Dissertationen zum Thema „Arc of the western Alps“

A partir de nouvelles observations structurales, nous proposons un nouveau modèle cinématique pour la Zone Houillère Briançonnaise, située au-dessus du front briançonnais (structure majeure des Alpes occidentales). Ce modèle est conforté par l'inversion des données de déformation finie, interpolées et visualisées à l'aide du modeleur 3D GOCAD (ENSG-LIAD Nancy). Les données de déformation finie ont été acquises par l'utilisation d'algorithmes d'analyse d'images, semi-automatisées au cours de ce travail. L'inversion a été effectuée à l'aide du logiciel FaultPack (Université Rennes 1). Le modèle cinématique qui résulte de ce travail se décompose en trois évènements principaux suivis d'un épisode extensif. L'évènement D1 traduit très probablement la subduction de l'océan piémontais au cours de laquelle, restant dans le prisme d'accrétion tectonique, la Zone Houillère subit un écaillage vers L'ouest alors qu'une partie de la Vanoise est entrainée dans la subduction. Au cours de D2, la subduction de la Vanoise se bloque et cette zone est alors charriée sur la Zone Houillère puis elle l'emboutit. L'évènement D3 est associé à l'indentation de la croûte européenne par le poinçon adriatique. Cette indentation est, entre autre, la cause de la surrection des Massifs cristallins externes qui réactive une surface correspondant au Front Briançonnais originel (décollement précoce D1 et/ou structure D2) en faille normale. Le rebroussement résultant de la surrection des Massifs cristallins externes produit une structure de type roll-over. L'évènement extensif D4, continuum plus superficiel de D3, provoque alors le basculement de tout l'édifice vers l'Ouest

La formation des arcs orogéniques résulte de plusieurs processus tectoniques ayant agi sur la configuration pré-orogénique, pré-collisionnelle ou sur la tectonique collisionelle. La formation de l’arc des Alpes occidentales est attribuée à l’indentation collisionnelle de la marge Européenne et du prisme orogénique par l’indenteur Adriatique. Cependant la direction d’indentation, sa composante rotationnelle et les mécanismes d’accommodation ne font pas consensus et de nombreux modèles cinématiques proposent des explications incompatibles entre elles, afin d’expliquer la géométrie arquée de la chaîne. L’évaluation des différents modèles de formation de l’arc des Alpes occidentales met en avant la probabilité de l’existence d’un proto-arc hérité de la phase de subduction et amplifié par l’indentation Adriatique, essentiellement vers le NW. Ces deux phases permettent d’expliquer la formation de l’arc à l’exception de sa terminaison méridionale E-W. En effet l’orientation des structures de l’arc de Castellane semble héritée des structures pyrénéo-provençales, antérieures à la collision Alpine et réactivées par une convergence N-S post-Tortonien (~12 Ma), sans lien direct avec la collision Alpine. Concernant la direction WNW-ESE des Alpes Ligures, elle semble être influencée par la rotation antihoraire de 50° des Apennins, liée au rollback du slab Adriatique, contemporain de l’ouverture du bassin Liguro-Provençal (23-15 Ma). Une compilation exhaustive des données de paléomagnétisme dans les Alpes a été construite et complétée par 11 sites de nouvelles données. L’étude des rotations d’axe vertical de ces données a permis de réfuter l’existence d’une rotation significative de la plaque Adriatique durant la collision Alpine. Les tests oroclinaux, réalisés à plusieurs échelles, mettent en évidence que l’arc des Alpes occidentales se développe sous l’effet de l’indentation vers le NW à partir d’un prisme orogénique déjà faiblement arqué avant la collision. La marge Européenne ne semble pas subir de rotation, impliquant une propagation d’un arc hérité de la marge passive Mésozoique. La géométrie actuelle de l’arc serait principalement contrôlée par la structure pré-collisionnelle de la marge Européenne que le prisme orogénique adopte sous l’effet de l’indentation Adriatique vers le NW. Sa terminaison méridionale aurait une histoire géodynamique différente. Elle serait le résultat de l’héritage pyrénéo-provençal avec réactivation Miocène dans la Zone Externe, et d’une rotation antihoraire de la Zone Interne, en lien avec l’orogénèse Apennine. L’indentation Adriatique, parfois interprétée comme principalement vers l’ouest, serait accommodée au Sud de l’arc par un décrochement senestre d’environs 50km selon la littérature. Cette interprétation est testée par une étude structurale de terrain, associée à une analyse géostatistique des trajectoires en carte des plans de schistosité et de stratigraphie. Les résultats semblent confirmer l’existence de décrochements senestres associés à une tectonique transpressive syn-collisionnelle. Cependant l’importance de ces décrochements paraît marginale en comparaison des 50 km de déplacement supposé. Cette analyse structurale a par ailleurs mis en évidence une déformation polyphasée dans le Dauphinois, associée à une mylonitisation localisée et caractérisée par un étirement N120°. L’évaluation des températures maximales par la méthode RSCM indique des Tmax supérieures à 350°C à la bordure Nord de l’Argentera, atteignant localement 400°C correspondant à un métamorphisme régional plus important que celui généralement attribué à ce secteur des Alpes. Ce métamorphisme est principalement associé à l’enfouissement tectonique par le passage des nappes internes sur le Dauphinois au début de la collision. Dans la région du massif de l’Argentera, les Tmax dans le Dauphinois correspondent à une profondeur d’enfouissement de 11 km au niveau du Front Pennique diminuant jusqu’à 4 km à l’aplomb de l’arc de Castellane
The formation of orogenic arcs results from several tectonic processes that may have affected the pre-orogenic, pre-collisional tectonic setting, or the collisional tectonic. The formation of the Western Alpine arc is classically attributed to collisional indentation of the European margin and the orogenic prism by the Adriatic indenter. However, the direction of indentation, its rotational component, or the accommodation mechanisms of this indentation are not agreed upon and the numerous kinematic models of the arcuate geometry of the chain are not compatible with one another. The evaluation of the different models of the formation of the Western Alpine arc allows to put forward the probability of the existence of a proto-arc inherited from the subduction phase, and amplified by the Adriatic indentation towards the NW. These two processes explain the formation of the Western Alpine arc, except for its E-W southern termination. Indeed, the orientation of the Castellane Arc seems to be mainly inherited from the Pyrenean-Provençal structures, preceding Alpine collision, and reactivated by post-Tortonian (~12 Ma) N-S convergence, not directly related to Alpine collision. Concerning the WNW-ESE direction of the southern termination of the arc, which forms the Ligurian Alps, it seems to have rotated counterclockwise by 50° with the northern Apennines, linked to the rollback of the Adriatic slab, contemporary with the opening of the Liguro-Provençal basin (23-15 Ma). A compilation of paleomagnetic data in the Alps was constructed and completed with 11 new data sites. The study of vertical-axis rotations, estimated by paleomagnetic analysis refutes the existence of significant rotation of the Adriatic plate during Alpine collision. Oroclinal tests, carried out at several scales, show that the Western Alpine arc develops under the effect of indentation towards the NW from an orogenic prism that is already weakly arcuate before the onset of collision. Furthermore, the European continental margin does not seem to undergo significant rotation, implying propagation of an arc that is mainly inherited from the Mesozoic passive margin. The present geometry of the Western Alpine arc would be mainly controlled by the pre-collisional structure of the European margin that the orogenic prism adopts under the effect of NW Adriatic indentation. Its southern termination would have a different geodynamic history. It would be the result of Pyrenean-Provençal inheritance, reactivated during the Miocene in the External Zone, and of an anti-clockwise rotation of the Internal Zone, linked to the Apennine orogeny. Adriatic indentation, sometimes assumed to follow a westward trajectory, would be accommodated to the south of the arc by a sinistral shear zone of about 50km according to literature. This interpretation is tested by a structural field study, associated with a geostatistical analysis of map trajectories of the schistosity and stratigraphy planes. The results confirm the existence of sinistral shear zones associated with syn-collisional transpressive tectonics. However, these faults seem to represent minor structures with respect to the accommodation of 50 km of displacement. This structural analysis has also highlighted polyphase deformation in the Dauphinois, associated with localised mylonitisation is characterised by a N120° stretching. The assessment of the maximum temperatures by the RSCM method indicates temperatures above 350°C at the northern edge of the Argentera, reaching locally 400°C, corresponding to a regional metamorphism that is more important than the one attributed to this sector of the Alps. This metamorphism is mainly associated with tectonic burial due to thrusting of the internal nappes on the Dauphinois at the beginning of collision. In the Argentera Massif region, the Tmax in the Dauphinois correspond to a burial depth of 11.3 ± 1 km at the level of the Pennine Front, decreasing to 4 km below the Castellane arc

