Late orogenic carboniferous extensions in the Variscan French Massif Central Michel Faure

To cite this version:

Michel Faure. Late orogenic carboniferous extensions in the Variscan French Massif Central. Tectonics, American Geophysical Union (AGU), 1995, 14 (1), pp.132-153. ￿10.1029/94TC02021￿. ￿insu-00716156￿

HAL Id: insu-00716156 https://hal-insu.archives-ouvertes.fr/insu-00716156 Submitted on 10 Jul 2012

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. TECTONICS, VOL. 14, NO. 1, PAGES 132-153, FEBRUARY 1995

Late orogenic carboniferous extensions in the Variscan French Massif Central

Michel Faure

InstitutUniversitaire de France,D6partement des Sciences de la Terre, Universit6d'Or16ans, Orl6ans, France

Abstract. The Variscan French Massif Central nappestacking and is responsiblefor partial meltingat deep experienced two successivestages of extension from Middle levels, induces gravitational instability. According to mass Carboniferous to Early Permian. In the northern Massif balancedeductions from the peripheralmolasse basins and the Central, the first stage began in the late Visean, immediately uplift rate, erosionalone appearsunable to denudethe main after nappe stacking, and is well recorded by Namurian- part of the uppercrust and to exhumethe deepestparts of the Westphalian synkinematic plutonism. The Middle orogen [Thompson and Ridley, 1987; Dewey, 1988]. Carboniferous leucogranites widespread in the NW Massif Extension removes the instability by reducing the thickness Central (Limousin and Sioule area) were emplaced within a of the crustalroot and allowing the continentalcrust to recover crustextending along a NE-SW direction.At the sametime, the a standard thickness of about 30 km. hanging wall or "Gu6ret extensionalallochton" moved toward The Variscan Belt of Western Europe is interpretedas a the SE. Several examples of the synextensionalplutonism are collisional belt between the northern Laurussia and southern also recognized in central Limousin: Saint Mathieu dome, La Gondwanacontinents [Dewey and Burke, 1973, Matte, 1986]. Porcherie, and Cornil leucogranites.These examples illustrate In the FrenchMassif Central,which belongsto Gondwana,the the relationshipbetween granite emplacementand crustal scale Variscanorogenic events range from Late Silurian(circa. 410 deformation characterized by NW-SE stretching and NE-SW Ma), which is the average age for the high-pressure shortening. In the central and southern Massif Central metamorphism,up to Late Carboniferous-EarlyPermian (ca (C6vennes,Chhtaigneraie, and Margeride areas), plutonismis 300 Ma) which is the depositiontime of the late-orogenic, dominantly granodioritic and exhibits the same structural frequently coal-bearing, intramontanemolassic sediments. features: NW-SE maximum stretchingand overturning to the Crustal thickeningin the Massif Central is well documented SE. Middle Carboniferous (Namurian-Westphalian)extension [Ledru et al., 1989 and referencestherein] by (1) southverging was parallel to the Variscan belt both in the Massif Central and deep seated metamorphic nappes, (2) high pressure southern Armorican area. This extensional regime was active metamorphism,and (3) crustalmelting and magmatism. from the late Visean in the north,while compression The width of similar tectoniczones is nearly 4 times larger dominated in the southernmostdomains (Montagne Noire and in the Massif Central than in the nearby Armorican massif. Pyren6es). The second extensional stage occurred from Late Such a sudden change, occurring within a few tens of Carboniferous to Early Permian. This event was responsible kilometers, suggestseither paleogeographicvariations that for the opening of intramontane coal basins, brittle are not recorded in the rock facies or differential modifications deformation in the upper crust, and ductile normal faulting of the initial compression-relatedstructures. The tectonic localized on the margin of cordieritegranite-migmatite domes. zonation seems to have been dilated in the Massif Central. Data from the coal basins show that the half-graben is the Indeed, extensionaltectonics due to gravity collapse of the dominant structural style, except for basins located along thickened crust has been described in many places of the submeridianal left-lateral faults which have pull-apart Massif Central, [e.g., Mattauer et al., 1988; Mdnard and geometries. Late Carboniferousextension occurred along the Molnar, 1988; Van den Driessche and Brun, 1989, 1991; NE-SW direction. The NE-SW maximum stretchingdirection Faure, 1989; Echtler and Malavieille, 1990; Malavieille et al., can be found in the whole Massif Central but is more developed 1990; Faure et al., 1990; Faure and Pons, 1991]. As pointed in the eastern part. The extensional direction is transverseto out by Faure and Becq-Giraudon [1993], post-Variscan the general trend of the belt, and top-to-the-NE shearing is extension in the Massif Central can be subdivided dominant. Correlations of these two extension directions with chronologically and structurally into two successivestages. neighboringVariscan massifsare discussed. The Middle Carboniferous(late Visean-Namurian-Westphalian, 330-315 Ma) extensionalperiod is characterizedin the north Introduction Massif Centralby a NW-SE maximumstretching direction and a NE-SW intermediate axis of finite strain. It is followed by a Late orogenic extension is presently recognized in most secondextensional stage in Late Carboniferous(Stephanian)- collisional belts. Crustal thickening, which is caused by Early Permian (Autunian) times characterizedby a NE-SW maximumstretching axis and a NW-SE intermediateaxis. This Copyright1995 by the AmericanGeophysical Union. paperaims to showthe spatialsuperposition and the temporal successionof the two extensionalregimes. New structuraldata Paper number 94TC02021. andprevious metamorphic, magmatic, and sedimentarydata are 0278-7407/95/94TC-02021 $10.00 synthetizedand reinterpreted in the frameof this model.

132 FAURE:VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL 133

The Middle Carboniferous NW-SE extension lineations occur in all the ductile faults that bound the Gu6ret allochton. Shear criteria consistentlyindicate a top-to-the-SE In spite of lack of Namurian-Westphaliansedimentary motion for the Gu6ret allochton with respect to the basins in the Massif Central, evidence for extensional surrounding units. tectonicsof this age is found in the ductile crust by a The time of the displacementis given by the radiometricage conspicuoussynmetamorphic deformation and the shapeof of the syntectonic leucogranitic plutons: 318_+5Ma (Rb-Sr numeroussyntectonic plutons (Figure 1). In the Massif methodon whole rock) for the Saint Sylvestrepluton [Duthou Central, two groups of Carboniferous granitoids are et al., 1984] and 324_+18 Ma (U-Pb method on zircon and recognized, namely, leucogranitesand granodiorite- monazite) for the Brame pluton [Hollinger et al., 1986]. The monzograniteassociations [Didier and Lameyre, 1969]. The syntectonic character of the leucogranitesis observed in the former is more common in the northern Massif Central, field, hand specimen, and thin section by the progressive whereasthe latter is widespreadin the centraland southern evolution at the scale of each pluton, from a mylonitic fabric parts.In the following,examples from several parts of the it, the pluton cortex to a magmaticfabric (i.e., orientedtexture Massif Central are presented to show how Middle with weak or no plastic strain) in the core [Molllet and Carboniferoussyntectonic granitoids recorded the regional Bouchez, 1982; Faure and Pons, 1991]. To the west and SW, extensionaldeformation during their emplacement. the Gu6ret extensional allochton is bounded by the Namurian leucogranitesof Brame, Saint Sylvestre, and Saint Goussaud. This last pluton is asymmetricwith a subverticalnortheastern Northern Massif Central side and a gently dipping southwesternone. It has an elliptical The Gu6ret extensional allochton. Ductile outcrop shape with its long axis parallel to the right-lateral extensionis well demonstratedin the NW part of the Massif AttUnes fault and also to the regional stretchinglineation in Central, or Limousin area. Northern Limousin (Figure 2) is the host rock mica schist.The Saint Goussaudpluton intrudes occupiedby the Gu6ret massif, which consistsof pre-upper the Devonian stack of nappes. The early tectonic contact Visean cordierite-bearing granitoid, migmatite, and gneiss between the mica schist and orthogneiss,which is parallel to radiometricallydated around360 Ma by the Rb-Sr methodon the regional foliation, is refolded by SW verging low-angle whole rock [Bertbier et al., 1979]. The granitoid and axial planar microfolds.These structuresare interpretedas drag metamorphic rocks that bear a subhorizontalfoliation are folds related to the emplacementof the Saint Goussaudmassif. geometrically underlain by flat-lying mica schist and gneiss The contactis thereforea detachmentfault (Figure 3). (Figure 3). To the east, the Gu6ret massifis sharplytruncated The Brame massif is the largest syntectonicleucogranite in by the Sillon Houiller fault. Due to the 60 to 70 km Stephanian northern Limousin. Structural analysis, in agreement with left-lateral offset of this fault, the Gu6ret massif and underlying gravimetric data lAudrain et al., 1989; Vasseuret al., 1990] Chavanon series are found in the Tr6ban massif and Sioule shows that the pluton is asymmetrically rooted to the east, metamorphic series respectively [Grolier and Letourneur, below the Gu6ret massif, and overturned to the west-NW. 1968; Grolier, 1971b]. Whatever its attitude, the foliation bears a N120øE to 130øE The Gu6ret massif is surroundedby the Plateaud'Aigurande mineral and stretchinglineation. The presentboundaries of the metamorphicseries to the north, the Brame-SaintSylvestre- Brame pluton are the Nantiat and Bussi•res-Madeleinenormal Saint Goussaudleucogranites to the west and SW and by the ductile faults to the west and east, respectively.In both cases, Chavanonmetamorphic series to the south(Figure 2). In this the leucogranite exhibits a mylonitic fabric with S-C last area, migmatite of the Gu6ret massif overlies biotite- structures.Hot slickensideson the C surfaceare parallel to the sillimanite gneiss and mica schist with a subhorizontal NE-SW stretching lineation. The relative displacement contactrefolded by NW-SE upright folds. In other places,the correspondsto a rise of the Brame pluton with respectto the boundarybetween the Gu6ret massifand the surroundingunits Gu6ret massif and the west Limousin metamorphic series. is marked by a subvertical ductile contact. The left-lateral Inside the pluton, magmatic structures formed before full Marche fault connects to the west with the N-S trending crystallization of the magma [e.g. Hutton, 1988; Paterson et Bussi•re-Madeleine fault, which, in turn, connects to the SE al., 1989] are parallel to the mylonitic ones. In particular,the with the NW-SE right-lateral,Arr•nes-La Courtine wrench fault mineral lineation exhibits the same NW-SE trend whatever its [Molllet and Lespinasse,1985; Lespinasseet al., 1986]. On presolidus or postsolidusorigin. the Gu6ret side of these faults, the sense of shear is sometimes The Plateau d'Aigurande metamorphic series. unclear becausean earlier ductile fabric formed in pre-Visean Ductile extensional tectonics of Namurian-Westphalianage, times is reactivated in the Middle Carboniferous deformation. related to the SE motion of the Gu6ret allochton, were first In fact, the Gu6ret massifappears surrounded and underlainby a described in the Plateau d'Aigurande metamorphic series and kilometer scale listtic surface,with vertical or high-angle dips the syntectonicleucogranites [Faure et al., 1990; Dumas et al., at ground level to the west becoming progressivelylower 1990]. The Plateau d'Aigurande is an antiformal stack of pre- angleto subhorizontalat depthand to the east(Figure 3). The Visean nappes[Quenardel and Rolin, 1984]. The WSW-ENE term "Gu6ret extensional allochton" [Faure and Pons, 1991] trending antiform is due to the emplacement of several will be used in the following. The Bussi•re-Madeleinefault leucogranite plutons (e.g., Crozant, Measnes, Orsennes, and exhibits a NW-SE (N120øE to N130øE) "hot striation" marked Crevant) dated around 312 Ma by the whole rock Rb-Sr method by muscovite, quartz ribbons, and sometimes sillimanite [Petitpierre and Duthou, 1980; Rolin et al., 1982]. The shape fibers. Its present geometryindicates a normal motion, the of the Plateau d'Aigurande plutons is characterized by a Gu6ret massif being downfaulted with respect to the Brame downward tail pointing to the SE, eccentricto the south, and granite. Similar NW-SE trendinghot slickensidesand mineral by a flat cap with recumbentperipheral lobes. Such a shapeis 134 FAURE:VARISCAN E)CI•NSIONS IN THE FRENCHMASSIF CENTRAL

4EI SinceyF.

MORVAN 2E BLISMES CALDEIRA /i EPINAC- .AUTUN

M6•rCEAU PLATEAU D'AIGURANDE • / TRER4N

46N 46N

ST-ETIENNE x

ST-MATt•EU ß x

VELAY BRIVE SCA

4. + + + + + + + + + + + + + FIGEAC

44N CARMAUX '/ 44N •ST-AFFR•QUE

2E ,SESSAC

50 Km

Gu6ret extensional allochton and

j.• CarboniferousPermian(dots)coal and basinsLate (black) f.'•• massifitseastern and equivalentLivradois ofareaTr6ban

'• MigmafiticMontagneNoffe)domes (Velay and [-•ß Otherundifferenciatedmetamorphic series +• MiddleCarboniferous (Namurian- Lower micaschist unit underlying Westphalian)granites • the Gu6ret extensional allochton Late Viseanvolcanic-sedimentary NW-SE stretchingdirection W••]rocks ("tufs anthracif•res") ofN. --• (Namurian-Westphalian) Massif Central > NE-SW stretchingdirection (Late Carboniferous-EarlyPermian)

Figure 1. General map of the FrenchMassif Central with emphasison the Carboniferousextensional structures.Abbreviations are E, Echassi&es;SCA, SainttChristophe d'Allier; V, Veinaz•s;and M, MarcoIts. FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL 135

1E 2E

.MONTMARAULT

x, y,, IOULE B-M F.

