significantly thinner rift sequence has not been drilled, it is likely that than for Atlantic rifted margins (Table 1), which it includes syntectonic clastic sediments interfin- is consistent with small amounts of partial gering with lacustrine limestones representing a melting of the mantle beneath the stretched good potential source-rock. Unlike basins zone, and consequently indicate low potential located on the thin-skinned extensional domain, temperature of the mantle (McKenzie & Bickle siliciclastic sediments derived from the erosion 1988; Chamot-Rooke et al. this volume). Table 1 of the surrounding crystalline and metasedimen- also suggests that the thin oceanic crust in the tary rocks of the basement (Fig. 7), have good basin off the Gulf of Lion margin and the reservoir potential, which make these grabens a average fast spreading rate correspond to values prime target for exploration (Vially & measured in back-arc basins, rather than in the Tr6moli6res 1996). Atlantic. Travel-time tomography in the western Mediterranean (Spakman et al. 1993) shows positive velocity anomaly (up to +2%) in the Atlantic-type margin versus back-arc upper 100 km thick interval centred on the Alg6ro-Proven~al basin, that characterizes an margin upper mantle with temperatures lower than a The width and thickness of the extended conti- radially symmetric velocity model of the Earth. nental crust, the amount of extension, and total Colder mantle may account for the abnormal subsidence of the Gulf of Lion margin does not subsidence of the basin, and could be a result of significantly diverge from these of non-volcanic the >40 Ma long subduction beneath this area. Atlantic-type margins (Table 1). However, Unlike Atlantic-type margins, in the Gulf of rifting is much younger, and the period of rifting Lion the maximum thickness of syn- and post-rift (at most 10 Ma) is shorter than in the Central sediments (7 kin) is located seawards of the con- and South Atlantic continental margins. Conse- tinent-ocean boundary. This is due to the large quently, rates of extension and of subsidence, input of terrigenous sediments that prograde averaged over the duration of margin evolution, across the margin, and reach the closed Alg6ro- are higher than for Atlantic margins. Provenqal oceanic basin. The basin is surrounded Previous subsidence analyses of the Gulf of by zones of active tectonics and of elevated relief, Lion margin consisted of backstripping of com- which induce intense erosion. In addition, the mercial wells (Watts & Ryan 1976; Steckler & Messinian event induced erosion of pre-Messin- Watts 1980), and later, of 2D backstripping of ian sediments deposited in the upper margin and GULF OF LION OVERVIEW 29

Fig. 8. Cartoon showing the asymmetry of the margins in a marginal basin formed by subduction retreat. The margin of the stable continent receives large clastic sedimentary influx from wide drainage areas, whereas the margin of the migrating micro-continent is fed by small drainage basins and volcanoes. Oceanic crust in the back-arc basin is formed by passive upwelling of the asthenosphere, generating small amounts of melt, and thus a thin crust. The upwelled asthenospheric cell follows the outward migration of the subduction, which involves faster cooling (and more important post-rift subsidence) of the divergent margin than on the migrating continental margin. High rates of subsidence and of sedimentation on the side of the stable continent lead, within less than 30 Ma, to the formation of a margin displaying features of a mature Atlantic- type margin. their resedimentation in the basin. The sedi- heat-flow pattern displaying higher values mentary prism is thicker across the NW margin towards the Sardinia margin (Lucazeau & of the Alg6ro-Proven~al basin (Gulf of Lion Mailh6 1986). The fast rate of thermal subsi- margin and Valencia Trough) than across the dence beneath the divergent Gulf of Lion margin Sardinian margin, reflecting the larger continen- results from low potential temperature of the tal drainage area in the NW, including the Rh6ne mantle (Chamot-Rooke et al. this volume). Sub- and Ebro river, whereas the Sardinian margin is sidence is accompanied by large flux of clastic characterized by smaller sedimentation rates, as sediments, which suggests that in spite of the a result of smaller drainage basins. young age of the margin, fast burial may be suf- Figure 8 represents a conceptual model of the ficient to promote maturation on the Gulf of Gulf of Lion margin. It developed as the diver- Lion margin (Burrus & Audebert 1990). In con- gent continental margin that remained fixed to a trast, the drifted Sardinia margin is characterized large continent, whereas the west Sardinia by a lower rate of subsidence and sedimentation. margin represents the margin of a small conti- Like marginal basins such as the Japan Sea and nental block carrying the volcanic arc that drifted the South China Sea, the Gulf of Lion margin has away. Asymmetric structure and development of the same tectonic setting as the Far East Siberia the back-arc