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Forel

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Rifting event magmatic event Hlgh cruatal Scattered

Keywords:

té il y a environ 3 Ma et se poursuit actuellement . Au sud, le MNT révèle un champ de failles dominant orienté N 135 °E, souligné en particulier par des alignements volcaniques tels l'Aubrac, le Velay ou le Dcves. Cette direction majeure a été acquise pendant le soulèvement de la partie sud qui a débuté il y a environ 10 Ma, bien avant le soulève ment plus récent de la partie nord. Ce soulèvement s'est ralenti entre 5,5 Ma et 3-3,5 Ma pour redevenir très actif de puis cette période jusqu'à l'actuel, comme dans la partie nord. L'épisode magmatique pré-rift, extrêmement limité en volume, est attribué à une flexure de la lithosphère européenne au moment des premières compressions alpines pendant le Paléocène. Cette flexure de la lithosphère est encore appa rente à l'échelle de la France grâce aux données de géophysique ou de géomorphologie dans le Massif central el le Morvan. L'épisode magmatique syn-rift, restreint au nord du Massif central. est clairement associé aux zones d'amin cissement crustal maximal résultant principalement de l'extension asymétrique E-W oligocène supérieur à miocène in férieur. Cet amincissement lithosphérique permet d'expliquer, par décompression du manteau, le faible taux de fusion partielle nécessaire pour rendre compte du volcanisme, localisé au nord. de !'Oligocène supérieur au Miocène infé rieur. L'orientation N-S dominante observée sur le MNT est un héritage de cette période d'extension E-W où les failles sub-méridiennes d'âge varisque ont été réactivées. L'épisode magmatique majeur est caractérisé par deux pics d'activi té qui sont synchrones des deux périodes de soulèvement maximal: au sud vers 10-5,5 Ma, puis au nord et au sud à partir de 3-3,5 Ma. Au sud. la radiographie de la croûte et de la limite lithosphère-asthénosphère obtenue par sismique classique et tomographie sismique montre que le manteau lithosphérique a subi un très fort amincissement tandis que l'épaisseur de la croûte est quasi-normale. L"anomalie thermique définie sous la lithosphère dans cette partie sud té moigne alors d'une érosion thermique de la base de la lithosphère, responsable du soulèvement isostatique et du pre mier pic d'activité magmatique. Un second épisode d'érosion thermique à la base de la lithosphère. plus diffus mais réparti du nord au sud, expliquerait la seconde période de soulèvement isostatique ainsi que le second pic d'activité magmatique enregistrés au nord et au sud à partir de 3.5 Ma. r North 1. Pre-rift magmatic event

The evolution of the Massif Central rift : spatio-temporal distribution of the volcanism

LAURENT MJCHON 1 and OLIVIER MERLE 1 Key 11•ords. -Massif Central. Ccnozoic, Rifling, DEM, Volcanism, Tcctonic

Abstrac1. -The Massif Central area is the larges! magmatic province of the West-European Rift system.The spa tial-temporal distribution of Te rtiary-Quaternary volcanism in the Massif Central, France, shows that three magma tic phases can be defined, each of them characterized by different volumes and different locations. The first event, termed the pre-rifl magmatic event, is very scarce and restricted to the north of the Massif Central. It is suggested that this cou Id result from lithospheric bending of the European lithosphere ahead of the incipient Alpine chain during the Pa leocene. The second event, termed the rifl-re/a1ed 111ag111a1ic event, is located in the north of the Massif Central only and is spatially connected with zones of high crustal thinning (i.e. the Limagne graben). It immediately follows Oligo cene graben formation and associated sedimentation, and is represented by more than 200 scattered monogenic edifi ces. This second event can be attributed to partial melting as a conscquence of lithospheric thinning that aff ected the north of the Massif Central during the rifting event. The Jack of volcanism in the south during the same period oftime is probably related to the very slight lithospheric thinning during the Oligocene. The third event, termed the major magmatic eve111, started fi r st in the south in the upper Miocene at about 15 Ma, well after the end of the sedimentation. lt is unrelated to any extensional event. This major magmatic event reached the north of the Massif Central at about 3.5 Ma, following a pause in volcanism of about 6 Ma after the rift-related magmatic event. These two episodes of the ma jor magmatic event are spatially and temporally associated with the two main periods of uplift, suggesting a eommon origin for volcanism and uplift processes. The major magmatic event can be attributed to late thermal erosion of the base of the lithosphere above a mantle diapir, as suggested by seismic tomography data. This general magma tic evolu tion drawn f r om data at the Massif Central scale may apply to the Eger graben as well, as the three magmatic events described in this study (prc:-rif11nagmatic event, 1ifting event and post-Miocene volcanic event) are also reported in the literature. This suggests that a single cause should ex plain the formation of the entire western European rift surroun ding the Alpine mountain belt.

Evolution du rift du Massif central : distribution spatio-temporelle du volcanisme

Mots clés. -Massif Central, Cénozoïque. Rifting. MNT, Volcanisme. Tectonique.

Résumé. -Le rift Ouest-européen correspond à un épisode d'extension lithosphérique qui s'est produit de )'Eocène su périeur jusqu'au Miocène inférieur. La direction d'extension est globalement perpendiculaire au front de la chaîne al pine et s'exprime, d'est en ouest, par la formation du graben de l'Eger, du graben du Rhin et des fossés d'effondrement du Massif central. Le Massif central est la plus importante province magmatique liée à cet épisode de rifting. Le volca nisme de cette province peut être séparé en trois épisodes successifs. 1. Episode de magmatisme pré-rift. Cet épisode correspond à 15 localités répertoriées presque exclusivement dans le nord du Massif central et datées du Paléocène à la fin de !'Eocène. 2. Episode de 111ag111a1is111e syn-rifl. La sédimentation oligocène s'est eITectuée à un niveau proche de celui de la mer et pratiquement sans manifestation volcanique. A l'échelle du rift (i.e. de la Bresse à la Limagne), l'ex tension symétrique de !'Eocène supérieur à !'Oligocène moyen est devenue asymétrique à partir de !'Oligocène supé rieur. L'épisode de magmatisme a débuté à ! 'Oligocène supérieur et s'est principalement développé au Miocène infërieur, pendant une quinzaine de millions d'années. Il est spatialement lié aux zones d'amincissement crustal maxi mal (fossé de la Limagne) et est absent de la partie sud du Massif central où les récentes données géophysiques mon trent que l'amincissement crustal est négligeable. 3. Episode de 111ag111a1isme 111aje111: Cet épisode a démarré au sud du Massif central près de 15 Ma d'années après la fin de la sédimentation oligocène. C'est l'épisode majeur avec le déve loppement des grandes provinces magmatiques du Cantal, du Velay ou de l'Aubrac. Une reprise plus tardive du volca nisme se produit dans la partie nord du Massif central, près de 6 Ma après la fin de l'épisode précédent dit syn-rift. Dans son ensemble, cet épisode majeur est caractérisé par deux pics d'activité: de 9 à 6,5 Ma uniquement au sud du Massif central, puis de 3,5 à 0,5 Ma tant au nord qu'au sud du Massif central. L'analyse du MNT permet de montrer que le nord du Massif central présente un champ de structures dominé par des failles nord-sud. Il s'agit de failles d'âge varisque réactivées en faille normale pendant l'extension et la création des fossés d'effondrement. L' étude des profils d'équilibre des rivières au passage de certaines failles ainsi que l'âge des coulées de lave actuellement en position de reliefs inversés montrent que le soulèvement dans cette partie nord a débu-

INTRODUCTION

The West-European plate was affected by a rifting episode f r om the upper Eocene to the early Miocene (Priabonian to Burdigal ian) [START_REF] Ziegler | -European Cenozoic rift system[END_REF][START_REF] Bois | Initiation and evolution of the Oligo-Mioccne rift basins of southwestcrn Europe : contribution of dccp scismic rencc tion profiling[END_REF]. Extension is roughly perpendicular to the Alpine f r ont and generated a graben system, which includes, from east to west, three main segments : the Eger graben of the Czech Republic, the Rhinegraben oî Gcrmany and the French Massif Central grabens. These three segments are characterised by the oc currence of a typical intraplate alkaline volcanism either within the grabens or along their margins [START_REF] Downes | Tertiary-Quaternary extension-related alkaline magmatism in western and central Europe[END_REF].

The Massif Central area is the largest magmatic pro vince of the West-European Rift system. The origin of the magmatism is still a malter of debate [START_REF] Downes | Tertiary-Quaternary extension-related alkaline magmatism in western and central Europe[END_REF]. The presence of a hot spot below the Massif Central has long been advocated [START_REF] Brousse R | Le volcanisme en France[END_REF]. Different hypo theses such as the upwelling of numerous associated mantle diapirs [START_REF] Lucazeau | Peridotite xenoliths in Massif Central basalts, France. Texturai and geophysical evi dence for asthenosphcric diapirism[END_REF] or a large aborted mantle plume located at depth and common to the entire western part of the European plate [START_REF] Hoernle | -Scismic and chemical evidencc for largc-scalc rnantle upwelling benea1h the castcrn Atlantic and western and central Europe[END_REF][START_REF] Granet | Massif central (France) : new constraints on the gco dynamical evolution from tclescismic tomography[END_REF] have been proposed. Tt bas also been suggested that the magmatism could result from a mantle destabilization induced by the Alpine collision [START_REF] Sobolev S | -Upper mantle 1empera1u res and lithosphere-asthenosphere system beneath the French Massif Central constraincd by scismic, gravity, petrologic and thermal observations[END_REF].

ln these previous studies, the link between extension and magmatism was not clearly established. The aim of this paper is to provide an interpretation of the spatio-temporal distribution of the volcanism by integrating recent geologi r cal, geophysical and geochemical data fom the Massif Central. This makes it possible to unravel the different ki nematic history between the northern and the southern part of the Massif Central during the Tertiary.

