TECTONICS, VOL. 15, NO. 6, PAGES 1192-1212, DECEMBER 1996

Low-angle crustal ramp and basin geometry in the Gulf of Lion passive margin: Oligocene-Aquitanian Vistrenque graben, SE France

A. Benedicto,1 P. Labaume,2 M. S6guret,1 and M. S6ranne1

Abstract. With more than 4000 m of Oligocene-Aquitanian elevation,the Vistrenquebasin fill recordsthe whole rifting sediments,the Vistrenquegraben (SE France) is the deepest episodebecause of its locationat the front of the Pyrenean synrift depocenterof the Gulf of Lion passivemargin, NW orogen. Mediterranean. Detailed analysis of industrial seismic reflection profiles and borehole data show that the steep 1. Introduction N•mes fault, which bounds the graben to the NW, forms at depth a low-angle (25ø) crustalramp. Along-strikechanges of In the Gulf of Lion passivemargin (SE France,Figure 1), it hangingwallgeometry allow us to infer along-strikechanges has recently been suggested that upper crustal extension of fault shape:A rollover structureand divergentOligocene- duringthe Oligocene-Aquitanianrifting was accommodatedby Aquitanianbasin fill are associatedwith a listric geometryof low-angle (250-30ø ) crustal ramps located in the shelf and the fault in the southernpart of the graben, while a pseudo- slopeof the margin,while the landwardpart was only affected rollover and compensationgraben result from a two-segments by thin-skinned Mesozoic cover d6collement (Figure 2) planar geometry of the fault in the northernpart. Preexisting [Sdranne et al., 1995]. The Nimes fault correspondsto the structures inherited from Mesozoic extension and Late boundarybetween the two domainsand controlsthe Vistrenque Cretaceous-Eocene Pyrenean thickening controlled the graben (Figure lb), the deepestsynrift depocenterof the location of the N•mes fault and the transfer zones which divide margin.The Vistrenquegraben is thusa strategicfeature for a the graben into different compartments. Since both better understandingof the margin evolution. Surprisingly,it hangingwall and fault profile are well constrained,restoration remainspractically unknown from a structuralpoint of view, techniquescan be used to estimatethe prerift topography.The despite the existence of abundant seismic reflection and Vistrenque graben was formed close to sealevel,but at the boreholedata accumulatedduring 40 yearsof oil researchand front of a > 1 km-high elevated area resulting from the salt exploitation.This paperpresents a new structuralmodel Pyrenean orogeny. In the studied transect,the N•mes fault for the Vistrenquegraben and the Nimes fault basedon the formedthe landward (NW) boundaryof thebasemenf faulted interpretationof the subsurfacedata. Basin model is analyzed domain of the margin. Extensionaldeformation was restricted in the regionalcontext of the Gulf of Lion marginin orderto to this domain during most of the rifting interval. Small better understand its kinematic evolution. amounts of extension were transmitted landward to Mesozoic Geometryof normal faults in passivemargins has been cover d6collementrooted in the N•mes fault, only during short largelydiscussed. Normal faultsflattening at depthassociated episodes, probably resulting from gravitational instability to crustaldetachments have classicallybeen proposedas main during margin collapse.The N•mes low-angle crustal ramp, as structuralfeature in the evolution of extensionalrifted margins well as the other crustal ramps of the margin of similar [Wernicke and Burchfield, 1982; Gibbs, 1984; Lister et al., orientation, are probably newly formed extensionalstructures 1991]. Nevertheless, relationships between near-surface rather than reactivated Pyrenean thrusts.Their activation at a steeplydipping and deeplow-angle detachments remains one low-angle may have been allowed by crustal weakening of the most discussedpoint. resulting from the previous Pyrenean thickening. Upper Linkage of surfacehigh-angle fault and deep low-angle crustal extensioncorresponding to the graben formation was detachmentcan (1) be listric [Verral, 1981; Wernicke and transmitted basinward through an intracrustal detachment, Burchfield, 1982; Gibbs, 1983; Davison, 1986; White et al., or/and distributed in the lower crust across the margin. In 1986; McClay and Ellis, 1987;Ellis and McClay, 1988], or contrastto the more stretchedareas of the margin which do not (2) by a set of planar faults with discrete change of dip display thick synrift series due to their initial high surface [McClay and Ellis, 1987; Faure and Cherrnette,1989], also known as kink-stylefault [Groshong,1989]. Analoguemodelling [Closs, 1969; McClay and Ellis, 1987; I G6ofiuides-Bassins-Eau,CNRS Universit6 Montpellier II, Ellis and McClay, 1988] and geometric/numericalmodels Montpellier, France. [Jackson and McKenzie, 1983; Gibbs, 1983; White et al., 2Laboratoirede G6ophysiqueInterne et Tectonophysique,CNRS 1986; Faure and Cherrnette,1989; Groshong,1989; Xiao and Universit6Joseph Fourier, Grenoble,France. Suppe, 1992] of normal faults systemsin the caseof faults flatteningat depth has pointedout the relationshipsbetween Copyright1996 by the AmericanGeophysical Union. hangingwall deformation and fault section shape. These different approachesconverge in the result that movement Papernumber 96TC01097. along a listric fault generatesa half grabenwith a divergent 0278-7407/96/96TC-01097512.00 basin fill towards the fault and a rollover structure of the

1192 BENEDICTOET AL.: EXTENSIONALCRUSTAL RAMP AND BASIN GEOMETRY 1193

•--•ß Mainextensional basins OutcroppingPaleozoic !/i•.• Preriftcover detached during Pyrenean orogeny NW limit of oceanic crust •.... Paleozoicinvolved inPyrenean orogeny NW limit of intermediate continental crust Lion

Prover•,al basin nque graben. (Fig.

44 Front , , Main normal faults + --•-- Pyrenean thrusts (Eocene) • Alpine thrusts (Mio-Pliocene) • Pyrenean thrust reactived + during Alpine compression + • Pyrenean compression <===:>Oligocene-Miocene extension + + +

43

' 50km '% ,, "";,',, q "'-.. ,' 42 .,_...... I 2 ø / 4 ø 5 ø 6 ø

Figure 1. (a)'Position of the Gulf of Lion margin in the Western Mediterraneanbasin. Stippled lines are prerift locationof the Corsica-Sardiniaand Balearic islands.(b) Simplified structuralframework of the Gulf of Lion passive margin and location of the Vistrenque graben, AB, Albs basin; HB, H6rault basin; MB, Manosque basin; NPFZ, North Pyrenean fault zone; Sh, Saint Hippolyte well; Va, Vacquibreswell; SM, SaintesMaries 101' Be, Beauducwell; Mo, Montpellier; Ni, N•mes; Ma, Marseille. Modified from S6ranneet al. [1995].

hangingwall. However, a planar tault generates a from the hangingwall shape as aids to field and seismic compensation graben with a horizontal basin fill and a interpretations.These techniquesconsider an undeformable pseudo-rolloverstructure of the hangingwall,these different footwall and different modes of deformation for the models are not always well differentiated in the literature hangingwall(see review and discussionby Dula [ 1991]). Fault dealing with normal faults systems [e.g., Gibbs, 1984; reconstruction techniques use a horizontal prerift surface Groshong, 1989; Dula, 1991; Xiao and Suppe, 1992; (datum) and the geometry of the hangingwall (top of the Withjack et al., 1995]. prerifi) to reconstruct the fault shape at depth or vice versa. If the relationshipsbetween fault and hangingwall/basin However, in the case of intracontinental basins, possible fill geometryin two dimensionappears well established,the inherited prerifi elevation (irregular topography) and erosion possiblealong-strike variation of the fault geometryhas not of both footwall and hangingwall must be taken into account been investigated and natural examples of such three- in analyzing the final geometry of the fault system. Besides, dimensional structures have not been described. prerift elevation may be responsible for erosion instead of Here, we analyze the three-dimensionalgeometry of the deposition during part of the activity of faults. In this case, Vistrenquegraben and the N•mesfault, pointingout the along- calculated tectonic extension from the observed final strikevariation of the fault geometryfrom listric to planarand geometry may be minimized, !eading to differencesin the resulting contrastingbasin geometries. extension ratios calculated from tectonics or subsidence at the Relationshipsbetween hangingwall and fault geometries scaleof the margin. have lead to differenttechniques to reconstructthe fault shape Our good control of hangingwalland fault geometryof the 1194 BENEDICTO ET AL.' FO('IENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY

Vistrenque graben allow us to use the classical fault • • •-o • c•o •c• • reconstructiontechniques, in an inverse way, to reconstruct the prerift relief, inherited from the prerift Pyreneanorogeny which affected the area. At the scale of the Gulf of Lion margin, constraint on Vistrenque graben and N•mes fault geometry gives valuable basisto discussthe role of a major crustalramp in the structure of a passivemargin. The Vistrenquegraben location allows to discuss the kinematic relationships between basementfault and cover d6collementdomains, and the genetic relationships between the low-angle crustal ramps of the margin and previous crustal thickening.

