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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.
51

Visco-elastic finite element analysis of subduction zones

Woodward, D. J. 1976 (has links)
A new visco-elastic finite element method is developed and applied to some of the processes in subduction zones. The effects of phase changes are simulated by deriving an equation of state for the mantle under mineralogical equilibrium. Using the elastic parameters determined from this equation, the stresses due to the contraction of the descending slab as it changes phase at the olivinespinel transition are shown to be about 8 x 10 (^8) n/m (^2). The phase changes are also shown to play an important role in the flexure and bending of the lithosphere from the earth's surface to plunge at 45 – 60o into the asthenosphere. The phase changes effectively reduce the bulk modulus and so the lithosphere bends more easily. The major bending is at 30 - 60 km depth where the stresses due to bending extend the area of phase transitions so that it extends throughout the thickness of the descending slab. Phase changes and fracture combine to reduce the flexural parameters of the lithosphere to that estimated from the shape of the outer-rise. Thin plate theory, however, is shown to be inapplicable to this region. Tensional stresses aligned parallel to the dip of the descending slab are shown to be necessary to maintain the large negative gravity anomaly associated with subduction zones. This applies in all subduction zones, and local stresses must be responsible for the earthquakes indicating down dip compression in the upper part of the descending slab. The shear zone between two converging plates can be adequately modelled in visco-elastic finite element analysis by a row of elements whose viscosity is 2-3 orders of magnitude lower than the surrounding rocks.
52

The geology of the western part of the Northern Range of Trinidad

Potter, Henry Clifford 1974 (has links)
The results of geological mapping in the western part of the Northern Range of Trinidad are shown on 1:25,000 maps. The stratigraphy of the Caribbean group, low grade metamorphics more than 2500 m thick, has been revised. Type sections of the Maraval, Maracas and chancellor formations are established. Correlation with neighbouring area is suggested. The Maraval formation is the oldest in the area, probably Upper Jurassic; it consists of more than 500 m marbles and limestones. The base is not seen. The Maracas formation, also probably Upper Jurassic, lies above the marvel formation. It is approximately 1500 m thick and consists of slates, quartzites and phyllites. Rare volcanic ashes are found. Sedimentary structures suggest palaeocurrent directions from north to south and deposition as turbidites. The chancellor formation lies above the maracas formation and is probably lower cretaceous in age. It is approximately 500 m thick and consists of bedded limestones and phyllites with rare quartzites. It has been divided into four members. On the island of Monos it contains channels of conglomerates in the upper limestones member. The laventille formation, lower cretaceous, lies along the south flank of the Northern Range and is less metamorphosed than its equivalent the chancellor formation. There are two facies – massive limestones and a predominantly shale succession. Evaporates occur. Autobrecciation occurs in the limestones and a zone of contemporaneous dislocation lies along the eastern edge of the massive limestones. The morvant beds consist of shales, sandstones and exotic blocks. They are probably the extension of the upper cretaceous galera formation. Recumbent anticlines overturned to the north form the main part of the Northern Range. Three phases of folding can be recognised. Thrusting occurs parallel to the exes of the folds. The El Pilar fault and similar major E-W faults developed later. These have been described as wrenches but horizontal displacement is unclear, although major vertical movement is described. The main metamorphism is believed to be tectonic, contemporary with the major folding at the end of the Lower Cretaceous. Further deformation occurred in Upper Cretaceous time. Faulting continued through the Tertiary.
53

The crustal structure of northern England

Swinburn, Peter M. 1975 (has links)
The crustal structure of Northern England was examined, using mainly the refraction technique. A three layer crustal model with interfaces at 0 to 3 km, 12 km, and 27 km was interpreted from the data. The lateral variations in thickness of the sedimentary cover was investigated by measuring Pg travel times from quarry blast sources recorded at mobile stations. The time term method was used to interpret the results and showed that the basement varied from 2i to 3h kin deep within the sedimentary troughs to less than 1km beneath the block regions. The interpretation of apparent velocities of crustal phases across Eskdalemuir and Rookhope arrays, and wide-angle reflections, suggested the existence of a lower crust between 12 and 27 km deep, with a velocity of about 6.5 km/sec. The upper crust (Pg about 5.7 km/sec) showed some evidence of velocity increasing with depth, especially the uppermost part of the granite beneath Rookhope array. A refraction line (N.E.R.L.) recorded the shots of the L.I.S.P.B. project throughout the northern Pennines. This line examined the variation in thickness of the main crustal layers across the principal structural units. It suggested that the Moho was approximately level beneath the region. This indicates that the relative elevation of the blocks was isostatically supported by low density granites. The upper mantle has a Pn velocity of about 8.05 km/sec and the character of the phase suggested a sub-moho structure such as an increase of velocity with depth. The surface wave dispersion of teleseismic events recorded at Eskdalemuir and Wolverton was examined. The interpretation suggested a crust 30 km thick and a shear velocity of 4.55 km/sec for the upper mantle.
54

