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RESEARCH THEMATIC: volcanic system evolution and eruptive dynamics

Reconstruction of the morpho-structural evolution and the eruptive dynamics of the Cantal massif ; relationship with the spatio-temporal evolution of the Massif Central volcanism (France)

Massif Central Cenozoic volcanic province

The Cantal massif, view toward SE from Puy Mary (1 783 m)

The Cantal massif, located in the French Cenozoic volcanic province, is the largest Miocene volcano in Europe. It is mainly composed of volcaniclastic breccias giving impressive dimensions and a shield-like morphology. Field observations, coupled with K-Ar datings, allowed us to reconstruct the morpho-structural evolution, the eruptive dynamics and the volcanic history of the massif. We first constrained a stratigraphic sequence which is identical all around the massif. Then, we show that it exists from chrono-stratigraphic, structural and geomorphologic evidences a 8 x 10 km central caldera ca. 8 Myrs ago occupied by a lake. The sedimentological and morphological study of the main breccia unit, the Large Breccia Flow, led us to define it as one of the largest syn-eruptive debris flow in the world. We estimate its volume at ca. 100 km3. We propose that this debris flow resulted from a major intracaldera lake eruption. The interaction between a juvenile magma and the water of the caldera lake, with a surteyan dynamics, initiated the debris flow that propagated on the external slopes of the volcano at 360° until 25 km from its source. This study opens outlets on one hand to volcanic hazards related to major intracaldera lake eruptions ; on the other hand, we show that it is possible to reconstruct the former source morphology of a several Myrs-old volcaniclastic breccia flow by studying the sedimentological and morphological criterion of its deposit.
Furthermore, the combination of new K-Ar datings with field observations allowed us to reconstruct the volcanic history of the Cézallier plateau (site of the youngest volcanic activity in the continental France), of the Aubrac plateau, and also the volcanism along the Sillon Houiller. Consequently, we established the spatio- temporal relationships between the Cantal massif and the adjacent volcanic provinces, highlighting the important role of the Hercynian inherited fractures in the magmas ascent. We thus confirm the spatio-temporal migration toward the north in Auvergne. Finally, the acquisition of 47 new K-Ar ages ranging from 12.8 Ma to 9 ka conveys new time constraints on the chronology of Massif Central volcanism by precising it spatio-temporal distribution.

Keywords : Cantal, Massif Central, volcanism, geochronology K-Ar, caldera, debris flow

Techniques : K-Ar dating (Cassignol-Gillot technique, 1986), DRX, laser granulometry, geomorphology, cartography

Supervisors : Pierre-Yves Gillot

MASTER'S DEGREE RESEARCH WORK

Geochemical and geochronological study of the Nyiragongo parasite cones (RDC) : significance in the volcanic complex evolution (2008)

supervisors : Pierre-Yves Gillot (LGMT)

Synthetic magma feeding model of the Nyiragongo complex within the continental crust based on geochemical data (Leibrandt, 2008)

The Nyiragongo volcano, situated in the Virunga volcanic field in the Western Branch of the East African Rift System, is one of the most active volcanoes on Earth and its lavas have a strong alkalinity and a very low saturation rate. It is a volcanic complex composed of three proximal main cones from which develop radial rift zones depending on the main regional tectonic directions. Above these fault systems grow parasites cones. The study of the parasites cones lavas major elements allows the discrimination between two groups of cones in each rift zone : cones I are less undersaturated than cones II. The trace elements study shows that parasite cones have a commun OIB-like source and lavas are issued from a very low partial melt around 1 %. The high Zr/Hf ratios indicate that the underlying mantle is metasomatised by carbonate-rich fluids. From the study of the evolution of La/Sm vs Sm/Yb, lavas could be issued from the melt of spinel and garnet peridotites, with possible presence of phlogopite.

Melt depth is about 70 km (2,2 GPa) depending on minerals domain of stability in the metasomatised mantle. Thus, lava source is single but with very low heterogeneities inferior to 1 km. Also, the lava isotopes study coupled with a synthesis of the geochemical and seismic datas on the East African Rift System show that the source presents characteristics of a mantle upwelling under the Western Branch rift. Finally, the parasite cones K-Ar dating (Cassignol-Gillot technique, 1982) indicates that the studied rift zones are less than 5 000 years. From the parasite cones and Nyiragongo main cones source characterisation and based on the literature, this work establishes a synthetic model of the complex magma feeding and of these magmas sources. Thus, magmas born in a metasomatised mantle that goes up in the lithosphere.

