No abstract available

Doctor of Philosophy(PhD) / Mid-ocean ridges are a fundamental but insufficiently understood component of the global plate tectonic system. Mid-ocean ridges control the landscape of the Earth's ocean basins through seafloor spreading and influence the evolution of overriding plate margins during midocean ridge subduction. The majority of new crust created at the surface of the Earth is formed at mid-ocean ridges and the accretion process strongly influences the morphology of the seafloor, which interacts with ocean currents and mixing to influence ocean circulation and regional and global climate. Seafloor spreading rates are well known to influence oceanic basement topography. However, I show that parameters such as mantle conditions and spreading obliquity also play significant roles in modulating seafloor topography. I find that high mantle temperatures are associated with smooth oceanic basement, while cold and/or depleted mantle is associated with rough basement topography. In addition spreading obliquities greater than > 45° lead to extreme seafloor roughness. These results provide a predictive framework for reconstructing the seafloor of ancient oceans, a fundamental input required for modelling ocean-mixing in palaeoclimate studies. The importance of being able to accurately predict the morphology of vanished ocean floor is demonstrated by a regional analysis of the Adare Trough, which shows through an analysis of seismic stratigraphy how a relatively rough bathymetric feature can strongly influence the flow of ocean bottom currents. As well as seafloor, mid-ocean ridges influence the composition and morphology of overriding plate margins as they are consumed by subduction, with implications for landscape and natural resources development. Mid-ocean ridge subduction also effects the morphology and composition of the overriding plate margin by influencing the tectonic regime experienced by the overriding plate margin and impacting on the volume, composition and timing of arc-volcanism. Investigation of the Wharton Ridge slab window that formed beneath Sundaland between 70 Ma and 43 Ma reveals that although the relative motion of an overriding plate margin is the dominant force effecting tectonic regime on the overriding plate margin, this can be overridden by extension caused by the underlying slab window. Mid-ocean ridge subduction can also affect the balance of global plate motions. A longstanding controversy in global tectonics concerns the ultimate driving forces that cause periodic plate reorganisations. I find strong evidence supporting the hypothesis that the plates themselves drive instabilities in the plate-mantle system rather than major mantle overturns being the driving mechanism. I find that rapid sub-parallel subduction of the Izanagi mid-ocean ridge and subsequent catastrophic slab break o_ likely precipitated a global plate reorganisation event that formed the Emperor-Hawaii bend, and the change in relative plate motion between Australia and Antarctica at approximately 50 Ma

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 122-136).

Thesis (Ph. D.)--Oregon State University, 2005. / Printout. Includes map in pocket. Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph. D.)--Texas A & M University, 1983. / Vita. Includes bibliographical references (leaves 110-120).

Thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1979. / Vita. Includes bibliographical references (p. 206-211).

Thesis (Ph. D.)--University of Sydney, 2008. / Includes graphs and tables. Includes list of publications. Title from title screen (viewed December 16, 2008). Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Division of Geology and Geophysics, School of Geosciences, Faculty of Science. Includes bibliographical references. Also available in print form.

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Manganese-iron oxide concretions are presently forming on Patrick Sill in upper Jervis Inlet. The marine geology of Patrick Sill and the adjoining basins (Queen's Reach and Princess Royal Reach) was studied to define the environment in which the concretions form. The river at the inlet head is the principal source of sediment to the upper basin. The average grain size of surficial bottom sediments within this basin decreases uniformly with distance from the source. Patrick Sill separates the upper from the lower basin. The sediment distribution pattern within the lower basin differs markedly from the upper basin as there is no dominant source of material but rather many localized sources. Abundant shallow marine faunal remains recovered in deep water sediment samples indicate that sediments deposited as deltas off river and stream mouths periodically slump to the basin floors. Geologic and optical turbidity information for the upper basin can best be explained by slumping from the delta at the inlet head with the initiation of turbidity or density currents. Patrick Sill appears to create a downstream barrier to this flow. The mineralogy of the bottom sediments indicates derivation from a granitic terrain. If this is so, the sediments presently being deposited in both basins are reworked glacial materials initially derived by glacial action outside the present watershed. Upper Jervis Inlet is mapped as lying within a roof pendant of pre-batholithic rocks, principally slates. Patrick Sill is thought to be a bedrock feature mantled with Pleistocene glacial material. The accumulation rate of recent sediments on the sill is low especially in the V-notch or medial depression. The manganese-iron oxide concretions are forming within the depression and apparently nowhere else in the study area. Also forming within the depression are crusts of iron oxide and what are tentatively identified as glauconite-montmorillonoid pellets. The concretions are thought to form by precipitation of manganese-iron oxides on pebbles and cobbles lying at the sediment water interface. The oxide materials are mobile in the reducing environment of the underlying clayey-sand sediment but precipitate on contact with the oxygenating environment of the surficial sediments. The iron crusts are thought to be forming on extensive rocky surfaces above the sediment water interface. The overall appearance and evidence of rapid formation of the crusts suggests they formed from a gel in sea water. Reserves of manganese-iron concretions on Patrick Sill were estimated to be 117 metric tons. Other deposits of concretions have recently been found in other inlets and in the Strait of Georgia but, to date, the extent of these has not been determined. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

