Ocean-continent convergence and subsequent continental collision are responsible for continental growth, mountain building, and severe tectonic events including volcanic eruptions and earthquake activity. They are also key driving forces behind the extensive thermal and compositional heterogeneities at crustal and mantle depths. Active subduction along the Calabrian Arc in southern Italy and the Hellenic Arc are examples of such collisional tectonics. The first part of this thesis examines the subduction related deformations within the crust beneath the southern Apennines. By modeling regional surface wave recordings of the largest temporary deployment in the southern Apennines, a lower-crustal/upper-mantle low-velocity volume extending down to 50 km beneath the mountain chain is identified. The magnitude (~ 0.4 km/s slower) and anisotropic nature (~ 10%) of the anomaly suggest the presence of hot and partially molten emplacement that may extend into the upper-crust towards Mt. Vulture, a once active volcano. Since the Apulian basement units are deformed during the compressional and consequent extensional events, our observations favor the ``thick-skin'' tectonic growth model for the region. In the deeper mantle, active processes are thermodynamically imprinted on the depth and strength of the phase transitions. This thesis examines more than 15000 SS precursors and provides the present-day reflectivity structure and topography associated with these phase transitions. Through case studies I present ample evidence for both slab penetration into the lower mantle (beneath the Hellenic Arc, Kurile Island and South America) and slab stagnation at the bottom of the Mantle Transition Zone (beneath the Tyrrhenian Sea and eastern China). Key findings include (1) thermal anomalies (~ 200 K) at the base of the MTZ, which represent the deep source for Cenozoic European Rift Zone, Mount Etna and Mount Cameroon volcanism, (2) significant depressions (by 20-40 km) at the bottom of the Mantle Transition Zone beneath subducting slabs, (3) a strong 520-km reflector near subducting slabs, (4) a weak and elevated (15-25 km) 410-km reflector within active deformation zones, (5) strong lower mantle reflectors (~ 900 km) while slabs penetrate into the lower mantle, and (6) consistency between the topography of a 300-km reflector and an exothermic phase transformation. / Geophysics

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This thesis focuses on two different topics in seismology: imaging the global structures of the mantle transition zone discontinuities and studying the site response effects in the Atlantic and Gulf Coastal Plain. Global structures of the mantle transition zone discontinuities provide important constraints on thermal structures and dynamic processes in the mid mantle. In this thesis, global topographic structures of the 410- and 660-km discontinuities are obtained from finite-frequency tomography of SS precursors. The finite-frequency sensitivities of SS waves and precursors are calculated based on a single-scattering (Born) approximation and can be used for data selection. The new global models show a number of smaller-scale features that were absent in back-projection models. Good correlation between the mantle transition zone thickness and wave speed variations suggests dominantly thermal origins for the lateral variations in the transition zone. The high-resolution global models of the 410- and 660-km discontinuities in this thesis show strong positive correlation beneath western North America and eastern Asia subduction zones with both discontinuities occurring at greater depths. Wavespeed and anisotropy models support vertical variations in thermal structure in the mid mantle, suggesting return flows from the lower mantle occur predominantly in the vicinity of stagnant slabs and the region overlying the stagnant slabs. In oceanic regions, the two discontinuities show a weak anti-correlation, indicating the existence of a secondary global far-field return flow. The Atlantic and Gulf Coastal Plain is covered by extensive Cretaceous and Cenozoic marine sediments. In this thesis, the site response effects of sediments in the Coastal Plain region relative to the reference condition outside that region are investigated using Lg and coda spectral ratios. The high-frequency attenuation factors (kappa) in the Coastal Plain are strongly correlated with the sediment thickness. At frequencies between 0.1-2.86 Hz, the Lg spectral ratio amplitudes are modeled as functions of frequency and thickness of the sediments in the Coastal Plain. Analysis of the residuals from the stochastic ground motion prediction method suggests that incorporating the site response effects as functions of sediment thickness may improve ground motion prediction models for the Coastal Plain region. / Doctor of Philosophy / The mantle transition zone is the region in the Earth’s interior between depths of ∼410 km and ∼660 