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Equivalent medium theories can be used to interpret seismic anisotropy in field seismic data to infer the properties of subsurface fractures. These theories analyse the seismic response of the rock in the long wavelength limit and relate the degree of anisotropy measured to the fracture properties. They have particular use in the hydrocarbon industry where extraction can be determined by both naturally and induced fractures. Validation is required to use these theories with more confidence in the commercial setting. One method for validation is through controlled laboratory seismic experiments. For the idealised fracture distributions found in these equivalent medium theories the laboratory experiments require rocks that can be built with a controlled fracture geometry. I present ultrasonic laboratory data from three different experiments of synthetic porous rocks containing controlled fracture geometries. I then analyse the data using suitable theory where possible. Despite the ultrasonic experiments violating equivalent medium criteria strong relationships between data and theory were found. The relationship between shear-wave splitting and fracture density was found to be highly robust. The dependence of shear-wave splitting on fluid saturation at 45° to the fracture normal was quantified for variations of fluid viscosity and bulk modulus and has direct implications for oil/water discrimination in fractured reservoirs. Based on a single fitting parameter from the water saturated data it was possible to accurately predict Thomsen’s anisotropy parameters, e and d for air and glycerin saturation. Predictions of g are independent of fluid saturation and model fitting and show strong agreement with the laboratory data.

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Deepwater fold and thrust belts (DWFTBs) have been recognized on most the world’s passive margins. They play a key role in the redistribution of gravitationally induced strain from the continental margin into the abyssal plain. Commonly they form tripartite systems linked by a detachment or detachments, containing: an updip extensional domain dominated by normal faulting, a downdip contractional domain dominated by folding and thrusts, and a transitional domain between. Typically they are commonly classified based upon their driving forces, geometry and detachment lithology. Systems concentrated on active margins and those detaching onto salt detachments are well described and modelled, but do not provide models applicable to other margin types. The geometry of thin shale detached DWFTBs on passive margins are poorly constrained. A wide variety of geometric arrangements are observable that do not conform to our current understanding of their formation. Through observation, interpretation and restoration of interpreted seismic profiles this study proposes new models for their formation and growth in both two and three dimensions. A consistent deficit of extensional versus compressional displacement in favour of extension has been observed in DWFTB systems. This study reveals this to be a consistent feature across DWFTBs and relates it to an early compactional phase of deformation in their development. This study then further investigates this phenomenon through field studies, seismic interpretation and the restoration of DWFTB structures at a range of scales. This study proposes this missing strain component is compensated for by the internal deformation, through folding and thrusting, of sediments within the DWFTB itself. Finally this thesis draws on the understanding gained from the entire margin to discuss what DWFTB development reveals about wider margin and tectonic scale processes.

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Continental extension causes rifting and thinning of the lithosphere that may result in breakup and eventually the initiation of seafloor spreading and passive continental margin development. Ambiguity exists regarding the roles of magmatism and structural inheritance during rifting and continental breakup during this process. This study focuses on the importance of these controls on the Mesozoic-Cenozoic separation between West Greenland and Eastern Canada. It is important to improve our knowledge of the processes that influenced breakup as the current understanding of these processes is limited and also to reduce hydrocarbon exploration risk in this tectonic setting. During this study passive margin processes were investigated using a variety of methodologies at a range of scales from that of conjugate margin pairs (Chapters 4 and 5), through margin and basin scale studies (Chapter 6) to the smallest scale on individual igneous intrusions (Chapter 7). At the largest scale an assessment of the magmatic and structural asymmetry between the conjugate margins of the Labrador Sea based primarily on field data and subsequent analysis near Makkovik, Labrador, but also other large-scale geophysical datasets demonstrated that early rifting was dominated by simple shear rather than pure shear. In such a scenario Labrador was have been the lower plate margin to the upper plate southwest Greenland margin. Further analysis of field observations indicated that rifting of the Labrador Sea region may have been aided by a favourably orientated basement metamorphic fabric and that observable onshore brittle deformation structures may be related to Mesozoic rifting. Further north in the Davis Strait, seismic interpretation at the margin and basin scale allowed a series of seismic surfaces, isochrons and a new offshore fault map to be produced. The results of this analysis demonstrated that the geometry of rift basins was primarily controlled by pre-existing structures, an assertion supported by observations of reactivation onshore in West Greenland. Finally, at the smallest scale, results of numerical modelling offshore Newfoundland demonstrated that even on non- volcanic passive margins, intrusive magmatism can influence thermal evolution. In addition, the presence of widespread igneous rocks on passive margins may be indicative of regional-scale thermal perturbations that should be considered in source rock maturation studies. Overall, the conclusion of this project is that both magmatism and structural inheritance have profoundly influenced the continental breakup between West Greenland and Eastern Canada, and that interplay between these two complex groups of mechanisms may have also contributed to the geological evolution of this area.