DISTRIBUTION OF THE YOLCANISM r

The Massif Central Cenozoic volcanism is spread out f om the Drevin in the north to the Escandorgue area in the south (fig. 1 and2). Volcanism started 65 Ma ago in the Paleocene and the most recent eruptions can be dated f r om the Holocene [START_REF] Bellon | Eocenc volcanic activity in Bourgogne[END_REF][START_REF] Brousse R | Le volcanisme en France[END_REF][START_REF] Vincent | Découverte d'un volcanisme paléocène en Auvergne: les maars de Menat et leurs annexes; é1t1des géologique et géo désique. -81111[END_REF][START_REF] Varet | Le volcanisme tertiaire et quaternaire en France[END_REF]Mergoil et Boivin, 1993;[START_REF] Nehlig | -Histoire géologique sim plifiée du volcan du Cantal[END_REF]. The classification into different magmatic provinces bas been attempted [Goër de Herve and Mergoil, 1971;[START_REF] Brousse R | Le volcanisme en France[END_REF]Maury and Varet, l 980 ;Mergoil and Boivin, 1993]. These studies clearly show that each magmatic province is characterised by some well defined periods of activity and that two main volcanic phases can be distinguished during the Cenozoic : the pre-rift and the post-rift volcanic phases.

This distinction in two periods can be significantly im proved and refined by compiling the different ages of the volcanic provinces, which are geographically listed f r om north to sou th (fig. 1 ). Such a classification allows the dis tinction of three main volcanic phases.

Pre-rift magmatic cvcnt

From the early Paleocene up to the end of the Eocene, a scarce and scattered volcanism, consisting of less than 15 necks and volcanoes, occurred in the northern part of the Massif Central. Only one magmatic manifestation is re ported from the southern part (fig 2a). ln the Charollais area, 7 volcanoes have been recognised dated f r om 64.5 to 36.7 Ma with K/Ar method [START_REF] Bellon | Eocenc volcanic activity in Bourgogne[END_REF]. Near Menat, several outcrops of basait around a maar have been dated from the Paleocene at around 60 Ma with the K/ Ar method [START_REF] Vincent | Découverte d'un volcanisme paléocène en Auvergne: les maars de Menat et leurs annexes; é1t1des géologique et géo désique. -81111[END_REF], a result consistent with paleontological data [START_REF] Kedves | -Quelques types sporomorphcs du bassin lignitifèrc de Menat. -Acla Uni[END_REF][START_REF]Sur Me11athotheri11111 et l'âge paléocène du gise ment de Menat (Puy de Dôme). ln : Problèmes actuels de pa léontologie[END_REF]. ln the Forez, the pre-rift magmatic activity is restricted to a single volcano, which erupted 62 Ma ago [START_REF] Lenoir | Nouvel les données géochronologiqucs. géochimiqucs et isotopiques sur le volcanisme du Forez: relation avec l'évo lution cénozoïque du manteau du Massif central[END_REF]. Here, the lava has a melilitite composition and corresponds to the most under-saturated magma type recorded in the Massif Central [START_REF] Hernandez | Données nouvelles sur la composition minéralo gique de la néphélinitc à mélilite de Marcoux (Forez)[END_REF]. Along the borders of the Limagne graben, near Vichy and Servant, two volcanoes (45.8 and 44 Ma) display inverted reliefs (Mont Péroux and the Champonier dyke) [START_REF] Brousse | Le volcanisme en France el en Europe limitrophe. -Guides géologiques régionaux[END_REF]]. The only pre-rift volcanic manifestation described in the south- [START_REF] Bellon | Eocenc volcanic activity in Bourgogne[END_REF][START_REF] Vincent | Découverte d'un volcanisme paléocène en Auvergne: les maars de Menat et leurs annexes; é1t1des géologique et géo désique. -81111[END_REF][START_REF] Baubron | Existence d'un volcanisme anté-néogène dans les Causses (Massif central français)[END_REF]: Cantagrel and Boivin. 1978;Bellon and Hernandez. 1979;[START_REF] Varet | Le volcanisme tertiaire et quaternaire en France[END_REF]Baubron andDemange, 1982: Cantagrcl andBaubron. 1983;Gas taud et al., 1983[START_REF] Chantepie | Le volcanisme basaltique miocène et pliocène dis persé dans la région de Clermont Ferrand et sur le plateau des Dômes. Etude pétrologique et géochronologiquc[END_REF]Goër et al., 1991 b: Mergoil andBoivin, 1993;Rochette et al .. 1993: Cheguer. 1996: Nchl ig et al .. 1999). B) Synthe tic representation of volcanism in the Massif Central.

Boxes 1 and 2 reprcscnt the l wo peaks of activity during the ma j or magmatie event. et al., 1974: Vince/li et al., 1977; Baubron et al., 1978; Cc111tagrel and Boil'i11. 1978: Bellon ami Hemandez. 1979: Ma111: r and Varet, 1980; Baubron a11d Demange. 1982: Ca11tagrel a11d Ba11bro11. 1983: Gastaud et al.. 1983: Chantepie. 1990: Goër et al .. 1991a: Mergoil a11d Boivi11. 1993: Rochelle et al .. 1993: Cheg11e1: 1996: Nel,lig et al .. 1999). 8) Représe11tatio11 sy11thétiq11e du 1•olcanis111e du Massi f cemral. Les caisso11s I et 2 rep1•ése111ent les deux pics d'aclivité durant la phase majeure de volcanisme.

ern part of the Massif Central is a basanitic neck in the Tassières hamlet near Séverac le Château (57 Ma) [START_REF] Baubron | Existence d'un volcanisme anté-néogène dans les Causses (Massif central français)[END_REF].

The above ages were obtained in the 1970s and the 1980s by the K/Ar method. New datings using the 40 Ari 39 Ar method, carried out in the Massif Central, clearly shows that previously published ages based on the KI Ar method must be interpreted with care. For example, the Marcoux melilitite, which has been previously dated at 49.8 Ma with the KI Ar method [Bellon and Hernandez, 1979), bas given an age of about 62 Ma using the .io Ar/ 39 Ar method [Lenoir el al., 2000). However, these data confi r m the time span of the pre-rift magmatism from Paleocene to Upper Eocene. The lavas of the pre-rift volcanism are highly under-saturated (alkali basait to melilitite), which strongly suggests small degrees of partial melting (< 5%) of a C02-and H20-rich mantle source [START_REF] Edgar | The gcncsis of alkalinc magmas with cmphasis on their source regions: infcrenccs from cxpcrimcntal studies[END_REF]Wil son et al., 1995).

Rifting magmatic event

From the upper Eocene to the end of the lower M iocene, the Massif Central was af f ected by east-west extension, which led to the formation of numerous grabens [Bergerat, 1985;Blès el al, 1989;Lacombe et al., 1993;Merle el al., 1998). ln the northern part of the Massif Central, the three most important grabens are N-S oriented. From east to west, they are the Bresse graben, the Roanne-Montbrison graben and the Limagne graben [fig. 1 in Merle el al., 1998). As shown f r om an experimental study, the geometry of the sys tem can be explained as resulting f r om symmetric exten sion during the upper Eocene and middle Oligocene [Merle el al., 1998;Michon and Merle, 2000). ln the upper Oligocene and to a lesser extent in the lower Miocene, sub sidence remained very active as indicated by the sedimenta tion rate in the western Limagne graben while there was a lull in Roanne-Montbrison and Bresse grabens [Merle et al., 1998;Hugueney et al., 1999). ln the south, the main sedimentary basins are Le Puy, Saint Flour and Aurillac basins but these are much less developed than in the north ern part. The distinction between well defined N-S grabens in the north and small and non-oriented basins in the south is reinforced by recent geophysical data [START_REF] Zeyen | Refraction-seismic investigations of the nothern Mas sif Central (France)[END_REF]. [START_REF] Zeyen | Refraction-seismic investigations of the nothern Mas sif Central (France)[END_REF]. The Moho is at 26 km depth in the west [START_REF] Zeyen | Refraction-seismic investigations of the nothern Mas sif Central (France)[END_REF]] and 29.5 km in the east [START_REF] Bergerat | Extcnsional tectonics and subsidence of the Bresse basin: an in1crprc1a1ion from ECORS data. -Mé111[END_REF], indicating a thinning of the crust to 23.3 km and 28 km below the Limagne and Bresse grabens, respectively. This yields 25 % crustal thinning beneath the Limagne graben and 10 % below the Bresse [Merle et al., 1998]. Ac cording to the sedimentation history, most of the 25%

C

crustal thinning in the Limagne graben can be considered to have occurred in the upper Oligocene and to a lesser extent in the lower Miocene.

In the southern part of the Massif Central, the Moho is flat-lying at a depth of about 28 km. The average topo graphie elevation of the Yariscan basement is about l 000 m. This suggests slight crustal thinning during the rifting event. This is correlated with low thickness of the Eo-Oligocene sediments, which never exceeds a few hundred meters.

These data reveal a strikingly different tectonic evolu tion between a northern part of the Massif Central where significant crustal thinning has occurred and the southern part where the rifting was limited. Despite the oc _ currenc _ e of a few volcanic minerais in some upper Rupelian sed1ments [START_REF] Devineau | Dynamique d'un rift intracontinental: enregistre ment par les variations isotopiques Sr-Nd dans la fraction détritique des sédiments[END_REF], the rifting event is mainly charac terised by the lack of volcanic activity during sedimentation both in the north and the south of the Massif Central.

