Coevolution of relief and landslides on a volcanic landscape. The bassin of Puy-en-Velay, Massif central, Alexandre Poiraud

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Alexandre Poiraud. Coevolution of relief and landslides on a volcanic landscape. The bassin of Puy- en-Velay, Massif central, France. Q8 - Quaternaire 8, Feb 2012, Clermont-Ferrand, France. 2012. ￿hal-01088301￿

HAL Id: hal-01088301 https://hal.archives-ouvertes.fr/hal-01088301 Submitted on 27 Nov 2014

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. COEVOLUTION OF RELIEF AND LANDSLIDES ON A VOLCANIC LANDSCAPE* The bassin of Puy-en-Velay, Massif central, France

G Alexandre Poiraud, PhD Thesis EOLAB GEOLAB, UMR-CNRS 6042 4, rue Ledru - 63057 Clermont-Ferrand [email protected]

CONTEXT AND PHILOSOPHY OF THE RESEARCH METHODOLOGY

. 20 years ago, the symposium "Morphogenic rythms . Morphometric analysis of 19 palaeolandslides in Volcanic Domain" at Clermont-Ferrand HOW VALLEYS AND LANDSLIDES . Morphometric analysis of 22 watersheds by mean of . PhD Thesis about the landslide hazard mapping EVOLVED TOGETHER ? multivariate statistics (PCA and HAC) in the bassin of the Puy-en-Velay (Massif central, France) . Dating of basaltic tables as a chronological indication of the beginning of a new erosion cycle (bibliography) . Linking the temporal short scale of engineering geology with the temporal long term of relief evolution

STUDY AREA AND PALAEOLANDSLIDES

3°36'0"E 3°48'0"E 4°0'0"E 4°12'0"E la Cueille Courtialoux Palaeolandslides Complex of Monastier la borne St Paulien " la Loire MEYGAL REGION ¹ 0 5 10 Km 45°6'0"N FAULT 45°6'0"N " la sumene 3°90E Le Puy DEVES REGION " la gagne 3 Cordat (les Boiroux) 45°N View of palaeolandslide complex of Monastier la 45°0'0"N 45°0'0"N " City Solignac " Grazac quarry The Borne Railway River SSO NNE Monastier stadium river " III II I Geology 800 ? la gazeille Miocene volcanism " III’ Plio-pleistocene volcanism II’ Acid volcanism 750 " VELAY REGION 44°54'0"N I’

44°54'0"N Tertiary/quaternary sediment Crystalline basement Elevation a.s.l. (m) 700 3°36'0"E 3°48'0"E 4°0'0"E 4°12'0"E 200 400 600 800 1000 Distance (m) 1200 basaltic plate reworked material Tertiary basin with sandy clay material talus scree quaternary deformed bedrock sliding boulder fluvial terrace Cross section of palaeolandslide fractured slidind boulder 3 periglacial formation ot St Vidal Two volcanic phases1 : 1- Miocene => VELAY REGION RELIEF INVERSION HUGE COMPLEX LANDSLIDE 2. Plio-pleistocene => DEVES REGION OVERSTEEPENING Initiation and reactivation during the Pleistocene and Holocene Rapid uplift during Pliocene with a great incision of hydrographic network 2

RESULTS AND INTERPRETATION

1 the morphometric difference between 2 morphometry of palaeolandslides 3 Interpretation and Deves and Velay and relationship with valley dimensions theorical model of coevolution

1 Longitudinal profile 200 Valley and landslide dimensions evolve together. When the astiers watershed is "young" and that relief is due to a simple gorge, 0.9 of principal rivers devès monastier_S1 H/L = 0.19 monastier_N monastier_S2 landslides are constrained by the limited dimensions of the valley. 0.8 Rivers from velay H/L = 0.31 chabanne_h prada H/L = 0.13 Devès rochaubert That is the "physiographic limit". The only way for landslide evolving Devès = convex profile 150 cueille malaval 0.7 (young watershed) chabanne_b is the retrogression processes witch cause the backdrop of basaltic. 0.6 The H/L of landslide is so dependent of valley dimensions. average age st-vidal 1.5/2Ma of watersheds 0.5 Velay = concave profile 100