BELLAC

BRAME y x ALL•ON 46N X LIMOGES MILLEVACHES CLERMONT. suss^c ß .FERRAND

$T-,•AUD MESSEIX

ST-MATHIEU

Sillon Houiller 20 KM 1E ß 3E

"'•"'"'"''" Permian deposits Lowermetamorphic unit Thrust '• Carboniferousdeposits Normal fault

• foliationMiddleCarboniferous trend when knowngranitoids with Stephanian-Autuniankinematics Namurian-Westphaliankinematics •• LatedepositsVisean (Tufs Anthracif•res) Pre-Late Visean kinematics Pre'lateUssel,Tr6ban, Visean St-Gervaisgranites (Gu6ret,d'Auvergne) Metamorphicrocks undifferenciated

Figure2. Structuralmap of the Limousinarea showing the polyphasedeformations: NW-SE pre-Visean deformationand NW-SE Namurian-Westphalian andNE-SW Stephanian-Autunian extensional structures. Some earlythrusts are reactivated as normalfaults. Granitic plutons are ST-G, Saint Goussaud, ST-S, Saint Sylvestre; PO,Porcherie; CR, Cornil;E, Echassi•res;PS, Pouzol-Servant; ST-GA, Saint Gervais d'Auvergne. B-MF is Bussi•res-Madeleinenormal fault. S-C is SainteChristine fault. Coal basins are Co, Commentry;Do, Doyet; Bo, Bosmoreau. 136 FAURE: VARISCAN EXTENSIONSIN THE FRENCH MASSIF CENTRAL

W E I SillonlHouiller B.Mi GUERET 0l(]) _.,.. Nantiat1/F. N•IF.I StephanJanAhuncoal basinI/ LateViseanStephaniandyke coalswarmbasin i•? ..,•:i::"•'•"'--""•'• ,..: -. ;• ,?.:.: •,:,:•., .;.;::- •ALimagne '•"'"' ' , GHAVAN(•SOULE NW SE•, NE SW • L•UCOe•AN/T• Marche F. , Arr•nes F. POUZOL-SERVANT MICROGRANITE PLATEAUD'AIGURANDE 0J •) GUERET (])le c•,,,,.LIMOUSIN •'"• Upper plate rocks (pre-Visean metamorphics) .'..:.,.r•:,-..,;,;•,',.,'?:;x:.',.:,'.)., • %' '.,,' -'. ß' •.:;.• "•'...... iil'"•"'"'"'"•"'Gu6ret extensional allochton -• Lowerplate mica schists L• SyntectonicNamurian-Westphalian granites Figure 3. Cross sectionsthrough the Gu6ret extensionalallochton.

in agreement with vertical shortening as the cause of the or retrogressiveshear bands. This event is recorded by K-Ar flattened top of the pluton, as is to be expectedfor magmatic measurementson hornblendewhich provide ages between330 bodies emplaced within an extending crust. The leucogranites and 300 Ma [Cantagrel, 1973]. have oblique tails rooted at about 3-4 km from the Marche The Chavanon and Sioule metamorphic series. fault, and they are overturnedto the NW. The NW-SE direction The Chavanon series and its eastern equivalent, the Sioule that appears to control the upper flow direction of the series, are characterizedby an inverted metamorphiczonation leucogranite is also the regional direction of maximum [Grolier, 1971a]. From top to bottom, the subhorizontal stretchingin the host rock. metamorphic rocks consist of cordierite granite, (Gu6ret and The northwestern margin of the plutons is locally Tr6ban granites respectively to each series), migmatite, mylonitized. S-C relationships along the N120øE trending biotite-sillimanite gneiss, two-mica-sillimanite paragneiss, stretching lineation in the mylonite indicate a downward and biotite-muscovite+ staurolite mica schist (Figure 2). The motion (to the NW) of the host rock with respect to the biotite-sillimanite gneiss and two-mica paragneissboundary granite. A similar motion is shown in the contact aureole by is interpreted as a thrust surface. The same polyphase asymmetric biotite pressureshadows around garnet and shear evolution is observed in both areas. The oldest deformation is bands and also in the mica schist-gneisshost rock even when characterizedby NE-SW (N30øE-60øE) trending mineral and contact metamorphismis absent. The same NW-SE stretching stretchinglineations and intrafolial folds. At the southernend lineation is observed in the oriented leucogranite and its of the Sioule area, the metamorphicfoliation makes contact mylonitic cortex, in the contact aureole, and in the mica schist with the vertically foliated Saint Gervais granodioriteby the and gneiss country rocks forming the lower unit (Figure 2) right-lateral ductile wrench fault of Sainte Christine [Belin, [Quenardel and Rolin, 1984; Faure et al., 1990]. Conversely, 1981]. The ductile deformationis dated as pre-Visean by the the upper unit of the Plateau d'Aigurandemetamorphic series, unconformityof late Visean volcanic sedimentaryrocks of the which consists of gneiss, amphibolite, and migmatite, "Tufs anthracifbres" formation [Grolier, 1971a; Vennat, exhibits a Devonian NE-SW stretchinglineation [Boutin and 1982]. The right-lateralwrench at the southernboundary of the Montigny, 1993]. However, this upper unit is devoid of the Chavanon series cannot be dated since late Visean rocks are NW-SE ductile deformation related to the Namurian- lacking. The flat-lying foliation of the Sioule and Chavanon Westphalian extension. Early thrusts are reworked as metamorphicseries also bears a NW-SE mineral and stretching "detachmentfaults." Along the northernflank of the antiform, lineation well developed in the lowermost mica schist in the detachment hanging wall, the Devonian foliation is sequenceand in the two-mica-sillimaniteparagneiss [Grolier, folded into post folial folds, overturned to the NW, that are 1971a; Feybesse and Teygey, 1987]. The kinematics of this interpreted as drag folds related to the northwestward NW-SE deformationwhich is coeval to the retrogressionof an decollementof the upper plate during extension(Figure 3). In earlier medium pressure-mediumtemperature metamorphism the footwall, kinematic analysis along the NW-SE lineation indicate top-to-the-SEshearing [Faure et al., 1993]. shows a divergent motion away from the antiform axis, The difference between the Sioule and Chavanon series is namely, top-to-the-NW shearingand top-to-the-SEshearing in the Carboniferous magmatism. In the Sioule series, the the northern and southern limbs, respectively [Faure et al., Echassibresleucogranite occupies the core of an antiform. A 1990]. This extensional ductile deformnationis coeval with a 4øAr/39Arage of 308+2Ma(Namurian) on lepidolite [Cheilletz conspicuous retrogression of the mica schist-gneiss lower et al., 1992] is in agreementwith Rb-Sr whole rock age of unit. Most of the shear criteria are marked by chloritized micas 300-312 Ma [Duthou and Pin, 1987]. The Echassi•respluton FAURE:VARISCAN EXTENSIONS IN THEFRENCH MASSIF CE• 137 has been interpretedas an injected pluton filling a N60øE left- --- • ..... 2E40 lateral pull-apart [Gagny et at., 1984]. However, this model I'Archambault does not fit with the asymmetric shape of the pluton. To the east and SE parts of the massif, the granite foliation is flat ß . ß , lying (dip about 10-20øE) and concordant with the host ' ."'.' staurolite mica schist, whereas to the west and NW, the ß foliation dips more steeply, about 40-60 ø, to the NW. In the horizontal plane, the massif has a slightly elliptical section with a NW-SE long axis. In the vertical NW-SE sectionplane, the pluton is rooted to the NW and overturned to the SE in agreementwith the Bouguer gravimetric anomaly for this area [Vigneresseet at., 1985]. Linear structuresare more difficult to V!]L.LEFRANCHE + + observe, but, as is usual in most plutons, the best developed + + joint set correspondsto the "crossjoint" [e.g., Balk, 1937; Marre, 1982], which is the plane normal to the stretching lineation. In the Echassi•resmassif, most of the joints (aplitic MONTVICQ+ 3E dykes, quartz veins, and ore bearing veins) are orientedN10øE to N30øE, in agreement with the Nl10-120øE stretching lineation in the host rocks. The three-dimensionalshape and the internal structureof the Echassi•resleucogranite show that it is a syntectonicbody which experiencedNW-SE stretching, yarea O<- deposit_ vertical shortening, and southeastwardoverturning during its Permianf•':':.'• emplacement. ELOYStephanJan coalbasin The Pouzol-Servant microgranite is also an asymmetric Montmaraultgraniticcomplex [+-•--• body with a steep western margin and a flat-lying eastern 50 km margin concordant with the surrounding mica schist. This Metamorphicbasemenl boundary is well observed along the Sioule valley. There, numerousshear criteria, such as sigmoidal quartzo-feldspathic NNE SSW BO•O• lensesand asymmetricboudins of microgranitedykes, indicate DOYET-MONTVICQ VILLEFRANCHE L'ARCHAMBAULT COMMENTRY a top-to-the-SEsense of shear[Faure et at., 1993]. The Pouzol- gr a n itc metamorphxc, rock ' s Stephanian Permian , Servantmicrogranite is an injectedmassif which was emplaced within a N-S to N20øE gash opened as the same time as the t'" + •' '" -" :-'•-' -'1 hypotheticaldecollement NW-SE stretching. Evidence of magmatism related to brittle 5 km stretchingis also provided by the numerousN20øE to N40øE Figure 4. Structural map of the Montmarault area and cross trending microgranite dykes of late Visean to Namurian age section from Commentry to Bourbon l'Archambault showing [Vennat, 1982]. the NE-SW extensionalsetting of the Late Carboniferouscoal The NW-SE structural trend is also the axial direction for deposits.Along the Sillon Houiller, the basinsare left-lateral millimeter to kilometer scale postfolial upright folds which pull-aparts, west of the Sillon Houiller the half-graben are assumedto be coeval with the stretchingparallel to their structure predominatesßCommentry, Doyet-Montvicq, and axes. Consequently, the strain ellipsoid is qualitatively Vi]lefranche basins opened upon metamorphicbasement and characterized by a vertical maximum shortening, a NW-SE the Middle Carboniferous Montmarault granite which maximum stretching and an intermediate NE-SW shortening experienceda synemplacementductile deformation.A ductile axis. Such a prolate shape of the strain ellipsoid is detachmentfault is assumedat depth. When known, foliation representativeof a constrictionalbulk finite strain. and lineation trends are put inside the Montmarault granite. The Montmarault area. Due to poor outcrop Solid arrows indicate the senseof shear of the upper part both conditions, the NE corner of the Gu6ret extensional allochton in the granite mylonite (southof Bourbonl'Archambault) and is the less studied part of the northern Limousin structure. the host rock (west of Saint-Eloi). There, the Gu6ret massif is intruded by the Montmarault granitic complex (Figures 2 and 4) which is a composite massif with mainly pink porphyritic granite and related small Namurian, which is the assutned age of the granite granodioriteand leucograniteplutons [Truland et at., 1990]. crystallization and before the deposition of Early Permian The Montmarault granite is petrologically similar to the "red (Autunian) sandstonethat coversthe foliated granite [Turland granites" of the Bourbonnaisarea, dated between 330 and 320 et al., 1990]. Inside the Montmarault granitic complex, NW- Ma by Rb-Sr and U-Pb methods[Binon and Pin, 1989]. The SE linear preferredorientations marked by elongatedxenoliths same Middle Carboniferousage is likely for the Montmarault have been recognized[Barbarin et al., 1985]; (also personal complex. The NW boundary of the eastern arm of the observations, 1993). In the metamorphic rocks, along the Montmarault pluton is mylonitized. Along the N150øœ eastern contact of the granite, our preliminary investigations trending stretching lineation, shear criteria indicate a show a top-to-the-SE sense of shear along the N120øE to downwardmotion of the host rock with respectto the granite N150øE stretchinglineation. Some additionalstructural data of (Figure 4). The mylonitization occurred during or after the the Montmarault complex will be discussed below when 138 FAURE:VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL dealing with the formation of coal basins and Stephanian- indicate top-to-the-SE motion. However, E-W to NW-SE Autunian extension. stretchinglineation is conspicuousin the Dronne mica schist. In conclusion, the Gu•ret extensional allochton is one of Most of the shear criteria which indicate a top-to-theW-SW the widest late orogenic structuresof the Massif Central. The senseof shear[Floc'h, 1983] predatethe deformationrelated to architectureof a syntectonicpluton results from the interplay the granite emplacement.Although it is unknown,the age of between regional tectonics and ballooning due to pluton this deformationis likely to have beenpre-Visean as discussed dynamics [Brun and Pons, 1981]. The northern Limousin below. leucogranites provide various examples of shape and The Porcherie leucogranite-Sussac antiform. In emplacement kinematics controlled by NW-SE stretching central Limousin, the Porcherie porphyritic leucogranite during the collapseof the Gu•ret allochton.The Brame pluton, (Figure 2) intrudes the metamorphic series in the Saint overturnedtoward the NW, experiencedan antithetic motion Germain-les-Belles synform [Ledru and Hottin, 1984]. The with respect to the general tectonic motion. Conversely, the foliation bears a well-marked E-W to NW-SE (N80øE to Echassi•res body behaved synthetically like the surrounding N120øE) mineral and stretching lineation, coeval to the rocks. The weak ellipticity of the Plateau d'Aigurande or magma crystallizationbut becomingpost-solidus in character Echassi•res plutons suggests that the ballooning force was along the margins. This deformation is a Namurian- strong enough to balance the regional stretching, whereas in Westphalian event, since U/Pb dating on monazite gives an the case of the Saint Goussaudpluton the ballooningforce was age of 317+3 Ma [Lafont and Respaut,1988]. unable to opposethe regional stressfield. NE of the Porcherie granite, a lower mica schist unit, West of the Nantiat fault, the metamorphic series also equivalent co the Dronne unit, outcropsin the core of the experienced ductile extensionaltectonics. North of Bellac, the Sussac brachyantiform (Figure 2) [Mouthier, 1976]. The pre-Devonian "Lanneau migmatite" is bound to the west by a contact between the mica schist unit and the Limousin normal ductile fault along which several small gneissic gneissesis classicallyinterpreted as a refoldedNW vergent leucogranites exhibit S-C structures (the west side moving thrust[e.g., Floc'h, 1983;Ledru et al., 1989]. Top-to-the-NW down) [Santallier and Floc'h, 1989]. East of the Oligocene shear criteria are observed along the NW-SE stretching graben of the "Limagne d'Allier," NW-SE trending ductile lineation.Although it cannotbe demonstratedhere, it is likely extension is not described. However, in the Livradois area that at least a part of these microstructures,which occur in (Figure 1), conspicuous dynamic retrogression of the chlorite-sericiteretrogressive conditions, were formedduring metamorphicseries [Forestier, 1963] and top-to-the-SEductile the Carboniferousextension. Consequently, the mica schist- shearing in syntectonicgranitic bodies [Mouctar, 1985] are gneiss thrust contact is consideredto have been reworked as a tentatively interpreted here as formed during the Middle normalfault with west sidemoving down. Carboniferous extension. Evidence for NE-SW synextension shortening. In the southernLimousin, postfolial NW-SE to E-W trending

