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Magnetic minerals are abundant within our Earth's crust and can retain, through one of a number of processes, a remanent magnetisation induced by the Earth's magnetic field. Analyses of palaeomagnetic samples have been used for the past fifty years to improve our understanding of many of the Earth's major processes. Recent studies utilising newly developed imaging techniques, namely holographic transmission electron microscopy, have for the first time allowed direct observations of the magnetic structure in palaeomagnetic samples on a nanoscale. It is commonly observed that igneous rocks contain closely packed magnetic lamellae with a non-magnetic matrix, a result of the chemical process of exsolution. However, the results of current micromagnetic models, generated to predict the magnetic structure within such samples, are not in agreement with these direct observations. The results do, however, show strong similarities to the direct observations. The discrepancies between the direct observations and micromagnetic models indicate a lack of understanding of the magnetic interactions within such samples. To examine this two distinct hypotheses have been tested. Firstly, the geometry of the system has been altered to examine the effect of this on the magnetic structure of the grains. Secondly, a multiphase model has been produced. This multiphase model allows the simulation of more complicated systems that include more than one magnetic material in direct contact. This multiphase model has allowed us to examine the effect of varying the exchange in these multiphase structures and its effect on the modelled magnetic structure. Further, this multiphase model has allowed us to examine theoretical systems involving combinations of magnetic materials commonly found in palaeomagnetic samples.

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.

The Heart Mountain Detachment (HMD) in Wyoming constitutes one of the largest known rock slides (3400 km2) on Earth. This detachment took place along the stratigraphic boundary between the Bighorn Dolomite at the hanging-wall and the Snowy Range Formation at the footwall. The slide resulted in the formation of an up to 3 m-thick carbonate ultracataclasite (CUC) at the base of the slide. The origin of the CUC and the nature of the triggering mechanism responsible for the initiation of the catastrophic movement have long been controversial. The most widely accepted theory is a mid-Eocene eruption in the Absaroka volcanic province that triggered rupture and subsequent detachment of Paleozoic rocks. Rapid sliding was facilitated by basal fluidization generated by thermo-mechanical decomposition of carbonate rocks. Here I present a proof of concept study addressing the question of the consistent magnetic fabrics observed in the CUC, as well as new observations indicating the discovery of mineral grains of volcanic origin within the CUC. Additionally, some constraints are placed on the thermo-chemical conditions operating at the base of this catastrophic landslide. Overall, the CUC displays an average magnetic susceptibility one order of magnitude higher (1803 . -6 [SI]) than the overlying Bighorn Dolomite (148 . -6 [SI]) and underlying Snowy Range Fm (636 . -6 [SI]). Anisotropy of magnetic susceptibility (AMS) data, field observations and microstructural analysis suggest that ferromagnetic (s.l) minerals in the CUC originate from the Bighorn Dolomite, the Tertiary volcanics and synkinematic thermal decomposition of pyrite into pyrrhotite and magnetite. Thermomagnetic investigations revealed a Curie temperature of 525 °C which suggests that magnetite is the dominant magnetic carrier mineral in the CUC. Energy Dispersive Spectroscopy analyses confirm that this magnetite has a relatively low ulvöspinel content. Magnetic hysteresis properties point to an average pseudo-single domain magnetic grain size or, alternatively, a mixture of single domain and multi-domain grains. The origin of AMS is magnetostatic, elucidated by a high degree of consistency between AMS directions and 3–D SPO directions. The anisotropy of magnetic susceptibility (AMS) directional data displays two dominant fabric orientations suggesting possible strain partitioning in the CUC. The consistency of magnetic fabrics suggests that the CUC followed a dominantly transpressive regime. The deformation mechanism causing the consistent AMS is a homogeneous passive rotation of magnetite grains. Microstructural analysis of iron bearing minerals suggests that a high oxygen fugacity was present in the CUC at the onset of the slide. Evidence for elevated temperatures in the CUC is shown by broken twins in calcite which form as result of dynamic recrystallization. High pore fluid in the CUC is indicated by CUC dikes intruding the hanging wall and by accretionary grains (lapilli). Finally, the presence of unserpentinized and a few weakly serpentinized olivine clasts in the CUC brings the "hot water" weakening mechanism proposed by Ahranov and Anders (2006) into question.