The lower crustal structure beneath the Western Alps -- including the Moho -- bears the signature of past and present geodynamic processes. It has been the subject of many studies until now. However, its current knowledge still leaves significant open questions. In order to derive new information, independent from previous determinations, here I wish to address this topic using a different method --- ambient seismic noise autocorrelation --- that is for the first time applied to reveal Moho depth in the Western Alps. Moho reflections are identified by picking reflectivity changes in ambient seismic noise autocorrelations. The seismic data is retrieved from more than 200 broadband seismic stations, from the China--Italy--France Alps (CIFALPS) linear seismic network, and from a subset of the AlpArray Seismic Network (AASN). The automatically-picked reflectivity changes along the CIFALPS transect in the southwestern Alps show the best results in the 0.5--1 Hz frequency band. The autocorrelation reflectivity profile of the CIFALPS transect shows a steeper subduction profile,~55 to ~70 km, of the European Plate underneath the Adriatic Plate. The dense spacing of the CIFALPS network facilitates the detection of lateral continuity of crustal structure, and of the Ivrea mantle wedge reaching shallow crustal depths in the southwestern Alps. The data of the AASN stations are filtered in the 0.4--1 and 0.5--1 Hz frequency bands. Although the majority of the stations give the same Moho depth for the different frequency bands, the few stations with different Moho depths shows the care that has to be taken when choosing the frequency band for filtering the autocorrelation stacks. The new Moho depth maps by using the AASN stations are a compilation of the first and second picked reflectivity changes. The results show the complex crust-mantle structure with clear differences between the northwestern and southwestern Alps.

Climate variability during the Holocene (the last 11700 calendar yr b2k) is relatively subtle whether compared with the higher amplitude characterizing the last glacial period. Nonetheless, several proxy data (e.g. pollen, plant macrofossils, glacier length variations), indicate detectable changes in temperature and/or moisture during this period. Retreating glaciers may uncover peat sequences that were once ice-buried. Palynological analysis of those organic deposits provides information on plant communities colonization in proglacial areas as well as long-distance vegetations signal. The aim of this study is to investigate the potential of high-altitude pollen records for vegetation history and climate reconstruction during the Holocene in the south-western Italian Alps. The study sites are located in the Rutor Glacier area (La Thuile Valley). Within this thesis, modern (a) and past (b) vegetation and climate conditions have been analyzed using different methodologies; a) Modern pollen rain, vegetation, climate and terrain parameters have been collected at 27 sampling sites placed along an altitudinal gradient from the village of Morgex (983 m asl) to the Rutor Glacier forefield (2668 m asl). This altitudinal training set has been designed (i) to provide a robust modern reference for reliable palaeoenvironmental and quantitative interpretations of past changes in vegetation composition; (ii) to integrate the newly-obtained pollen spectra into a large, continental dataset of modern pollen samples (EMPD - European Modern Pollen Database) for quantitative climate reconstructions. b) New paleoecological, stratigraphical and geochronological data have been obtained from peat deposits (Lac dans la Roche peat bog, 2594 m asl) and integrated with the well-known buried peat sequences exposed at 2510 m asl by the '80es Rutor Glacier retreat. The result is a composite paleoecological record covering most of the Holocene. At the base of the composite sequence, meltwater glacier sediments testify to the early Holocene development of a proglacial lake, before Abies alba expansion in the western Italian Alps. These sediments bear the palynological evidence of a primary plant succession testifying to an ecological mechanism of colonization on deglaciated terrains. Moreover, macrofossil analysis highlighted the local presence of alpine dwarf - shrubland species (e.g. Salix cfr. foetida). Pinus cembra occurs rather densely in the alpine belt from ca. 8000 yrs cal. BP, representing the beginning of the Holocene thermal maximum in the high Alps. Between 8000 - 4000 yrs cal. BP timberline was higher than today. A radiocarbon age obtained from a Pinus cembra wood fragment suggests the occurrence of pines as high as ca. 2600 m asl at around 5650 yrs cal BP. The dated wood fragment cannot have been long-distance transported, thus it indicates the presence of this species in situ. Moreover, paleobotanical insights and comparison with climate threshold of modern treeline (ca. 9,3° C at 2400 m asl), suggest a positive treeline shift of almost 150 m. Preliminary pollen-inferred Tjul reconstructions show higher values (up to 2 - 3°C) than today for this interval. At around 4000 yrs cal. BP Picea abies and Alnus viridis started to expand. A progressive natural/climatic driven decline of Pinus cembra took place. Pollen inferred Tjul shows a decreasing trend between 2900 – 2000 yrs cal BP and possibly a short-lived cold event starting at ca. 1140 yrs cal. BP, pre-dating the onset of the LIA (Little Ice Age). A subsequent arolla pine forests reduction can be attributed to LIA glacier advance (max occurred between 1751 – 1864 AD), which is testified in the upper part of the sequence by fluvioglacial deposits.

On-going population growth and urbanization increasingly force people to occupy environments where natural processes intensely affect the landscape, by way of potentially hazardous natural events. Tectonic plate boundaries, active volcanic regions and rapidly uplifting mountain ranges are prominent examples of geomorphically hazardous areas which today accommodate some of the world’s largest cities. These areas are often affected by more than one hazard such as volcanic eruptions, earthquakes, landslides, tsunamis, floods, storms and wildfires, which frequently interact with each other increasing the total impact on communities. Despite progress in natural hazards research over the last two decades, the increasing losses from natural disasters highlight the limitations of existing methodologies to effectively mitigate the adverse effects of natural hazards. A major limitation is the lack of effective hazard and risk assessments incorporating hazard interactions and cascade effects. Most commonly, the assessment of risks related to different hazards is carried out through independent analyses, adopting different procedures and time-space resolutions. Such approaches make the comparison of risks from different hazard sources extremely difficult, and the implicit assumption of independence of the risk sources leads to neglect of possible interactions among hazard processes. As a result the full hazard potential is likely to be underestimated and lead to inadequate mitigation measures or land-use planning. Therefore there is a pressing need to improve hazard and risk assessments and mitigation strategies especially in highly dynamic environments affected by multiple hazards. A prominent example of such an environment is the western Southern Alps of New Zealand. The region is located along an actively deforming plate boundary and is subject to high rates of uplift, erosion and orographically-enhanced precipitation that drive a range of interrelated geomorphic processes and consequent hazards. Furthermore, the region is an increasingly popular tourist destination with growing visitor numbers and the prospect for future development, significantly increasing societal vulnerability and the likelihood of serious impacts from potential hazards. Therefore the mountainous landscape of the western Southern Alps is an ideal area for studying the interaction between a range of interrelated geomorphic hazards and human activity. In an effort to address these issues this research has developed an approach for the analysis of geomorphic hazards in highly dynamic environments with particular focus on tectonically-active mountains using the western Southern Alps as a study area. The approach aims to provide a framework comprising the stages required to perform multi-hazard and risk analyses and inform land-use planning. This aim was approached through four main objectives integrating quantitative geomorphology, hazard assessments and GIS. The first objective was to identify the dominant geomorphic processes, their spatial distribution and interrelationships and explore their implications in hazard assessment and modelling. This was achieved through regional geomorphic analysis focusing on catchment morphometry and the structure of the drainage networks. This analysis revealed the strong influence and interactions between frequent landslides / debris-flows, glaciers, orographic precipitation and spatially-variable uplift rates on the landscape evolution of the western Southern Alps, which supports the need for hazard assessment approaches incorporating the interrelationships between different processes and accounting for potential event cascades. The second and third objectives were to assess the regional susceptibility to rainfall-generated shallow landslides and river floods respectively, as these phenomena are most often responsible for extensive damage to property and infrastructure, injury, and loss of lives in mountainous environments. To achieve these objectives a series of GIS-based models was developed, applied and evaluated in the western Southern Alps. Evaluation results based on historical records indicated that the susceptibility assessment of shallow landslides and river floods using the proposed GIS-based models is feasible. The output from the landslide model delineates the regional spatial variation of shallow landslide susceptibility and potential runout zones while the results from the flood modelling illustrate the hydrologic response of major ungauged catchments in the study area and identify flood-prone areas. Both outputs provide critical insights for land-use planning. Finally, a multi-hazard analysis approach was developed by combining the findings from the previous objectives based on the concepts of interaction and emergent properties (cascade effects) inherent in complex systems. The integrated analysis of shallow landslides, river floods and expected ground shaking from a M8 plate-boundary fault (Alpine fault) earthquake revealed the areas with the highest and lowest total susceptibilities. Areas characterized by the highest total susceptibility require to be prioritized in terms of hazard mitigation, and areas with very low total susceptibility may be suitable locations for future development. This doctoral research project contributes to the field of hazard research, and particularly to geomorphic hazard analyses in highly dynamic environments such as tectonically active mountains, aiming to inform land-use planning in the context of sustainable hazard mitigation.