Height (m) laussonne But, with the widening of valley, due to the backdrop of basaltic (mature watershed) ceyssoux dolaizon_N2 0.4 relative similar dolaizon_S1 dolaizon_S2 plate border by landsliding, the physiographic limit disappears. The normalized elevation savin H/L ratio 0.3 geometry of landslides is then more dependent of local factors than 50 the physiographic limits. 0.2 Rivers from dolaizon_N1 higher dimensions Velay 8/10Ma 0.1 for Velay landslides High incision, convex profile

0 0 with waterfall, simple 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 200 400 600 800 1000 1200 1400 network Normalized distance Lenght (m) 0.4 concentrated oversteepening decrease of H/L H/L > 0.25, high physiographic 0.35 with increasing of constraint 37% of variance Retournac 25% of variance " " dolaizon_N1 malaval valley width for distance from the exutory age of watershed 0.3 ceyssoux watershed lenghtening physiographic development dolaizon_S1 Deves landslides St Paulien St Paulien dolaizon_N2 low backdrop intensity " PCA " PCA 0.25 dolaizon_S2 astiers Pertuis coordinates axe F1 coordinates axe F2 " "Pertuis -4.55 - -1.83 -2.78 - -1.25

Puy-loudes H/L chabanne_h " -1.82 - 0.68 Puy-loudes" -1.24 - -0.60 ceyssac monastier_S2 no relationships St Julien St Julien cueille " 0.69 - 1.67 " -0.59 - 1.22 0.2 Puy-ville Puy-ville " " chabanne_b 1.68 - 3.05 1.23 - 4.17 monastier_S1 for Velay landslides monastier_N High incision, convex profile river river savin prada " city " city 0.15 devès st-vidal laussonne with waterfall, network Solignac " "Solignac velay

Monastier Monastier densification and lenghtening " " 0.1 Cayres " Estables "Cayres " "Estables Alleyrac + physiographic limits - expression of local complexity + " "Alleyrac ¹ ¹ 0.05 concentrated oversteepening 0 2.5 5 10 0 2.5 5 10 0 500 1000 1500 2000 2500 3000 km km widht of valley (m) H/L > 0.20, high physiographic

250 constraint

Retournac Retournac " 10% of variance " devès velay positive relation circularity ratio Hierarchical high backdrop intensity Ascending St Paulien St Paulien 200 between lenghtening " PCA " Classification st-vidal Pertuis coordinatesaxe F3 Pertuis valley widening (deves border) " " circus rochaubert of landslide and -3.24 - -0.83 1 - Loire axe Puy-loudes Puy-loudes morphology " -0.82 - 0.18 " 2 - Granitic watersheds St Julien St Julien backdrop for Deves " 0.19 - 0.89 " 3 - Deves watersheds 150 "Puy-ville "Puy-ville Low incision, concave profile 0.90 - 2.02 4 - Velay watersheds river river without waterfall, network " " city city (m) no relationships ceyssac "Solignac "Solignac monastier_S1 densification and lenghtening 100 monastier_S2 prada for Velay landslides Monastier" Monastier" Cayres chabanne_b "Cayres Estables " Estables " " monastier_N cueille malaval Alleyrac "Alleyrac " dolaizon_S2 low physiographic constraint, 50 savin chabanne_h dolaizon_S1 ¹ ¹ ceyssoux 0 2.5 5 10 0 2.5 5 10 increase of local constraint km km astiers dolaizon_N2 release laussonne theorical backdrop of basaltic plate border of physiographic limits dolaizon_N1 0 low backdrop intensity and 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 H/L general downwearing

Aknowledgement

Thanks to Emmanuelle Defive for the numerous discussions we had on the topic

* in reference to A. Jefferson, G.E. Grant, S.L. Lewis and S.T. Lancaster (2010) - Coevolution of hydrology and topography on a basalt landscape in Oregon Cascade Range, USA. Earth Surface Processes and Landforms, 35, 803-816 1 - Mergoil J. and Boivin P. (1993) - Le Velay, son volcanisme et les formations associées : notice à la carte au 1/100000. Géologie de la France, 3, 96p. 2 - Defive E. (1996) - L'encaissement du réseau hydrographique dans le bassin supérieur de la Loire - contribution à l'étude des rythmes d'évolution géomorphologique en moyenne montagne volcanisée. Thèse de doctorat, Paris I, Panthéon-Sorbonne, 551 p. 3 - Poiraud A. and Defive E. (2011) - Morphology and geomorphological significance of relict landslides in the Tertiary basin of Puy-en-Velay (Massif Central, France). Géomorphologie : relief, processus, environnement, 3,