Constructions et destructions du stratovolcan du Cantal
Authors: Nehlig Pierre , Leyrit Herve , Dardon Arnaud , Freour Gwenael , de Goer de Herve Alain , Huguet David , Thieblemont Denis 
Published: June 2005

The Cantal (France) stratovolcano, which is 70 km in diameter and extends 2500 km 2 , is the largest perialpine stratovolcano. Due to its size and the abundance of breccia, it has never before been the subject of a comprehensive synthesis, despite being considered in more than 30 doctoral theses and over 200 scientific papers, memoirs and reports. An intensive research project, which integrates a synthesis of existing published and unpublished data and new geological, geochemical, geophysical and geochronological data, along with 1:25,000-scale mapping of the central part of the stratovolcano, has led to the production of the first 1:50,000-scale map of the central part of the volcano and a 1:100,000-scale map of the entire volcano. The present mapping and analytical work has led to an entirely new conceptual view of the geological history of the stratovolcano and to a reinterpretation of the previously defined stratigraphic units and their volcanological significance. This paper presents a brief geological history, focussing on the abundant primary and secondary breccia (lahar and debris-avalanche deposits) that make up most of the volcano, and reviews a number of dogmas and uncertainties concerning the volcano and its evolution. The stratovolcano was emplaced between 13 and 2 Ma on an uplifted Hercynian basement associated with Oligocene sedimentary basins. The overall geometry of the Cantal stratovolcano is rather simple, composed of a central trachyandesitic volcano surrounded by debris-avalanche and debris-flow deposits sandwiched between two basaltic lava flows. Basaltic lava erupted first, between 13 and 7 Ma, with a peak activity around 9 Ma. Trachyandesitic lava with minor trachyte and rhyolite was erupted towards the end of the basaltic activity, between 10 and 6.5 Ma, although mainly between 8.5 and 7 Ma. This episode led to the construction of a high stratovolcano and its associated laharic apron. The edifice collapsed several times and produced gigantic debris-avalanche deposits that are widespread in the Cantal and as far as 40 km from its centre. The last stages of trachyandesitic activity were synchronous with the emplacement of phonolitic domes between 7.5 and 5.5 Ma. This intrusive event was followed by extensive basaltic lava flows that covered most of the Cantal. The present geometry of the Cantal volcano is the result of these phases of construction and cataclysmic destruction followed by intense glacial and periglacial erosion. The ages of emplacement of the debris-avalanche deposits are now well constrained by abundant isotopic data obtained from the overlying, underlying and included blocks. They imply that several large debris-avalanches affected the flanks of the Cantal volcano between 8.0 and 6.8 Ma. The deposits are in chronological order and separated by episodes of volcanic construction: -- the deposits in the north and east (Rhues, Veronne, Impradine, Santoire, Alagnon Chevade valleys), dated at before 7,4 Ma, form a highly discontinuous, thin eroded layer that is overlain by a thick volcanoclastic laharic piedmont derived from the subsequent phases of volcanic construction; -- the deposits in the west (Marilhou, Mars, Maronne, Aspre, Bertrande valleys) are dated at between 7.2 et 7.4 Ma; -- the deposits in the southwest (Doire, Authre, Jordanne, Cere and Epie valleys) are dated at between 7.4 and 6.8 Ma; -- the deposits in the south (Goul and Brezons valleys) younger than 7.1 Ma and emplaced before the Cere deposit. The absence of a laharic unit on top of the southwestern debris-avalanche deposits is in agreement with this succession of volcanic construction and destruction, as it implies the absence of any major volcanic construction after the last gravitational collapse. All the other sectors are characterized by thick debris-flow deposits overlying the debris-avalanche deposits. This chronological succession of events invalidates the previously proposed debris-avalanche chronologies. The present-day total volume of debris-avalanche deposits is around 245 km 3 for a total volcanic volume of 385 km 3 . Individual debris-avalanche bodies have volumes of several tens of km 3 . Well-characterized prehistoric and historic debris-avalanche bodies have height/length ratios around 0.1. Taking this good correlation into account suggests altitudes above 3000 m for the Cantal paleovolcano and explains the high paleoslopes observed in its central part. Previous models required the existence of a gigantic caldera ("fosse volcano-tectonique") in the central part of the volcano to account for the abundant "pyroclastic rocks" now interpreted as debris-avalanche deposits. This caldera and smaller ones were geophysically and geochronologically documented. New geophysical and geological expertise, however, has revealed the absence of such features. The detailed mapping has shown that the Cantal stratovolcano is mainly the result of several phases of construction and destruction over a relatively short period from 8.5 to 6.5 Ma. The construction phases led to the edification, over several hundred thousand years, of trachyandesitic volcanoes (25 km in diameter and more than 3000 m high) surrounded by debris deposits (laharic piedmont, 40 km in diameter). Due to the high viscosity of the trachyandesitic material, each construction phase resulted in major gravitational collapse, causing a large debris avalanche talus (70 km in diameter) around the central volcano. The last collapse in the southwest was not followed by a construction event, as indicated by the absence of overlying debris-flow deposits and by the flat morphology sealed by the upper basaltic flows.