The Cogne magnetite deposit (Western Alps, Italy) is the largest in a series of apatite and sulphide-free magnetite orebodies that are hosted in serpentinites belonging to western Alpine ophiolitic units. The nearly endmember composition of magnetite, which is unusual for an ultramafic setting, and the relatively high tonnage of the deposit (18 ∙ 10^6 tons at 45-50 wt% Fe) make Cogne an intriguing case study to explore magnetite-forming processes in ophiolites. The Cogne magnetite shows variable textures, including nodular ores, veins and fine-grained disseminations in serpentinites after mantle peridotites and totally serpentinized melt-impregnated peridotites (troctolites). An increase in Co/Ni ratio from magnetite-poor serpentinized peridotites (0.05) to nodular ores (>1) is observed. Trace element analyses of magnetite from different sites and lithologies by laser ablation inductively coupled plasma mass spectrometry indicate that magnetites have typically hydrothermal compositions, characterized by high Mg and Mn (median values up to ~24100 and ~5000 ppm, respectively), and low Cr, Ti and V (median values up to ~30, ~570 and ~60 ppm, respectively). Moreover, the variations in trace element compositions distinguish magnetite that has hydrothermal fluid-controlled composition [highest (Mg, Mn, Co, Zn)/Ni ratios] from magnetite whose composition is affected by host-rock chemistry (highest Ni ± Ti ± V). U-Th-Pb dating of magnetite-associated uraninite constrains the formation of the deposit to the Late Jurassic (ca. 150 Ma), during an advanced stage of the opening of the Alpine Tethys. Thermodynamic modelling of fluid-rock interactions indicates that fluids produced by seawater–peridotite or seawater–Fe-gabbro are not sufficiently Fe-rich to account for the formation of the Cogne deposit. This suggests that fractionation processes such as phase separation were critical to generate hydrothermal fluids capable to precipitate large amounts of magnetite in various types of ultramafic host-rocks. The oceanic setting and geochemical and mineralogical similarities with some modern ultramafic-hosted volcanogenic massive sulphide deposits on mid-ocean ridges suggest that the exposed mineralized section at Cogne may represent the deep segment of a seafloor, high-temperature (~300–400°C) hydrothermal system. The occurrence of similar magnetite enrichments in present-day oceanic settings could contribute to explain the presence of significant magnetic anomalies centred on active and inactive ultramafic-hosted hydrothermal fields.
Il giacimento di magnetite di Cogne (Alpi Occidentali, Italia) è il più grande tra le mineralizzazioni a magnetite prive di apatite e solfuri che sono ospitate nelle serpentiniti appartenenti alle unità ofiolitiche delle Alpi Occidentali. La magnetite di composizione prossima al termine puro, insolita in rocce ultramafiche, e il tonnellaggio significativo del giacimento (18 ∙ 10^6 tonnellate con concentrazioni di Fe del 45-50% in peso), rendono Cogne un interessante caso studio per indagare i processi responsabili della formazione di magnetite nelle ofioliti. La magnetite di Cogne si presenta come minerale nodulare, vene e disseminazioni in serpentiniti derivanti da peridotiti di mantello e in peridotiti impregnate da fuso (troctoliti) totalmente serpentinizzate. Il rapporto Co/Ni aumenta a partire dalle serpentiniti povere in magnetite (0.05) fino al minerale nodulare (>1). L'analisi degli elementi in traccia nella magnetite proveniente da differenti siti e litologie, ottenuta tramite laser ablation inductively coupled plasma mass spectrometry, indica che la magnetite ha una composizione tipicamente idrotermale, caratterizzata da alte concentrazioni di Mg e Mn (valori mediani fino a ~24100 e ~5000 ppm, rispettivamente) e bassi Cr, Ti e V (valori mediani fino a ~30, ~570 e ~60 ppm, rispettivamente). Inoltre, le variazioni nel contenuto di elementi in traccia distinguono la magnetite che ha una composizione controllata dal fluido idrotermale [alti rapporti (Mg, Mn, Co, Zn)/Ni] dalla magnetite la cui composizione risente della geochimica della roccia incassante (alti Ni ± Ti ± V). La datazione radiometrica con il metodo U-Th-Pb dell'uraninite associata alla magnetite vincola l'età della formazione del giacimento al Giurassico superiore (circa 150 Ma), durante uno stadio avanzato dell'apertura della Tetide alpina. La modellazione termodinamica delle interazioni fluido-roccia indica che i fluidi risultanti dalle reazioni acqua marina-peridotite e acqua marina-Fe-gabbro non sono sufficientemente ricchi in Fe per generare il giacimento di Cogne. Ciò suggerisce che processi di frazionamento, come la separazione di fase, furono di cruciale importanza per produrre fluidi idrotermali in grado di precipitare grandi quantità di magnetite in varie tipologie di rocce incassanti ultramafiche. Il contesto oceanico e le somiglianze geochimiche e mineralogiche con alcuni moderni depositi vucanogenici a solfuri massivi di dorsale oceanica ospitati in ultramafiti suggeriscono che la sezione mineralizzata di Cogne possa rappresentare il segmento profondo di un sistema idrotermale di fondale oceanico di alta temperatura (~300-400°C). La presenza di analoghe concentrazioni di magnetite nella litosfera oceanica attuale potrebbe contribuire a spiegare l'esistenza di significative anomalie magnetiche situate in corrispondenza di sistemi idrotermali idrotermali sia attivi che inattivi impostati su rocce ultramafiche.

2012/2013
Small-size ground-based telescopes can effectively be used to look for transiting rocky planets around nearby low-mass M stars using the photometric transit method. Since 2008, a consortium of the Astrophysical Observatory of Torino (OATo-INAF) and the Astronomical Observatory of the Autonomous Region of Aosta Valley (OAVdA) have been preparing for the long-term photometric survey APACHE (A PAthway toward the Characterization of Habitable Earths), aimed at finding transiting small-size planets around thousands of nearby early and mid-M dwarfs. APACHE uses an array of five dedicated and identical 40-cm Ritchey-Chretien telescopes and its routine science operations started at the beginning of summer 2012. Here I present the results of the `pilot study', a year-long photometric monitoring campaign of a sample of 23 nearby dM stars, and of the APACHE survey first year data. In these studies, I set out to (i) demonstrate the sensitivity to > 2 Rearth transiting planets with periods of up to a few days around our programme stars, through a two-fold approach that combines a characterization of the statistical noise properties of our photometry with the determination of transit detection probabilities via simulations; and (ii), where possible, improves our knowledge of some astrophysical properties (e.g. activity, rotation) of our targets by combining our differential photometric measurements with spectroscopic information from the long-term programme GAPS with the HARPS-N spectrograph on the Telescopio Nazionale Galileo. Furthermore, cool M dwarfs within a few tens of parsecs from the Sun are becoming the focus of dedicated observational programs in the realm of exoplanet astrophysics that will make use of astrometric measurements. I present numerical simulations to gauge the Gaia potential for precision astrometry of exoplanets orbiting a sample of known dM stars within ~ 30 pc from the Sun. I then investigate some aspects of the synergy between the astrometric data expected from the Gaia mission on nearby M dwarfs and the APACHE program.
XXV Ciclo
1985

PhD Thesis about the study and morpho-evolution analysis of Mont de la Saxe landslide by geomorphometry and geomorphology approach on Terrestrial Laser Scanner dataset. Detailed analysis of DTMs and a discussion of their various applications and derived geothematic/geomorphometric products and application is a recent goal become necessary due to the high volume of “3D” data.We are in an historical moment where “coarse” 3D representations (10 to 50 m) are overtaken by the “high resolution” models (1cm to 1m). These new methodologies and techniques ( Structure for Motion techniques) are useful to define geomorphological and morphodynamic models of study areas.Now it's important to work in integrated methods for geomorphological and models analysis and hazard motion maps in order to define the real effectiveness of Landform definition and modification of the present topography through high-resolution techniques , also for reconstructing recent geodynamic history of the area.The goal is to define data, methods and elaboration useful to represent as best as possible, out Morphometric dynamism analysis.

