Detailed investigation of upper mantle seismic discontinuities beneath the Northern Atlantic

Seismological observations confirmed seismic discontinuities in the Earth's upper mantle, where physical parameters such as density and velocity change. The most prominent discontinuities in the upper mantle are located at ~410 and ~660 km depth. High-pressure high-temperature experiments showed that these discontinuities are associated with solid-solid phase transformations of olivine to its high-pressure/high-temperature polymorphs, which are highly sensitive to temperature and mineralogical variations. Therefore, combining the seismological observations and results of mineralogical experiments may enable us to interpret the lateral depth variation of those discontinuities, in terms of thermal and compositional state of the mantle.This thesis investigates the structure of Earth's upper mantle seismic discontinuities beneath the Northern Atlantic. Reflections of compressional and shear waves, which reflect off the underside of the seismic discontinuities, are used to infer their depth variation beneath this area (PP and SS precursors). The seismograms used in this study were collected and analyzed by using seismic array techniques. The source-receiver configuration used in this study sample the North Atlantic and surrounding regions with a number of crossing ray paths allowing for a better resolution of the discontinuities.The observed depressed 410 km discontinuity beneath the hotspots of Azores, Canaries and Cape Verdes and elevated 660 km discontinuity beneath some parts of the Northern Atlantic, lead us to make assumptions on the dynamic state of the mantle beneath this oceanic area. The results support the hypothesis of existence of a large upwelling beneath the southern part of Northern Atlantic, ponding beneath the 660 km discontinuity. From this upwelling three secondary plumes may originate, penetrate into the mantle transition zone and appear as the Azores, Canaries and Cape Verde hotspots at the surface. Moreover, recent results of mineralogical experiments are used to convert the 410 km and 660 km discontinuity depth to excess temperatures beneath the hotspots. Furthermore, the apparent absence of the 660 km discontinuity could indicate mineralogical variations or the presence of hotter material beneath the hotspots.The detected underside reflections from the 410 km discontinuity were investigated in terms of polarity behaviour compared to ones reflected off the surface. "Same" and "opposite" polarity behaviours were detected and were inspected with regard to regional or travel path dependence. The observed polarities depend on epicentral distance that cannot be explained by standard Earth models. Using Zoeppritz equations, a large number of combinations of density, compressional and shear wave velocities for both olivine and wadsleyite layers above and below the 410 km discontinuity were tested. The best fitting models were then interpreted in terms of the water and iron content in the mantle transition zone in this region and we find that an increase of water can explain the observations but a combined effect of water and iron is also possible.IIIn a third study, the effect of deformation of olivine just above the 410 km discontinuity, on polarities and amplitudes of precursors is investigated. The reflection coefficient modelling shows that the type of the deformation system such as compression or shear, has a considerable effect on the amplitude and polarity of the SS underside reflections but little effect on P waves other than amplitude variations. Furthermore, the difference in amplitude and polarity pattern between different deformation geometries allows to make assumptions on the possible deformation mechanisms above the mantle transition zone.This thesis shows that using all waveform, travel time and polarity information of underside reflections in high-resolution studies, provides a better understanding of mineralogical variations, temperature and deformation mechanisms for the mantle transition zone and in turn allows a better description of mantle dynamics.