Understanding fault zone evolution is crucial to appreciating how deformation mechanisms may change in time and space. This is of particular importance when considering seismic slip, with its potential for human hazard. Histories of fault evolution and reactivation are recorded as overprinted structures in ancient fault zones, now exhumed from seismogenic depths. Recognition of ancient seismicity is aided by the occurrence of pseudotachylyte, a solidified frictional melt generated at seismic slip speeds along faults. Because pseudotachylytes form on similar timescales to the duration of seismic slip, they capture a snapshot of earthquake parameters such as temperature, depth, strength, magnitude and stress drop. The Outer Hebrides Fault Zone, UK, was repeatedly reactivated during long-lived collision and bears widespread pseudotachylyte. It is used in this thesis as a case study in which to constrain the seismic history. Slip directions on pseudotachylyte faults are identified using field observations supported by microstructural evidence. The depth and temperature of faulting, and the coseismic temperature rise, are studied using the composition and microstructures in the pseudotachylyte veins, whilst experimentally produced melts further understanding of the control of lithology on coseismic fault strength. Finally, the static strength and the dynamic weakening are derived from further field observations. Seismicity occurred at ≥ 10 km, somewhat deeper than has been previously thought, and was initially scattered diffusely around the fault zone on small, strong faults. Magnitudes are recorded up to MW 6.3 and static stress drops from 1.5-8.8 MPa. The lithology hosting the fault is shown to control the coseismic strength of the pseudotachylyte-bearing fault both through the melt composition and through the development of fault roughness. Overall, results show that seismicity in the Outer Hebrides occurred throughout a long convergence history because fault weakening, slip localisation and fluid influx were heterogeneously distributed around the fault zone.

The development of magmatism during the transition from continental rifting to sea-floor spreading remains poorly understood as most rifted margins are now inactive and buried beneath thick sediments. Dabbahu and the basaltic Erta Ale volcanoes situated within the actively rifting Afar region of northern Ethiopia, offer a unique opportunity to address this problem. The nature of rhyolitic volcanism, the sub-volcanic system, and their relationship to basaltic magmas involved in dyking events is investigated at Dabbahu volcano and forms the main subject of this study. The evolution of the volcano is constrained through 9 new 40Ar/39Ar dates and geochemical analysis of 93 samples, supplemented by a new geological map. Dabbahu has been active for over 65,000 years, erupting basalts through to evolved pantellerites. Modelling shows the evolved magmas were derived through protracted (>80%) fractional crystallisation. A shallow magma storage region (1-5 km) is confirmed through melt inclusion analysis, and further constrained with seismic and InSAR data. However, it is proposed that this region is a temporary site of pre-eruptive storage for evolved products and the site of differentiation lies at depth within the crust (14-20 km). Field evidence indicates that magmas were not erupted in fractionation sequence and mixing between cogenetic magmas is recorded on all scales. The input of new magma may have been a key eruption trigger at Dabbahu throughout its history, as was the case in the most recent 2005 eruption at the Da’Ure vent following dyke injection. Erta Ale is a basaltic shield within the most northerly magmatic segment of the same name, and contains one of the world’s longest lived lava lakes. An overview of the events of the November 2010 eruption in this remote area has been constructed through a combination of ground observation and remote sensing. Analysis of zero age lavas provides new insights into this unique volcano