The "rift-related magmatic event" started in the late Oligocene at a time when sedimentation was on the decline. It lasted about 15 Ma up to the end of the lower Miocene. Peperitic deposits interbedded in the upper Oligocene sedi ments (Hourrière et al., 1998] prove that this scattered mag matic event is coeval with the asymmetric phase of rifting. The volcanism is located in the northern part of the Massif Central only and is totally unknown in the southern part (fig. 1 ). Moreover, it is restricted to the western area of the northern part, which indicates that it is spatially connected with the area of high crustal thinning, that is the Limagne graben.

More than 200 monogenic scattered volcanoes erupted during this period mainly within the Limagne graben (s./.) or the Forez areas (fig. 2b). Two volcanic manifestations only have been reported far away f r om the graben areas near the Variscan Sillon Houiller fouit (fig 2b). The magmatism is mainly characterised by alkaline basaltic to nephelinitic lavas (85 % of the total volume) and the _ only magmas exhibiting a differentiation trend are located in the Comté area, in the heart of the Limagne graben [START_REF] Tricot | Le volcanisme dia Comté d'Auvergne (MCF). -Thèse 3 c cycle[END_REF] (fig. 2b). Recent 40 Ar! 39 Ar dating from this area indi-

Major magmatic event

The beginning of the major magmatic event is not coeval in the southern and northern parts of the Massif Central. It started in the south, in areas that lack any volcanism before that period, except the Paleocene Tassières neck (fig 1 and 2a). Eruptions took place from the upper Miocene, at least 15 Ma after the end of the sedimentation. In the north, the major magmatic event started after a gap of about 6 Ma in the upper Miocene. The volume of erupted magma dunng this period is much larger than during any previous mag matic events, and is estimated from 1000 to 1500 km 3 .

As revealed by the estimation of volumes erupted dur ing this period, the major magmatic event was characterised by two peaks of activity (fig. 1 b). The fi r st peak can be dated from 9 to 6 Ma and is associated with the building of the largest magmatic provinces of the Massif Central (Cantal, Aubrac, Velay, Causses and Coirons), following a N l 20 °E/N l 40 °E average trend (Cantal (85 % of the trachy andesitic stratovolcano ages between 9-6 Ma), Aubrac (8, 7-6 Ma), South Velay (83 % of the ages between 9-6 Ma), Causses (9.2-5.8 Ma) and Coirons (7.9-6.2 Ma)) [Baubron and Demange, 1982;Mergoil and Boivin, 1993;[START_REF] Féraud | Geochronological and structural study of' Tcrtiairy and Quaternary dykes in southern France and Sardinia : an examplc of the utilization of dykes swanns as pa leostress indicator[END_REF][START_REF] Fréour | Modèle d'évolution volcano-structural du Cantal: Rôle et répartition des déstabilisations de nancs en zone proxi male (Cantal -France)[END_REF]] (fig 2c). This huge volume of volcanic activity is restricted to a N 11 ü°E oriented elliptical area in the south (fig. 2c) where the Oligocene crustal thinning was negligible (<< 5 %). There is a very good correlation between the location and orienta tion of these large magmatic provinces in the field and the mantle thermal anomaly beneath as deduced by seismic studies [Granet el al., 1995;[START_REF] Sobolev S | -Upper mantle 1empera1u res and lithosphere-asthenosphere system beneath the French Massif Central constraincd by scismic, gravity, petrologic and thermal observations[END_REF] (fig. 3).

ln contrast to the first peak, the second peak of volca nism occurred both in the north and the sou th of the Massif Central. From 3.5 Ma to 0.5 Ma, it led both to the building of new provinces such as Escandorgue, Monts Dore and Devès, and the reactivation of old magmatic provinces (Limagne, Causses, Velay and Sioule). The spatial distribu tion of this second peak is more elongated in the north-south direction (f r om the Chaîne des Puys to the Escandorgue) than the fi r st peak but has a quite similar east-west width. The main volcanic provinces formed dur ing this period are the Monts Dore and Sancy stratovolcanoes and the Devès basaltic shield. The other provinces are characterised by limited monogenic activity (Escandorgue, Velay) during which relatively small vo lumes of lava were erupted. Following this second peak, the last eruptions in the Massif Central occurred f r om the upper Pleistocene to the present-day in the Chaîne des Puys, the Cézallier and in the Ardèche area (Bas Vivarais) [Goer et al., 1991a;Rochette el al., 1993].

Erupted magmas are sub-alkaline (basait to rhyolite) or alkaline (basanite to phonolite) intraplate lavas. The geo chemical and Sr-Nd-Pb isotopie of the most primitive mafic magmas from the Cantal stratovolcano can be modelled by mixing partial melts of: (i) an asthenospheric component with isotopie composition be tween depleted mantle and high p (HJMU) endmembers for which there is no need to invoke the existence of a deep mantle plume and (ii) a lithospheric component derived from the old Variscan lithosphere [START_REF] Downes | Tertiary-Quaternary extension-related alkaline magmatism in western and central Europe[END_REF].

TIMING OF UPLIFT

Marine to laguno-lacustrine Eo-Oligocene sediments can be seen at elevations of several hundred metres (up to 1000 m), which clearly indicates post-Oligocene uplift of the whole area. ln the studied zone, this uplift post-dates that of the southern margin of the Massif Central in the Cévennes area [START_REF] Rey R | Biostratigraphic des formations sédimentaires du Cantal[END_REF]. lt occurred from the Upper Miocene (about 10 Ma) with a maximum of regional dom ing f r om the Upper Pliocene (3.5 Ma) [Derruau, 1971;[START_REF] Griel | L'évolution du !Vtassif central français[END_REF].

ln the south, it has been shown that the uplift in the Velay, Margeride and southern Forez started in the Upper Miocene, 10 Ma ago, and led to the formation of the main trend of topography observed nowadays [START_REF] Upton | Les monts du Forez[END_REF][START_REF] Griel | L'évolution du !Vtassif central français[END_REF]Goër de Herve and Etienne, 1991;[START_REF] Cantagrel | Chronologie de l'cncaisscmcnt du réseau hydrographique en domaine volcanisé: l'exemple de la haute vallée de la Loire[END_REF]]. From 5.5 to 3-3 .5 Ma, the superimposi tion of the volcanic formations in the Allagnon valley and in the Velay area suggests that uplift was limited [Goër de Herve and Etienne, 1991;[START_REF] Augendre | Les cnseigne111en1s des marqueurs en paléogéo graphie dans une aire volcanisée : l'exe111ple de la vallée de l'Allagnon en aval de Joursac[END_REF]. Following this period, doming became again very active and led to impor tant inverted relief of volcanic formations (e.g. the Cézallier eastern border, the Allagnon and Allier valley and the Escandorgue).

ln the north, some very clear fault traces and the study of river entrenchment, together with the cross-cutting rela tionships with well-dated Cenozoic volcanic formations, demonstrates that the main upl i ft of the northern part of the Massif Central postdates 3-3.5 Ma. Firstly, departures from equilibrium profiles, as in the Forez area (fig. 4), at the crossing of fault traces along streams running in the same lithology cannot be ascribed to the Oligocene sedimenta tion period but demonstrates very recent differential uplift. Secondly, from the Lower Miocene to the Upper Pliocene, this uplift was slight or even inactive as lava tlows from that period of time are now found superposed, which indi cates that they flowed out within the same paleo-valleys. The occurrence of several Pliocene volcanic formations several hundred metres above the present day watercourses clearly shows a strong excavation of the rivers since 3-3.5 Ma.

PRESENT-DA Y STRUCTURAL TRENDS

Using a Digital Elevation Model (DEM with a 50 m resolu tion step), it can be shown that the northern Massif Central displays a N-S oriented fault pattern whereas the southern part displays a N 135 °E oriented fault pattern (fig. 3a). Both directions result f r om older Variscan tectonics, as shown by structural studies in other areas of the Massif Central that lack any imprint of the rifting episode [Feybesse, 1981].

Although the prominant directions of the major faults in the north are commonly N-S or near N-S, some oblique orientations are reported either in the Limagne graben or within the Hercynian basement area on the edge of the graben. ln the Combrailles area (NW of Clermont-Ferrand), uplift is indicated by a downtbrow of more than one hun dred metres of Variscan basement through reactivation of well defined Variscan faults such as the Sillon Houiller or through the reactivation of some previously undescribed oblique networks such as the Morge faulted zone (N55 °E) (fig 5). The Morge faulted zone is one of the most seismic areas of the Massif Central [Dorel et al.,I 995] possibly re sulting fr om residual or continuing activity of the uplift process.

In the south, the DEM analysis reveals a prominant N J 35 °E oriented fouit pattern underlined by several volca nic alignments (Aubrac, Devès and Velay), the elliptic shape of the Cantal stratovolcano and the trend of the Margeride horst (fig. 3a). A single N045 °E direction can also be defined east of the Velay, a common Variscan direc tion in this area.

DISCUSSION

The pre-ri ft volcanism is present throughout the entire West-European Rift system and is arranged roughly parai lei to the Alpine front (fig. 6). It is scarce and characterised by a few tens of outcrops of primary magmas ranging f r om al kali basait to very undersaturated lavas such as the melilitites of Marcoux, Essey-la-Côte, Grand Valtin and Bohemia [START_REF] Wilson | Melilitites: partial melts of the thermal boundary layer? -Co111rib[END_REF]. This volcanism started dur ing the late Cretaceous and mainly occurred in the Paleocene and Eocene. At the European scale, this period (i.e. a round the Cretaceous-Tertiary transition) corresponds to the early Alpine compression and to the peak of the high-pressure metamorphism in the Sesia zone [see Merle and Michon, 2001].

