The present thesis is concerned with the volcanogenic sulphide deposits associated with the Troodos ophiolite, Cyprus. Extensive petrological, chemical and field data are presented and these suggest that the ophiolite has formed on a spreading axis which was located above a subduction zone. This tectonic regime was responsible for the enrichment of the Troodos primary magma in silica and the large ion lithophile elements. Formation of the Sheeted Complex and Lower Pillow Lavas is envisaged to have taken place close to the axis of spreading. The lavas are extrusive products of subaxial chambers which exhibited both open and closed system fractionation. The Upper Pillow Lavas are considered to be derived from the same primary magma as the other members of the ophiolite but exhibit only limited fractionation. The difference between the two suites is attributed to extrusion of the Upper Pillow Lavas on the flanks of the spreading axis, from diapirically rising magma chambers. A detailed geochemical study of the hydrothermal metamorphism of the ophiolite suggests extensive mobility of components in the Sheeted Complex. All geochemical changes are in agreement with experimental and field evidence of seawater-rock interaction; they suggest enrichment of the rocks in magnesium and sodium and extensive leaching of calcium, silica, barium and zinc. Strontium is locally leached and concentrated in secondary minerals. Calculations of element fluxes during hydrothermal metamorphism suggest that the amounts leached from the Sheeted Complex can account for the amounts deposited in the sulphide deposits and metalliferous sediments (umbers). A hydrothermal model is proposed according to which seawater trapped within the Sheeted Complex is heated under the influence of the geothermal gradient, enriched in metallic and other elements by breakdown of primary silicates and finally driven off by the intrusion of magma chambers at high levels in the crust. These chambers are deduced to be those which formed the Upper Pillow Lavas, as inferred from the association of sulphide deposits with the initial stages of eruption of these lavas. Examination of the field occurrence of several sulphide deposits suggests that these are most often localised by faults which were inherited from the tensional regime of the spreading axis. Localisation at sites of structural intersections is also described. The size of the deposits is determined to a great extent by the structural disturbance in the area in which they occur, smaller deposits being favoured at strongly tectonised areas. In the absence of tectonism ore localisation may take place within the lava pile. It is concluded that the presence of a heat source is more important than structural disturbance in the formation of deposits. The results suggest, also, that there was only one period of sulphide-producing hydrothermal activity and this is temporally placed at the early stages of extrusion of the Upper Pillow Lavas. The mineralogical characteristics of deposits are described, with emphasis on the behaviour of the hydrothermal fluids at discharge sites. The main conclusions suggest that extensive interaction of the fluids with the wall rocks and seawater takes place and this determines the final mineralogical character of the deposits and the formation of copper-rich and copper-poor orebodies. The extent of interaction is determined to a large extent by the structure and results to the formation of extensive alteration zones in deposits which formed deep within the lavas. Exchange between the fluids and wall rocks is mainly characterised by enrichment of the surrounding lavas in magnesium and sodium, depletion in calcium and strong enrichment in barium and potassium. The mineralized lavas are depleted in alkalies and enriched in magnesium. The implications of the above conclusions on exploration are briefly discussed, with emphasis on the future application of geochemical methods in the search for sulphide deposits.

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Archaeomagnetic studies were carried out on fired and sediment samples collected from predominantly 'rescue' British archaeological sites. Four of the fired sites were suitable for refraction and wall movement studies. The results showed no clear evidence for systematic wall movements or refraction. The scattered directions were explained in terms of local movements, inhomogeneous refraction, small scale anisotropy, fabric anisotropy and the presence of iron objects. The grain size and colour of some of the sediments were studied and it was found that the presence of shards affects the magnetization of the sediments but this will depend on the percentage of shards and their magnetization. No direct relation was found between the intensity and the stability of the sediments The brown and red samples appear to have the highest intensities and relatively higher stabilities. The accumulated British and French archaeomagnetic data were assessed and presented as an archaeomagnetic data bank which was re-evaluated. Revised archaeomagnetic curves were plotted for both Britain and Paris. A total of 96 samples were collected from 19 Iraqi archaeological sites. These were used to build an initial archaeomagnetic curve for Iraq. A comparison of different palaeointensity methods on specimens taken from a single Roman brick, 3 other Roman bricks, and on nine Iraqi samples showed that the Thellier and Kono and Ueno methods give the most reliable results. Magnetic characterization of 28 Iraqi obsidian artefacts (using their magnetizations and refractive indices) indicated that all samples except one came from a single source.

