Les Brèches d’Urdach, témoins de l’exhumation du manteau pyrénéen dans un escarpement de faille vraconnien-cénomanien inférieur (zone nord-pyrénéenne, Pyrénées-Atlantiques, France)

Abstract

On passive margins, mantle denudation is generally closely related to detachment tectonics and extreme crustal thinning (Whitmarch et al., 2001; Manatschal, 2004). These processes generate crust and mantle mixed deposits under deep marine conditions (turbidites, debrites and olistolites). This dynamical model has recently been proposed to explain the supposed exhumation of the Pyrenean lherzolites at the base of the Albian flysch infillings, the same in the Aulus Basin (Lagabrielle and Bodinier, 2008) as in the Mauléon one (Jammes, 2009; Jammes et al., 2009, 2010 ; Lagabrielle et al., 2010). In the Aulus Basin (fig. 1), this model of submarine mantle exhumation and erosion failed. A recent work shows that the breccias interpreted as Albian flysch materials reworking the lherzolite (Lagabrielle and Bodinier, 2008) really involve two kinds of breccias: 1 - the classical “Lherz Breccias” or cataclastites related to the Cenomanian (95?Ma) intrusion of mantle slices within the Jurassic marbles, heated up to 500-600?°C (Vielzeuf and Kornprobst, 1984; Ternet et al., 1997; Colchen et al., 1997; Fabries et al., 1998); 2 - the newly described “Coumettes Breccias” which really correspond to horizontally bedded continental karstic infillings following the Late Cretaceous tectonic inversion of the Basin (Debroas et al., 2010). In the Mauléon Basin (fig. 1), on the contrary, mantle exhumation and erosion are obvious in the Urdach area where a serpentinized lherzolite body is really reworked in the famous “Urdach Breccias” (Schoeffler et al., 1964; Casteras, 1970). These materials also involve Paleozoic (gneisses, schists including andalousite, sillimanite, muscovite and biotite, quartzites, granites and pegmatites) and Mesozoic (Jurassic and Early Cretaceous limestones and dolostones) elements. But in this case different interpretations of the modes of exhumation of the lherzolite body have been distinguished. The competitive interpretations. The Urdach Breccias have been successively considered as a mylonitized tectonic sheet (Viennot, 1927) or as a “wildflysch” located at the base of the Cenomanian flysch unconformably covering a ridge made up of lherzolite and Paleozoic formations (Tisin, 1963; Schoeffler et al., 1964; Deloffre, 1965; Casteras, 1970). Then, following petrographic works related to the lherzolite body (Monchoux, 1970; Gaudichet, 1974), different authors (Roux, 1983; Souquet et al., 1985) observed a transition between the breccias and the lateral Flysch noir, Vraconnian to Lower Cenomanian in age. They interpreted the breccias as a prismatic, 200 m thick fault scarp accumulation, very similar to other examples known in different Mid-Cretaceous Pyrenean Flysch noir basins. In recent times, the Urdach Breccias have been first considered as part of a “Flysch à blocs” involved in an Albian, north-vergent tectonic sheet (Duée et al., 1984; Fortané et al., 1986). But this hypothesis has no field reality (Canérot and Delavaux, 1986; Canérot and Debroas, 1988). More recently, Jammes (2009) and Jammes et al. (2009, 2010) consider that the Urdach lherzolite body and its unconformable cover (Urdach Breccias) overlap the vertical flysch mechanically. The presence of ophicalcite deposits at the top of the lherzolite indicates syn-sedimentary mantle denudation through detachment process, at the bottom of the Albian flysch basin. In another hypothesis (Lagabrielle et al., 2010), the Urdach Breccias accumulate at the base of a north dipping normal fault crossing the flysch which overlaps the peridotites. This conclusion of local mantle exhumation related to extreme thinning of the Paleozoic crust is newly extended to the whole Chaînons Bearnais zone. New observations and proposed interpretation. A recent mapping of the Urdach area (fig. 2), allows us to conclude that the Urdach Breccias are autochtonous as they interfinger with the vertical, N-S oriented, Vraconnian to Lower Cenomanian Flysch noir (fig. 4). They really correspond to thick (near 1 km) prismatic accumulation involving different-originated (abundant Paleozoic and scarse Mesozoic) and sized (from grain to olistoliths; fig. 3) elements related to a northern or northeastern fault zone. Their contact with the lherzolites being mechanical, they do not represent the unconformable cover of these neightbouring, mantle originated rocks. The Urdach Breccias evidence one of the numerous fault-scarp deposits well known in the Flysch noir formation, throughout the Pyrenees (Souquet et al., 1985; Bilotte et al., 1987; Debroas, 1982, 1990). In our interpretation, the Mid-Cretaceous Urdach Breccias wedge must be related to an active, syn-sedimentary, transverse fault zone which separated the eastern Mail Arrouy rising block, from the western Tardets, subsiding one (fig. 6). On the first unit, the thickness (more than 3?000?m) of the Mesozoic sedimentary cover close to the Ossau Valley, near Arudy, and the westward strong stratigraphic and erosion wedges (from Upper Aptian up to Middle Cenomanian) indicate northeastward tilting (Canérot and Delavaux, 1986; Canérot, 2008). Then, close to the Aspe Valley, at the top of the Mail Arrouy block, the pinching out process leads to a contact between the Cenomano-Turonian carbonate flysch and the Mid-Jurassic limestones and dolostones. This geometry points out really the proximity of the Urdach fault scarp zone which is responsible for mantle denudation and related ophicalcites between two Paleozoic and Mesozoic crust-bearing blocks. After a period of active sedimentation, the fault structure seems to be inherited or reactivated towards the end of the Turonian as it corresponds to the eastern edge of the extended (90 km long and up to 60 m thick) Col d’Osquich megaturbidite (Debroas et al., 1983; Mulder et al., 2009). Then, during the Pyrenean tectonic period, N-S transpression converted the Urdach lherzolite into an uprooted slice mechanically separating the western Urdach Breccias from the eastern Cenomano-Turonian carbonate flysch of the Bugangue Forest. So, our new interpretation of the Urdach lherzolite-bearing structure does not fit in the recent hypothesis of detachment tectonics and extreme thinning leading to local (Jammes, 2009; Jammes et al., 2009, 2010) or regional (Lagabrielle et al., 2010) crust disparition during the formation of the Albo-Cenomanian Mauléon flysch basin. The results exposed in the present work evidence the presence, during this period, of a more or less thick crust including lower paleozoic materials, above the lherzolite bodies, the same in the Urdach example as in the Pic Saraillé one. They also testify the importance of the Pyrenean compression phase and the following erosion step in the final exhumation of the mantle-originated peridotites.