2. Geological Framework The Gulf of Lion passivemargin correspondsto the NW margin of the Proven9albasin (Figure l a) which openeddue to Oligocene-Aquitanian rifting and Burdigalian oceanic accretion associated with the southeastward drift of the Corsica-Sardinia block [Auzendeet al., 1973; Biju-Duval et al., 1979; Rehault et al., 1984; Burrus, 1984]. This extensionalepisode corresponded to back-arc openingin the context of African and Europeanplate convergence[Olivet et al., 1984; Rehault et al., 1984; Maillard and Mauffret, 1993]. The Gulf of Lion margin formed at the expense of a Paleozoicbasement and Triassicto Eocenecover. It comprises Oligocene to Aquitanian basins overlain by Miocene (post- Aquitanian) to Plio-Quaternarysequences (Figure lb) [Lefevre, 1980; Arthaud et al., 1980; Gorini et al., 1993; Guennoc et al., 1994]. The Oligocene-Aquitanian sediments are classically consideredto represent the rifting stage and the post-Aquitanian to Plio-Quaternary deposits of the postrift stage [Burrus, 1984; Gorini et al., 1993]. The margin is boundedto the NW by the C6vennesfault, at the edge of the Massif Central. The NE boundary corresponds to the Ar16sienne transfer zone [Gorini et al., 1993' Guennoc et al., 1994] which separates the continental margin from the intracontinental rift of the Rh6ne valley between the C6vennesand Durancefaults (Figure 1). The N•mes fault, which boundsthe Vistrenquegraben to the NW, represents the northeastern boundary of a low-angle faulted basement domain (Figure 2) [Sdranne et al., 1995] which extends basinward (mainly offshore). The structureof this domain is well illustrated on the Etude Continental et Oceaniquepar Reflexionet RefractionSismique (ECORS) deep seismic reflection profiles [de Voogd et al., 1991] and on industrial seismic reflection lines [Gorini et al., 1993]. The landward(northwestern) part of the margin,between the N•mes and C6vennes faults, corresponds to a thin-skinned extensional domain, characterizedby half grabens associated with listtic faults which detach in the Triassic shales and evaporites[Stiranne et al., 1995]. The rift structuresof the Gulf of Lion margin overprintolder structureswhich resultfrom a complexevolution: 1. Mesozoic extension resulted in the formation of the SE France basin (part of the Alpine Tethyan margin) [Beaudrirnont and Dubois, 1977; Curnelle and Dubois, 1986].The SE basin was controlled by NE-SW and E-W trendingmajor faults inheritedfrom the late Paleozoicstrike- slip faulting [Arthaud and Matte, 1975]. More than 10 km of BENEDICrO ET AL.: EXTENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY 1195

Triassic to Cretaceous sediments accumulated in the central passing upward to lacustrine limestones alternating with part of the basin,bounded to the NW by the C6vennesfault and marls, sandstones,and conglomerates;(2) the "S6rie Rouge" to the southby an E-W trendinguplifted domain of which the (200 m thick), comprising palustrine red clays and northernboundary lies close to the present-daycoastline. gypsiferous marls with several intercalations of marls and 2. Late Cretaceous to Eocene N-S Pyrenean compression sandstones;(3) the "S6rie Calcar6o-salif'ere"(900 m thick), resulted in the formation of the E-W trending Pyreneanbelt formed by rhythmic halite (exploitedin Parrapon),anhydrite, (Figure l a), which extendedfrom the present-dayrange across and marly clay calcareousdeposits. the Gulf of Lion area. In the Corbinres region, the Pyrenean The preciseage of the Oligocenesediments remains poorly structuralzones are transferrednorthwestward with respectto known due to the absence of fossils in the continental facies. the present-dayPyrenean range (Figure lb). During Pyrenean The "S6rieCalcar6o-salif'ere" is attributedto the Stampianon compression, NE-SW trending Mesozoic faults were the base of correlationwith the saliferousdeposits in the reactivated as sinistral strike-slip faults and E-W trending Rh6ne Valley area. The age of the underlying"Oligocene" faults as N verging thrusts [Arthaud and $•guret, 1981; series remains unknown. The Chattian has not been Ternpier, 1987]. Basementfrontal thrustramps were developed recognizedin the Vistrenque graben, but is presentin the close to the present-daycoastline (Figure lb). To the north, outcroppingbasins of adjacentareas [Cavelief et al., 1984]. the Mesozoic cover was detached above the Triassic shales and The Oligocene series are overlain by 800 to 1500 m of evaporitesand affectedby E-W trendingfolds and thrusts. sediments attributed to the Aquitanian on the base of Timing of the transition from Pyrenean compressionto foraminifera fauna (Calina, 1983 in Valette [1991]), divided Oligocene-Aquitanian extension is still poorly known. into three monotonousseries. The Lower Aquitanian series Northeast of the Ar16sienne transfer zone, in the Al•s and consists of lagoonal to coastal sediments made of Manosquebasins (Figure lb), middle Ludian (Upper Eocene) nonfossiliferous-variegatedclays with rare intercalationsof continental sediments display extensional growth structures calcareousmudstones and sandstones.These depositsare and are classicallyinterpreted as the oldest synrift sediments overlain by marine clays with rare intercalations of [Cavelier et al., 1984]. Southwest of the Ar16sienne transfer limestonescorresponding to the Middle Aquitanian. The zone, in the Montpellier region, the same sedimentsbelong to Upper Aquitanianseries consists of a coastalclay-sandstone the prerift successionand are involved in the latest Pyrenean coarsening-up succession, with few lagoonal dolomitic folding. In this area, the oldest synrift sediments are limestone and lignite beds. The Oligocene-Aquitanian continentaldeposits attributed to the middle Stampianon the successionindicates a transgressivetrend from the Stampian base of vertebrate fauna correlated with the P19-P20 to the Middle Aquitanian,whereas the UpperAquitanian series foraminifera zones of Blow (about 34 to 32 Ma, Crochet correspondsto a regressivephase. [1984]). This suggestsa diachronousonset of the rifting, The Aquitanianis unconformablyoverlain by transgressive older in the north and younger southward,that is toward the Burdigalianmarine calcarenitespassing upward to Langhian front of the Pyreneanrange. and Tortonian marine marls. In the Rh6ne Valley, this calcarenite-marl successionis classically consideredas a 3. Stratigraphy in the Vistrenque Graben transgressivetrend. These deposits are truncatedby a regional erosional surface of Messinian age [Beaufort et al., 1954; The general stratigraphy of the Vistrenque graben and Dernarcq, 1970] relatedto the drying out of the Mediterranean adjacent areas is established from 48 petroleum research sea [Ryan and Cita, 1978]. The erosional surface is buried boreholes, 25 of which have reached the pre-Oligocene under Pliocene marine clays and marls passingupward to successionthat forms the basin substratum(Figure 3). The Quaternaryfluvial depositsof the Rh6ne delta. most complete and best known basin fill sequencehas been By analogywith the geodynamicalinterpretation of Bessis drilled in the Pierrefeu well (P in Figure 3), where the and Burrus [1986] for the offshorearea, the unconformity Oligocene to Quaternary successionis 4920 m thick (Caline, between the Aquitanian and Burdigalian observed in the 1983 in Valette [ 1991]) and in the neighbouringParrapon salt Vistrenquegraben wells and seismiclines was interpretedby exploitation boreholes (PR in Figure 3, Valette [1991]; Valette [1991] and Valette and Benedicto [1995] as the Valette and Benedicto [1995]). breakup unconformity separating Oligocene-Aquitanian On the base of different pre-Oligocene stratigraphy two synriftbasin fill from Burdigalianto recentpostrift sequences. areas can be distinguishedfrom boreholes(Figure 3): in the This interpretationis discussedin later sections,on the basis north of the basin, the top of the pre-Oligocene consistsof of the new seismicdata presented in thispaper. Neocomian (Valanginian-Berriasian) marine carbonates, whereas in the south it consists of Jurassic marine carbonates 4. Structure of the VistrenqueGraben locally overlain by Upper Cretaceousfluvial facies (Marette well, M in Figure 3) or/and continental Eocene carbonates The first structuralinterpretation of the Vistrenquegraben, (Baumelleswell, B in Figure 3). based only on borehole and gravimetric data, assumeda The Oligocene sedimentsform a successionof continental symmetric graben bounded by steeply dipping faults to lagoonal deposits with alternations of fluvial and involving the Paleozoic basement[Arthaud et al., 1980]. In a lacustrine-palustrinefacies. Accordingto Valette [1991], this detailedstudy of the Parraponsalt exploitationarea, seismic successionincludes from the bottom to the top: (1) the "S6rie interpretationled Valette [1991] to proposethat the main Grise" (2000 m thick), consisting of dark clays and structuralcontrol of this part of the grabenwas exerted by the sandstoneswith lignite intercalations in the lower part, N•mesfault, interpretedas a listric fault flatteningat depth. 1196 BENEDICTOET AL.:EXTENSIONAL CRUSTAL RAMP AND BASINGEOMETRY

i i Mainnormal taults Topof pre-Olig. inwells - - SG seismicprofiles VGseismic profiles • Preriftnotreached PRseismic profiles •j• LowerCretaceous !lillilITransferOutcropping zonepre-Oligoc. 41• Jurassic,covered bylocally Upper I • / • I Oligo-Aquitanianbasin fill Cretaceous/ Eocene

*"'"'•Outcropping post-Aquit.Plio-Quaternary Miocene v35 Vaunage

L1 LJ •J•

Figure 3. Structural framework of the Vistrenque graben showing location of data base and cross sections.Boreholes cited in the text or reportedon line drawings(Plate 1) are: V3, Vaunage3; LJ, La Jassette; L1, Lunel 1; L2, Lunel 2; L3, Lunel 3; A, Aubord; SV, Saint Veran; P, Pierrefeu; A3, Albaron 3; A7, Albaron 7; A101, Albaron 101; M, Marette; M1, Montcalm 1; M2, Montcalm 2; B, Baumelles; V, Vaccar•s; SM, SaintesMaries 101. PR, Parraponsalt exploitation;t.z., transferzone; a, b and c, crosssections of Figure 6.

