One of the most controversial and unresolved questions regarding the evolution of the Earth is the question of when did modern subduction-driven plate tectonics begin? Although the magmatic record implies that subduction-driven plate tectonics began in the Archean c. 3 Ga, the lack of high pressure, low temperature rocks in the pre-Neoproterozoic metamorphic record suggests that the onset of modern subduction occurred after c. 800 Ma. To offer a way forward with the debate, and to look beyond the Neoproterozoic for evidence of such events, this thesis aims to develop a novel way of utilising rutile, a robust accessory mineral that is stable over a large P-T range and commonly found within a wide variety of high grade metamorphic rocks. In this contribution, rutiles from Neoproterozoic blueschists, eclogites and ultrahigh-pressure terranes (Syros, Greece and the Western Alps), and Archean ultrahigh-temperature granulites (Antarctica), have been investigated for mineral inclusions using electron probe microanalysis and Raman spectroscopy. Trace element analysis of rutile has also been carried out using laser ablation inductively coupled plasma mass spectrometry to determine metamorphic temperatures of formation using Zr-in-rutile geothermometry. In the blueschists and eclogites, rutile is shown to contain numerous inclusions of high-pressure minerals such as glaucophane, lawsonite, omphacite and garnet, as well as diagnostic ultrahigh-pressure minerals, including the first reported occurrence of exceptionally preserved monomineralic coesite in rutile from the Dora-Maira massif, Western Alps. The chemical comparison between inclusion and matrix phases show that inclusions generally represent peak metamorphic assemblages; although prograde phases not found in the matrix have also been identified implying that rutile grows continuously during prograde burial and traps mineralogic evidence of this evolution. Pressure-temperature estimates obtained from mineral inclusions using conventional geothermobarometry and average pressure-temperature calculations, when used in conjunction with Zr-in-rutile geothermometry of the host rutile, can provide additional constraints on the metamorphic conditions of the host rock. Furthermore, the preliminary investigation of mineral inclusions within detrital rutiles from river sediments collected in the Po Plain, Western Alps and their comparative chemistry to inclusion and matrix phases within in-situ samples from the nearby Sesia Zone and Monviso metaophiolite, demonstrates the potential of using mineral inclusions in sediment provenance analysis. In the Archean granulites, rutile grains which occur in contact with or adjacent to zircon are found to record temperatures lower than expected as post-peak fluid-rock interaction resulted in significant zircon recrystallization and the variable resetting of zirconium concentrations in rutile. However, rutile grains found as inclusions within orthopyroxene have been shielded from post-peak diffusional resetting and record UHT conditions. In addition, rutile is found to contain inclusions of kyanite, sillimanite, quartz and corundum, demonstrating that rutile has the capacity to preserve mineralogic evidence of ultra-high temperature metamorphism. While the discovery of inclusions in Archean granulites demonstrates that rutile can preserve evidence of metamorphism in old crust, inclusions were not present in all samples leading to new questions as to the formation and growth of rutile. Overall, this study demonstrates that rutile is an excellent repository for mineral inclusions and that the study of mineral inclusions in rutile may profoundly change how we investigate and recover evidence of both high-pressure and high-temperature events in both detrital populations and partially retrogressed samples.

The sedimentary rocks of Sleat of Skye,are represented by Torridonian sandstones and shales and Cambrian quartzites and dolomites with some dolomitic shales and shales. The upper part of the Torridonian,consists of a thick pile of sandstones, poorly stratified and in places strongly contorted. Grain size varies and often shows a bimodal fequency distribution.The major constituents include quartz, feldspars mica and sericitic or clayey material, ore and lithic fragments. The lower part of the Torridonian, contains numerous bands of shale, which in Skye, appear at definite stratigraphical positional, thus making subdivision possible. These shales and sandy shales display well developed statification and show occasional ripple bedding.In places features of flowage and loading may be observed. The Cambrian quartzites overlie unconformably the Torridonian the lower part is strongly false bedded and the upper contains numerous small tubes, believed to have resulted from the burrowing; action of worms. This rock unit is followed by the thin bands of Seroulite Grit of orthoquartzitic composition and icoid Shales. The uppermost part of the succession consists of the Durness Limestones,represented in this area by fine to coarse grained dolomites with numerous small scale sedimentary structures. Some parts of the area are covered by breccias believed to be of sedimentary origin. The ground has been affected by at least three periods of folding,two of which were accompanied also by sets of minor folds.Strong thrusting subdivided the area into several slices, the structures may however be correlated across the thrust planes.

