• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 634
  • 527
  • 257
  • 3
  • 2
  • 1
  • Tagged with
  • 3084
  • 734
  • 245
  • 234
  • 233
  • 233
  • 131
  • 126
  • 120
  • 115
  • 113
  • 113
  • 109
  • 109
  • 99
  • 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.

A geophysical investigation of the south-east Greenland continental margin

Featherstone, P. S. 1976 (has links)
During the summers of 1973 and 1974 geophysical observations were made, aboard R.R.S. Shackleton, across the south east Greenland continental margin, between 58 and 65 N. The thesis describes the reduction and interpretation of the magnetic, bathymetric, gravimetric, and deep seismic reflection data and gives details of the digital deconvolution and C.D.P. stacking techniques developed for processing the reflection data. The magnetic results indicate that, south of 63 N., anomaly 24 is the earliest recognisable oceanic magnetic anomaly. North of 63 N., anomalies 22-24 cut out against the margin, and a complementary widening of ocean floor of this age, on the opposite Rockall margin, north of Hatton Bank, indicates that a local westward migration of the spreading axis occurred, north of 63 N., shortly after the split. Igneous intrusives, outcropping on the rise, post-date the continental split by several million years, indicating that the volcanic activity of East Greenland may have occurred some time after continental separation started. Airgun and sparker profiles show three major sediment groups. Two groups of Tertiary age are separated by an erosional unconformity, beneath the rise north of 62 N. The upper sediments are interpreted as contour current deposits of Miocene and later age, and the lower sediments as lithified oozes of about Eocene age. Below these Tertiary sediments, older, seaward dipping reflectors occur between anomaly 24 and the scarp. These are interpreted as Mesozoic sediments overlying subsided continental crust. The oceanic-continental crustal boundary, as recognised from magnetic anomalies, occurs to the east of this subsided region and lies up to 80 km seaward of the scarp, which is an erosional feature cut by contour currents. Gravity profiles indicate that the main change in crustal thickness beneath the margin lies up to 80 km landward of the scarp north of 63.5 N; but corresponds more nearly with the scarp further south.

Marine geophysical studies between northwest Scotland and the Faeroe Plateau

Himsworth, Eldred Michael 1973 (has links)
A marine geophysical survey of the northern Rockall Trough including the Banks to the north and northwest, the Wyville-Thomson Rise and the Hebridean continental shelf was carried out in 1970 and 1971. Gravity, magnetic and seismic reflection data indicates that the central Rockall Trough is underlain by about 5 km of sediment overlying a normal oceanic crust. The sedimentary thickness decreases to about 3 km and the crust becomes anomalously thick at the northern end of the Trough. Gravity and magnetic interpretation suggests that the Faeroe-Shetland channel is also underlain by anomalously thick oceanic crust. Gravity interpretation indicates that George Bligh, Bill Bailey's and Faeroe Bank are underlain by crust of continental thickness. The Wyville-Thomson Rise, which connects Faeroe Bank to the Scottish continental margin, is composed of two basement ridges of pre-Lower Oligocene age shrouded by sediments up to 1.5 km thick. The north easterly ridge is continuous from the Bank to the continental margin but the south westerly ridge terminates about 50 km from the margin. Magnetic and gravity evidence indicates that the ridges are composed of igneous material and that crustal thickening occurs beneath the ridges. An intrusive complex of unknown age lies beneath the southwest flank of the Rise. Gravity, magnetic and bathymetric interpretation indicates that the Hebridean continental shelf is underlain by Lewisian basement. Gravity and magnetic interpretation indicates that a NNE-SSW trending sedimentary basin about 1.5 km deep and with a partial covering of Tertiary lavas lies between Lewis and the Flannan Isles. Tertiary intrusive complexes, recognisable by their magnetic, gravitational and bathymetric effects, are present beneath St. Kilda, below the continental slope 75 km northwest of St. Kilda and beneath the shelf kO km north-northwest of the Butt of Lewis

Crust and upper mantle structure in the region of Barbados and the Lesser Antilles