The buckling of the lithosphere as a response to com pression was recently studied by numerical modelling [START_REF] Cloetingh | Lithosphere folding: Primary response 10 compression'? (from central Asia to Paris basin)[END_REF]. For a 300 Ma lithosphere, that is the inferred age of the European Variscan lithosphere, re sults show that the buckling can be preserved during several tens of million years. They are also in good agreement with geological studies at the scale of the Massif Central. Grav ity data show the existence of crustal ridges which have been attributed to lithospheric buckling at about 60 Ma as a result of early Alpine compression [Lefort and Argawal, 1996]. Southeast of the Paris Basin, the doming is concen tric around the Alpine belt and reveals a curved uplifted zone, which goes from the north of the Massif Central to the south of the Rhine graben [Lefort and Agarwal, 1996, Fig I O]. We believe significant that this concentric ridge around the Alpine belt corresponds to the location of the pre-rift volcanism. As an attempt to interpret this pre-rift volcanism, whose spatial distribution is not related to the main Limagne and Rhine grabens but with a large-scale crustal undulation, we propose that it may result from a low to very low degrees of mantle melting induced by lithospheric doming and adiabatic decompression during the early Alpine compressive event. The activity of the pre-rift volcanism up to the Eocene could be explained by the preservation of the lithospheric buckling. This hypothe-sis is reinforced by geomorphological studies which show a general uplift of the Massif Central and Morvan area at the same period [ [START_REF] Griel | L'évolution du !Vtassif central français[END_REF][START_REF] Wyns R | Contraintes géologiques et géomorphologiques à l'his toire de la mobilité verticale de la lithosphère continentale en Europe de l'ouest depuis le Crétacé; relation avec la genèse des grabens et du volcanisme tertiaires[END_REF].

The rifting volcanic event, which is contemporaneous with the upper Oligocene-lower Miocene rift sedimenta tion, occurred in the west of the northern part of the Massif Central. This magmatic phase is clearly related in space and time to the asymmetric rifting episode, which induced significant crustal thinning in the west, mostly below the Limagne graben (Merle et al., 1998]. Assuming that high crustal thinning results f r om the stretching of the whole lithosphere, the scattered volcanic activity recorded f r om the upper Oligocene to lower Miocene may be induced by low degrees of mantle melting (< 10%) due to mantle de compression during extension. This interpretation is rein forced by the lack of volcanism in the southern part where very slight crustal thinning occurred during the Oligocene. The scattered volcanic event is then interpreted as a conse quence of the rifting episode in the northern part of the Massif Central. In this area, the main structures which formed during this rifting event are N-S oriented, mostly perpendicular to the extension direction [Bergerat, 1985], and the present structure is probably inherited from that pe riod.

A fter a Jack of volcanism both in the northern and southern parts, voluminous magmatic activity started at 14 Ma and 5.5 Ma ago in the southern and northern parts, res pectively. This major volcanic event is characterised by a more or Jess continuous activity up to the present�day and can be divided into two volcanic peaks. These two peaks are spatially and temporally associated with the two main peri ods of uplift, suggesting a common origin for volcanism and upl i ft processes.

ln the south, it is probably significant to consider the parallelism between the trend of major faults, the long axis of the two principal magmatic provinces (Cantal and Devès-Velay) and the thermal anomaly orientation (fig 3). The elongated thermal anomaly just below the Cantal and Devès-Velay volcanic provinces suggests that the major volcanic event was induced by this significant asthenospheric upwelling south of the main rifting area. In contrast with lithospheric mantle thinning deduced f r om the seismic tomography, slight crustal thinning in the same area indicates that upwelling of the litbosphere/ asthenosphere boundary does not result from Eo-Oligocene extension but is probably due to a late process of thermo-mechanical ero-sion a long the I ithosphere Such a thermal erosion at the lithosphere base would have caused (i) the major mag matic event and (ii) the uplift of the volcanic area as a re sult of isostatic adjustment.

The occurrence of two peaks of magrnatism and uplift cou Id be attributed to two episodes of thermal erosion at the base of the lithosphere spaced by about 5 Ma. The first epi sode, f r om 9 Ma to 6 Ma, was restricted to the southern part of the Massif Central whereas the second one, f r om 3.5 to 0.5 Ma, was less pronounced, but more widespread : f r om the Chaîne des Puys in the nortb to the Escandorgue in the sou th.

This general magmatic evolution drawn from data at the Massif Central scale may apply to the Eger graben as well, as the three magmatic events described in this study (pre-rift magmatic event, rifting event and post-Miocene volcanic event) are also reported in the literature [Bellon and Kopecky, 1977;Dudek and Elias, 1984) (fig 7). This suggests that a single cause should explain the formation of the entire western European rift surrounding the Alpine mountain belt.

CONCLUSIONS

This study on the spatio-temporal distribution of volcanism in the Massif Central makes it possible to propose three successive magmatic events during the Tertiary evolution of the Massif Central.

1. Pre-rift magmatic event. -This volcanic phase is ex tremely small, but common to the entire West-European Rift system [Bellon and Kopecky, 1977;[START_REF] Vincent | Découverte d'un volcanisme paléocène en Auvergne: les maars de Menat et leurs annexes; é1t1des géologique et géo désique. -81111[END_REF]Lippolt, 1983;Brousse and Lefèvre, 1990). The pa rallelism between the pre-rift volcanic belt and the Alpine f r ont cou Id be explained by a flexure of the lithosphere dur ing early Alpine late Cretaceous-Paleocene compression.

2. Rifting magmatic event. -From the Priabonian up to the end of the early Miocene, three main N-S grabens were initiated in the northern part of the Massif Central. ln the upper 01 igocene, extension remained active in the west only, leading to a significant crustal thinning below the western Limagne graben (up to 25%). This lithospheric stretching induced a scattered late Oligocene-lower Mio cene volcanic phase located f r om the Roanne-Montbrison graben in the east to the Sillon Houiller in the west. ln the south, a very slight crustal thinning occurred during the Oligocene rifting period. Tt is considered that this thinning was not large enough to trigger volcanism, as it occurred in the north during the lower Miocene.

Major magmatic event.

-This volcanic episode, which started 14 Ma ago in the south and 5.5 Ma ago in the North, is characterised by two peaks of volcanism (9-6 and 3.5-0.5 Ma). Timing and structural features of the major magmatic event indicate that it is unrelated to Eo-Oligocene extensional processes. The fi r st peak of this major mag matic event, which occurred in the sou th of the Massif Cen tral only, is coeval with an episode of uplift and is attributed to a thermal erosion along the base of the litho sphere. The second peak, which is also coeval with an epi sode of uplift, would be induced by a similar process, but below a larger area, from north to south, than for the first peak.

Acknowledgments. -The authors want to thank Olivier Lacombe, Jean-Christophe Maurin and Marjorie Wilson for their critical reviews.

  FIG.1. -A ) Spatio-temporal distribution of the volcanism in the Massif Central area: the ri fting cvcnt include both the sedi111en1ation/ex1ensional period and the scattered volcanism rccorded in the north. Note the lack of volcanism aftcr the sedimentation pcriod in the south. For cxplanation sec the tcxt Established from radiomctric data[START_REF] Bellon | Eocenc volcanic activity in Bourgogne[END_REF][START_REF] Vincent | Découverte d'un volcanisme paléocène en Auvergne: les maars de Menat et leurs annexes; é1t1des géologique et géo désique. -81111[END_REF][START_REF] Baubron | Existence d'un volcanisme anté-néogène dans les Causses (Massif central français)[END_REF]: Cantagrel and Boivin. 1978; Bellon and Hernandez. 1979;[START_REF] Varet | Le volcanisme tertiaire et quaternaire en France[END_REF] Baubron and Demange, 1982: Cantagrcl and Baubron. 1983; Gas taud et al., 1983[START_REF] Chantepie | Le volcanisme basaltique miocène et pliocène dis persé dans la région de Clermont Ferrand et sur le plateau des Dômes. Etude pétrologique et géochronologiquc[END_REF] Goër et al., 1991 b: Mergoil and Boivin, 1993; Rochette et al .. 1993: Cheguer. 1996: Nchl ig et al .. 1999). B) Synthe tic representation of volcanism in the Massif Central.

FIG. 1

 1 FIG.1. -A) Réparlitio11 spatio-temporelle du volca11isme : ! 'épisode de rifti11g inclut les périodes d •extensio11 et de sédime11tc11io11. et la phase de vole<1-11isme dispersé dans la partie nord. Noter l 'abse11ce de 1•olcc111is111e après la période de sédi111e111atio11 dam la moitié sud. [D "après les 11•m•c111xde 8ello11 et al., 1974: Vince/li et al., 1977; Baubron et al., 1978; Cc111tagrel and Boil'i11. 1978: Bellon ami Hemandez. 1979: Ma111: r [START_REF] Varet | Le volcanisme tertiaire et quaternaire en France[END_REF] Baubron a11d Demange. 1982: Ca11tagrel a11d Ba11bro11. 1983: Gastaud et al.. 1983[START_REF] Chantepie | Le volcanisme basaltique miocène et pliocène dis persé dans la région de Clermont Ferrand et sur le plateau des Dômes. Etude pétrologique et géochronologiquc[END_REF]: Goër et al .. 1991a: Mergoil a11d Boivi11. 1993: Rochelle et al .. 1993: Cheg11e1: 1996: Nel,lig et al .. 1999). 8) Représe11tatio11 sy11thétiq11e du 1•olcanis111e du Massi f cemral. Les caisso11s I et 2 rep1•ése111ent les deux pics d'aclivité durant la phase majeure de volcanisme.

D

  FIG. 2. -Schematic maps showing the spatial distribution of the Massif Central volcanism. A , B and C represent the three di fferent volcanic cvents. In C, the N 110 °E elliptical area corresponds 10 the zone affected by the first peak or volcanism. FIG. 2. -Cartes schématiques de la distribution spatiale du volcanisme du Massif cen tral. A, B et C représen ten t les trois épisodes volcaniques. En C, l'ellipse orientée N 11 < f E représente la zone couverte par le premier pic de volcanisme.

  between 20 and 22 Ma with a climax at about 21 Ma[Cheguer, 1996].