Central and Southern Limousin uprightfolds have long beenrecognized [e.g., Grolier, 1971b; Floc'h, 1983]. From north to south the folds are Sussac, Besides the Gu•ret allochton, which shows the structural Meuzac, and Tulle antiforms, and the Saint Germain-les-Belles relationship between granite emplacement and regional and Uzerchesynforms (Figure 2). The Limousinleucogranites extensional tectonic setting, several isolated areas in central always outcrop in the core of the antiforms. It is obvious for and southernLimousin also support this interpretation.They the Saint Mathieu dome and the Cornil granodiorite.This will be briefly examined. pluton(CR in Figure2) occupiesthe coreof the Tulle antiform. The Saint Mathieu dome. In the westernmost part of Rb-Srdating on wholerock gives a Namurian-Westphalianage the Limousin, the Saint Mathieu area exhibits a subcircular of 317+15 Ma [Bernard-Griffiths and Vachette, 1970]. structurepartly hidden below the Mesozoic cover (Figure 2). Emplacement kinematics of the Cornil granite is not This dome reworks the pre-Visean stack of nappes. The available, but preliminary survey showsthe occurrenceof a lowermost unit or Dronne mica schistis intruded by two types NW-SE minerallineation and magmatictextures. The upright of Namurian-Westphalian granitoids [Floc'h, 1979, 1983], folding is coeval to the greenschistfacies retrogressionof namely, the Piegut-Pluvier porphyritic granodiorite dated mica schist and gneiss of the lower unit. Chlorite-albite- around 325517 Ma (Rb-Sr method on whole rock) and the Saint actinoliteassemblages formed at the expenseof higher-grade Mathieu-Saint Saud leucogranite,dated around 315+17 Ma by minerals. In the present state of knowledge, an extensional the samemethod [Duthou and Dutreuil, 1978]. The leucogranite settingfor the Cornil granite emplacementappears the most that surroundsthe Saint Mathieu dome presentsa well marked likely. The Porcherie pluton occupiesan antiformal second- magmatic mineral preferred orientationevolving to a mylonite order fold within the Saint Germain-les-Bellessynform. No along the granite boundaries. Whereas the foliation strike large granitic massif appearsin the Meuzac antiform;however, marks the annular shape of the dome, the mineral and few leucogranitemasses are found, and the existenceof an stretchinglineations trend consistentlyNW-SE with a variable underlyingbody is suspectedon geophysicalgrounds. The dip all around the dome. S-C relationshipsand other shear same geometryis describedfor the plateau d'Aigurandeand criteria show a top-to-the-NW motion along the NW side of the Sioulemetamorphic series (see above, Figure 3) andhas been dome (the host rock moves downwards with respect to the consideredas evidence for a diapiric emplacementof the granite). There, the granite-metamorphiccontact is reworked leucogranites [Lameyre, 1982, 1984]. The pluton rise is by a left-lateral strike-slip fault in brittle conditions. Along responsiblefor refolding the stack of nappesand the thrust the easternside of the dome,the Saint Saudleucogranite and its contacts.The lower mica schistunit •s presentlyobserved in peripheralaplitic dykes are foliated and lineated;shear criteria tectonic "windows" which are in fact the "footwall" of FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL 139

extensionalallochtons coeval to granite emplacement.The interference, already considered in the case of the north Limousin area, between granite ballooning and regional deformationis responsiblefor the bulk postmetamorphic structureof the southLimousin. On one hand,the plutonism createsthe domingof the metamorphicseries, and on the other hand, the regional strain field, characterizedby NE-SW Mende shorteningand NW-SE stretching,is responsiblefor the NW- SE orientationof the uprightfolding (Figure 5). When the regional shorteningis weaker than the ballooningpush, a brachyantiform,as in the caseof the Saint Mathieu dome, will Florac' develop. MONT LOZERE The Argentat fault and Massif de Millevaches. It has long been proposed[Grolier, 1971b;Lameyre, 1982; Floc'h, 1983] that the Argentatfault is a verticalthrust contact between the Limousin nappes and the Millevaches metamorphicseries (Figure 2). The Argentatfault has also beenconsidered as a normalfault [Lameyre,1984; Ledru and Autran, 1987; Mattauer et al., 1988]. Along the fault, ,?•" Fig.7 MONTCALM myloniticfoliation dips steeply(60-70 ø) to the westand bears a stretchinglineation trending to N330øEon average.Shear bandsand othercriteria indicate a dextral-normalmotion, the Limousin gneiss moved downwardwith respectto the Millevachesmassif [Feix et al., 1987]. Althoughearlier deformations cannot be ruled out, this ductile deformation -- 10km likely took place in Namuriantimes sincemylonitization ST-GUIRALI, , I LIRONStudy are •'••rn occurswithin retrogressive conditions in the Limousingneiss • Mesozoiccover (temperatureof 300-400øC,pressure of 2kb,Feix et al., 1987). ....[--•aureoleBoundary aroundofmetamorphic the plutons

Moreover,mylonite pebbles similar to the myloniticrocks .•--• Coalconglomeratesbasin with (dots) basal • Aigoual-St-GuiraI-LironMontLozb. re-Bome plutons observedalong the fault are foundin the Stephaniancoal l-----tC6vennes micaschists • withinternal structure basinsthat are distributedalong the Argentatfault. A Microgranitedykes widespreadcataclastic deformation responsible for a normal- Figure 6. Structural map of the C6vennesarea with emphasis sinistral motion overprints the ductile deformation of the late structuresrelated to granite emplacement.Bar is [Labernardii•re,1970]. The tectonicsignificance of thisbrittle stretching lineation related to pluton emplacement' arrows structurewill be discussedin the followingsection. indicate the sense of shear of the upper part, both in the Eastof theArgentat fault, the Millevaches massif presents a granite and the host rock. widevariety of metamorphicand granitic rocks. Leucogranites

are among the youngestplutonic rocks; a few Rb-Sr datingson whole rock indicate late Visean to Namurian ages (335 to 330 Ma [Monier, 1980]). The leucogranitefrequently occupies the core of N-S to NW-SE antiforms. When present, the planar preferred orientation of granite minerals is generally parallel to the host rock foliation, and a N-S to NW-SE mineral lineation has been recognized. This structural pattern is correlated with the diapiric emplacementof leucograniteand the ductile dextral-normal motion of the Argentat fault (Figure 5).

Southern and Central Massif Central

The C6vennes area. The southeastern part of the Massif Central, or C6vennes area (Figure 1), is a wide mica schist domain which experiencedprograde greenschistfacies metamorphism and nappe stacking in Late Devonian-Early Carboniferous.The two plutonsof Mont Loz•re and Aigoual- Saint Guiral-Liron intrude the subhorizontal mica schist series Figure 5. Schematic interpretation of the south Limousin (Figure 6). The emplacement of these massifs has been showing the relationships between postfoliation upright variously interpreted as synorogenic, late tectonic, or post folding, pluton emplacement,and normal-rightlateral wrench tectonic[e.g., Gbze, 1949; De Waard, 1949; Prager, 1965; Van motion along the Argentat fault. Moort, 1966] (also work of F. Arthaud, discussed by 140 FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL

Alabouvette [1988]). The synorogenicinterpretation was in parameter, ranging from 1.6 to 15, indicates a constrictional fact understoodas coeval with a south verging compression. finite strain (Figure 8). Brittle stretching shown by The Aigoual-Saint Guiral-Liron massif is made solely of boudinagedxenoliths and K feldsparmegacrysts is foundin the porphyriticgranodiorite, whereas the Mont Loz•re consistsof Pont-de-Montvert pluton. Submeridianal cross joints are several petrographic facies among which the porphyritic conspicuous'some of them, in the Mont Loz•re pluton, form granodiorite of Pont-de-Montvert forms the largest pluton large N-S trendingcliffs. N-S to N30øE trendingaplitic dykes [Van Moort, 1966]. The Borne massif, which is left laterally are, by far, the most abundant of the joint sets. They offset to the north by the Villefort fault, correspondsto the correspondto crossjoints [Balk, 1937; Marre, 1982]. As the easterntermination of the Pont-de-Montvertpluton. Due to its filling is the same granitic material, it is likely that stretching richnessin structuralmarkers, such as schlieren,aplitic joints, occurred during crystallization of the pluton. In the xenoiiths,and K feldsparmegacrysts, the porphyriticfacies is northwesternend of the Aigoual massif, microgranitictexture the most suitable for structural analysis [Fernandez, 1977; predominates.This massif terminatesin the north at kilometer Mialhe, 1980; Faure et al., 1992]. to meter-scale N-S trending microgranite dykes (Figure 6). The Mont Loz•re and Aigoual-Saint Guiral-Liron plutons This submeridianalto NE-SW dyke swarm is consistentwith an present common structural features. The foliations make E-W E-W to NW-SE stretching. The microgranite veins can be elongatedellipses, the long axis of which are parallel to the compared to tension gashes opened by NW-SE brittle mapped shape of the two massifs.In the Mont Loz•re massif, stretching and filled by injected microgranite during the the sameplanar and linear mineral preferredorientations cross Carboniferous extension. through the lithological boundaries. This shows that the A N80øE to N150øE lineation is well developed in the synemplacementdeformations of all the facies are coeval. The contact metamorphic aureole. North of Le Vigan, cherty plutons have an asymmetric shape in the submeridianal limestone presents a well-developed E-W boudinage, but verticalplane [Fabre, 1877]; in their northernside, the granite macroscopicevidence of stretchingis rare. In the field, this foliation dips gently to the north, whereas it is subvertical lineation appears as a crenulation and mineral lineation of along the southernside (Figure 7). The same asymmetryis biotite and andalusite; under the microscope, boudinaged found in the mica schist in close vicinity of the plutons. andalusiteand quartz pressureshadows at the extremities of Along the northern side, the N90øE to N130øE trending biotite porphyroblastsalso show stretchingin this direction. centimeterto meter folds are overturnedto the southand along Several generationsof contactminerals can be recognizedon the southern side, upright microfolds without overturning the basis of the degreeof their preferredorientation. The latest predominate.These folds, which consistentlyindicate a N-S andalusitecrystals are randomly oriented, but earlier ones are shortening,progressively fade out away from the granite. As stretched and others are involved in the crenulation wrinkles. proposedin other examples[e.g., Pons et al., 1991; Paterson Therefore contact minerals are syntectonic to posttectonic et al., 1991], here also we interpretthe microfoldsobserved in with respectto graniteemplacement. Along the northernand the surroundingsof the granitic massifs as due to pluton southernsides of the two massifs, shear criteria (pressure ballooning and not to a compressionalevent of regional shadows,sigmoidal biotite, shear bands, and asymmetric extent. quartz lenses)dominantly indicate a top-to-the-eastsense of Inside the granite, the mineral lineation, defined by shear.However, even in gneissosegranite where stretching xenolith and biotite clots, has a near E-W trend. The strain lineation is well marked, shear criteria are difficult to assess. ellipsoid has been calculated using the shape of mafic These observationsare in agreement with a significant xenoliths. All the points fall in the prolate field; the Flinn componentof coaxial strain(i.e., stretchingwithout shearing) $ hi

St-Guiral porphyroidgranite EarlyPaleozoic Contactaureole • Sp • C•vennesmica schists (LeVigan series) _S p/ , \ , ._.