Following a major crustal rifting episode centred on the Krafla volcanic system in Northeast Iceland, the local seismicity was monitored for three months using a dense network of single component seismometers. Initial earthquake locations were computed for 489 local events using a one-dimensional velocity model derived from seismic refraction data. Activity was concentrated in clusters beneath a geothermal area within the Krafla caldera and below the Bjarnarflag geothermal well field to the south, and in a narrow linear zone coincident with a site of recent dyke injection. Events of magnitudes -2.4 to 2.1 were located. The b-value for the entire dataset is 0.77±0.10, and is lower for events at Bjarnarflag than for events in the dyke injection zone. Seismicity was continuous. A simultaneous inversion of 1771 P-wave arrival times was carried out to calculate the 3-d velocity structure and refine hypocentral locations. The derivative weight sum was used to identify the well-resolved volume. High velocity bodies at depth beneath the rim of the caldera are high density intrusives, probably gabbros. Volumes of low velocity coincide with zones of geothermal exploitation within the caldera and beneath Bjarnarflag, and result from intense fracturing and hydrothermal alteration. After relocation through the three-dimensional velocity structure, the spatial distribution of hypocentres is more focussed and considerably shallower than before. This led to a closer correlation between zones of seismicity and detailed features within the source volumes, such as geothermal reservoirs, fault surfaces and zones of known recent magmatic intrusion. Focal mechanisms were determined using P-wave polarity data for 153 of the best located earth quakes, of which 139 have double couple solutions. Fourteen events were non-double couple, 4 of which could be solved as either opening or closing tensile cracks assuming small circle nodal lines. The biasing effect of using incorrect hypocentres and an over-simplified velocity structure were investigated by 3-d ray tracing. The effects on ray angles are large and variable. The impact of these effects on double couple solutions is generally small, but can be critical for the confident identification of non-double couple events. Seismicity in geothermal areas results from cooling and fracturing of hot intrusives at depth, and fault surfaces are marked by enhanced seismicity when they provide efficient migratory paths for geothermal fluids. Seismicity may be induced partially by geothermal mining. The stress field in the Krafla volcanic system at the time of the survey was variable along its length, as was the mode of fracturing. The stress field at Bjarnarflag was chaotic. In the geothermal area within the caldera σ(_3) was perpendicular to the plate boundary, and in the dyke injection zone σ(_1) was perpendicular to the plate boundary. Such heterogeneity in the stress field is contrary to what might be expected at a spreading axis, and is attributed to variation in regional extensional stress release during the recent rifting episode.

A geologic-kinematic model for the evolution of the Gulf of Mexico and Caribbean region is built within a framework provided by a detailed Late Paleozoic (Alleghenian) plate reconstruction and a revised North American (NOAM) and South American (SOAM) relative motion history. From the Middle Jurassic to the Campanian, SOAM migrated east-southeast from NOAM. From the Carapanian to the Eocene. Little or no NOAM-SOAM relative motion occurred, although minor sinistral transpression is suggested. Since the Eocene, minor west-northwest convergence between NOAM and SOAM has occurred along pre-existing fracture zones. Three stages of evolution are recognized which correlate with these phases of relative motion. Stage 1: mainly carbonate shelves fringed the Gulf of Mexico and "Proto-Caribbean" passive rifted margins, during plate separation. Stage 2: the Caribbean Plate (CARIB) progressively entered the NOAM-SOAM gap from the Pacific by subduction of Proto-Caribbean crust beneath the Greater Antilles, Stage 3: CARIB migrated east by 1200 km, subducting Proto-Caribbean crust and forming the Lesser Antilles Arc, Transform faults have dissected the original Greater Antilles Arc, and nappes in the Venezuelan Andes have been emplaced southeastwards onto the northern SOAM margin, diachronously from west to east. Field work done in Dominican Republic, both near Puerto Plata and in the southwest sector, indicates that 1) Cuba and northern and central Hispaniola are parts of one original Greater Antilles arc, 2) this arc collided with the Bahamas in the Late Paleocene=Mid Eocene, and 3) Hispaniola has been assembled by strike-slip juxtaposition of terranes from the west.

