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Post-eruption evolution of maar lakes and potential instability: The Lake Pavin case study, French Massif Central

Abstract : Few studies have focused on the post-eruption evolution of maar craters and associated lake level changes, al-though breaching of the crater rim poses mass-flow andflood hazards long after the initial eruption. LakePavin is a maar lake in the French Massif central formed by an eruption c. 6700 years ago. The maar is perchedon the edge of a glacially-shaped valley and the rim overlooks the surrounding subdued landscape. The rim isbreached to the north by an overflow channel, and to the south the maar has cut into theflank of the Montchalscoria cone and lavaflows. Incision of the lake outlet led to the growth of an alluvial fan that pushed the CouzePavin River towards the north side of the Gelat valley.Two lake terraces within the crater are used to determine stages in the evolution of the lake. The highstand Stage1 terrace +18 m above the current lake level indicates that the post-eruption lake surface and volume was 15%and 38% larger than the present-day lake, respectively, prior to the 5-7th Century CE. The poorly preserved Stage2 bench, +4.35 m on the west side of the natural dam, indicates that incision paused between Stage 1 andachievement of present lake level (Stage 3) before the 18th Century. Four units of streamflow deposits(c. 0.087 km2in area and 0.14–0.18 km3in volume) that formed the fan at the mouth of the outlet in the Gelatvalley have been14C dated to between the 9th and 16th Century. From the fan sedimentology, we infer thatthe lake level dropped slowly between highstand Stages 1 and 2, when the outlet cut down through the rim sad-dle. The lake level drop accelerated between Stage 2 and Stage 3 before the 14th Century, inducing small debrisflows. The latter may have resulted from heavy rainstorms, but there is no evidence for catastrophic events. Step-wise downcutting of the north rim saddle was also governed by contrasted properties of the deposits forming thedam. The current lake level has been stable since at least the early 18th Century, but the long-term stability of thedam is at risk
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Jean-Claude Thouret, P. Boivin, D. Miallier, F. Donnadieu, J.-P. Dumoulin, et al.. Post-eruption evolution of maar lakes and potential instability: The Lake Pavin case study, French Massif Central. Geomorphology, Elsevier, 2021, 382, pp.107663. ⟨10.1016/j.geomorph.2021.107663⟩. ⟨hal-03164557⟩



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