  . 3. _ A) Struc1ural setting of the northern and the soulhern parts of the Massif Central. I n black, elevat(on higher than 1000 _ m. _ Prominent orienta tions are Nl35'E in the south and N-S in the north. B) Schematic map present1ng the onentauon of the Eo-Ohgocenc _ crustal th1nnmg Ill the north (1) and the thermal anomaly orientation in the south (2). Lithosphere asthenosphere boundary (LAB) topography g1ven 111 ktlometres [fr om Sobolev et al., �;i 1 t _ A) Cadre structural des parties nord et sud du Massi f Central. En noir som représentées les aftill'.des supérie '. ll"es à 10 � 0 ! "• le � orie11t � 1tio ! 1s don;ilw111es de fa moitié sud sont N l 3S'E et la partie nord est marquée par des directions N-S. 8) Carte s � hema _ t,que p1 : esentan ! I one ! 1tat1on de I 0111111cissement crus ta/ éo-ofigocène dans le nord ( 1) et f 'orientation de / 'anomalie thermique dans le sud(2) l11111te /1t/10sphere-asthenosphere (LA B) en kdo mètres [d'après Sobolev et al., /997).

  FIG. 4. -Topographie profiles of two streams running aeross the Forez fàult. Departures from the cquilibrium profiles of the streams when crossing the fault indicate recent vertical movemcnts, as already suggcsted by the very clear fault trace of' the fault on the DEM. A.G.: Ambert graben. FIG. 4. -Prof ils topographiques de deux ruisseaux recoupant la/aille du Fore=. les ruptures de pentes des prof ils aux passages de la/aille du Fore= indi quent des 111011w:111ents 1•ertica11x récents. Celle rectonique est égale111e111 suggérée pa,• le tracé très net de la faille du Forez sur le MNT. A. G. : Fossé d'Amberr.

  FIG. S. -Digital Elevation Model (DEM) of the Combrailles area. The Sillon houiller and the Morge faults bound a horst on which the Monts Dore and Chaîne des Puys volcanic provinces were built up. FIG. 5. -Modèle 1111111érique de 1erroi11 (MNT) de la régio11 des Combrailles. le Sillo11 houiller el la/aille de la Morge borde111 1111 hors! sur lequel se sot11 co11s1r11iles les provi11ces volcaniques des 1110111s Dores el de la choî11e des P uy s.

  -... �--=LJ='--'L.:.J =::.., .

D

  FIG. 7. -Comparative Tcniary evolution of the Massif Central and of the Eger graben. ln the Massif Central, data from the northcrn and southern pan are gathered in the same diagram and the major magmatic cvcnt is schema1ically represented by the two pcaks ofactivity (1 and 2). Note the strong similari ties between these two segments of the West-European riït, separated by several hundred kilomctres. The similarity indicates an evolution linked tocom mon regional factors. FIG. 7. -Comparaiso11 de I 'évolutio11 tertic,ire du Massi f ce11t/'GI et du graben de l'Ege,: Da11s le Massi f central, les données des parties sud et nord so111 rassemblées e11 1111 ((lbleau unique et la phase majeure de volcanisme est représentée par les deux pics de magma1is111e (I et 2). Noter les.fortes similitu des d'évolution entre ces deux segments du rifi Ouest-européen. séparés par plusieurs centaines de kilomètres.

- Major volcanlc event (Upper Mlocene/present) 1 Slou� 2 SillooHCM.l.11kr 3 Clli!ncdcl Puy, 4 Monu Dore 5 Ctr.allier 6 C.,,c,J 7 �• bl8UJ.it1duPvy 8 Vctayorifflul 9 eo,,.. 10 Allbrlc 11 Bu Vivvai, 12 Cavua 13e

..-.,..


Olivier Merle

Laurent Michon

The formation of the West European rift : A new model as exemplified by the Massif Central area

Keywords: Massif Central, Alps, West European Rift, Tectonic, Volcanism, Cenozoic

Abstracl. -In this paper, we use mainly field data from the Massif Central area, which have been presented in a com panion paper [Michon and Merle, 2001), to discuss the origin and the evolution of the West European Rift system. Tt is shown that the tectonic event in the Tertiary is two-stage. The overall geological evolution reveal a tectonic paradoxe as the first stage strongly suggests passive ri fting, whereas the second stage displays the tirst stage of active rifting. ln the north, crustal thinning, graben formation and sedimentation at sea level without volcanism during the Lower Oligo cene, followed by scattered volcanism in a thinned area during Upper Oligocene and Lower Miocene, represent the classical evolution of a rift resulting from extensional stresses within the Iithosphere (i.e. passive rifting). In the sou th, thinning of the lithospheric mantle associated with doming and volcanism in the Upper Miocene, together with the Jack of crustal thinning, may be easily interpreted in terms of the first stage of active rifting due to the ascent of a mantle plume. This active rifting process would have been inhibited before stretching of the crust, as asthenospheric rise associated with uplift and volcanism are the only tectonic events observed. The diachronism of these two events is emphasized by two clearly distinct orientations of crustal thinning in the north and mantle lithospheric thinning in the south. To understand this tectonic paradox, a new mode! is discussed taking into account the Tertiary evolution of the Alpine chain. lt is shown that the formation of a deep Iithospheric root may have important mechanical consequences on the adjacent lithosphere. The downward gravitational force acting on the descending slab may induce coeval exten sion in the surrounding lithosphere. This cou Id trigger graben formation and laguno-marine sedimentation at sea level followed by volcanism as expected for passive rifting. Concurrently, the descending lithospheric tlow induces a tlow pattern in the asthenosphere which can bring up hot mantle to the base of the adjacent lithosphere. Slow thermal ero sion of the base of the lithosphere may lead to a late-stage volcanism and uplift as expected for active rifting. thosphérique s'accompagne également d'un autre processus. Lorsque la racine lithosphérique se forme, elle prend la place de l'asthénosphère et provoque son déplacement latéral. Ceci induit un t1ux asthénosphérique, ascendant au ni veau de la lithosphère adjacente, qui contrebalance le mouvement descendant lié à la racine. La cellule ainsi créée amène de l'asthénosphère chaude à la base de la lithosphère adjacente. Si le processus connaît une ampleur suffisante, pendant une durée de temps de plusieurs millions d'années, une érosion thermique de la base de la lithosphère adja cente se produit, conduisant à un soulèvement isostatique et une phase de volcanisme, d'une manière tout à fait sem blable à ce qui est observé dans le cas d'un rift actif. La création de la racine lithosphérique d'une chaîne de montagne peut produire ainsi deux ef f ets dans la lithosphère adjacente: une extension lithosphérique et une érosion thermique à la base de la lithosphère. Ces deux effets ne sont pas synchrones et le second est différé dans le temps. Dès que la ra cine lithosphérique commence à se former, la force verticale agissant sur cette racine peut engendrer l'extension dans la lithosphère adjacente. En revanche, l'érosion thermique est un phénomène tardif qui nécessite (i) que la racine at teigne une certaine profondeur pour engendrer un t1ux asthénosphérique conséquent et (ii) un temps relativement long pour réaliser une érosion thermique significative. Le paradoxe tectonique enregistré dans le Massif central peut être ré solu en considérant la formation de la profonde racine alpine. D'une façon tout à fait semblable à l'évolution en deux temps découlant du modèle proposé, le Massif central est d'abord affecté par un rifting de type passif suivi quelques millions d'années plus tard par les premiers stades d'un rift de type actif.

La formation du rift Ouest-européen : un nouveau modèle tectonique

Mols-clés. -Massif Central, Alpes, Rift Ouest-européen, Tectonique, Volcanisme, Cénozoïque.

Résumé. -La synthèse des données géologique montre une évolution tout à fait différente entre le nord et le sud du Massif central. Le nord du Massif central est principalement caractérisé à l '01 igocène par un fort amincissement crus tal, la formation de fossé d'eff ondrement au niveau de la mer avec une sédimentation épaisse (jusqu'à 3000 m) sans volcanisme, suivie de la fin de !'Oligocène supérieur au Miocène inférieur d'un volcanisme localisé dans les zones de plus fort amincissement crustal. Cette évolution est typique d'un rift de type passif. Au sud, l'évolution tectonique est caractérisée au Miocène supérieur par un très fort amincissement de la lithosphère mantellique sans amincissement crustal, un soulèvement et une importante phase magmatique. Cette évolution est cohérente avec celle attendue lors des premiers stades d'un rift de type actif. Le diachronisme de ces deux événements est souligné par l'orientation N-S de l'amincissement crustal au nord et 1 'orientation N l 3S'E de 1 'amincissement de la lithosphère mantellique au sud. Le modèle proposé pour expliquer ce paradoxe part de l'hypothèse que cette évolution procède d'une cause unique, qui se trouve dans la formation de la chaîne alpine immédiatement plus à l'est. La chaîne alpine est spatialement et tempo rellement connectée au Rift Ouest-européen. Celui-ci comprend, d'est en ouest, le graben de l'Eger, le graben du Rhin et les fossés d'effondrement du Massif central, l'ensemble du système de grabens étant disposés concentriquement au tour du front alpin. La déformation dans les Alpes démarre à !'Eocène comme en témoigne les données géochronologi ques récentes sur le métamorphisme de haute pression dans les zones les plus internes de la chaîne. La progression vers l'ouest des déformations est attestée par les données stratigraphiques qui montrent que le chevauchement du Briançonnais sur la zone dauphinoise date du début de !'Oligocène. La formation de ce chevauchement crustal majeur de la chaîne alpine est ainsi contemporaine de la formation des grabens et de la sédimentation dans le Massif central. Deux questions se posent: la formation d'une chaîne de montagne peut-elle générer dans la lithosphère adjacente un épisode d'extension? les modalités de cette extension peuvent-elles se dérouler en deux étapes successives, la pre mière ayant les caractéristiques d'un rift passif et la seconde les caractéristiques d'un rift actif? Pendant la collision continentale, le découplage de la croûte et du manteau entraîne la formation d'une profonde racine constituée de man teau lithosphérique. Des simulations numériques onl montré que la force gravitaire de cette racine lourde, plus dense que l'asthénosphère environnante, engendrait une compression dans la croûte sus-jacente et de l'extension dans la croûte adjacente. Un tel système est à même de dépasser la résistance en extension de la lithosphère adjacente et de provoquer un épisode de rifting qui présentera les caractéristiques d'un rift de type passif. La création de la racine li-

INTRODUCTION

The formation of the West European rift remains a subject of great debate. As a continental rift, its origin may be at tributed to two different tectonic processes. The first hy pothesis involves continental rifting, which would be entirely governed by lithosphere extension. Such rifts, r which are termed passive rifts, result f om extensional fail ure of the continental lithosphere, where the main cause is to be found in far-field stresses generated at plate bound aries. The second hypothesis refers to a continental rift ini tiated above a mantle plume or diapir. This second type, usually termed active rifting, is associated with thermal thinning of the base of the lithosphere due to heat advection at the lithosphere-asthenosphere boundary (LA B).

