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Les Brèches d’Urdach, témoins de l’exhumation du manteau pyrénéen dans un escarpement de faille vraconnien-cénomanien inférieur Elie-Jean Debroas (1) (zone nord-pyrénéenne, Pyrénées- Joseph Canérot (2) Atlantiques, France) Michel Bilotte (3) The Urdach Breccias evidence the exhumation of the Pyrenean mantle in a Vraconnian to Lower Cenomanian fault scarp (North Pyrenean zone, Pyrénées-Atlantiques, France) Géologie de la France, n° 2, 2010, p. 53-64, 6 fig. Mots-clés : Brèches, Flysch noir, Lherzolite, Vraconnien-Cénomanien, Urdach, Pyrénées-Atlantiques. Keywords: Breccias, Flysch noir, Lherzolite, Vraconnian-Cenomanian, Urdach, Pyrénées-Atlantiques. Résumé précisent également l’agencement des brèches dans un prisme épais d’un millier de mètres. Vers l’ouest, ce prisme Dans la zone nord-pyrénéenne, les « Brèches de s’interstratifie dans le Flysch noir vraconnien-cénomanien Lherz » et les « Brèches d’Urdach » sont depuis longtemps inférieur du fossé subsident de Tardets alors qu’à l’Est une connues pour les éléments de lherzolite qu’elles zone de failles synsédimentaires normales le sépare de la renferment. La présence de ces éléments a récemment tête surélevée d’un bloc du Mail Arrouy basculé de l’Aptien conduit à envisager que ces brèches témoignaient de supérieur au Cénomanien inférieur. Il apparaît ainsi que l’exhumation et de la resédimentation albo-cénomanienne contrairement aux interprétations récentes, l’exhumation des péridotites mantelliques sur le plancher sous-marin sous-marine des lherzolites d’Urdach s’est produite des bassins d’Aulus (Lherz, Ariège) et de Mauléon localement au niveau des escarpements vraconniens à (Urdach, Pyrénées Atlantiques). cénomaniens inférieurs de cette zone de failles. Dans ces Dans le bassin d’Aulus, un nouvel examen des Brèches escarpements affleurait également une croûte continentale de Lherz nous a permis d’infirmer cette interprétation et amincie mais encore d’épaisseur non négligeable puisque donc le modèle d’exhumation du manteau qu’elle son érosion a alimenté la grande majorité des 1 000 m des soutenait. Il nous a amené à distinguer deux types de Brèches d’Urdach. brèches. Les « Brèches de Lherz » proprement dites, les Extended abstract plus développées, sont dépourvues de structures sédimentaires et conformément à l’interprétation classique, On passive margins, mantle denudation is generally elles sont bien cataclastiques et pénécontemporaines du closely related to detachment tectonics and extreme crustal métamorphisme pyrénéen mésocrétacé. Les « Brèches thinning (Whitmarch et al., 2001; Manatschal, 2004). These des Coumettes » très localisées ont été nouvellement processes generate crust and mantle mixed deposits under définies pour désigner le remplissage continental de puits deep marine conditions (turbidites, debrites and olistolites). karstiques dont les stratifications encore proches de This dynamical model has recently been proposed to l’horizontale prouvent qu’elles sont postérieures à explain the supposed exhumation of the Pyrenean l’inversion structurale fini-crétacée du bassin d’Aulus. lherzolites at the base of the Albian flysch infillings, the same in the Aulus Basin (Lagabrielle and Bodinier, 2008) À l’inverse, dans le bassin de Mauléon, les résultats de as in the Mauléon one (Jammes, 2009; Jammes et al., la nouvelle cartographie des Brèches d’Urdach, exposés 2009, 2010 ; Lagabrielle et al., 2010). dans la présente note, vérifient que ces brèches sont bien interstratifiées dans le Flysch noir et donc sédimentaires, In the Aulus Basin (fig. 1), this model of submarine sous-marines et albo-cénomaniennes. Mais, ces résultats mantle exhumation and erosion failed. A recent work (1) Université de Toulouse, UPS (SVT-OMP), LMTG, 14 Avenue Edouard Belin, F-31400, France, [email protected] (2) Université de Toulouse, UPS (SVT-OMP), LMTG. Adresse personnelle : 37, avenue de Cousse, F-31750 Escalquens (3) Université de Toulouse, UPS (SVT-OMP), LMTG, LECP, 39 Allées Jules Guesde, F-31400, France * Manuscrit présenté le 25 juin 2010, accepté le 19 novembre 2010 GÉOLOGIE DE LA FRANCE, N° 2, 2010 53 LES BRÈCHES D’URDACH (PYRÉNÉES-ATLANTIQUES, FRANCE) shows that the breccias interpreted as Albian flysch they interfinger with the vertical, N-S oriented, Vraconnian materials reworking the lherzolite (Lagabrielle and Bodinier, to Lower Cenomanian Flysch noir (fig. 4). They really 2008) really involve two kinds of breccias: 1 - the classical correspond to thick (near 1 km) prismatic accumulation “Lherz Breccias” or cataclastites related to the Cenomanian involving different-originated (abundant Paleozoic and (95 Ma) intrusion of mantle slices within the Jurassic scarse Mesozoic) and sized (from grain to olistoliths; fig. 3) marbles, heated up to 500-600 °C (Vielzeuf and