The aim of this thesis is to use cosmogenic isotope analysis to date a late-glacial stillstand in the eastern Highlands of Scotland. The dates link the extent of the local Cairngorm ice cap and the Scottish Ice Sheet at a particular stage of degiaciation. The constraints provided by such dates add insight to the process of degiaciation and the climatic conditions at the time, The geomorphoiogical and sedimentological evidence suggests that the stillstand lasted around 1 ka and was the last of two or three such stages that affected the region. Cosmogenic 10Be results show that the stillstand occurred at 16-15 ka BP and was synchronous across the massif, Correlations between the Cairngorm 10Be ages, data provided by North Atlantic ocean cores, and the Greenland ice core data suggest that the stillstand in the Cairngorms occurred at the same time as Heinrich 1, the final major iceberg ratting event in the North Atlantic at the end of the last glaciation. independent ,4C and tephrochronoiogicai dating from Loch Etteridge within the Spey Valley show that degiaciation proceeded rapidly after the stillstand. This latter data has been used to provide constraints on erosion rates of granite surfaces in the Cairngorms which are less than 10 mm/ka. Two further implications arise from this work, First the last stage of valley glaciation in the Cairngorms occurred at 16 - 15 ka BP, and thus any Loch Lomond Stadial event (Younger Dryas equivalent) in the Cairngorms was confined to the high corries. Second, the dating of the stillstand shows that the eastern flank of the main Scottish Ice Sheet had retreated to 40% of its LGM extent by the time of the Heinrich 1 Event.

Constraining the dynamic feedback between deforming porous media and fluid is crucial for understanding hydrocarbon reservoirs, CO2 storage sites and other evolving porous media. In particular, predicting complex fault architecture at depth, currently relies on deterministic algorithms, that do not take account of these dynamic coupling. For instance, creation of permeability due to fracturing may permit fluid flow to enter a fault zone, resulting in cementation (strengthening) or alteration (weakening) of the fault and host rocks. The resulting changes in rock strength may enhance or retard further fracturing and may even result in a switch of deformation mechanism. Temporal and spatial evolution of fluid flow through faulted porous rocks has been studied in a field site in SE Utah, USA. The field area presents a well-exposed fault system that contains evidence for flow of multiple phases of groundwater with varying chemistries and flow of hydrocarbons. By detailed field mapping and microstructural observations of the fault rocks and of the evidence for fluid flow (e.g. bleaching and hydrocarbon staining) the fluid flow history and evolving flow properties of the rocks has been unravelled. The 6km long fault presents an erosional scarp of up to 20m high at its centre. This scarp is dissected with canyons that permit cross-sectional views of the fault and associated alteration which has been mapped. The field area contains two general classes of lithology. In porous sandstones, deformation is accommodated by deformation bands and fractures. In tight limestones and siltstones deformation is accommodated by fracturing and the formation of clay-rich fault rocks. Evidence for multiple fluid flow events can be observed. Hydrocarbon staining is confined to the coarsest grained layers in the sandstones, and to fractures in all lithologies. Bleaching surrounds fractures in the fault damage zone and along bedding in the fault zone. This thesis presents evidence for a reduction in porosity due to increased burial and cementation to be the biggest influence on deformation mechanisms for fault growth and evolution. Evidence of the evolving structural and hydrogeological properties of the fault zone are explored. This thesis discusses how such data can be used to improve the predictive capability of fault zone properties at depth where the faults have accommodated post-faulting fluid flow.

No description available.