The initial stages of these two different types of rift are thought to be significantly different, which allows distinc tion in the field [START_REF] Park R | Geological structures and moving plates[END_REF]. Plume-generated rifts are as sociated with crustal doming and abundant volcanism in their initial stages. Graben formation and sedimentation oc curs later, when the doming reaches a critical level to initi ate extensional faulting within the crust. ln contrast, far-field stress-generated rifts start with graben formation and sedimentation close to sea level and develop doming and volcanism at a later stage.

Tn this paper, we use mainly field data from the Massif central area, which have been presented in a companion pa per [Michon and Merle, 2001], to discuss the origin and the evolution of the West European Rift system. As exempli fied by the evolution of the Massif Central rift, we show that this extension is closely related to the formation of the Alps. A new tectonic model is proposed which takes into account the role of the deep lithospheric root created during mountain building processes in the deformation of the sur rounding lithosphere.

THE TECTONIC PARADOX OF THE MASSIF CENTRAL

In this study, we do not deal with the very sparse volcanic manifestations which occurred in the Paleocene at around 60-65 Ma [Vincent et al, 1977]. We believe that this rare pre-rift volcanism may be attributed to the flexure of the lithosphere associated with the first compressions recorded in the Alps [see the discussion in Michon and Merle, 2001], as testified by the oldest radiometric data at about 65 Ma in the Sesia Austro-Alpine unit [Froitzheim et al., 1996;Rubatto et al., 1999] and geophysical data about the buck ling of the French lithosphere during the Paleocene [i.e. [START_REF] Lefort | Gravity evidence for Alpine buckl ing of the crust bencath the Paris Basin[END_REF].

Synthesis of the main geological data reveals a strik ingly different structural evolution between the northern and the southern parts of the Massif Central. Tt is useful to summarize these differences as they reveal the fondamental constraints that must be taken into account before propos ing a general tectonic model.

The northern part of the Massif Central is characterized by the following structural features :

1) grabens are well developed and display a clear north-south orientation. Sedimentation rates have been high with up to 3500 m in the Limagne graben;

2) upper Eocene and Oligocene sedimentation occurred close to sea level, as demonstrated by repeated marine in cursions [START_REF]Etudes géologiques sur la Limagne (Auvergne). -Thèse d'Etat[END_REF][START_REF] Gorin | Etude palynostratigraphique de sédiments paléogènes de la grande Limagne (Massif central. France). -Thèse 3[END_REF][START_REF] Chauve | L'Est de la France (Vosges. fossé rhénan[END_REF]Rat, 1984;Briot and Poidevin, 1998;[START_REF] Bodergat A | Incursions marines dans l'environnement lacustre du rift oligo cène de Limagne (Massif central. France): apport des organis mes halophiles et des isotopes du strontium; datation par les mammi fèrcs[END_REF];

3) crustal thinning is locally important, up to 25% in the Limagne graben [Merle et al., 1998]. It is spatially con nected with the main grabens, and is especially visible be low the Limagne and the Bresse grabens [e.g. [START_REF] Bergerat | Extensional tectonics and subsidence of the Bresse basin : an interprctation f r om ECORS data[END_REF][START_REF] Truffert | Structures crustalcs sous le Jura et la Bresse: con traintes sismiques et gravimétriqucs le long des profils ECORS Bresse-Jura et Alpes Il[END_REF]Zeyen et al., 1997];

4) there is a linear relationship between the thickness of Oligocene sediments in the grabens and the amount of crustal thinning, showing that crustal thinning took place in the Oligocene and was achieved when the sedimentation stopped [Fig. 6 in Merle et al., 1998]; 5) the sedimentation period lacked accompanying vol canism. Volcanism started by the end of the sedimentation and was mainly developed in the lower Miocene; 6) this scattered volcanism occurred in the west, espe cial ly in the Limagne graben, that is in the zone of highest crustal thinning [Fig. 2 in Michon and Merle, 200 l]; 7) a second magmatic event started after a gap of about 6 Ma in the upper Miocene. This second magmatic event is also restricted to the west, but main ly occurred in an area where geophysical data have revealed no crustal thinning [Zeyen et al., 1997] (e.g. The Chaîne des Puys, The Mont Dore, The Sioule, etc). It is coeval with regional doming [Michon and Merle, 2001].

The southern part of the Massif Central is characterized by the following structural features :

1) basins are not preferentially oriented or bounded by a N 135 °E oriented horst zone. Upper Eocene-01 igocene se dimentation is never as thick as in the north, with a maxi mum thickness of a few hundred meters;

2) sedimentation mainly occurred at sea level [Dangearcl, 1933;[START_REF] Rey R | Biostratigraphie des formations sédimentaires du Cantal[END_REF]Turland et al., 1994].

3) the Moho is flat-lying at a depth of about 28 km [Zeyen et al., 1997]. According to the average elevation at about 1000 m of the Variscan basement, the crust is about 29 km thick and crustal thinning is considered to be negli gible;

4) the lithospheric mantle is strongly thinned, up to 65 %, a long a direction which is nearly perpendicular to the orientation of crustal thinning recorcled in the north [Granet et al., 1995 a and b;Sobolev et al., 1996] [Fig. 3 in Michon andMerle, 2001]; 5) volcanism started in the upper Miocene more than 10 Ma after the end of the main phase of sedimentation [Michon and Merle, 2001]. lt is associated with huge vo lumes of magma and is undisputably the major volcanic phase in the Massif Central; 6) doming of the area is shown to be coeval with this major magmatic event [e.g. Le Grief, 1988;Goër de Herve and Etienne, 1991;Defive and Cantagrel, 1998].

When comparing the north and the south of the Massif Central, these data reveal a tectonic paradox. The Oligocene and lower Miocene history in the north of the Massif Central presents all the characteristics of passive rifting whereas the upper Miocene to present-day history in the sou th strongly suggests active rifting (fig. l ).

ln the north, crustal thinning, graben formation and sedimentation at sea level without volcanism during the Lower Oligocene, followed by scattered volcanism in a thinned area during Upper Oligocene and Lower Miocene, represent the classical evolution of a rift resulting from extensional stresses within the I ithosphere.

ln the soutl1, thinning of the lithospheric mantle associ ated with doming and volcanism in the Upper Miocene, to gether with the lack of crustal thinning, may be easily interpreted in terms of the first stage of active rifting due to the ascent of a mantle plume. This active rifting process 40 35 30 25

20

would have been inhibited be fore stretching of the crust, as asthenospheric rise associated with uplift and volcanism are the only tectonic events observed. The diachronism of these two events is emphasized by two clearly distinct ori entations of crustal thinning in the north and mantle lithospheric thinning in the south.

The negligible crustal thinning in the south explains well the Jack of volcanism following the Upper Eocene Oligocene sedimentation period. This merely indicates that the Oligocene passive rifting event recorded in the north has weakly affected the south, and that the lithospheric stretching was not strong enough at that time to induce par tial melting in the mantle. In the same way, the late second magmatic event is recorded in the north in a non-thinned crustal area, after an interruption of volcanism for several millions years. This suggests that the southern active rifting phase has propagated to the north in the very late Tertiary.

According to this interpretation, we face the following dilemna. lt could be considered that these two rifting events result f r om two different processes which are completely unrelated. However, two superimposed unrelated rifting events in the Te rtiary would be a surprising coincidence, ail the more because the same tectonic evolution is recorded far away from the Massif Central area in the Eger graben of the Czech Republic [Michon and Merle, 2001]. Another al ternative would be to consider that the whole Tertiary his tory of the Massif Central area and the Eger graben result f r om a single cause at the scale of western Europe. We dis cuss this hypothesis in the following section.

THE ALPS AS A SINGLE CAUSE

Several models have been proposed to explain the forma tion of the West European rift [e.g. Bergerat, 1985;[START_REF] Bles | Successive post-Variscan stress fields in the French Massif Central and its borders (Western Europcan plate) : comparison with geodyna mic data[END_REF][START_REF] Ziegler | European Cenozoic rif't system[END_REF]Lacombe et al., 1993]. Among them, the compression associated with the late stages of the Pyrenean orogeny has been advocated as a source for far-field stresses capable of reactivating the Permo-Carbon iferous fracture systems. In such an hypothesis, the forma tion of the Eger graben in the Bohemian Massif (fig. 2), far away f r om the Pyrenees, which reveals a tectonic evolution surprisingly similar to that of the Massif Central [Michon and Merle, 2001], is poorly understood. ln a search of a single cause at the scale of plate tecto nics which can explain not only the Oligocene rifting but also the following asthenospheric rise and related magmatism, it is clear that the Alps should be considered as the best candidate because this mountain chain is tempo rally and spatially connected witb the entire West European Rift system (fig. 2).