Thesis: S.M., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 101-104).
We developed a (almost) fully automatic earthquake detection and location method to study seismicity on regional scales based on array-processing techniques. We combined the beam-formed network response with template matched-filtering to enhance detection capabilities. We applied our method to the study of the seismicity of the Western Alps, and we claim that it can be applied in many different contexts to quickly get high quality earthquake catalogs. The study of the seismicity of the Western Alps revealed continuous seismic activity, organized into background and strongly clustered seismicity. We detected 9,018 seismic events from August 2012 to August 2013, outperforming the reference catalog that accounts for 1,698 events in the same period. Comparisons between our catalog and reference catalogs/ studies show that we retrieve well the main features of the region. We also studied source parameter differences between background seismicity earthquakes and clustered seismicity earthquakes. Using spectral ratios of collocated events, we measured the seismic moments Mo and corner frequencies f, for earthquakes detected with a subset of 11 template events. We found that background seismicity earthquakes follow the scaling law ... usually associated with self-similar earthquakes, whereas clustered earthquakes strongly deviate from this scaling law. Our observations show that the corner frequencies exhibit little dependency on the seismic moments, following a scaling law around ... We conclude that, in our study region, the degree of clustering seems to be driven by the rupture mechanism.
by Eric Beaucé.
S.M.

Soils formed from ultramafic rocks are normally by pH values close to neutrality, a high base status and are usually rich in Mg, Fe and heavy metals. The low Ca/Mg ratio and the high heavy metal content could cause toxic effects in the biological communities. Plant communities, in particular, are usually different from nearby areas with different substrates and rich in endemisms and adapted species and subspecies. Despite their great environmental and ecological interest, pedological and ecological properties of mountain or boreal soils developed on similar substrates have seldom been studied worldwide. 198 soil pits (associated with phytosociological surveys) have been opened and analyzed in the ophiolitic area of Mont Avic Natural Park (Val d’Aosta, Western Alps, Italy), beween 900 and 2900 m above see level. Soils formed from ultramafic, mafic rocks and calcschists have been observed, in order to recognize the most ecologically important soil factors. The results show that soil properties are related with altitude and slope aspect in forest habitats, while the effect of substrate becomes important above timberline. Strong leaching in forest soils, related to high acidity and to the podzolization process, decrease the total and bioavailable heavy metal contents, above the treeline pedogenic and geomorphic processes release and accumulate large quantities of potentially hazardous trace elements. The plant communities strictly depend on the edaphic properties above the treeline, while in the forest habitats the differences caused by substrate are less discernible. Microbial and microarthropodal communities suffer stress caused by heavy metals in forest soils, while at the alpine level non significant statistical or ecological correlation are visible. Heavy metals (Ni, in particular) are the most important edaphic properties in differentiating plant communities on different substrata, while the Ca/Mg ratio (usually considered the most influencing soil properties on ultramafic soils) has no particular ecological effect.

Understanding the controls on the distribution and type of hydrothermal venting in modern oceanic spreading environments is key to developing tools for exploration and understanding the metallogeny of ancient massive sulfide deposits. Compared to mid-ocean ridges, subduction zones are characterized by additional tectonic complexities, including arc-ridge collisions, arc rotations, pre-existing structures, and variable distances to the arc. This thesis addresses the question, “How do tectonic complexities associated with subduction influence the structure and volcanic evolution of a back-arc basin, and how do they affect the distribution and type of hydrothermal venting?” A multi-scaled approach was used to address this question in the nascent back-arc region of the New Hebrides and in the more advanced stages of opening of the Mariana back-arc basin. In the New Hebrides, an arc-ridge collision segmented the volcanic front and affected the southern and northern back-arc regions in different ways. In the southern Coriolis Troughs (CT), voluminous eruptions are closely linked to the ridge collision, forming a large shield volcano in the near-arc region (Nifonea Volcano). The caldera-hosted eruptions produced high-temperature but short-lived magmatic-hydrothermal activity restricted to the shield volcano. In the northern Jean Charcot Troughs (JCT), ridge collision caused a reversal in the rotation of the arc, reducing extension in the south and increasing extension in the north. Unlike the CT, extension in the JCT is strongly affected by pre-existing structures, which form irregular widely-spaced grabens and volcanic ridges and magmatism in the central part of the back-arc. Here, hydrothermal venting is focused along deeply penetrating faults, associated with widespread tectonic extension. Detailed studies of the mineralogy and geochemistry of the ore and alteration at the Tinakula deposit reveal that massive sulfide accumulation in the region dominated by tectonic extension is characterized by longer-lived, lower-temperature venting than at Nifonea. Hydrothermal activity in the JCT at Tinakula is dominated by (1) long-lived heat from an underlying magma source; (2) fluid circulation along a fissure with long-lived or reactivated permeability; (3) enrichment in fluid-mobile elements such as Ba that are transported at low temperature; (4) mixing of cold seawater with hydrothermal fluids within the permeable volcaniclastic substrate and at the seafloor; (5) water depth controls on maximum hydrothermal vent temperatures; and (6) reduced permeability of the host volcaniclastic succession at the site of mineralization caused by precipitation of alteration minerals and sulfates, focusing fluid flow. The different styles of volcanic and hydrothermal activity closely resemble those of mid-ocean ridge environments in areas that are dominated by tectonic rather than magmatic extension. A comparison with the more advanced stages of rifting and segmentation of the Mariana back-arc demonstrates that Mid-Ocean Ridge (MOR)-type structural and magmatic controls on hydrothermal activity are important during all stages of back-arc basin evolution. This work highlights the diversity of volcanic eruption styles and hydrothermal venting from the earliest stages of back-arc rifting to the advanced stages of basin opening and shows that processes normally associated with MOR-type spreading are directly analogous to back-arc basin systems. However, additional tectonic complexities (e.g., ridge-arc collisions) have a major impact on the location and type of magmatic and hydrothermal activity at back-arc spreading centers, with important implications for understanding ancient volcanic-hosted massive sulfide deposits that mainly formed in back-arc basins.

