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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

The geology of the region between the Alness River and the Dornoch Firth

Armstrong, Matthew 1964 (has links)
The stratigraphy, petrography and structure of the Old Red Sandstone between the Alness River and the Dornoch Firth have been examined in detail.
32

Towards using seismic anisotropy to interpret ductile deformation in mafic lower crust

Tatham, Daniel John 2008 (has links)
The lower crust forms an important geodynamic control in continental tectonics and the communication and coupling of kinematics between surface and deep-Earth processes. An understanding of the relationship between seismic properties, finite strain and fabric orientation thus provides a useful tool in the remote sensing and interpretation of deformation in the lower crust. This thesis outlines a work-flow model by which the seismic properties of a single and representative lower crustal lithology can be calculated and calibrated against finite strain from petrofabric development across a strain gradient. The work-flow model constitutes a multi-disciplinary approach, incorporating field mapping and sample collection, experimental petrofabric determination, and seismic modelling. A review of compositional estimates of the deep crust, including xenoliths, exposed sections and estimates from wide-angle seismic profiles, indicates the importance of mafic lithologies. The Laxfordian-age high-grade shear zone at Upper Badcall, NW Scotland, exhibits a strain gradient in a deformed doleritic Scourie dyke (Lewisian complex) that intersects the zone at a high angle. From an analysis of field data from detailed mapping, the shear zone is shown to be characterised by generally simple shear, but where the tectonic movement direction varies transversely across the shear zone. Calculation of the strain profile across the deformation zone gives shear strains, y up to 57, but with y < 15 being perhaps more realistic. Cumulative displacements total ~1000m left-laterally, and ~600m vertical displacement, north-side up. Nine samples were collected across the shear zone in the mafic dyke, representing a strain gradient from undeformed protolith to the highest recorded stains. The sample suite is characterised as a hornblende-plagioclase-quartz aggregate that develops macroscopic planar and linear fabrics with strain, from an essentially isotropic protolith. Quantification of the aggregate lattice preferred orientation (LPO) using electron backscatter diffraction (EBSD) showed the dominance of fabric development in the hornblende phase, with (100) poles clustering forming normal to the foliation plane and [001] axes parallel to the tectonic X direction. Plagioclase and quartz retained random fabrics from the wall-rock protolith with increasing finite strain. The hornblende LPO fabric, described by the texture index, J, shows a positive logarithmic relationship with strain, where LPO intensity saturated by y ~10. The strain-calibrated quantitative petrofabric description of each sample is used to calculate their aggregate elasticity tensors (Cij) via a Voigt-Reuss-Bill average, and from which seismic properties are derived using Christoffel's equation. Hence, a framework of petrofabric- and strain-calibrated seismic properties is described for a strain gradient in a representative high-grade mafic lithology. P-wave anisotropies up to ~10% are-recorded in the most deformed samples with Vsmax typically between 6.42-6.63kms/-1. S-wave anisotropies record up to 7.23% AV, in the most deformed samples, with Vpmax ranging between 3.62-3.75kms-1 for all samples. The relationship between petrofabric-derived seismic anisotropy and finite strain across the sample suite show a positive relationship, approximated by a logarithmic function, whereby P- and S-wave anisotropy exhibit a steep positive gradient with strain up to y~10. The sample-wise framework of petrofabric- and strain-calibrated seismic properties is interpolated to estimate the continuum relationship between seismic properties, finite strain and petrofabric orientation. In a move to illustrate the application of results in seismic and structural modelling, case study models of crustal deformation are presented for the eastern Basin and Range province, the North Sea rift, and Tibet. Models are promising in their ability to differentiate between regions of lower crust characterised by a uniform mafic composition but different finite strain state and/or petrofabric geometry, although multiple seismic survey methods may be needed to fully interpret results in terms of strain and fabric orientation. In summary, a multidisciplinary approach combining field mapping and sampling, petrofabric characterisation with EBSD, and seismic modelling provides an efficient and reproducible work-flow for the determination of petrofabric-derived strain-calibrated seismic properties of lower crustal materials.
33

