>Magister Scientiae - MSc / A magmatic Ni-Cu (±Co-Zn) sulphide deposit, named the Hondekloof prospect, is present in the Kliprand area at the border between the Northern Cape and Western Cape Provinces of South Africa. The deposit occurs in the central part of the polyphase deformed and highly metamorphosed Garies Terrane, in the Namaqua Sector, along the south-western margin of the Mesoproterozoic Namaqua-Natal Metamorphic Province. Given the sub-economic concentrations yielded from evaluation of three of its known massive-sulphide lenses evaluated, the Hondekloof prospect has received relatively little consideration in terms of ongoing scientific research. Consequently, many aspects related to the genesis, classification and tectonic evolution of the deposit, to date, remain relatively unclear and unknown. The present contribution has therefore been geared to addressing some of those issues in view of the new data obtained on the country rocks and host rocks to the mineralization.Six exploration boreholes were logged, sampled and examined at the deposit site in Kliprand. A total of seven host rocks, namely meta-gabbronorite, biotite gneiss, feldspathic-biotite-garnet gneiss, pink gneiss, meta-syenite as well as enderbite along with a garnetiferous quartzofeldspathic rock occur in association with the sulphide mineralization. The origin, protoliths and tectonic settings of the host lithological units were determined and discussed in terms of modern plate tectonic principles. The meta-gabbronorite (the actual ore host), which had a magmatic protolith and forms part of the pre- to syn-tectonic Oorkraal Suite, displays the chemical characteristics of a depleted mantle origin (source of MORB-like melt), generated within a continental tectonic rift environment. Country rocks with sedimentary protoliths have chemical characteristics indicating a psammitic, felsic to intermediate provenance deposited within a regional subduction-related tectonic setting. A number of geochemical indices, of which the combination of element-ratios (such as Fe2O3/Al2O3 vs. TiO2/Al2O3, as well as Cr vs. Ni, amongst others) created as exploration vectors towards mineralisation, have shown potential for pointing towards the direction of the mineralization. Based on classification schemes of magmatic Ni sulphide deposits, it is has been suggested that the Hondekloof prospect could be classified as a low MgO, conduit-type, disseminated magmatic nickel sulphide deposit.

The Southeastern US is an ideal location to explore the interactions between mountain-building, rifting, magmatism and intraplate deformation. It experienced the formation of the Southern Appalachians over multiple episodes of orogenesis, continental rifting that formed the South Georgia Rift Basin, and widespread magmatism associated with the Central Atlantic Magmatic Province (CAMP). CAMP was followed by the breakup of Pangea, but the suture between Laurentia and Gondwana from the Appalachian orogeny is preserved in the crust of the Southeastern US. Intraplate seismicity indicates ongoing deformation in the Southeastern US today, but the mechanisms controlling this seismicity are poorly understood. This thesis uses seismic constraints to examine the tectonic history of the Southeastern United States (US). We use new wide-angle refraction seismic data to model crustal and upper mantle velocities in order to investigate the structures formed by mountain-building, rifting, and magmatism. Broadband seismic data are then used to detect and characterize earthquakes in the central Georgia-South Carolina region. Wide-angle seismic data were collected on three profiles crossing major geological features in Georgia to investigate the tectonic history of the Southeastern United States as a part of the SUwanee Suture and GA Rift basin experiment (SUGAR) project. We model VP and VS of the crust and upper mantle on SUGAR Line 2, which extends from the Inner Piedmont to the Georgia coast. We identify a north to south decrease in upper crustal VP/VS at the Higgins-Zietz magnetic boundary, which other recent studies have identified as the location of the suture between Laurentia and Gondwana. This boundary also lies near the northwestern edge of the South Georgia Rift Basin, the southeastern termination of the low velocity zone interpreted as the Appalachian detachment, and localized crustal thinning. Together, these results provide new evidence in support of the Alleghanian suture being located at the Higgins-Zietz magnetic boundary, and suggest that this orogenic boundary influenced the location of subsequent rifting. We compare the VP structures of two SUGAR wide-angle transects that cross western and eastern segments of the South Georgia Rift, respectively. Elevated (>7.0 km/s) lower crustal velocities are observed where the rift basin sedimentary fill is thickest and the crust is most thinned. The quantities of mafic magmatic intrusions are consistent with decompression melting at modestly elevated mantle potential temperatures, such as those estimated for CAMP intrusions. These results suggest that, in contrast with the widespread CAMP-aged magmatism at the Earth’s surface, lower crustal magmatic intrusions in the Southeastern US are limited and localized in areas that experienced extension. These new constraints on the velocity structure and tectonic history of the Southeastern United States are then applied to understand earthquakes in the region today. Using broadband seismic data, we find that earthquakes southeast of the Eastern Tennessee Seismic Zone are concentrated within the Carolina Terrane, a particularly heterogeneous accreted terrane of the Southern Appalachians. Within this terrane, seismicity concentrates near rivers and reservoirs, including a sequence of earthquakes in 2013 associated with an increase in water levels at Thurmond Lake on the Georgia-South Carolina border. Focal mechanisms suggest that the earthquakes are occurring on structures that are oblique to the trend of the Appalachians that are more optimally oriented in the modern stress regime.