This Ph.D. Thesis is intended as a multidisciplinary research work on the Zermatt-Saas (Western Alps) serpentinites and associated carbonated rocks. The main goal of this thesis is to decipher the carbonation processes involving serpentinites and the concomitant element mobilization, while also attempting at defining the serpentinites’ subduction-to emplacement- evolution. To address these aims, field work activities, spanning from acquisition of macro-structural data to sampling of representative lithologies, and detailed petrography were carried out on a sequence of serpentinites, carbonated serpentinites and listvenites from the Mount Avic Massif (Zermatt-Saas Zone, Aosta Valley, Western Alps) and neighbouring area of Vercoche. On the selected data set a wide range of geochemical analyses were carried out not only to define the bulk rock compositions but also to trace, by means of EMPA, LA-ICP-MS, µ-Raman applied to microstructurally selected sites, variations and chemical gradients of elements relevant for this study (i.e., redox sensitive elements such as C and S, but also major and trace elements). In order to decipher the subduction-to emplacement evolution of serpentinites, detailed petrology and geochemical investigations were applied to unravel the signature of fluid-rock interaction during subduction. Moreover, by integrating structural, petrological and geochemical results with thermodynamic modelling, the P-T-(t)-d paths representative of the evolution of serpentinites were obtained. Serpentinites from the Mount Avic massif retain structural and geochemical fingerprints of a long-lived evolution. Mantle (and oceanic) relics can be easily distinguished from the prograde metamorphic recrystallized phases: clinopyroxene, olivine and Cr-cores within magnetite preserve their mantle texture and, in part, their original composition. These features suggest that the serpentinites derived from mantle peridotites, mainly represented by harzburgite, that were exhumed and exposed at the Mesozoic Tethyan Ocean floor prior to alpine subduction. In this context, mantle peridotites likely interacted with seawater that enhanced the pervasive serpentinization and caused the progressive replacement of olivine and pyroxene by serpentine-lizardite with “mesh” and “bastite” textures, respectively. Bulk rock trace and REE patterns obtained on serpentinites as well as in situ concentrations of trace and rare Earth elements on serpentinite forming minerals suggest that the Mount Avic serpentinite still largely retains its oceanic geochemical signature despite intense Alpine subduction metamorphism. At the onset of the Tethyan ocean closure due to convergence between the Paleo-Europe and Paleo-Adria continental plates, the serpentinites underwent prograde metamorphism during Alpine subduction. Different mineral assemblages marking superposed fabrics are variably retained within the serpentinites. In foliated serpentinites, despite the pervasive overprinting provided by S2 foliation, relics of pre-D2 structures are preserved. In particular, Ti-chondrodite, clinopyroxene, olivine, antigorite and magnetite-bearing assemblage, marking pre-to early D1 metamorphic stage, attest for peak UHP conditions attained at Pmin = 2.8-2.85 GPa for T = 600-645°C. The crystallization of Ti-clinohumite along Ti-Chondrodite rims during D1 deformation stage, suggest that serpentinites, after reaching the peak metamorphic conditions, experienced a quasi-isothermal decompression from pre-to early D1 conditions, still within typical Alpine subduction geothermal gradient of 6.5-8.0°C/km. Detailed LA-ICP-MS analyses carried out on clinopyroxene, humite, olivine and serpentine minerals showed that Zr, Ti, Hf and Sr elements were mobilized during the transition from the mantle to the subduction environment. The lizardite-antigorite transition phase was accompanied by Sr, Cl and alkalis release in an open system which likely occurred in early oceanic stage and was complete at T>390°C. Differently, the partitioning of Ti, Hf, Zr between clinopyroxenes and Ti-clinohumite supports the idea that mantle clinopyroxene breakdown and successive high-pressure clinopyroxene and Ti-clinohumite crystallization took place in a closed space environment. During the retrograde path, the serpentinites re-equilibrated at Pmin = 2.4-1.7 GPa and T = 510-610°C; at these conditions, D2 deformation stage caused the crystallization of Ti-clinohumite and Ti-chondrodite in textural equilibrium along S2 foliation planes together with olivine, clinopyroxene, antigorite and magnetite. Locally, veins and shear zones marked by clinopyroxene, olivine and Ti-clinohumite postdating S2 foliation, attest that during syn-to-post-D2 exhumation hot geothermal gradients (20°C/km), indicative of greenschist facies re-equilibration, were reached. These new results provide unprecedented insights on the geochemistry, petrology and P-T peak conditions at which the Mount Avic serpentinites (Zermatt-Saas Zone) re-equilibrated during the alpine subduction and exhumation and allow for a comparison with the UHP serpentinite slices belonging to the Cignana Lake and Valtournanche (Créton) units of the Zermatt-Saas Zone north of the Aosta-Ranzola fault. Serpentinites of the Mount Avic massif underwent carbonation likely during their retrograde evolution, which caused the formation of carbonate-bearing lithologies comprising carbonated serpentinites and listvenites. These metasomatized rocks represent valuable records of CO2 sequestration enhanced by fluid-rock interaction mainly focused along serpentinite shear zones. Fluid-rock interaction, responsible for the genesis and evolution of carbonated serpentinites and listvenites, is attested not only by macro-scale, but also by micro-scale structural features. Petrographic observations suggest that the Mount Avic serpentinites, after having experienced D2 and post-D2 stages at high-pressure and greenschist facies conditions, respectively, likely reached lower T and P conditions while being progressively infiltrated by CO2-rich fluids. Ti-clinohumite relics within partially carbonated serpentinite, presently pseudomorphically replaced by olivine and ilmenite and the presence of Ti-clinohumite not yet replaced by olivine and ilmenite, but still in equilibrium with olivine and clinopyroxene, constrain the P-T conditions attained by serpentinites prior to carbonation and supports the hypothesis that the initiation of carbonation of serpentinites took place just before, or even simultaneously with, Ti-Chu breakdown, likely during the early retrograde greenschist facies re-equilibration of serpentinites. Based on thermodynamic modelling, the equilibrium P-T conditions for listvenitization are constrained at 0.3GPa and 300°C and the minimum amount of CO2, required for the infiltrating fluid to produce the listvenite mineral assemblage is XCO2 (min) = 0.02. Upon fluxing of CO2-bearing fluids, carbonated serpentinites are progressively transformed into listvenites as evidenced by the crystallization of quartz as a product of talc breakdown, making-up matrices together with aggregates of magnesite and fuchsite fibres, attesting for the presence of K in the infiltrating fluid. The presence of Cr-spinel, magnetite and mesh texture relics in listvenites is regarded as a clear petrogenetic relationship with serpentinites. Electrolytic fluid infiltration modelling suggests that the Mount Avic listvenites attained low T and very high fluid/rock (F/R) ratios: prolonged fluid infiltration at low temperatures causes the solubility of quartz to increases to the point that part of the SiO2 present in the listvenite matrix is dissolved in the fluid and then precipitated in veins which diffusively characterize listvenites. Furthermore, with progressive fluid infiltration, MgO/SiO2 ratio increases up to a point (at F/R between 6 and 7 and ca. 38-42% CO2), where the infiltrating fluid removes CO2 from the system. The dissolution of carbonate and concurrent silicification of the altered rock, once all the talc has reacted with CO2,aq to form magnesite and quartz, is thermodynamically predicted at T ≤ 200°C under prolonged fluid influx. The petrographic investigation on sulfides allowed to depict a chemical gradient: from serpentinites to listvenites the sulfide abundance decreases in accordance with the decrease in S (wt%) abundance observed by measured bulk rock values as well as predicted by thermodynamic models. In serpentinites the sulfur-poor heazlewoodite and pentlandite are associated with awaruite Ni-Fe alloy, in accordance with the observations that Fe-Ni alloys tend to be preferentially found in peridotites that have been only partially serpentinized. Passing to carbonated serpentinites the sulfide abundances progressively decreases and their composition changes. In particular, in carbonated serpentinites, neither Ni-Fe alloys nor pentlandite are identified while sulfur-poor heazlewoodite forms aggregates with sulfur-rich Co-bearing NiS. Finally, the transition from carbonated serpentinites to listvenites is marked by the limited presence of small-grained pentlandite and haezlewoodite, and absence of alloys while magnetite is progressively replaced by hematite displaying martite texture. The pseudomorphic martitic replacement of magnetite by hematite due to magnetite oxidation, observed exclusively within listvenites, and not in carbonated serpentinites where sulfides are still present, suggests that once the sulfides are completely dissolved, the fluid continues to evolve towards high oxygen fugacity (fO2) conditions. Prior to fluid infiltration, serpentinites attained very low fO2 and fS2 as constrained by the presence of Ni-Fe alloys (i.e., awaruite) and sulfur-poor assemblage (heazlewoodite and pentlandite) in magnetite-bearing antigorite-serpentinites. Within the successive steps of fluid infiltration, the redox conditions of the system changed: the passage from serpentinites to carbonated serpentinites is marked by more oxidising conditions, by a marked increase in Fe3+/Fetotal ratios and by the stability of heazlewoodite with sulfur-rich Co-bearing NiS (millerite). The transition from carbonated serpentinites to listvenites is marked by the consumption of sulfides and the concomitant constant increase in fO2. It is likely that sulfides in the Mount Avic carbonated serpentinites and listvenites were dissolved by sulfate-dominant fluids in oxidising rocks. Listvenites are predicted to be stable at high-fO2, above the MH (magnetite-hematite) buffer where hematite is expected to be stable, consistently with petrographic observations of pseudomorphic replacement of magnetite by martite-textured hematite. Interestingly, nickel shows high mobility during the carbonation process, passing from being retained in sulfides and alloys in serpentinites (awaruite, pentlandite and heazlewoodite) to form rare millerite in carbonated serpentinite and listvenites and to be concentrated in trevoritic cores within spinels in listvenites. The comparison between serpentinites and listvenites by means of trace elements allows to evaluate that the compatible elements (V, Sc, Zn, Cr, Ni and Cu) were redistributed between the two rock types suggesting a mainly closed-system behaviour during listvenitization and internal cycling of such components from the host minerals in the serpentinite. Differently, higher incompatible (K, Rb, Sr, Ba Nb) and fluid mobile elements concentrations in listvenites suggest that these elements were sourced externally from the serpentinites. Moreover, Co zoning within sulfides in carbonated serpentinites may reflect the interaction of the host rock with saline fluids, whose provenance could be attributed to dehydration reactions affecting serpentinites or to fluids released upon metasediments devolatilization. Based on stable carbon and oxygen isotope compositions of magnesite, it is proposed that the CO2-rich fluids promoting serpentinite carbonation derived from devolatilization of metasedimentary rocks in the subduction zone while metaperidotites were already facing early exhumation conditions during their early retrograde evolution. During syn-to-post D2 early retrograde path serpentinites experienced strain conditions favourable for the formation of shear zones; such structures served as preferred pathways for CO2-rich fluid circulation and enhanced the carbonation of serpentinites and the formation of listvenites. Only in Oligocene times, when the post-emplacement tectonics was active, the rheological contrast between serpentinites and listvenites worked as weakness surface that facilitated the nucleation of the normal fault along which listvenites and carbonated serpentinite crop out. The combination of field, macro and micro-structural, petrological and geochemical data with thermodynamic modelling, provide new and unprecedented insights 1) on the P-T-(t)-d history of the serpentinites from the Mount Avic: this represents the first attempt at defining a P-T stability field for the Zermatt-Saas serpentinites south of the Aosta-Ranzola fault in the Western Alps; 2) on the interaction of CO2-bearing fluids with serpentinites to form carbonated serpentinites and listvenites and how this process controls element mobilization, variations of redox sensitive elements and carbonation.

