The asthenosphere, a weak layer beneath the rigid lithosphere, plays a fundamental role in the operation of plate tectonics and mantle convection. While this layer is often characterized by low seismic velocity and high seismic attenuation, the global structure of the asthenosphere remains poorly understood. In this dissertation, twelve years of SS precursors reflected off the top and bottom of the asthenosphere, namely, the LAB and the 220-km discontinuity, are processed to investigate the boundaries of the asthenosphere at a global scale. Finite-frequency sensitivities are used in tomography to account for wave diffraction effects that cannot be modeled in global ray-theoretical tomography. Strong SS precursors reflected off the LAB and the 220-km discontinuity are observed across the global oceans and continents. In oceanic regions, the LAB is characterized by a large velocity drop of about 12.5%, which can be explained by 1.5%-2% partial melt in the oceanic asthenosphere. The depth of the Lithosphere Asthenosphere Boundary is about 120 km, and its average depth is independent of seafloor age. This observation supports the existence of a constant-thickness plate in the global oceans. The base of the asthenosphere is imaged at a depth of about 250 km in both oceanic and continental areas, with a velocity jump of about ∼ 7% across the interface. This finding suggests that the asthenosphere in oceanic and continental regions share the same defining mechanism. The depth perturbations of the oceanic 220-km discontinuity roughly follow the seafloor age contours. The 220-km topography is smoother beneath slower-spreading seafloors while it becomes rougher beneath faster-spreading seafloors. In addition, the roughness of the 220-km discontinuity increases rapidly with spreading rate at slow spreading seafloors, whereas the increase in roughness is much slower at fast spreading seafloors. This observation indicates that the thermal and compositional structures of seafloors formed at spreading centers may have a long-lasting impact on asthenospheric convections. In continental regions, a broad correlation is observed between the 220-km discontinuity depth structure and surface tectonics. For example, the 220-km discontinuity depth is shallower along the southern border of the Eurasian plate as well as the Pacific subduction zones. However, there is no apparent correlation between 3-D seismic wavespeed in the upper mantle and the depths of the 220-km discontinuity, indicating that secular cooling has minimum impact on the base of the asthenosphere. / Doctor of Philosophy / In classic plate tectonic theory, the outermost shell of the Earth consists of a small number of rigid plates (lithosphere) moving horizontally on the mechanically weak asthenosphere. In the classic half space cooling (HSC) model, the lithosphere is formed by gradual cooling of the hot mantle. Therefore, the thickness of the plate depends on the age of the seafloor. The problem with the HSC model is that bathymetry and heat flow measurements at old seafloors do not follow its predicted age dependence. A modified theory, called plate cooling model, can better explain those geophysical observations by assuming additional heat at the base of an oceanic plate with a constant thickness of about 125 km. However, such a constant-thickness plate has not been observed in seismology. In this thesis, the asthenosphere boundaries are imaged using a global dataset of seismic waves reflected off the Earth's internal boundaries. Strong reflections from the top of the asthenosphere are observed across all major oceans. The amplitudes of the SS precursors can be explained by 1.5%-2% of partial melt in the asthenosphere. The average boundary depths are independent of seafloor age, and this observation supports the existence of a constant-thickness plate in the global oceans with a complex origin. The 220-km discontinuity, also called the Lehmann Discontinuity, was incorporated in the Preliminary Reference Earth Model in the 1980's to represent the base of the asthenosphere. However, the presence and nature of this boundary have remained controversial, particularly in the oceanic regions. In contrast to many studies which suggest the 220-km discontinuity does not exist in the global oceans, SS precursors reflected from this interface are observed across the oceanic regions in this thesis. Furthermore, there is a positive correlation between the topography of the 220-km discontinuity and seafloor spreading rate. Specifically, the 220-km discontinuity is smoother beneath slower-spreading seafloors and much rougher beneath faster-spreading seafloors. In addition, the roughness increases faster at slowerspreading seafloors while much more gradual at faster-spreading seafloors. This indicates a close connection between seafloor spreading and mantle convections in the asthenosphere, and seafloors have permanent memories of their birth places. Different melting processes at slow and fast spreading centers produce seafloors with different physical and chemical properties, modulating convections in the asthenosphere and ultimately shaping the topography of the 220-km discontinuity. Reflections from the 220-km discontinuity are also observed across the global continental regions. In addition, the 220-km discontinuity beneath the continents is comparable to that under oceanic regions in terms of their average depth (∼ 250 km) and velocity contrast across the discontinuity (∼ 7%). In continental regions, there is a general connection between the 220-km depth structure and plate tectonics. For example, the boundary is shallower along the southern border of the Eurasian plate from the Mediterranean region to East Asia where mountain belts were formed as a result of collision between the Eurasian plate and the Nubian, Arabian and Indian plates. Depth perturbations of the 220-km discontinuity are also observed along the Pacific subduction zones including the Cascadia Subduction Zone, Peru-Chile Trench and Japan-Kuril Kamchatka Trench. In addition, depth anomalies are mapped in the interior of continents, for example, along the foothills of high topography in the interior of the Eurasian plate, which may be controlled by far-field convection associated with the convergent processes at the plate boundaries.

