This thesis investigates the interactions between tectonics and erosion in the eastern Himalaya through the study of paleo-Brahmaputra deposits in the foreland basin. Sediment depositional dating of two sedimentary sections is performed using magnetostratigraphy, apatite fission-track and luminescence dating. Provenance analysis using zircon and apatite U-Pb dating allows the reconstruction of the Miocene-Quaternary paleo-drainage of the Brahmaputra River and the documentation of the tectonic evolution of two poorly understood Himalayan features: the Shillong Plateau and the Namche Barwa Syntaxis. The Shillong Plateau is the only elevated topography in the Himalayan foreland and the timing of its surface uplift is debated. Decoupling between of the time of rock exhumation and surface uplift has been explained by differences in rock erodibilities of the plateau between the Shillong Precambrian basement and the overlying Cenozoic sedimentary rock. New detrital zircon U-Pb data and lithospheric stress field modelling presented here date the rise of the Plateau between 5.2 Ma and 4.4 Ma leading to the redirection of the Brahmaputra River at that time, and the role of tectonics in the rise of the plateau is invoked. The Namche Barwa syntaxis is located at the eastern Himalayan termination and its development is widely debated. It has been subjected to anomalously young (< 10 Ma) peak metamorphism, and unusually high exhumation rates (up to 10 mm/yr), in comparison with the Himalayan main arc of the orogen. However, the timing of the onset of rapid exhumation of the Namche Barwa syntaxis is poorly constrained (between 10 and 3 Ma). Focusing on the proximal detrital record of material eroded from the syntaxis, new rutile U-Pb, white-mica 40Ar/39Ar and zircon fission-track data, together with published data are incorporated in a thermokinematic model which suggests an older onset ( > 10 Ma) of rapid exhumation, and at high but not extreme rates (< 5 mm/yr).

The structural geometry and stratigraphic development of continental rifts provides an important insight on the underlying lithospheric processes, seismic hazard assessment, and hydro-carbon prospectivety. Many continental rift basins have been exposed to multiple phases of extension and have been studied extensively in the field, subsurface and using analogue models. However, parameters such as the impact of pre-existing structures, rheology heterogeneity, and thermal perturbation size on the rift geometry evolution are not yet well understood. Furthermore, due to the limited dimension of the data available and the methods applied, the temporal growth character of the fault segments and systems that make up a rift margin covering fault array is not yet well-constrained. The East Shetland Basin is located on the western margin of the north Viking Graben, northern North Sea, and comprises a full fault array, and it is generally assumed that this basin developed in response to two phases of extension, separated by a period of tectonic quiescence. This study uses recently available, high resolution subsurface data that covers the full extent of the basin, and in contrast with previous work, this work proposes that the fault array in the East Shetland Basin gradually developed during a single-phase extension event. Highly detailed qualitative and quantitative seismic analyses show a continuous, diachronous development of the fault array, not only between individual faults but also between the fault segments along strike of the large basin- covering faults from pre-Triassic-to-Late Jurassic. This diachroneity is also observed in the basin-wide, well analyses of chronostratigraphically constrained depositional units. Moreover, a regional assessment of the tectono-stratigraphic relationship implies that the fault activity rate was equal or less than the sedimentation rate during Late Triassic-to-Late Jurassic. Pre-existing normal faults show limited impact on the structural geometry development of the basin, as most pre-Triassic faults get buried or cross-cut during later fault activity and fault reactivation is limited. This study suggests that a changing geometry of the underlying thermal perturbation as predicted by previous numerical models, affected basin- wide strain distribution over time and, therefore, the structural evolution of the fault array in the East Shetland Basin. The diachronous development of a fault array complicates the basin- wide usage of the conventional discrete tectonic stages (i.e., pre- syn- and post-rift) and the assumption of synchronous fault array growth mainly during the syn-rift stage. This work suggests the usage of rift activity maxima and minima when describing fault array development during a first-order syn-rift stage. The work presented here implies that the investigation of fault populations is scale dependent: besides crustal-scale parameters, at fault array scale, lithospheric-scale variables also affect the geometry and temporal development. The results yielded by this study are at the resolution that allow direct comparison to predictions by current numerical models, however, caution is required for the application of predictions from analogue rift growth models, which do not include the role of lithospheric thermal and rheological evolution.