For the same area, Valette and Benedicto [1995] proposeda and depth-convertedseismic lines has been carried out to test two-segmentsplanar geometryfor the N•mes fault. Here, we the consistencyof the seismic interoretation and validates the show that the N•mes fault corresponds to a low-angle use of the unmigrated sections for the interpretation of the basementramp along the whole Vistrenquegraben, and that structuralstyle. Seismic facies (Figure 4) are best visible and along-strikevariations of fault shapeimply major changesin correlatablein the Saint Gilles (SG) unmigratedsurvey. hangingwall geometry. The seismicinterpretation is summarizedin the line drawings presentedin Figure 5. Isobath/isopachmaps (Figure 4.1. Seismicinterpretation 5) and depth cross sections(Figure 6) have been constructed Data. The industrial multichannel seismic reflection from seismic and bore hole data. Depth conversion of the surveys(presented in Table 1) and boreholesused in this study seismic data has been done using the seismic stack velocities, are located in Figure 3. Comparisonof unmigrated,migrated but only at points where the stack velocities were in BENEDICTO ET AL.: EXTF./qSIONAL CRUSTAL RAMP AND BASIN GEOMETRY 1197

Table 1. SeismicReflection Data Usedin this Work (Locationin Figure 3)

Survey Company SeismicLines Processing

Saint Gilles ESSO-REP,1969 SG.1 to 18 Satan (SG) SG.2, 3, 6, 8, 9, 10& 11 Migrationby Eurafrep,1986 SG.I, 2 &7 Migrationby Elf-Atochem,1975 SG.4 & 9 Migrationand depth convertion by IFP, 1994 Vauvert- Eurafrep,1986 VG. 1 to 8 Satan Gallician (VG) VG.I, 3,5 &7 Migrationby Eurafrep,1986

Parrapon Elf-Atochem,1986-87 PR. 1 to 7 Migrated (PR) [Valette,1991 ]

agreementwith velocityanalysis in the Pierrefeu,Parrapon- 18, and La Galine-1 wells [Valette, 1991]. Seismic stratigraphy. The seismic stratigraphy used in this work is derived from that proposedby Valette [1991] for the Parraponsurvey (PR in Figure 3). Validity of Valette's seismic stratigraphyis ensuredby the gocclquality and dense grid of PR lines, and correlation using geophysicallogs and vertical seismic profiles in closely spaced wells. Although Valette's seismicsequences were defined in a small area in the northern part of the Vistrenque graben, they have proved to have excellent lateral continuity acrossthe whole graben. In this study, we considerfive seismic stratigraphicunits, a to e (units b to e are showed in Figure 4), characterizedby specific seismic facies patterns and bounded by 4 major discontinuities, 1 to 4 (discontinuities 2 to 4 are shown in Figure 4). Although some of these units may correspondto seismic sequencesin the senseof Mitchurn [1977], they are referred to as units and not sequencesdue to the difficulty of defining the precise nature of some of the boundarieson the seismic lines. The typical seismic facies are found in the central part of the graben. Their characteristics become progressively less distinct southeastwarddue (1) to lateral thinning and facies change of the basin fill and (2) to seismic noise resultingfrom the Messinianerosional surface. The five seismic units correlate well with the litho-biostratigraphic units drilled in boreholes. Unit a (sequence1 by Valette [1991]) is truncatedupward by discontinuity 1 and exhibits a variable opaque to chaotic seismic facies. Unit a corresponds to the pre-Oligocene succession, and discontinuity 1 to the top of the basin Figure 4. Typical pattern of seismicstratigcaphic units substratum(isobath map b in Figure 5). and discontinuities used for seismic interpretation; 2, Unit b (sequences2 to 5 by Valette [1991]) is concordant discontinuity2 (Aquitanian/Oligocene);3, discontinuity3 with or onlaps discontinuity 1 and is topped by concordant (post-Aquitanianunconformity, Burdigalian/Aquitanian); 4, discontinuity2. In the southernpart of the Vistrenquegraben, discontinuity4 (Messinian erosion, Pliocene/Miocene);b, unit b is made up of an alternanceof high- and low-amplitude unit b (Oligocene);c, unit c (Aquitanian);d, unit d (post- NW divergent reflectors.In the central part of the graben the AquitanianMiocene); e, unit e (Pliocene);bI, groupb I ("S6rie reflectorsare subparallel and form three groups: bI, bii andblii, Grise");bii, groupbii ("S6rieRouge"); bii I, groupbill ("S6rie from the bottom to the top. Here bii shows low-amplitude Calcareo-salif'ere").Discontinuity 1 (top of the pre-Oligocene) reflectors or transparent facies between high amplitude andunit a (pre-Oligocene)are not visiblein thissection of the reflectorsof bI, andbill,. A brightcontinuous reflector forms unmigratedline SG2. the top of biiI. In the northernpart of the Vistrenquegraben, ,. • '-..-•.' ,•. ;•; •' • ======'•".-':-:':•.::•{•{•:.-:...::;.".-'•ig'.-:-:.-"•;.,::::::::•.::-..::::::i•i;i:•..::::..:..:.::•:::.:':':":'"-•'.:;:::

• E"'"'""""•'•'.---...•:½i:i:::"-':--'4-----:%: '"'" • '••

,,c: . 2' ...... :;:,•;½:'*:';':'-:-'"•ii':""':-'":::•-:---'-"•i • o • -,-, • • ',-,

= c: 0,,, o-___. .L,00 • • "=2 E • •

-.-- n-, _•,•-',• ,• I::: 0 ->,,•'- ."•4 c•.'• • "-' •'

"'-= " • ol

m o o • • -w-.. o •'½;/ •2_-_••----- • • •,.• _,/ o 0 o o

• 0 '- •_. • (p 0 -'" r- c I•• '• I o o ,,, =o . • • • • •

"E:- .../ *.., • • -,•- .--:.::-: ,•.e ,•. '."4

"•""--. ß '. , '% '":":•----."-'--'-':•.•,•"::--....':,-"--- .•....'.":.4.'.:..'.-'..*..--':•;• I• k •'-_,/.•,// ' '.:."'x.%%,\',"'.' •'•• ,•,.2..• I :•':•'::'o'. "•'••' •", •::";":;'" fr'"'""'•'"':"":•:•••• ...... :;,:,:...-';:.,,::•-;,......

_ .. BENEDICTO ET AL.: EXTENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY 1199

NW Si= NE Vauvert compartment•'-,.•3• ;--I extension =8750-9000m !--;.-• .,"• La Jassette (p. 3km) St. Veran (p. 1kin) Albaron 101 (p. Skm) Vaunage3 (p.6km) Aubord1 (p. 1kin) Pierrefeu Albaron3 I Vaccar•s

• d6collement•

•;•d/;Jurasic , ..,, basininverted during the -o '...... ' .'. -, . • •o'•.• .:;~, • -• - , •mes •au•r '•', .... 2, ' .... '. • - '..'• '. ' '- 'Mesozoic E-W fa, Pyrenean• ßß. compression...... •; • • . •..: -,'• •• ..... '.•, • •* •..-ß...... ; •• • .....•: 5 ,, • . ,f, •'•.• whichcontroled erosion of ,-, - -. ß • •. , •...... -, .- • ,-- •, • ,.'.•. .' ...... ,.. ,z •, • •.' . ' ' 'Cretaceousto the south, rotated and "• • reactivatedasa N-vergingthrust during: the Pyrenean compression compartmentMarsillargues•'•= e•=; --lextension =9150-9500m }--;•.. • J• J• II Vistre7uegraben j• • Petit Rh6ne• graben Okra break-upunconformity Messinianerosion

...... 1

2

3

4 5•-- Marly 6 Lias Mesozoic NTmesfault -, 7

8 E-WPyrenean 9 lO the Pyrenean compression

•,,•,•:•t'•extension =3600.4100m ! •| extension=4600ml--;._-•,",• Marette PetitRh6ne graben CO rn p art rn e n t VistreiquegrabenMarette break-up unconformityAiguesMortes I Stes.Marie,102 Messinian erosion

Mesozoic Nimes fault

2•.. -ø;, '• SW

• Plio-Quaternary POSTRIFT SYNRIFT '"':•...... ß Miocene pot-Aquitanian /F" i!:i...'"'"'•'"'" AquitanianOligocene

• Lower Cretaceous PRERIFT !:-',"."1Eocene r-T'-i upperJurassic | ! Middle and • Triassic •-:• Paleozoic Lower Jurassic