This thesis focuses on calculation of finite and progressive deformation in different tectonic scales using 2D numerical models with application to natural cases. Essentially, two major tectonic areas have been covered: a) salt tectonics and b) upper mantle deformation due to interaction between the lithosphere and asthenosphere. The focus in salt tectonics lies on deformation within down-built diapirs consisting of a source layer feeding a vertical stem. Three deformation regimes have been identified within the salt: (I) a squeezing channel flow underneath the overburden, (II) a corner flow underneath the stem, and (III) a pure channel flow within the stem. The results of the model show that the deformation pattern within the stem of a diapir (e.g. symmetric or asymmetric) can reveal information on different rates of salt supplies from the source layer (e.g. observed in Klodowa-diapir, Poland). Composite rock salt rheology results in strong localization and amplification of the strain along the salt layer boundaries in comparison to Newtonian rock salt. Flow and fold structures of passive marker lines are directly correlated to natural folds within a salt diapir. In case of the upper mantle, focus lies on deformation and resulting lattice preferred orientation (LPO) underneath an oceanic plate. Sensitivity of deformation and seismic anisotropy on rheology, grain size (d), temperature (T), and kinematics (v) has been investigated. The results of the model show that the mechanical lithosphere-asthenosphere boundary is strongly controlled by T and less so by v or d. A higher strain concentration within the asthenosphere (e.g. for smaller potential mantle temperatures, higher plate velocities, or smaller d) indicates a weaker coupling between the plate and the underlying mantle, which becomes stronger with the age of the plate. A Poiseuille flow within the asthenosphere, significantly affects the deformation and LPO in the upper mantle. The results of the model show, that deformation in the upper mantle at a certain distance away from the ridge depends on the absolute velocity in the asthenosphere. However, only in cases of a driving upper mantle base does the seismic anisotropy and delay times reach values within the range of natural data.

Le travail réalisé pendant cette thèse a permis d'apporter de nouvelles contraintes sur le développement et la distribution de la déformation dans le manteau supérieur et plus particulièrement au niveau des grandes limites de plaques décrochantes. Grâce à l'apport de l'expérience USArray et d'une dizaine d'années d'enregistrements sismologiques supplémentaires, nous avons pu étudier, de manière précise, les variations d'anisotropie dans le voisinage de la Faille de San Andreas. Nous avons confirmé et étendu l'observation de deux couches anisotropes sous cette limite de plaque. On y observe une première couche localisée dans la lithosphère marquant la déformation induite à la limite de plaque, et une autre, asthénosphérique, cohérente avec l'anisotropie observée loin de la faille et d'origine plus discutée. Nous avons montré que la zone de déformation associée aux failles de San Andreas, Calaveras et d'Hayward a, vraisemblablement, une largeur d'au moins 40 kilomètres en base de lithosphère, sous chacune de ces failles. Nous avons ensuite procédé à la modélisation thermomécanique (ADELI) de la migration d'une limite de plaques décrochante couplée à une modélisation du développement de fabriques cristallographiques par une approche viscoplastique auto-cohérente (VPSC). Ceci nous a permis d'y observer le développement de la déformation et les conséquences des possibles interactions entre la déformation décrochante en surface et le cisaillement en base de lithosphère dû au déplacement horizontal des plaques. Les propriétés élastiques déduites des