The structural and metamorphic evolution of the Darjeeling-Sikkim Himalaya, a “classical” region of inverted metamorphism in the Himalaya, has been investigated by field studies combined with 2-D numerical modelling of the thermal evolution. In the Himalaya, an orogen-wide zone of inverted isograds is spatially associated with the Main Central Thrust (MCT). In the Sikkim-Darjeeling region, an inverted metamorphic <i>field</i> gradient is indicated by garnet-grade rocks in the upper Lesser Himalaya (LH), which increase in grade to sillimanite + K-feldspar assemblages in the middle to upper structural levels of the Higher Himalayan Crystallines (HHC). Metamorphic breaks in the “Barrovian sequence” have been established between the garnet- and sillimanite-bearing rocks in the Darjeeling region and between the kyanite-staurolite schists and biotite-sillimanite schists in Sikkim. Since the accurate location of the MCT is critical to constraining the metamorphic evolution of the Higher and Lesser Himalaya, a number of criteria are used in defining the MCT zone in this region. These include lithologic contrasts, increase in non-coaxial deformation features towards the MCT zone and geomorphology. The MCT forms a zone of distributed ductile deformation that has propagated southwards with time, resulting in a 3-10 km wide zone, containing rocks from both the Higher and Lesser Himalaya. Four episodes of deformation and two metamorphic events have been identified in the HHC. Textural evidence and garnet zoning profiles indicate a single episode of prograde metamorphism, but four deformation events in the MCT zone and the LH. Garnet zoning profiles from the HHC indicate retrograde equilibrium. M₁ resulted in a peak assemblage of prismatic sillimanite + K-feldspar as well as muscovite dehydration melting resulting in millimetre to centimetre scale leucosomes, while M₂ is associated with rapid exhumation of the HHC during simultaneous movement along the MCT and the South Tibetan Detachment System (STDS) forming decompression textures in metabasic boudins and pelites.

This thesis details the results of an investigation into the behaviour of slopes of liquefiable sand under earthquake loading and the influence of these laterally spreading slopes on inclusions such as pile foundations passing through them. A study of the behaviour of these slopes has been carried out using the techniques of dynamic centrifuge modelling. Eight tests were carried out on laterally spreading slopes and a further five on slopes containing instrumented pile foundations. Each model was subjected to a sinusoidal input motion using a Stored Angular Momentum earthquake actuator, causing liquefaction of the sand and lateral spreading of the slope. Data from instruments measuring acceleration, fluid pressure, total stress and bending moment were logged during the earthquake and analysed to reveal information relating to the performance of these slopes during earthquakes. The experiments highlighted the importance of the dilation of liquefied soil to the behaviour of liquefiable slopes. Slope movements were limited by dilation during each cycle of the earthquake which prevented significant soil flow velocities from building up and large pressures were applied to piles from the liquefied soil owing to dilation of soil close to the pile foundation. It was shown that these large lateral forces were not wholly transmitted into bending moments due to the dynamic response of the piles, but this could be the cause of significant pile damage in other situations. It was also seen that present design methods are non-conservative for both the induced bending moments and the applied lateral loads. Further research is needed to develop better design guidance for this situation. The data from these experiments was compared with the results of a number of numerical models constructed during this work in order to simplify the prediction of the behaviour of these slopes. The displacements of the slopes were compared with those predicted using a Newmarkian sliding block approach modified to include the effects of excess pore pressures. This was shown to give reasonable agreement with centrifuge data, though it requires input of a measured or predicted time-history of excess pore pressure to calculate threshold accelerations.

