Theses defended

2021 2020 2019 2018 2017 2016 2015



Méric HAUGMARD - 14th October

"Nonlinear determination of hypocentral and structural parameters: application to the intracontinental seismicity of the Armorican Massif"

The most widely used inversion schemes to determine local or regional earthquake sources (epicenter, depth and origin time), are based on iterative linearized least-squares algorithms and strongly depend on an a priori knowledge. We introduce a joint structure-source determination approach for one or several events. This procedure manages to get away from dependence on an initial solution and minimizes the influence of poorly known seismic velocity structure used for calculating the propagation time. The exploration relies on parallel Markov chain Monte Carlo techniques, which sample models within a broad range of parameters, distributed according to the posterior distributions. The arrival time uncertainties of direct and refracted waves on the Mohorovičić discontinuity, defined by hand-picking, are fully exploited by Bayesian inference and lead the nonlinear determination. The analysis of covariances between parameters and the projections of the structure parameter variations on the source parameter space provide information about the influence of the seismic network geometry (sparse and/or azimuthally inhomogeneous), or an inappropriate velocity structure. This procedure is applied to the moderate and diffuse local seismicity of the Armorican Massif (western France), an extensive outcrop of the Variscan basement, which crustal structure is apprehended with large uncertainties. The accurate location of 350 earthquakes, with a set of more than 7,000 onset time of seismic-waves recorded by permanent and temporary networks between 2011 and 2013, sheds new light on the seismicity and the structure of the Armorican Massif.

Geoffrey AERTGEERTS - 25th January

"Petrological study of the Champtoceaux and Audierne complex ophiolitic relics: characterisation of fibrous minerals in mafic and ultramafic rocks"

Relics of ophiolitic units of the south Armorican Massif belonging to the Champtoceaux and Audierne complexes are interpreted as inherited from the Galice-Massif central ocean which separated Gondwana and the Armorica microplate. A petrological and geochemical study shows that all these rocks belong to a suprasubduction ligurian type ophiolite. Two ophiolites were recognized in the Champtoceaux complex. The Drain-Pont de Barel Devonian ophiolite is characterized by spinel lherzolites, cumulates and N-Type MORB mafic rocks. It could have been set up in the Saint-Georges-sur-Loire back arc basin. The Folies Siffait ophiolite could be composed of refertilized harzburgites, cumulates and N-Type MORB mafic rocks. It could be an intraoceanic Silurian ophiolite. In the Audierne complex, the ophiolite is Ordovician and constituted by strongly refertilized spinel harzburgites, sometimes lherzolites, cumulates and N-Type MORB mafic rocks. This ophiolite could belong to the Galice-Massif central ocean. All the peridotites are strongly serpentinized by sea-water rock interaction. During this process, chrysotile cross-fiber veins have been probably formed by fluids oversaturation. All mafic rocks were strongly deformed during a main event which could be late Devonian to Carboniferous. During the late stage of this event, fluid circulation would have caused rock fracturation and acicular to fibrous amphiboles transformation.

Ianis GAUDOT - 22nd January

"Analysis of seismic ambient wavefield cross-correlations - Application to the tomography of western France"

The continuous excitation of the Earth's surface due to oceanic wave -seafloor coupling, is widely used for both analyzing the seismic ambient wavefield properties and for imaging the deep structures. A new statistical approach based on the phase redundancy of cross-correlations is developed to provide quantitative informations on the ubiquitous seismic signal. Two dense arrays of broadband seismometers have been temporarily deployed over the southern and western France, and over the northern Spain, providing a good opportunity to reveal the crustal and uppermost mantle features beneath this region. A Monte Carlo Markov chain inversion algorithm is used to translate, for each station pair, the frequency dependent energy diagram of the ambient noise cross-correlation, into probability density distribution of the shear wave velocity as a function of depth. The 1D velocity profiles are then combined together using an anisotropic regionalization scheme, to build a 3D velocity model with an unprecedented lateral resolution of 75 km. The isotropic velocity maps correlate well with the geology up to 10 km depth. While negative velocity anomalies are associated with the sedimentary basins, the crystalline Variscan basement displays positive anomalies. At 20 km depth, the Bay of Biscay exhibits positive velocity anomalies, whose eastern boundaries can be interpreted as the ocean-continent transition. The overall crustal structure below the Armorican Massif appears to be heterogenous at the subregional scale.

Alice HAZOTTE - 15th January

"Role of bacterial metabolites in the mobilization of a caesium doped illite: mechanistic study and application to phytoextraction"

Following nuclear accidents, various radionuclides such as cesium-137 (Cs) are dispersed in the atmosphere before being deposited on the soil. In order to eliminate Cs accumulated in the upper soil horizon, both in situ and at lower cost, phytoextraction is proving to be a promising method. The combination of bioaugmentation of soil and phytoextraction may reduce the treatment duration by increasing the fraction of Cs mobilized by the plant. Understanding the mechanisms that governs the Cs-soil-plant-bacteria interactions is the subject of this thesis. The first part of the thesis concerns the bioaccessibility of a purified and Cs-doped illite, considering the direct or indirect actions of bacterial metabolites (citric and oxalic acids, desferrioxamine mesylate, acetohydroxamic acid and pyoverdine (PVD) produced by P. fluorescens). PVD desorbs up to 45% of Cs from illite through direct (ion exchange) and indirect (illite weathering) mechanisms. In the second part, red clover (Trifolium pratense), chosen as accumulator plant, is hydroponically grown, with or without PVD. Without PVD, 10.0 μmol of Cs per g of dry matter (DM) are accumulated in roots and 5.9 μmol.g-1 DM in its aboveground parts while in its presence, Cs accumulation by red clover is reduced by 25 to 70% but the translocation factor was higher (0.5 without PVD and 1.1 with PVD). In the third part, the red clover is grown in pots in the presence or absence of PVD. The results are similar to those measured in hydroponics (118 Cs μmol.g-1 MS in roots and 40 μmol.g-1 MS in the aboveground parts). Keywords: cesium, illite, microbial weathering, phytoextraction, Pseudomonas fluorescens, pyoverdine, siderophores, Trifolium pratense.

Marion NACHON - 11th January

"Martian surface chemistry within Gale crater, using ChemCam/Curiosity elementary data and laboratory analyses"

The Mars Science Laboratory (MSL) Curiosity rover landed on Mars on August 6th 2012. Its objective is the understanding of the primitive martian environment, by investigating the ancient sediments within Gale crater. Onboard, the ChemCam instrument uses, for the first time in the planetary exploration history, the Laser-Induced Breakdown Spectroscopy (LIBS) technique, coupled to a Remote Micro-Imager (RMI). During this thesis we have beneficiated from the ChemCam spectral and imaging data, and we have coupled the elemental composition provided by the laser with the geological context. We focused on diagenetic targets i.e. formed posterior to the sediments deposition), and especially on calcium sulfate veins. Also, we have performed laboratory analyses in order to constrain the ChemCam analyses on Mars, and studied two volatile elements of special interest for martian geochemistry : sulfur and phosphorus. Finally, part of this thesis activity was dedicated to ChemCam data acquisition, by participating to the rover operations, i.e. scientific data planification and treatment performed daily.