Figure 7. Cross sectionthrough the Saint Guiral granite(adapted from work of Arthaudas discussedby Alabouvette[1988]). SR is regionalfoliation in the C6vennesmica schists;Sp is pluton-relatedfoliation in the contact aur6ole. FAURE:VARISCAN EXTENSIONS INTHE FRENCH MASSIF CENTRAL 141

The ageof thesestructures is unknown,but theirgeometry and kinematicscorrespond to normalductile faults with the SE side movingdown. West of Al•s, the Montcalmmassif (Figure 6) presentsan anomalyin this consistentNW-SE trend.In this area, the andalusitemica schistsof the contactaureole have a N50øE lineation with top-to-the-NE senseof shear (normal geometry).This unusualpattern is interpretedhere as due to normal motion along the Villefort fault. The NE stretching suggeststhat this deformation might be relatedto the Late Carboniferousopening of the C6vennescoal basins (see below). On the basis of available Rb-Sr dates on whole rock and biotite or feldsparseparates, the age of the C6vennespluton rangesfrom 280 to 315 Ma [Hamet and Mattauer, 1977; Vialette and Sabourdy, 1977; Mialhe, 1980]. The youngest radiometric ages (280-290 Ma, Autunian) are certainly too young, since Stephanian-Permian detrital rocks overlie the Pont-de-Montvert pluton [Deroin et al., 1990]. A Middle Carboniferousage, close to the Rb-Sr whole rock age of 315+5 Ma in the Borne massif [Mialhe, 1980], is likely. Such a date Figure 8. Flinn diagramshowing the prolateshape of the would be in good agreement with the extensional setting mafic xenoliths. Triangles are Mont Loz•re; squares are proposedabove and also with the chronologicaland structural Aigoual-SaintGuiral massifs measured by J.FBabinault and I. data for the Margeride-Ch•taigneraiearea. Lucas. The Margeride-Chhtaigneraie area. The Margeride massif is the largest post-Visean plutonic complex in the Massif Central (Figure 1). The available radiometric ages (U- in the medial part of the plutons.In the westerntermination of Pb zircon of 334+7 Ma [Lafont and Respaut, 1988], Rb-Sr the Aigoual-Saint Guiral-Liron massif, the lineation dips whole rock of 322+12 Ma [Couturid et al., 1979], and U-Pb steeply toward the NW (N330øE), and the host rock moved monazite of 314+3 Ma [Pin, 1980]) indicate a staggering of downward. The eastern end of the Mont Loz•re massif can be magmatic events from late Visean to Westphalian, in observed around the Borne massif. There the lineation dips agreementwith the variety of granitic types [Couturid, 1977]. about 50-60øE, host mica schists moved downward with The main facies is a porphyritic monzogranite with several respectto the granite [Faure et al., 1992]. Along the northern and NW boundaries of the Mont Loz•re, the nonporphyritic granite is locally mylonitized [Fabre, 1877]. S-C relationshipsindicate right-lateral and normal motions (host -50 rock moving downwardsto the NW) in the north and NW sides, -40 respectively (Figure 6). As a whole, the kinematic picture related to granite emplacementindicates an E-W stretchingand divergent kinematics at the extremities of the plutons. '50, However, the top-to-the-SE shear is dominant over the top-to- the-NW shear. These kinematics indicate that the two plutons are overturned to the SE; the opposite shear sensesat their westernextremities are related to pluton ascentaround the root zone. The Bouguer gravimetric anomaly map for this area shows that the lowest minima are located in the western part of both massifs(Figure 9). This suggeststhat the granitesare rooted to the west, in agreementwith the kinematics. A sketch of the general three-dimensionalshape of the C6vennes plutons is depicted in Figure 10. Although a vertical shorteningcoeval with granite emplacementcannot be strictly proven, the shape IE 10 km

of the plutons, their internal structure, the E-W ductile -3O stretching, and emplacement-relatedkinematics better agree with an extensionaltectonic setting than a compressionalone in which N-S stretchinglineation would be expected. In the mica schistsoutside the contactmetamorphic aureole, NW-SE trending structures are rare. However, meter to Figure 9. Bouguer gravity anomaly map of the C6vennes millimeter scale upright microfolds and crenulations are area. Dashed line is the the boundary of the Mesozoic cover. described [De Waard, 1949]. In the SE part, top-to-the-SE Note that the minima (in mGal) are located in the western part penetrativeshear bands are locally well developed(Figure 6). of the two plutons. 142 FAURE:VARISCAN EXTENSIONS IN THE FRENCHMASSIF CENTRAL

ductile deformations are found on a subhorizontal foliation, the relatedcompressional or extensionalsetting is difficultto settle.Owing to the spaceproblem, the emplacementof sucha thick body,around 10 km, is easierto understandin termsof extensional tectonics. Between the western end of the Margeride pluton, or Entraygues appendix, and the Sillon Houiller, the Chhtaigneraiemetamorphic series is intruded by the granitic bodies of Omps, Boisset, MarcoIts, Veinaz•s, and Soulaque from west to east (Figures 1 and 11). All these massifs are porphyritic monzogranites petrologically similar to the Margeride pluton [Joubert, 1978; Feybesse, 1981]. Rb-Sr whole rock dates of 320 Ma, 315+15 Ma, 313+13 Ma, and 292+10 Ma for the Veinaz•s, MarcoIts, Omps, and Boisset granitesrespectively, (the ageshave been recalculated with )• = Figure10. Inferredshape of theC6vennes plutons and their 1.42 10-• year-1),support this comparison[Vivier and emplacementkinematics. Lasserre, 1973; Duthou et al., 1986]. The western side of the Omps granite experienced a postsolidus brittle-ductile deformation related to the wrench faulting along the Sillon NE-SW trending rows of small leucogranite bodies. The Houiller [Feybesse, 1981]. However, the internal structureof Margeridemassif is a subhorizontalslice 4 to 8 km thick that the Omps and Boissetplutons and NW-SE stretchinglineation extendslaterally several kilometers below the host rock mica in the host rock suggest that granite emplacementcould be schists. Gravimetric data show NNE-SSW trending light related to the Middle Carboniferous extension before the anomalies which are interpreted as feeder zones of the activity of the Sillon Houiller fault. Margeride granite [Petrequin, 1979; Bayer et al., 1982]. The Among the Namurian-Westphalian granites, the Veinaz•s NNE-SSW direction is subperpendicularto the maximum pluton (Figure 11) presents peculiar features. This massif stretchingdirection of Middle Carboniferousage recognized exhibits a stronglyanisometric shape with a NE-SW long axis all over the Massif Central. The laccolithic shape of the almost perpendicularto the usual NW-SE stretchingtrend of Margeride massif [Couturid, 1977] with its recumbentmargins the Namurian-Westphalian extension. In contrast to other is in agreementwith vertical shortening.According to Laboue plutons,andalusite, biotite, or cordierirein the contactmica [1982], the granitic fabric was formed during pluton schist are almost unoriented; static crystallization is emplacement. Structural analysis shows that N60øE and dominant.When presentin the contactaureole, the foliationis N140øE imbricate fabrics are widespread,but internal structure at high angle to the graniteboundary but is parallelto the of the Margeride massif is presentlynot available. Our local regionalfoliation. The Veinaz•s plutoncrosscuts the regional observations along the northern contact show that contact structuressuch as NW-SE foliation, NE-SW lineation, and early metamorphic minerals such as andalusite or biotite are thrusts [Joubert, 1978; Bogdanoff et al., 1989] without any sometimes oriented N120-130øE. Shear criteria indicate a structuralaccordance. Within the pluton, the planar structure, normal fault motion (the host rock is downfaultedwith respect markedby xenolithsand biotite, dips steeplyand followsthe to the granite)in agreementwith the kinematicsestablished in plutonshape; lineation is rare and subvertical.In thin section, the granitic northern margin [Laboue, 1982]. Moreover, as quartz is weakly deformed by unduloseextinction and already pointed out in the case of the Saint Mathieu area, polygonization,and zonedplagioclase with syneusisfigure is deformation related to granite emplacement is weak and common. These textural features indicate that the mineral subparallelto an older lineationof sillimaniteor biotite [Burg preferredorientation in the Veinaz•s pluton was acquired and Matte, 1978] with which it must not be confused. In the before the full crystallizationof the magma[e.g., Hutton, thermal aureole of the southern margin of the pluton, mica 1988; Patersonet al., 1989]. Along the margins,mylonite is schists exhibit a subhorizontal foliation with NW-SE mineral completelylacking. The centerof the foliationellipse is lineations of biotite, albite, and cordierite. Although the locatedto the NE, which correspondsto the biotite-cordierite contact minerals are oblique to the regional foliation, they are leucocraticfacies. The Bouguergravity anomaly[Joubert, deformed and indicate a top-to-the-NW shear. Due to 1978] showsa minimumalso centered in the NE endof the submeridianalfolding relatedto the Velay dome (Figure 1), the plutonsuggesting that this area is thepluton root zone. These laccolith footwall may be observed in the eastern margin. lines of evidencesupport our conclusionthat the Veinaz•s There, plastic deformation of quartz is limited to the graniteis an injectedpluton that filled a NE-SWtrending melanocraticfacies in the basal part of the pluton. In the same kilometer scale "tensiongash" which openedin agreement area, the Saint Christophed'Allier leucogranite(SCA in Figure with the Namurian-Westphalianstrain field. 1) exhibits a well marked sub-solidusfabric with a NW-SE As in the Limousin, in the southern and central Massif mineral and stretching lineation. Evidence of divergent Central, most of the Middle Carboniferous porphyritic s,hearing,namely, top-to-the-NW shear in theNW partand monzogranites present synemplacement structures top-to-the-SEin the SE partcan be observed.Top-to-the-NW characterizedby a NW-SE stretching.Granite shape, internal shearbands are also well developedin orthogneisswithin the structures,and emplacementkinematics are in agreementwith underlyingcountry rocks. Moreover, as the magmaticand an extensional setting. FAURE:VARISCAN EXTENSIONS IN THE FRENCHMASSIF CENTRAL 143

Omps N,20 • 2ø30 Boisset Marcol•s + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

44 ø 45

',<,45 /+

• 2O

MONTSALVY 45 50 30 20 .40 ,x

8O

10 Km

2 ø 30

•...... Contact aureole ---l_•foliationMain facies trendof the Veinaz•s granite with

Main quartz veins i- '+ ++ ,1 centralBiotite-cordierite facies leucocratic Pre-Visean thrust Micaschist host rock with foliation trend Compression related pre-Visean iineation with sense of shear of the upper part. Figure 11. Structuralmap of the Veinaz•spluton in the Chataigneraiearea showing its crustalscale "tension gash"shape, mapped with the collaborationof A. Chauvet.

Middle Carboniferous Extension Versus Massif Central exhibit the structuralfeatures established for Compression syntectonic plutons [e.g., Hutton, 1988; Paterson et al., 1991; Pons et al., 1991]. At the scale of the Massif Central, In the Massif Central, the emplacementof the Namurian- the NW-SE trend corresponds to the maximum stretching Westphalian granitoids follows a tectonic control. All the direction. The northwestwardor southeastwardoverturning of plutons discussedabove have a mineral preferred orientation each individual pluton dependson the local tectonic setting. that began to form during the crystallization of the magma The NE-SW direction correspondsto a shorteningdirection as under a regional strain and continued in the postsolidusstate shown by large-scale upright folding and the crenulation after complete crystallization [Hutton, 1988]. The planar and lineation. Evidence for vertical shortening can be inferred linear structuresare in continuity with the regional ones in the from the pluton shapes, in particular from the peripheral host rock. Frequently, the pluton marginsare mylonitized, and recumbentlobes. Top-to-the-SE displacementas in the Gu6ret emplacement-relatedplastic strain can be also recognized in extensional allochton and the C6vennes plutons appears the metamorphic aureole. In spite of a lack of precise dominant at the NW and SE extremities of the Massif Central, radiometric dating of igneous and contact metamorphic whereastop-to-the-NW shearis well developedin the southern minerals, the Middle Carboniferous plutons of the French Limousin. Metamorphic retrogressionof the lowermost mica 144 FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL

schist unit also occurred during the synmagmaticNW-SE the Massif Central. Due to its greater mobility, the stretching. These lines of evidence show that regional emplacementof the volcanicmagma in the uppercrust would deformation, granite emplacement, and retrogressionare precedethe slowest granitic ascent. coevaland relatedto the decompressionof the stackof nappes. However, in late Visean times, extensional tectonics did not An extensionalsetting is most likely to accountfor all these prevail all over the belt (Figure 13). In the southernmostarea, events. or Montagne Noire (Figure 1), kilometer scale south verging Although crustal thinning was active in Namurian- recumbent folds were emplaced in the latest Visean-early Westphaliantimes, the onset of this tectonicregime is not Namurian [Feist and Galtier, 1985]. The Paleozoic series is fully constrainedby the plutonism.Indeed, the late Visean is a underlain by mica schist and gneiss of the "axial zone." It is major chronologicalboundary in the history of the Massif likely that during the recumbent folding of the sedimentary Central. In the north, around Roanne, the late Visean "Tufs series, the Proterozoic-Early Cambrian substratum also anthracif•res" series is characterizedby the associationof underwent a compressionalevent recorded by (1) a Namurian sedimentaryand volcanic rocks dominatedby acidic lavas, 4øAr/39Arage of 316_+4Ma frombanded gneiss [Maluski et al., ignimbrite, and pyroclasticrocks. Near Boen (Figure 12), 1991], (2) a medium pressure/medium temperature microgranitedykes, granophyre,and granite are closely metamorphism(i.e., eclogitic facies and kyanite) [Demange, associatedwith the volcanic-sedimentaryseries. Petrological 1985], and (3) possibly, ductile deformation [Van den and geochemicalwork [e.g., Leistel and Gagny, 1984] has Driessche and Brun, 1991; Cassard et al., 1993]. Evidence for shownthat all the magmaticrocks are cogenetic.They belong compression at this time is also reported in the Pyren6es to a singlevolcanic plutonic event formed by crustalmelting. [Vissers, 1992]. Therefore, as suggestedby Mattauer et al. The effusivepart was emplaced along NE-SW trendingfractures [1988], compressionin the outer zone is coeval to extension that form a NE-SW graben,the "Loire synclinoriumtrough." in the inner zone. This tectonic pattern is similar to the The granite-granophyre-microgranite dykes present a Himalayas [Burg et al., 1984] or the northern Apennines subvertical N70øE trending planar mineral preferred [Carmignani and Kligfield, 1990] where graniticplutons were orientation corresponding to magma flow during their emplaced within an extensional setting while compression emplacement [Leistel and Gagny, 1984]. Thus the tectonic was still active in the foreland. In the Massif Central, the settingof the late Visean magmatismis in agreementwith a transition zone from compressional to extensional tectonics NW-SE stretchingof the crust.Although occurring in brittle is not precisely located since it moved southwardwith time. In conditions, this deformation can be considered as the Namurian-Westphalian times, this transition zone lay beginningof the Middle Carboniferousextensional regime in approximately around the Margeride massif. This may account