The uplift history of the Tibetan Plateau is poorly constrained in part due to its complex and extended tectonic history. This study uses basin analysis, stable isotope analysis, magnetostratigraphy, detrital zircon U-Pb dating, and paleoaltimetry, and frequency analysis to reconstruct the tectonic, spatial, and environmental evolution of the Zhada basin in southwestern Tibet since the late Miocene. The Zhada Formation, which occupies the Zhada basin and consists of ~ 850 m of fluvial, alluvial fan, eolian, and lacustrine sediments, is undeformed and lies in angular unconformity above Tethyan sedimentary sequence strata. The most negative Miocene δ¹⁸Opsw (paleo-surface water) values reconstructed from aquatic gastropods are significantly more negative than the most negative modern δ¹⁸O(sw) (surface water) values. In the absence of any known climate change which would have produced this difference, we interpret it as indicating a decrease in elevation in the catchment between the late Miocene and the present. Basin analysis indicates that the decrease in elevation was accomplished by two low-angle detachment faults which root beneath the Zhada basin and exhume mid-crustal rocks. This exhumation results from ongoing arc-parallel extension and provides accommodation for Zhada basin fill. Sequence stratigraphy shows that the basin evolved from an overfilled to an underfilled basin but that further evolution was truncated by an abrupt return to overfilled, incising conditions. This evolution is linked to progressive damming of the paleo-Sutlej River. During the underfilled portion of basin evolution, depositional environments were strongly influenced by Milancovitch cyclicity: particularly at the precession and eccentricity frequencies.

In questa tesi si applica un’indagine multidisciplinare guidata dall’analisi strutturale per studiare l’evoluzione tettono-metamorfica di rocce ultrafemiche appartenenti a unità ofiolitiche affiorati nella parte occidentale delle Alpi o intrappolate nel basamento prealpino durante la collisione varisica. L’analisi si focalizza sulla Zona Zermatt-Saas (ZSZ) in alta Valtournanche (AO), al contatto di questa zona e la parte esterna del bordo meridionale della Zona Sesia Lanzo nella Valli del Tesso e del Tessuolo (TO) e nel Massiccio dell’Argentera, in alta Valle Gesso (CN). Le prime due aree si trovano nella Zona Piemontese che include unità derivanti da litosfera oceanica ristrutturate durante la convergenza alpina. L’ultima zona è situata nei Massicci Cristallini Esterni delle Alpi che includono in prevalenza rocce di origine continentale con scarsi relitti di rocce di probabile origine oceanica, ristrutturate e pervasivamente riequilibrate durante il ciclo Varisico. L’analisi meso- e micro-strutturale è stata integrata dall’analisi minero-chimica, dalle stime termo-barometriche e dalla modellazione petrologica delle associazioni di minerali metamorfici all’equilibrio, dalla datazione radiometrica dei fabric e dall’analisi geochimica di micro-domini selezionati. Le serpentinite di Créton in Valtournanche sono state interpretate come una scaglia di litosfera originariamente adiacente alla dorsale medio oceanica della Tetide Alpina, metasomatizzata e percolata da Fe- e Mg-gabbri, che ha raggiunto condizioni di ultra alta pressione (UHP) durante la convergenza Alpina, come registrato dalle associazioni a Ti-chondrodite e Ti-clinohumite, precedentemente allo sviluppo della foliazione di alta pressione, dominante a scala regionale, e datata a 60-70 Ma. Il paragone delle condizioni metamorfiche registrate con un modello numerico di subduzione Alpina ha convalidato l’evoluzione ricostruita e ha permesso di restaurarne la posizione nel sistema di subduzione in funzione dell’evoluzione termo-barica dedotta per queste rocce. La serpentinite di Gias Vej in Valle del Tesso ha registrato un’evoluzione strutturale comune alle rocce ofiolitiche e continentali ad essa adiacenti e ha raggiunto un picco metamorfico eclogitico, come testimoniato dalle paragenesi di alta pressione a Ti-clinohumite, durante la subduzione Alpina. Presso il Lago Brocan dell’alta valle Gesso in Argentera, boudins di serpentinite e diopsidite, associati a boudins di anfibolite e marmi, si trovano avvolti e allineati dalla foliazione migmatitica regionale di età tardo-Varisica. Essi verosimilmente rappresentano, analogamente alle rocce Alpine studiate, i relitti della sutura dell’oceano Reico, riequilibrati in condizioni di alta temperatura-bassa pressione e trasposti durante la collisione Varisica ed il successivo collasso tardo orogenico. In conclusione, i risultati ottenuti mostrano come l’utilizzo integrato di tecniche laboratoriali e modellistiche, basate su un solido lavoro di terreno, possano descrivere e individuare unità tettono-metamorfiche nei domini di affinità oceanica in catene montuose risultanti da uno o più cicli di Wilson e dare un contributo alla ricostruzione delle loro evoluzione geodinamica, anche quando scaglie di crosta oceanica sono frammentate e disperse nella cicatrice profonda di un’antica sutura continentale.
The tectono-metamorphic evolution of serpentinites and associated rocks has been investigated in the Alpine ophiolitic Piemontese Zone (PZ) – in the Zermatt-Saas Zone (ZSZ) and near the Sesia-Lanzo Zone (SLZ) rim – and in the Variscan migmatites of the Argentera External Crystalline Massif (ECM). Materials selected for laboratory work contain sequences of meso- and microstructural imprints containing parts of the tectonic evolution of both mono- and poly-orogenic environments (in our case the Piemontese Zone in the Penninic of the Western Alps and the Argentera EMC – Provençal domain of the Alpine collisional front at the Alpine belt termination within the Western Mediterranean). In the Zermatt-Saas Zone serpentinite of Valtournanche, meso- and microstructural analyses have been coupled with petrological investigation, geochemistry, and radiometric dating. In Valtournanche, Créton serpentinite has been interpreted as a slice of mid-ocean ridge lithosphere, affected by gabbroic percolation and hydrothermalism, deeply involved in the Alpine subduction complex, reaching UHP conditions (2.9-3.3 GPa and 600-630 °C) prior to be exhumed at HP conditions 60-70 Ma and incorporated in a mix of slices of oceanic material of heterogeneous origin and metamorphic evolution. Gias Vej serpentinite registered Eclogite facies conditions and was coupled with slices of continental material at the southern border of the Sesia Lanzo Zone before the record of Pmax conditions. At Lake Brocan in Valle Gesso, remnants of serpentinised spinel lherzolite and diopsidite are suggested to represent a most probable vestigial suture zone of the Rheic Ocean in the External Crystalline Massif of Argentera; this relict survived repeated transpositions and dismembering during migmatisation of the deep Variscan crust related to Variscan continental collision. The obtained results indicate that investigation of ultramafic rocks by a structure-driven multidisciplinary approach, can unravel the most complete memory of the divergent and convergent tectonic evolution of old oceans. Similar investigation strategies of laboratory procedures, based on solid structural fieldwork, may more diffusely support circumscription of tectonic units in ocean-derived sequences and contribute to redefine their translational tectonic trajectories during mountain-building processes.

