Geological Sciences Theses and Dissertations

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Browsing Geological Sciences Theses and Dissertations by Subject "Anisotropy"

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    New Perspectives on Mid-Ocean Ridge Magmatic Systems and Deformation in the Uppermost Oceanic Mantle from Active- and Passive-Source Seismic Imaging in Cascadia
    (University of Oregon, 2019-01-11) VanderBeek, Brandon; Toomey, Douglas
    In this dissertation, I use seismic imaging methods to constrain the evolution of the oceanic upper mantle across the Juan de Fuca (JdF) and Gorda plates. This work begins by studying the geometry of the mantle magmatic system and patterns of mantle flow beneath the northern JdF ridge in relation to ridge-parallel changes in accretionary processes. I find that the dynamics of lithospheric rifting exert the primary control on the distribution of shallow mantle melts and variations in crustal thickness and composition. The orientation of mantle divergence beneath the JdF ridge, as inferred from seismic anisotropy, is oblique to the overlying plate divergence direction. Similar observations made at the East Pacific Rise and Mid-Atlantic ridge suggest plate motions alone do not control mantle flow patterns. On the contrary, stresses exerted at the base of the plate by the asthenospheric flow field may contribute to changes in plate motion prompting a reorientation of oceanic spreading segments. The mantle anisotropic fabric of the JdF plate interior is then investigated to identify whether the rotated mantle flow field observed beneath the JdF ridge persisted throughout the recent geologic past. However, observations suggest that the anisotropic structure created at the ridge partially reorganizes off-axis obscuring the paleo-flow geometry. Next, I focus on how the physical state of the oceanic lithosphere evolves with time. Using local earthquake arrival times I test whether the seismic velocity structure of the upper mantle lithosphere is thermally controlled or dominated by heterogeneities introduced upon accretion at the ridge or by subsequent deformation off axis. Despite extensive surficial evidence of faulting across the Gorda plate, deformation appears to be restricted to crustal depths and mantle velocities are explained by conductive cooling. In contrast, the velocity structure of the JdF plate is inconsistent with conductively-cooled mantle. Hydration of the mantle lithosphere associated with tectonic discontinuities is invoked to explain anomalously slow P-wave speeds. Lastly, a joint inversion of teleseismic body and surface wave data is proposed to image the geometry of mantle upwelling and melt production beneath the JdF and Gorda Ridges. This dissertation includes previously published and unpublished coauthored material.