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

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

France

Alexandre Poiraud

To cite this version:

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�

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apoiraud@yahoo.fr

COEVOLUTION OF RELIEF AND LANDSLIDES ON A VOLCANIC LANDSCAPE*

The bassin of Puy-en-Velay, Massif central, France

Alexandre Poiraud, PhD Thesis GEOLAB, UMR-CNRS 6042

4, rue Ledru - 63057 Clermont-Ferrand

GEOLAB

CONTEXT AND PHILOSOPHY OF THE RESEARCH

. 20 years ago, the symposium "Morphogenic rythms in Volcanic Domain" at Clermont-Ferrand

. PhD Thesis about the landslide hazard mapping in the bassin of the Puy-en-Velay (Massif central, France)

. Linking the temporal short scale of engineering geology with the temporal long term of relief evolution

METHODOLOGY

. Morphometric analysis of 19 palaeolandslides

. Morphometric analysis of 22 watersheds by mean of multivariate statistics (PCA and HAC)

. Dating of basaltic tables as a chronological indication of the beginning of a new erosion cycle (bibliography)

HOW VALLEYS AND LANDSLIDES EVOLVED TOGETHER ?

STUDY AREA AND PALAEOLANDSLIDES

RESULTS AND INTERPRETATION

1 the morphometric diﬀerence between

Deves and Velay 2 morphometry of palaeolandslides

and relationship with valley dimensions 3 Interpretation and

theorical model of coevolution

"

la borne

la gagne la Loire

la gazeille la sumene

la laussonne Le Puy

Loudes

Cayres

Solignac

Alleyrac Monastier St Paulien

3°36'0"E 3°36'0"E

3°48'0"E 3°48'0"E

4°0'0"E 4°0'0"E

4°12'0"E 4°12'0"E

44°54'0"N 44°54'0"N

45°0'0"N 45°0'0"N

45°6'0"N 45°6'0"N

" City

River

Geology

Miocene volcanism

Plio-pleistocene volcanism Acid volcanism

Tertiary/quaternary sediment Crystalline basement

¹

0 5 10

Km

DEVES REGION

VELA

Y REGION MEYGAL

REGION

3°90E

45°N

Tertiary basin with sandy clay material

Two volcanic phases :

1- Miocene => VELAY REGION

2. Plio-pleistocene => DEVES REGION Rapid uplift during Pliocene

with a great incision of hydrographic network

1

2

la Cueille

Cordat (les Boiroux) Courtialoux

FAULT

Palaeolandslides Complex of Monastier

Grazac

stadium quarry The Borne

river Railway

800 750

700

200 400 600 800 1000 1200

Elevation a.s.l. (m)

Distance (m)

SSO NNE

basaltic plate talus scree sliding boulder

fractured slidind boulder

quaternary deformed bedrock fluvial terrace

periglacial formation

III II

III’ II’

I ?

I’

reworked material View of palaeolandslide complex of Monastier

Cross section of palaeolandslide ot St Vidal

HUGE COMPLEX LANDSLIDE

Initiation and reactivation during the Pleistocene and Holocene RELIEF INVERSION

OVERSTEEPENING

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Rivers from Devès

Rivers from Velay

normalized elevation

Normalized distance

1.5/2Ma

8/10Ma

Longitudinal proﬁle of principal rivers

circularity ratio 10% of variance

37% of variance 25% of variance

st-vidal ceyssac

dolaizon_S1

dolaizon_N1

dolaizon_N2 ceyssoux

dolaizon_S2

cueille

malaval chabanne_b

rochaubert chabanne_h

monastier_N prada

monastier_S1 monastier_S2

savin

laussonne

astiers

0 50 100 150 200

0 200 400 600 800 1000 1200 1400

Lenght (m)

Height (m)

devès

velay _{H/L = 0.31} ^{H/L = 0.19} _{H/L = 0.13}

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

0 500 1000 1500 2000 2500 3000

widht of valley (m)

H/L

devès

velay ^st-vidal

ceyssac dolaizon_S1

dolaizon_N1

dolaizon_N2 ceyssoux

dolaizon_S2

cueille malaval

chabanne_b chabanne_h

monastier_N prada

monastier_S1 monastier_S2

savin

laussonne astiers

physiographic limits expression of local complexity

+ - +

ceyssac

ceyssoux dolaizon_N2

dolaizon_N1 dolaizon_S1 dolaizon_S2

st-vidal

astiers chabanne_b

chabanne_h cueille laussonne

malaval monastier_N

monastier_S1

monastier_S2 prada

rochaubert

savin

0 50 100 150 200 250

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

H/L theorical backdrop of basaltic plate border (m)

devès velay

circus morphology

valley widening (deves border)

release

of physiographic limits

relative similar H/L ratio

higher dimensions for Velay landslides

decrease of H/L with increasing of valley width for Deves landslides no relationships for Velay landslides

positive relation

between lenghtening of landslide and

backdrop for Deves no relationships for Velay landslides

Valley and landslide dimensions evolve together. When the watershed is "young" and that relief is due to a simple gorge, landslides are constrained by the limited dimensions of the valley.

That is the "physiographic limit". The only way for landslide evolving is the retrogression processes witch cause the backdrop of basaltic.

The H/L of landslide is so dependent of valley dimensions.

But, with the widening of valley, due to the backdrop of basaltic plate border by landsliding, the physiographic limit disappears. The geometry of landslides is then more dependent of local factors than the physiographic limits.

High incision, convex proﬁle with waterfall, simple

network

concentrated oversteepening H/L > 0.25, high physiographic constraint

low backdrop intensity

High incision, convex proﬁle with waterfall, network

densiﬁcation and lenghtening concentrated oversteepening H/L > 0.20, high physiographic constraint

high backdrop intensity

Low incision, concave proﬁle without waterfall, network densiﬁcation and lenghtening low physiographic constraint, increase of local constraint low backdrop intensity and general downwearing

* 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, Aknowledgement

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

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