Exposed rhyolitic dykes at eroded volcanoes arguably provide in situ records of conduit processes during rhyolitic eruptions, thus bridging the gap between surface and sub-surface processes. This study involved micro- to macro-scale analysis of the textures and water content within shallow (emplacement depths <500 m) rhyolitic dykes at two Icelandic central volcanoes. It is demonstrated that dyke propagation commenced with the intrusion of gascharged currents that were laden with particles, and that the distribution of intruded particles and degree of magmatic overpressure required for dyke propagation were governed by the country rock permeability and strength, with pre-existing fractures playing a pivotal governing role. During this stage of dyke evolution significant amounts of exsolved gas may have escaped. Furthermore, during later magma emplacement within the dyke interiors, particles that were intruded and deposited during the initial phase were sometimes preserved at the dyke margins, forming dykemarginal external tuffisite veins, which would have been capable of facilitating persistent outgassing during dyke growth. It is further demonstrated that following initial dyke-opening, geochemically homogenous dykes grew via the incremental emplacement of magma, with fluctuations in the shallow-dyke permeability occurring via bubble collapse, and this is deemed to have been critical in dictating pressure within the deeper magma source region and fragmentation. Of further significance, it is also shown that shear deformation was localised during magma emplacement, with localised vesiculation occurring along emplacement boundary layers via viscous heating, which temporarily promoted magma ascent, but with later bubble collapse culminating in brittle failure of bubble-free magma, after shear zone migration. However, in some instances high strain rates during viscous bubble deformation resulted in ductile-brittle transitions, with resultant slip triggering micro-tensile failure of bubbly magma, as the slipped magmatic plug experienced decompression. This tensile failure probably occurred distal to shear zones, where bubbles where relatively isolated. Interlinking of the micro-cracks formed extensive internal tuffisite vein networks, which acted as efficient outgassing pathways, given their access to significant quantities of preexsolved volatiles. The models presented in this thesis are relevant to the conduit processes that take place during rhyolitic eruptions; insight is provided into how rhyolitic magma ascends through the shallow (<500 m deep) crust and also into how the magma deforms during its ascent and into the processes that govern magmatic outgassing.

The Palaeozoic North American continent of Laurentia had a margin to the Iapetus Ocean which evolved from a late Precambrian/Cambrian rift to a passive margin and Mid/Late Palaeozoic orogenic belt. A number of sedimentary basins existed in the Laurentian continental interior, situated some distance from this margin. These intracratonic basins have proved controversial for a number of reasons; they are typically circular to sub-circular in outcrop and isopach maps and have prolonged patterns of subsidence. This dissertation describes detailed subsidence analysis of 3 intracratonic basins (the Illinois, Michigan, and Hudson Bay basins) and describes a pilot study of a fourth, the Williston Basin. It also describes the subsidence history of the Iapetean margin. It offers new insights into the mechanisms driving their subsidence and the relationships between the interior basins and the cratonic margins. North America was chosen for the study of intracratonic basins, as it offers excellent datasets and detailed stratigraphy. However, such basins exist on many continents and understanding of other basins should follow from this North American work. In order to investigate subsidence histories, a large database of stratigraphic sections has been compiled from outcrop and oil exploration boreholes. A standard backstripping technique has been applied and the resulting subsidence profiles interpreted in the light of existing structural frameworks, incorporating information such as seismic, gravity and magnetic anomaly data. In areas where independent evidence suggests that basin initiation was caused by lithospheric extension, subsidence data have been inverted for strain rate and the amount of extension quantified. Along the cratonic margin, new basinal areas are identified, such as central Texas and Alabama/Tennessee. An inferred rift arm, the Tobosa Basin of west Texas, is shown to have been absent during Early Palaeozoic times.