structural style and evolution of the gulf of lion oligo ... · structural style and evolution of...

I~UTTERWORT H I~IIE I N E M A N N

Marine and Petroleum Geology, Vol, 12, No. 8, pp. 809-820, 1995 Copyright © 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0264-8172/95 $10.00 + 0.00

Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting: role of the Pyrenean orogeny

M. S6ranne*, A. Benedicto and P. Labaum$ Gdofluides-Bassins-Eau, CNRS-Univisitd Montpellier 2, 34095 Montpellier cedex 05, France

C. Truffertt and G. Pascal Laboratoire de G@ologie, Ecole Normale Sup@rieure, 24 rue Lhomond, 75231 Paris, France

Received I September 1994; revised lOJanuary 1995; accepted 9 May 1995

The Gulf of Lion margin results from the Oligo-Aquitanian rifting and Burdigalian crustal separation between continental Europe and Corsica-Sardinia. Immediately before the onset of extension, the area of the Gulf of Lion was affected by the Pyrenean orogeny which controlled the structural style of the evolving margin. During extension, the foreland of the Pyrenean orogen was affected by extensional thin-skinned tectonics. The d6collement level ramped down into the basement, in areas where the latter was thickened during orogeny. In this intermediate part, the margin was extended by several crustal-scale low-angle faults, which generated small amounts of syn-rift sedimentation compared with the accumulation of post-rift sediments. However, more than 4 km of syn-rift sediments were deposited in the Camargue basin, which is located at the transition between thin- and thick-skinned extensional systems. Kinematic restorations and stratigraphy suggest a pre-rift surface elevation above sea-level of at least 1 km in the intermediate part of the margin, which is in agreement with reduced syn-rift sedimentation. The slope area extends seaward of the North Pyrenean Fault, a terrane boundary inherited from the Pyrenean collision. This part of the margin was stretched by seaward dipping low-angle block tilting of the upper crust, and antithetic lower crustal and sub-crustal detachment. The lithospheric structures inherited from the Pyrenean orogeny exerted a strong control on the kinematics of the rifting and on the distribution and history of subsidence. Such parameters need to be integrated in the definition of pre-rift initial conditions in future basin-modelling of the Gulf of Lion.

Keywords: Pyrenean orogeny; Gulf of Lion; rifting

Continental passive margin studies have been the topic of numerous studies, in parallel with the development of tectonic and subsidence models (McKenzie, 1978; Le Pichon and Sibuet, 1981; Lister et al., 1991). It is now commonly agreed that intra-continental rifts evolve towards passive continental margins, and that the initial stages of subsidence are followed by thermally controlled subsidence. However, from the late 1980s onwards, evidence for extensional collapse and late orogenic extension in general (S6ranne and Malavieille, 1994) has drawn attention to a novel type of extension.

* Correspondence to: Dr M. SEranne tPresent address: BRGM, Service G6ologique National, BP 6009, 45060 Od6ans, France ~: Present address: Laboratoire de Geophysique Interne et Tectono- physique, CNRS-Universit6 Joseph Fourier, BP 53, 38041 Grenoble, France

Extension at the expense of an orogenic lithosphere has important bearings on the choice of initial conditions for subsidence models: thickness, surface elevation, thermal regime and theology are strongly dependent on the pre-rift history. The thermal evolution of associated sedimentary basins and the maturation of organic matter are expected to reflect the effects of geotherms perturbed by the orogenic phase, whereas the in- homogeneous distribution of crustal masses inherited from the mountain-building process determines the location and the geometry of the extensional structures.

The Gulf of Lion continental passive margin results from the Late Oligocene-Miocene rifting and later south-eastward drift of the Corsica-Sardinia block (Figure 1). The margin truncates the eastwards continuation of the Pyrenean range formed during the Palaeo- gene; extensional structures are superimposed onto the thrusts and folds of the Pyrenean foreland. Exploration for hydrocarbons triggered the first geological investi-

Marine and Petroleum Geology 1995 Volume 12 Number 8 809

Structural style and evolution of the Gulf of Lion: M. Seranne et al.

' MassifCentra, ~b~ , ~ ~ ' / ~

~# 41l~* ~of| i d rls e i I I etltltltltltltlt~ ~ ' - . . . . . . . " '