Theoretical studies of the geodynamics of accretion boundaries in plate tectonics

Kusznir, N. J. 1976 (has links)
Various aspects of the physical processes occurring at the accretion plate boundary in plate tectonics have been investigated. Regional stresses have been investigated, arising from lateral density contrasts in the ocean lithosphere. Elastic, visco-elastic and elastic/visco-elastic models predict regional stresses in the ocean basin of the order of 0.25 kb. Investigation of the thermal stresses created in the oceanic lithosphere as a consequence of the cooling of the ocean lithosphere as it moves away from the ridge axis, shows that tensional stresses occur in the upper lithosphere and compressional stresses in the lower lithosphere. An elastic/viscous model of the lithosphere predicts deviatoric stresses of the order of 3 kb. in the upper crust. The temperature distribution beneath the ocean ridge with magma solidifying to form crustal layer 3 has been investigated. Numerical models show that the width of the magma chamber and the thickness of the dyke complex depends on half spreading rate. If there is significant crystal settling, the width of the chamber is predicted to be considerably reduced. A critical half spreading rate of 0.45 cm/yr is predicted, below which the intruded material solidifies instantaneously. Computations support Cann's petrological model. Investigation of the magnitude of the stresses caused by the buoyancy of a magma chamber in the lower crust at the ridge axis suggest that the magma chamber is unable to cause crustal fracture and is, alone, a dynamically stable structure. The additional stresses due to the upthrust of molten upper mantle material is required to cause crustal fracture and a zone of fracture of less than 5 km wide is predicted. The stress field created in the oceanic lithosphere by a mantle plume has been calculated analytically. Estimates of the plume dimensions and velocity suggested by Morgan are predicted to be just sufficient to cause fracture of the lithosphere above the plume axis.
55

The crustal structure of East Africa through earthquake seismology

Maguire, P. K. H. 1974 (has links)
First arrivals of local and regional earthquakes, recorded at the Kaptagat array station on the western flank of the Gregory Rift Valley in Kenya, have been analysed to determine the crustal structure westward from the Rift axis. Apparent velocities have been interpreted in terms of crustal velocities. Azimuth and epicentral distance of event has provided lateral and vertical control on crustal boundaries. Normal shield crust, underlain by normal Moho is concluded to exist on the western flank of the Rift Valley. From the satisfactory propagation of S(_n) from events originating around the western Rift, normal Moho is considered to exist across the East African Plateau. A massive mantle derived intrusion is concluded to have penetrated the crust to within approximately 8 km of the Rift floor. This intrusion reaches to within about 7 km of the Elgeyo escarpment, the major Rift Valley boundary fault immediately to the east of Kaptagat. In order to aid second arrival analysis, a synthetic seismogram programme based on short wavelength asymptotic theory for reflected and refracted waves has been written. A qualitative analysis of real and synthetic seismograms of events originating to the west of Kaptagat, suggests that the crustal structure is more complex than the simple two- layered model initially produced. It is considered that the structure beneath the Gregory Rift is more equivalent to that beneath ocean Rift Systems than that beneath continental Rifts. However, rather than being a direct extension of the "World Rift System, the Eastern Rift and associated Kenya dome should be considered as a single physical unft.
56

Crustal structure of the eastern Caribbean in the region of the Lesser Antilles and Aves Ridge

Kearey, Philip 1973 (has links)
The results of two marine gravity, magnetic and seismic reflection surveys in the eastern Caribbean are presented, the principal structures surveyed being the Venezuela Basin, Aves Ridge, Grenada Trough and Lesser Antilles island arc. The data are presented in the form of charts of bathymetry, free air anomalies, Bouguer anomalies and total field magnetic anomalies. The Venezuela Basin is a broad, extensive basin with a sediment thickness in excess of 1.5 km. At its eastern margin it exhibits small negative free air anomalies and is in approximate isostatic equilibrium. The Aves Ridge is an elevated linear volcanic prominence parallel to the Lesser Antilles characterised by positive free air anomalies. Its typical form is of two basement ridges marking its western and eastern flanks enclosing a sediment filled trough. The western ridges are shown to be more dense and less magnetic than the main body of the Aves Ridge and may represent a series of intrusive bodies. The Aves Ridge exhibits strong magnetic-gravity correlations. It is underlain by a root attributed to depression of the Moho and lowest crustal layer and exhibits negative isostatic anomalies. The Grenada Trough is a basin of subdued topography in the south and more rugged topography in the north. It contains a sediment thickness in excess of 2 km. and is characterised by predominantly negative free air anomalies. It is underlain by an elevated Moho and exhibits predominantly negative isostatic anomalies. The Lesser Antilles lie on a narrow elevated ridge rising abruptly out of the Grenada Trough. The island arc exhibits extreme free air gravity gradients. It is underlain by a root attributed to depression of the Moho and is the site of a linear belt of positive isostatic anomalies. Possible modes of origin of the Aves Ridge are discussed.
57

