In this dissertation, the large-scale geophysical behaviour of the Earth, Venus and Mars are compared, using data collected by the Magellan spaceprobe (for Venus) and the Viking and Mars Global Surveyor (MGS) probes (for Mars). Neither Venus nor Mars show evidence of plate tectonics operating at the present day. On Venus, the lack of water means the frictional resistance at faults and the viscous drag on the base of the moving lithospheric plates are too high to be overcome by the driving forces for plate tectonics. The high elastic thickness of Mars results in a large frictional resistance to fault motion, although the faults themselves are probably no stronger than those on the Earth, and means large compressive stresses are required to initiate subduction. The likely high viscosity of the martian mantle, a consequence of its probable dryness and low temperature, may also result in large drag forces on the base of the lithosphere. Plate tectonics may have operated in the past on both planets, providing a possible explanation for the rapid resurfacing of Venus required by the crater counts and the linear magnetic anomalies recently discovered on Mars.

A theory of pile failure, based on buckling instability is proposed in this thesis. The main postulate of this theory is that if piles are too slender they require lateral support from the surrounding soil if they are to avoid buckling instability. During earthquake-induced liquefaction, the soil surrounding the pile loses effective confining stress and can no longer offer sufficient support to the pile. A slender pile may then buckle sideways in the direction of least elastic bending stiffness pushing aside the initially liquefied soil, and eventually rupturing under the increased bending moment and shear force. Lateral loading due to slope movement, inertia or out-of-straightness increases lateral deflections, which in turn induces plasticity in the pile and reduces the buckling load, promoting more rapid collapse. These lateral loads are, however, secondary to the basic requirements that piles in liquefiable soil must be checked against Euler's buckling. This theory has been formulated based on a study of fifteen case histories of pile foundation performance and verified using dynamic centrifuge tests. Analytical studies also support this theory of pile failure. A hypothesis of post-buckling pile-soil interaction is also developed to fit the centrifuge test data. Centrifuge tests were designed in level ground to avoid the effects of lateral spreading and the main aim was to study the effect of axial load as soil liquefies. The failure mode observed in the tests was very similar to those observed in the field in laterally spreading soil. It is concluded in this thesis that it is not necessary to invoke lateral spreading of the soil to cause a pile to collapse. The pile may even collapse before lateral spreading starts. The key parameter identified to distinguish whether the pile pushes the soil (buckling) or the soil pushes the pile (lateral spreading) is the slenderness ratio of the pile in the liquefiable region. The critical value of this parameter is approximately 50. In summary, it has been shown that the current codes of practice for pile design omit considerations necessary to avoid buckling in event of soil liquefaction. These codes are inadequate and buckling needs to be addressed. It has been identified that many of the structures designed based on the current codes of practice may be unsafe and may need retrofitting. Therefore, a design method is proposed taking into consideration the buckling effect.

Quantitative analysis of the tectonic evolution of the North Sea has tended to concentrate on the Mesozoic and Cenozoic rifting phases (Sclater & Christie, 1983; Barton & Wood, 1984; Latin & White, 1993). A Permo-Triassic phase of lithospheric extension has been recently interpreted from tectonic subsidence data from the northern North Sea (Hellinger <I>et al</I>., 1989; White & Latin, 1993; Roberts <I>et al</I>., 1995) and speculative suggestions on the influence of this phase on estimates of successive phases of rifting have been made (Hendrie <I>et al</I>., 1993). The aim of this dissertation is to examine the late Palaeozoic/early Mesozoic geological history of the southern North Sea. 631 one-dimensional stratigraphic sections were modelled by backstripping and strain rate inversion. A model of lithospheric extension accounts successfully for the observed tectonic subsidence. No previous study has undertaken a comprehensive quantification of Permo-Triassic lithospheric extension within the North Sea. Six phases of lithospheric extension in the southern North Sea, Danish Basin, Wessex Basin and East Irish Sea basin have been interpreted by modelling of water-loaded subsidence data: (i) Late Carboniferous (approximately 320 to 296 Ma), (ii) Late Permian-Early Triassic (256 to 240 Ma), (iii) Early Jurassic (208 Ma), (iv) Mid-Jurassic-Early Cretaceous (180 to 155 Ma), (v) Late Cretaceous-Early Tertiary (90 to 50 Ma) and (vi) Late Tertiary (25 to 2.5 Ma). Lithospheric stretching in the Late Permian-Early Triassic commenced at 256 Ma, and shows an acceleration in tectonic subsidence at 245 Ma. The average duration of rifting is 15 Ma and the average stretching factor, β, is 1.15. In localised areas, such as the Sole Pit Basin, β=1.16 to 1.20. In this work it has been shown that the lithospheric plate model is a valid approximation for the structure of continental lithosphere. Inverse modelling of observed subsidence data from the southern North Sea is used to estimate (i) the lithospheric thickness; (ii) the linear coefficient of thermal expansion and (iii) the basal plate temperature. The values obtained (120 ± 5 km, (3.65 ± 0.25) x 10<SUP>-5</SUP> °C<SUP>-1</SUP> and 1425 ± 125°C, respectively) are consistent with those of Parsons & Sclater, (1977).