Figure I Location of the Gulf of Lion within the western Mediterranean, with respect to the Pyrenean fold and thrust belt. The pre-rift position of Corsica, Sardinia and the Balearics according to R6hault et al. (1984) is indicated (dotted lines) as well as the boundaries of accreted oceanic crust (grey lines). The deep seismic reflection profiles ECORS-Pyrenees and ECORS- CROP Gulf of Lion are located on this map

gations in the Gulf of Lion, with drilling of several exploratory wells (Cravatte et al., 1974) and syntheses of existing geological and geophysical data (Arthaud et al., 1981; Arthaud and S6guret, 1981). Different subsidence models have been tested against the borehole data in an attempt to explain the anomalously high post-rift subsidence (Steckler and Watts, 1980; Burrus and Foucher, 1986; Bessis, 1986; Kooi et al., 1992). However, these studies assumed a rather 'standard' pre-rift configuration of the lithosphere. Several papers have suggested a genetic relationship between localized Oligocene rifting and the overall convergence of the African and European plates (e.g. Tapponnier, 1977), whereas others have proposed that the Gulf of Lion was a marginal basin above the north-west subduction of the African plate (e.g. R6hault et al., 1984). The area of the Gulf of Lion was chosen for a ECORS deep seismic survey (Burrus et al., 1987; de Voogd et al., 1991), the preliminary results of which have been integrated into regional studies (Gorini et al., 1993). In this paper, we analyse variations in tectonic style across the onshore and offshore part of the continental margin and we investigate the possible role that Pyrenean orogenic structures played in the localization and kinematics of extension in the Gulf of Lion margin.

The structure of the onshore extensional system was assessed by compilation of the published (BRGM) and unpublished geological maps. Additional detailed mapping was carried out in specific areas (e.g. Phili- bert, 1992). Data from onshore industrial boreholes and the nine offshore exploratory wells (Cravatte et al., 1974; Gorini et al., 1993) were integrated. Industrial multichannel seismic reflection surveys in the H6rault, Al~s and Camargue basins (Figure 2) allowed con- straint of the structure at depth. The structural and deep crustal configuration of the offshore part of the margin is defined by the ECORS-CROP deep seismic survey shot in 1988 across the north-west Mediterranean from the Gulf of Lion to Sardinia (de Voogd et al., 1991), which has been reprocessed (Pascal et al., 1994; unpublished data) and by a refraction survey consisting

of 11 expanded spread profiles distributed throughout the eastern part of the Gulf of Lion (Le Douaran et al., 1984, reprocessed by Pascal et al., 1993).

Pre-rift geological setting The major pre-rift structures and stratigraphic features of the study area are now summarized, special attention being paid to those that exerted a strong control on the development of the Oligocene extension.

Late Variscan tectonics The continental basement (Figure 2) consists of Palaeo- zoic rocks deformed during the Variscan orogeny (Devonian to Late Carboniferous). Late Variscan extension activated NE trending and N-S oriented strike slip faults and E - W extensional faults (Arthaud and Matte, 1975), which controlled the formation of Carboniferous and Permian continental basins (Echtler and Malavieille, 1990).

M e s o z o i c extensional ' SE Basin ' Shales and evaporites of the Mid-Triassic period reflect a major transgression over the deeply eroded and peneplained Palaeozoic basement. In the Gulf of Lion area, the continental to marine facies distribution shows that Triassic sediments were deposited in a triangular-shaped bay open to the east, limited to the north-west by the Massif Central and to the south by a continental elevated area extending into the present day Gulf of Lion. This palaeogeographical setting con- tinued throughout the Jurassic, allowing deposition of a succession of marine carbonates and shale intervals, thickening towards the east in the 'Sud-Est Basin' (Beaudrimont and Dubois, 1977). The Jurassic sequence deposition was controlled by the Liassic Tethys rifting, which is expressed by syn-sedimentary extensional faults along the NE trending Massif Central margin (Giot et al., 1991) and E - W trending normal faults along the southern margin, from Provence to Languedoc (Figure 2). Within this triangular-shaped basin, the top of the block-faulted Palaeozoic dips about 10 ° towards the axis of the synform. The Meso- zoic sedimentary cover was thin and discontinuous over the southern Corsica-Sardinia continental block.

Cretaceous wrenching and inversion During early Late Cretaceous opening of the Bay of Biscay and North Atlantic, the eastward drift of the Iberian block was accompanied by left-lateral shear along the North Pyrenean Fault Zone (NPFZ, Figures 1 and 2), crustal thinning along the southern European margin and development of E - W trending strike-slip basins and ridges and HT-LP metamorphism (Choukroune and Mattauer, 1978; Golberg et al., 1986). This zone of shear, separating Europe and Iberia-Corsica-Sardinia extends across the Gulf of Lion. It is likely that the metamorphism of Albian age documented in the basement of the GLP2 well (Figure 2) points to the eastwards continuation of the NPFZ (Gorini, 1993). The deep and narrow basin observed north of GLP2 (Figure 2) could therefore be a Late Cretaceous strike-slip basin reactivated during Cenozoic rifting. Left-lateral motion of Iberia-

810 M a r i n e and P e t r o l e u m G e o l o g y 1995 Vo lume 12 Number 8


Corsica-Sardinia is also responsible for the partial inversion of the Mesozoic basin in Provence and Languedoc. Aerial erosion with westwards increasing amplitude caused the truncation and karstification of Early Cretaceous sediments across uplifted blocks and the deposition of bauxites. In the study area, Early Cretaceous series were partly preserved north of E - W trending basement faults located between Provence and Languedoc.