basin is related to the outward margin and the Pearl River Basin. migration of the Sardinia continent, induced by the subduction retreat. Southeast migration of Tectonic evolution in the NW the subduction involves the parallel displace- ment of the asthenospheric cell beneath the Mediterranean framework accretionary ridge in the back-arc basin. This Physical experiments of the formation of back- may account for the present-day asymmetric arc basins show that tensile stress occurring in 30 M. SI~RANNE GULF OF LION OVERVIEW 31 the overriding plate is insufficient to break retreat proceeded during this time. Continuous apart the lithosphere if it does not contain a location of the calc-alkaline volcanic arc in weak zone (Shemenda 1994). In the case of the western and central Sardinia from Late Gulf of Lion margin, rifting was superimposed Rupelian through to Serravalian strongly sug- onto an orogenically thickened zone. Onset of gests that the subducted slab kept the same dip extension in the area (late Rupelian, 28-30 Ma) through time. During this interval the Gulf of post-dates the end of the Pyrenean orogeny by Lion margin underwent thermal subsidence some 10 Ma. Post-orogenic relaxation of the over a wide area as witnessed by the Burdigalian geotherms perturbed during latest Cre- marine transgression on the shelf and hinterland taceous-Bartonian (>30 Ma long) thickening (see Fig. 3). event allowed a significant decrease of the inte- During Serravalian-Tortonian times back-arc grated strength of the lithosphere, during 10 extension and oceanic accretion in the Alg6ro- Ma (Cloetingh et al. 1995), leading to the pres- Provencal basin gave way to rifting between Sar- ence of a weak zone underlying the Pyrenees at dinia and Calabria. Concomitantly, the calc- the onset of rifting. Roll-back of the subduction alkaline volcanic arc jumped from Sardinia to an in Late Rupelian induced tensile stress in the area in the present Tyrrhenian sea. As a result, overriding continental lithosphere, which was the volcanic arc lies closer to the trench, sug- expressed by rifting centred on the weak oro- gesting a steepening of the subduction angle. genic zone. This zone is distinct from the vol- Steepening of the subduction must have canic arc set in Sardinia. It is also distinct from increased throughout Late Miocene, as present- the area located in the upper margin, which was day seismicity and tomography image a slab involved in the earlier rifting of the West Euro- dipping up to 70 ~ to the NW beneath the Cal- pean plate, from the Rhine graben to Camar- abrian arc (Spakman et al. 1993; Selvaggi & gue. However, Late Rupelian back-arc Chiarabba 1995). Sinking of the subducted slab extension in the Gulf of Lion overlaps to the in the mantle is accompanied by emplacement of north with the West European rift system, and an asthenospheric wedge at the junction of the reactivates grabens, such as the Camargue overriding plate and the subducted lithosphere. basin, that were initiated during Priabonian Rifting of the Gulf of Lion margin appears as time under E-W extensional stresses. the first stage of a continuous process of subduc- During Late Rupelian to Aquitanian times, tion retreat over 900 km, in ESE direction, the area previously occupied by the Pyrenean developing from Late Oligocene to Present, range was stretched and thinned. Extension and while Africa (at the longitude of Algiers) had a lithospheric thinning was driven by the subduc- northward motion of 350 km. tion retreat and eastward migration of Sar- dinia-Calabria, corresponding to the conditions Conclusion of 'passive rifting' (Keen 1985). This is in agreement with (1)the non-volcanic nature of the The IBS-Gulf of Lion project allowed us to margins, (2) the >200 km width of the rift and realise a modern synthesis of data on a Euro- (3) the rapid cooling and thermal contraction pean basin within the Alpine-Mediterranean away from the rift axis. realm, and gave an opportunity to re-evaluate Continental break-up occurred at the Aqui- the petroleum potential of this area. The tanian-Burdigalian transition, some 50 km IBS-Gulf of Lion project offered an opportunity southeast of the axis of the rift, as evidenced by to reassess the petroleum evaluation of parts of the wider Gulf of Lion margin compared to the the margin, and provided new geological con- West Sardinian Margin. This asymmetry prob- straints on the identification of petroleum ably reflects the southeastwards retreat of the systems (Mascle et al. 1996; Vially & Tr6moli~res subduction and migration of Sardinia-Calabria 1996; Mascle & Vially this volume). Regional during the 10 Ma long rifting interval. reviews (e.g. Deville et aL 1994; Mascle et al. Oceanic accretion in the Alg6ro-Provenqal 1994) have stressed that even if the exploration basin spanned the entire Burdigalian and prob- in the area had little success in the past, the full ably extended into the Langhian, as discussed by evaluation of the onshore and offshore Gulf of Chamot-Rooke et al. (this volume). Subduction Lion margin still remains to be done.