Mafic-ultramafic igneous rocks from the Early Proterozoic intra-cratonic Cuddapah Basin of southern India have been studied for their petrological and geochemical characteristics. A field reconnaissance survey was accompanied by sample collection from several stratigraphic horizons in the sedimentary basin. <SUP>40</SUP>Ar-<SUP>39</SUP>Ar laser fusion age determinations on phlogopite mica in the Tadpatri mafic-ultramafic sills have provided firm constraints on the age of the earliest phase of the mafic magmatism in the basin at 1.9 Ga. Petrographical and mineralogical studies of the Vempalle have Tadpatri sills from the Cuddapah Basin have provided some insight into the complex magma chamber processes that may have modified their parental melt compositions. This has been further investigated by studying variations in the major, trace and rare-earth element concentrations of the lavas and sills. Geochemical modelling using the major and trace element concentrations of mafic rocks from the Cuddapah Basin has provided firm constraints for the melt generation processes during the Early Proterozoic. Both forward and inverse geochemical modelling of the lavas and sills suggest that the former were generated by ~ 8-12% partial melting of a lherzolite mantle source whereas the sills were generated by larger degrees (~ 12-18%) of partial melting at a mantle potential temperature (T<SUB>p</SUB>) of ~ 1500<SUP>o</SUP>C. The thickness of the mechanical boundary layer predicted by the geochemical modelling, after lithospheric stretching, is 70 km with a minimum initial lithospheric thickness of 120 km. This corresponds to a lithospheric stretching factor (<I>β</I>) of 1.6-1.8.

Metamorphic Studies in Adula. The Central Alpine Adula Nappe is the largest of the Penninic basement nappes, and was subducted to between 60 and 80km depths in a south-directed subduction zone during the continental collision of Europe (Adula) and Africa in the Tertiary period. High pressures during subduction resulted in a regional eclogite facies metamorphism. Subsequent high-temperature metamorphism during decompression destroyed most of the high-pressure assemblages which were preserved mostly as relicts in mafic boudins and rare pelitic outcrops. High-pressure metabasic rocks show a regional variation in mineral assemblages consistent with increasing metamorphic pressures and temperatures from north to south. In the north of Adula high-pressure amphibolites preserve the assemblage garnet-barroisite/or glaucophane-epidote-quartz-rutile±clinozoisite±phengite±paragonite± carbonate±omphacite. The incidence of omphacite increases in these rocks from north to south and some high-pressure amphibolites show transitional characteristic towards eclogites. In central parts of Adula metabasic rocks preserve the eclogite facies assemblage garnet-omphacite-rutile±kyanite±amphibole±phengite±paragonite±carbonate ±clinozoisite/zoisite±quartz. The incidence of kyanite in eclogites increases from north to south while micas and amphiboles become less common. A sequence of four main phases of deformation affected all investigated parts of central and northern Adula but only the first two phases were associated with eclogite facies metamorphism. The first (Sorreda) phase of deformation was associated with the subduction of Adula and the prograde route into eclogite facies. The second (Zapport) phase of large scale isoclinal folding and development of the main foliation began under eclogite facies conditions and continued during decompression.

The kinematics of the Turkish-Iranian Plateau are studied using information from the focal mechanisms of earthquakes, observations of the geomorphology associated with active faulting, and published GPS measurements. Combining these sources of data makes it possible to examine how the velocity field is accommodated by active faulting. A band of previously unrecognised oblique normal faults is described, rotations about vertical axes are shown to be occurring in the northern plateau, and the age of initiation of the current configuration of faulting is estimated. The dynamics of continental deformation are then considered, in a series of studies of parts of the India-Asia collision zone. The observed surface velocities are found to be consistent with viscous flow in response to gravitational body forces, and the importance of the lower boundary condition is discussed. Deformation maps for common rock-forming minerals show modelling results to be consistent with laboratory measurements of the rheology of minerals. Gravitationally-driven flow provides an explanation for the occurrence of normal-faulting earthquakes in the southern Tibetan Plateau, and for the formation of the Eastern Himalayan Syntaxis. The final part of this thesis combines the two approaches described above. The kinematics of the southeastern margin of the Tibetan Plateau are examined in detail, and numerical modelling is used to suggest the origins of the observed velocity field. It is found that the long-wavelength deformation is driven by pressure gradients in the crust resulting from topographic slopes, and that horizontal surface velocities alone cannot be used to distinguish between two possible modes of deformation.