The West European Rift system displays a series of grabens or volcanic manifestations which roughly surround the Alpine chain. From east to west, these include the Eger graben, the Swabian Jura, the Rhine graben and the Massif Central (fig. 2). To the south of the Massif Central, tbe present-day structure of grabens and the volcanism are re lated to the opening of a marginal basin during the counter clockwise rotation of the Corso-Sardinia micro-plate [Channel! and Mareschal, 1989;Carminati et al., 1998;[START_REF] Seranne | The Gulf of Lion continental margin (NW Mcditer ranean) revisited by IBS: an overview[END_REF], an event whicb overprinted the earlier sedi mentary basins due to the West European rift stricto sensu. This rifting event results from a completely different plate kinematic history than that of the West European Rift sys tem and is best referred to as the North-West Mediterranean Rift [Seranne and Merle, 1999].

In the West European Rift system, incipient basins can be dated from the Priabonian (late Eocene) as this period corresponds to the beginning of the sedimentation in the Massif Central, the Rhine graben and the Eger graben The formation of the Frontal Penninic thrust is usually considered to occur at the beginning of the Oligocene [START_REF] Tricart | Le chevauchement de la zone briançonnaise au sud-est du Pelvoux : clé des rapports zone externe/zone internes dans les Alpes occidentale. -8111[END_REF]. According to this major event, a sedimenta tion of «Red Molasse» took place in the Dauphinois zone during the Oligocene [Kerckhove, 1980]. These molassic sediments result from the erosion of the high relief formed at that time in the internai domain.

Geochronological records are still a matter of contro versy in the Alps. Recent data seem to indicate that HP metamorphism in the most internai zones, with the excep tion of the Austro-Alpine Sezia zone, occurred in the range f r om 50 to 30 Ma [e.g. Monié and Philippot, 1989;Ti Iton et al., 1991;[START_REF] Bowtel | Sm-Nd isotopie evidence on the age of the eclogitisation in the Zcrrnau-Saas ophiolites[END_REF]Duchêne et al., 1997[START_REF] Rubatto | Jurassic formation and Eocene subduction of the Zcrmatt-Saas-Fcc ophiolites: impli cations for the geodynamics evolution of the Central and Wes tern Alps. -Co11trib[END_REF]. Despite the uncertainties linked to geochronological methods, these ages from the most inter nai zones, together with the migration of crustal deforma tion f r om east to west, fit quite well with the late Eocene/early Oligocene defonnatïon recorded at the bound ary between the internai and the externat domain (i.e. Fron tal Penninic thrust). From these data, it can be concluded that the major deformation in the Alps started in the middle Eocene in the most internai units and reached the boundary with the external domain in the end of the Priabonian or the beginning of the Rupelian.

The beginning of crustal thinning and graben formation recorded in the north of the Massif Central and the Rhine graben appears to be coeval with crustal thickening and ma jor crustal wedge overthrusting in the western Alps. What must be explained is the juxtaposition of two lithospheric domains, which exhibit at the same time extension and compression. The questions are the following: can the for mation of a mountain chain generate extension in the sur rounding lithosphere? Can the deformation 111 the surrounding lithosphere be two-phase and mimic tirst a passive rifting and then an active rifting?

THE ROLE OF THE LITHOSPHERIC ROOT

To answer these questions, the deformation associated with the collision of two lithospheric plates must be consid ered in its entirety. ln most mountain chains, the deforma tion in the crust and the mantle lithosphere is decoupled along a major shear zone located at the interface between the lower crust and the mantle. Such a process, which was fi r st proposed for the Himalayan chain [Mattauer, 1986], leads to the formation of both an upper stack of crustal wedges and a lower mantle lithospheric root. The depth of this lithospheric root is linked to the amount of shortening following the continental collision.

The formation of a mantle I ithospheric root has impor tant mechanical consequences. lt has been known for a long time that the mantle lithosphere is denser that the asthenosphere. This difference in densities is the key pa rameter which explains the sinking of oceanic lithosphere in subduction zones. lt is responsible for the downward body force usually referred to as the trencl1 pull [e.g. [START_REF] Turcotte | Geodynamics. Application of continuum physics 10 gcological problems[END_REF]. ln the same way, the mantle root in continental colli sion is denser than the surrounding asthenosphere. It fol lows that this slab exerts on the whole collisional system a downward gravitational body force. This downward body force is usually considered as sufficient to maintain the convergent belt in a state of compression even after conver gence has ceased [Fleitout and Froidevaux, 1982].

Numerical simulations have been conducted to study the effect and the deformation associated with this major vertical body force [e.g. [START_REF] Fleitout | Modélisation des contraintes tectoniques et des ins tabilités thermomécaniques dans la lithosphère[END_REF]Channell and Mareschal, 1989]. lt has been shown that a mantle lithospheric root is pulled down into the asthenosphere ge nerating a zone of crustal compression flanked by zones of crustal extension. These numerical models explain the com pression in the collision zone and contemporaneous exten sion in adjacent areas [Mc Kenzie, 1978;Fleitout and Froidevaux, 1982;Channell and Mareschal, 1989]. Signifi cant extensional deformation may be expected as the extensional strength of the lithosphere as a whole is conside rably less than its compressive strength [START_REF] Kusznir | The extensional strength of the conti nental lithosphere: its dcpcndance on geothermal gradient and crustal composition and thickncss[END_REF]. The extension should occurr in a way similar to that expected for passive rifting.

As an interesting result of the numerical simulation, the deformation in the adjacent lithosphere is dependant of the asymmetry of the lithospheric thickening [Channell and Mareschal, 1989]. Asymmetric lithospheric thickening leads to enhanced extension in the adjacent I ithosphere but on one tlank only of the collision zone, the other tlank beeing alrnost devoided of extensional stresses (fig. 3).

THE ASTHENOSPHERIC FLOW

Another process follows from the formation of the rnantle lithospheric root. lts consequence on adjacent lithosphere has not been fully studied yet but is clearly apparent in nu rnerical simulations. When sinking into the asthenosphere, the rnantle I ithospheric root replaces the asthenosphere and expells it away f r orn the root. This induces an asthenospheric tlow.

The general tlow pattern f r om numerical rnodelling clearly shows that downward movement of the rnantle lithospheric root is counterbalanced by upward motion of the asthenosphere below the adjacent I ithosphere [Channcll and Mareschal, 1989] (fig. 3). Considering that the litho sphere is sufficiently cool to behave rigidly, the tlow pat tern is restricted within the asthenosphere, which exhibits a tluid-like behavior on geologic time scales.

This process brings upward to the base of the I itho sphere sorne asthenospheric mantle, which is hotter than the temperature usually considered to define the lower boundary of the lithosphere, that is about l 300 °C. Jf such a process takes place over a sufficient time span, thinning of the lithospheric mantle by thermal erosion may occur, lead ing to volcanism and uplift by isostatic adjustment, in a way similar to active rifting (fig. 3).

Again, when the lithospheric thickening is asymrnetric, the resulting flow pattern is also strongly asyrnmetric and the ascent of the asthenosphere occurs on one tlank of the collision zone only, as shown by numerical simulations [Channel and Mareschal, 1989] (fig. 3).

THE MODEL

It has been shown that the formation of a mountain chain may have important mechanical consequences in the sur rounding lithosphere [START_REF] Fleitout | Modélisation des contraintes tectoniques et des ins tabilités thermomécaniques dans la lithosphère[END_REF]Channel and Mareschal, 1989). Two different processes may result f r om the creation of a lithospheric root: lithospheric extension and thermal erosion along the base of the lithosphere.

It is doubtful that these two processes are simultaneous. Once the process of the formation of the lithospheric root is on the way, a downward gravitational body force acts on the descending slab. This induces immediate extensional stresses in the adjacent lithosphere, which may eventually overcome its strength and produce whole-lithosphere fail ure.

By contrast, thermal erosion of the adjacent lithosphere should be a late process for two different reasons. Firstly, the lithospheric root must reach a certain depth to induce significant asthenospheric tlow. As shown from numerical modelling [Channell and Mareschal, 1989], the amplitude of this flow is directly proportional to the volume of the asthenosphere replaced by mantle I ithosphere. Secondly, as a thermal effect, it needs time for the hot asthenosphere brought up to the base of the lithosphere to induce thermal erosion. As a result, the passive rifting recorded in the adja cent lithosphere should predate the active rifting due to the asthenospheric flow.

We propose to solve the tectonic paradox described in the Massif Central area by taking into account the mechani cal decoupling between the crust and the mantle during the formation of the Alpine chain. In the Alps, such a process certainly occurred as demonstrated by geophysical data which reveal a deep mantle lithospheric root over 220 km thick [ e.g. Panza and Muel Ier, 1979;[START_REF] Spakman | The upper mantle structure in the central Euro pean-Mediterrancan region[END_REF]Mueller and Panza, 1986;Babuska et al., 1990). This corres ponds to the Alpine "verschluckungszone" defined by Laubscher in 1971. The root is in marked contrast with the LAB depth of about 90-100 km to the west of the Sillon Houiller, a stable domain since the Hercynian orogeny which was not-affected by the rifting event [START_REF] Lucazeau | -lnterpretation of heat now data in the French Massif central[END_REF]Sobolev et al., 1996].

As expected in the process under consideration, the rift ing event in the Massif Central resulting from the formation of the Alps is two-fold. It initially resembles passive rifting and is followed a few millions years later by a second stage which mimics the first stage of an active rifting.