Westbrook, Graham K. 1973 (has links)
The Lesser Antilles form one of only two island arcs that occur in the Atlantic Ocean. Bathymetric, gravity, magnetic, and seismic reflection data were collected by HMS HECLA during 1971 in an area bounded by latitudes 12 54’ N and 13 54' N, and longitudes 57 W and 61 10' W, including the islands of St. Lucia, St. Vincent, and Barbados. These data are reduced and interpreted in conjunction with seismic refraction data from an experiment run in 1972 and data from other published and unpublished sources. The structure of the upper sedimentary layers is derived from the seismic reflection records. The crustal structure is modelled two dimensionally using a non-linear optimisation technique to fit the observed gravity and seismic refraction data. The island arc and the Barbados Ridge are examined in detail, and the nature of a ridge running eastward from St. Lucia into the Atlantic Ocean basin is investigated. Magnetic anomalies are treated by direct modelling, magnetic to gravity transformation, and analysis of the power spectrum. The seismicity of the eastern Caribbean is considered with respect to possible plate motions, and maps of focal depth and energy release are presented. The mechanism and causes of subduction beneath the Lesser Antilles are discussed. The possible gravity anomaly caused by subducted lithosphere is estimated and its effect on the determination of the crustal structure examined. The influence of the relative motions between the North and South American plates on the development of the Caribbean and the Lesser Antilles is studied. Some ideas on the origin and growth of the Barbados Ridge and the island arc are put forward. Geophysical data profiles, and computer programs for reduction and interpretation of data are presented in appendices.

A seismic refraction study of the crustal structure of North West Scotland and adjacent continental margin

Armour, A. R. 1977 (has links)
In the summer of 1975, the Department of Geological Sciences, University of Durham, carried out a long range refraction project using explosions fired in Rockall Trough and the Hebridean Shelf areas. Temporary recording stations were situated in the North-west Highlands and Islands of Scotland. The data was supplemented by recordings from the permanent networks of Scotland. Plus-minus, time-term, velocity filtering and particle motion processing methods were applied to the data. Wherever possible, results from gravity and magnetic studies and seismic reflection profiles were used to compliment the interpretations. On the Hebridean Shelf, small sedimentary basins and lateral variations of basement velocity are shown which correlate with earlier gravity and magnetic interpretations. Low average crustal velocities on the shelf, west of the Hebrides, give crustal thicknesses of about 25 km but east of the Hebrides higher average crustal velocities give estimates of about 30 km. A mid-crustal refractor is not clearly observed but may be at a depth of about 18 km on the outer shelf. The Minch area shows large P(_n) time-terms but this does not necessarily signify thickened crust since they may be attributed to velocity anomalies within the crust. Continental crustal thinning and transition to oeanic crust at the Rockall Trough margin takes place over a narrow zone of about 50 km width. Crustal thicknesses in Rockall Trough, at 58 N, are between 7 and 12 km and further north, near Rosemary Bank, are between 12 and 24 km.

Stress analysis of the lithosphere

Dean, D. S. 1972 (has links)
Computer programs have been written to perform finite clement calculations with 3-noded elements for the cases of plane strain, plane stress and axisyrmetry and to produce data for these programs. The latter is specifically orientateo towards (geophysical models and includes a facility for quasi-horizontal boundaries. These have been used to investigate the stress field ansociateo with sedimentary basins and continental margins. From those calculations, the preferential formation of normal faults on. and to the continental side of margins, with the subsequent formation of deep sedimentary basins, is explained. Quantitative studies on the diffusion of stresses through mathematical models of the lithosphere and asthenospherc have given time constants of 200,000 years for plates of the size of the Pacific. Simulated failure has been built into this model and leads to the prediction of a 10 yearly failure cycle with energy releases of 10(^27)ergs. The finite element models have been extended cope with dynamic cases and used to calculate the natural frequencies if free vibration of a sedimentary basin. For a Rayleigh type response the fundamental period has been shown to be directly proportional to the derth of the basin. The natural frequencies calculated are of the same order as those measured in surface waves generated by earthquakes and a Fraunhofer absorption effect is suggested.