In this thesis, I investigate the seismic structure of oceanic lithosphere and asthenosphere with a particular focus on seismic anisotropy, using high-resolution surface waves recorded on ocean-bottom seismometers (OBS) in the Pacific and Atlantic Oceans. The NoMelt (~70 Ma) and Young OBS Research into Convecting Asthenosphere (ORCA) (~43 Ma) OBS experiments located in the central and south Pacific, respectively, provide a detailed picture of ``typical'' oceanic lithosphere and asthenosphere and offer an unprecedented opportunity to investigate the age dependence of oceanic upper mantle structure. The Eastern North American Margin Community Seismic Experiment (ENAM-CSE) OBS array located just offshore the Eastern U.S. captures the transition from continental rifting during Pangea to normal seafloor spreading, representing significantly slower spreading rates. Collectively, this work represents a diverse set of observations that improve our understanding of seafloor spreading, present-day mantle dynamics, and ocean basin evolution. At NoMelt, which represents pristine relatively unaltered oceanic mantle, we observe strong azimuthal anisotropy in the lithosphere that correlates with corner-flow induced shear during seafloor spreading. We observe perhaps the first clear Love-wave azimuthal anisotropy that, in addition to co-located Rayleigh-wave and active source Pn constraints, provides a novel in-situ estimate of the complete elastic tensor of the oceanic lithosphere. Comparing this observed anisotropy to a database of laboratory and naturally deformed olivine samples from the literature leads us to infer an alternative ``D-type'' fabric associated with grain-size sensitive deformation, rather than the commonly assumed A-type fabric. This has vast implications for our understanding of grain-scale deformation active at mid-ocean ridges and subsequent thermo-rheological evolution of the lithosphere. At both NoMelt and YoungORCA we observe radial anisotropy in the lithosphere with Vsh > Vsv indicating subhorizontal fabric, in contrast to some recent global models. We also observe azimuthal anisotropy in the lithosphere that parallels the fossil-spreading direction. Estimates of radial anisotropy in the crust at both locations are the first of their kind and suggest horizontal layering and/or shearing associated with the crustal accretion process. Both experiments show asthenospheric anisotropy that is significantly rotated from current-day absolute plate motion as well as rotated from one another, at odds with the typical expectation of plate-induced shearing. This observation is consistent with small-scale density- or pressure-driven convection beneath the Pacific basin that varies in orientation over a length scale of at most ~2000 km and likely shorter. By directly comparing shear velocities at YoungORCA and NoMelt, we show that the half-space cooling model can account for most (~75%) of the sublithospheric velocity difference between the two location when anelastic effects are accounted for. The unaccounted for ~25% velocity reduction at YoungORCA is consistent with lithospheric reheating, perhaps related to upwelling of hot mantle from small-scale convection or its proximity to the Marquesas hotspot. While lithospheric anisotropy is parallel to the fossil-seafloor-spreading direction at both fast-spreading Pacific locations, it is perpendicular to spreading at the ENAM-CSE in the northwest Atlantic where spreading was ultra-slow to slow. Instead, anisotropy correlates with paleo absolute plate motion at the time of Pangea rifting ~180–195 Ma. We propose that ultra-slow-spreading environments, such as the early Atlantic, primarily record plate-motion modified fabric in the lithosphere rather than typical seafloor spreading fabric. Furthermore, slow shear velocities in the lithosphere may indicate that normal seafloor spreading did not initiate until ~170 Ma, 10–25 Myr after the initiation of continental rifting, revising previous estimates. Alternatively, it may shed new light on melt extraction at ultra-slow spreading environments.