The 8-9 million year old volcanic complex of Oxnadalur is host to large-volume basalt flows, small and large volume rhyolite ash and lava flows, and a gabbroic intrusion. Both the plagioclase and pyroxene phenocrysts of the basalt are larger in size in the younger flows. The rhyolite ashes contain no primary crystals, but numerous basalt xenoliths and pumice fragments. The rhyolite lava flows are banded, with only the oldest containing phenocrysts of sanidine and plagioclase. One rhyolite flow is a mingled hybrid of two glasses, each containing plagioclase, pyroxene, and hornblende. Whole rock major and trace element analyses indicate a mixing trend among all of the units in the complex; yet abundant xenoliths in the ashes make this less data less dependable. In situ major and trace element analyses were performed via electron microprobe show two distinct populations in the variation diagrams, with the basalts and rhyolites separated by a compositional gap. Electron microprobe analyses also show that the plagioclase of the basalts and the gabbro are normally zoned with distinct calcic cores and sodic rims; this is also true for the mingled hybrid flow. Rare earth element analyses done via laser ablation inductively coupled plasma mass spectrometry, show that the phenocrysts are enriched in the light and depleted in the heavy rare earth elements. Rare earth element abundances in the glasses have a trend similar to that of ocean island basalt rather than that of mid ocean ridge basalt. Plagioclase geothermometry and amphibole geobarometry indicate that the magma chambers were replenished by new batches of melt and may have existed at a shallow level in the crust just prior to being erupted. Oxygen isotope ratios are depleted compared to those of typical mid ocean ridge basalts, typically indicating that the source melt was partially melted from a hydrothermally altered layer in the crust. As the δ18O values are whole rock, the depletion may be the result of any sub solidus interaction with low δ18O water. The data indicate that multiple shallow reservoirs evolved separately, with limited communication while being intruded by new magma throughout the lifespan of the complex.

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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.

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