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Les Pyrénées alpines sud-orientales (France, Espagne) – essai de synthèse

Abstract : In the eastern Pyrenees, the Axial Zone (AZ) shows a complex Alpine structure partly obscured by the Neogene tectonics, more particularly in the vallespir area. Five south-verging, in sequence thrusts are identified in the Hercynian basement.From N to S and top to bottom, there are : three major thrusts (the well established Aspres thrust, the more recently recognized Canigou and Vallespir thrusts) and two minor thrusts (Albères and Roc de France). These contacts delimit six tectonic units, an upper (Aspres), two middle (Canigou and Vallespir) and three lower (Albères, Roc de France and St-Laurent-de-Cerdans). A similar structure is reported further southwest in the Ribes de Freser area. The rhombic pattern drawn by the Alpine E-W, NW-SE and NE-SW directed thrusts in the eastern Axial Zone largely results from pre-alpine heritage (inverted late-Hercynian and Cretaceous normal faults, late-Hercynian mylonitic zones reactivated during the Alpine events). A S-dipping monocline affecting this thrust system limits the AZ and the South-Pyrenean Zone (SPZ), the AZ being raised with respect to the Alpine cover rocks of the SPZ. This monocline corresponds to the short limb of a large ramp anticline linked to the emplacement of the lowermost unit (St-Laurent-de-Cerdans). The SPZ is made up of the Cadí units overlain by allochthonous units (from W to E : Pedraforca, Coustouges, Bac Grillera, Biure and Empordà). The Cadí units (SPZ) together with the associated middle basement units (AZ) form the Canigou nappe, a major unit below which are visible the lower basement units (AZ) in the Albères tectonic window (and in the small Freser window). The South-Pyrenean allochthons moved from N to S and their origin is likely to be found on the basement of the Canigou nappe (AZ). Except the Coustouges, lower Bac Grillera and (?) Biure small allochtons that may be rooted in the Aspres thrust (AZ), it is not possible to root the other allochtons (especially the large Pedraforca unit) in the AZ due to lack of suitable Alpine thrusts. Therefore, we are obliged to root them farther north, i.e. to the north of the North Pyrenean Fault (NPF). It is the same for the large South-Pyrenean Central Unit in the Central Pyrenees. Athough it invalidates the model commonly accepted since 25 years for the interpretation of the "ECORS-Pyrenees" seismic profile (with a large antiformal stack in the AZ), we show that this new interpretation is reasonable on two conditions: (i) an early (Lower Eocene s.l.), major, south-verging tangential tectonic regime should have existed in the central-eastern Pyrenees, which carried the European part of the chain (now the NPZ) together with the Cretaceous rift onto the South-Iberian margin (i.e. the AZ); this is clearly visible in the westernmost Pyrenees (Béarn, Aragon); (ii) this thrust system should have been modified by the late (Upper Eocene s.l.) formation and uplift of the AZ, due to the combined effects of the bordering monocline to the South and the NPF to the North. If this were the case, the present-day AZ forms a tectonic window under a system of upper units (NPZ and southern allochtons).
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Bernard Laumonier. Les Pyrénées alpines sud-orientales (France, Espagne) – essai de synthèse. Revue de Géologie pyrénéenne, 2015. ⟨hal-01302546⟩



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