4E30

/

4E

v / / ROANNE © / v 46 N •,46N v v v v v v v V ',,, v vV v'Lv v V &?x v ' V V v

v v

Ignimbrite Granite Tuff -- Oranophyre 46 N45 46 N45 BOEN

!•:EURS 10Km I 4E 4E30

Figure 12. Late Viseanextensional setting of the ignimbriteand tuff formations(Tufs anthracif•res)in the Loire graben(adapted from Leisteland Gagny [1984]). FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL 145

COMPRESSIONI EXTENSION,,

I ?LA TEA U D ' AIG URAND E

N. LIMOUSIN BRAME ST-SYLVESTRE

A ECtlASSIERES

PORCHERIE CORNIL

S. LIMOUSIN PIEGUT ST-MATHIEU

OMPS VEINAZES , .. CHATAIGNERAIE B OISSffF MARCOLES

MARGERIDE u • u

BORNE MT-LOZERE CEVENNES

AIGOUAL

MONTAGNENOIRE A A A

' ' , COMPRESSION •WRENCH EXTENSION i I I I ' ' 'J "'=''1='"' "' "1• ' i'

3•5 345 325 315 45 295 285 275 VISEAN NAMURIAN STEPHANIAN 'rourASA ws'rr,A[S AurUrA

Figure 13. Timing of granite emplacementin the Massif Central in relation with the N. to S. migration of stressfields. A andU standfor 4øAr/39Ar and U/Pb methods, respectively' others are Rb/Sr ages recalculated with3. = 1.4210-11 year-1 (references in the text).

for the difficulty in clarifying the tectonic setting of this Echtler and Malavieille, 1990; Malavieille et al., 1990]. This pluton. Indeed, accordingto radiometricages, it is conceivable deformation is responsiblefor intramontanecoal basins and that early Margeridegranites were emplacedwhen compression granite-gneiss domes. This section emphasizes structural was still active, and the latest leucogranitic intrusions were featuresshowing that the maximumstretching trends NE-SW. emplacedduring the onset of extension. The NW-SE trend is a conspicuousstructural trend in the Massif Central, but it cannot be systematicallyattributed to NE-SW Extensional Features in the Upper Crust Middle Carboniferousextension. NW-SE wrench faults [Ledru Opening dynamics of the coal basins. In the and Autran, 1987] and NW-SE stretchinglineation lying on a Massif Central, Late Carboniferous (Stephanian) to early subhorizontalfoliation, for example in the Gu6ret massif and Permian (Autunian) sediments consist of continental to Limousin metamorphic series [Floc'h, 1983; Bouchez and lacustrine deposits trapped within several tens of basins Jover, 1986] formed in pre-late Visean times, are documented. ranging from less than one to several hundred square When chronological markers are lacking, the distinction kilometers (Figure 1). There is presently some uncertainty between the pre-Visean and Namurian-Westphalian regarding the precise location of the Stephanian-Autunian deformationsis difficult to assess.One key is given by the boundarydue to the isolatedcharacter of the basinsand the study of the PT path of metamorphicrocks since their pre- slow evolution of plants used as stratigraphic indicators. Visean deformationoccurs with a progressivemetamorphism Indeed, Autunian-type spores are found inside typical and their retrogression occurs during extensional tectonics Stephanian deposits, even in the type locality of Saint during uplift of the nappes[e.g., Faure et al., 1990, 1993]. Etienne, and Stephanian-typeferns are found inside the The Late Carboniferous-Early Permian Extension Autunian formations of the Autun basin [Broutin et al., 1986]. Therefore the accuratedating of deformationphases and their Stephanian-Autunian extensional tectonics have been correlation from one basin to anotherare questionable. already recognized [e.g., Mattauer et al., 1988; Mdnard and The boundarybetween the StephanJanand Permiandeposits Molnar, 1988; Van den Driessche and Brun, 1989; 1991; is in some placesconformable (e.g., Brive and Lucenay),and 146 FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL

in other it is unconformable;(e.g., Aumance,Decazeville, and Although border faults are hidden below the Mesozoic rocks, Lod•ve) the latter occurrenceshave been taken as the mark of a radial extension is invoked as opening dynamics for the tectonic event [Grolier, 1971b]. However, evidence for a Aumance basin. compressionalevent associatedwith an erosivediscontinuity Our preliminarydata show that the Ahun basin(Figure 2) is is lacking. Presently, the angular unconformityis better also a half-grabencontrolled by a NE-SW openingdirection. understoodin terms of tilted blocks [Van den Driessche and The main difference from the northern Gu6ret coal fields is that Brun, 1989; Legrand et al., 1991] the bordernormal fault dips to the SW. The structuralcontrol of the openingand sedimentaryinfill Stephaniannappes were recognizedin the C6vennescoal of the late Paleozoicbasins is well established[Vetter, 1986]. basin [Gras, 1970] and taken as evidence for a Carboniferous Several stress fields have been derived on the basis of relative compression [Bibs et al., 1989]. These nappes are chronology of brittle structures and "Stephanian" reinterpretedas gravity-drivensheets that slid along the coal stratigraphy.Emphasis has been placed on the strike-slip measures [Ddldnin et al., 1988]. Although west verging motionassumed to be dueto a NW-SE compressivestress field. reversefaults of unknownage do exist, they merelyindicate a In this scheme,coal and clastic depositsfilled pull-apart NNW-SSE shortening.The C6vennesbasin is divided into basins[Arthaud and Matte, 1975;Gdlard et al., 1986;Bonijoly westernand easternsubbasins by the Rouvergueridge, which and Castaing,1984; Lerouge, 1988;Bibs et al., 1989].In fact, formed before or during depositionof the Late Carboniferous the pull-apartmodel is limited to N-S trendingfaults, and the rocks. The westernsubbasin is a half-grabenbounded by a dominantstructural pattern of theseis the half-graben.This normalfault to the-NE, but a normaloffset along the Villefort geometryis in agreementwith an extensionalsetting [Gibbs, fault cannotbe ruled out, as suggestedby the top-to-the-NE 1984]. The azimuth of the border faults allows a classification ductiledeformation west of Al•s (Figure6). The structureof the [Faureand Becq-Giraudon,1993]. C6vennesbasin is in agreementwith NE-SW stretchingand The submeridianal faults: In the Late Carboniferous, NW-SE shortening. the Sillon Houiller and the Argentatfault movedas brittleleft- E-W trending faults: The Graissessacbasin has also a lateral wrenchfaults along which severalsmall pull-apart half-grabengeometry with a southernE-W trendingnormal- basinsopened (Figure 2) [Labernardibre,1970]. Structuraland dextralborder fault (slickensidestrend N50øE). Structurein the sedimentological studies in the Saint Eloi, Messeix, Late Carboniferous sediments shows that the basin infill is Decazeville, Bosmoreau, and Argentat basins show the relatedto the activityof this fault [Becq-Giraudon, 1973; syntectoniccharacter of the sedimentinfill. The openingof Echtlerand Malavieille,1990]. In the Lod•vebasin, Early pull-apartbasins along the N-S trendingfaults is in agreement Permianbeds unconformably overlie the Stephaniandeposits. with a NNE-SSW (to NE-SW) maximum stretchingdirection As the sameNE-SW to N-S maximumstretching direction is [Gdlard et a/.,1986; Robert et al., 1988]. In somecases, (e.g., found in Permian rocks [e.g., Santouil, 1980], this Decazeville, Messeix, Saint Eloi) en 6chelon folds indicate a unconformityis now interpretedin termsof synsedimentary NW-SE shorteningin agreement with a N-S trending left- intrabasinaltilt [Van den Driesscheand Brun, 1989].Farther handed pull-apart. N-S folds indicate an E-W to NE-SW north,the openingof the SaintAffrique Permian basin, which shortening;however, becauseof lack of stratigraphicmarkers, presentssimilar structures, is controlledby the reactivationof the age of this secondaryfolding is unknown. In the Messeix Variscanthrusts [Legrand et al., 1991]. basin, folding occurred after coalification, which itself is NE-SW trendingfaults: This trendis well developed related to the Late Carboniferousthermal regime [Robertet al., in the easternMassif Central.It controlsthe openingof 1988]. Thus a post-250 Ma (Late Permian) age for the E-W basinssuch as SaintEtienne, Blanzy-Montceau, and Epinac- shorteningappears likely. Autun.The SaintEtienne basin is boundedby thenormal fault NW-SE trending faults: The Commentry, Doyet- of Pilat characterized by N-S to NE-SW slickensides Montvicq, Deneuille, and Aumance basins (Figure 4) have a [Malavieilleet al., 1990]. The Stephanian"nappes" in the typical half-graben structure,documented by mining [Fayol, SaintEtienne basin must be reinterpretedas thin skinsheets 1888; Freytet, 1960]. The border fault is systematically due to gravitysliding. In the Blanzy-Montceaubasin, normal- located to the SW side of these basins. There, decameter to dextralfaults control the depositionof coalmeasures [Valid et hectometerscale brittle shearzones, formed at the expenseof al., 1988]. The early extensionaldeformation structures are Devonian metamorphics and the Middle Carboniferous erasedby a Tertiary deformation[Feys and Gand, 1983]. Montmarault granite, bear NE-SW downdippingslickensides. Structuraldata from the Epinac-Autunbasin are scarce. This structure is in agreementwith the tilted block pattern Accordingto Marteau [1983],a NW-SEto E-W shortening found in extensional basins. The arrangement of detrital characterizesthe late Stephaniantectonics, whereasN-S material close to the border fault supports a synsedimentary Autunianextension is responsiblefor a half-grabenstructure activity. The Aumance basin is the northernmostStephanian- with an E-W trendingnormal fault, located southof the Autun Autunian basin of this area. The unconformitybetween NE-SW basin. folded "Stephanian"and flat-lying "Autunian" sandstonewas Brittle deformation outside the coal basins. interpreted as evidence for a deformation phase [Grolier, Late Carboniferous-EarlyPermian brittle deformationin the 1971b]. However, NW-SE shortening does not support a upper crust is widespreadin the Massif Central, for instancein compressionalevent since vertical shorteningis predominant. the Marche [Lerouge and Quenardel, 1985], Millevaches Lower Permian clastic formations are synextensionaldeposits [Labernardibre, 1970], Ch•taigneraie [Bibs et al., 1982], controlled by two sets of NE-SW and NNW-SSE trending Margeride[Dutartre, 1981].Normal faults, microgranite dykes, normal faults [Bonnion et al., 1983; Turland et al., 1990]. joints, and quartz veins have been used to derive a succession FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL 147 of stress fields. According to the authors mentioned above, metamorphism (4-5 kbar and 800_+50øC)developed at the early-middle and late Stephaniantimes are characterizedin the expense of the Devonian structures, through migmatization Massif Central by N-S to E-W compression, respectively, and anatexis [Montel et al., 1992]. The cordierite (_+garnet) followed by a N-S Autunian extension. These stress field Late CarboniferousVelay granite dated at 298-274 Ma on the determinationsdo not provide the {•2-{•1/{•3-{• 1 ratio needed basis of whole rock Rb-Sr age [Caen-Vachetteet ai., 1982] is to estimate the relative magnitude of the three principal an anatectic massif coeval to the doming. Extensional stresses. The brittle strain data allow estimation of a N-S to tectonics are well documented in the northern margin of the NE-SW maximum finite elongation and an E-W to NW-SE Velaydome, along the Pilat detachement fault. The 4øAr/39Ar maximum finite shortening.Strain estimatealong the vertical dating supports a late Westphalian-Stephanianage for the axis is generally not considered. cooling and uplift of the metamorphicpile [Costa, 1991]. The In the NE Massif Central, the Late Carboniferous-Permian structureof the Velay dome is the result of the interferenceof a acidic volcanism of Blismes (Figure 1), occurred within an radial shortening due to the horizontal expansion of the elliptical caldera with a NNW-SSE long axis [Carpena et al., granite during its emplacement and a vertical shortening 1984]. Microtectonic analysis of the brittle deformation relatedto the regionallate Variscanextension. [Lagarde et al., shows that before Mesozoic tectonics, the volcaniclastic 1993]. sequencerecords a Permian extensionalstress field with a N-S The axial zone of the Montagne Noire is a gneissic dome maximum stretching direction and a Late Carboniferous recognizedsince G•ze [1949]. Cordierite-garnetgranitoids and extensionalstress field with N130øE and N40øE trendsfor {•1 migmatites occupy the core of an ENE-WSW elongated and {•3 respectively [Carpena et al., 1984]. These directions elliptical structure. N50øE to N70øE stretching lineation is are the orientation of the maximum and minimum horizontal conspicuous in both the migmatite and gneiss-mica schist stresses,the vertical principal stress being in fact {•1. The envelope. Divergent kinematics, namely, top-to-the-NE and smalldifference in modulusbetween {• 1 and•2 may accountfor top-to-the-SW shears at the NE and SW terminations of the the high frequency of strike-slip faults together with normal dome, respectively, and coaxial strain between are well faults. known. The axial zone dome is variously interpreted as a In spite of the difficulty of accuratedating, late Paleozoic basementantiformal stack, diapir, or strike-slip-relatedor pure brittle deformations in the Massif Central are consistent with extensional core complex [e.g., Schuilling, 1960; Beaud, an extensional stress field characterized by NE-SW to N-S 1985; Faure and Cottereau, 1988; Echtler and Malavieille, maximum stretching. 1990; Van den Driessche and Brun, 1989, 1991; Cassard et al. 1993]. Indeed, if Late Carboniferous ductile deformation in the Extensional Ductile Deformation in the Lower footwall of the Graissessacbasin is supportedby structuraland Crust radiometricdata, in thedome core the 4øAr/39Ar age of 316_+4 The Late Carboniferous ductile deformation is localized Ma on biotite [Maluski et al., 1991] indicates also a Namurian within granite-migmatitedomes (Figure 1). event coeval with recumbentfolding before doming. Therefore The Velay dome is the largest late orogenic deep seated the Late Carboniferous extension followed a Middle structurein the Massif Central (sccDupraz and Didier, [1988] Carboniferouscompression in the axial zone of the Montagne for an extensive bibliography). A high temperature Noire.

