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/\ dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg for the Degree of Master of Science. Johannesburg 1994. / This dissertation investigates the upper mantle discontinuities beneath South Africa by analysing seismic waves converted from p to Sv recorded at an array of broad band seismometers. Events, with desirable characteristics recorded at each station are transformed to a suitable coordinate system, normalised, and stacked to enhance the weak converted phases. Additional low pass filters are also applied to improve the signal to noise characteristics of the data. No data suitable to processing were found for stations outside the Kaapvaal craton Despite additional problems related to the application of sub-optimum numerical routines to a limited data base, by combining results with those from an independent study two models were derived for the '400 km' and '670 km' discontinuities beneath the Craton. Models for the '670 km' discontinuity accord with a simplified global earth model while models for the '400 km' discontinuity indicate the presence of a low shear velocity layer not supported in a global earth model. Addltlonal broad band seismic data must be acquired to investigate this phenomenon further. / AC2017

One of the defining characteristics of hotspot volcanism is the presence of a long-lived, linear chain of age-progressive volcanoes created by the movement of the lithosphere over a stationary melting anomaly. However, the spatial distribution of volcanism at hotspots is often complex and highly variable suggesting that the relationship between magma generation and magma transport at hotspots is poorly understood. Here, I present the results of the first systematic quantitative characterization of the spatial distribution of volcanism at oceanic hotspots. In the first study I develop a novel methodology to characterize the across-strike distribution of volcanism at hotspots and apply it to a catalog of 40 oceanic hotpots. I find that only 25% (10/40) of hotspots exhibit the simple single-peak profile predicted by geodynamic models of melt generation in mantle plumes. The remaining 75% (30/40) of hotspots exhibit a dual- or multi-peak pattern. In the second study, I focus on the across-strike distribution of volcanism at the oceanic hotspots that are sourced by a deep-rooted mantle plume. 14 out of the 15 consensus plume-fed hotspots exhibit a dual-peaked across-strike profile. The spacing between these peaks display a strong negative correlation with lithospheric age, in direct contrast to models of inter-volcanic spacing controlled by elastic plate thickness. This relation suggests a different mechanism controls volcanic spacing at plume-fed hotspots. In the third chapter, I investigate variations in the average topographic profiles over time along the two longest and best-constrained oceanic hotspot tracks: Hawaii and Louisville. I find that the dual-peak across-strike profile of volcanism is a persistent feature at the Louisville hotspot over the entire length of the track examined (spanning a period of more than 65 Myr). In contrast, the dual-peak profile of volcanism at Hawaii is only evident along the most recent portion of the track (i.e., over the last 5 Myr). In total, this thesis represents a significant step foreword in the collective understanding of hotspot volcanism, and introduces a new diagnostic tool for analysis of hotspot influenced seafloor topography.

This dissertation consists of four chapters, each of which is either published in a peer-reviewed journal, or in submission. These chapters developed from the testing of the hypothesis that the lithospheric mantle contains significant magnetic regions that contribute to the magnetism observed/measured, either at or close to the Earth’s surface, or from satellite data. Chapter 1 “Eight good reasons why the mantle could be magnetic” (2014) published in Tectonophysics by Ferré, Friedman, Martín-Hernández, Feinberg, Till, Ionov and Conder, addresses the motivation for this project and establishes the probability of upper mantle contribution to magnetic anomalies. My role with this manuscript was to produce figures using my previously collected data (Figures 2, 4, and 6), compile known data on the magnetic properties of minerals in mantle peridotites (Table 1), provide discussion for and against each argument made, and edited the manuscript. Chapter 2 “Remanent magnetization in fresh xenoliths derived from combined demagnetization experiments: Magnetic mineralogy, origin and implications for mantle sources of magnetic anomalies” (2014) published in Tectonophysics by Martín-Hernández, Ferré, and Friedman, investigates the natural remanent magnetization of mantle xenoliths. Notably, it establishes that the natural remanent magnetization of these xenoliths is derived from a thermoremanent magnetization (primary) and not from chemical remanent magnetization (secondary) origin. My primary role in this study was to provide preliminary magnetic and petrologic data and analysis of the samples. Secondary responsibilities were to prepare the samples, edit the manuscript and provide discussion on the results. Chapter 3 “Craton vs. rift uppermost mantle contributions to magnetic anomalies in the United States interior” (2014) published in Tectonophysics by Friedman, Feinberg, Ferré, Demory, Martín-Hernández, Conder, and Rochette begins to compare magnetic properties across different tectonic settings. The metasomatized cratonic upper mantle of the United States interior contains ferromagnetic phases that exist at temperatures lower than the Curie temperature. This upper mantle would likely contribute to magnetic anomalies. Alternatively, the high geotherm and sulfide-rich mantle near the Rio Grande Rift precludes this area from mantle contribution to magnetic anomalies. As first author I prepared samples, ran experiments, processed data, produced figures, wrote the manuscript and applied for funding. Chapter 4 “What is magnetic in the mantle wedge?” (2015) submitted to Geology, examines the mantle wedge beneath multiple island arcs. Magnetic anomalies in island arc settings have been attributed to a serpentinized mantle wedge. While this material is not available to test, metasomatized mantle, common to the mantle wedge, is available. Metasomatized mantle is mostly paramagnetic, and thus supports that stepwise dehydration of a subducting slab may produce positive and negative anomalies in the mantle wedge. As first author I prepared samples, ran experiments, processed data, produced figures, wrote the manuscript and applied for funding.

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