A geophysical investigation of the Nova Scotian continental shelf

Goodacre, Alan Kenneth 1976 (has links)
A major east-west structural discontinuity, termed the Minas Basin - Chedabucto Bay - Orpheus fault zone, transects Nova Scotia and the adjacent Nova Scotian continental shelf. To the north of this discontinuity, the geophysical evidence indicates structural continuity between the late Precambrian basement rocks of south-eastern newfound lane and Eastern Cape Breton Island. The generally dense basement rocks of the northern Scotian Shelf are pierced by intrusions of magnetic granite and traversed by linear belts of volcanic rocks. The basement is depressed into a regional east-west trough which is filled by Palaeozoic and younger sedimentary rocks. To the south of the major east-west discontinuity, the southern Scotian Shelf consists of a south easterly dipping early Palaeozoic basement complex intruded by granite and overlain by a wedge of late Palaeozoic and younger sedimentary rocks which reach their maximum thickness east of Sable Island. The most notable feature on the Nova Scotian continental shelf is the Orpheus Graben which lies along the Minas Basin - Chedabucto Bay - Orpheus fault zone and which is filled with Mesozoic sedimentary rocks. Many of the basement structural features on the Nova Scotian continental shelf appear to be products of continental collisions during the early Palaeozoic Era while other features are probably due to continental separation in the late Palaeozoic and Mesozoic Eras. In particular, the Minas Basin - Chedabucto Bay - Orpheus fault zone was probably initiated in Siluro-Devonian times as the result of the thrusting of the north-western portion of Africa into the late Precambrian rocks of the Avalon (Acado-Baltic) Platform of south-eastern New Brunswick, eastern Cape Breton Island and south-eastern, Newfoundland. Later on, the Orpheus Graben was formed along the existing zone of weakness probably in Triassic or Jurassic time as a result of continental break-up and the formation of the present-day Atlantic Ocean.

A seismic refraction study of crustal structure between the Faeroe Isles and Scotland

Smith, Peter Joseph 1974 (has links)
A seismic refraction project known as the North Atlantic Seismic Project (NASP) was carried out in the NE North Atlantic between Scotland and Iceland during July 1972. This thesis presents the results and interpretation of the data obtained between Scotland and the Faeroe Islands. The first arrival travel time data was analysed by firstly fitting straight line segments by least squares, and secondly by time term analysis. The shot-station configuration of the project favoured time term analysis as this method combines the large quantity of data obtained, and the interpretation is not limited by apparent velocities. Amplitude measurements were made on some of the data in order to positively identify the large amplitude secondary arrivals observed as the reflected phase from the Moho, PmP. This phase was used to supplement the crustal structure information obtained from the first arrivals, and theoretical travel times have been calculated for these reflections. Little use has-been made of S wave arrivals. Two main crustal layers were established beneath the Scottish shelf with a Moho depth of 25-26 km. The upper basement layer is at a depth of about 2-3 km beneath mainly Palaeozoic sediments (5.0 km s(^-1)) and has a P wave velocity of 6.1 km s(^-1). It is interpreted as Lewisian schists and gneisses. The lower layer at a depth of about 9 km has a P wave velocity of 6.48 km s(-1) and is interpreted in terms of granulite facies Lewisian material. A normal Moho Pn velocity of 7.99 km s(^-1) was found. A continental crustal thickness of about 30 km was determined beneath the Faeroe Plateau. There is quite an abrupt transition in the basement material between the north west and south-east regions of the Plateau. The material in the north-west has a velocity of about 6.1 km s(^-1) and is interpreted as normal continental metamorphic rocks such as gneisses. In the south-east the velocity of about 5.5- 5.6 km(^-1) is interpreted in terms of slates, and the transition between the two as a change in metamorphic grade. The crustal structure was not well determined beneath the Faeroe/Shetland Channel but the region appears to be underlain by anomalous oceanic material with a Moho depth of 11-18 km. Two main crustal layers were identified with velocities of about 4.65 km s(^-1) and 6.l6 km s(^-1), and at depths of about 2.2 km and 7.9 km respectively.

Estimations of Q from seismic Rayleigh waves

Burton, Paul W. 1973 (has links)
The specific attenuation factor, Q(_γ)(^-1), has been estimated from seismic Rayleigh waves in the frequency range 0.015-0.11 Hz. The 95% confidence limits determine a narrow region around all estimates. The observational data consists of digitised Rayleigh wave traces from film chips of the long period vertical component instruments of the WWSSN stations. Events used are nuclear explosions in Movaya Zemlya, the Lop Nor region of China (Southern Sinkiang Province) and the Aleutian Islands. The group velocity and spectral amplitudes are obtained for each seismogram using an improved "multiple filter technique Q(^-1)(_ γ) is estimated by a least squares regression fit to the subsequent amplitude distance plots. Values of Q(^-1)(_ γ) are generally larger when determined from Novaya Zemlya (.004) than for the Lop Nor test site (.003). The largest values of (.009) are found at low frequencies (0.02 Hz), implying a zone of high dissipation in the upper mantle sampled by these frequencies alone.-1The observed values of Q(^-1)*_ Q(^-1)(_ γ) are directly inverted using an extended Monte-Carlo technique - "Hedgehog". This successfully inverted the data from Novaya Zemlya revealing a region of high dissipation coincident with the low velocity zone, although low velocity is not assumed. The inversion model shows Q(^-1)(_a) = .002, Q(^-1)(_β) = .0045 for the uppermost 120 km and Q(^-1)(_a) = .007, Q(^-1)(_β) = .015 (Q(_a) = 140, Q(_β) = 65) in the absorption zone below 120 km.