/ / •YET AUMANCE / ' -! •(• ST-ELOI

Figure 14. Schematicrepresentation of the Late Carboniferous-EarlyPermian extensiondue to doming and basinsopening. •e west to eastincrease of extensionrate is accomodatedby the Sillon Houiller and Argentat fault. 148 FAURE: VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL

Discussion The Middle CarboniferousNW-SE extensionis quite different. It is a diachronous event that started in late Visean in The Late Carboniferous-Early Permian extension, the northern Massif Central and later reached the C6vennes recognizedin the whole Massif Central, is responsiblefor the area (Figure 15). In late Visean, in the Montagne Noire, massif's present width. NE-SW maximum stretching, recumbent folds emplaced with a NNE-SSW stretching transverseto the general trend of the belt, increasesfrom west lineation but elongation occurred also along the E-W to east (Figure 14). In the eastern part, extension caused direction.The related strainellipsoid lies in the oblatefield. In doming and opening of more than 10 coal basins. In the the north Massif Central, the finite strain ellipsoid has a Limousin area, brittle NE-SW stretching is recorded in the prolate shape, since NW-SE maximum stretchingis coeval basement of the coal basins, but neither granite-migmatite with NE-SW shortening. In terms of finite strain, the dome nor ductile deformation of late Carboniferous age is relationships between NW-SE extension in the north Massif reported.West of the Sillon Houiller, coal basinsare few and Centraland NNE-SSW compressionin the MontagneNoire can are smaller than those to the east. The submeridianal strike- be qualitativelyseen as an exchangebetween the X and Y finite slip faults of the Sillon Houiller and the Argentat fault might strain axes in the horizontal plane; the axis of maximum be interpretedas tear faults to accomodatethe variable amount shorteningZ remains vertical. of extensionon both parts of the Massif Central [Burg et al., Although ductile normal faults with N-NE to S-SW 1990]. A minimum estimate of stretchingis provided by the stretchinghave been recentlydescribed in the southernpart of cumulativelength of the pull-apartbasins parallel to the fault. the Armorican massif [Gapais et al., 1993], the Armorican In the case of the Argentat fault it is about 10 km, whereasit massifis characterized,in Carboniferoustime, by right-lateral reaches more than 95 km for the Sillon Houiller. The Sillon strike-slip motion and synkinematic granite emplacement. Houiller that stretchesalong more than 500 km is the major Along the southern Armorican shear zone, maximum late orogenic structureof the Massif Central [Grolier and shorteningtrends NE-SW, and vertical finite elongation is Letourneur, 1968]. However, ductile deformation related to small since deformationis close to simple shear[Berthd et al., wrenching is rare, and a pre-Stephanianactivity of the Sillon 1979]. The transition from wrench to extensional tectonics Houiller is not demonstrated. Therefore, as the initiation of can be seen as an exchange between Y and Z axes in the such a large fault in Late Carboniferoustime only seemsvery vertical plane normal to the trajectoryfollowed by the X axis unlikely, we tentatively suggesthere that, owing to its N20øE alongthe Armoricanshear zone (Figure 15). trend, the "Proto-Sillon Houiller" was initiated during Middle Late orogenic extensionaltectonics are not limited to the Carboniferousas a normal fault. Our preliminaryresults in the Massif Central. To the east, ductile normal faulting and Montmaraultarea (Figure 4) supportthis interpretation. syntectonicplutonism of Visean to Namurian age is found in

I

LATE VISEAN • Y • K-1 NAMURIAN

WESTPHALIAN O

STEPHANIAN y, C lqoi•e X

Figure 15. Relationshipsbetween wrench, extension, and compressionin the southernArmorican-Massif Central branch of the Variscan belt. Late Visean, Namurian, Westphalian, and Stephaniancorrespond to the ageof beginingof extensionwhich is shiftedto the southwith time. Solidareas are the mainCarboniferous plutonsused as crustalstrain markers. Bar indicatesthe stretchingdirection; arrows show the senseof shearof the upper part. FAURE:VARISCAN EXTENSIONSIN THE FRENCHMASSIF CENTRAL ! 49

MiddleCarboniferous extension

Middle Carboniferouscompression Late Carboniferous-EarlyPermian extension

Armorican %•.// • X shearzones

Schwarzwald

Cdvennes F.

Figure 16. The two stagesof extensionaltectonics of the Massif Central locatedwithin the W. European VariscanBelt. Note the discrepancyin azimuthfor the Late Carboniferous-EarlyPermian extension directions in the Massif Central and Pyr6n6es.

the Vosgesand Schwarzwald[Rey et al., 1991;Echtler and crustal strain markers, extension is well recorded by its Chauvet, 1991]. However the extension direction is at structural,metamorphic, and plutonic effects in the deep crust. variance: E-W in Schwarzwald, NW-SE in eastern Vosges and A conspicuousductile stretchingoccurred along the NW-SE NE-SW in western Vosges. Late orogenic extensional trend and in the same time, NE-SW horizontal shortening tectonics are also described in the Variscan Pyr6n6es [e.g., producedlarge-scale antiforms and synforms.These two finite Vissers, 1992], but a discrepancyremains with respectto the strain directions controlled the emplacementand shapeof the extensional pattern in the other Variscan massifs. In the intrusions. The synkinematic granitoids exhibit typical Pyren6es,extension started during the early Stephanianwith a features such as asymmetric shape, recumbent peripheral NW-SE maximum stretchingdirection. This trend is almost lobes, and NW-SE lineation formed in subsolidus to perpendicularto the trend describedin the Massif Central postsolidusstates, which reflect the dynamicsof the extending (Figure 16). Systematicstructural studies and accuratedating of crust. these events are still needed. The Middle Carboniferous extension is a diachronous event which propagatedsouthward. The temporal transitionbetween Conclusion the two orthogonalextensional tectonic regimesremains to be studiedfrom the point of view of stressand strain fields. Two Severallines of evidencesupport a Late Carboniferous-Early possibilities are tentatively suggested here for the strain Permian extensional stress field in the French Massif Central. history. First, the X and Y axes of the finite strain ellipsoid The NE-SW trend is the maximum stretchingdirection that which lay in the horizontalplane rotated aroundthe vertical Z controlled(1) the openingof intramontanecoal basinswhich axis from Middle Carboniferous(late Visean) to Early Permian exhibit a typical half graben geometry, (2) the formation of (Autunian). Second, the finite strain axes remained fixed in pull-apart troughs along the tear faults, (3) the uplift of direction but varied in modulus; thus the X and Y strains granite-migmatite domes, (4) the localization of ductile underwent a transient stage of flattening strain (X=Y). The deformationalong normal or normal-wrenchfaults, and (•) the understandingof the Variscan extensionsin the Massif Central brittle faulting widespreadin the upper plate metamorphic will be greatly improved by the increase of the accuracy of rocks. At the scale of the whole massif, extension appearsto radiometric dating. be asymmetricsince top to the NE shearis predominant.Last, the Carboniferous-Permianunconformity is not evidence for a compressionalevent in the Massif Central. This tectonic style has been comparedto the Basin and Range Province of the Acknowledgments.I am particularlyindebted to J. Pons,who showed me the importanceof granitetectonics in the studyof mountainbelts, western United States [M•nard and MoInar, 1988; Malavieille, andto J.-F. Becq-Giraudon,who provideda lot of informationon coal 1993]. basins. This paper benefited from discussionswith J. Grolier, M. The Middle Carboniferous extensional tectonic regime Mattauer,O. Monod, and C. Pin. K. C. Burke and S. Patersonimproved a presents contrasting characters.In spite of lack of upper first draft of the manuscript. 150 FAURE:VARISCAN EXTENSIONS IN THE FRENCHMASSIF CENTRAL

References

Alabouvette,B., Noticeexplicative de la feuille Bits, J.L., D. Bonijoly,C. Castaing,and Y. Sahara alg6rien, Ann. Fac. Sci., Univ. "Le Vigan" h 1/50000, 68 pp., 6dition du Gros,Successive post-Variscan stress field Clermont,50, 324 pp., 1973. BRGM, Orl6ans, France, 1988. in the French Massif Central and its borders Carmignani, L. and R. Kligfield, Crustal Arthaud, F., and P. Matte, Les d6crochements (WesternEuropean plate): Comparison with extension in the northern Apennines:The tardi-hercyniensdu sud-ouestde l'Europe. geodynamicdata, Tectonophysics, 169, 79- transitionfrom compressionto extensionin G6om6trie et essai de reconstitution des 111, 1989. the Alpi Apuane complex, Tectonics, 9, conditions de la d6formation, Bogdanoff,S., J.L. Cirodde,and M. Donnot, 1275-1303, 1990. Tectonophysics,25, 139-171, 1975. The nappesof La Chataigneraie,southwest Carpena,J., J. Doubinger,R. Gu6rin,J. Juteau, Audrain, J., J.L. Vigneresse,M. Cunet, and M. MassifCentral, France, Tectonophysics, 157, and M. Monnier, Le volcanisme acide de Friedrich,ModUle gravim6trique et mise en 69-79, 1989. l'Ouest Morvan dans son cadre g6ologique: place du complexe granitique Bonijoly,D., andC. Castaing,Fracturation et caract6risationg6ochimique, structurale et hyperalumineuxde St-Sylvestre {Massif gen•se des bassinsst6phaniens du Massif chronologiquede mise en place, Bull. Soc. Central franqais), C. R. Acad. Sci., Ser. 2, Centralfranqais en r•gimecompressif, Ann. Geol. Fr., 26, 839-859, 1984. 309, 1907-1914, 1989. $oc. Geol. Nord, 103, 187-189, 1984. Cassard,D., J.L. Feybesse,and J.L. Lescuyer, Balk, R., Structuralbehaviour of igneousrocks, Bonnion, S., L. Courel, J.P. G61ard,and Y. Variscan crustal thickening, extensionand Mere. Geol.Soc. Am., 5, 177 pp., 1937. Paquette,L'organisation des d6potsde late overstacking during the Namurian- Barbarin, B., J.M. Belin, A. Fernandez, J. Charbon et des st6riles dans le bassin de Westphalianin the westernMontagne Noire Grolier, A. Lacour, and M. Turland, l'Aumance (Allier): tectonique (France), Tectonophysics,222, 33-53, 1993. Observationsde p6trologiestructurale sur le syns6dimentaireet syndiag6n6tique,Mere. Cheilletz,A., D.A. Archibald,M. Cuney,and B. granite de Montmarault (Allier, Puy de Geol. Univ. Dijon, 8, 87-97, 1983. Charoy,Ages 39Ar/4øAr du leucogranite•t D6me), Gdol. Fr., 4, 381-388, 1985. Bouchez,J.L., and O. Jover,Le Massif Central: topaze-16pidolite de Beauvoir et des Bayer, R., J.P. Couturi6, and G. Vasseur, Un chevauchementde type himalayenvers pegmatites sodomithiques de Ch6deville R6sultatsg6ophysiques r6cents sur le granite l'ouest-nord-ouest,C. R. Acad. $ci., $er. 2, (Nord du Massif Central, France). de la Margeride,Ann. Gdophys.,38, 431- 302, 675-680, 1986. Significationp6trologique et g6odynamique, 447, 1982. Boutin,R., andR. Montigny,Datation 39Ar/4øAr C. R. Acad. Sci., Ser. 2, 305, 1167-1173, Beaud, F., Etude structurale de la zone axiale des amphibolitesdu complexeleptyno- 1992. de la zoneaxiale orientalede la Montagne amphibolique du plateau d'Aigurande: Costa, S., East-west diachronism of the Noire (sud du Massif Central franqais), collision varisque•t 390 Ma dans le Nord- collisional stage in the French massif Th•se 3 ø cycle, 191 pp., Univ. de Ouest du Massif Central franqais, C. R. Central: implications for the European Montpellier,Montpellier, France, 1985. Acad. $ci., Ser. 2, 316, 1391-1398, 1993. Variscan Orogen, Geodin. Acta, 5, 51-68, Becq-Giraudon,J.F., Etude g6ologiquedu Broutin,J. J. Doubinger,J. Langiaux,and D. 1991. bassin houiller de Graissessac,Bull. Bur. Primey, Cons6quencesde la coexistencede Couturi6, J.P., Le massif granitiquede la Rech. Gdol. Min. Fr., I, 3, 151-163, 1973. flores • caract•resst6phaniens et autuniens Margeride (Massif Central franqais),Ann. Belin, J.M., Le massif de St-Gervais dans les bassins limniques d'Europe Sci. Univ.Clermont, 62, 319 pp., 1977. d'Auvergne (Massif Central franqais). occidentale,Mere. $oc. Geol. Fr., 149, 15- Couturi6, J.P., M. Vachette-Caen, and Y. Evolution d'un ensemble granitique 25, 1986. Vialette, Age namurien d'un laccolithe composite,Th•se 3øcycle,151 pp., Univ. de Brun,J.P., and J. Pons,Strain patterns of pluton granitiquediff6renci6 par gravit6: le granite Clermont-Ferrand, Clermont-Ferrand, emplacementin a crust undergoingnon- de la Margeride(Massif Centralfran•ais), France, 1981. coaxial deformation, Sierra Morena, C. R. Acad. Sci., Ser. 2, 289, 449-452, 1979. Bernard-Gdffiths,J., and M. Vachette,Age southernSpain, J. Struct.Geol., 3, 219-229, D616nin,P., J. Clermont6,L. Courel,M. Dumain Cambriendes migmatitesde l'anticlinal de 1981. and J. Laversanne, Remise en cause des Tulle((Massif Central franqais) et ses Burg, J.P., and P. Matte, A cross-section chardagesdans le bassinhouiller st6phanien relations avec l'Sge du granite dit "tardi- throughthe French Massif Central and the des C6vennes(Gard, France), C. R. Acad. migmatitique"de type Cornil, C. R. Acad. scopeof its Variscangeodynamic evolution, Sci. Ser. 2, 307, 1237-1243, 1988. Sci., Ser.2, 270, 916-919, 1970. Z. Dtsch. Geol. Ges., 109, 429-460, 1978. Demange,M., The eclogitefacies rock of the Berth6, D., P. Choukrouneand P. Jegouzo, Burg, J.P., M. Brunel,D. Gapais,G.M. Chen, Montagne Noire, France, Chem. Geol., 50, Orthogneiss, mylonite and non-coaxial andG.H. Liu, Deformationof leucogranites 173-188, 1985. deformationof granites:The exampleof the of the crystalline Main Central Sheet in Deroin, J.P., J.F. Becq-Giraudonand A. Prost, South armorican shear Zone, J. Struct. southernTibet, J. Struct.Geol., 6, 535-542, Fracturationtardi-hercynienne et d6tritisme Geol., 1, 31-42, 1979. 1984. associ6: chronologiedes 6v6nementsdans Berthier, F., J.L. Duthou, and M.Roques, Burg, J.P., J.P. Brun, and J. Van den Driessche, les massifsgranitiques c6venols d6croch6s Datationg6ochronologique Rb/Sr surroches Le Sillon Houiller du Massif Central par la faille de Villefort (Mt-Loz•re et totales du granite de Gu6ret (Massif franqais: faille de transfert pendant Borne, Massif Central, France), C. R. Acad. Central). Age fini-D6vonien de mise en l'amincissement crustal de la chaine Sci., Ser. 2,311,865-871, 1990. place de l'un de ses facies types, Bull. Bur. varisque?,C. R. Acad. Sci., Ser. 2, 311,147- De Waard, D., Tectonicsof the Mt-Aigoual Rech. Gdol. Min. Fr., /, , 31-42, 1979. 152, 1990. pluton in the southeasternC6vennes, Proc. Binon, M., and C. Pin, G6ochronologieRb-Sr et Caen-Vachette,M., J.P. Couturi6, and J. Didier, K. Ned. Akad. Wet., 62, 381-403, 1949. U-Pb des granitesdu Mayet-de-Montagneet Ages radiom6triques des granites Dewey, J., Extensionalcollapse of orogens, des Bois Noirs, Montagne bourbonnaise anatectiqueet tardimigmatitiquedu Velay, Tectonics,7, 1123-1139, 1988. (Massif Central), Bull. Soc. Geol. Fr., 8, (Massif Central franqais),C. R. Acad. Sci., Dewey, J. and K.C. Burke, Tibetan, Variscan 695-703, 1989. Ser. 2, 294, 135-138, 1982. and Precambrian basement reactivation: Bl•s, J.L., D. Bonijoly, and Y. Gros, Apport de Cantagrel,J.M., Significationdes ages •t l'argon Productsof continentalcollision, J. Geol., l'analyse structurale•t la connaissancedes d6termin6ssur amphibolesdans les socles 81,683-692, 1973. g•tes filoniens, Bull. Bur. Rech. Geol. Min. m6tamorphiquesanciens: application au Didier, J., and J. Lameyre,Les granitesdu Fr., 2, 4, 427-436, 1982. Massif Central franqais et • l'Aleskod, MassifCentral fran•ais: 6tude compar6e des FAURE:VARISCAN EXTENSIONS 1N THE FRENCH MASSIF CENTRAL 15 1