The dynamic upper zone of the Earth, particularly at subduction zones, is responsible for volcanism, earthquakes and other natural hazards, large-scale elemental cycles, and the formation of economically viable mineral deposits. However, our knowledge of processes during subduction is hindered by the challenge of in-situ studies and limited exhumation of (ultra)high-pressure metamorphic rocks. Topics like the stress state and fluid-rock interaction at depth are of particular interest, yet poorly understood. The aim of this thesis is to utilize and further develop the use of garnets and inclusions therein as tool of unravelling metamorphic conditions, fluid-rock interaction, deformation, stress, and strain, in systems where these are otherwise unobtainable. The ultrahigh-pressure metamorphic Lago di Cignana unit (LCU) in the Western Alps of Italy is the focal point of this thesis. This locality provides an example of fluid-rich metamorphic rocks within the subduction zone, an ideal setting to improve our combined understanding of the extent and interaction of stress state, deformation, and fluid-rock interaction. This thesis combines petrological, microstructural, geochemical, and mineralogical aspects of garnet, rutile, quartz, and zircon, to gain a better understanding of the processes at work during (ultra)high-pressure metamorphism. Five studies are presented in this work, each on a different aspect of the intersection between fluid-rock interaction, deformation, metamorphism, and stress. The first study highlights an extreme case of fluid-rock interaction in a long-lived fluid pathway. Fluids derived from dehydrating serpentinites led to high amounts of dissolution of matrix minerals, resulting in the accumulation of garnet. Quartz inclusions in garnet that grew coeval with garnet pressure solution indicate that the system was at low differential stress. The fluids that circulated through these garnetites resulted in unique microstructures that reveal a history of pressure solution, grain-boundary migration, radial fracturing, and partial replacement accommodated by infiltrating fluids. The second study focuses on the microstructural aspects of the garnet in this system, showing that the preferred shape orientation has locally recorded a change in relative stress orientation, but also that local microstructures can be correlated to this shape preferred orientation, possibly indicating complex internal stresses within the garnetite. Evidence for grain-boundary migration is observed in all studied garnet-rich lithologies in the LCU while it is almost never observed elsewhere, suggesting a unique case of interaction between garnets and a fluid. The third study focuses on zircon and coesite inclusions trapped within garnet that was fractured and sealed, aiming to understand the resetting of elastic strains in the zircon and how coesite was preserved despite this fracturing occurring in the quartz stability field. The low-pressure conditions of fracturing are corroborated by elastic geothermometry on the zircon inclusions. The fourth study approaches the same system as in the previous chapters from a perspective of trace elements. The growth of garnet is studied by their rare earth element composition, and the trace element compositions of sub-micron zircon and rutile inclusions within garnet cores is calculated based on contaminated garnet measurements. The latter allowed for Zr-in-rutile thermometry to be applied, supporting low-pressure retrograde conditions of garnet fracturing and related garnet alteration. The fifth study targets rutile rather than garnet, combining a microstructural study with inclusions and trace elements to reveal that rutile in an UHP omphacite vein formed around the HP-UHP boundary and subsequently deformed. Low-angle boundaries that formed during this deformation then acted as fast diffusion pathways for trace elements, as is revealed by atom probe tomography.
The dynamic upper zone of the Earth, particularly at subduction zones, is responsible for volcanism, earthquakes and other natural hazards, large-scale elemental cycles, and the formation of economically viable mineral deposits. However, our knowledge of processes during subduction is hindered by the challenge of in-situ studies and limited exhumation of (ultra)high-pressure metamorphic rocks. Topics like the stress state and fluid-rock interaction at depth are of particular interest, yet poorly understood. The aim of this thesis is to utilize and further develop the use of garnets and inclusions therein as tool of unravelling metamorphic conditions, fluid-rock interaction, deformation, stress, and strain, in systems where these are otherwise unobtainable. The ultrahigh-pressure metamorphic Lago di Cignana unit (LCU) in the Western Alps of Italy is the focal point of this thesis. This locality provides an example of fluid-rich metamorphic rocks within the subduction zone, an ideal setting to improve our combined understanding of the extent and interaction of stress state, deformation, and fluid-rock interaction. This thesis combines petrological, microstructural, geochemical, and mineralogical aspects of garnet, rutile, quartz, and zircon, to gain a better understanding of the processes at work during (ultra)high-pressure metamorphism. Five studies are presented in this work, each on a different aspect of the intersection between fluid-rock interaction, deformation, metamorphism, and stress. The first study highlights an extreme case of fluid-rock interaction in a long-lived fluid pathway. Fluids derived from dehydrating serpentinites led to high amounts of dissolution of matrix minerals, resulting in the accumulation of garnet. Quartz inclusions in garnet that grew coeval with garnet pressure solution indicate that the system was at low differential stress. The fluids that circulated through these garnetites resulted in unique microstructures that reveal a history of pressure solution, grain-boundary migration, radial fracturing, and partial replacement accommodated by infiltrating fluids. The second study focuses on the microstructural aspects of the garnet in this system, showing that the preferred shape orientation has locally recorded a change in relative stress orientation, but also that local microstructures can be correlated to this shape preferred orientation, possibly indicating complex internal stresses within the garnetite. Evidence for grain-boundary migration is observed in all studied garnet-rich lithologies in the LCU while it is almost never observed elsewhere, suggesting a unique case of interaction between garnets and a fluid. The third study focuses on zircon and coesite inclusions trapped within garnet that was fractured and sealed, aiming to understand the resetting of elastic strains in the zircon and how coesite was preserved despite this fracturing occurring in the quartz stability field. The low-pressure conditions of fracturing are corroborated by elastic geothermometry on the zircon inclusions. The fourth study approaches the same system as in the previous chapters from a perspective of trace elements. The growth of garnet is studied by their rare earth element composition, and the trace element compositions of sub-micron zircon and rutile inclusions within garnet cores is calculated based on contaminated garnet measurements. The latter allowed for Zr-in-rutile thermometry to be applied, supporting low-pressure retrograde conditions of garnet fracturing and related garnet alteration. The fifth study targets rutile rather than garnet, combining a microstructural study with inclusions and trace elements to reveal that rutile in an UHP omphacite vein formed around the HP-UHP boundary and subsequently deformed. Low-angle boundaries that formed during this deformation then acted as fast diffusion pathways for trace elements, as is revealed by atom probe tomography.

Les Alpes occidentales, résultat de la collision entre la plaque Eurasie et le promontoire apulien de la plaque Afrique, traversent l'Europe sur près de 1200 km. C'est l'une des chaînes de montagnes les mieux étudiées au monde, notamment par des méthodes d'imagerie géophysique. Celles-ci ont permis de réaliser plusieurs grands profils d'échelle crustale par sismique active. Parallèlement, la sismicité de magnitude relativement modérée a motivé le déploiement de réseaux sismologiques denses permettant de localiser plusieurs milliers d'évènements par an. Ces données apportent énormément d'information sur la géodynamique actuelle des Alpes et ont servi à réaliser plusieurs tomographies. L'ensemble de ces travaux permet une bonne compréhension de la chaîne, cependant des incertitudes persistent motivant la présente étude dont l'ambition est de réaliser une tomographie de la lithosphère des Alpes occidentales.Notre étude s'appuie sur plus de 791000 temps d'arrivée d'ondes P et S émises par plus de 36000 séismes locaux et enregistrées par 375 stations. Le domaine d'étude de 456x414 km2 couvre le sud-est de la France, le nord-ouest de l'Italie et la majeure partie de la Suisse. Dans ce domaine, la majorité des séismes a lieu dans les premiers 15 km de la croûte et grande partie des temps d'arrivée correspondent à des ondes réfractées au toit du manteau. Cela permet d'obtenir une résolution convenable à la fois dans la croûte et dans le toit du manteau. L'intérêt d'utiliser un grand nombre de données est double : cela assure une couverture relativement complète du domaine et améliore par la loi des grands nombres la précision du modèle déterminé. Toutefois, ce type de jeu de données nécessite un traitement adapté pour gérer les inévitables données aberrantes.La tomographie par temps d'arrivée de séismes locaux de la lithosphère a été réalisée à l'aide du code INSIGHT, développé au cours de cette étude à partir d'un code de V. Monteiller et B. Valette. Le modèle est constitué des valeurs de vP et vP/vS en chaque nœud d'une grille 3D à taille de maille constante, des localisations et des paramètres d'effets de site analogues aux « corrections statiques » de la prospection sismique. Le modèle vP a priori pour la croûte et le manteau supérieur est une fonction continue de la profondeur. Les localisations initiales des foyers ont été obtenues à l'aide du code LOCIN développé pour cette étude et permettant de déterminer une densité de probabilité par recherche sur grille. Les temps de propagation sont déterminés pour les premières arrivées en intégrant la lenteur le long des rais ; la géométrie de ceux-ci est déterminée par le maximum du gradient des temps de propagation, eux-mêmes calculés par la résolution de l'équation eikonale par différences finies. L'inversion est menée par une approche de moindres carrés non-linéaires, basée sur une description stochastique des données et des paramètres du modèle.La topographie du Moho est déduite de ce modèle de tomographie en prenant le maximum du gradient de vP obtenu entre les isovitesses 7,3 et 7,6 km/s. Cette information est complétée par des modèles issus de précédentes études. La large proportion d'ondes réfractées de notre lot de données permet un niveau de détails relativement fin. Ce modèle du Moho est ensuite introduit comme interface a priori dans un nouveau processus de tomographie dans lequel les paramètres vP et vP/vS de la croûte et du manteau sont décorrélés. La discontinuité du Moho est mieux modélisée et les ondes réfractées sont mieux déterminées. Cette approche permet par ailleurs de calculer les temps de propagation des ondes directes lorsque celles-ci arrivent après les ondes réfractées : plus de 100000 temps d'arrivées sont ainsi ajoutés aux données et la résolution dans la croûte est améliorée.Ces deux tomographies, la topographie du Moho et les localisations fines apportent de nouvelles informations sur les structures profondes des Alpes occidentales
The Alpine chain, which stretches in the middle of Europe across six countries, is probably the most studied mountain range in the world. Geology and metamorphism studies contributed for a large part to the current understanding of the geodynamics and history of this region. Since the second half of the 20th century, geophysical methods were employed to study its lithosphere and several crustal cross-sections where performed, mainly using controlled-source seismology. In parallel, dense seismic networks were also deployed in France, Italy, and Switzerland in order to study the usually low-magnitude activity of the western Alps. Over the past 25 years, these networks have permitted to locate tens of thousands of local earthquakes. In the last two decades, local or regional tomographic studies have been conducted using subsets of this data, which substantially improved our understanding of the deep structure of the Alps.Here, and based on 36,000 seismic events, 375 stations and more than 791,000 P and S-waves arrival times, we performed a tomographic study on a 456x414 km2 area covering the western Alps. Even if most of these earthquakes occurred within the first 15 km beneath surface, a large part of the data is composed of refracted-waves, letting us insight the deep structure of the crust. The interest of such a large dataset relies on the accuracy ensured by the law of large numbers, but the unavoidable presence of outliers requires a specific approach in order to handle it. The a priori earthquake locations were computed using the LOCIN algorithm developed in this study, which is basically a grid-search algorithm combined with a probabilistic approach.Tomography of the crust and upper mantle based on travel-times analysis was conducted using the INSIGHT algorithm which was developed in this study (based on a V. Monteiller and B. Valette algorithm). Our model consists of a set of vP and vP/vS values given at each node of a three-dimensional, regularly-spaced grid, which constitutes the inversion grid. Transition between crust and mantle is modelled by a continuous change in velocity, as we do not introduce any a priori information on the Moho interface. Earthquake locations and site-effect residuals at each station (analogous to "static corrections" in seismic prospecting) are also determined in the process. The forward computation of travel times in the 3D model is performed by integrating slowness along the rays, which are determined by a finite-difference resolution of the eikonal equation. Inversion is carried out using a non-linear least-squares approach based on a stochastic description of data and model. The smoothing and damping parameters are adjusted by means of L-curves analysis.The Moho topography is determined by matching two informations: (i) the maximum of the vP gradient within this preliminary tomographic model, taken in a 7.3-7.6~km/s range and (ii) information provided by previous studies to fix Moho depth in the border area of our study zone, where our model is poorly resolved. As our tomographic model relies on a large set of refracted waves, the Moho topography we build is detailed and presents interesting new insights for the western Alps. This Moho interface is then used as an a priori discontinuity in a new tomography process. Parameters within the crust and the upper mantle are then decorrelated, letting refracted-waves to be more correctly modelled. By this approach, we are able to compute not only the first- but also the second-arrival travel-time which corresponds to the direct wave in the crust for focus-station distances greater than 100-125 km. This allows us to add more than 100,000 new data to our dataset, which of course improves the resolution in the crust.Both tomographic models, the Moho topography and the earthquake relocations provides new evidences and constraints on the deep structure of the western Alps