Marine electromagnetic (EM) geophysics is an up-and-coming branch of the geosciences that is allowing for the advancement in our understanding of key properties of the oceanic lithosphere and subduction dynamics, particularly in how deformation manifests geophysically and how it evolves through time and under various conditions. This dissertation focuses on two unique marine EM data sets collected at the Hikurangi subduction zone, New Zealand, and on 33 Ma Pacific lithosphere. Analysis of the former, which constitutes the bulk of this dissertation, offers the first kilometer-scale characterization of offshore, margin-wide electrical resistivity variations at a subduction zone and provides an electrical framework for discussing the potential causes of along-strike differences in megathrust slip at the Hikurangi Margin. The latter data set is used to constrain electrical anisotropy of the shallow lithosphere, which enables an interpretation of the deformation history of normal oceanic lithosphere. Chapter 2 of this dissertation gives a brief overview of the physical underpinnings of EM methods with attention given to the marine magnetotelluric (MT) and controlled-source electromagnetic (CSEM) methods. Maxwell's equations are reviewed and the relevant derivations leading to the temporal and spatial behavior of EM waves for the frequencies used in this dissertation (~0.001--0.1 Hz) are presented. Chapter 3 focuses on the tectonic background of the Hikurangi Margin and on processing of the MT and CSEM data. Interest in the Hikurangi Margin has arisen both because of its proximity to the inhabitants of New Zealand and due to the recognition of several properties that vary along the strike of the margin. The most intriguing of those variations, and most concerning from a natural hazard perspective, are the along-strike change in interseismic coupling and slow slip event (SSE) occurrence, with stronger coupling and deeper, infrequent SSEs realized in the southern Hikurangi Margin and weaker coupling and shallower, more frequent SSEs in the north. Several proposed causes of these variations are cited, including differences in sediment thickness and roughness of the incoming plate, changes in the plate interface geometry, and the effect of geological terranes in the forearc on pore pressure. But the degree to which any or all of these factors affect interseismic coupling remains an open question. The remainder of Chapter 3 is devoted to detailing the steps involved in processing the marine MT and CSEM data. A workflow for optimizing MT response function estimation is presented and improvements to the marine CSEM processing scheme are described. In Chapter 4 of this dissertation, inversions of the data collected at the southern Hikurangi Margin are presented, and these resistivity models are compared with co-located seismic data. Individual inversions of the CSEM and MT data along with joint inversion of the two data sets highlights the distinct sensitivities and resolving capabilities of each data type. A thick (4--6 km) sediment package covers the Hikurangi Plateau of the incoming plate. The plateau itself is evident as a dipping resistor (>10 Ω-m) that approximately corresponds with the seismically interpreted depth of the Hikurangi Plateau. Resistors in the shallow forearc are interpreted as free gas or gas hydrate, which is prevalent at the Hikurangi Margin. A resistive anomaly beneath one of two main ridges appears to comprise the footwall of a thrust fault, which potentially implies a high permeability system that allows for preferential dewatering of the footwall. Using available P-wave velocity data for this region, equations relating resistivity to velocity are derived. The resistivity presented in Chapter 4 and Archie's law are used to derive porosity models of the southern Hikurangi profile in Chapter 5. Vertical compaction is shown to dominate trends in porosity. A reference compaction porosity model is approximated and removed from the resistivity-derived porosity model in order to identify porosity trends distinct from compaction. A deepening in the negative porosity anomaly of the shallow incoming plate sediments as they approach the trench suggests these sediments experience compression several kilometers seaward of the main frontal thrust. This could represent the early stages of protothrust zone development. An increasingly positive porosity anomaly observed in the sedimentary unit just above the Hikurangi Plateau as it nears the trench may indicate heightened fluid overpressures in an incipient décollement. In Chapter 6 of this dissertation, inversions of the central Hikurangi Margin are shown and discussed. Compared to resistivity in the southern Hikurangi Margin, the forearc and incoming plate of the central Hikurangi Margin are more complex in their resistivity structure, possibly due to the impact of rougher seafloor. Extensive evidence for free gas or gas hydrates is found as shallow resistive anomalies in these models. Other anomalous resistors may correspond to exhumed terranes in the forearc. Anomalous forearc conductors could indicate sediment underplating or damage zones associated with subducting topography. Chapter 7 shows the resistivity and porosity of the northern Hikurangi Margin and offers the first detailed electrical image of a seamount prior to and during subduction. The seamount on the incoming plate is shown to have a thin, resistive cap that traps a conductive matrix of porous volcaniclastics and altered material over a resistive core. Again applying Archie's law to estimate porosity from resistivity reveals that the seamount will allow ~3.2--4.7x more water than normal, unfaulted oceanic lithosphere to subduct with the seamount. In the forearc, a sharp, resistive peak on the slab is interpreted as the core of a subducting seamount. This cone of high resistivity lies directly beneath a prominent conductive anomaly in the upper plate. Burst-type repeating earthquakes and other seismicity from a recent SSE cluster in and around this conductive anomaly, which seems to implicate the subducting seamount in the generation of fluid-rich damage zones in the forearc. The interaction of the subducting topography with the upper plate will thus alter the effective normal stress at the plate interface by modulating fluid overpressure. The results in this chapter show that subducting topography can transport large volumes of water to the forearc and that such topography is able to severely modify the structure and physical conditions of the upper plate, which may influence the location and timing of SSEs. Finally, Chapter 8 provides a robust constraint on the electrical azimuthal anisotropy of oceanic lithosphere. The data for this chapter were collected in a region of oceanic lithosphere removed from the influence of plate boundaries and intraplate volcanism. The survey design was chosen to maximize azimuthal coverage so as to constrain the directional dependence of resistivity. Inversions of the data resulted in an anisotropic resistivity model wherein the crust is ~18-36x more conductive in the paleo mid-ocean ridge direction than the perpendicular paleo-spreading direction. In the uppermost mantle conductivity is ~29x higher in the paleo-spreading direction. The crustal anisotropy is interpreted to result from sub-vertical porosity created by ridge parallel normal faulting during extension of the young crust and thermal stress-driven cracking from cooling of mature crust. Anisotropy in the uppermost mantle implies that shearing of mantle olivine during plate formation generates a strong electrical signal that is preserved as the plate ages. Reanalysis of EM data collected offshore Nicaragua suggests that the Pacific Plate electrical anisotropy is not a local anomaly but rather may be prevalent throughout oceanic lithosphere.