km. The structure of the mantle transition zone plays an important role in understanding temperature variations and mass exchanges in the interior of the Earth. This dissertation aims at resolving depth variations of the top and bottom boundaries of the mantle transition zone at a global scale using underside reflected seismic waves. The advanced method used here resolved stronger small-scale depth variations of the boundaries than a conventional method using the same dataset. The two mantle transition zone boundaries both occur at depths greater than the global average beneath eastern Asia and western North America where cold oceanic lithosphere subducted under the continents. This positively correlated behaviors of the two boundaries agree with a scenario where cold subducted slabs have been horizontally deflected and stagnant above the bottom boundary of the mantle transition zone while hot materials beneath the mantle transition zone flow upwards due to the stagnant slabs penetrating the bottom boundary of the mantle transition zone. This dissertation also provides an examination of the differences between response of earthquake ground shaking in the Atlantic and Gulf Coastal Plain and that outside the Coastal Plain using seismic-wave spectral ratios. Ground shaking in the Coastal Plain is found to be amplified at low frequencies and de-amplified at high frequencies relative to that outside the Coastal Plain due to the extensive marine sediments in the Coastal Plain region. The amplification and attenuation factors can be estimated from spectral ratios and are found to be strongly correlated with the sediment thickness in the Coastal Plain. The spectral ratio functions derived in this dissertation may be adopted by studies on analyzing the seismic hazard in the Central and Eastern United States.

L’évolution des assemblages pétrologiques avec l’augmentation de pression et de température est couramment perçue à l’équilibre et figée dans le temps. Le développement des sources synchrotron de rayons X permet à présent de mesurer in situ et en temps résolu les vitesses de transformations minéralogiques à haute pression (HP), haute température (HT). Cette thèse présente l’utilisation de cette technologie dans 2 contextes géologiques. (i) Le potentiel sismogène de la déstabilisation des minéraux hydratés dans les plaques en subduction est vérifié. Les cinétiques de déshydratation du talc, de la phase à 10Å et de l’antigorite ont été mesurées à HP-HT en système clos. Nous avons identifié que l’antigorite se déshydrate en passant par un stade intermédiaire. Toutes les vitesses de libération de fluides associées sont plus rapides que la déformation visqueuse des roches et sont donc compatibles avec le déclenchement de rupture. (ii) Les cinétiques de transition olivine-ringwoodite ont été déterminées dans la loupe de costabilité pour des compositions riches en Fe. Elles mettent en évidence une amorphisation partielle de l'olivine en début de transformation. Cela pourrait perturber de manière significative la vitesse des ondes sismiques lors de leur passage au niveau de la zone de transition mantellique. Par ailleurs, les temps caractéristiques de réaction et la réduction conséquente de la taille des grains, indiquent qu’une telle transition de phase induit une atténuation sismique importante. Ces résultats expérimentaux in situ HP-HT révèlent des mécanismes originaux de transition de phase et contribuent ainsi à une meilleure compréhension des modèles géodynamiques / The evolution of petrological assemblies with increasing pressure and temperature is commonly perceived at equilibrium and fixed within time. The development of X-ray synchrotron sources now enable to measure in situ, time-resolved rates of mineralogical transformations at high pressure (HP), high temperature (HT). This thesis presents the application of this technology in two geological settings. (i) The seismogenic ability of breakdown of hydrated minerals within the subducting slab is checked. The dehydration kinetics of talc, 10Å phase and antigorite were measured at HP-HT in a closed system. We have found that antigorite dehydrates through an intermediate stage. All associated rates of released fluids are faster than the viscous deformation of rocks and are therefore compatible with the trigger of rupture. (ii) The kinetics of olivineringwoodite transition were determined within the co-stability loop for Fe-rich compositions. They show a partial amorphization of olivine at the beginning of the transformation. This could significantly affect the velocities of seismic waves when crossing the mantle transition zone. Moreover, the characteristic times of this reaction and the substantial reduction in grain size, indicate that such a phase transition may induce a significant seismic attenuation. These in situ HP-HT experimental results reveal novel mechanisms of phase transition and thus contribute to a better understanding of geodynamic models