In order to understand the evolution of deep sedimentary basins, the spatial and temporal development of the Bonaparte Basin has been studied in detail. This basin was chosen because of the quantity and quality of data available. Over 7,500 km of seismic data, including 2,500 km of deep seismic data imaging to 14 seconds two-way-travel-time, were interpreted in an attempt to constrain the depth and structure of basement across the entire basin area. A database of over 100 exploration wells, across the Bonaparte Basin and North West Shelf, was used to determine the ages and lithologies of a total of 19 interpreted seismic horizons. Results for the Bonaparte Basin show that the current global average values of the expected subsidence and strain rate variation for sedimentary basins, are heavily skewed towards a minima since the global data sample only the areas of sedimentary basins where basement <4 km. The Bonaparte Basin data lie in the same trend as the global data, but extend the range of the expected limits of <I>β</I>. However, these data do not exceed the maximum strain rate defined by the global data, i.e. ~10<SUP>-15</SUP>s<SUP>-1</SUP>. This continuity suggests that the global estimates for subsidence and strain rate are not representative of the full range recorded in sedimentary basins worldwide. A more accurate range in this global variation is obtained if the total area covered by sedimentary basins can be quantified. This method of quantifying deep sedimentary basins is generic, provided adequate seismic data which image the entire sedimentary succession to basement and well data to constrain the interpreted seismic horizons, are available.

No description available.

The study of the structure and composition of passive continental margins provides an insight into the mechanisms of continental rifting and its evolution to seafloor spreading. Wide-angle and normal-incidence seismic data acquired during July-August, 1995 (RRS <I>Discovery</I> cruise 215), has been used to determine the structure of the ocean-continent transition in the southern Iberia Abyssal Plain. A basement and mantle velocity structure is derived by P-wave travel-time and synthetic seismogram modelling along a 320 km long wide-angle seismic profile coincident with the pre-existing deep seismic reflection profile IAM-9 located at ~40°20'N. A series of five short (~60 km) wide-angle profiles and coincident MCS reflection sections constrain the structure of the peridotite ridge, an enigmatic basement feature identified to form part of the ocean-continent transition along the West Iberia Atlantic margin. The structure of thinned continental crust, close to its suspected oceanward limit, is imaged along four intersecting depth migrated MCS seismic profiles that lie close to the Ocean Drilling Program (ODP) Legs 149 and 173 transect of basement drill sites. On the IAM-9 wide-angle profile, a 190 km wide ocean-continent transition zone (OCT) is observed. The OCT includes a pair of overlapping peridotite ridges, and is bounded by oceanic crust and landward by fault bounded blocks of continental crust. The continental crust thins from 28 km to 7 km below the continental slope, over a lateral distance of 80 km.

This dissertation describes the use of a number of seismic techniques to probe further the crustal and uppermost mantle shear velocity structure of the collision zone and the undeformed Indian shield to the south. A study of Rayleigh wave fundamental mode group velocity dispersion curves for 4054 receiver-source paths across India, Tibet and surrounding regions is used to obtain high-resolution group velocity maps between 10s and 70s. The dataset provides a higher frequency content than previous global studies and, with the inclusion of long paths up to ~5000km, bridges the gap between regional and global studies. This provides better constraints on whole crustal structure. Higher frequency P to S receiver functions are used to resolve the position of the major interface beneath seismic stations across the region; most importantly the crust-mantle boundary. Joint inversion of receiver function data and group velocity dispersion data limits the non-uniqueness inherent in receiver function inversion which is highly sensitive to a depth-velocity trade-off. The receiver function study is divided into two parts, defined by geographical area. Firstly, data from a number of broadband stations deployed over the course of this research in West Bengal and Sikkim are analysed alongside data from the INDEPTHII deployment which provides a northwards extension of the profile into Tibet. Data from previous experiments in nearby Nepal and Bhutan are studied in order to give a more complete picture of the crustal structure of this region of the Himalayas. Secondly, receiver function data for a large number of stations across the South Indian shield are revisited to provide an improved and coherent picture of variations in crustal thickness across the different geological terrains. Finally, dispersion curves extracted from the series of group velocity maps produced for the region are inverted for shear velocity structure.