leucograniteset granodiorites,Contrib. Faure, M., A.E. Prost, and E. Lasn½, and J.M. Leistel, Contr61e structural de la Mineral.Petrol., 24, 219-238,1969. D6formation ductile extensive d'fige m6tallogen•sedans la r6g•on d'Echassi•res Dumas, E., M. Faure, and J. Pons, namuro-westphalien dans le plateau par une zone de cisaillement r6gional L'architecturedes plutonsleucogranitiques d'Aigurande,Massif Central fran•ais, Bull. orient6e N60E, Massif Central franqais, C. du plateaud'Aigurande et l'amincissement Soc. Geol. Fr., 8, 189-197, 1990. R. Acad. Sci., Ser. 2,298, 459-462, 1984. crustaltardi-varisque, C. R. Acad.Sci., Ser. Faure, M., J. Pons,and J.F. Babinault,Le pluton Gapais, D., J.L. Lagarde, C Le Corre, P. 2,310, 1533-1539, 1990. du Pont-de-Montvert : un granite Jegouzo, A. Casas-Sainz, and J. Van den Dupraz, J., and J. Didier, Le complexe syntectoniqueextravas6 vers l'est pendantle Driessche, La zone de cisaillement de anatectique du Velay (Massif Central d6s6paississementcrustal varisque du Massif Quiberon: t6moin d'extension de la chaine fran•ais):structure d'ensemble et 6volution Central franqais, C. R. Acad. Sci., Ser. 2, varisque en Bretagne m6ridionale au g6ologique,Geol. Fr., 4, 73-88,1988. 315, 201-208, 1992. Carbonif•re, C. R. Acad. Sci., Ser 2, 316, Dutartre,P. Etude de la fracturationdu granite Faure, M., J. Grolier, and J. Pons, Extensional 1123-1129, 1993. de la Margeride,Doc. Bur. Rech.Geol. Min. ductile tectonicsin the Sioule metamorphic G61ard, J.P., C. Castaing, D. Bonijoly, and J. Fr., 41,344 pp., 1981. series (Variscan French Massif Central), Grolier, Structureet dynamiquede quelques Duthou, J.L., and J.P. Dutreuil, Age Namuro- Geol. Rundsch., 82, 461-474, 1993. bassins houillers limniques du Massif westphaliendes granitoYdes de Piegutet de Fayol, H., La th6orie des deltas et la fortnation Central, Mem. Soc. Geol. Fr., 149, 57-72, St-Mathieu (Limousin) et dualit6 de leur du bassin de Commentry, Bull. Soc. Geol. 1986. origine,C. R. Acad.Sci., Set. D, 286, 925- Fr., 16, 968-979, 1888. G•ze, B., Etude g6ologique de la Montagne 927, 1978. Feist, R., and J. Galtier, D6couverte de flores Noire et des C6vennes m6ridionales, Mere. Duthou,J.L., and C. Pin, Etudeisotopique Rb/Sr d'•ge namurien probable dans le flysch h Soc. Gdol. Fr., 62, 215 pp., 1949. de l'apexgranitique d'Echassi•res, Geol. Fr., olistolithes de Cabri•res (H6rault). Gibbs, A.D., Structural evolution of extensional 2-3, 63-67, 1987. Implicationssur la dur6ede la s6dimentation basins margins, J. Geol. Soc. London, 141, Duthou,J.L., Cantagrel,J.M., Didier,J., andY. synorog6nique dans la Montagne Noire 609-620, 1984. Vialette, Paleozoic granitoids from the (Francem6ridionale), C. R. Acad. Sci.,Ser. Gras, H., Contributionh l'6tude g6ologiquedu French Massif Central: Age and origin 2,300, 207-212, 1985. bassin houiller des C6vennes, Bull. Bur. studiedby 87-Rb/87-Srsystem, Phys. Earth Feix, I., P.L. Gulllot, S. Miyashita, G. Bossi•re, Rech. Geol. Min. Fr., 1, 7-31, 1970. Planet. Inter., 35, 131-144, 1984. and J.P. Floc'h, Argumentsen faveur d'un Grolier, J., Contributionh l'6tudeg6ologique des Duthou,J.L., S. Bogdanoff,J.L. Cirrode, and V. 6pisode majeur en cisaillementdextre le s6riescristallophylliennes inverses du Massif Nicolas,Chronolgie Rb-Sr du granitede long de la faille d'Argentat,Massif Central, Central franqais: la s6rie de la Sioule (Puy Veinaz•s environnant le gisement Cons6quences.C. R. Acad.Sci., Ser. 2,305, de D6me, Allier), Mere. BRGM, 64, 163 pp., wolframif•rede Leucamp(Cantal, Massif 473-476, 1987. 1971a. Central franqais),paper presentedat the Fernandez, A., Sur la structure et la mise en Grolier, J., La tectonique du socle hercynien 11th R6union Sciences de la Terre, place du granite porphyroidedu Pont-de- dans le Massif Central, in Symposium J. Clermont-Ferrand,25-27 March, 1986. Montvert (Mont Loz•re, Massif Central Jung: Gdologie,gdomorphologie et structure Echtler, H., and A. Chauvet,Carboniferous franqais), C. R. Somm.Soc. Geol. Fr., 19, profonde du Massif Central Francais, pp. convergenceand subsequentcrustal 151-154, 1977. 215-268, Plein Air Service, Clermont- extensionin the SouthernSchwarzwald (SW Feybessse,J.L., Tectoniqueet microtectonique Ferrand, 1971b. Germany),Geodin. Acta, 5, 37-49,1991. de la r6gionde Laroquebrou(Cantal, Massif Grolier, J., and J. Letourneur, L'6volution Echtler, H., and J. Malavieille, Extensional Central, Franqais).R61e de la d6formation tectonique du grand Sillon Houiller du tectonics,basement uplift and Stephano- ductile et 6volution du Sillon Houiller, Thb,se Massif Central francais, Proc. 23rd. Int. Permiancollapse basin in a late Variscan 3ø cycle, 250 pp., Univ. Clermont-Fcrrand, Geol. Congr.,/, 107-116, 1968. metamorphic core complex (Montagne Clermont-Ferrand, France, 1981. Hamet, J., and M. Mattauer, Age hercynien, Noire, Southern Massif Central), Feybessse,J.L., and M. Teygey, Evolution d6termin6 par la m6thodeRb-Sr du granite Tectonophysics,177, 125-138,1990. tectonom6tamorphique d6vonienne et de l'Aigoual, Cons6quencesstructurales, Fabre,G., Surl'fige et la constitutiondes r6gions carbonifb,re de la s6rie de la Sioule, Geol. C.R. Sorere. Soc. Geol. Fr., 2, 80-84, 1977. schisteusesdu G6vaudan et des C6vennes, Fr., 2-3, 33-41, 1987. Hollinger, P., M. Cuney, M. Friedrich, and L. Bull. Soc. Geol. Ft., 5, 399-409, 1877. Feys, R., and G. Gand, Gisementhouiller du Turpin, Age carbonif•re de l'Unit6 de Faure, M., L'amincissementcrustal dans la Creusot-unetectonique de serragedans le Brame du complexe granitique chainevarisque h partirde la d6formation sillon permo-houillerde Blanzy-Montceau, peralumineuxde St-Sylvestre(NO du Massif ductiledes leucogranites du Limousin,C. R. Geol. Fr., 2, 97-122, 1983. Central) d6fini par les donn6esisotopiques Acad.Sci., Set. 2, 309, 1839-1845,1989. Floc'h, J.P., Le m6tamorphismeet la mise en U-Pb sur zircon et monazite, C. R. Acad. Faure, M., and J.F. Becq-Giraudon,Sur la place des granitesdu d6me de St-Mathieu Sci., Ser. 2, 303, 1309-1314, 1986. successiondes 6pisodesextensifs au cours (Limousin occidental), Bull. Bur. Rech. Hutton, D.H.W., Granite emplacement dudes-6paississement carbonif•re du Massif Geol. Min. Fr., 1, 89-107, 1979. mechanisms and tectonic control: Inferences Centralfranqais, C. R. Acad.Sci., Ser. 2, Floc'h, J.P., Une traverse de la branche from deformation studies, Trans. R. Soc. 316, 967-973, 1993. lig6rienne de l'orog•ne varisque, de Edinburgh,Earth Sci., 79, 245-255, 1988. Faure, M., and N. Cottereau, Donn6es l'Aquitaine /t la zone broy6e d'Argentat Joubert,M., Etude p6trographiquestructurale et cin6matiquessur la miseen placedu d6me (Massif Central francais), Th•se, 445 pp., m6tallog6niquede la Ch•taigneraie, Massif migmatitiquecarbonif•re moyen de la zone Univ. Limoges,Limoges, France, 1983. Central franqais, Th•se 3ø cycle, 210 pp., axialede la MontagneNoire (Massif Central Forestier, F.H., M6tamorphismehercynien et Univ. Clermont-Ferrand, Clermont-Ferrand, franqais),C. R. Acad.Sci., Ser. 2, 307,1787- ant6-hercyniendans le bassindu haut-Allier France, 1978. 1794, 1988. (Massif Central Franqais),Bull. Serv. Carte Labernardi•re,H., La tectoniquecassante du Faure, M., and J. Pons, Crustal thinning Geol. Fr., 271,521-805, 1963. socle hercynien dans la r6gion de recordedby the shapeof the Namurian- Freytet,P., Stratigraphieet tectoniquedu Bassin Bourganeuf,Bull. Bur. Rech. Geol. Min. Fr., Westphalianleucogranite in the Variscan Houiller de Montvicq (Allier), Bull. Soc. 2, 29-33, 1970. belt of the Northwest Massif Central, Geol. Fr., 2, 621-629, 1960. Laboue, M., Etude structurale du Massif France,Geology, 19, 730-733,1991. Gagny,C., G. Courrioux,O. Cuenin,T. Jacquot, granitiquede la Margeride,Th•se 3ø cycle, 152 FAURE: VARISCAN EXTENSIONSIN THE FRENCHMASSIF CENTRAL