Figure6. Crosssections through the Vistrenque graben. Note the compensation graben geometry in crosssection (Figure 6a) and the rollover geometry in cross sections (Figure 6b) and (Figure 6c) (location in Figure 3). 1200 BENEDICTO ET AL.: EXTENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY unit b passesto subparalleldisrupted reflectors, locally to top of the Paleozoic series, based on lateral correlations chaotic facies, making difficult the differentiationof the three between seismic lines and the Castries and La Jassettewells (C groups.Unit b correspondsto the Oligocenedeposits. The bi and LJ in Figure 3) in the footwall. Northwest of the N•mes groupcorrelates with the "S6rieGrise", the bxigroup with the fault (footwall), these reflectorsare segmentedand their upper "S6rie Rouge" and the bxiI group with the "S6rie Calcar6o- envelope gently dips southeastwards,suggesting that the salif•re". Discontinuity 2 correspondsto the boundary Paleozoic is lowered southeastwardby one or several steeply betweenthe Oligoceneand Aquitanian series (Figure 5d). dippingnormal faults (SG 3, 4 and in Plate 1). Unit c (sequence6 by Valette[ 1991]) onlapsdiscontinuity The N•mes fault is thus characterizedin seismicprofiles by 2 toward the $E and is truncatedupward by the erosional a steepSE dipping ramp in the upperpart and by a low-angle discontinuity 3. Unit c is characterizedby a transparent (25ø) SE dippingramp which affectsthe Paleozoicbasement at seismicfacies with a few brightreflections of high-amplitude depth. Depth-convertedsections show that the low-angle ramp at the top, and a few easily correlatablehigh amplitude is defined down to about 8 km (Figure 6). Isobath mapping reflectorsin the middle part. It showsrapid lateral seismic (Figure 5a) shows that the low-angle ramp is affected by facies changes.In the southernpart of the Vistrenquegraben, along-strike depth changes which define NW-SE trending unit c showsa NW divergentpattern, while in the southern lateral ramps. The regional direction of extension indicates part showsa subparallelone. This unit c correspondsto the that the N•mes fault ramp was a frontal ramp in the rift fault Aquitanian deposits, and the erosional discontinuity 3 system(Figure lb). correspondsto the breakupunconformity defined by Valette Contrasting hangingwall geometries. T w o [ 1991] and Valetteand Benedicto[ 1995] (Figure5e). distinct geometriesof the Oligocene-Aquitanianbasin fill and Unit d (sequences7 and 8 by Valette [1991]) is concordant its pre-Oligocenesubstratum are seenon the seismiclines and with or onlapsdiscontinuity 3 and is truncatedupward by the isopach/isobathmaps (Figures 5 and 6). erosionaldiscontinuity 4. The lower part comprisesa groupof (1) In the southernpart, the Vistrenquegraben showsan bright subparallel and continuousreflectors of moderate asymmetric geometry corresponding to a hangingwall amplitude.The upperpart showshummocky reflections and rollover structure of the basin substratumand Oligocene- frequentlydisrupted configuration. The lower part corresponds Aquitanian infill units against the N•mes fault. On the to the transgressiveBurdigalian postrift deposits[Gorini et southeasternside of the graben, the Oligocene-Aquitanian al., 1993] and the upper part to the post-BurdigalianMiocene basin fill dips northwestward and displaying a pattern succession (Langhian to Tortonian). Discontinuity 4 divergenttoward the NW (VG4-VG7, SG4-SG9 and SG5-SG10 correspondsto the regionalMessinian erosional surface. in Plate 1). The NW dipping reflectorsof the Oligoceneunit Unit e (sequence9 by Valette [1991]) onlapsdiscontinuity abut againstthe low-angle part of the N•mes fault, while the 4. It is mainly transparentin $G lines and showssubparallel Aquitanian unit dips slightly to the SE againstthe steep part discontinuousreflectors in VG lines, with local hummocky of the fault, defining a slight synclinetruncated upward by the clinoforms. Unit e corresponds to the Plio-Quaternary post-Aquitanianunconformity (SG4 in Plate 1). At the crestof succession. the rollover, reflectorsare offset by a SE dippingfault forming Geometry of the N•mes fault. Truncated reflectors of the northwesternboundary of anotherasymmetric half graben, the hangingwall and footwall, as well as reflectors the Petit Rh6ne graben (SG9 and 10 in Plate 1). Details of its corresponding to the fault itself allow accurate seismic geometry are maskedby seismicartefacts related to velocity identification of the Ntmes fault in the whole surveyed area contrasts associated with the Messinian erosional surface, but (Figures5a and 6). seismic correlations define the Petit Rh6ne graben infill as The upperpart of the N•mesfault, downto about2 s (twt), is Oligocenein age (group bi). The Oligocene-Aquitanianbasin a SE dipping steep surface (about 70ø in depth-converted fill units extend southeastwardbeyond the rollover crest with a sections; Figure 6) defined by truncated reflectors of the horizontalpattern and reducedthickness. hangingwall (Oligocene, Aquitanian, and post-Aquitanian (2) In the northernpart of the Vistrenquegraben, the basin Miocene) and footwall (pre-Oligoceneunit) (SG2, SG3, SG4, fill units also form a slight syncline,but with a different,more and SG 5 in Plate 1). symmetricgeometry than in the southernpart (SG2 and SG3 in The deep portionof the N•mes fault is a $E dippinglow- Plate 1). In the axis of the graben, the Oligocene-Aquitanian anglesurface (about 25 ø in depth-convertedsections; Figure 6) units showsa subparalleland subhorizontalpattern. In the SE, definedby (1) reflectorsrelated to the fault itself, visiblein all both the pre-Oligoceneand Oligocene-Aquitanianunits show a lines between about 2 and 3 s (twt), and (2) truncatedreflectors confused image with disrupted reflectors resulting from of the hangingwalland footwall. In the footwall, most of the faulting and dip slightly to the NW. Dipmeter logs from the lines show bright continuoussubhorizontal reflectors between Pierrefeu and Parrapon wells show that faults are parallel, 2 and 3 s (twt) beneath the central part of the Vistrenque closely spacedand dip 70 ø to the NW [Valette, 1991; Valette graben.To the SE, thesereflectors are either truncatedby the and Benedicto, 1995]. In the NW, the Oligocene-Aquitanian SE dippingreflectors of the N•mesfault or interruptedagainst units dip slightly to the SE and are affectedby steep(70 ø) SE the NW-tilted hangingwallreflectors (e.g., $G3 and 4 in Plate dipping normal faults syntheticwith the N•mes fault. 1). In the latter case, the limit between both groups of In the whole Vistrenquegraben, the NW dipping basin fill reflectors is aligned with the N•mes fault reflectors. The units, on the southeasternside, are also affected by several truncation of the deep horizontal reflectors thus defines a antithetic d6collementswhich correspondto the deel• part of footwall ramp in the low-angle part of the fault. The deep NW dippinglistric faults (VG7, SG9, and SG10 in Plate 1). In horizontal reflectorsare interpretedas being within or at the the southernpart of the graben,the associatedminor rollovers BENEDICTO ET AL.: EXTF3qSIONALCRUSTAL RAMP AND BASIN GEOMETRY 1201 in the Oligocene-Aquitanianunits are truncatedby the post- folding of the hangingwallunits of a SW dippinglateral ramp Aquitanian unconformity, indicating that most of the of the Nimes fault. Thus, the Vauvert compartmentoverlies the movement occurred during the Oligocene-Aquitanian deepestlevel of detachmentof the Nimes fault. extension. However, the base of the post-Aquitanian The strike lines VG2 and SG1 also showthat lateral ramps successionis locally affectedby late movementsof the listric of the antithetic dtcollements which affect the Oligocene- faults (VG7 in Plate 1). In the northernarea, dtcollementsare Aquitanian units occur at the level of the Gallician and not visible on the seismic lines, but have been revealed by the Arltsienne transfer zones. detailedstudy of the Parraponsalt exploitationarea [Valette, 1991; Valette and Benedicto, 1995]. 4.2. RelationshipsBetween the Nimes Fault The contrastinggeometries of the Oligocene-Aquitanian Profile and Hanging Wall Geometry basin fill observedalong the strike of the Nimes fault in On seismic lines (Plate 1), the N•mes fault is characterized seismiclines allow to differentiatethree compartmentsin the in the whole Vistrenque graben by a low-angle profile at Vistrenquegraben, namely, from the SW to the NE, the depth. However, hangingwallgeometry varies (1) along-strike Marette,Marsillargues, and Vauvertcompartments (Figure 3). for the basin substratumand Oligocene-Aquitanianunits, and The Marette and Marsillarguescompartments are characterized (2) vertically, between the Oligocene-Aquitanian and post- by the divergent Oligocene-Aquitanianbasin fill above a Aquitanian Miocene units. In this section, we discuss the rollover. They differ by the thicknessof the Oligoceneunit, relationshipsbetween fault profile and variable hangingwall thinnerin the Marette compartment.The Vauvert compartment geometries. is characterizedby a horizontal pattern of the Oligocene- For the Oligocene-Aquitanian period, the Marette and Aquitanianreflectors in the axis of the graben, and steep Marsillarguescompartments, on the one hand, and the Vauvert basinwarddipping faults on the SE borderof the graben. compartment, on the other hand, show a fundamental The post-Aquitanian successionis not affected by difference in hangingwall geometry, which can be explained hangingwalltilting and grabencompartimentalization. This by along-strikechange of the shapeof the curvaturebetween subhorizontal succession rests unconformably over the steepand low-angleparts of the N•mes fault. previouslytilted and erodedunderlying units. Although it is Flattening at depth of a normal fault may be achievedby thicker above the Vistrenque graben, it extends out of it, either a progressivecurvature, that is, a listtic fault sensu rapidly thinning up to disappearnorthwestward above the stricto, or an abrupt change of dip, that is, a two-segments footwall of the Nimes fault (Figure 5e). A recentunpublished planar fault (Figure 7). Each case implies a specificgeometry seismicsurvey in the northernpart of the Vistrenquegraben of the hangingwall.Movement along a listric fault generatesa suggeststhat the upper part of the fault truncatesthe post- classicalhangingwall rollover geometryand a divergentbasin Aquitanian Miocene unit and is sealed by the Messinian fill toward the fault [e.g., Verrall, 1981; Gibbs, 1983; erosional surface (M. Perrissol, personal communication, Davison, 1986; White et al., 1986]. Movement along a two- 1995). segment planar fault generates a compensation graben, Transfer zones between the Vistrenque graben induced by synthetic and antithetic faults [Faure and compartments. The three compartmentsdifferentiated in Chermette, 1989]. For small offset along the major fault, the the graben are separated by zones of accommodation basin substratumof the hangingwall forms a symmetric, fiat subperpendicularto the Nimesfault (Figures3 and 5b). The Aigues-Mortes transfer zone separatesthe Marette and Marsillarguescompartments. On the SG6 strikeline (Plate 1), TWO-SEGMENTS it correspondsto a NE-dippinglow-angle fault whichbranches PLANAR FAULT LISTRIC FAULT on the Nimes fault. Its activity was responsiblefor the thicker Oligoceneunit in the Marsillarguescompartment. It remained inactive during Aquitanian times. The Gallician transferzone separatesthe Marsillarguesand Vauvert compartments.On the VG2 strikeline (Plate 1), it correspondsto a NW trendinganticlinal warping of boththe basinsubstratum and Oligocene-Aquitanianunits. Oligocene- Aquitanianreflectors show a slightdivergent geometry toward the NE, and the anticline is truncatedby the post-Aquitanian cl ...... unconformity.The anticline was developedabove a SE compensationgraben rollover trendinglateral ramp of the Nimesfault whichgently deepens northwardbeneath the Vauvertcompartment (Figure 5a). c2 To the NE, the Vauvertcompartment boundary corresponds to the Arltsienne transferzone (Figure lb). The SG1 strike pseudo-rollover rollover line (Plate 1) showsthat this boundarycorresponds to a NE upwardflexure of both the basinsubstratum and Oligocene- Figure 7. Two types of fault shape with their associated Aquitanianunits. On this flexure,the Oligoceneunit presents hangingwall and basin fill geometries,modified from Faure a divergentgeometry toward the SW andonlaps northeastward and Cherrnette[1989]. (a) initial geometry,(b) potentialvoid onto an erosional surface at the top of the pre-Oligocene. resultingfrom hangingwalldisplacement, and (c) evolutionof Seismicdata suggests.that the flexure resultsfrom forced hangingwalland basin fill geometries. 1202 BENEDICTO ET AL.: EXTENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY

NW SE

0km - 0km I .__ Pliocene _ 1

_

4- 4

6

7 7

8 8

9 9

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Aquitanian Oligocene Present elevationof the post-Pyrenean el ext=5 km ext =4,5 kmI • erosionalsurface (suposed total extension = 9,5 km topfootwallcutoff) Depth of the present topfootwallcutoff