fabriques cristallographiques modélisées montrent que de telles interactions existent et provoquent, sous la limite de plaques, une rotation des orientations cristallographiques avec la profondeur. Le signal associé à ces rotations progressives n'est toutefois pas cohérent avec la présence de deux couches d'anisotropie comme proposée sous la faille de San Andreas. Nous pensons par conséquent qu'il existe, sous la Californie, une zone de découplage entre la lithosphère et l'asthénosphère, permettant d'individualiser une déformation lithosphérique d'une déformation asthénosphérique. Nous estimons, en outre, que l'anisotropie observée dans l'asthénosphère sous la Californie ne peut être expliquée seulement par le cisaillement induit par le déplacement de la lithosphère Nord Amérique. En effet, les propriétés anisotropes obtenues par modélisation à partir d'une plaque se déplaçant dans une direction et une vitesse proche de celle de la plaque Amérique du Nord montrent qu'on ne peut espérer guère plus que quelques dixièmes de seconde de délai au bout de 10 Ma de déplacement. Les déphasages mesurés en Californie étant de l'ordre de 1,5 s, il est donc nécessaire d'invoquer la présence d'écoulements mantelliques actifs sous cette région / This work provides new constraints on the development and on the distribution of the deformation in the upper mantle and particularly beneath transform plate boundaries. USArray experiment and the remarkable increase of the dataset in California for the past ten years allowed us to scrutinize the lateral variations of the anisotropy in the vicinity of the San Andreas Fault zone. We have confirmed and increased the detection of two layers of anisotropy beneath this plate boundary. The first layer, located in the lithosphere, is related to the deformation induced at the fault, and the other one, located in the asthenosphere, is coherent with the anisotropy observed far from it, its origin is however less clear. We show that the deformation zone associated both to the San Andreas, Calaveras and Hayward Faults, is likely 40 km wide at 70 km depth. We then performed numerical thermomechanical modeling (ADELI) of the displacement of a transform plate boundary associated with the computation of the development of crystallographic fabrics using a viscoplastic self-consistent approach (VPSC). We analyzed the distribution of the deformation in the model ant looked after the possible interactions at depth between deformation caused at surface by the strike-slip dynamic of the fault and the shearing at the base of the lithosphere caused by the horizontal displacement of the plates. Elastic properties derived from the crystallographic fabrics modeled, show that such interactions exist and induce, beneath the fault zone, a progressive rotation of the crystallographic fabrics with depth. Seismological signature of these smooth rotations is however not relevant with the presence of two anisotropic layers as proposed beneath California. We thus consider that a decoupling zone exists between the lithosphere and the asthenosphere beneath the California to account for the sharp separation between a lithospheric and an asthenospheric deformation. We furthermore estimate that anisotropy observed far form the San Andreas Fault in California cannot be explained only by the drag of the asthenosphere by the North America lithosphere as proposed in our article. Indeed, we can only expect few tenths of second of splitting delay from the anisotropic properties derived from the numerical modeling of a plate moving in the same direction and in the same velocity than the North American lithosphere only for 10 Ma of displacement. As delays observed in California rather reach 1.5 s, anisotropy in this region thus requires the existence of an active asthenospheric flow to be explained.