154 pp., Univ. Clermont-Ferrand,Clermont- Range and the Late PaleozoicVariscan belt, cristallophylliennes de la r6gion de Ferrand, France, 1982. Tectonics,12, 1115-1130, 1993. Ch•teauneuf-la-For•t (Haut Limousin, Lafont,J.M., andJ.P. Respaut, G6ochronologie Malavieille, J., P. Guihot, S. Costa, J.M. Massif Central fran•ais), Th•se 3ø cycle, U-Pb et leucogranitesvarisques: cas des Lardeaux,and V. Gardien,Collapse of the 163 pp., Univ. Lyon, Lyon, France,1976. massifs de Grandrieu (Loz•re) et de la thickened Variscan crust in the French Paterson,S., R., R.H. Vernon, and O.T. Tobish, Porcherie (Limousin), Massif Central Massif Central: Mont Pilat extensional shear A review of criteria for the identification of franqais,Bull. Mineral., 111,225-237, 1988. zone and St.-Etienne Late Carboniferous magmatic and tectonic foliations in Lagarde, J.L., C. Dallain, O. Merle, and P. basin,Tectonophysics, 177, 139-149,1990. granitoids, J. Struct. Geol., 11, 349-363, Ledru, Champs de deformation associ6s•t Maluski, H., S. Costa, and H. Echtler, Late 1989. l'expansion horizontale de magmas Variscantectonic evolution by thinningof Paterson, S., R., R.H. Vernon, and T. K. Fowler, granitiques.exemple des granites hercyniens earlier thickenedcrust, an 39Ar/40Ar study Aureole tectonics, in Contact du Velay, Massif Central, C. R. Acad. Sci., of the Montagne Noire, SouthernMassif Metamorphism. edited by D.M. Kerrick, Ser. 2, 316, 653-659, 1993. Central, France, Lithos, 26, 287-304, 1991. Rev. Mineral., 26, 673-722, 1991. Lameyre, J., Contribution•t la g6ologiedu Marre, J., Mgthodesd'analyse structurale des Petitpierre, E., and J.L. Duthou, Age Limousin: Arguments pour des fenEtres graniwrdes, 128 pp., Bureaude Recherches Westphalien par la methode Rb/Sr du ouvertesdans un grand chardagepar des G6ologiqueset MiniSres, Orleans, France, leucogranite de Crevant, Plateau diapirs leucogranitiques,C. R. Acad. Sci., 1982. d'Aigurande(Massif Central fran•ais), C. R. Ser. 2, 294, 1237-1240, 1982. Marteau, P., Le bassin permo-carbonif•re Acad. Sci., Ser. 2, 291, 163-166, 1980. Lameyre, J., Contribution•t la g6ologiedu d'Autun,Doc. B. R. G. M., 64, 197p.,1983 Petrequin,M., Etude gravim6triquedu massif Limousin (II), Les leucogranites Mattauer, M., M. Brunel, and P. Matte, Failles de la Margeride et de sa bordure finicarbonif•reset le modulehimalayen, C. normales ductiles et grands m6ddionale,Th•se 3ø cycle, 128 pp., Univ. R. Acad. Sci., Ser. 2, 298, 895-900, 1984. chevauchements:Une nouvelle analogie Montpellier, Montpellier,France, 1979. Ledru P., and A. Autran, L'6dification de la entrel'Himalaya et la chainehercynienne du Pin, C., Donn6es microstructurales sur les chainevarisque dans le Limousin.R61e de la Massif Central fran•ais, C. R Acad. Sci., terrainsm6tamorphiques de la s6rie du Lot, faille d'Argentat •t la limite Limousin- Ser. 2, 306, 671-676, 1988. Bull. Bur. Rech. Geol. Min. Fr., 1, (4), 293- Milevaches,Doc. BRGM, 140, 51-81, 1987. Matte, P., Tectonicsand plate tectonicsmodel 313, 1980. Ledru, P., and A.M. Hottin, Le chevauchement for the Variscan belt of Europe, Pons, J., P. Debat, C. Oudin, and J. Valero, du Haut-Limousin: coupede St-Germain- Tectonophysics,126, 329-374, 1986. Emplacementkinematics of the syntectonic les-Belles,Doc. BRGM, 81-3, 87-106, 1984. M6nard, G. and P. Molnar, Collapse of a Sarayagranite (Eastern Senegal), Bull. Soc. Ledru, P., J.M. Lardeaux, D. Santallier, A. Hercynian Tibetan Plateau into a late Geol. Fr., 162, 1075-1082, 1991. Autran, J.M. Quenardel, J.P. Floc'h, G. Paleozoic European Basin and Range Prager, M., Observationssur la mise en place Lerouge, N. Maillet, J. Marchand, and A. province, Nature, 334, 235-237, 1988. du granite de St-Guiral-Liron (C6vennes Ploquin, Oh sontpass6es les nappesdans le Mialhe, J., Le massif granitiquede la Borne: m6ddionales), Bull. Soc. Geol. Fr., 7, 479- Massif Central franqais? Bull. Soc. Geol. 6tude p6trographique, g6ochimique, 486, 1965. Fr., 8, 605-618, 1989. g6ochronologiqueet structurale,Th•se 3ø Quenardel, J.M., and P. Rolin, Paleozoic Legrand, X., J.C. Soula, and J.P. Rolando, Un cycle, 170 pp., Univ. Clermont-Ferrand, evolution of the Plateau d'Aigurande(NE module de d6veloppement d'un roll-over Clermont-Ferrand, France, 1980. Massif Central, France), in Variscan complexe:la borduresud du bassinde Saint- Mollier, B., and J.L. Bouchez, Structuration Tectonics of the North Atlantic Region Affdque (Sud du Massif Central franqais), magmatique du complexe granitique de edited by D.H.W. Hutton and D.J. C. R. Acad. Sci., Ser. 2, 312, 1625-1631, Brame-St Sylvestre-StGoussaud (Limousin, Sanderson,Geol. Soc. Spec.Publ., London, 1991. Massif Central francais), C.R. Acad. Sci., 14, 47-61, 1984. Leistel, J.M., and C. Gagny, Mise en evidence Ser. 2, 294, 1329-1334, 1982. Rey, P., J.P. Burg, and J.M. Caron, Middle and d'une fosse volcano-tectoniqueau Vis6en Mollier, B., and M. Lespinasse,D6formation Late Carboniferous extension in the sup6rieur dans le NE du Massif Central magmatique et plastique en limite nord du Variscan Belt: Structuraland petrological franqais,Rev. Geol. Dyn. Geogr. Phys.,25, granite de Saint-Sylvestre (Nord-Ouest du evidencesfrom the VosgesMassif (Eastern 19-31, 1984. Massif Central francais): La faille France), Geodin. Acta, 5, 17-36, 1991. Lerouge, G., Tectogenbsecompar6e de deux d'Arr•nes-Ouzilly, C.R. Acad. Sci., Ser. 2, Robert, P., C. Berquer,L. Courel, B. Kubler, segments de la cha•ne hercynienne: Le 300, 681 686, 1985. and P. Robert, Anomaliethermique pr6coce Massif Central franqaisseptentrional et le Monier, G., P6trologiedes granitoYdesdu Sud- dans le bassin houiller st6phanien de suddu MassifArmoricain, Mgm. Geodiff, 2, Millevaches, (Massif Central francais). Messeix-Singles, Massif Central fran•ais, 363 pp., Univ. Paris-Sud,Orsay, 1988. Min6ralogie, g6ochimie, g6ochronologie, Sci. Geol. Strasbourg,41,333-349, 1988. Lerouge, G., and J.M. Quenardel,Chronologie Th•se 3ø cycle, 288 pp., Univ. Clermont- Rolin, P., J.L. Duthou, and J.M. Quenardel, des 6v6nementstectoniques dans le NW du Ferrand, Clermont-Ferrand,France, 1980. Datation Rb/Sr des leucogranitesde Crozant Massif Central fran•ais et le Sud du bassin Montel, J.M., C. Madgnac, P. Barbey,and M. et d'Orsennes:cons6quences sur l'•ge de la de Pads du Carbonifbre inf6deur au Plio- Pichavant, Thermobarometryand granite dernitrephase de tectoniquetangentielle du Quaternaire, C. R. Acad. Sci., Ser. 2, 301, genesis:The hercynianlow P, high T Velay Plateau d'Aigurande(NW du Massif Central 621-626, 1985. anatecticdome (French Massif Central), J. fran•ais), C. R. Acad. Sci., Ser. 2, 294, 799- Lespinasse,M., B. Mollier, J. Delair, and Y. Metamorph. Geol., 10, 1-15, 1992. 802, 1982. Bladier, Structuration tangentielle et Mouctar, K., Mise en 6vidence de cisaillements Santallier, D. and J.P. Floc'h, Tectonique chevauchements carbonif•res dans les ductiles tangentielset d6crochantsdans le tangentielle et d6crochements ductiles leucogranites du NW du Massif Central Sud Livradois, (Massif Central fran•ais); d6vono-carbonif•res superpos6sdans la fran•ais: L'exemple des failles de Bussibres- Cons6quencespour le "m6tamorphisme r6gion de Bellac (nord-ouest du Massif Madeleine et d'Arr•nes-Ouzilly' C. R. Acad. Livradois", C. R. Acad. Sci., Ser. 2, 301, 189- Central fran•ais), C. R. Acad. Sci., Ser. 2, Sci., Ser. 2, 303, 1575-1580, 1986. 193, 1985. 309, 1419-1424, 1989. Malavieille, J., Late orogenic extension in Mouthier, B., Lithostratigraphie ct Santouil, G., Tectonique et microtectonique mountainbelts: Insightsfrom the Basin and m6tamorphisme des formations compar6ede la distensionpermienne et de FAURE: VARISCAN EXTENSIONSIN THE FRENCH MASSIF CENTRAL 153

l'6volutionpost-triasique dans les bassinsde modb•lede l'extensionpa16ozoYque sup6rieur France, Mere. Soc. Geol. Ft., 149, 7-14, Lodb•ve, St-Affrique et Rodez (France), dans le Sud du Massif Central, C. R. Acad. 1986. Thb•se3 ø cycle, 74 pp., Univ. Montpellier, Sci., Ser. 2, 309, 1607-1613, 1989. Vialette, Y., and G. Sabourdy,Age et origine Montpellier, France, 1980. Van den Driessche,J., and J.P. Brun, Tectonic des granitoYdesdu Mont Loz/•re dans le Schuilling,R.D., Le d6megneissique de l'Agout evolution of of the Montagne Noire Massif Central franqais, C. R. Serum.Sec. (Tarn et H6rault), Mere. Sec. Geol. Fr., 39, (SouthernFrench Massif Central): a model Geol. Fr., 19, 127-132, 1977. 1-59, 1960. of extensionalgneiss dome, Geedin. Acta, 5, Vigneresse,J.L., V. Menichetti, and M. Cannat, Thompson,A.B., and J.R. Ridley, Pressure- 1-2, 85-99, 1991. Interpr6tation gravim6trique du massif Temperature-time (P-T-t) histories of Van Moon, J.C.,Les roches cristallophylliennes granitique d'Echassib•res, Programme orogenicbelts, Philos. Trans. R. Sec. London des C6venneset les rochesplutoniques du G6ologie Profonde de la France, Dec. A, 321, 27-45, 1987. Mont Lozb•re,Ann. Fac. Sci..Univ. Clermont, B.R.G.M., 95-8, 81-91, 1985. Turland, M., V. Mathis, J. Grolier, G. Monnier, 31,272 pp., 1966. Vissers, R.L.M., Variscan extension in the G. Crois6, P. Debriette, D. Milhau, F. Vasseur,G., A. Dupis,J. Gallart, and G. Robin, Pyren6es,Tectonics, 11, 1369-1384, 1992. Mercier-Batard, J.P. Carrou6, M. Piboule, Donn6es g6ophysiquessur la structure du Vivier, G., and M. Lasserre,Age hercyniendes and N. Deb6glia,Notice explicative,Carte massif leucogranitiquedu Limousin, Bull. granitesde la Chfitaigneraie(SW du Massif g6ol. France, (1/50000), feuille Bourbon Sec. Geol. Fr., 8, Vl, 3-11, 1990. Central fran•ais), Bull. Sec. Geol. Fr., 15, l'Archambault,Montluqon, Montmarault, 82 Vennat, G., Un exemple de relations 283-287, 1973. pp., 6dition du BRGM, Orl6ans, France, volcanisme plutonisme : chronologie. 1990. petrologie dynamisme d'un complexe M. Faure, D6partement des Sciencesde la Vail6, B. L., Courel, and J.P. G61ard, Les volcano-plutonique dans la r6gion de Terre, Universit6 d'Or16ans, 45067 Orl6ans marqueursde la tectoniquesyns6dimentaire Gannat-Les Ancizes (Massif Central Cedex 02, France. [email protected] et syndiag6n6tiquedans le bassinst6phanien franqais), Thb•se3 ø cycle, 155 pp., Univ. orleans.fr h r6gime cisaillant de Blanzy-Montceau Clermont-Ferrand, Clermont-Ferrand, (Massif Central, France), Bull. Sec. Geol. France, 1982. (Received March 2, 1994; Fr., 8, 529-540, 1988. Vetter, P., Les formations limniques du revisedJuly 26, 1994; Van den Driessche, J., and J.P. Brun, Un Carbonifb•resup6rieur et de l'Autunien en acceptedAugust 9, 1994.)