Résumé
Le Plateau des Dômes surplombe le graben de la Limagne de plusieurs centaines de mètres et supporte une chaîne volcanique quaternaire orientée nord-sud, la Chaîne des Puys (CdP), constituée d’environ 80 édifices répartis linéairement sur une trentaine de kilomètres. Ce plateau est traversé par un réseau de failles crustales où se reconnaissent les grandes familles varisques, représentées dans le Massif central par des failles emblématiques telles que le Sillon Houiller, la Faille des Cévennes ou la Faille de Villefort. Depuis son émersion définitive vers la fin du Crétacé, le Plateau des Dômes a été principalement affecté par deux événements tectoniques : la formation du Rift Ouest Européen (ROE), avec son volcanisme associé, et une surrection tardive qui se poursuit actuellement.
Par une synthèse des données existantes et une réappréciation des cartes géologiques et de l’analyse de la topographie (analyse des MNT : Modèles Numériques de Terrain), cet article montre que la structuration tertiaire et quaternaire du Plateau des Dômes s’est entièrement effectuée par la réactivation de quelques failles varisques majeures qui ont 1) permis le basculement du socle lors de la formation des grabens, 2) assuré sa surrection différentielle par rapport à d’autres secteurs limitrophes et 3) contrôlé l’ascension des magmas vers la surface. Une évolution structurale du plateau est proposée résumant les étapes majeures depuis l’Eocène.
Mots-clés : Plateau des Dômes, Chaîne des Puys, Graben de Limagne, Rift Ouest Européen, Volcanisme, Soulèvement.
Extended abstract
The Plateau des Dômes overhangs the Limagne graben by several hundred metres and represents the basement of a Quaternary volcanic chain oriented north-south, the Chaîne des Puys (CdP), consisting of about 80 volcanic edifices spread linearly over thirty kilometers. Since its final emersion in the late Cretaceous, the Plateau des Dôme has been mainly affected by two tectonic events: the formation of the West European Rift (ROE), with its associated volcanism, and a late surrection that is still ongoing. To unravel the Tertiary and Quaternary geological history of the Plateau, a structural approach was defined. At the scale of the Massif central, four families of Variscan crustal faults can be described : 1. the Sillon Houiller family oriented N20E (dir.SH), 2. the Cévennes fault family oriented NE-SW (dir.C), 3. the Villefort fault family oriented N160-170E (dir.V) and 4. the Armorican family, which oscillates between N110E and N140E (dir.A) due to the orientation of the North and South Armorican Shear Zone. This network of crustal faults can be observed at any scale.
It is shown that the Plateau des Dômes is crossed by such a network of variscan faults, which was reactivated during the Tertiary and Quaternary tectonic events occurring in the studied zone. No new faults were created in response to the different stress patterns that prevailed in the Tertiary and Quaternary periods.
The main conclusions of this study can be summarized as follows :
1. The Variscan basement of the Plateau des Dômes was tilted westwards at the end of the Eocene by the formation of the Olby half-graben controlled by the reactivation of the Pontgibaud fault [FdP (dir.SH)]. It was limited to the east by the Limagne fault (FdL), which reactivated two Variscan directions (dir.SH and dir.V), the only two compatible with the east-west extension. At that time, the future plateau was emerged, but at an altitude close to sea level.
2. The cessation of the FdP and the continued activity of the FdL in the Oligocene caused the eastern edge of the future plateau to be dissected and lowered down by the activation of FdL's secondary (dir.SH) faults. The future plateau remained close to sea level. The main outlines of its present topography were acquired at the end of the sedimentation period.
3. The plateau was uplifted shortly before the emission of the last Montdorian lava flows, dated at 3.5/3 Ma. To the east, the FdL was not reactivated during the regional uplift and the Plateau des Dômes appears as such due to erosion of the Limagne weak sediments, which cleared the fault scarp. The Plateau des Dômes is part of a larger ensemble (i.e. a larger plateau) bounded by the Morge dir.C fault to the north and the Sillon Houiller to the west. Through the coeval reactivation of these two faults, this large north-western quadrant has undergone differential uplift bringing it to a high position relative to areas further north and west. To the south, the Plateau des Dômes is dominated by the large Monts Dore stratovolcano. In this sector, a series of parallel dir.A faults, oriented N110E, as well as the reactivation of the Nébouzat fault, have ensured the differential surrection of the basement in a way similar to successive steps of a staircase.
4. The volcanoes of the CdP are aligned along a main fault (the Beauregard fault), which is the backbone of the chain, and along several secondary faults, dir.SH to the NE and dir.A to the SE, all connected to the main fault. The lava flows follow paleo valleys that were dug along the weak zones constituted by the Variscan faults. The north of the plateau, dominated by dir.SH faults, exhibits lava flows that are generally oriented NE-SW, in sharp contrast to the south, dominated by dir.A faults, where the flows are oriented NW-SE.
Keywords: Plateau des Dômes, Chaîne des Puys, Limagne graben, West European Rift, Uplift.
Dernière mise à jour le 17.03.2023