Energy released from the Earth’s crust in the form of earthquakes commonly follows a powerlaw gamma type probability distribution. This spontaneous organisation is in apparent contradiction to the second law of thermodynamics that states that a system should naturally evolve to a state of maximum disorder or entropy. However, developments in the field of modern thermodynamics suggest that some systems can undergo organisation locally, at the expense of increasing disorganisation (or entropy) globally through a process of entropy production. The primary aim of this thesis is to investigate self-organisation in the Earth’s seismogenic lithosphere as a driven, far-from-equilibrium, self-organising ‘dissipative structure’ in a very near critical steady-state and the underlying general mechanisms involved. The secondary aim is to test in more detail the applicability of the Bak, Tang and Wiesenfeld (BTW) model of Self-Organised Criticality (SOC) in describing Earth’s seismicity. This is done by: 1. Mathematical derivation of analytical solutions for system energy and entropy using the tools of equilibrium statistical mechanics; 2. The study of conservative and non-conservative versions of the BTW numerical model and 3. Analysis of temporal and spatial properties of earthquake data from the Harvard Centroid Moment Tensor catalogue and the Global Heat Flow Database. The modified gamma distribution predicts analytically that entropy S is related to the energy probability distribution scaling exponent B and the expectation of the logarithm of seismic energy hlnEi in the form of the gamma entropy equation S » BhlnEi. This solution is con- firmed for both numerical model results and real earthquake data. Phase diagrams of B vs. hlnEi suggest that the universality in B need not be maintained for a system to remain critical provided there is a corresponding change in hlnEi and S. The power-law systems examined are different from equilibrium systems since the critical points do not occur at global maximum entropy. For the dissipative BTW model at a steady-state, the externally radiated energy follows out-of-equilibrium power-law gamma type statistics, but, the internal energy has two icharacteristics that are indicative of equilibrium systems; a Gaussian type energy probability distribution and a Brownian noise power-spectrum for the internal energy fluctuations. This suggests an observer dependency in assessing criticality. The internal and external entropies calculated for the model are negatively correlated suggesting that driven systems self-organise at the expense of increasing entropy globally through a process of dissipation. A power-law dependency of mean radiated energy hEi on dissipation 1¡® is confirmed for a locally driven dissipative system in the form hEi » (1¡®)¡0:975. The BTW model shows spatial heterogeneity whilst maintaining universality in contradiction to previous assumptions. The quantitative analysis of real data reveals that earthquakes are more predictable spatially then temporally. Regionalisation using the Flinn-Engdahl classification shows that mid-ocean ridges are more organised (lower entropy) than subduction zones. A regional study of three different scaling exponents suggests that universality in earthquake scaling is violated, in contradiction to the original model of SOC. A model of self-organised sub-criticality (SOSC) is proposed as an alternative model for Earth seismicity. Overall, the results suggest that the tools of equilibrium thermodynamics can be applied to a steady-state far-from-equilibrium system such as the Earth’s seismogenic lithosphere, and that the resulting self-organisation occurs at the expense of maximising dissipation and hence entropy production.

The effect of scale on shear strength is studied by performing direct shear tests on different sized samples of large joints. A rubber moulding system is used to obtain impressions of the roughness from a variety of natural joint surfaces. A brittle model material is developed and used to cast several sets of identical interlocking specimens, which are in turn subdivided into sets of equidimensional joint block samples, each of the sets representing a different average block size or joint length. All sample sizes are tested in the same relative direction of shearing and under precisely the same level of normal stress. A total of eleven joints displaying a large variety of surface roughness is included in the experimental programme. The results from those tests show that peak shear strength is a strongly scale-dependent variable. Scale effects are most pronounced in cases of rough undulating joints, whereas they are virtually absent for relatively planar joints. The key factor behind the changing behaviour and strength with increasing scale is the involvement of different sizes of asperities in controlling the peak behaviour of different lengths of joints. An important consequence is a decrease in the value of roughness coefficient (JRC) and wall strength (JCS) operating at larger scale. Use of those indices may enable realistic allowance to be made. for the scale effect in peak shear strength. The normal deformability of rock joints is investigated by conducting loading/unloading and repeated load cycling tests on a wide ranging variety of fresh and weathered joints in five different rocks. The shear deformability is studied by performing direct shear tests at different levels of normal stress in a portable shear box. Several mismatched joints are also tested under compression. In the present work the analytical representation of the stress deformation curves is considered in detail. Quantitative relationships between maximum closure, aperture, wall strength and roughness are defined.