The geology of the Blaenau Ffestiniog area, Merionethshire

Bromley, A. V. 1964 (has links)
No description available.
34

The structure of the upper mantle beneath East Africa

Forth, Philip A. 1975 (has links)
This study concerns the analysis of arrivals at the Kaptagat array in Kenya set up by the Durham University Department of Geological Sciences. The array consisted of ten short period seismometers and was sited 10 km vest of the Elgeyo escarpment, which forms the western boundary of the Gregory Rift. Onset time analysis was used to determine the slowness and azimuth of approach for teleseisraic arrivals. Large deviations from the expected slowness and azimuth are found and it as shown that the major cause of these anomalies is the low velocity upper mantle which is assumed to exist beneath the Kenyan domal uplift. A preliminary analysis of 60 teleseismic arrivals shows that the anomalies cannot be explained in terms of a single plane interface or any plane structure. It is shown that if the data is to be explained by a single boundary then the structure must thin both to the north and west of the station. The structure was thus assumed to be caused by a structure which is ellipsoidal in plan and hyperbolic in section and optimum structures were calculated for various assumed velocities. Relative teleseismic P-wave delay time data between Kaptagat and Bulawayo, which confirms the presence of low material beneath the station, was, reinterpreted on the basis of the model proposed and was found to be consistent with a depth to the bottom of the structure between 150 and 300 km. The arrival of a phase corresponding to a reflection from the top of the proposed structure was searched for in the records from local earthquakes using velocity filtering techniques but no consistent arrival could be identified with confidence. A preliminary study showed that the postulated seismic structure was not inconsistent with the available gravity data.
35

Data processing and computer techniques for marine seismic interpretation

Birch, Roger W. J. 1973 (has links)
This work is divided into two sections, the first containing results and interpretations from marine seismic reflection profiling performed by Durham University (1972) in a region to the north of the Faeroe Islands, and the second containing theories for the removal of multiple reflection effects from marine seismic records by means of digital data processing techniques. The seismic profiling investigations were carried out to ascertain the geological structure causing the gravity 'low' north of the Faeroes which had previously been proposed by Bott, Browitt and Stacey (1971) to be caused by an infilled valley. Results from the 1972 survey shows that this infilled valley has a limited aerial extent containing relatively large basement undulations. Further work was carried out to obtain information about the sedimentary' sequencies and to try and correlate these with sediments in surrounding regions where data had been obtained by previous workers. The profiling work (1972) indicated three major sequencies within the sedimentary column with an overall thickening of sediments away from the uplifted areas of the Iceland - Faeroe Ridge and Faeroe Islands. The data processing section deals principally with the removal of multiple reflections from marine seismic records. An introduction is given to the basic concepts involved throughout this work, and includes a description of noise theory and types of multiple reflections encountered in marine seismic profiling. Some previous methods for multiple elimination are improved upon and then two new techniques are developed, applied to seismic sections, and finally a comparison made between the techniques used. All programs are written in FORTRAN IV for use on the IBM 360 computer, and for displaying purposes, facilities available with the Durham IBM 1130 plotting system were used.
36

Structure and petrochemistry of the Reykjadalur central volcano and the surrounding areas, midwest Iceland

Johannesson, Haukur 1975 (has links)
The main geological feature of the research area is a Tertiary flood basalt pile, about 2.7 km thick, within which are three Tertiary central volcanoes. The "basement" consists of faulted and tilted basalts, probably about 12-13 m.y. old. These basalts were overlain by the Hredavatn sedimentary horizon, 7 m.y. ago, followed by lavas of the Hallarrauli central volcano which was active from 6.7 to 6.0 m.y, ago. The time sequence of the volcanism was of acid ignimbrites, then intermediate flows, and finally thin tholeiite flows. The volcano did not evolve the structural or geothermal characteristics usually associated with central volcanoes. The Reykjadalur central volcano became active about 5.8 m.y. ago and is situated on a series of thick tholeiite flows which overlie unconformably the Hallarrauli central volcano. Voluminous acidic and some intermediate rocks formed first and were succeeded by thin tholeiite flows. A collapse caldera, 10 km in diameter, then formed and was filled with volcanics. After the formation of the caldera, a basic to acid cone sheet swarm, about 20 km in diameter and concentric to the caldera, intruded the country rocks. About 4.3-4.4 m.y, ago, soon before the volcano became extinct, a massive icecap covered the volcano and its environs; the Holtavorduheidi sedimentary horizon covered the area east and southeast of the volcano at this time. The Laugardalur central volcano consists of a caldera and its volcanic filling; it may be parasitic to the Reykjadalur central volcano. Later, 3.4 m.y, ago, acid tephra was erupted from the Litla Baula vent and the Baula acid intrusion was emplacedat this time. Volcanic activity was renewed about 2 m.y. ago, represented by the Snjofjoll series and a few more recent volcanics. The Tertiary basalts are tholeiitic. The rocks from the central volcanoes show a trimodal volume distribution, with basalts and rhyolites more abundant than rocks of intermediate compositions. Three distinct breaks occur in the chemical properties : between the olivine tholeiites and the quartz tholeiites; between the quartz tholeiites and the basaltic icelandites; and within the dacites. The basalts were derived from the Upper Mantle, olivine tholeiites being produced by a greater degree of partial melting and/or at greater depths than the quartz tholeiites. On volume and time relations and on geocheraical grounds, the acid and intermediate volcanics are considered to be derived by two-stage melting of the Icelandic lower crust. The acid magmas were generated first, followed by intermediate magmas from partially melting the residuum at a higher temperature. Although this process is fuelled by basaltic intrusions within the lower crust, the late appearance of basalts in the central, volcanoes is attributed to the density-depth relations between the crust and the rising magma.
37