Abstract The lithospheric conductivity in the westernmost Fennoscandia has been studied using magnetotelluric (MT) data. The western margin of Fennoscandia was significantly affected in Paleozoic by the Caledonian orogeny and later by the rifting of Laurentia and the opening of the Atlantic Ocean c. 80 Ma ago. Magnetotelluric studies have been carried out in two target areas in southern Norway and in western Fennoscandia. The first study resulted in 2-D geoelectric models of two profiles stretching from Oslo to the Norwegian coast. The interpretation suggests that the basement is in general very resistive with a few conductive upper crustal layers, representing the alum shales, and middle crustal conductors possibly imaging the remnants of the closed ocean basins. A more extensive MT study was performed within the project "Magnetotellurics in the Scandes". Measurements were carried in summers of 2011 to 2013, resulting in an array of 279 MT sites. The data allowed us to derive 2-D geoelectric models for the crust and upper mantle as well as 3-D models for the crust. The inversions revealed a resistive upper crust and a conductive lower crust, two upper crustal conductors in the Skellefteå and Kittilä districts, highly conducting alum shales in the Caledonides and a conductive upper crust beneath the Lofoten peninsula. The thickness of the lithosphere is around 200 km in the north and 300 km in the south-west. The Palaeoproterozoic lithosphere is the thickest, not the Archaean, on contrary to a generally accepted hypothesis. A better image of the lithosphere will help to evaluate the proposed mechanisms of the exhumation of the Scandinavian Mountains. The theoretical part of this study is the development of a new multi-resolution approach to 3-D electromagnetic (EM) modelling. Three-dimensional modelling of MT data requires enormous computational resources because of the huge number of data and model parameters. The development of the multi-resolution forward solver is based on the fact that a finer grid resolution is often required near the surface. On the other hand, the EM fields propagate in a diffusive manner and can be sufficiently well described on a grid that becomes coarser with depth. Tests showed that the total run time can be reduced by five times and the memory requirements by three times compared with the standard staggered grid forward solver. / Tiivistelmä Olemme tutkineet litosfäärin sähkönjohtavuutta Fennoskandian länsiosassa magnetotelluurisen (MT) menetelmän avulla. Fennoskandian länsireuna muokkautui merkittävästi paleotsooisena aikana Kaledonidien vuorijonopoimutuksessa sekä myöhemmin mesotsooisena aikana Laurentia-mantereen repeytyessä ja Atlantin valtameren syntyessä noin 80 miljoonaa vuotta sitten. MT-tutkimukset tehtiin Etelä-Norjassa ja Fennoskandian luoteisosassa. Ensimmäisessä tutkimuksessa kallioperän sähkönjohtavuutta kuvattiin kaksiulotteisilla (2-D) johtavuusmalleilla, jotka ulottuvat Oslosta Norjan rannikolle. Mallien tulkinta viittaa siihen, että maan kuori on pääosin hyvin eristävä lukuun ottamatta muutamaa kuoren ylä- ja keskiosassa olevaa johdekerrosta. Yläkuoren johteet edustavat alunaliuskeita ja keskikuoren johteet todennäköisesti suljetuissa merialtaissa syntyneitä hiilipitoisia sedimenttikerrostumia. Laajempi MT-tutkimus tehtiin ”Magnetotellurics in the Scandes” -hankkeessa. Mittauksia tehtiin 279 mittauspisteessä kesinä 2011–2013. Saadun aineiston avulla voitiin laatia 2-D inversiomallit kuoresta ja ylävaipasta sekä 3-D inversiomalli kuoresta. Tulosten mukaan täällä kuoren yläosa on eristävä kun taas kuoren alaosa on sähköä hyvin johtava. Edellisen lisäksi malleissa näkyy yläkuoren johtavat muodostumat Skellefteån ja Kittilän alueilla, korkean johtavuuden alunaliuskeet Kaledonidien alueella sekä johde Lofoottien alla. Litosfäärin paksuus on noin 200 km mittausverkon pohjoisosassa ja noin 300 km lounaassa. Tämän mukaan litosfääri on paksuin varhaisproterotsooisen litosfäärin alueella, ei arkeeisen litosfäärin alueella vastoin yleistä hypoteesia. Tutkimuksen teoreettisessa osassa kehitettiin sähkömagneettiseen mallinnukseen uusi monitasoiseen diskretisointiin perustuva menetelmä. MT-aineiston 3-D käänteisongelman ratkaisu ja siihen liittyvä suora mallintaminen vaativat suuren laskennallisen kapasiteetin, koska havaintojen ja mallin kuvaamiseen tarvittavien parametrien määrä on erittäin suuri. Moniresoluutio-algoritmi perustuu siihen, että mallin hienojakoisempaa diskretisointia tarvitaan yleensä lähellä maan pintaa kun taas syvemmälle edettäessä, sähkömagneettisen aallon diffuusin etenemisen vuoksi, malli voi olla karkeampi. Tietokonesimulaatioiden mukaan suoritusaika on viidennes ja muistitarve kolmannes verrattuna tavanomaiseen suoran laskennan ”staggered grid” -diskretisointiin.