Pyrenean orogeny Following left-lateral wrenching, the Late Senonian- Palaeogene convergence and collision of the Iberia- Corsica-Sardinia block with the southern margin of the European plate resulted in the Pyrenean orogeny. The belt stretches E - W from the Bay of Biscay to the Gulf of Lion, and from Languedoc to Provence through the NE trending Corbi6res transfer zone (Mascle et al., 1994). The structure of the western segment, i.e. the present day Pyrenees, is now well constrained by ECORS deep seismic reflection (Choukroune and ECORS-Team, 1989; Roure et al., 1989). This profile displays a wide, thickened, south-verging thrust belt on

the Iberian part, which is separated by the NPFZ from the narrower, north-verging European counterpart lying above a subcrustal lithospheric wedge. Shortening of about 100 km is associated with crustal thickening and the development of roots about 50 km below the Iberian plate. In contrast, the eastern segment of the Pyrenees is less well constrained, as most of this thrust belt is now lying under the Gulf of Lion, with only the northern foreland being exposed. The asymmetrical structure of the Pyrenees seems to have flipped across the NE trending transfer zone: wide north-verging, deformed areas extend north of the NPFZ, whereas palinspastic reconstruction of Sardinia (poorly deformed during the Pyrenean orogeny) in its pre-rift position does not allow for a wide southern domain.

The similarity of the Languedoc-Provence belt to the present day southern Pyrenean foreland has already been noted by Arthaud and S6guret (1981). Thin- skinned tectonics above the d6collement level of the Triassic evaporites involves thrusting or folding, depending on the thickness of the Mesozoic series. The direction of thin-skinned thrusting (Figure 2) varies from north-westward in the NE trending transfer zone

Main extensional basins Pre-rift cover deformed during Pyrenean orogeny

Outcropping Paleozoic Paleozoic of the Pyrenean Axial Zone

l r Oceanic crust " I = Intermediate continental crust IIIII Transfer zone • l'--r Main normal faults

Pyrenean thrusts (Eocene) + Z - \ "v=v" Alpine thrusts (Miocene) +

Pyrenean thrusts reactived + . . ~ during Alpine compression~l~ + " Pyreneancompression + ~ Oligocene extension

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Figure 2 Structural map of the Gulf of Lion area, showing the superimposition of the Oligo-Aquitanian extensional structures over the Pyrenean (Eocene) compressional structures. Note the nearly perpendicular directions of compressional arid extensional deformat ions . NFPZ, North Pyrenean Fault Zone; Av., Avignon; Ma, Marseille; Mo, Montpellier; Na, Narbonne; Ni, N~mes; Pe, Perpignan; To, Toulon; AB, Al~s basin; AiB, Aix basin; CB, Camargue basin; GC, Graben Central; GFB, Grand Faraman basin; HB, H~rault basin; and MB, Manosque basin. Offshore boreholes: Ag, Agde marine; Au, Autan 1 ; Be, Beauduc; Ca, Calmar; Ci, Cicindelle; GLP2, Golfe du Lion Profond 2; Mi, Mistral; Ra, Rascasse; Si, Sirocco; and Tr, Tramontane. Regional sect ions s h o w n in this paper are located on this map. ESPs 201 to 205 are indicated along the ECORS 'NW' profile. Geometry of the offshore syn-rift structures modified from Gorini (1993). Directions of compress ion and extension from Arthaud and S6guret (1981), Arthaud et al, (1981), Arthaud and Pistre (1993), Tempier (1987) and Hippolite et al. (1993)


Structural style and evolution of the Gulf of Lion: M. S~ranne et al.

(Gorini et al., 1991; Mascle et al., 1994), to northwards in Montpellier area (Arthaud and S6guret, 1981) and in Provence (Tempier, 1987). In Languedoc and in Provence, Pyrenean foreland structures account for about 25-30 km of shortening (Tempier, 1987). This value is derived from the deformed Mesozoic strata and does not take into account most of the shortening in the Palaeozoic basement of the hinterland (which lacks Mesozoic cover) nor the south-verging, southern part of the belt. Thus it may be assumed that shortening across the Languedoc-Provence segment was of the same order of magnitude as in the Pyrenees, resulting in similar values of crustal thickening.