elements related to a northern or northeastern fault zone. Kornprobst, 1984; Ternet et al., 1997; Colchen et al., 1997; Their contact with the lherzolites being mechanical, they do Fabries et al., 1998); 2 - the newly described “Coumettes not represent the unconformable cover of these Breccias” which really correspond to horizontally bedded neightbouring, mantle originated rocks. The Urdach continental karstic infillings following the Late Cretaceous tectonic inversion of the Basin (Debroas et al., 2010). Breccias evidence one of the numerous fault-scarp deposits well known in the Flysch noir formation, In the Mauléon Basin (fig. 1), on the contrary, mantle throughout the Pyrenees (Souquet et al., 1985; Bilotte et exhumation and erosion are obvious in the Urdach area al., 1987; Debroas, 1982, 1990). where a serpentinized lherzolite body is really reworked in the famous “Urdach Breccias” (Schoeffler et al., 1964; In our interpretation, the Mid-Cretaceous Urdach Casteras, 1970). These materials also involve Paleozoic Breccias wedge must be related to an active, syn- (gneisses, schists including andalousite, sillimanite, sedimentary, transverse fault zone which separated the muscovite and biotite, quartzites, granites and pegmatites) eastern Mail Arrouy rising block, from the western Tardets, and Mesozoic (Jurassic and Early Cretaceous limestones subsiding one (fig. 6). On the first unit, the thickness (more and dolostones) elements. But in this case different than 3 000 m) of the Mesozoic sedimentary cover close to interpretations of the modes of exhumation of the lherzolite the Ossau Valley, near Arudy, and the westward strong body have been distinguished. stratigraphic and erosion wedges (from Upper Aptian up to The competitive interpretations. The Urdach Middle Cenomanian) indicate northeastward tilting Breccias have been successively considered as a (Canérot and Delavaux, 1986; Canérot, 2008). Then, close mylonitized tectonic sheet (Viennot, 1927) or as a to the Aspe Valley, at the top of the Mail Arrouy block, the “wildflysch” located at the base of the Cenomanian flysch pinching out process leads to a contact between the unconformably covering a ridge made up of lherzolite and Cenomano-Turonian carbonate flysch and the Mid- Paleozoic formations (Tisin, 1963; Schoeffler et al., 1964; Jurassic limestones and dolostones. This geometry points Deloffre, 1965; Casteras, 1970). Then, following out really the proximity of the Urdach fault scarp zone petrographic works related to the lherzolite body which is responsible for mantle denudation and related (Monchoux, 1970; Gaudichet, 1974), different authors ophicalcites between two Paleozoic and Mesozoic crust- (Roux, 1983; Souquet et al., 1985) observed a transition bearing blocks. between the breccias and the lateral Flysch noir, Vraconnian to Lower Cenomanian in age. They interpreted After a period of active sedimentation, the fault structure the breccias as a prismatic, 200 m thick fault scarp seems to be inherited or reactivated towards the end of the accumulation, very similar to other examples known in Turonian as it corresponds to the eastern edge of the different Mid-Cretaceous Pyrenean Flysch noir basins. extended (90 km long and up to 60 m thick) Col d’Osquich In recent times, the Urdach Breccias have been first megaturbidite (Debroas et al., 1983; Mulder et al., 2009). considered as part of a “Flysch à blocs” involved in an Then, during the Pyrenean tectonic period, N-S Albian, north-vergent tectonic sheet (Duée et al., 1984; transpression converted the Urdach lherzolite into an Fortané et al., 1986). But this hypothesis has no field reality uprooted slice mechanically separating the western Urdach (Canérot and Delavaux, 1986; Canérot and Debroas, Breccias from the eastern Cenomano-Turonian carbonate 1988). More recently, Jammes (2009) and Jammes et al. flysch of the Bugangue Forest. (2009, 2010) consider that the Urdach lherzolite body and its unconformable cover (Urdach Breccias) overlap the So, our new interpretation of the Urdach lherzolite- vertical flysch mechanically. The presence of ophicalcite bearing structure does not fit in the recent hypothesis of deposits at the top of the lherzolite indicates syn- detachment tectonics and extreme thinning leading to local sedimentary mantle denudation through detachment (Jammes, 2009; Jammes et al., 2009, 2010) or regional process, at the bottom of the Albian flysch basin. In another (Lagabrielle et al., 2010) crust disparition during the hypothesis (Lagabrielle et al., 2010), the Urdach Breccias formation of the Albo-Cenomanian Mauléon flysch basin. accumulate at the base of a north dipping normal fault The results exposed in the present work evidence the crossing the flysch which overlaps the peridotites.

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