Fault pseudotachylytes commonly form by frictional melting due to seismic slip. These fine-grained clastic rocks result from melt quenching and may show a high concentration of fine ferromagnetic grains. These grains are potentially excellent recorders of the rock natural remanent magnetization (NRM). The magnetization processes of fault pseudotachylytes are complex and may include the following: i) near coseismic thermal remanent magnetization (TRM) acquired upon cooling of the melt; ii) coseismic lightning induced remanent magnetization (LIRM) caused by earthquake lightnings (EQL); iii) post seismic chemical remanent magnetization (CRM) related to both devitrification and alteration. Deciphering these magnetization components is crucial to the interpretation of paleointensities to see if coseismic phenomena such as EQL’s were recorded within these rocks. Hence the paleomagnetic record of fault pseudotachylytes provides an independent set of new constraints on coseismic events. Fault pseudotachylytes from the Santa Rosa Mountains, California host a magnetic assemblage dominated by stoichiometric magnetite, formed from the breakdown of ferromagnesian silicates and melt oxidation at high temperature. Magnetite grain size in these pseudotachylytes compares to that of magnetite formed in friction experiments. Paleomagnetic data on these 59 Ma-old fault rocks reveal not only anomalous magnetization directions, inconsistent with the coseismic geomagnetic field, but also anomalously high magnetization intensities. Here we discuss results of rock magnetism and paleointensity experiments designed to quantify the intensity of coseismic magnetizing fields. The REM’ paleointensity method, previously tested on meteorites, is particularly well suited to investigate NRMs resulting from non-conventional and multiple magnetization processes. Overall findings indicate an isothermal remanent magnetization (IRM) in some, but not all, specimens taken from four different Santa Rosa pseudotachylyte samples. The cause of this IRM may be attributed to an LIRM produced by ground lightning (less likely), LIRM produced by an EQL (more likely), or a VRM imparted during laboratory preparation (not likely). The anomalously high NRM recorded in a few specimens points to LIRM as the most likely explanation for the dominant origin of magnetization.

Quaternary magnetic studies have provided the conceptual framework to bridge magnetic studies into ancient systems. In cases where environmental materials have been subjected to diagenetic alteration two questions come to mind: 1) What part of the magnetic signal is preserved in the rocks; and 2) can the preserved signal be used to infer/identify magnetic patterns that are characteristic of the depositional, post-depositional, and/or diagenetic environment. Analyses of multi-parameter magnetic experiments conducted on upper Mississippian and lower Pennsylvanian continental successions reveal that distinct depositional, pedogenic, and diagenetic magnetic patterns can be separated and identified. Evidence for a primary depositional signal in several of the upper Mississippian lithofacies is identified by a detrital remanence component attributed to source-area-derived magnetite/titanomagnetite. Red and gray vertisols preserve a Mississippian pedogenic signal characterized by magnetic enrichment, depletion, and amalgamation patterns that are associated with the removal and transport of Fe-rich clays as well as vertical mixing by shrink-swell mechanisms. These well-developed vertisols are interpreted to reflect significant hiatuses in sedimentation associated with prolonged exposure on interfluve/floodplain surfaces that may correlative with incised valleys (lowstand surface of erosion). Similarly, in lower Pennsylvanian quartz arenite facies, early siderite cementation zones as well as conglomerate lags with distinctive magnetic characteristics are thought to reflect periods of prolonged exposure and to define unconformities within compound valley fills. / Master of Science

Geomagnetic excursions, brief deviations in geomagnetic field behaviour from that expected during 'normal' secular variation, remain some of the most enigmatic features of geomagnetic field behaviour. This thesis presents high-resolution records of geomagnetic excursions recorded at the Blake-Bahama Outer Ridge in the Western North Atlantic. The highest resolution record yet of the Blake geomagnetic excursion (~125 ka) is measured in three cores from Ocean Drilling Program Site 1062 (ODP Leg 172). These cores have sufficiently high sedimentation rates (>10 cm ka^-1) to allow detailed reconstruction of the field behaviour at these sites during the excursions. Previous reconstructions of geomagnetic field behaviour during excursions from marine cores have been limited by low-resolution age models. This thesis discusses a new approach, whereby measurements of excess ²³⁰Th (²³⁰Th_xs) are used to constrain relative variations in sedimentation rate. Modifications are suggested to the methods previously used to calculate the concentration of ²³⁰Th_xs and a new MATLAB® program is developed and described that allows rapid and flexible calculation of ²³⁰Th_xs. Using this new approach, the duration (6.5±1.3 kyr) and age (129-122 ka) of the Blake excursion are accurately constrained. A palaeomagnetic study is also conducted on two ODP Sites, 1061 and 1062 on the Blake-Bahama Outer Ridge to obtain a high-resolution record of the Laschamp geomagnetic excursion (~41 ka). The Blake excursion is found to be of 'long' duration (6.5±1.3 kyr) whilst the Laschamp excursion is relatively short (<400 years) showing that excursions do not have a characteristic duration, linked to the conductivity of the inner core, but instead occupy a continuous range of durations. The records of both the Blake excursion and the Laschamp excursion from the Blake-Bahama Ridge sites also show rapid transitions to excursional geomagnetic pole positions (less than 500 years), much faster than often quoted for full geomagnetic reversals. Based on current estimates for reversal durations, this would imply that excursions and reversals are controlled by different processes.