E - W faults inherited from the Late Cretaceous inversion were reactivated as north-verging thrusts (Arthaud and S6guret, 1981; Tempier, 1987). Base- ment frontal ramps are found close to the present day coastline. The Cap Sici6 thrust (Figure 2, near Toulon), which displays a minimum shortening of 8 km, is the only exposed basement thrust, but basement thrusts are also present in wells (e.g. Cicindelle) and inferred in the construction of seismically controlled sections of southern Carmargue. The ECORS profile also displays a set of SE dipping reflectors that cannot be related to the extensional system, which we interpret as the trace of north-verging thrusts or lateral ramps. Basement ramps are associated with a network of NE trending lateral ramps accommodating left-lateral strike-slip movement (Mauffret and Gennessaux, 1989). Thrust- ing and folding in Provence and Languedoc is commonly dated as Lutecian-Bartonian. However, thrusting may have started earlier, when the syn- tectonic breccia of Cuisian age (Early Eocene) were deposited in front of the Montpellier thrust.

Structure of the landward part of the margin

The landwardmost part of the margin extends south- east of the C6vennes fault which controlled the formation of the Oligocene H6rault and Al~s half-graben basins (Figure 2). They are the larger basins active during the post-Pyrenean rifling phase, the other extensional structures being very small half-graben.

The H6rault basin (Maerten, 1994; Maerten and S6ranne, in press) (Figures 2 and 3) displays a NW dipping monoclinal series unconformably overlying the north-verging Eocene Montpellier thrust. The onset of syn-rift sedimentation occurred during the Stampian in small fault-bounded basins now deeply buried below later syn-rift Aquitanian alluvial plain deposits. Syn-rift basin-fill gives evidence for an overall north-westward migration of depocentres. Burdigalian marine sediments unconformably overlie the previous deposits and onlap pre-rift basement to the SE. Extensional faulting affects the pre-rift Mesozoic cover detached above the Palaeozoic basement, which remained undeformed during rifting. Faulting propagated north-westwards from the Triassic d6collement level, which passed downdip into - - and reactivated in an extensional regime - - the basement ramp of the Montpellier thrust (Figure 3). Depocentres distribution in the H6rault basin provides evidence for a NNW-trending transfer fault across the basin (Maerten and S6ranne, in press), which can also be traced in the Gulf of Lion (Gorini, 1993) and that we informally name the S6toise transfer

zone (Figure 2). .The Al~s basin (Figure 2) shows a rollover and a

syn-tectonic diverging basin-fill, indicating the control by an extensional listric fault that is superimposed onto, and partly reactivates, the C6vennes Fault. Recon- struction of the Oligocene fault shows that it detaches in the Triassic level, whereas earlier studies suggest the existence of a structurally lower detachement in the Carboniferous (Roure et al., 1992). Syn-rift sedimentation unconformably overlies the syn-tectonic compressional Middle Eocene sediments. Extension started earlier (latest Eocene) than in the rest of Languedoc and extended into the Late Oligocene (Alabouvette and Cavelier, 1984). The exclusively continental basin- fill consists of alluvial fans along the NW margin that pass to silt and lacustrine limestones in the axis of the basin. The maturity of organic matter within pre-rift late Cretaceous shales located in the hanging wall suggests a late uplift of several hundred metres (A. Mascle, pers. comm.) that is related to Alpine inversion, and which is not detailed here (see Roure et al., 1992). To the SW, the basin is associated with a NNW trending transfer fault zone, which is identified in the Palaeozoic footwall and whose structural and stratigraphic effects can be recognized across the whole margin (Figure 2). It corresponds to the Arl6sienne transfer fault zone (Gorini, 1993). North-east of this transform zone, the only significant syn-rift basins are located along the C6vennes fault (Al6s basin) and close to the Durance fault (Manosque and Aix basins, Figure 2). Seismic reflection data clearly indicate that these basins have been formed on listric faults that detach above the Hercynian basement (Biondi et al., 1992; Roure et al., 1992) and there is no Oligocene basin overlying the thick and tabular Mesozoic cover which could indicate basement ramps located between the C6vennes and Durance faults. The mechanism by which the continental crust is extended north-east of the Arlesienne transfer fault zone remains an open question.

Les Matelles basin example Les Matelles basin (Figure 4, location on Figure 3) is an asymmetrical NE-trending syncline. The SE limb displays the complete regional stratigraphic series from Mesozoic marine carbonate up to the latest Eocene alluvial deposits ('Ludian', previously dated Early Oligocene (Alabouvette and Cavelier, 1984)). The NW steep limb corresponds to a hanging wall flat resting against the footwall ramp of the Les Matelles fault. The NW limb displays a thin interval of lower Cretaceous marly limestones and lower Eocene marls, and the complete middle Eocene limestone. The uppermost Eocene interval onlaps toward the NW and is not exposed in the NW limb. This gives evidence for pre- Oligocene downthrow of the SE block of Les Matelles fault (Figure 4), due to previous Pyrenean left-lateral strike-slip (Arthaud and S6guret, 1981). The syncline is cored by coarse breccia, which locally unconformably overlie the NW steep limb in a typical syn-tectonic progressive unconformity pattern. These coarse and angular breccia were generated along the NW active margin of the basin at the expense of Lower Cretaceous or Middle Eocene limestones; there is no significant input from the presently exposed Upper Jurassic lime-

