Le massif du Cantal, localise dans la province volcanique Cenozoique du Massif Central (France), est le plus grand complexe volcanique Miocene d’Europe. Il est majoritairement constitue de breches volcanoclastiques qui lui conferent des dimensions impressionnantes et une morphologie surbaissee. Des observations de terrain, couplees a des datations K-Ar, nous ont permis de reconstituer l’evolution morpho-structurale, la dynamique eruptive et l’histoire volcanique du massif. Apres avoir contraint dans l’espace et dans le temps une sequence stratigraphique identique tout autour du massif, nous montrons qu’il existe des evidences chrono-stratigraphiques, structurales et geomorphologiques en faveur de la formation d’une caldeira centrale de 8 x 10 km il y a environ 8 Ma occupee par un lac. L’etude morphologique et sedimentologique de la principale unite brechique du massif, le Grand Ecoulement Brechique, nous a conduit a le definir comme l’un des plus importants debris flow syn-eruptifs connus au monde dont nous estimons un volume de l’ordre de 100 km3. Nous proposons que cet ecoulement resulte d’une eruption sous-lacustre intracaldeira majeure. L’interaction entre un magma juvenile et l’eau du lac de caldeira, selon un dynamisme surtseyen, a initie la formation du debris flow qui s’est propage sur les pentes externes du volcan a 360° jusqu’a plus de 25 km de sa source. Cette etude ouvre des perspectives d’une part quant aux risques volcaniques lies aux eruptions sous-lacustres intracalderiques majeures, et d’autre part quant a la possibilite de reconstituer la morphologie passee de la source d’un ecoulement brechique volcanoclastique âge de plusieurs Ma, par l’etude de son depot selon des criteres sedimentologiques et morphologiques.Dans un deuxieme temps, la combinaison de nouvelles datations K-Ar et d’observations de terrain nous ont permis de reconstituer l’histoire volcanique du plateau du Cezallier, siege de la plus jeune activite volcanique de France metropolitaine, du plateau de l’Aubrac, et egalement la chronologie du volcanisme le long du Sillon Houiller. Nous avons pu ainsi etablir les relations spatio-temporelles entre le massif du Cantal et les provinces volcaniques adjacentes revelant le role important des fractures heritees de l’Hercynien pour la remontee des magmas. Nous confirmons ainsi une migration spatio-temporelle vers le nord du volcanisme en Auvergne. Finalement, l’acquisition au cours de ce travail de 47 nouveaux âges K-Ar de 12,8 Ma a 9 ka complete la chronologie du volcanisme du Massif Central en precisant sa distribution spatio-temporelle.
The morphometry of volcanic ash produced by explosive eruptions yields ample information on fragmentation processes (e.g. magmatic vs magma–water interactions), and on transport and sedimentation mechanisms. Most previous works on volcanic clast morphometry focused on the Apparent (2D-)Projected shape of ASH grains, here termed APASH, to infer processes and eruptive styles. However, textural analyses of ash grains has remained a long and tedious task that made such approaches inappropriate for eruption surveillance duties. In this work we show that new technological advances on automated dispersion of granular materials imaged with a camera-coupled microscope and enhanced computer capabilities enable fast and high resolution image acquisition of thousands of ash grains that resolve this limitation. With a morpho-grainsizer designed for such fast and routine measurements we perform a series of APASH analyses on selected ash fractions of tephra deposits from known eruptive styles. We record the size, aspect ratio, circularity and convexity of APASH images and assess resolution, reproducibility, minimum population size, and total analytical duration, and offer recommendations for the reporting of APASH data for inter-laboratory comparisons. To avoid fractal geometry concerns, our analyses are carried out at constant size range (250–300 μm) and optical magnification (× 5) on ~ 3000 grains per samples collected from homogenized samples. Results from the andesitic 1999-ongoing eruption of Tungurahua volcano (Ecuador) show that ash particles from the moderate 2001 phase are relatively equant and convex in shape, while the stronger 2006 subplinian phase produced ash grains with more elongated, less circular and less convex APASH signatures. Ash grains from a basaltic scoria cone-forming eruption show even more ragged APASH characteristics. Overall, our protocol allows obtaining accurate and reproducible morphometric measurements that reveal subtle variations of the morphological signature, and the short duration (1.8 hours) of the whole analytical process renders high resolution analyses of ash shape achievable for volcano surveillance applications. This research ultimately aims to set up a morphometric database of APASH results for well-defined eruptive styles, in order to interpret on a short term basis any APASH data from active volcanoes for monitoring purposes.
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