Ultramafic rocks – that are usually located deep below the Earth's surface – are occasionally exhumed by the motion of tectonic plates. The massive chemical disequilibrium that exists between these exposed rocks and the surface waters and atmosphere leads to geologically rapid reactions that consume water and CO₂, binding them to form secondary hydrated/carbonated solid minerals that are found extensively in continental exposures (ophiolites) and at the seafloor near mid-ocean ridges. Pervasive fracturing and faulting in oceanic lithosphere generates pathways for fluids to access and react with rocks that are in some cases located down to depths of tens of kilometers. Over time, the large volumes of fluids and volatiles that are bound up in crustal and upper mantle rocks via such reactions are eventually subducted to extreme depths where subsequent fluid release can trigger melting, arc volcanism and seismic activity. In addition to their geophysical importance, these reactions are also considered to be critical for the survival of organisms in deep sea hydrothermal systems, and a potential source in the origin of life hypothesis. The natural transfer of atmospheric CO₂ to stable, solid carbonate minerals has, in recent years, motivated a large research effort towards investigating its potential as a large-scale carbon sequestration alternative. Understanding the geophysical impact and environmental potential of these reactions and their related processes requires knowledge of their basic physical and chemical behavior. Because of the difficulties of observing these processes in real-time, either experimentally or in the field, there has been a heavy reliance on hypothetical arguments that have been driven by observations in natural rocks. The observations paint a very complex picture – involving an interplay between reaction, fluid flow and fracturing – that is not easily explained by simple model descriptions. Although there has been increasing interest in modeling this class of problems in recent years, to date there remains a considerable gap between the theory and computational framework that is required for a consistent model description. A major theme in said models is their omission of poro-mechanical effects and complications arising from clogging of pore space with precipitating minerals. Both of these are necessary ingredients for a consistent model; however, they require a more complex description that is based on coupled multiphase continuum mechanics, reactive transport, and potentially brittle failure. Each of these components is a technical challenge in its own right, requiring development of novel theory and computation that integrates them in a suitable manner. The overall goals and themes of this thesis are aimed at closing this gap. To this end, I develop a modeling framework and computational tools that are capable of describing reactive flow in brittle media, with a specific focus on fluid-mineral reactions in near-surface ultramafic rock environments. The exposition of this framework is split into 3 separate chapters that build on one other in increments of complexity. Specifically, Chapter 1 presents a poromechanics-based description of coupled fluid flow, mass transfer and solid deformation for a simplified hydration reaction. This model is extended in Chapter 2 to incorporate cracking by adopting modern developments in computational fracture mechanics. Finally, in Chapter 3 I extend the set of reactions to support mixed H₂O-CO₂ fluids by leveraging recently developed tools in computational thermodynamics. Along the way I present a number of numerical model simulations that develop intuition and draw comparisons with natural observations, whilst remaining mindful of its limitations and areas for improvement. Overall, this work represents progress towards better understanding of physical and chemical feedbacks of reactive-brittle processes in the Earth's near-surface and the potential for large-scale carbon sequestration.

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Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Bibliography: leaves 198-220. / by Richard H. Burroughs, III. / Ph.D.

Thesis. 1976. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography: leaves 51-58. / by Jeremy D. Duschenes. / M.S.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Vita. / Includes bibliographies. / by Robert Sherman Detrick, Jr. / Ph.D.