812 Marine and Petroleum Geology 1995 Volume 12 Number 8

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stone of the footwall. The small (500 m radius) alluvial fans interfinger with marly shales and conglomerate channel-fill belonging to an axial south-westward flow- ing braided 'fluvial system, or with lacustrine carbonate deposits in the NE end of the basin (Crochet, 1984). These sediments are biostratigraphically well dated as latest Stampian and span a time interval less than one million years (Grambast, 1962; Rey, 1962; Crochet, 1984) The structure of the basin is complicated by the occurrence of slivers of Lutecian limestone - - some tens of metres thick and several kilometres along-strike - - within the syn-rift sediments (Figure 4a). The Lutecian imbricate thrusts overlie and deform the lower part of the Oligocene breccia and they are sealed by the same formation. Clast composition and distribution within the breccia suggests that a significant part of the clasts are derived from the Lutecian thrusts. The deformation pattern at the front of the thrusts indicates a SE vergence and shows that they have been emplaced in the basin during sedimentation of the syn-rift Oligocene (Philibert, 1992). The structure of the basin can be accounted for by thin-skinned extensional tectonics, characterized by a ramp in the massive Upper Jurassic limestones, a fiat in the Lower Cretaceous marly limestones and an emerging ramp in the Middle Eocene limestones (Figure 4b). Seismic reflection and rollover geometry indicate that the ramp across the Upper Jurassic (Les Matelles Fault) is a listric fault that detaches in the Triassic series at a depth of 4000 m. During extension along the Les Matelles Fault, the Lutecian hanging wall flat was transported towards the ramp and formed the hanging wall syncline, where detritus was trapped. Part of the hanging wall flat became detached from the ramp/flat system and was gravitationally emplaced into the basin as an olistolithe, and subsequently eroded and overlain by syn-tectonic syn-rift sedimentation. The imbricate thrusts represent successive reactivations of the surface of gravitational slide, following successive increases of accommodation in the basin due to the activity of the Les Matelles Fault. The structural and stratigraphic

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model of the Les Matelles hanging wall syncline basin applies to most small-scale Oligocene basins of Languedoc.

Structure of the intermediate part of the margin

The intermediate part of the margin extends over 100 km from the Nimes Fault to the trace of the NPFZ in the Gulf of Lion (Figure 2). The thin-skinned extensional system of the NW end of the margin, ramps down into the Nimes Fault. This NE trending fault strikes normal to the extensional direction and acts as a frontal ramp. On this transect, three major normal faults involve the Palaeozoic basement and formed several syn-rift graben. The pre-rift surface corresponds to the Pyrenean erosional unconformity that exhumes allochthonous Mesozoic cover beneath Camargue and Palaeozoic basement beneath the gulf. This intermediate zone is characterized by a large thickness of post-rift series that is not proportional to the thin and discontinuous syn-rift series nor to the amount of crustal thinning. However, due to its structural position on the margin, the 4000 m deep Carmargue basin (Figure 2) stands out as an exception in the group of syn-rift basins.

Camargue basin The Camargue basin is subdivided into the NE trending Vistrenque graben and the associated Petit Rh6ne graben (or Petite Camargue in the offshore along strike continuation) (Figures 2 and 3). The over 4000 m thick Stampian to Aquitanian syn-rift series unconformably overlies Mesozoic carbonates. Stampian sequences comprise thick continental to iagoonal shales and silts overlain by evaporitic sediments characterizing playa environments. The structural repetition of evaporites results from basinward gravity thrusting above the rollover during syn-rift sedimentation (Valette, 1991; Valette and Benedicto, 1995). Overlying Aquitanian shales and silts were deposited in littoral to very shallow marine environments. The entire syn-rift series was thus deposited close to sea level, indicating that sedimentation always balanced accommodation. It further indicates that surface elevation in the Vistrenque basin was close to sea level from Early Oligocene times onwards. A Burdigalian marine sequence onlaps an erosional unconformity developed under subaerial conditions (Valette, 1991), which is interpreted as the break-up unconformity. The upper part of the basin-fill is truncated by the erosional surface corresponding to the Messinian lowstand of the Mediterranean; this unconformity is onlapped by the transgressive Pliocene to Present series.

The structure of the Vistrenque graben is controlled by the NE trending, SE dipping Nimes Fault. Sections across the south-western part of the graben display a typical rollover due to the concave-upward profile of the bounding fault in the upper 3 km of the crust (Figure 3), whereas sections across the north-eastern part of the graben display compensation graben due to a planar attitude of the fault (Valette and Benedicto, 1995). Syn-rift series of the Vistrenque basin extend over the entire down-faulted block, beyond the hanging wall crest. This indicates subsidence due to the low angle (25 °) Nimes Fault, which is planar from 3 km


Structural style and evolution of the Gulf of Lion." M. S~ranne et al.