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9 H=V 10 Figure 8. (a) Simplified line drawing of depth-convertedlines SG4 and SG9 (by IFP, 1994). (b) Sketch showingcalculation of extensionin the Vistrenquegraben through the crosssection b of Figure 6, using the inclined-shear((• = 60ø) technique.Calculation is madeusing the simplifiedline drawingof a, consideringno postrifttectonic movement and after restorationof the Petit Rh6ne graben.el, value of extensionusing the presentelevation of the preservedpost-Pyrenean erosional surface to locate the original footwall cutoff (1); e2, minimizedvalue of extensionif the presenttop footwall cutoff visiblein seismicdata is used (2). Taking a possiblemaximum postrift vertical tectonic movementfor this sectionof about 500m, extensionresults between 350m (using 1) and 300m (using 2) smaller. Taking compactioninto accountfor calculatingthe distributionof extensionbetween the Oligoceneand Aquitanianwould give a highervalue for the Oligocene and a lower value for the Aquitanianextension.

bottom syncline, and the basin fill displays an horizontal fault is not imagedby seismiclines (SG4 and SG5 in Plate 1), parallel pattern.When the offset along the major fault is large but the hangingwallrollover and divergentbasin fill indicates enough to allow. the basin substratumhangingwall cut off to a listric geometry for the fault which passesfrom 70ø SE reach the low-angle segment of the fault, shearing of the upsectionto 250-30ø SE at depth(Figure 6b and c). hangingwallfaulted blocks inducesa basinwardtilting of the In contrast, the horizontal basin fill and the faulted SE basin substratumresulting in a pseudo-rollovergeometry. margin in the Vauvert compartmentsuggest a two-segments In the Marette and Marsillargues compartments, the planar geometryof the N•mes fault. This geometryis rather transitionbetween the steep and low-angle parts of the N•mes well constrainedby hangingwalland footwall cutoffs(SG2 and BENEDICTOET AL.: F3('IENSIONALCRUSTAL RAMP AND BAS1NGEOMETRY 1203

SG3 in Plate 1). The fault dip changesfrom about 70ø SE Our structuralanalysis emphasizes the different tectonicsof upsection to about 15ø at depth (Figure 6a). The Oligocene the basin between the Oligocene-Aquitanian and post- reflectors rest directly on the low-angle segmentof the fault, Aquitanian periods. This analysis supportsthe interpretation indicating that the hangingwall pseudo-rollover stage has of the post-Aquitanian discontinuity in terms of "breakup been reached. unconformity",separating (1) a synrift Oligocene-Aquitanian The along-strikevariation of the Nimes fault shapeis well episode related to pluri-kilometric extension along a low- imaged by the isobath map of the Nimes fault (Figure 5a), angle crustalramp, from (2) a postrift post-Aquitanianepisode where the southernlistric and the northerntwo-parts planar characterized by a combination of large-scale erosions fault sectors are well differentiated. The related change in (breakup unconformity and Messinian erosion) and vertical hangingwallgeometry is well imaged by the isobathmap of accommodation compensatedby steeply dipping basement the Aquitanianbase (Figure 5d) wherecontours draw a faultward faults. Although precise postrift tectonic movement is dip surfacein the listric fault sectorand a synclinepattern in difficult to appreciate with available data, its maximum the two-partsplanar fault sector. possible vertical amplitude would be about 1000 m (in the The Nimes fault differs from the theoreticalmodels of Figure southof the graben,Figure 5e), correspondingto a maximum 8 by the fact that the deep part is not horizontal but showsa horizontal extension of about 500 m. low-angle inclination. This difference did not influence the structural style of the hangingwall but introduced a vertical 4.3. Amount of Synrift (O!igocene-Aquitanian) componentof hangingwall displacement,responsible for the Extension depositionof the Oligocene-Aquitaniansediments SE of the rollover crest. The amount of extension calculated on a fault flattening at The SE dip of the Oligocene-Aquitaniandeposits along the depth depends on the assumed modes of hangingwall NW border of the grabenis interpretedas being related not to deformation.For instance,assuming hangingwall deformation the particular geometry of the fault, but to differential by simple shear [Verrall, 1981; Gibbs, 1983; White et al., compactionof the underlyingsediments [White et al., 1986]. 1986; Faure and Cherrnette, 1989; Dula, 1991], varying the In the three compartments,the antithetic d6collementsin angle of shear(c0 from 45ø to 90ø inducesdifferences in the the Oligocene-Aquitanian successionare interpreted as the estimated horizontal extension ranging from 100 to 300% basal surfacesof gravity slidestoward the deepestpart of the [White et al., 1986]. basin, resulting from the hangingwall tilting induced by the In the case of the Vistrenquegraben, we have calculatedthe Nimes fault. value of synrift extension for each compartmentby restoring The distribution and subhorizontal geometry of the post- the top of the pre-Oligocene(prerift) succession.We assumea Aquitanian Miocene cannot be explainedby the activity of a 60 ø inclined shear deformation, compatible with the classical low-angle fault at depth. This implies that the low-angle angle of fracturingof rocks [Faure and Cherrnette,1989] and basementramp of the N•mes fault was no longer active after which provides a first order approximationin calculating net the Aquitanian. However, the base of the post-Aquitanian extension(Figure 8). depositsis locatedat 1000 to 1500 m deep in the Vistrenque Whatever mode of deformation of the hangingwall and graben while it is outcroppingnorthwestward on the footwall restoration technique is used, calculating extension in the of the N•mes fault (Figures 5e and 6). This differencemust be Vistrenque graben poses two problems: (1) the present top explained.Three possibilitiescan be considered:(1) tectonic footwall cutoff visible on seismiclines may not correspondto activity of the steepbasement faults observedon seismiclines the original cutoff which may have been eroded during in the footwail of the N•mes fault ßthese deep faults may have extension and the formation of the post-Aquitanian breakup propagatedupward and reutilized the steepupper part of the unconformity. Extension calculated using the present top Nimes fault, (2) compaction of the Oligocene-Aquitanian footwall cutoff will be a minimum as is shownin Figure 8b. In sediments,and (3) burial of a topographyinherited from the the case of the Vistrenquegraben, we assumethat the original post-Aquitanianerosion (paleo-fault). top footwall cutoff was locatedat 225m abovethe presentsea Differential compaction of the Oligocene-Aquitanian level (1 in Figure 8b). This level correspondsto the average sediments may explain the gentle syncline geometry of the elevation of the preserved subhorizontal post-Pyrenean post-AquitanianMiocene unit in the centerof the graben,but erosion surface in the Nimes fault footwall (Vaunage area, cannot explain its important thickness in the whole area Figure 3); and (2) possiblepost-Aquitanian (postrift) tectonic located SE of the Nimes fault including sectors where movement must be restored before calculation of the synrift thickness of the Oligocene-Aquitanian deposits is reduced extension.As this movement is not precisely known, we have (compareSG4 and SG5 in Plate 1). calculatedthe possibleextreme values of synrift extensionfor Since the Nimes fault is sealed by the Messinian erosion, both minimum (0 m) and maximum (1000 m) possiblepostrift tectonicactivity of steepdeep faults would have only occurred vertical movementsin each compartment. during the post-AquitanianMiocene, the Pliocene-Quaternary Calculatedextension (Figure 6) is comprisedbetween 8750 units occupying the space created later by the Messinian m (400 m postrift movement) and 9000 m (no postrift erosion. However, respective importance of offset by post- movement) acrossthe Vauvert compartment,9200 m (500 m Aquitanian tectonic movement and topographic offset postrift movement)and 9500 m (no postrift movement)across inherited from the post-Aquitanian erosion cannot be the Marsillargues compartmentand 3600 m (1000 m postrift differentiated with available data. movement) and 4400 m (no postrift movement) across the 1204 BENEDICTO ET AL.: EX'IENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY

Marette compartment.Extension has also been calculatedin 5.1. Reconstruction of the Prerift (Pre-Oligocene) the Petit Rh6ne graben,resulting in valuesof about2200 m Topography across the Marsillargues compartmentand 4600 m in the Marette compartment. Classical fault reconstructiontechniques are commonlyused These valuesof extensionpoint out the role of the Aigues- to calculate the fault geometry from the hangingwall Mortes transferzone which separatestwo compartmentswith geometry.This implies the definition of a prerift level datum, notably different amountsof extension,explaining the thin usually assumedto be a horizontal line at the elevation of the Oligoceneseries in the Marette compartment.On the other top footwall cutoff. In the case of intramontanebasins, it is hand, the similar values in the Marsillargues and Vauvert obvious that the prerift surfacewas not horizontal becauseof compartmentsindicate that the different structuralstyle important initial morphology. In this case, reconstruction between the southern and northern parts of the Vistrenque techniquescan be usedin an inverseway for the reconstruction grabenwas not relatedto a changein the amountof extension of the prerift topographyif both the fault and hangingwall across the Gallician transfer zone. geometriesare well constrained. We have appliedthis approachto the Vistrenquegraben in order to evaluatea possibleprerift topographyinherited from 5. Structural Inheritance and Origin of the Nimes the Pyreneanorogeny (Figure 9). The recentreprocessing and depth conversionof lines SG4 and SG9 (by the IFP, 1994) Fault Low-Angle Crustal Ramp defined precisely the N•mes fault and hangingwallgeometries The Vistrenquegraben formed in an areapreviously affected (Figure 8a). The extensionrestored is 9500 m corresponding by Mesozoic extension (SE France basin) and by Upper to the maximum horizontalextension as explainedbefore. In a Cretaceous-Eocenecompression (Pyrenean orogeny). The first step, the Petit Rh6ne graben was restored using the N•mesfault is thoughtto have actedas a normalfault during simple shear ((• = 60 ø) technique. According to this the Mesozoic extensionand a sinistralstrike-slip fault during reconstruction, the Petit Rh6ne fault is listtic and flattens at the Pyrenean compression [Arthaud and Sdguret, 1981]. about 4800 m (marly Lias). In a second step, we have Evaluationof the role of this structuralheritage is important reconstructed the prerift topography using three different in the understandingof the origin and compartmentalization techniques,the constantbed length [Davison, 1986], constant of the graben. Specifically, we have investigatedthe possible heave (• = 90ø) [Verrall, 1981; Gibbs, 1983] and simpleshear existence of an inherited topography and the correlation of (• = 60 ø) [White et al., 1986; Faure, 1990; Dula, 1991] Tertiary extensionalstructures with preexistingstructures. techniques(Figure 9).

topfootwallcutoff 225m Okm

1

2 - - 2

3 3

4 4

5 5

6 6

7 7

8 8

9 -9

_

10 -10 Figure 9. Prerift relief reconstructionthrough the cross section b in Figure 6 using three different reconstructiontechniques: inclined shear ((• = 60ø), constantheave, and constantbed length. (a) Theoretical geometryand depth of the top hangingwallassuming an horizontalprerift level datum at 225 m high and usingthe inclinedshear technique ((• = 60ø). Note the verticalamplitude of the relief in the threecases. BENEDICTO ET AL.: EXTE2qSIONAL CRUSTAL RAMP AND BASIN GEOMETRY 1205

Present structuration Prerift structuration

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• •...... LowerCretaceous "• Mesozoicnormal fault rotated/reutilised as thrusl '--'--I"'"":'•'• ß Jurassic -,--,-• MesozoicPyreneannormalstrike-slip faultfault • Pyreneanrelief • Pyreneanthrust •i I!•';:•;• ....I Oligocene-AquitanianUpper Cretaceous/Eocene NimesonfaultJurassic offset -•-'• -•, Pyreneananticline - overturned anticline Figure 10. Structuralschemes showing (a) presentlocation of the prerifi structuresin the Vistrenque grabenarea, and (b) locationof the prerifi structuresbefore the Oligocene-Aquitanianextension.