In this thesis, I use multiple techniques to investigate the active faulting and deformation of the Altay Mountains, Western Mongolia. The Altay are an intracontinental transpressional mountain range, which are deforming in the far-field of the India-Asia collision. An anastomosing network of dextral faults strikes NNW-SSE, and accommodates NE-SW oriented shortening by rotating anticlockwise about vertical axes. I begin by characterising the Altay faults, and add to what is already known about their surface expression with new observations of active faulting and three previously undescribed ancient earthquake ruptures. I use ¹⁰Be cosmogenic dating and uranium-series dating on pedogenic carbonate to estimate the average Quaternary rate of slip for two of the major fault zones in the Altay. The slip rate on the Ölgiy fault is constrained to 0.3-2.1 mm/yr^-1. Results from the Hovd fault are ambiguous, demonstrating the complications encountered with application of Quaternary dating techniques. I measure palaeomagnetic directions from Cretaceous to Pliocene-aged sediments in the eastern Altay to constrain the degree of anticlockwise rotation. Results from thermal demagnetisation of specimens indicate that the eastern Altay has not undergone significant rotation, in contrast with previous studies from the Siberian Altay that reveal almost 40 degrees of anticlockwise rotation. This suggests that the eastern-most Altay fault is too young to have experienced significant rotation, or is kinematically different from the Siberian Altay. I apply apatite fission track (AFT) dating and track length modeling to the central Altay. Results from AFT dating show rapid cooling in the late Cretaceous due to the distal assembly of Central Asia, suggesting that there was pre-existing topography at the start of the Late Cenozoic phase of deformation, the timing of which is constrained to have initiated at least 20 Myr ago. My work demonstrates that combining results from techniques that cover a variety of time scales quantifies the evolution of active faulting and deformation in the region.