Present-day situation

NW SE Messinian erosion

Syn-rift fillin~ Pyrenean Thrust

NTmes Fault ~ ~ ? ~ ~ [ O~,~:~5km

Ramp-flat fault No ore-rift relief

Ramp fault Pre-rift relief =1250m

Figure 5 Sketch of a simplified present day section in the Camargue basin, showing the geometrical relationships between extensional fault, previous thrust and partially preserved syn-rift infill (sediments deposited close to sea level). The two possible fault profiles imply two pre-rift hanging wall topographies

downwards and that penetrates the basement to at least 10-km depth (Figure 3). In contrast, the Petit-RhOne basin that developed in the hanging wall of the Nimes Fault is characterized by a lack of syn-rift sedimentation beyond the rollover crest. The bounding fault is a listric ramp which flattens in the Triassic dOcollement and it is associated further to the SE with a ramp in the basement.

Pre-rift restoration An E - W structural high extending along the shoreline corresponds to a Pyrenean thrust in the Mesozoic

cover. Such a high in the hanging wall of the N~mes Fault may (i) be the result of the flattening of the fault profile at depth or (ii) pre-date the extensional structure (Figure 5). The Messinian erosion prevents observing the geometry of the syn-rift sequences against this high. Restoration of the extensional fault in the case of a planar low angle fault (as suggested by seismic reflection) indicates a pre-rift relief south-east of the Vistrenque basin, in excess of 1000 m, that would be inherited from northwards Pyrenean thrusting. Thrusting and erosional denudation of the Palaeo- zoic basement becomes prominent south of the Camargue basin. Pre-rift sedimentary cover is extremely reduced in the nearshore boreholes and Palaeozoic rocks subcrop beneath syn- or post-rift sequences in the offshore wells. Restoration of the extensional motion on the three major basement faults in Camargue (Figure 6) points to a pre-rift topography having an elevation of about 2000 m. These estimates possibly include errors due to (i) the lack of isostatic compensation and (ii) the lack of a precise pre-rift topographic reference datum as the area has been later truncated by the Messinian erosion; however, they provide a valu- able indication of the location and order of magnitude of the pre-rift relief.

Crustal sections From the shoreline to an area between ESP201 and ESP202, the ECORS profile (Figure 7A) displays a flat Moho reflection, at about 20 km depth, and a 5 km thick reflective lower crust. The reflections corresponding to the major faults that affect the upper crust either disappear above or merge with the reflective crust (Figure 714). Major faults dip basinward, with the exception of the steep fault bounding the Vistrenque basin, at the NW end of ECORS. This landward dip-

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Structural style and evolution of the Gulf of Lion: IV/. S~ranne et al.

ping fault could be interpreted as an antithetic fault of the Nimes Fault (Guennoc et al., 1994). However, the available data do not suggest the existence of a major SE dipping normal fault between the NW end of ECORS and onshore outcrops. The Nimes Fault seems to die out south-eastwards before it reaches the S6toise transfer zone and deformation is taken up by the landward-dipping normal fault. According to the ECORS section, the fault bounding the Petite Camargue basin has a listric profile rooted in a mid-crustal level, which is in agreement with the rollover of the pre-rift. This Petite Camargue Fault correlates north-eastwards with the fault bounding the Petit Rh6ne basin (Figure 7B). Transition from a listric basement ramp profile in the SW (Figure 7A) to a listric-flat-basement ramp in the NE (Figure 7B) is achieved through the reactivation of the Pyrenean thrust as lateral ramp, north of Cicindelle well. In addition, a major NE trending, low-angle (25°), intra-crustal planar reflector has been mapped offshore (Gorini et al., 1993) bounding the Grand Faraman basin (Figure 7A), and which correlates with the major extensional fault beneath Beauduc well (Figure 7B).