During the late Silurian to early Devonian, end-Caledonian sinistral transpression caused movement on a series of N to NE trending strike slip faults that transected the Dalradian terrain. One of this set of faults, the Tyndrum Fault, was active from the end of the Caledonian orogeny to after Permo-Carboniferous times. Detailed mapping of fault zone fractures and veins has revealed a long and complicated movement history involving both sinistral and dextral strike-slip fault movement. Reactivation involved both the formation of new structures and the utilisation of old structures The Tyndrum Fault hosts both economic lead-zinc and gold-silver-telluride mineralisation. The gold-silver mineralisation occurred in a sinistral strikeslip fault set adjacent to the Tyndrum Fault probably before 410Ma. The leadzinc mineralisation occurred at - 340 Ma in a dextral strike-slip movement and is hosted by the Tyndrum fault plane and in subparallel fractures. Many episodes of quartz mineralisation have also occurred. Individual mineralisation episodes show a cyclic character, with hydrothermal brecciation and cataclastic events. When distinctive mineralising fluids were pumped through the fault, individual groups of cycles may be recognised. Several cycles were responsible for the gold mineralisation. An initial hydrothermal brecciation deposited quartz and pyrite. Electrum was adsorbed onto this pyrite in a following cataclastic event, and several more cycles of hydrothermal brecciation and cataclasis occurred. The lead-zinc mineralisation occurred by a similar mechanism, and these cycles may be related to the earthquake process. Stable isotopic ratios from veins containing the gold mineralisation have been determined. 0180 values for quartz ranged from +10 to +15 %0. oD values from water in fluid inclusions ranged from -91 to -34%0. 034 S val ues ranged from +0.25 to +8%0 for pyrite. Later lead-zinc veins in the vicinity gave 034S values of +50/00 for galena. Fluid inclusion studies indicated the gold-bearing veins were formed from C02 -rich, low salinity, unmixed fluids. The 't values of. 290-350°C probably represent true trapping temperatures. The isotopic ratios of the mineralising fluid were calculated as between -1.0 to +6.80/00 fa r sulphur and for oxygen as between +3.6 to 6.8%0. Comparison with nearby .porphyry-style mineralisation suggests an original magmatic origin for the mineralising fluid but there may be an influence from a meteoric component. The sulphur isotopic ratios suggest a mainly country rock source, though a magmatic component is also apparent.

Little is known about how the internal structures of explosive calderas vary with time, and whether this can affect magma differentiation, eruption behaviour and periodicity. Some exhumed calderas underwent a single piecemeal collapse – e.g. Scafell caldera,UK (Branney and Kokelaar, 1994), however this study tests whether some calderas become more intensely fractured with time due to successive distinct caldera-collapse eruptions. This scenario might lead to more frequent, smaller eruptions over time (noncyclic); magma leakage through an increasingly fractured volcano might also lead to less evolved compositions with time due to shorter residence times (Ferriz and Mahood, 1984). This could have important implications for predicting future catastrophic eruptions. This study revisits the modern ~20 km diameter hydrothermally active Los Humeros caldera (México) where this hypothesis was first formulated, to see how well the structural evolution can be reconstructed, and whether changes in structure affected the styles and periodicity of eruptions. Detailed structural mapping, documentation of draping and cross-cutting field relations, together with logging, petrography, XRF major and trace element geochemistry, 40Ar-39Ar and radiocarbon dating of the pyroclastic stratigraphy has revealed that: (1) eruptions were less frequent after the second major caldera collapse (the ~10 km diameter nested Los Potreros caldera); (2) both calderaforming and smaller explosive eruption volumes decreased by an order of magnitude over ~460 ka; (3) changes in eruption periodicity and magnitude were driven by the rate of magma replenishment, possibly superimposed over caldera cyclicity; (4) the last ~230 ka records the repeated tapping, fractionation and replenishment/re-heating of an evolved magma body; (5) the arrival of exotic, primitive magma triggered the second caldera-forming eruption and caused extensive post-caldera collapse magma mixing; (6) mapping has revealed many structural features that conform to observations at ancient calderas and analogue models; (7) Los Potreros caldera subsided in two discrete stages.

No description available.

No description available.