Diamonds from the Sao Luiz alluvial deposit, Brazil, have been found to occlude syngenetic inclusions whose associations are evidence for formation in the mantle transition zone and lower mantle (Wilding, 1990; Harte and Harris, 1994). These diamonds represent the most extensive sample of deep mantle available to date, and have been subjected to detailed study. Five principal associations are identified. One association involves orange garnet inclusions (with diopside and pyrrhotite) which contain a significant pyroxene component in solid-solution (majorite component) indicating formation in the transition zone (Akaogi and Akimoto, 1977). Trends of major element composition against Si content are most consistent with formation within the deepest regions of the transition zone where equilibrium with perovskite structured CaSiO3 (CaSi-Pvk) is envisaged (Irifune and Ringwood, 1987). The remaining associations all involve MgO - FeO (fPer) and are also believed to have been in equilibrium with CaSiO3 composition inclusions. The association of fPer and (Mg, Fe)Si03 (LM I) is understood to have formed at pressures of >24 GPa (Yagi et al., 1978), within the lower mantle, where (Mg, Fe)SiO3 adopts a perovskite structure (MgSi-Pvk) at pressures above the breakdown of (Mg, Fe)2SiO4 ringwoodite. Indeed, all the broadly pyroxene composition phases recovered in association with fPer are envisaged to have formed with perovskite structures. The LM I association also includes grains of broadly pyrope-almandine composition with high Fe3+ content (Fe3+/EFe=-0.7) and very low Ca (<0.15 wt% CaO) and depleted rare earth element (REE) concentrations consistent with equilibrium with REE-phyllic CaSi-Pvk. This new mineral is shown to adopt a tetragonal I :F -2d structure and is referred to provisionally as 'TAPP' (tetragonal almandine-pyrope phase). Given the propensity for MgSi-Pvk to adopt the entirety of the likely lower mantle A12O3 budget within its structure at depths over 820 km (e. g. Kesson et al., 1995), and the stability of an A1203-involving association at depths of 720-820km (Irifune et al., 1996), TAPP is believed to form in aluminous bulk compositions in the depth region, 670-720km. A deeper association of fPer, aluminous and Fe 3'-rich MgSi-Pvk and A1203 (ruby) from Sao Luiz diamonds forms a third (LM H) association. The remaining two associations have characteristics indicative of formation in the deepest regions of the transition zone. An association (LM III) of low Ca-garnet with a small majoritic component, a previously unrecorded C2/c structured Al-Ca-Na-Few-rich magnesium silicate (with 11,5 and 6 wt% A1203, CaO and Na2O respectively) and fPer is reported. Trace element compositions of this garnet are found to be transitional between majoritic garnet (Harte, 1992) and TAPP. The final association, found in a single diamond involves a (Mg, Fe)2SiO4 composition inclusion, fPer and TAPP (UM/LM association), and is suggestive of formation within the range 460-720km depending on bulk composition (Jeanloz and Thompson, 1983). Also identified from Sao Luiz is the first recorded sapphire inclusion in diamond. Change in cell parameters on release of two fPer inclusions (one from Guinea, West Africa) have been measured and interpreted on the basis of expected mantle geotherms and physical properties of compressibility and expansivity. Depths of formation of -300km are inferred which, on correction due to the fractured and plastically deformed nature of the diamond hosts, extend to within the lower mantle. The very low Fe 3+ content of fPer and the large Fe 3+ content of aluminous MgSi-Pvk inclusions additionally support formation at high pressure (McCammon et al., 1995 and McCammon, 1997). Furthermore, the presence of significant quantities of magnesioferrite as inclusions in many fPer inclusions is consistent with the high Fe3+ content of associated phases and indicates relatively oxidised conditions of formation. Partitioning of Fe, Ni and Mg between fPer and MgSi-Pvk is indicative of high temperature (>2000K) within the lower mantle which suggests a steep thermal gradient at 670km and hence a thermal boundary layer between the upper and lower mantle. This observation, in addition to indications from associations of a compositional distinction between upper mantle and lower mantle, supports separate regimes of mantle convection. The diamonds themselves show cathodoluminescencep atterns indicative of a complex interplay of growth and resorption. Transition zone stones show a range in nitrogen content from <15 to 311ppm, and are highly aggregated indicating a long, high temperature history. Lower mantle stones are even more deficient in nitrogen (mostly Type II diamond), and show a very tight clustering of 513C composition around -5%o. Given ranges of up to 9%o within single stones, precipitation under fluctuating conditions within a homogeneous reservoir is concluded. Values for 815N of -6 and -5.2%o have been obtained for an upper / lower mantle boundary sourced stone. Thermoelastic modelling is applied to a variety of deep mantle phases and it is concluded that, with a thermal boundary between upper and lower mantle, there exists a narrow depth region just below 670km where many phases, (particularly diamond) are gravitationally stabilised. Diamond moving within the circulatory system of the lower mantle will, therefore, tend to pond in this region. Exhumation from the deep mantle is believed to have been relatively swift due to the lack of: reequilibration of composite grains; complete exsolution of majoritic garnet; and recombination of magnesioferrite with Per. A regime of transportation by upwelling mantle plume is envisaged. The dominance within thin cratonic areas amongst world-wide locations of deep mantle diamonds is also discussed. This observation is interpreted in terms of thin cratonic areas being suitably reduced to stabilise diamond at shallow depths, unlike in oceanic settings where diamond bums to form C02- Additionally, the crust in thin cratonic regions is not suitable for formation of lithospheric diamond and so the deep population of stones is not outnumbered by shallow sourced diamonds.