A seismic refraction survey of the earth's crust beneath the Lesser Antilles

Boynton, Colin Harford 1974 (has links)
Seismic refraction data from the Lesser Antilles Seismic Project of April 1972 are analysed to delineate the crustal layers beneath the Lesser Antilles Island Arc, Grenada Trough, Tobago Trough and Barbados Ridge. First arrival travel times from 23 of the LASP recording stations are analysed by the classical reversed-profile, modified ‘plus-minus’ and time-term methods of interpretation. Two crustal layers of velocities about 6.2km s(^-1) and 7.0km s(^-1) are found beneath the Lesser Antilles ridge. It is suggested that the upper layer is of largely andesitic composition and that the lower layer represents the old oceanic crust which has been intruded with basic magma from the upper mantle beneath the arc. The presence of two crustal layers is common to most island arcs. Shear-wave velocities and Poisson's ratio for the upper crust beneath the arc are estimated, and these suggest that no large magma chambers are present above about 5km. The crust of the Tobago Trough is shown to consist of a layer of velocity 7.0km s (^-1), overlain by a thick pile of sediments and ? metamorphics, This structure shows closer affinities to 'Atlantic' crust than to 'Caribbean' crust. The results from the Grenada Trough are shown to be consistent with previous interpretations of the crustal structure. These results indicate that there may have been an earlier period of subduction at a site some distance west of the present Lesser Antilles subduction zone. This may be related to the possible formation of the Aves Ridge as an active island arc in late Cretaceous – early Tertiary times.
38

Upper mantle structure using P wave data from an East African array station

Blackhouse, Richard W. 1972 (has links)
A seismic array of ten short period seismometers has been set up at Kaptagat, 10 km west of the Elgeyo escarpment in Kenya, by the Durham University Geology Department. This study concerns the analysis of teleseismic arrivals. The method of onset time analysis is used to calculate values of dT/dΔ and azimuth of approach for 34 events from epi central distances of 18 - 99º. Large systematic variations in slowness and azimuth are observed as a function of the great circle azimuth to the epicentre. These are similar to those found at other arrays but are of unusually high amplitude. Prom consideration of onset time residuals and the velocities of regional earthquake arrivals at Kaptagat it appears that these effects are not caused by variations in near-surface structure or by errors in the array geometry. An explanation is developed in terms of sharply dipping interfaces beneath the array, and the preferred model is of a mantle low velocity zone with sloping boundaries underneath the Gregory Rift. This anomalous zone attenuates rapidly westward to sink below mantle material typical of the stable areas of Africa. P-wave delay times are measured relative to Bulawayo for 78 events in the distance range 24 - 99º. Substantial values confirm the presence of a considerable low velocity body beneath the array and the absence of large variations with azimuth implies ultra-low velocities with relatively moderate dips on the zone boundaries. A reinterpretation of Rayleigh wave phase and group velocities for the AAE-NAI path is made and found to be compatible with the top of the low velocity mantle material sinking eastward from the Gregory Rift underneath normal shield-type topmost mantle. Although the model derived is probably not continuous throughout the East African rift system, the upper mantle structure beneath the Gregory Rift is similar to that suggested to exist below oceanic ridges.
39