Fig. 9. Suggested geodynamic evolution of the NW Mediterranean from Priabonian to Present along a section across the present-day Gulf of Lion, Alg6ro-Proven~al basin, Sardinia, Tyrrhenian Sea, Calabria and Ionian Sea. This curved line of section is parallel to the direction of extension of each successive rifted basin. See text for comments. 32 M. SI~RANNE

The study produced a much improved know- References ledge of the onshore margin, and a decisive ARTHAUD, F. & SI2GURET, M. 1981. Les structures advance in the understanding of the geodynam- pyr6n6ennes du Languedoc et du Golfe du Lion ics of the NW Mediterranean. The study charac- (Sud de la France). Bulletin de la SociOtO terized a number of new extensional GOologique de France, 23, 51-63. sedimentary basins. Tectonics and kinematics of ---, Mt~GARD, F. & SI~GURET, M. 1977. Cadre tec- an extending crust were studied at various scale tonique de quelques bassins s6dimentaires. Bull- (hundreds of metres to hundreds of kilometres). etin des Centres de Recherche Exploration- Through the development of ECORS data, new Production Elf Aquitaine, 1, 147-188. insight into the structure and processes of defor- , OGLER, M. & SI~GURET, M. 1981. G6ologie et mation of the lower crust were established, in g6ophysique du Golfe du Lion et de sa bordure nord. Bulletin du BRGM, (2), 1, 175-193. particular denudation of the lower crust at the AUZENDE, J. M., BONNIN, J. & OLIVET, J. L. 1973. The ocean-continent transition. Methodology of 3D origin of the Western Mediterranean basin. gravity inversion proved successful in a region Journal of the Geological Society, London, 19, where data is very unevenly distributed and of 607-620. extremely varying quality. There is a number of BARTRINA, M. T., CABRERA, L. L., JURADO, M. L., such regions where the methodology tested in GU1MERA, J. & ROCA, E. 1992. Cenozoic evol- the Gulf of Lion can be applied. ution of the central Catalan margin (Valencia Although the Gulf of Lion margin displays Trough, western Mediterranean). Tectono- structural and stratigraphic features similar to physics, 203, 219-248. BELLON, H. 1976. Sdries magmatiques ndogOnes et qua- 'Atlantic-type' margins, its structure and evol- ternaires du pourtour de la MOditerrande occiden- ution corresponds to that of a rifted margin of a tale, comparOes dans leur cadre g~ochronologique large continent, formed during the opening of a - Implications gdodynamiques. Thbse 3eme marginal basin. Specific features, such as struc- Cycle, Paris Sud - Orsay. tural partitioning in the margin, thin oceanic & BROUSSE, R. 1977. Le magmatisme crust in the basin, high rates of subsidence and pdrim6diterran6en occidental. Essai de synth6se. sedimentation in the lower slope and basin, and Bulletin de la SociOtO Gdologique de France, 19, asymmetry with the conjugate margin can be 469--480. accounted for by a tectonic model of a back-arc BENEDICTO, A. 1996. ModOles tectono-sddimentaires de bassins en extension et style structural de la marge basin developed above a retreating subduction passive du Golfe du Lion (SE France). Th~se de zone. Tectonic evolution of the Gulf of Lion Doctorat, Univ. Montpellier 2. margin corresponds to the early stage of a con- --, LABAUME,P., SI~GURET,M. & SI~RANNE,M. 1996. tinuous process of southeastward retreat of sub- Low-angle crustal ramp and basin geometry in duction or roll back that has taken place in the the Gulf of Lion passive margin: the Oligo- Alpine-Mediterranean region since Oligocene. cene-Aquitanian Vistrenque graben, SE France. For that reason, the results and methodologies Tectonics, 15, 1192-1212. of IBS-Gulf of Lion can easily be exported to --, St~GURET, M. & LABAUME, P. 1999. Interaction other Tertiary Alpine-Mediterranean basins. between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: Further afield, the results and methodologies of example of the Oligocene Matelles Basin, Gulf of IBS-Gulf of Lion could be applied to marginal Lion margin (S.E. France). This" volume. basins of the western Pacific, Central America BERGERAT, F. 1985. 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