Synthetic magma feeding model of the Nyiragongo complex from the superior mantle based on geochemical and geodynamical data (Leibrandt, 2008)

COMMUNICATIONS

> Articles:

Leibrandt S., Gillot P.-Y. – Identification of a central caldera on the Cantal volcano (France): insights
for a new evolution model, in prep. (to be submitted to JVGR)
Leibrandt S., Gillot P.-Y. – Large volcaniclastic debris flow resulting from a plinian caldera lake eruption: case study from the Miocene Cantal volcano (France), in prep. (to be submitted to Geology)

> Meetings:

Leibrandt S., Gillot P.-Y., Hildenbrand A. - Emplacement of large volcaniclastic breccia volumes in relationship with a central caldera: volcanic history of the Cantal massif (France), EGU meeting, Vienna (Austria), 2011.
Leibrandt S., Gillot P.-Y., Hildenbrand A. - Évolution du massif du Cantal : relation entre la formation d'une caldeira centrale et la mise en place de large volumes de brèches volcanoclastiques, RST Bordeaux (France), 2010.
Leibrandt S., Quidelleur X. - Volcanism, climate and mass extinctions : a cause and effect relationship ?, 11e colloque pluridisciplinaire, Université Paris Sud (France), 2010.
Leibrandt S., Gillot P.-Y., Hildenbrand A., Leocat E. - Genesis and emplacement of the large debris flow of the Cantal volcano (France), VMSG (http://www.vmsg.org.uk/), Bournemouth (UK), 2009.
Leibrandt S., Gillot P.-Y., Hildenbrand A. - The French Massif Central volcanism : spatio-temporal extension, tectonic control and magmatic variability, Journée des Doctorants, Université Paris Sud (France), 2009.

> Conferences:

"Behind the scenes of the Montserrat Volcano Observatory - Monitoring techniques of the Soufrière Hills Volcano" Société de Volcanologie de Genève, Musuem of Natural History of Geneva (Switzerland), February 2009.

"Do supervolcanoes threaten humanity?" Uranoscope, Ozoir-la-Ferrière (France), June 2012.


TEACHING

- 1st yr General Sciences : Introduction to cartography.
- 2nd yr Biology : lessons of general geology + cartography, rocks and minerals. In charge of 45 students.
- Fieldtrips : "Volcanology of Cantal, Mont Dore an Chaîne des Puys (Auvergne, France)" (3rd yr Earth Sciences), "Volcanology of Auvergne (France)" (4th yr Biology-Geology), "Geomorphology in a volcanic field" (4th yr Earth Sciences).


EDUCATION

2008-2011: PhD in Volcanology - Université Paris Sud

2007-2008 : Master (2nd year) Volcanology ("ESV parcours volcanologie") - Université Paris Sud

2006-2007 : Master (1st year) Earth and Universe Sciences - Université Paris Sud

2003-2006 : Bachelor's degree in Earth Sciences - Université Paris Sud

VOLCANO OBSERVATORY EXPERIENCE

September 2008 : "volunteer student" at the Montserrat Volcano Observatory (MVO, West Indies) directed by Nicolas Fournier (Seismic Research Center, West Indies). "Techniques of monitoring and gas study at Soufrière Hills Volcano (Montserrat, West Indies)"

FIELD EXPERIENCE

...on active volcanoes : Italy (Etna, Vesuvius, Phlegrean Camps, Ischia, Vulcano, Stromboli), Greece (Santorini), Costa Rica (Arenal, Poas, Irazu, Ricon de la Vieja, Turrialba), Antilles (Montagne Pelée, Montserrat), New Zealand (Ruapehu, Tongariro, Ngauruhoe, Rangitoto), Iceland.

OTHER SKILLS

- Animation/leader diploma
- Mountain guide diploma
- Scientific animations about general geology and volcanoes
- Guide on Massif Central volcanoes and on the Italian active volcanoes