Both the constantbed length and constantheave techniques 5.2. Prerift Structures result in a depressionin the axis of the basin and relief (1500 m and 1200 m, respectively) above the rollover. The simple Nimes fault. The thickening of the Jurassic sequences shear techniquegenerates only 1500 m of topographyabove between the Castries well (2020 m) in the footwall and the the rollover which progressivelyjoins the footwall surface.It Albaron 101 well (3313 m) in the hangingwall (A101 in is significant that in the three cases (1) a positive relief is Figure 3), and the horizontal synsedimentaryoffset of Aptian- generatedin the SE part of the section, and (2) the relative Cenomanian facies northeast of Nimes [Monier and Ferry, amplitudeof the relief betweenthe lowest and highestpoints 1987; Masse et al., 1990] attest to the existence of a is about 1200-1500 m independentof the surfacegeometry. Mesozoic Nimes fault during the formation of the SE France Although these estimatespossibly include errors due to the basin. lack of isostatic compensation, they provide a valuable By analogy with the steep geometry of other basement indicationof the locationand order of magnitudeof the prerift faults of the SE France Mesozoic basin [Route et al., 1992, relief. We interpret this topographyas the front of the eastern Maerten and S•ranne, 1995], we proposethat the Nimes fault continuationof the Pyreneanbelt (Figures lb and 10). was a SE facing steeplydipping fault systemresponsible for a The restorationpresented in Figure 9 is made assumingno 1000 to 2000 m offset of the top of the Paleozoic basement postrift movement on the Nimes fault. Introducing a postrift during the Mesozoic. This fault systemis representedon the movementwould result in a front of prerift relief closerto the seismic lines by the steep faults which offset the top of the surface trace of the Nimes fault, without changing the Paleozoic in the footwall of the upper part of the low-angle magnitudeof the relief. crustalramp. Orientationof the steepMesozoic Nimes fault is The front of the Pyrenean prerift relief has also been likely to have favored its reactivationas a sinistral strike-slip inferred from the restorationof the southeasternpart of the fault during the Pyrenean orogeny, but seismic data do not crosssection c (Figure 6) showedby S•ranne et al. [1995].In give evidence for such movement. this restoration, the structural high drilled in the Saintes Transfer zones. The Arl•sienne •ransfer zone has •een Maries 102 well (top of Late Jurassicdrilled at 115 m depth; interpreted as a regional feature whose structural and SM in Figure 3) is interpretedas a residualrelief relatedto the stratigraphicaleffects can be recognizedacross the whole Gulf uplift of the Mesozoic successionby a N verging Pyrenean of Lion margin [Gorini, 1993], althoughit is not well imaged thrust (Figure 10). in seismicprofiles. It correspondsto the SE continuationof a 1206 BENEDICTO ET AL.: EXTENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY major late Hercyniansinistral strike-slip fault outcroppingin the Massif Central (Figure lb). The Gallician transferzone separatesthe Lower Cretaceous prerift seriesin the north from the Jurassicprerift rocks directly overlainby Upper Cretaceousor youngerdeposits in the south.This transferzone is alignedwith E-W trending,N verging Pyreneanthrusts, the Montpellier thrust to the west, andsouth-Alpilles thrust to the east,which display the same stratigraphicdifference between their footwall and their hangingwall(Figures lb and 10). This suggeststhat (1) the Gallician transfer zone was controlled by this thrust system, and (2) the latter was formed at the level of a N facing Cretaceousnormal fault responsiblefor the stratigraphic difference between the southern and northern blocks (Saint Gilles fault in Figure 10). Thickening of the Mesozoic successionand deepeningof the top of the Paleozoicbasement northof this fault were probablyresponsible for the formation of the transfer zone, and the difference in extensional structuralstyle betweenthe Vauvertcompartment to the north and the Marsillarguescompartment to the south. A similar origin is proposedfor the N verging thrusts inferredat the northernedge of the SaintesMaries high (Figure "•:..."-•Mesozoic cover d6collementdomain 6c), explaining the northward thickening of the Jurassic '• Low-angle basementramp domain successionbetween the Saintes Maries well (2485 m) and the • Zoneundeformed during extension Albaron 101 well (> 3583 m) (Saintes Maries fault in Figure , , Extensional cover fault 10). The westward continuation of this fault may have • Extensional basement fault IIII Transfer zone controlledthe southernboundary of the Marette compartment. T Frontof basementPyrenean thrust Although this boundaryoccurs outside the seismicsurveys ==•m Pyreneanbasement ramp reactivated as oblique ramp during extension studiedin this paper,its locationis inferredfrom the different • Direction of extension structural style between the Marette compartmentand the Figure 11. Location of the Vistrenquegraben with respect offshorecontinuation of the Vistrenquegraben imaged on the to (1) the domains of different extensional structural style of NW-ECORS seismic profile [de Voogd et al., 1991]. On the Gulf of Lion margin, and (2) the Pyreneanthrust front. anotherhand, there is no evidenceof a prerift control on the Modified from Sdranne et al. [1995]. Discussionin the text. Aigues-Mortestransfer zone.

5.3. Origin of the NimesFault Low-AngleCrustal Ramp basement faults, as discussed above for the Nimes fault, we The low-anglegeometry of the Nimesfault at depthis conclude that the extensionallow-angle crustal ramps were similarto that of the otherssynrift basement faults observed most probably newly formed at the onset of the Oligocene- in seismiclines in theoffshore part of themargin [de Voogdet Aquitanian rifting; the new formation has probably been al., 1991, Gorini et al., 1993]. Thus the originof the NE favored by crustal weakening associatedwith the previous trending extensionallow-angle crustal ramps must be Pyrenean thickening. discussedon a regionalscale. However, the Vistrenquearea that forms a triangular zone At this scale,the basement-faulteddomain during rifting between the N•mes fault, the Ar16sienne transfer zone and the matchesthe area of crustalthickening during the previous Pyrenean frontal thrust, stands as an exception in this Pyreneanorogeny (Figures lb and 11), suggestingthat the structuralframework (Figure 11). Although the graben was latter played a major role in controllingthe formationof the formed by an extensionalbasement ramp, it standsoutside the extensionallow-angle crustal ramps [Gorini et al., 1993; area of previous Pyreneanthickening and relief (Figure 9). Sdranneet al., 1995].The orogeniclithosphere was probably This suggeststhat developmentof the extensional system weakenedand extensionwas preferentiallylocalized in the beyond the thickened area was more stable than reactivationof thickenedareas. However, the Pyreneanthrusts were E-W the frontal thrust in an extensionalregime. The extensional trending,implying that the NE trendingextensional crustal systemdeveloped 10 to 20 km northwestwarduntil it reached ramps cannot correspondto reactivated thrusts. The frontal preexisting major discontinuitiesin the basement; namely, rampsof the thrustsmay have been reactivatedas oblique the steeply dipping precursorNimes fault and the Ar16sienne extensionalramps [Sdranneet al., 1995], exceptin the transfer zone. Corbib.res transferzone (Figure lb) where the local SW-NE 6. Discussion trending of thrusts allowed their reactivation as frontal extensionalramps [Gorini et al., 1991]. The Vistrenquegraben, the deepestdepocenter of the Gulf of Sincethe Mesozoicfaults were probablysteeply dipping Lion passivemargin, is located(1) at the boundarybetween

Castriestype series Albaron 101 type series NW Pyrenean relief SE 1209

Okm

2

3

4 N-vergent g a) 5 Pyrenean thrust Mesozoic Nimes Fault 6

?