ODP Leg 125 recovered peridotites from Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc. The peridotites recovered comprise about 95% harzburgites and about 5% dunites, which are variably serpentinised (mostly 60-100%). The Leg 125 peridotites represent some of the first extant peridotites recovered from a forearc selling. A detailed petrographic, mineral and bulk-rock chemical study of the peridotites has been undertaken in order to elucidate information about melting and fluid processes in the forearc mantle wedge. The harzburgites are highly refractory in terms of their mineralogy and geochemistry. They have low modal clinopyroxene, highly magnesian olivine (Mg# = 91.1-93.6) and orthopyroxene (Mg# = 91.6-93.2) and chrome-rich spinels (Cr# = 60-80) and very low incompatible element contents (Ti <80 ppm). Furthermore they are more refractory than the most depleted abyssal peridotite, which suggests that the harzburgites can be interpreted as residues to extensive partial melting (>20%). The Ti concentrations in the clinopyroxene indicate that the harzburgites are residues to -25% fractional melting. However, petrographic and other geochemical information show that the Leg 125 harzburgites have had a complicated melting and enrichment history. Many samples have olivine fabrics which are interpreted as having formed beneath a spreading ridge. Orthopyroxenes have lobate grain boundaries often associated with fresh olivine neoblasts. This texture is interpreted as showing the incongruent melting of orthopyroxene, a process which happens at low pressures (-3 kb) and high water pressures. These two types of textures indicate that the peridotites have had a two stage melting history. Moreover, the V concentrations in the clinopyroxenes can be explained by -15% partial melting at low oxygen fugacities (FMQ-1), followed by 5- 10% melting at high oxygen fugacities (FMQ+1). Oxygen thermobarometry calculations are in accordance with the peridotites last equilibrating under oxygen fugacities of greater than FMQ+1.The bulk-rocks have chondrite-normalised REE patterns showing extreme U-shapes with [La/Sm](_N) ratios in the range 5.03-250.0 and [Sm/Yb](_N) ratios in the range 0.05 to 0.25; several samples have possible small positive Eu anomalies. On extended chondrite-normalised plots the bulk-rocks also show enrichments in Sr and Zr relative to their neighbouring REEs and are enriched in LREE, Rb, Cs, Ba, Sm, and Eu relative to abyssal peridotites. Covariation diagrams based on clinopyroxene data show that Sr, Ce, Nd, Sm, Eu and Zr are enriched in the clinopyroxenes and that the enrichment took place during or after melting. The enrichment component is most likely a melt derived from the underlying subduction zone. A multistage melting and enrichment model is proposed for the peridotites where they first melt 10-15% beneath a spreading ridge. The resulting depleted spinel Iherzolite is enriched and then melted again 10- 15% above the subduction zone to produce the spatially associated boninites. A final enrichment event takes place during and after this melting event to produce the characteristic trace element enrichments in the Leg 125 peridotites.

The RESTE Project was an integrated geophysical-geological study of the Estremadura, southwestern Portugal. The core of the programme consisted of the acquisition and analysis of microearthquake data. This was complemented by an investigation of the structural evolution of the sedimentary basins of the Estremadura. The geological evolution of the Lusitanian Basin was strongly marked by the reactivation of Palaeozoic basement faults, in response to a sequence of tectonic events: opening of the Central Atlantic, opening of the North Atlantic and Alpine convergence between Africa and Eurasia. The current tectonics are regarded as a subdued continuation of the Miocene deformation (Betic Orogeny), and the "tectonic memory" revealed by the Lusitanian and Lower Tagus Basins is explored to characterize the current tectonic processes. Strike-slip tectonics are identified as a dominant feature of several stages of the evolution of the basins, with particular relevance during the Miocene. The technique of "backstripping" is applied to well data, to constrain the history of vertical movement in the basins. This analysis highlighted the pre mature truncation, in the Late Jurassic, of a normal passive-margin evolution. Tectonic unstability caused the structural inversion of areas within the basins, and seems to have inhibited the predictable thermal subsidence. The rifting process, initially taking place at the Lusitanian Basin, jumped westwards in the Late Jurassic. Crustal underplating and the activity of transfer faults are in-voked as possible explanations for the subsequent deformation of the aborted rift. An upper-plate margin configuration is in good agreement with several observations. The tendency for structural inversion continued throughout theCretaceous, and with the onset of the Alpine convergence in the Turonian the control of the tectonic activity seems to have switched from the Atlantic to the Mediterranean. This reinforced the tectonic unstability, marked by magmatic activity and by a regional upwarp that was to last until the Eocene. Of particular interest was the behaviour, during the Late Cretaceous, of the Setúbal Peninsula sub-basin, which seems to have tilted towards the NW as a block, with a hinge line along the present Lower Tagus Valley. When sedimentation was resumed in the Eocene, a pattern of differential vertical