A number of reflectors which are apparently un- related to Cenozoic graben (below ESP201) and are observed at middle crustal levels (SE of Sirocco) are interpreted as Pyrenean north-verging thrusts. In addition, the tectonic contact which accommodates the thin-skinned extension in the H6rault basin corresponds to the basement ramp of the previous Pyrenean Montpellier thrust (motion oblique to the section in Figure 7A). Major low-angle extensional faults bound half-graben, the sedimentary fill of which displays divergent and partly onlapping reflectors interpreted as syn-tectonic sediments. They are truncated by a break- up unconformity attributed to the earliest Burdigalian (Valette and Benedicto, 1995) or the latest Aquitanian (Gorini et al., 1993). Depth conversion of the profile using the velocity stacks indicates syn-rift sediments with a maximum thickness of 1000 m in the offshore Vistrenque and Petite Camargue basins, and less in the more seaward syn-rift depocentres. This is far less than what is measured in the onshore Camargue basin. On the parallel section (Figure 7B) the thin-skinned tectonics zone displays only one large syn-rift basin in the Camargue. This section is characterized by the three major SE dipping basement faults described earlier, in the hanging wall of which a structurally high Palaeozoic basement is directly overlain by post-rift sediments. In spite of SE dipping extensional faults, the top of the basement is located at a higher structural level in the hanging wall than in the footwall. This is in agreement with pre-extension basement culmination and erosion of the Mesozoic cover in the offshore domain of the Gulf of Lion. Pre-rift surface elevation can easily account for the lack or very small amount of syn-rift sediments in this part of the margin. If continental basins were formed on the collapsing mountain belt, they were rapidly cannibalized. There is thus no stratigraphic record of the early stages of rifting (Oligocene) in the areas of surface pre-rift elevation. In contrast, where surface elevation was close to sea level from the onset of extension as in Camargue, the complete syn-rift sequence has been preserved. The presence of the deepest syn-rift depocentre localized in

the Camargue basin can be accounted for by the com- bined effects of (i) preservation of syn-rift sedimentation north of the elevated area of the Pyrenean range and (ii) control by a crustal-scale low-angle extensional fault which allowed a large amount of accommodation. The Central Graben (Figure 2), the large syn-rift depocentre in the SW of the Gulf of Lion (Gorini et al., 1993; Guennoc et al., 1994) lies in a similar structural position.

Structure of the basinward part of the margin

The basinward part of the continental margin, imaged on the ECORS profile (Figure 7A), is limited by a deep and narrow basin probably controlled by a SE dipping steep fault. This narrow basin is located in the eastward, along-strike continuation of the North Pyrenean Fault (see Figure 2). Newly reprocessed pre-stack migration of the ECORS profile (Pascal et al., 1994; unpublished data) displays the structures of this part of the margin (Foldout 1). It is characterized by tilted blocks bounded by low-angle normal faults. Half- graben contain tilted and diverging reflectors, interpreted as syn-rift sediments. The basal unconformity is unclear. Depending on the interpretations, there is at least 1000 m and a maximum of 2000 m of syn-rift in the deepest depocentre. Syn-rift series are truncated by a prominent post-rift unconformity. Syn-rift sedimentation did not keep up with faulting as the crest of the tilted blocks stand above the post-rift unconformity, resulting in elevation of the tilted block crests above the syn-rift basins. The post-rift unconformity is overlain by a post-rift sequence onlapping to the NW onto the basement highs. The GLP2 borehole located on the side of a structural high drilled the upper part of this transgressive sequence. It indicates that the first sediments onlapping the basement at this point (late Burdigalian) are continental and that they very rapidly pass upwards into open marine series (Gorini, 1993). This implies that: (i) the diachronous lower boundary of the transgressive post-rift sequence corresponds to a coastal onlap on the flank of an emergent land present- ing high topographies; (ii) the syn-rift Oligo-Aqui- tanian sediments were deposited in continental environment; and (iii) the rate of post-rift subsidence was very fast.

This 60 km wide zone coincides with a sharp rise of the Moho discontinuity from 20 km (beneath GLP2) to 15 km (at ESP 203) and a decrease in crustal thickness from 15 to 5 km. In this area, the lower crust does not display the typical laminated seismic facies as in the intermediate zone. The upper crust is affected by tilted blocks controlled by low-angle normal faults dipping 30-35 ° towards the basin (Foldout 1). These faults seem to detach in a group of very shallow dipping reflectors located about 2 -3 km beneath the top of the fault blocks, which could correlate landwards with the R reflector that cuts the top of the basement near GLP2 well (de Voogd et al., 1991). At variance with the S reflector which is imaged on the northern Bay of Biscay Margin (Le Pichon and Barbier, 1987), the R reflector cuts down-section towards the basin, synthetically to the normal faults bounding the tilted blocks. It lies within a domain of typical crustal velocities (6.2 km/s), differing in this respect from the S reflector observed on



the Galicia margin, where it is interpreted as the tectonic boundary between crustal material in the hanging wall and serpentinized peridotites in the footwall (Boillot et al., 1988). We interpret the R reflector as an extensional intra-crustal shear zone into which the low angle normal faults are rooted, and which allows stretching of the upper crustal level. Estimates of the extension across the low-angle faults indicate horizontal extension of about 60% above the R reflector, which cannot account alone for the whole crustal thinning observed on this part of the ECORS profile.