Les eaux des sources artésiennes de Salins-les-Thermes (vallée de la Tarentaise, Savoie) sont thermales (39°C) et hyperminéralisées (19,4g/l) contenant du NaCl (14,6g/l) et du CaSO4 (3,6g/l), ainsi que de nombreux éléments mineurs et traces (Sr, Si, Br, Fe, Li, As, Rb, Mn, U…) et du CO2 (230mg/l). Cette étude a pour objectifs d’améliorer les connaissances sur le fonctionnement global du système hydrothermal en contexte alpin et plus précisément de la zone d’émergence située en fond de vallée. Une approche interdisciplinaire associant la géologie, l’hydrodynamique et l’hydrogéochimie a été menée.À partir de données bibliographiques et des reconnaissances de terrain, l’étude géologique a permis de caractériser les potentiels hydrogéologiques des formations du secteur et d’identifier la structure drainante pour l’ascension des eaux souterraines. Celle-ci se fait le long de la bordure SE du contact chevauchant divisant l’extrémité sud de l’unité de Moûtiers et recoupant la vallée de Salins au niveau des sources.L’étude hydrodynamique repose sur une analyse des variations, à long terme et lors de perturbations provoquées, des paramètres hydrodynamiques (débit et niveau d’eau) et physico-chimiques (température et conductivité) des eaux de l’ensemble des sources du site et des réservoirs de surface. Le fonctionnement hydrodynamique de la zone d’émergence consiste en un équilibre de pression entre la remontée thermominérale et les réservoirs de surface (système alluvial et réservoir de versant). Cet équilibre est sensible au système d’exploitation et contrôle la répartition des flux aux différentes émergences.Une étude des interactions eaux-roches et eaux-gaz a été menée à partir d’analyses sur les éléments majeurs, mineurs et traces des eaux et des précipités de boues, ainsi que des analyses isotopiques sur les eaux (δ18O, δ2H et δ13CCMTD), pour l’ensemble des eaux thermominérales et froides présentes sur le site. La minéralisation des eaux souterraines est acquise au sein des calcaires dolomitiques et des quartzites de l’unité de Moûtiers, en bordure des évaporites (anhydrite, gypse et halite) qui jalonnent le contact chevauchant. Le circuit souterrain atteindrait une profondeur de 2km environ, ce qui permettrait aux eaux d’acquérir une température de 60°C et de drainer du CO2 magmatique issu du socle. La faible minéralisation des eaux du système alluvial et du versant n’a pas permis de les distinguer dans les phénomènes de mélange avec les eaux thermominérales.Cette thèse apporte de nouvelles hypothèses sur la zone d’alimentation et des connaissances complémentaires sur le fonctionnement du système. Les apports sur la zone d’émergence ont permis d’évaluer la sensibilité de la ressource thermominérale et de proposer des préconisations pour la mise en place d’un forage profond aux acteurs de l’activité thermale (propriétaire et exploitant) menant un projet de développement pour une gestion durable de l’exploitation de cette ressource thermominérale
Artesian springs waters of Salins-les-Thermes (Tarentaise Valley, Savoie) are thermal (39°C) and hypermineralized (19,4g/l) with NaCl (14,6g/l), CaSO4 (3,6g/l), many minor and trace elements (Sr, Si, Br, Fe, Li, As, Rb, Mn, U…) and CO2 (230mg/l). This study aims to improve knowledge on the functioning of the hydrothermal system in alpine setting and more precisely on the springs area in the valley. An interdisciplinary approach associating geology, hydrogeochemistry, and hydrodynamic was carried out.The literature data and geological field work allowed to determine the hydrogeological potential of local rock formations and to identify the deep tectonic fault which drains thermal groundwater towards the surface. Thermal groundwater rises along the thrust fault which divides the south of the Moûtiers unit and intersects the Salins valley in the spring area.The hydrodynamic study is built on variation analysis of hydrodynamic (water level, flow rate) and of physico-chemical (temperature, conductivity) parameters in water from thermal springs and subsurface aquifers, in long-term and during induced disturbance. The hydrodynamic functioning of the springs area consists of a pressure balance between the hydrothermal upwelling and subsurface aquifers (alluvial system and valley side rocks). This relationship is sensitive to the exploitation process of the thermal water and controls mixing processus at springs.Study of water-rock and water-gas interactions was carried out from analysis of major, minor and trace elements in water and in precipitated muds in addition with isotopic analyses (δ18O, δ2H et δ13CCMTD), for deep thermal and cold shallow groundwaters. The mineralization of deep groundwater results of interaction with dolomitic limestone and quartzite of the Moûtiers unit and evaporites (anhydrite, gypsum and halite) present along thrusts. The deep circuit would reach 2km depth, allowing groundwater to acquire a temperature of 60°C and CO2 magmatic from basement. The poorly mineralized both alluvial and valley side waters have not been differenciated in mixing processes with thermal groundwaters.This thesis provides news hypotheses about the infiltration area and additional knowledge on the functioning of the hydrothermal system. Contributions on the springs area allowed to estimate the sensitivity of resource and to propose recommendations to operators of the thermal activity to develop a deep borehole for sustainable management of the resource

La chaine alpine est un domaine a croute epaissie pour lequel le deplacement impose aux limites est absorbe alternativement par deux mecanismes de raccourcissement. Le premier correspond au glissement d'ecailles crustales sur des discontinuites, le second a une deformation interne des ecailles quand les glissements se bloquent. Ce modele s'applique egalement aux deformations actuelles et permet d'integrer aussi bien les donnees sur la sismicite (mecanismes au foyer) que sur les mouvements verticaux (surrection actuelle)