Vibroseis encoding

Bernhardt, Thomas 1977 (has links)
The FM signals, called sweeps, used in the Vibroseis method of seismic exploration show a considerable amount of energy in the sidelobes after correlation detection. These sidelobes represent signal generated noise and if not kept low in amplitude they miggit mask subsequent reflections, thereby reducing the detection capability of the Vibroseis system. The purpose of this research has been to investigate new coded signal design techniques for the use with Vibroseis, in order to achieve sidelobe suppression. Some of the codes examined have already been known to radar and communication theory, whilst some codes are original developments of this research exercise. Binary and quaternary complementary series are found to be especially suitable for a Vibroseis encoding technique. A new and simple algorithm for the generation of quaternary series from known binary complementary sequences is given and the concept of correlation matrices is introduced to complementary series, permitting signal design in the detection window. The encoded Vibroseis input signals were tested on a computer and showed perfect sidelobe suppression a certain distance away from the main compressed pulse, when detected by a matched filter. Field tests with the coded signals were conducted, taking advantage of a computerized Vibroseis field system. The tests showed promising results. However, it became clear that the vibrator control devices will have to be adjusted to the transmission of such sophisti cated signals, in order to allow substantially better results than in the conventional Vibroseis system. A ‘Continuous Vibroseis Transmission System’ is suggested, transmitting energy during the normal listening period. Such a system has been developed witht h help of so-called ‘Mutually-Orthogonal-Complementay Sets of Sequences’ and although not yet practically tested its anticipated advantages and disadvantages are described. Finally, ‘Predistortion’ as a method of Vibroseis signal design is examined. Providing the correct predestortion parameters are chosen, the signal-to-correlation noise ratio an be increased. A spectrum whitening effect observed an addition of selected perdistorted sweeps can be of advantage in a quaternary comlementary coded Viboseis system, permitting an optimal wavelet design in the detection window.
40

A seismic investigation of the lithosphere of the Gregory rift

Savage, John Edward Graham 1979 (has links)
During 1976 and the first six months of 1977, the Department of Geological Sciences at Durham University maintained networks of temporary seismic stations over the southeast flank of the Kenya dome and in the central section of the Gregory rift. At each station, signals from local and teleseismic events were recorded from a three component set of seisometers onto magnetic tape. Recorder generated timecode, and BBC GMT pips recorded alongside, enable reproduced seismograms to be timed accurately. Waveform matching of replayed teleseismic P-wave arrivals enabled relative onset times to be obtained with great accuracy. Delay times were obtained for each of the 24 stations, also with high relative accuracy. It is shown that the significantly larger delay times obtained for stations near the culmination of the dome must be due to the presence of anomalously low P-wave velocity material in the upper mantle. A localised trough in the pattern of delay times over the rift and coincident with the positive axial Bouguer anomaly is shown to be due to the presence of anomalously high P-wave velocity material within the crust. Preliminary interpretations assume horizontal layering beneath each station. Flat bottomed models, assuming a uniform anomalous zone velocity of 7.5 km/sec are derived for profiles running southeast over the flank of the dome and across the rift. Interpretations for the flank show a sharply increasing thickening of the anomalous zone towards the rift, with a secondary thickening near or under Mt. Kilimanjaro. The rift profile shows that the anomalous zone penetrates the crust to within about 20 km of the surface. A depth of 120 km is deduced for the base of the anomalous zone, but this may be in error due to systematic error in the baseline of station delays. To circumvent the significant errors associated with the assumption of horizontal layering, a three-dimensional ray tracing technique is devised. Flat bottomed models are derived assuming uniform anomalous zone velocities of 7.5 and 7.0 km/sec. The 7.0 km/sec model shows a thinner and shallower anomalous zone, but the overall shapes of these models are in good agreement with the preliminary models. Deficiences in the ray tracing technique are discussed and it is shown that the parameters characterising the three-dimensional models are not well controlled. Suggestions are made for improving the technique. The models are all consistent with the theory that upward perturbation of the 1ithosphere-asthenosphere boundary, giving rise to magmatic activity, thinning of the lithosphere and domal uplift, is the primary cause of rifting.

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