Development of geoelectrical techniques for investigation and monitoring of landfills

George, Andrew 2006 (has links)
A review of geophysical applications to investigation and monitoring of landfills identified that the geoelectrical methods, such as ERT, SP and EM, are ideally suited because much information can be inferred from the electrical characteristics of landfill waste. The review identified scope for further development due to constraints of geophysical surveying along the ground surface. In particular, the ERT method suffers a loss of data accuracy and resolution with increasing depth of investigation. These constraints have provided the main focus for interest during this research project. A field test site was examined on a reconnaissance basis using a multi- method geophysical approach to obtain the best possible sub-surface characterisation and interpretation in the absence of intrusive calibration data. On the basis of results obtained a model was produced showing combined geoelectrical response, which may be used to plan further detailed investigations including conventional intrusive site examination. It is further demonstrated that the effectiveness of ERT in a landfill setting lies with the arrangement of measurement electrodes and application of specialised electrode address configurations. In an active landfill setting, basal electrodes installed within the cell drainage medium prior to waste emplacement were used in conjunction with ground surface electrodes and the address configuration applied across the array pair. Delineation of basal leachate accumulation and differentiation from perched tables within the waste profile was possible. In a closed landfill setting, where restorative capping and leachate extraction was scheduled, a system for ERT monitoring was established by installing vertical electrode arrays during routine drilling and emplacement of gas wells. In this setting, baseline sub surface geoelectrical characteristics were identified against which the effects of landfill capping and leachate extraction were assessed through variance in model resistivities and percentage resistivity change. This research has provided recommendations for geophysical monitoring best practice, which may help site stakeholders to achieve a more effective management of leachate control systems, to assess the effectiveness of restorative strategies, and to demonstrate legislative compliance with a greater degree of certainty.
58

Geology of the Easdale Slate Group of the islands of South Lorne, Argyll

Batterham, P. M. 1971 (has links)
No description available.
59

The structural geology of the Sierra de Moreno, northern Chile

Skarmeta, Jorge J. 1983 (has links)
No description available.
60

Palaeomagnetic studies on rock formations in the High Atlas and Anti-Atlas regions of Morocco

Hailwood, E. A. 1972 (has links)
The first part of this thesis (Chapters I to 8) describes the results of palaeomagnetic studies on samples collected from the High- Atlas and Anti-Atlas regions of Morocco. New palaeomagnetic pole positions are described from formations of Late Precambrian, Lower Cambrian, Devonian, Triassic, Jurassic and Cretaceous ages. The new Moroccan Late Precambrian and Lower Cambrian poles are shownto be more consistent with reliable Lower Palaeozoic poles of the same age from North America than ones from Southeast Africa, when the two continents are plotted on a Bullard-type reconstruction. It is concluded that a small part of Northwest Africa, including Morocco, may originally have been attached to North America, and that the rest of Africa was widely separated from North America at that time. Collision between Africa and Eastern North America probably occurred during the Carboniferous period, resulting in the formation of the Hercynian fold-belts along the present-day North Atlantic margins of the two continents and the welding of extreme Northwest Africa to the rest of the continent. Subsequent separation probably took place along a slightly different line, so that Morocco was left attached to Africa. Radiometric dating results from Moroccan igneous intrusions paralleling the local coastline yielded a mean age of 183 my, i. e. Lower Juraszic. other evidence suggests that the Mesozoic rifting between North America and Northwest Africa began at this time, and it is considered likely that the intrusion of the Moroccan Lower Jurassic igneous bodies was directly related to this event. Palaeomagnetic results from the Moroccan Cretaceous sediments indicate a gradual widening of the North Atlantic Ocean during the course of the Mesozoic. There are no significant differences between mean Triassic, Jurassic and Cretaceous poles from Morocco, and corresponding poles from South-east and Central Africa, and it is concluded that no significant post-Triassic relative tectonic movement has occurrcd between North-west and South-east Africa. The second part of the thesis (Chapters 9 and 10) describes an attempt at the AF demagnetisation of sediments by means of alternating fields of strangths up to 5000 Oe. Experiments were performed to investigate the effects of small asymmetries (up to 5%) in waveform of the applied alternating field, and it was shown that asymmetrics present in 'the natural AC mains supply at the Close House Laboratory at certain times, of the day are sufficiently high to produce appreciable AMM's in rock samples, if no sample rotation system is used during the demao-natisation process

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