8

9

Triassic 10

N-Montpellierbasins Vistrenquegraben Petit Rh6ne graben I I I [• extension750m displacedprecursor andNfmesdeformed fault [•extension1100f

Okra

2

3

4 b) ramplinking 5 ";"•'•Basin fill cover d•col.-crustal•

• extension3000m • extension1100m Okra Okm

1 1

2

3

4

o 5 o c) 6 ? Basinfill rid;rI

8 o

t• extension750m displacedanddeformed

Okra

1

2

3

4 d) 5 Basinfill 6 7

6

9

10 t• extension5000m

.

c-

e) '•,-----"•...... Basin fill rider2

breakup postrifttectonic movement = 500 m c- f)

Figure 12. Model of kinematicrelationships between the low-anglebasement faulted and cover ddcollement domains. 1210 BENEDICTO ET AL.: EXTENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY the basement faulted and the cover d6collement domains of the surface and, consequently,the Vistrenque graben acted as a margin, (2) close to the transfer zone separating the classicalhalf graben (c and e in Figure 12). The new formed continental margin frora the Rh6ne valley intracontinental upper segment of the fault propagatedNW of the previous rift, and (3) in the foreland of Pyrenean crustal thickening high-angle segment(through-going normal fault, Withjack et (Figure lb). These characteristicsmake the Vistrenquegraben al. [1990]), generatingsuccessive "riders" [Gibbs, 1984] (c a key structurefor understandingmajor aspectsof the origin and e in Figure 12). This is necessarybecause d6collement and kinematicsof the margin extensionalstructures. activity implies southeastwarddisplacement, deformation and abandonmentof the upperpart of the Nimes fault. For clarity, only two episodesare representedin Figure 12. However, the 6.1. Kinematic RelationshipsBetween the lack of visible riders on the seismiclines suggeststhat actual Basement Faulted and Cover D6coilement deformationinvolved a greaternumber of episodes,resulting Domains of the Margin in a greater number of smaller riders, unresolvableby seismic reflection. The grabensof the north Montpellierarea are mainly filled by lower Oligocene sediments (Stampian), but the upper The model implies that (1) the NW limit of the basement- Oligocene(Chattian) is also presentin someof them (e.g., the faulted domain (Nimes crustalramp) of the margin remained fixed during the whole rifting period, and (2) this limit formed Sommi•res basin, Cavelief et al. [1984]). Although the age of the landwardboundary of the deformationdomain during most the earliest synrift sedimentsin the Vistrenquegraben is not of the time. Cover d6collement correspondedto short events precisely known, it is Stampian or older, and synrift when a small amount of extension was transmitted to more sedimentationcontinued through to Aquitanian.This implies external areas. This may have been induced by gravitational (1) that the cover d6collement in the external part of the instability on the basement slope, possibly associatedwith margin was active during the Oligocene period of activity of changes of regional tilting during margin development the Nimes fault crustalramp, and (2) that it was rootedin the (isostatic or thermal adjusttnents). latter. However, in the Vistrenque graben transect, the total Oligocene extensionbetween the Nimes and C6vennesfaults 6.2. Why is the VistrenqueGraben the Deepest calculatedfrom crosssection restoration is only 1.5 km, while it is at least between 9000 and 9500 m on the Nimes fault Synrift Depocenterof the Margin? (Figure 8). This implies that only a fraction of the Oligocene The regional section(Figure 2) showsthat the Vistrenque extension on the crustal ramp was transmitted to the graben is the major synrift basin of the margin along this d6collement, whereas the other fraction was transmitted to the transect. Southeast of the Vistrenque graben, the major emergentpart of the Nimes fault. extensional detachments identified in seismic lines under the On the basis of these remarks, a schematic model of continental shelf have not generatedlarge grabenson their relationships between the Nimes fault and the cover hangingwall,whereas the highly stretchedareas underlying d6collementis proposedon Figure 12. In the NW part of the the continental slope support only thin and discontinuous section, the listric faults detaching into the Triassic level are synrift sedimentation [de Voogd et al., 1991]. This is due to synthesized as a single "north Montpellier graben" (total the prerift high surfaceelevation inherited from the Pyrenean extension: 1,5 km). The initial structure (Figure 12a) is orogeny.If continentalbasins were formedon the collapsing characterizedby (1) a steeply dipping Mesozoic fault which mountainbelt, they were rapidly cannibalized.There is thus separatesa thin Mesozoic cover in the NW from a thick one in little stratigraphicrecord of the early stagesof rifting in the the SE, and (2) an hypothesizedPyrenean thrust responsible areasof prerift relief [Sdranneet al., 1995]. This may explain for the prerift relief. During extension, the footwall remains the discrepancybetween the amount of extension estimated undeformedand the hangingwalldeforms by antitheticshear at from crustalthickness and that estimatedfrom basingeometry an angle of 60 ø. The amounts of extension applied at each [Burrus, 1987; Steckler and Watts, 1980]. On the other end of stage are adjustedin order to fit the finite extensiondetermined the section, in the zone of the north Montpellier basins, from the present-daysection. The timing of the successive sediment thickness was limited by the small amount of stages is constrained by the recognition of syntectonic extension(1.5 km) and shallowdepth of the d6collement. sedimentation in the different basins. Stages b, c, and d In contrast,the Vistrenquegraben is (1) controlledby a correspondto the Oligoceneperiod, stagee correspondsto the major crustalramp activeduring the whole rifting history(9 to Aquitanian, and stage f correspondsto a possible postrift 9.5 km of extension),and (2) located in the zone where initial (post-Aquitanian Miocene) movement. For simplicity, we surfaceelevation progressively sloped down to sealevel at the have considered that the Petit Rh6ne graben was formed front of the Pyrenean belt (Figure 9), which allowed the independentlyfrom the Nimes fault during the early stagesof accumulationof a completeand exceptionallythick synrift rifting. In Figure 12, formation of the Petit Rh6ne graben is sequence. arbitrarily distributedin stageb. Data on Moho depth [Chamot-Rookeet al., 1995; Gaulier et We infer that during the Oligocene, distribution of the al., 1995] indicate the lack of Moho uplift below the movementbetween the d6collementand the upper part of the Vistrenquegraben (Figure 2) and its offshorecontinuation [de Nimesfault was achievedby alternatingepisodes: (1) whenthe Voogdet al., 1991]. This is in agreementwith the asymmetric d6collementwas active and ramped down into the low-angle structureof the extensionalsystem controlled by low-angle basementramp, the Vistrenque graben acted as a ramp basin detachments[Lister et al, 1991]. Upper crustalextension on (hangingwallsyncline), without an emergingactive fault at its the low-angle ramps transferredbasinwards to the zone of boundary(b and d in Figure 12); (2) when the d6collementwas continental breakup by an intracrustal detachment or/and inactive, the Nimes fault propagatedupward and breachedthe distributedin lower crustextension across the wholemargin. BENEDICTOET AL.: EXTENSIONAL CRUSTALRAMP AND BASIN GEOMETRY 1211

7. Conclusions during rifting becauseit was involved (displacedand deformed) in the hangingwall of the cover d6collement. The lower Detailed structural interpretation of the subsurfacedata in basementpart of the steeply dipping precursorwas reactivated the Vistrenque graben demonstratesthat the Nimes fault, during the postrift period, reutilizing the upper part of the consideredpreviously as a steeplydipping basement fault, was emergent Nimes fault, allowing deposition of thick at depth a low-angle basement ramp during the Oligocene- subhorizontalunits above the tilted synrift units. Aquitanian rifting. This case study of low-angle crustal fault The low-anglecrustal ramp of the Nimes fault, as well as the associated with the formation of extensional sedimentary other ramps of the margin of similar orientation, are basin adds to the on-going discussion of the activity of interpreted as newly formed structureswhose formation was normal faults with a shallower dip than predicted by allowed by crustal weakening resulting from the previous mechanics.The present study emphasizesthe role of lateral Pyreneanthickening, rather than reactivatedPyrenean thrusts. changes of fault shape in controlling the variations of Upper crustal extension corresponding to the graben hangingwall geometry: a listric geometry of the fault in the formation was not locally compensated, but transmitted southernpart of the graben generateda rollover and divergent basinward to the zone of continental breakup by an Oligocene-Aquitanianbasin fill, while a two-segmentsplanar intracrustal detachment, or/and distributed in the lower crust geometry of the fault in the northern part resulted in the acrossthe margin. developmentof a pseudo-rolloverand compensationgraben. Within the margin, the Vistrenque graben is a distinctive Areas with different structural style are separatedby transfer feature due to the Mesozoic extensionand Pyreneanorogeny zones superimposedonto preexisting faults inherited from structuralheritage. The SE deepeningof the basementacross Mesozoic extension and Pyrenean compression.The origin of the precursorNimes fault controlled the emergenceof the along-strikevarying geometryof the low-angle Nimes fault is newly formed extensionalramp; the crustal thickening and difficult to establish.It probably reflects along-strikevarying surfaceelevation in the Gulf of Lion made the Vistrenque structuralpatterns acrossthe steeply dipping precursorof the graben,located outsidethe orogenicfront, the only one whose Nimes fault. sedimentaryfill recordsthe whole rifting episode,resulting in In the studied transect, synrift extensional deformation of the deepestdepocenter of the margin. the margin was achievedby several large basementramps, the Nimes fault being the landwardmostone. Small amounts of Acknowledgments. This work was supportedby the European extension were transmittedlandward during short intervals of Community DGXII (contractJOULE II - CEC Project nø PL 920287 "IntegratedBasin Studies"). We are gratefulto Coparex,Elf-Aquitaine, time to cover d6collement probably resulting from Elf-Atochem and ESSO-Rep for providing data for this study and gravitational instability during margin collapse. especiallyto ESSO-Repand Coparexfor authorizedpublication of line Kinematic modelling shows that the preexisting drawingsof seismiclines. IFP hasreprocessed lines SG2 andSG4. Ideas (Mesozoic) steeply dipping Nimes fault controlled (1) the developed in this contribution benefited from discussionswith A. location of the ramp linking the landward cover d6collement Mascle. We are grateful to A. Gibbs, R. Collier, and an anonymous and the low-angle crustal ramp, and (2) the emergenceof the reviewerfor their insightfuland detailedcomments which improvedthe Oligocene-Aquitanian fault; but its upper part was not active paper.