Crustal and upper mantle structures in oceanic regions

Haigh, B. I. R. 1973 (has links)
Bathymetric data from the North Atlantic and other oceans reveal that, in addition to the well-documented variations of structure which occur at right angles to the ocean ridge crests, variations of structure also exist parallel to the strikes of the ridges. A thermal model of sea-floor spreading, together with data concerning possible mineralogical phase changes, is used to generate synthetic ocean ridge topography. Comparison of this with observed bathymetric data indicates that a peridotitic composition including water is favoured for at least the top half of the lithosphere, and allows a picture of the variations of structure in the upper mantle to be built up. It is found that lateral inhomogenities in mantle temperatures are able to explain both the variation of ocean ridge dimensions and the uplift of different parts of the ocean basins relative to sea level which are evidenced by observed data. An empirical relationship between the calculated thicknesses of the lithosphere and the depths of the ocean basins is suggested. A major area of raised temperatures may underly the North Atlantic, stretching from the Azores to a focus at Iceland. There is evidence for the existence of other regional anomalies in mantle temperatures, but none of the magnitude of that suggested to lie beneath the northern North Atlantic. The time taken for thicker lithospheres to cool to equilibrium following their formation at the ocean ridges imposes a fundamental limitation on the capabilities of the model, and makes undisturbed bathymetric profiles essential if reliable measurements are to be made in areas of lower mantle temperatures. The results support suggestions that large scale mantle convection, not necessarily related geographically to the ridge crests, may occur, and indicate that active replenishment of the thermal anomalies may be required to sustain them over geologically significant periods of time.

The geology and petrology of the Pre-Cambrian basement between Sirdal and Åseral, Vest Agder, Norway

Leake, R. C. 1972 (has links)
The field relations and petrography of the rocks of the PreCambrian basement complex between Sirdal and Aseral, comprising two series of high-grade metamorphic gneisses separated by a structural discontinuity, syntectonic granites, intrusive quartz monzonites with thermal metamorphic aureoles and basic dykes, are described. During orogeny the gneisses were subjected to intense poly-phase deformation, three regional and two localised phases of which have been recognised. Minor fold relics within augen gneiss in the lower gneiss sequence suggest that this rock was involved in earlier deformation. the climax of metamorphic crystallisation occurred at the low-pressure granulite facies-amphibolite facies boundary with mineral parageneses corresponding closely with the sillimanite-cordierite-orthoclase subfacies of the Abukuma-type cordierite amphibolite facies except for the additional occurrence of orthopyroxene. Major and trace elements X.R.F. analyses of gneissic and some igneous rocks are presented. These data reveal significant differences between basic rocks of the two gneiss series, basic gneisses with different mineral assemblages and to a lesser extent different lithostatigraphical units in the upper gneiss series. Electron microprobe analyses of alkali feldspar, plagioclase, biotite, hornblende, clinopyroxenes, orthopyroxene, sphene, magnetite, ilmenite, chlorite, and garnet from several rock types are presented. With the exceptions of alkali feldspar, magnetite and ilmenite all minerals are chemically homogeneous and represent original equilibrium compositions. The chemical inhomogeneity of alkali feldspar resulted from post-crystallisation leaching and redistribution of alkalies, resistance to which is related to grain size. Equilibrium during original feldspar crystallisation is indicated by the restricted composition of plagioclase coexisting with alkali feldspar. The distribution of titanium and magnesium between coexisting silicates indicates equilibrium compositions, influenced by oxygen fugacity, the nature of the coexisting iron oxides and the tetrahedral aluminium content of the hydrous phases in addition to the rock composition. The application of several means of multicomponent paragenesis analysis reveals that the various mineral assemblages can be interpreted in terms of variations in major element rock composition and oxygen fugacity. The widespread molybdenite mineralisation is considered to have been transported from depth in siliceous hydrothermal solutions into the gneisses, especially where the strike of the gneissic layering coincided with deep fractures. Fixing of the metal as sulphide occurred particularly in the vicinity of pre-existing fahlband sulphide due to release of sulphur in the local environment of increased oxygen fugacity.

Page generated in 0.0493 seconds