Near the base of the crustal section a 5-20 ° landward dipping reflector T is visible on the prestack migration as well as on the initial processing (e.g. Gorini et al., 1993). It links the 20 km deep Moho beneath ESP202 to a 12 km deep level below ESP203, where, according to the ESP203 velocity determinations, it lies within crustal material (Foldout 1). The T reflector therefore offsets the Moho discontinuity by 5 km between the GLP2 and ESP203 locations. Mapping of this reflector shows that it is parallel to the margin and that it is con- sistently located above the seismic Moho (Pascal et al., 1993). We interpret T as a shallow dipping extensional fault that truncates the upper lithospheric mantle and the lower crust. Blockfaulting and seaward dipping shear zones in the upper crust, together with antithetic extensional detachment in the lower crust, may account for the extreme thinning of this zone. Consequently, this interpretation requires that the crust located sea- wards of the T reflector is composed of lower crust and upper lithospheric mantle, exhumed in a fashion similar to that proposed for the Galicia margin (Boillot et al., 1988). Alternatively, the T reflector may be compared with the lower crustal and subcrustal reflectors observed on BIRPS profiles which are interpreted as shear zones transfering the bulk strain from upper crustal faults to mantle faults (Reston, 1990).

Basinwards of the emergence of the T reflector, the crust is characterized by a constant thickness (4 km). According to Pascal et al. (1993), velocities from expanded spread profiles suggest the existence of a crust with intermediate characters. It could be either a lower continental crust, stretched and intruded by mafic volcanics, or serpentinized peridotites at the top of the mantle, denuded by a landward dipping extensional detachment (T reflector). Typical oceanic crust is found in the basin beyond ESP207.

Discussion and conclusion

The extensional structural style of the Gulf of Lion passive continental margin changes considerably from the landward end to the continent-ocean transition (Figure 8). The zoning of the Oligo-Miocene extensional province reflects the zoning of the Pyrenean structures: the extensional thin-skinned system over- prints the external thin-skinned thrust system which characterizes the northern foreland of the Pyrenees. South-west of the Arlesienne transfer fault the thin- skinned extensional system ramps down into the basement at the Nimes fault. Figure 8 also shows that the Pyrenean inherited frontal ramps were reactivated as oblique extensional ramps along a restricted E - W segment near the S6toise transform and linking shallow

T h i n - s k i n n e d extensional tectonics

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to deep extensional ramps. The intermediate zone, corresponding to thick-skinned faulting, developed in the area of previous Pyrenean crustal thickening, where the Mesozoic cover was removed by erosion. The evidence for surface elevation in that area accounts for the rather thin syn-rift series. Stretching resulted firstly in the collapse of the elevated relief without syn-rift sediment preservation. At the periphery of the topographic high where surface elevation was close to sea level (e.g. Camargue), stretching was associated with sedimentation from the onset of rifting, allowing for the formation of the thickest syn-rift series. Note that the extensional faults in this area are trending NE, therefore they are not reactivated Pyrenean thrusts. They are either neoformed faults or inherited lateral ramps of the contractional system. The zone of low-angle block faulting characterized by extreme continental stretching and very thick post-rift series is located south of the NPFZ, e.g. the Iberia-Sardinia-Corsica plate. The structural style and the subsidence pattern change across the terrane boundary between the European (north of the NPFZ) and the Iberia-Sardinia-Corsica plate. The present day structure of the Pyrenees


St ruc tu ra l s ty le a n d evo lu t i on o f the Gu l f o f L ion : M. S#ranne et al.

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(Roure et al., 1989) could be taken as a model for the pre-rift configuration of the Gulf of Lion (Figure 9). We suggest that the pre-rift crustal and lithospheric configuration were different on each side of the NPFZ. For the same value of 13, extension of the thickened and elevated European continental crust led to a moderately thinned crust (intermediate part of the margin), whereas extension of the Sardinia-Corsica continental block, little thickened during Pyrenean orogeny, led to an extremely thinned continental crust (basinward part of the crust).

The existence of an orogenically destabilized lithosphere at the onset of extension has considerable bearing on the mode of crustal extension and, consequently, on the choice of pre-rift initial conditions. Future basin modelling of the Gulf of Lion must integrate the presence of an elevated and thickened continental crust before rifting, as well as crustal in- homogeneities inherited from the Pyrenean orogeny.

Acknowledgements This work is supported by the European Community DGXII (contract JOULE II - CEC Project n ° PL 920287 Integrated Basin Studies). Elf Aquitaine, Esso- Rep, Total, Coparex and Elf-Atochem have provided data for this study. The ideas contained in this contribution benefited from discussions with N. Chamot- Rooke, J. M. Gaulier, A. Mascle, M. S~guret and R. Vially. We are grateful to H. Kooi, P. A. Ziegler, A. Perez-Estaun and an anonymous reviewer for their in- sightful and detailed comments which improved the paper, and to S. Cloetingh for his editorial work.

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Burrus, J. and Foucher, J. P. (1986) Contribution to the thermal regime of the Proven£al Basin based on flumed heat flow surveys and previous investigations Tectonophysics 128, 303- 334

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