According to geochronological data on high pressure terranes in the Alps, crustal thickening started in the middle Eocene. Tt appears that the I ithospheric root cou Id have been deep enough to generate extension at the time when the Frontal Penninic thrust developped (i.e. end of Priabonian or beginning of Rupelian). Then, the time avail able to create a significant lithospheric root capable of gen erating extension in the adjacent lithosphere would be r about 20 Ma, fom the middle Eocene to the end of the Eocene. The resulting effect in the Massif Central area is the formation of grabens at sea level which lack volcanism (fig. 4a). Stretching of the lithosphere took place over 10 Ma until the end of the Oligocene. From the upper Oligocene, decompression partial melting occurred in the zones of greatest lithospheric thinning (i.e. below the Limagne graben). This scattered volcanism lasted about 15 Ma up to the Upper Miocene. We consider that this first event of passive rifting ends in the Upper Miocene with the end of volcanic activity.

The flow pattern in the asthenosphere due to the cre ation of the lithospheric root in the Alps induces thermal erosion along the base of the adjacent lithosphere in the south of the Massif Central about 15 Ma after the end of the sedimentation period. This thermal erosion was accompa nied by intense volcanism and uplift of the wbole area (fig. 4b). This second event of active rifting propagated to the north of the Massif Central generating a renewal of vol canism after an interruption of about 6 Ma. As expected, this second phase of volcanism in the north is unrelated to previous zones of crustal stretching and mainly occurred in zones of normal crustal thickness (fig. 4b).

The question arises as to why the active rifting event started earlier in the south than in the north of the Massif Central and was significantly less developed in the north. Furthermore, to generate the second stage of magmatism, it is necessary to produce a thermal anomaly in the upper mantle of about 200 °C [Granet el al., 1995b;Sobolev el al., 1996]. This requires that the lithospheric root triggers an upwelling from a deep thermal boundary layer in the upper mantle. We feel that the answer to these questions needs further studies of the tlow pattern associated with the sink ing of a lithospheric root. Such an understanding of the deep processes involved during the creation of a lithospheric root might be achieved by the means of numer ical and analogue modelling.

It has been stressed in a companion paper [Michon and Merle, 2001] that the sedimentary and magmatic evolution of the Eger graben is surprinsingly similar to that recorded in the Massif Central. This is hardly a coïncidence. When looking at a map of the depth of the lithospheric root in the Alps (fig. 5), it appears that the Alpine root is strongly non linear. This may explain the three major segments of the West European rift, which are linked by large scale trans form zones (i.e. The Rhin-Saône and Franconie transform zones). Likewise, reactivation of Variscan faults may ex plain the sligbt deviation of these grabens from perfect par allelism around the Alpine belt.

CONCLUDING REMARKS

From our study of the Massif Central area, we conclude that the West European Rift system may result from the co eval formation of the deep lithospheric root in the Alpine chain. Such a process would induce a two-stage rifting event in the adjacent lithosphere, as shown in the Massif Central area and the Eger graben. The first stage resembles passive rifting whereas the second stage mimics the begin ning of an active rifting. This bypothesis explains most data gathered from the north and the sou th of the Massif Central area. Further studies on the deep structure of the Rhine graben and the Eger graben should show if the similar tem poral evolution of graben formation, sedimentation and vol canism recorded in these areas can be attributed to the same overall tectonic process.

Ack11owledg111e11ts. -The authors want to thank Olivier Lacombe, Jean-Christophe Maurin and Marjorie Wilson for their critical reviews.

  FIG.1. -Summary of main gcological data that allow us LO propose a two-fold evolution of the rifting process during the Tcniary. Scdi111en1a1ion al sca lcvel followed by scauercd volcanism lits with a passive rifting evenl. whereas the major phase ofvolcanism associated with uplift fits with an acti.ie rif ting cvcnt (explanation in the 1ex1). FIG. 1. -Rés11111é des pri11cipa/es do1111ées géologiques per111et1a111 de proposer 1111e double él'0/111io11 du processus de rifti11g au cours du Tertiaire. Au 11ord. fa sédi111e11tatio11 au 11irea11 de fa 111e1: suivie d'1111 vofca11is111e dispersé s •accorde (ll•ec 1111 modèle de rift pass(f En rewmche, au sud, fa phase ma jeure de volca11is111e associée à 1111e s11rrec1ion s'accorde (11•ec 1111e évo/111io11 de 1ype rift acti f (c.f expfica1io11 dans le texte).

D

  FIG. 2. -Simplilied geological map showing the close spatial rclationship be1wcen the Alps and the West Europcan Rift system. Mosl structural fcaturcs of the rifts, including the Massif Central arca, 1he Rhinegraben, the Swabian Jura and the Eger Graben arc concentric around the Alpine chain. FIG. 2. -Carte géologique simplifiée 1110111ra11t / 'élroite relai ion spaliale entre les Alpes et le rift Ouest-européen. les principaux segments du rifl (le Massif central. le graben du Rhin. le Jura souabe et le graben de /'Eger) sont disposés conce11triqueme111 autour de la chaîne alpine.

  . The major episode of graben formation occurred during the Oligocene. Deposition lasted until the end of the Oligocene. In a few areas, some residual basins of small ex tent reveal ongoing sedimentation during the Aquitanian [Gerbe et al., 1 998; Hugueney et al., l 999]. The beginning of the major phase of deformation in the r Alps can be dated f om stratigraphie records. During the Eocene, a large paleogeograpbical domain including the Briançonnais and the sub-Briançonnais zones of the Alps corresponded to a zone of widespread deep marine sedi mentation [e.g. Kerchove, 1980]. This so-called Nummulit ic sedimentation ended in the late Eocene with olistostrome formations, immediately followed by nappe emplacement. Most olistostromes in the sub-Briançonnais and Briançonnais zones can be dated from the Priabonian [e.g. Kerckhove,[START_REF] Kerckjiove C | La zone du nysch dans les nappes de I' Embru nais-Ubaye[END_REF] Bravard et al., 1981; Kerckhove and Pa iris, 1986; Barféty et al., 1992].

  FIG.3. -Schematic rcprcsentation of the rolc of the lithospheric root in triggcring rifting in the adjacent lithosphcrc[af'tcr Channel and Mareschal. 1989]. The downward gravitational force of the dense root induces exten sion ( 1) in the adjacent lithosphcre and is responsible for a deformation event which resembles passive rifiing. The mantlc 11ow duc 10 the sinking of the root brings hot asthenosphere 10 the base of the adjacent lithos pherc. This induces thermal crosion (2) leading tO a defonnation cvent which rcsembles active rifiing (2). FIG.3. -Représe111atio11 schématique du rôle de la raci11e li1hosphérique dans la création d'un épisode de rifting dans la lithosphère adjacenle[d'après Chcnmel and Mareschal. 1989]. La.force grm•itaire de la racine li1hosphériq11e de11se i11d11it de I •exlension ( 1) dans la lithosphère adja cente et est responsable de/ 'épisode de déformatio11 de t) 1 Je rift passif Le flux as1hé11osphériq11e d1i à l'enfoncement de la racine lithosphériq11e en traîne d11 malérie/ mantellique chaud à la base de la li1hosp/1ère adja ceme. Ceci induit une érosion thermique (2) et 1111 épisode de d4formation de type rift actif (2).

  FIG.4. -The two-fold evolu1ion of the rifting event in the Massif Central area as explained by the deep lithospherie root or 1he Alps. A) The forma tion of the lithospherie root in Eocene-Oligocene 1ime has indueed exten sion in the Massif Central, which started by sedimentation ai sea level followcd by scauercd volcanism in the late Oligoeene and lower Miocenc. At 1ha1 1i111e, the root is supposed to be deeper and steeper than nowadays as it is coeval with subduction of crustal slices (UHP and HP metamor phism). which were cxhumed laler and outcrop now at the surface. Ther mal erosion of this root may also be assumed since the Oligocene 8) The formation of the lithospherie root has induced a mantle now, which resul-1ed in thermal erosion at the base of the lithosphere trigerring volcanism and uplift of the whole area from the Upper Miocene 10 the present-day (further explanaiion in 1he text). FIG.4.-Double évo/111ion de l'épisode de rifling dans le Massif cen1ral liée à la créa1ion de la profonde racine li1hosphériq11e des Alpes. A) la .forma1ion de la racine li1hosphériq11e à I Eo-0/igocène a indui1 de I •exlen sion dans le Massi f ce111ra/, qui s 'es1 d "abord 111anifes1ée par une sédimen lalion au niveau de la me,; puis par une phase de volcanisme dispersé à /'Oligocène supérieur el au Miocène inférie111: A celle époque. la racine élail probablemenl plus pro. fonde qu 'ac1uellemen1 el é1ai1 associée à une subd11c1ion cr11s1ale, comme le 111onlre le mélamorphisme de u/Jra-ha111e pression dans les 1111i1és crus/ales les plus i111emes de la chaîne. 8) la for malion de la racine lilhosphérique a ind11i1 1111 flux as1hénosphériq11e sous le Massi f cenlral qui a pour ejfel une érosion thermique de la li1hosphère e1 le développement de la phase volcanique majeure contemporaine d'une s11rrec1ion du Miocène supérieur à l 'ac111el (cf explications dans le 1exte).

  FIG. 5. -Depth to the thermal lithosphere-asthenosphere boundary beneath the Alps showing the localisation of the deep lithospheric root with respect t> the three main segments of the West European Rift (depth of the lithosphere-asthenosphere boundary after Babuska et al. [ 1990)). FIG. 5. -Carte de la profondeur du LAB dans les Alpes montrant la position de la profonde racine lithosphérique par rapport aux trois segments princi paux du Rift Ouest-européen (profondeur de la racine /itltosphérique d'après Babuska et al. [1990}).