References

Arthaud, F., and P. Matte, Les d6crochements Bessis, F., and J. Burrus, Etude de la Liguro-Proven•al Basin, AAPG lnt. Con.f, tardi-hercyniens du SW de l'Europe. subsidencede la marge du Golfe du Lion Nice, 1995. G6om6trie et essai de reconstitution des (M6diterran6e occidentale), Bull. Cent. Closs,E., Experimentalanalysis of Gulf Coast conditions de la d6formation, Rech. Explor.-Prod. Elf-Aquitaine,10 (1), fracturepatterns, AAPG Bull., 52, 420-444, Tectonophysics,25, 139-171, 1975. 123-141, 1986. 1968. Arthaud, F., M. Ogler, and M. S6guret, Biju-Duval, B., J. Letouzey,and L. Montardet, Crochet, J.Y., G6ologie et pa16ontologiede la G6ologieet g6ophysiquedu Golfe du Lion Variety of marginsand deepbasins in the partie septentrionaledu foss6oligocb•ne des et de sa bordure nord, Bull. Bur. Res. Geol. Mediterranean, AAPG Bull., 29, 293-317, Matelles (H6rault, sud de la France), Ggol. Min. Fr., s. 2 (1), 175-193, 1980. 1979. Fr., 1, 91-104, 1984. Arthaud, F., and M. S6guret, Les structures Burrus, J., Contribution to a geodynamic Curnelle, R., and P. Dubois, Evolution des pyr6n6enesdu Languedocet du Golfe du synthesisof the Provencal basin (North- grands bassins s6dimentaires fran•ais' Lion (sud de la France), Bull. Soc. Ggol. western Mediterranean), Mar. Geol., 55, bassinsde Paris, d'Aquitaineet du Sud-Est, Fr., s. 7 (23), 1-63, 1981. 247-269, 1984. Bull. Soc. Ggol. Fr., 8, 4, 526-546, 1986. Auzende, J.M., J. Bonnin, and J.L. Olivet, The Burrus, J., Review of geodynamicmodels for Davison, I., Listric normal fault profiles: origin of the Western Mediterraneanbasin, extensional basins; The paradox of calculation using bed-legth balance and J. Geol. Soc. London, 129, 607-620, 1973. stretchingin the gulf of Lions, Bull. Soc. fault displacement,J. Struct. Geol., 8, 209- Beaudrimont, A.F., and P. Dubois, Un bassin Gdol. Fra., s. 8 (5), 377-393, 1987. 210, 1986. m6sog6endu domainep6ri-Alpin: le sud-est Cavelier, C. (coord.) et al., Pa16og•ne,in de Voogd, B. et al., First deep seismic de la France, Bull. Cent. Rech. Explor.- Synth•se Gdologique du Sud-Est de la reflection transectfrom the gulf of Lions to Prod. E.IfAquitaine, 1, 261-308, 1977. France, edited by S. Debrand-Passard,S. Sardinia(ECORS-CROP profiles in western Beaufort, L., J. Bruneau, A. Crepin and Y. Courbouleix, and M.J. Lienhardt, PP. Bull. Mediterranean),in ContinentalLithosphere: Julian,Ampleur de l'6rosionpontienne et du Res. Geol. Min. Fr., 125, 389-468, 1984. Deep SeismicReflections, Geod. Ser., vol. comblementplioc•ne en Camargue,Bull. Chamot-Rooke,N., F. Jestin, R. Vially, and 22, edited by PP. AUG. Washington,D.C., Soc. Ggol. Fr., s. 6 (4), 157-184, 1954. J.M. Gaulier, Finite kinematics of the 1991. 1212 BENEDICTO ET AL.: EXTENSIONAL CRUSTAL RAMP AND BASIN GEOMETRY

Demarq,G., Etude stratigraphiquedu Miocene Lister, G.S., M.A. Etheridge, and P.A. orogeny, Mar. Petrol. Geol., 12 (8), 809- rhodanien, Mdm. Bull. Res. Geol. Min. Fr., Symonds, Detachment models for the 820, 1995. 61,257 p, 1970. formation of passivecontinenetal margins, Steckler, M.S. and A.B. Watts, Nature, The Dula, W.F., Geometric models of listric normal Tectonics,10 (5), 1038-1064, 1991. Gulf of Lion: Subsidenceof a young faults and rollover folds, AAPG Bull., 75, Maerten, L. and M. S6ranne, Extensional continental margin, Nature, 28, 135-430, 1609-1625, 1991. tectonics in the Oligo-Miocene H6rault 1978. Faure, J.-L., and J.-C. Chermette, Deformation basin (SE France), Gulf of Lion, Bull. Soc. Tempier, C., ModUle nouveau de mise en of tilted blocks, consequenceson block Gdol. Fr., 6, 739-749. 1995. placedes structuresprovenqales, Bull. Soc. geometry and extension measurements, Maillard, A., and A. Mauffret, Structure et Gdol. Fra., s. 8 (3), 409-628, 1987. Bull. Soc. Gdol. Fr., s. 8 (5), 461-476, volcanisme de la fosse de Valence, Bull. Valette, M., Etude structuraledu gisement 1989. Soc. Gdol. Fr., 164, 365-383, 1993. salif'ereoligoc•ne de Vauvert (Gard), th•se Gaulier, J.M., N. Chamot-Rooke, and F. Jestin, Masse, J.-P., P.-J.L. Masse, and G. Tronchetti, doctorat,Univ. MontpellierII, 1991. Analysisof the isostaticequilibrium in the Variations s6dimentaires sous contr61e Valette, M. and A. Benedicto, Gulf of Lion (France), Implicationsfor the tectonique durant l'Aptien sup6rieur- Chevauchements gravitaires geodynamicand thermal evolution, AAPG C6nomanien moyen ?• l'articulation des halotectoniquesdans le bassindistensif de Int. Conf., Nice, 1995. blocsprovengal et languedocien(SE de la Camargue(Marge du Golfe du Lion, SE de Gibbs, A.D., Balanced cross section France): cadre pa16oc6anographiqueet la France), Bull. Soc. Gdol. Fr., 166 (2), construction from seismic sections in areas implicationspa16og6ographiques, Bull. Soc. 137-147, 1995. of extensional tectonics, J. Struct. Geol., 5, Gdol. Fr., s. 8 (6), 963-971, 1990. Verrall, P., Structural interpretation with 153-160, 1983. McClay, K.R., andP.G. Ellis, Analoguemodels applicationto North Sea problems,Joint Gibbs, A.D., Structural evolution of of extensional fault geometries, in Assoc.œor Petrol. Explor. Courses(UK), 3, extensional basin margins, J. Soc. Geol. ContinentalExtensional Tectonics, edited by 1981. London, 141,609-620, 1984 M.P. Coward, J.F. Dewey, and P.L. Wernicke, B., and B.C. Burchfiel, Modes of Gorini, C., P. Viallard, and J. Deramond, Hancock, Geol. Soc. Speci.Pub., 28, 109- extensional tectonics, J. Struct. Geol., 4, ModUle d'inversionstructurale n6gative: La 125, 1987. 105-115, 1982. • tectoniqueextensive post-nappe du foss6de Mitchum, R.M., Seismic stratigraphy and White, N.J., J.A. Jackson, and D.P. McKenzie, Narbonne-Sigean(Corbinres, SE France), global changesof sea level, 1: Glossaryof The relationshipbetween the geometryof C.R. Acad. Sci. Paris, s. 2 (312), 1013-1019, terms used in seismic stratigraphy, in normal faults and that of the sedimentary 1991. Seismic Stratigraphy - Applications to layers in their hangingwalls, J. Struct. Gorini, C., A. Le Marrec, and A, Mauffret, Hydrocarbon Exploration, editedby C. E. Geol., 8, 897-909, 1986. Contribution to the structural and Payton,AAPG Mem., 26, 205-212, 1977. Withjack, M. O., J. Olson, and E. Peterson, sedimentaryhistory of the gulf of Lions Monier, P., and S. Ferry, Mise en 6vidence Experimentalmodels of extensionalforced (western Mediterranean), from the ECORS d'un haut-fond pr6-urgonien dans le folds, AAPG Bull., 74, 1038-1054, 1990. profiles,industrial seismic profiles and well Barr6mien du mont Ventoux, r61e Withjack, M. O., Q. T. Islam, and P.R. La data, Bull, Soc. Gdol. Fr., 164, 353-363, s6dimentaire de la faille de Nimes, Bull. Pointe,Normal faultsand their hangingwall 1993. Soc. Gdol. Fr., s. 8 (3), 191-193, 1987. deformation:an experimentalstudy, AAPG Groshong, R.H., Half graben structures: Olivet, J.L., P. Beuzard, J.M. Auzende and J. Bull., 79, 1-18, 1995. Balanced models of extensional fault-bend Bonnin,Cin6matique de l'AtlantiqueNord et Xiao, H., and J. Suppe, Origin of rollover, folds, Geol. Soc. Am. Bull., 101, 96-105, Central, Publ. Cent. Nat. de la Rech. Sci., AAPG Bull., 76, 509-529, 1992. 1989. 54, 1-107, 1984. Guennoc P, N. Debaglia, C. Gorini, A. Le Rehault, J.P., G. Boillot, and A. Mauffret, The Marrec and A. Mauffret, Anatomie d'une western Mediterranean basin geological margepassive jeune (Golfe du Lion - Sud evolution, Mari.. Geol., 55, 447-477, 1984. A. Benedicto,M. S6guret;and M. S6ranne, France) : Apports des donn6es Route, F., J.-P. Brun, B. Colletta, and J. Van G6ofluides-Bassins-Eau (UMR 5569), CNRS- g6ophysiques,Bull. Cent.Rech. Explor.- Den Driessche, Geometry and kinematics Universit6 Montpellier II, cc.57, 34095 Prod. Elf Aquitaine,18, 33-57, 1994. of extensional structures in the Alpine Montpellier Cedex 5, France. (e-mail: Ellis, P.G., and K.R. McClay, Listric Foreland Basin of southeastern France, J. toni@ dstu.univ-montp2.fr; bassins @ dstu.univ- extensional fault systems - results of Struct. Geol., 14, 503-519, 1992. montp2.fr;seranne@ dstu.univ-montp2.fr) analoguemodel experiments,Basin Res., 1, Ryan W.B.F and M.B. Cita, The nature and P. Labaume, Laboratoirede G6ophysique 55-70, 1988. distribution of Messinian erosional surface. Interne et Tectonophysique,CNRS-Universit6 Jackson, J., and D. MCKenzie, The Indication of a several-kilometer-deep JosephFourier, BP 53, 38041 GrenobleCedex geometrical evolution of normal fault Mediterranean in the Miocene, Mar. Geol., 9, France. (e-mail: [email protected] systems,J. Struct. Geol., 5, 471-482, 1983. 2 7, 193-230, 1978. gr.fr) Lefevre, D., Evolution morphologique et S6ranne, M., A. Benedicto, C. Truffert, G. structurale du Golfe du Lion: Essai de Pascal, and P. Labaume, Structural style (ReceivedSeptember 12, 1995; traitement statistiquedes donn6es,th•se and evolution of the Gulf of Lion Oligo- revised March 29, 1996; doctorat3 ø cycle, Univ. ParisVI, 1980. Miocene rifting : Role of the Pyrenean acceptedApril 1, 1996.)