Le champ magnétique terrestre présente une vaste gamme de variations temporelles, de l'année à plusieurs millions d'années. J'ai étudié au cours de ma thèse divers aspects de ces fluctuations, des variations séculaires récentes aux renversements.La première partie de ma thèse porte sur l'archéomagnétisme, discipline qui permet de retracer l'évolution temporelle du champ magnétique terrestre au cours des derniers millénaires, principalement à partir des matériaux archéologiques. J'ai étudié deux jeux d'échantillons provenant pour l'un de la Grèce (période néolithique, 6800-3200 avant J.C.) et pour l'autre du Mexique (Palenque, période Maya Classique 320-840 après J.C.), dans le but d'acquérir de nouvelles données d'archéointensité et de mieux contraindre la variation séculaire du champ magnétique terrestre. La comparaison de mes données avec celles disponibles dans la bibliographie et avec les différents modèles globaux et régionaux a mis en évidence que les variations séculaires au Néolithique en Grèce et au cours du premier millénaire en Amérique Centrale sont très mal définies. Mes données suggèrent que des composantes locales, non prisent en compte dans les modèles globaux peuvent exister au niveau ces régions. Elles renforcent l'intérêt de développer des modèles régionaux précis, qui nécessitent un développement en harmoniques sphériques à un degré plus élevé. Pour ce faire, l'acquisition de nouvelles données de haute qualité est un élément majeur.La deuxième partie traite de la description d'un renversement du champ magnétique terrestre. Cette étude a été basée sur deux points précis: étudier les directions transitionnelles afin d'apporter de nouvelles contraintes sur le possible confinement longitudinal des pôles géomagnétiques virtuels (PGV) d'une part, et d'autre part vérifier des paléointensités transitionnelles obtenues sur la séquence volcanique d'Akaroa (Nouvelle Zélande), dont l'intensité est significativement supérieure à celles des intensités avant et après le renversement. Nous avons ré-échantillonné cette séquence, et l'évolution directionnelle obtenue pour ce renversement est une succession complexe de polarités N-T-R-T-N-T-R. Les PGV obtenus semblent se regrouper sous deux bandes longitudinales sous l'Australie et l'Amérique, ce qui renforce l'hypothèse d'une interaction entre le manteau et le noyau sur plusieurs millions d'années. Suite à une étude de minéralogie magnétique, j'ai sélectionné les échantillons susceptibles de fournir une valeur de paléointensité par les méthodes de Thellier et du multispecimen. Les paléointensités obtenues sont relativement faibles (environ 20 microT) au cours du changement de polarité et forte à la fin de la séquence. Mon interprétation, basée à la fois sur les valeurs de l'intensité du champ et sur les données radiochronologiques montrant que la séquence s'est mise en place très rapidement, est de considérer que seul le renversement C4Ar.1n-C4Ar.1r a été enregistré dans cette séquence. Dans cette hypothèse, le renversement montre un cheminement complexe comparable à d'autres renversements enregistrés dans l'hémisphère nord (Steens Mountain par exemple), incluant un phénomène de rebond avant de se stabiliser. La troisième partie de ma thèse est consacrée au développement d'une nouvelle méthodologie et d'un nouvel appareillage pour déterminer des paléointensités. Le faible taux de réussite des expériences de paléointensité de l'étude précédente m'a poussé à m'intéresser au protocole multispecimen, qui peut s'appliquer aux échantillons possédant un comportement polydomaine. L'inconvénient technique majeur de cette méthode réside dans l'application du champ le long de l'aimantation naturelle, difficile à réaliser avec précision dans les fours standards. Pour ce faire, nous avons décidé de développer des porte-échantillons permettant d'orienter les échantillons dans l'espace pour le four standard et un prototype de four à chauffage ultra-rapide particulièrement bie / The Earth's magnetic field shows a large range of temporal variations from the year to several million years. I studied during my PhD thesis several aspects of these fluctuations, from recent secular variations to reversals.The first part of my manuscript deals with archeomagnetism, a discipline that allows to track the temporal variations of the Earth's magnetic field through millennia, mainly from archeological materials. I studied two sets of samples, one from Greece (Neolithic period 6800-3200 B.C.) and the other from Mexico (Palenque, Maya Classic period 320-840 A.D.), to acquire new archeointensity data in order to better constrain the secular variation of the geomagnetic field. By comparing my data with those available in the literature and with the various global and regional models, I showed that the secular variations during the Neolithic in Greece and during the first millennium in Central America are poorly defined. My data suggest that local components, not described by global models, may exist in these regions. They reinforce the importance of developing specific regional models, which require development in higher spherical harmonic degree. As a consequence, the acquisition of new high quality data is of main importance. The second part presents the description of a geomagnetic field reversal. This work was based on two points: first by studying transitional directions to provide new constraints on the possible preferred longitudinal paths of virtual geomagnetic poles (VGPs) and second by checking transitional paleointensities obtained on a volcanic sequence in Akaroa volcano (New Zealand). Indeed the transitional field intensity is significantly higher than the field intensity before and after the reversal. We re-sampled this sequence, and the directional results show a complex sequence of N-T-R-T-N-T-R polarity. The transitional VGPs obtained are clustered in two longitudinal bands through Australia and America. This observation seems to reinforce the assumption of a core-mantle interaction over several million years. Following a rock magnetic study, I selected samples that could provide a value for the Thellier and multispecimen paleointensity methods. The obtained paleointensity are relatively low (about 20 microT) during the polarity change and strong at the end of the sequence.Based both on the field strength values and on the radiochronological ages, showing that the sequence was erupted in a very short time, I suggest that only the C4Ar.1n-C4Ar.1r reversal was recorded in this sequence. In this assumption, the reversal shows a complex path comparable to other reversals recorded in the northern hemisphere (for example the Steens Mountain), including a rebound before stabilizing.Finally the last part is devoted to the development of a new methodology and a new apparatus to determine absolute paleointensity. Following the low success rate of paleointensity experiments from the previous study, I decided to test the multispecimen protocol, which can be applied to samples yielding a predominant multidomaine behavior. The main technical drawback of this method lies in the application of the laboratory field along the natural remanent magnetization, a difficult task to perform accurately in standard paleointensity ovens. Thus, we decided to adapt sample holders from our standard oven in order to allow the sample orientation in space and to develop an ultra-fast heating oven prototype particularly well-suited for this method, allowing to apply the laboratory field in the 3 dimensions. I checked the different multispecimen protocols on historical lavas from Reunion and Etna volcano, yielding very different magnetic mineralogies. For all flows, I obtained paleointensities very close from the expected values, regardless from the magnetic mineralogy, revealing the feasibility of our apparatus and the promising interest of the method. The application of various corrections on the statistical estimation o