movement was established, with some areas continuing to undergo inversion while nearby areas subsided. This pattern characterized the Cainozoic evolution of the basins, and probably still applies to the neotectonic deformation. The activity of strike-slip basement faults, reactivated under the compressive regime caused by the Afro-Eurasian conver-gence, is proposed as the best explanation for the Miocene deformation, with particular relevance for the Lower Tagus Valley. The RESTE Microearthquake Survey is described, and the data acquired with the RESTE network are analysed. The local earthquakes are accurately located, and focal mechanism solutions are obtained for some of them. This information is used to discuss a neotectonic model for the Lower Tagus Valley. In view of their small magnitudes (l.1< M(_L) < 3.8), the focal mechanisms of these events cannot be interpreted directly in terms of the current tectonics. Such small events are usually local readjustments to previous episodes of deformation. However, such features as the along-strike reversal of the polarity of vertical motion or the coexistence at the same region of different types of source mechanism are diagnostic of strike-slip deformation. This model was supported by the occurrence of a macroearthquake (M(_D)=3.8) with an interpreted source mechanism of sinistral strike-slip. The alignment of four hypocentres along the direction of the Lower Tagus Valley, with a compatible orientation of the interpreted nodal planes, supports the existence of a crustal fracture associated with the Valley. The hypocentral depths of the recorded events reach 20 km, showing that the basement faults responsible for the seismicity affect at least the entire upper crust. Since the limited existing data suggest a high level of heat flow in the Lusitanian Basin, the depths reached by the microearthquakes may indicate an abnormally thick seismogenic layer. An investigation of the broad velocity structure of the lithosphere underneath the RESTE Network using the technique of teleseismic tomographic inversion suggested a correlation between Moho undulations and the inversion of areas of the Lusitanian Basin, and this may indicate that the controlling faults cut the entire crust. In order to provide a rationale for the intraplate seismicity of western Portugal, the neotectonics of Iberia are discussed, and a new kinematic model, centred on the idea of continental extrusion, is proposed. According to the model, a continental block formed by Iberia and northern Morocco is being pushed west wards by the convergence between Africa and Eurasia. The resistance offered by the oceanic parts of the plates varies across the East Azores Transform, leading to dextral shear in the Betic Range. The regional stress field induced by the continental convergence can explain the reactivation, in a simple-shear regime, of basement faults of Hercynian orientation, in particular that proposed for the Lower Tagus Valley.

The importance of pre-existing country rock structure in controlling the siting and geometry of intrusive bodies has been overlooked by recent workers, with most studies tending to concentrate on the effect of tectonic processes. This thesis describes three Caledonian plutons emplaced at intermediate crustal levels, the Thorr Granite, Co. Donegal, the Ratagain Complex, Inverness-shire and the Loch Loyal Syenite Complex, Sutherland. None of these plutons appear to have been emplaced in association with large tectonic strains. In all three cases, data collected from both the country rocks and the plutons demonstrates that regional tectonics alone cannot account for the observed shape, fabric evolution and mode of emplacement. Instead these features are more directly controlled by the interaction of the pre-existing structural architecture in the country rocks with fault and shear zones. Thus, whilst tectonic forces act as the catalyst in initiating the creation of space into which magma can be emplaced, the pre-existing structural architecture will control the ultimate form of the pluton. The plutons described in this thesis can, therefore, be considered to represent an intermediate stage between the sheeted intrusions that are emplaced in association with active shear zones, for example the Main Donegal Granite, and high level, passively emplaced plutons, such as the Rosses Granite. The evolution of deformation fabrics within the three plutons is described in detail. In all cases magmatic state deformation fabrics are predominant. However, most of these magmatic state fabrics are aligned parallel to the intrusion margins; this is particularly true in the case of the Thorr Pluton. Such a geometry could be accounted for by invoking the presence of large shear strains at the time of fabric formation. However, in the absence of evidence for large shear strains in the country rocks, it is proposed that such fabric geometries may be produced as a result of localised coaxial strain component in response to the body forces, or 'buoyancy head', exerted by the magma acting across the intrusion walls. Finally, in studying the kinematics of intrusion of the Thorr Pluton, a new technique for determining shear sense in rocks deformed in the magmatic state has been applied. The data collected from the application of this technique was found to corroborate the shear sense data collected from the envelope rocks. In this instance, the technique was a valuable aid to kinematic analysis, and ultimately to deducing an emplacement model for the Thorr Pluton.