The Kimmeridgian of England was characterised by the deposition of a widespread and frequently organic-rich mudrock formation, the Kimmeridge Clay. Significant lateral thickness variations are attributable to active faulting which is occasionally recorded within the sediments as a number--of earthquake-generated structures. The slowest subsidence rates occurred on the London platform and depositional depths increased away from this positive feature both to the north and the south. Organic-rich shales preferentially accumulated in the deeper basinal areas where the thickest sequences accumulated. Even in the deepest locations storms were able to influence the depositional environment. This is reflected by a range of tempestite beds and by the highly opportunistic character of the benthic fauna which responded to the brief storm-induced oxygenation events. Palaeocommunity analysis indicates that a number of variables were controlling the faunal distribution. Substrate consistency, environmental stability and oxygen levels all interacted to produce a diverse range of community types composed of a few euryoxic, opportunistic molluscs. A major change occurs in the middle of the Upper Kimmeridge Clay when softground faunas are replaced by firmground faunas. This appears to reflect a marked reduction in sedimentation rate related to a sudden increase in aridity at this time. Sandy marginal marine facies were developed on the London Platform throughout much of the Kimmeridgian; they are best seen at outcrop in the cliff sections of the Boulonnais. Facies range from shallow marine glauconitic sandstones with intensive bioturbation and a fauna of large bivalves, to shell-poor shoreface sandstones with well preserved sedimentary structures. Analysis of this latter facies indicates that storms and waves were the two principal sources of hydrodynamic energy. Tides were probably of negligible importance in the Kimmeridgian epeiric seas. Local cyclicity in the Boulonnais sediments appears to be due to small-scale fluctuations of narrow, nearshore facies belts; they are not related to the more ubiquitous shale: mudstone rhythms which characterise the offshore mudrocks. Investigation of eustatic sea level changes indicates a Jurassic highstand of sea level in the Lower Kimmeridgian followed by a slow progressive fall through the Upper Kimmeridgian. The highstand appears to have produced relatively deep water conditions in many epeiric settings throughout the world which led to organic-rich shale formation. Finer scale eustatic sea level changes are more equivocal although a highstand may have occurred in the Eudoxus zone. No substantiating evidence was found for the fine scale eustatic fluctuations recently proposed by the Vail team.

The Jura Quartzite, which lies stratigraphically above the Bonahaven Dolomite and the Port Askaig (=Eocambrian) Tillite, is interpreted as a tidal-shelf deposit. A coarse facies, consisting of beach and tidal dune sediments, and several fine facies, comprising storm sediments, are recognised. It is the relationships between these facies that are critical to their interpretation. The proportion of fine facies increases towards the NNE, parallel with the palaeocurrent means. A model for tidal-shelf sedimentation is developed in which storms are the dominant factor in the preservation of dunes as well as in the production of storm layers.

This thesis integrates structural, metamorphic and geochronological data to critically examine key aspects of the tectonothermal evolution of the mid-crustal portion of the Himalayan orogen . in the Makalu - Kharta region of southern Tibet. U(-Th)-Pb ages indicate that in the southern area prograde metamorphism of the mid-crustal Greater Himalayan Series (GHS) peaked at least as early as -38.9 Ma. Subsequent sillimanite grade metamorphism at -28.0 Ma was followed by at least two phases of leucogranite emplacement at -20.8 Ma and -16.7 Ma. To the north, approximately 40 km in the down-tectonic transport direction, prograde metamorphism was occurring at -25.4 Ma and lasted until -16.1 Ma. Immediately following metamorphism, two phases of leucogranite were emplaced at -15.2 Ma and -12.6 Ma, with an intervening phase of ductile deformation. These two outcrops record a northward propagation of peak metamorphism, leucogranite intrusion and deformation events that are -5 - 7 Ma younger in the down-tectonic transport direction. The top of the GHS is bounded by the South Tibetan Detachment System (STDS), a large-scale, north-dipping low-angle normal fault characterised by a -1 OOOm thick zone of distributed ductile shear. Macro- and micro-structural observations indicate that this segment of the STDS records a progression from ductile- to brittle-deformation without development of a single discrete detachment fault, and therefore provides important insight into the early ductile history of the STDS and associated exhumation of the GHS. Lying to the east and structurally beneath the GHS, the Ama Drime massif (ADM) is an anomalous region of elevated topography extending -70km northward from the crest of the High Himalaya. The ADM is bounded by north-striking normal-faults and is cored by 1800 Ma. gneisses, granulitized mafic eclogites and -12 Ma syn-kinematic leucosomes that record crustal-scale east-west extension and ductile exhumation of mid-crustal material that was enhanced by focused surface denudation acting on the Himalayan range front.

No description available.